IAM - 22523 Notes Industrial Ac Machines PDF Free Download

 

Unit 1 Three phase Induction Motor

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Unit 2 Single Phase Induction Motor Industrial Ac

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Unit-2 Single Phase Induction Motors 2.1 Double field revolving theory, principle of making these motors self start. 2.2 Construction and working: Resistance start induction run, capacitor start induction run, capacitor start capacitor run, shaded pole, repulsion type, series motor, universal motor, hysteresis motor. 2.3 Torque-speed characteristics for all of the above motors. 2.4 Motor selection for different applications as per the load torque speed requirements. 2.5Maintenance of single phase induction motors 2.1 Double field revolving theory, principle of making these motors self start. Why single phase induction motors are not self starting: Reason for single phase induction motor doesn't have a self starting torque: OR When single phase AC supply is given to main winding it produces alternating flux. According to double field revolving theory, alternating flux can be represented by two opposite rotating flux of half magnitude. These oppositely rotating flux induce current in rotor & there interaction produces two opposite torque hence the net torque is Zero and the rotor remains standstill. Hence Single-phase induction motor is not self starting. OR When single phase A.C supply is applied across the single phase stator winding, an alternating field is produced. The axis of this field is stationary in horizontal direction. The alternating field will induce an emf in the rotor conductors by transformer action. Since the rotor has closed circuit, current will flow through the rotor conductors. Due to induced emf and current in the rotor conductors the force experienced by the upper conductors of the rotor will be downward and the force experienced by the lower conductors of the rotor will be upward .The two sets of force will cancel each other and the rotor will experience no torque .Therefore single phase motors are not self starting. Double Field Revolving Theory: The sinusoidal alternating single phase supply given to the winding of the single phase motor produces an alternating magnetic field in the air gap around the rotor. But a sinusoidal alternating single phase field, having oscillating nature, can be expressed as the sum of two oppositely rotating fields (ɸf forward rotating field & ɸb backward rotating field) having the same angular speed as the alternating field but having constant magnitude of half the amplitude of the alternating field(Ferrari’s principle).The fields ɸf and ɸb are the forward and backward rotating components each of constant magnitude of ɸ1m/2. The speed of rotation is ‘ω’ radians per second. Hence the resultant of the addition of these two fields is given by taking and adding the components along the vertical and horizontal axis. The horizontal component sum is zero as they are equal and opposite in direction at all times. The resultant is along the vertical axis always for the given configuration but varies sinusoidal as seen below. Thus representation of an alternating magnetic field in terms of two oppositely rotating fields is the concept of Double revolving field theory. Both the rotating fields are cut by rotor conductors, emfs are induced, rotor currents flow and according to basic motor principle torques are produced on the rotor. However, since the fields are oppositely rotating, the torques produced on the rotor are also opposite to each other. At start, (t = 0) (Fig. a) these two torques are equal in magnitude but opposite in direction. Each torque tries to rotate the rotor in its own direction. Thus the net torque experienced by the rotor is zero at start, hence the single phase induction motors are not self-starting. Starting Methods of a Single Phase Induction Motor: The Single Phase Motor is not self starting and hence needs an auxiliary means or equipment to start the single phase induction motor. Mechanical methods are impractical and, therefore the motor is started temporarily converting it into two phase motor. Single phase Induction motors are usually classified according to the auxiliary means used to start the motor. They are classified according to the starting methods. 2.2 Construction and working: Resistance start induction run, capacitor start induction run, capacitor start capacitor run, shaded pole, repulsion type, series motor, universal motor, hysteresis motor. 2.3 Torque-speed characteristics for all of the above motors. 1) Resistance start induction run motor: The Sp lit Phase Motor is also known as a Resistance Start Motor. It has a single cage rotor, and its stator has two windings known as main winding and starting winding. Both the windings are displaced 90 degrees in space. The main winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance. A resistor is connected in series with the auxiliary winding. The current in the two windings is not equal as a result the rotating field is not uniform. Hence, the starting torque is small, of the order of 1.5 to 2 times of the started running torque. At the starting of the motor both the windings are connected in parallel. As soon as the motor reaches the speed of about 70 to 80 % of the synchronous speed the starting winding is disconnected automatically from the supply mains. A centrifugal switch is used to disconnect the starting winding. Therefore, the switch opens and disconnects the auxiliary winding from the supply, making the motor runs on the main winding only. The phasor d iagram of the Split Phase Induction Motor is shown below. The current in the main winding (IM) lag behind the supply voltage V almost by the 90-degree angle. The current in the auxiliary winding IA is approximately in phase with the line voltage. Thus, there exists the time difference between the currents of the two windings. The time phase difference ϕ is not 90 degrees, but of the order of 30 degrees. This phase difference is enough to produce a rotating magnetic field. The Torque Speed Characteristic of the Split Phase motor is shown below. Here, n 0 is the point at which the centrifugal switch operates. The starting torque of the resistance start motor is about 1.5 times of the full load torque. The maximum torque is about 2.5 times of the full load torque at about 75% of the synchronous speed. The starting current of the motor is high about 7 to 8 times of the full load value. The direction of the Resistance Start motor can be reversed by reversing the line connection of either the main winding or the starting winding. Applications of Sp lit Phase Induction Motor: 1. Used in the washing machine, and air conditioning fans. 2. The motors are used in mixer grinder, floor polishers. 3. Blowers, Centrifugal pumps 4. Drilling and lathe machine. 2) Capacitor start induction run 1-phase induction motor: In these motors one capacitor is connected in series with the auxiliary winding along with centrifugal switch. Thus this winding along with the capacitor remains energized at starting conditions. Capacitor used serves the purpose of obtaining necessary phase displacement at the time of starting. At certain speed the centrifugal switch gets opened due to centrifugal force and the capacitor gets disconnected. The Phasor Diagram of the Capacitor Start motor is shown below. IM is the current in the main winding which is lagging the auxiliary current IA by 90 degrees as shown in the phasor diagram above. Thus, a single phase supply current is split into two phases. The two windings are displaced apart by 90 degrees electrical, and their MMF’s are equal in magnitude but 90 degrees apart in time phase. The motor acts as a balanced two-phase motor. As the motor approaches its rated speed, the auxiliary winding and the starting capacitor is disconnected automatically by the centrifugal switch provided on the shaft of the motor. Characteristics of the Capacitor Start Motor The capacitor starts motor develops a much higher starting torque of about 3 to 4.5 times of the full load torque. To obtain a high starting torque, the two conditions are essential. They are as follows:-  The Starting capacitor value must be large.  The valve of the starting winding resistance must be low. The electrolytic capacitors of the order of the 250 µF are used because of the high Var rating of the capacitor requirement. The Torque Speed Characteristic of the motor is shown below. The characteristic shows that the starting torque is high. The cost of this motor is more as compared to the split phase motor because of the additional cost of the capacitor. The Capacitor start motor can be revers ed by first bringing the motor to rest condition and then reversing the connections of one of the windings. Applications of the Capacitor Start Motor 1. These motors are used for the loads of higher inertia where frequent starting is required. 2. Used in pumps and compressors 3. Used in the refrigerator and air conditioner compressors. 4. They are also used for conveyors and machine tools. 3) Capacitor start and capacitor run single phase induction motor: The Capacitor Start Capacitor Run Motor has a cage rotor, and its stator has two windings known as Main and Auxiliary Windings. The two windings are displaced 90 degrees in space. There are two capacitors in this method one is used at the time of the starting and is known as starting capacitor. The other one is used for continuous running of the motor and is known as RUN capacitor. So this motor is named as Capacitor Start Capacitor Run Motor. This motor is also known as Two Value Capacitor Motor. Connection diagram of the Two valve Capacitor Motor is shown below Working: There are two capacitors in this motor represented by CS and CR. At the starting, the two capacitors are connected in parallel. The Capacitor Cs is the Starting capacitor is short time rated. It is almost electrolytic. A large amount of current id required to obtain the starting torque. Therefore, the value of the capacitive reactance X should be low in the starting winding. Since, XA = 1/2πfCA, the value of the starting capacitor should be large. The rated line current is smaller than the starting current at the normal operating condition of the motor. Hence, the value of the capacitive reactance should be large. Since, XR = 1/2πfCR,the value of the run capacitor should be small As the motor reaches the synchronous speed, the starting capacitor Cs is disconnected from the circuit by a centrifugal swit ch Sc. The capacitor CR is connected permanently in the circuit and thus it is known as RUN Capacitor. The run capacitor is long time rated and is made of oil filled paper. The figure below shows the Phasor Diagram of the Capacitor Start Capacitor Run Motor. Fig(a) shows the phasor diagram when at the starting both the capacitor are in the circuit and ϕ > 90⁰. Fig (b) shows the phasor when the starting capacitor is disconnected, and ϕ becomes equal to 90⁰. The Torque Speed Characteristic of a Two Value Capacitor Motor is shown below. This type of motor is quiet and smooth running. They have higher efficiency than the motors that run on the main windings only. They are used for loads of higher inertia requiring frequent starts where the maximum pull-out torque and efficiency required are higher. Capacitor start and capacitor run single phase induction Motors are used in pumping equipment, refrigeration, air compressors, etc. 4) Shaded pole induction motor: When single phase supply is applied across the stator winding, an alternating field is created. The flux distribution is non-uniform due to shading bands on the poles. The shading band acts as a single turn coil and when links with alternating flux, emf is induced in it. The emf circulates current as it is simply a short circuit. The current produces the magnetic flux in the shaded part of pole to oppose the cause of its production which is the change in the alternating flux produced by the winding of motor. Now consider three different instants of time t1, t2, t3 of the flux wave to examine the effect of shading band as shown in the figure.  At instant t1: The flux is positive and rising; hence the shading band current produces its own flux to oppose the rising main flux. Due to this opposition, the net flux in shaded portion of pole is lesser than that in un-shaded portion. Thus the magnetic axis lies in the un-shaded portion and away from shaded portion.  At instant t2: The flux is maximum; the rate of change of flux is zero. So the shading band emf and current are zero. Thus the flux distribution among shaded and un-shaded portion is equal. The magnetic axis lies in the center of the pole  At instant t3: The flux is positive but decreasing, hence according to Lenz’s rule, the shading band emf and current try to oppose the fall in the main flux. So the shading band current produces its own flux which aids the main flux. Since shading band produces aiding flux in shaded portion, the strength of flux in shaded portion increases and the magnetic axis lies in the shaded portion. Thus it is seen that as time passes, the magnetic neutral axis shifts from left to right in every half cycle, from non-shaded area ofpole to the shaded area of the pole. This gives to some extent a rotating field effect which is sufficient to provide starting torque to squirrel cage rotor and rotor rotates Applications of the Shaded Pole Induction Motor This type of motor is used to drive the devices which require low starting torque. The various applications of the Shaded Poles Motor are as follows:- 1. They are suitable for small devices like relays and fans because of its low cost and easy starting. 2. Used in exhaust fans, hair dryers and also in table fans. 3. Used in air conditioning and refrigeration equipment and cooling fans. 4. Record players, tape recorders, projectors, photocopying machines. 5. Used for starting electronic clo cks and single-phase synchronous timing motors. 5) Working principle of AC series motor: Schematic diagram of an A.C Series motor: OR Working Principle of AC Series Motor: AC series motors are also known as the modified dc series motor as their construction is very similar to that of the dc series motor. An ac supply will produce a unidirectional torque because the direction of both the currents (i.e. armature current and field current) reverses at the same time. Due to presence of alternating current, eddy currents are induced in the yoke and field cores which results in excessive heating of the yoke and field cores. Due to the high inductance of the field and the armature circuit, the power factor would become very low. There is sparking at the brushes of the dc series motor. In this type of motor, the compensating winding has no interconnection with the armature circuit of the motor. In this case, a transformer action will take place as the armature winding will act as primary winding of the transformer and the compensation winding will acts as a secondary winding. The current in the compensating winding will be in phase opposition to the current in the armature winding. Speed-torque characteristics of AC series motor: The torque speed characteristics of ac series motor is shown in figure. It is clear that AC series motor develops high torque at low speed and vice versa. It is because an increase in torque requires an increasing armature current which is also the field current in series motor. The result is that the flux is strengthened and hence speed drops(as N α 1/ φ). The main features of the torquespeed characteristics of A.C series motor are as below: 1) It has high starting torque because initially T α(Ia)2. 2) It is variable speed motor and automatically adjusts speed as load changes. 3) For DC series motor the torque obtained is somewhat high than AC series motor. Applications of A.C Series Motor: 1. Where high starting torque is required 2. Stone Crushing Machine 3. Washing Machines. 4. Mixers and grinders 5. Food processors. 6. Small drilling Machines. 7. In Electric Traction 6) Universal motor The motor which operates on both AC and DC supply is called universal motor. Construction of Universal Motor: The construction of universal motor is just similar to DC motor. It consists of a stator on which field poles are mounted. The Field coils are wound on the field poles. However, the whole magnetic path comprising stator field circuit and also rotor or armature is laminated. Lamination is necessary to minimize the eddy currents which induce while operating on AC. The rotary armature is of wound type having straight or skewed slots and commutator with brushes resting on it. The commutation on AC is poorer than that for DC because of the current induced in the armature coils. For that reason brushes used are having high resistance. Working of Universal Motor: A universal motor works on either DC or single phase AC supply. When the universal motor is fed with a DC supply, the current flows in the field winding and it produces magnetic field in the air gap. The same current also flows through the armature conductors. According to basic motor principle, when a current carrying conductor is placed in the magnetic field, it experiences a mechanical force. Thus mechanical force is exerted on the current carrying armature conductors and torque is produced on rotor. Therefore the rotor starts to rotate. When fed with AC supply, it still produces unidirectional torque. Because armature winding and field winding are connected in series, they carry same current. Hence, as polarity of AC voltage changes and current reverses its direction, the direction of current in armature conductors and magnetic field in the air-gap reverses at the same time. The direction of magnetic field and the direction of armature current reverse in such away that the direction of force experienced by armature conductors remains same. Thus unidirectional torque is produced and motor continues to run in the same direction. As motor works on AC or DC supply, it is referred as Universal motor. Speed-torque characteristics of Universal Motor: As torque increases speed decreases, the characteristics is similar with DC series motor Application of Universal Motor 1) Mixer 2) Food processor 3) Heavy duty machine tools 4) Grinder 5) Vacuum cleaners 6) Refrigerators 7) Driving sewing machines 8) Electric Shavers 9) Hair dryers 10) Small Fans 11) Cloth washing machine 12) Portable tools like blowers, drilling machine, polishers etc 7) Hysteresis Motor A Hysteresis Motor is a synchronous motor with a uniform air gap and without DC excitation. It operates both in single and three phase supply. The Torque in a Hysteresis Motor is produced due to hysteresis and eddy current induced in the rotor by the action of the rotating flux of the stator windings. Construction of Stator of Hysteresis Motor The stator of the hysteresis motor produces a rotating magnetic field and is almost similar to the stator of the induction motor. Thus, the stator of the motor is connected either to single supply or to the three phase supply. The three phase motor produces more uniform rotating field as compared to that of the single phase supply. The stator winding of the single-phase hysteresis motor is made of permanent split capacitor type or shaded pole type. The capacitor is used with an auxiliary winding in order to produce a uniform field. Construction of Rotor of Hysteresis Motor The rotor of the hysteresis motor consists of the core of aluminium or some other nonmagnetic material which carries a layer of special magnetic material. The figure below shows the rotor of the hysteresis motor. The outer layer has a number of thin rings forming a laminated rotor. The rotor of the motor is a smooth cylinder, and it does not carry any windings. The ring is made of hard chrome or cobalt steel having a large hysteresis loop as shown in the figure below. Operation of a Hysteresis Motor The following illustration shows the basic functioning of a hysteresis motor. When supply is given applied to the stator, a rotating magnetic field is produced. This magnetic field magnetises the rotor ring and induces pole within it. Due to the hysteresis loss in the rotor, the induced rotor flux lags behind the rotating stator flux. The angle δ between the stator magnetic field BS and the rotor magnetic field BR is responsible for the production of the torque. The angle δ depends on the shape of the hysteresis loop and not on the frequency. Thus, the value of Coercive force and residual flux density of the magnetic material should be large. The ideal material would have a rectangular hysteresis loop as shown by loop 1 in the hysteresis loop figure. The stator magnetic field produces Eddy currents in the rotor. As a result, they produce their own magnetic field. Torque Speed characteristic of Hysteresis Motor The speed torque curve of the motor is shown below. Curve 1 is the ideal curve, and the curve 2 is the practical hysteresis motor curve. The torque-speed characteristic of the hysteresis motor is different from an induction motor. Since, at the synchronous speed, the torque developed by an induction motor becomes zero, whereas in the hysteresis motor the torque is constant at all the speed even at the synchronous speed. Thus, from the curve, it is seen that the locked rotor, starting and pull out torque is equal. This type of motor is smoothest running, quietest single phase motor and is used for quality sound reproduction equipment like record players, tape recorders, etc. It is also employed in electric clocks and other timing devices. 2.4 Motor selection for different applications as per the load torque speed requirements. Write any two applications of each of the following : i) Shaded pole IM ii) Capacitor start induction run iii) Resistance start induction run iv) Capacitor start capacitor run. 2.5 Maintenance of single phase induction motors Maintenance can be classified into two groups: 1. Restorative (Repairs) Maintenance: It is the most primary type of repairs. It arises after a fault being carried out. Restorative Maintenance has many disadvantages like lesser time for the useful life of the machine, waste of valuable energy, etc. It is also known as Corrective maintenance. 2. Protective Maintenance: This type of maintenance form can be referred to the schedule of planned maintenance actions (i.e. scheduled maintenance) aimed at the prevention of breakdowns and failures. Examples of Protective maintenance are changing of oil, greasing, tightening of the belt, changing of filters, etc. It can also be defined as “anything that increases the life of equipment, and helps it runs more efficiently.” Further, it can be divided into two subgroups of activities  Continues monitoring;  Periodic measurements or predictive techniques. Protective maintenance will generally involve lubricating, cleaning and check for sparking brushes, vibration, loose belts, high temperature and unusual noises. So a planned inspection and maintenance is needed for vast of electrical equipment to keep in proper working condition. Considering the above discussion most common faults that can be avoided through the adoption of condition monitoring methods: Stator Winding Faults: Normally a consequence of overheating, contaminations, etc, possibly causing shorted turns, shorted coils (same phase), phase or coil to ground and single phasing. Such failures create stator electrical imbalance as well as vibrations in the current harmonic content. Bearing Faults: It can be caused by incorrect lubrication, mechanical stresses, wrong assembling, etc. They can affect all the bearing parts such as inner and outer races, cage and balls or rolls. Rotor Faults: These are usually caused by broken bars or broken end rings, rotor misalignment and imbalance. The primary focus of problems in a three-phase induction motor is in their stators and their supports. The leading causes of failures are superheating, imperfections in the isolation, mechanical bearings, and electrical failures. So the following inspection schedule (may vary depending on the type of machines and importance of that machine) must be carried out of Induction Motors. The maintenance program for every week : 1. Examine commutator and brushes. 2. Examine the starter switch, fuses, and other controls; tighten loose connections. 3. See that machine brought up to rated speed in normal time or not. 4. Check the level of oil in bearings. The maintenance program for every five/six months: 1. Clean motor thoroughly, blowing out dirt from windings, and wipe commutator and brushes. 2. Check brushes and replace any that are more than half worn 3. Examine brush holders, and clean them if dirty. Make sure that brushes ride free in the holders. 4. Drain, wash out and replace oil in sleeve bearings. 5. Check grease in a ball or roller bearings. 6. See that all covers, and belt and gear guards are in place, in good order, and securely fastened. 7. Inspect and tighten connections on motor and control. The maintenance program for every year : 1. Clean out and renew grease in ball or roller bearing housings. 2. Clean out magnetic dirt that may be clinging to poles. 3. Check clearance between shaft and journal boxes of the sleeve bearing motors to prevent operation with worn bearings. 4. Clean out undercut slots in the commutator. Check the commutator for smoothness. 5. Examine connections between commutator and armature coils. 6. Test insulation by meg-ohm meter. 7. Check air gap.

 

 

Unit 3 Three Phase Alternator Industrial Ac Machines 

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Unit-3 Three Phase Alternator 3.1 Principle of working, moving and stationary armatures. 3.2 Constructional details functions, rotor constructions Windings: Single and Double layer. 3.3 E.M.F. equation of Alternator with numerical by considering short pitch factor and distribution factor. 3.4 Alternator loading: Factors affecting the terminal voltage of alternator; 3.5 Armature resistance and leakage reactance drops. Armature reaction at various power factors and synchronous impedance. 3.6 Voltage regulation: direct loading and synchronous impedance methods. 3.7 Maintenance of alternators 3.1 Principle of working, moving and stationary armatures. https://youtu.be/tiKH48EMgKE The working principle of an alternator is very simple. It is just like the basic principle of DC generator. It also depends upon Faraday’s law of electromagnetic induction which says the current is induced in the conductor inside a magnetic field when there is a relative motion between that conductor and the magnetic field. For understanding working of alternator let us think about a single rectangular turn placed in between two opposite magnetic poles as shown above. Say this single turn loop ABCD can rotate against axis a-b. Suppose this loop starts rotating clockwise. After 90o rotation the side AB or conductor AB of the loop comes in front of S-pole and conductor CD comes in front of N-pole. At this position the tangential motion of the conductor AB is just perpendicular to the magnetic flux lines from N to S pole. Hence, the rate of flux cutting by the conductor AB is maximum here and for that flux cutting there will be an induced current in the conductor AB and the direction of the induced current can be determined by Fleming’s right-hand rule. As per this rule the direction of this current will be from A to B. At the same time conductor CD comes under N pole and here also if we apply Fleming right-hand rule we will get the direction of induced current and it will be from C to D. Now after clockwise rotation of another 90o the turn ABCD comes at the vertical position as shown below. At this position tangential motion of conductor AB and CD is just parallel to the magnetic flux lines, hence there will be no flux cutting that is no current in the conductor. While the turn ABCD comes from a horizontal position to a vertical position, the angle between flux lines and direction of motion of conductor, reduces from 90o to 0o and consequently the induced current in the turn is reduced to zero from its maximum value. After another clockwise rotation of 90o the turn again comes to horizontal position, and here conductor AB comes under N-pole and CD comes under S-pole, and here if we again apply Fleming right-hand rule, we will see that induced current in conductor AB, is from point B to A and induced current in the conductor CD is from D to C. As at this position the turn comes at a horizontal position from its vertical position, the current in the conductors comes to its maximum value from zero. That means current is circulating in the close turn from point B to A, from A to D, from D to C and from C to B, provided the loop is closed although it is not shown here. That means the current is in reverse of that of the previous horizontal position when the current was circulating as A → B → C → D → A. While the turn further proceeds to its vertical position the current is again reduced to zero. So if the turn continues to rotate the current in turn continually alternate its direction. During every full revolution of the turn, the current in turn gradually reaches to its maximum value then reduces to zero and then again it comes to its maximum value but in opposite direction and again it comes to zero. In this way, the current completes one full sine wave cycle during each 360o revolution of the turn. So, we have seen how alternating current is produced in a turn is rotated inside a magnetic field. From this, we will now come to the actual working principle of an alternator. Now we place one stationary brush on each slip ring. If we connect two terminals of an external load with these two brushes, we will get an alternating current in the load. This is our elementary model of an alternator. Having understood the very basic principle of an alternator, let us now have an insight into its basic operational principle of a practical alternator. During the discussion of the basic working principle of an alternator, we have considered that the magnetic field is stationary and conductors (armature) is rotating. But generally in practical construction of alternator, armature conductors are stationary and field magnets rotate between them. The rotor of an alternator or a synchronous generator is mechanically coupled to the shaft or the turbine blades, which is made to rotate at synchronous speed Ns under some mechanical force results in magnetic flux cutting of the stationary armature conductors housed on the stator. As a direct consequence of this flux cutting an induced emf and current starts to flow through the armature conductors which first flow in one direction for the first half cycle and then in the other direction for the second half cycle for each winding with a definite time lag of 120o due to the space displaced arrangement of 120o between them as shown in the figure below. This particular phenomenon results in three-phase power flow out of the alternator which is then transmitted to the distribution stations for domestic and industrial uses. 3.2 Constructional details functions, rotor constructions Windings: Single and Double layer. Construction Of AC Synchronous Generator (Alternator) Salient pole type alternator The main parts of an alternator, obviously, consists of a stator and a rotor. But, unlike other machines, in most of the alternators, field exciters are rotating and the armature coil is stationary. Stator: Unlike in a DC machine, the stator of an alternator is not meant to serve a path for magnetic flux. Instead, the stator is used for holding armature winding. The stator core is made up of lamination of steel alloys or magnetic iron, to minimize the eddy current losses The construction of an alternator consists of field poles placed on the rotating fixture of the machine. An alternator is made up of two main parts: a rotor and a stator. The rotor rotates in the stator, and the field poles get projected onto the rotor body of the alternator. The armature conductors are housed on the stator. An alternating three-phase voltage represented by aa’, bb’, cc’ is induced in the armature conductors thus resulting in the generation of three-phase electrical power. All modern electrical power generating stations use this technology for generation of three-phase power, and as a result, an alternator (also known as a synchronous generator) has made itself a subject of great importance and interest for power engineers. An alternator is basically a type of AC generator. The field poles are made to rotate at synchronous speed Ns = 120 f/P for effective power generation. Where, f signifies the alternating current frequency and the P represents the number of poles. In most practical construction of alternator, it is installed with a stationary armature winding and a rotating field unlike in the case of DC generator where the arrangement is exactly opposite. This modification is made to cope with the very high power of the order of few 100 Megawatts produced in an AC generator contrary to that of a DC generator. To accommodate such high power the conductor weighs and dimensions naturally have to be increased for optimum performance. For this reason is it beneficial to replace these high power armature windings by low power field windings, which is also consequently of much lighter weight, thus reducing the centrifugal force required to turn the rotor and permitting higher speed limits. There are mainly two types of rotors used in construction of alternator: 1. Salient pole type. 2. Cylindrical rotor type. Salient Pole Type The term salient means protruding or projecting. The salient pole type of rotor is generally used for slow speed machines having large diameters and relatively small axial lengths. The poles, in this case, are made of thick laminated steel sections riveted together and attached to a rotor with the help of joint. An alternator as mentioned earlier is mostly responsible for generation of very high electrical power. To enable that, the mechanical input given to the machine in terms of rotating torque must also be very high. This high torque value results in oscillation or hunting effect of the alternator or synchronous generator. To prevent these oscillations from going beyond bounds the damper winding is provided in the pole faces as shown in the figure. The damper windings are basically copper bars short-circuited at both ends are placed in the holes made in the pole axis. When the alternator is driven at a steady speed, the relative velocity of the damping winding with respect to the main field will be zero. But as soon as it departs from the synchronous speed there will be relative motion between the damper winding and the main field which is always rotating at synchronous speed. This relative difference will induce the current in them which will exert a torque on the field poles in such a way as to bring the alternator back to synchronous speed operation. The salient feature of pole field structure has the following special feature1. They have a large horizontal diameter compared to a shorter axial length. 2. The pole shoes covers only about 2/3rd of pole pitch. 3. Poles are laminated to reduce eddy current loss. 4. The salient pole type motor is generally used for low-speed operations of around 100 to 400 rpm, and they are used in power stations with hydraulic turbines or diesel engines. Salient pole alternators driven by water turbines are called hydro-alternators or hydro generators. Cylindrical Rotor Type The cylindrical rotor is generally used for very high speed operation and employed in steam turbine driven alternators like turbogenerators. The machines are built in a number of ratings from 10 MVA to over 1500 MVA. The cylindrical rotor type machine has a uniform length in all directions, giving a cylindrical shape to the rotor thus providing uniform flux cutting in all directions. The rotor, in this case, consists of a smooth solid steel cylinder, having a number of slots along its outer periphery for hosting the field coils. The cylindrical rotor alternators are generally designed for 2-pole type giving very high speed of Why Armature Winding Is Stationary In An Alternator?  At high voltages, it is easier to insulate the stationary armature winding, which may be as high as 11 kV or even more in some cases.  The generated high voltage output can be directly taken out from the stationary armature. Whereas for a rotary armature, there will be large brush contact drop at higher voltages, also the sparking at the brush surface will be a problem to look after.  If the field exciter winding is placed in the rotor, low voltage DC can be transferred safely to the exciter winding via slip-rings.  The armature winding can be braced well, to prevent deformation caused by high centrifugal force if it was in the rotor. Construction of Synchronous generator or alternator: In Synchronous generator or alternators the stationary winding is called 'stator' while the rotating winding is called 'Rotor'. Stator: The stator in the synchronous generator is a stationary armature.This consists of a core and the slots to hold the armature winding similar to the armature of a d.c generator.The stator core uses a laminated construction.It is built up of special steel stampings insulated from each other with varnish or paper.The laminated construction is basically to keep down eddy current losses. Generally choice of material is steel to keep down hysteresis losses.The entire core is fabricated in a frame made of steel plates.The core has slots on its periphery for housing the armature conductors.The frame does not carry any flux and serves as the support to the core.Ventilation is maintained with the help of holes cast in the frame. Rotor: There are two types of rotors used in the synchronous generators or alternators: 1) Salient pole rotor 2)Smooth cylindrical rotor 1) Salient pole rotor: This is also called projected pole type as all the poles are projected out from the surface of the rotor.The poles are built up of thick steel laminations.The poles are bolted to the rotor as shown in the figure.The pole face has been given a specific shape.The field winding is provided on the pole shoe.These rotors have large diameters and small axial lengths. The limiting factor for the size of the rotor is the centrifugal force acting on the rotating member of the machine. As the mechanical strength of salient pole type is less, this is preferred for low-speed alternators ranging from 125 r.p.m to 50 r.p.m.The prime movers used to drive such rotor are generally water turbines and I.C. engines. 2)Smooth cylindrical rotor: This is also called non-salient type or non-projected pole type or round rotor.This rotor consists of a smooth solid steel cylinder, having a number of slots to accommodate the field coil.These slots are covered at the top with the help of steel or manganese wedges.The unslotted portions of the cylinder itself act as the poles.The poles are not projecting out and the surface of the rotor is smooth which maintains a uniform air gap between stator and rotor. These rotors have small diameters and large axial lengths.This is to keep peripheral speed within limits.The main advantage of this type is that these are mechanically very strong and thus preferred for highspeed alternators ranging between 1500 to 3000 r.p.m. Such highspeed alternators are called 'turbo-alternators'.The prime movers used to drive such type of rotors are generally steam turbines, electric motors. Working Principle of Synchronous generator : The alternators work on the principle of electromagnetic induction. When there is a relative motion between the conductors and the flux, emf gets induced in the conductors. The dc generators also work on the same principle. The only difference in the practical synchronous generator and a dc generator is that in an alternator the conductors are stationary and field is rotating. But for understanding, the purpose we can always consider relative motion of conductors w.r.t the flux produced by the field winding. Consider a relative motion of a single conductor under the magnetic field produced by two stationary poles. The magnetic axis of two poles produced by field is vertical, shown dotted in below figure. Let conductor starts rotating from position 1.at this instant, the entire velocity component is parallel to the flux lines. Hence there is no cutting of flux lines by the conductor. So d@/dt at this instant is zero and hence induced emf in the conductor is also zero. As the conductor moves from position 1 to position 2, the part of the velocity component becomes perpendicular to the flux lines and proportional to that, emf gets induced in the conductor. The magnitude of such an induced emf increases as conductor moves from position 1 to 2. At position 2, the entire velocity component is perpendicular to the flux lines. Hence there exists cutting of the flux lines. And at this instant, the induced emf in the conductor is at its maximum. As the position of conductor changes from 2 to 3, the velocity component perpendicular to the flux starts decreasing and hence induced emf magnitude also starts decreasing.At position 3, again the entire velocity component is parallel to the flux lines and hence at this instant induced emf in the conductor is zero. As the conductor moves from 3 to 4, velocity component perpendicular to the flux lines again starts increasing. But the direction of velocity component now is opposite to the direction of velocity component existing during themovement of the conductor from position 1 to 2.Hence an induced emf in the conductor increase but in the opposite direction. At position 4, it achieves maxima in the opposite direction, as the entire velocity component becomes perpendicular to flux lines. Again from position 4 to 1, induced emf decreases and finally at the position again becomes zero. This cycle continues as conductor rotates at a certain speed. So if we plot the magnitudes of the induced emf against the time, we get an alternating nature of the induced emf shown figure above. This is the working principle of Synchronous generator or Alternator. Compare salient pole and cylindrical rotor alternator Explain the essential difference between cylindrical (smooth) and salient pole rotor used in large alternators. What type of rotor would you expect to find in:(i) A-2-pole machine (ii) A-12-pole machine Type of rotor would expected to find in: (i) A-2-pole machine:-Cylindrical (smooth) rotor (ii) (ii) A-12-pole machine:-Salient pole rotor Types of Armature Windings in Alternator: The different types of armature windings in alternators are, 1) Single layer and double layer winding 2) Full pitch and short pitch winding 3) Concentrated and distributed winding Let us see the details of each classification. 1) Single Layer and Double Layer Winding : If a slot consists of only one coil side, winding is said to be a single layer. This is shown in figure(a). While there are two coil sides per slot, one, at the bottom and one at the top the winding is called double layer as shown in figure(b).A lot of space gets wasted in single layer hence in practice generally double layer winding is preferred. 2) Full Pitch and Short Pitch Winding: As seen earlier, one pole pitch is 180° electrical. The value of 'n', slots per pole indicates how many slots are contributing 180° electrical phase difference. So if coil side in one slot is connected to a coil side in another slot which is one pole pitch distance away from the first slot, the winding is said to be full pitch winding and coil is called full pitch coil. For example, in 2 poles, 18 slots alternator, the pole pitch is n = 18/2 = 9 slots. So if coil side in slot No. 1 is connected to coil side in slot No. 10 such that two slots No. 1 and No. 10 are one pole pitch or n slots or 180° electrical apart, the coil is called full pitch coil. Here we can define one more term related to a coil called coil span. Coil Span: It is the distance on the periphery of the armature, between two coil sides of a coil. It is usually expressed in terms of number of slots or degrees electrical. So if coil span is 'n' slots or 180° electrical the coil is called 180° full pitch coil. This is shown in the figure to left. As against this if coils are used in such a way that coil span is slightly less than a pole pitch i.e. less than 180° electrical, the coils are called, short pitched coils or fractional pitched coils.Generally, coils are shorted by one or two slots. So in 18 slots, 2 pole alternator instead of connecting a coil side in slot No 1 to slot No.10, it is connected to a coil side in slot No.9 or slot No. 8, the coil is said to be short pitched coil and winding are called short pitch winding.This is shown in the below figure. Short pitch coils Advantages of Short Pitch Coils: In actual practice, short pitch coils are used as it has following advantages, 1) The length required for the end connections of coils is less i.e. the inactive length of winding is less. So less copper is required. Hence economical. 2) Short pitching eliminates high frequency harmonics which distort the sinusoidal nature of e.m.f. Hence waveform of an induced e.m.f. is more sinusoidal due to short pitching. 3) As high frequency harmonics get eliminated, eddy current and hysteresis losses which depend on frequency also get minimised. This increases the efficiency. 3) Concentrated and distributed winding: In three phase alternators, we have seen that there are three different sets of windings, each for a phase. So depending upon the total number of slots and number of poles, we have certain slots per phase available under each pole. This is denoted as 'm'. m = Slots per pole per phase = n/number of phases = n/3 (generally no. of phases is 3) For example in 18 slots, 2 pole alternator we have, 8 n = 18/2 = 9 and m = 9/3 So we have 3 slots per pole per phase available. Now let 'x' number of conductors per phase are to be placed under one pole. And we have 3 slots per pole per phase available. But if all 'x' conductors per phase are placed in one slot keeping remaining 2 slots per pole per phase empty then the winding is called concentrated winding. Key Point: So in a concentrated winding, all conductors or coils belonging to a phase are placed in one slot under every pole. But in practice, an attempt is always made to use all the 'm' slots per pole per phase available for distribution of the winding. So if 'x' conductors per phase are distributed amongst the 3 slots per phase available under every pole, the winding is called distributed winding. So in distributed type of winding all the coils belonging to a phase are well distributed over the 'm' slots per phase, under every pole. Distributed winding makes the waveform of the induced e.m.f. more sinusoidal in nature. Also in concentrated winding due to a large number of conductors per slot, heat dissipation is poor. Key Point: So in practice, double layer, short pitched and distributed type of armature winding is preferred for the alternators. Full pitch coils are to be used so if phase 1 says R is started in slot 1, it is to be connected to a coil in slot 7. so that coil span will be 6 slots i.e. 'n' slots i.e. 1 pole pitch.As distributed winding is to be used, both the slots per pole per phase (m = 2) available are to be used to place the coils. And all coils for one phase are to be connected in series. So from slot No.7 we have to connect it to coil slot No.2 and slot No.2 second end to slot No.8 and so on.After finishing all slots per phase available under the first pair of pole, we will connect the coil to slot No.13 under next pole and winding will be repeated in a similar fashion. The starting end Rs and final end Rf winding for R-phase are taken out finally. Connections for R-phase only are shown in the below figure. Now, we want to have a phase difference of 120° between 'R' and 'Y'. Each slot contributes 30° as β = 30°.So start of 'Y' phase should be 120° apart from the start of 'R' i.e. 4 slots away from the start of R. So start of 'Y' will be in slot 5 and will get connected to slot No.11 to have full pitch coil. Similarly, the start of 'B' will be further 120° apart from 'Y' i.e. 4 slots apart start of 'Y' i.e. will be in slot No.9 and will continue similar to 'R'. Finally, all six terminals of three sets will be brought out which are connected either in star or delta to get three ends R, Y and B outside to get three phase supply. The entire winding diagram with star connected windings is shown in the below figure. 3.3 E.M.F. equation of Alternator with numerical by considering short pitch factor and distribution factor. Pitch Factor or Coil Span Factor (Kc): In practice, short pitch coils are preferred. So coil is formed by connecting one coil side to another which is less than one pole pitch away. So actual coil span is less than 180°. The coil is generally shorted by one or two slots. Key Point: The angle by which coils are short pitched is called angle of short pitch denoted as 'α'. α = Angle by which coils are short pitched. Angle of short p As coils are shorted in terms of the number of slots i.e. either by one slot, two slots and so on and slot angle is β then the angle of short pitch is always a multiple of the slot angle β. α = β x Number of slots by which coils are short pitched or α = (180° - Actual coil span of the coils) This is shown on the left side figure. Now let E be the induced emf in each coil side. If the coil is full pitch coil, the induced emf in each coil side help each other. Coil connections are such that both will try to set up a current in the same direction in the external circuit. Hence the resultant emf across a coil will be algebraic sum of the two. ER = E+ E = 2E Now the coil is short pitched by angle α, the two emf in two coil sides r longer remains in phase from external, circuit point of view. Hence the resultant emf is also no longer remains the algebraic sum of the two but becomes a phasor sum of the two as shown in the figure to the left. Obviously, ER in such a case will be less than what it is in case of full pitch coil. From the geometry of the above figure, we can write, AC is perpendicular drawn on OB bisecting OB. 1(OC) = l(CB) = ER/2 and ∠BOA = α/2 cos(α/2) = OC/OA = ER/2E ER = 2Ecos(α/2) This is the resultant emf in case of a short pitch coil which depends on the angle of short pitch 'α'. Key Point: Now the factor by which, induced emf gets reduced due to short pitching called pitch factor or coil span factor denoted by Kc. It is defined as the ratio of resultant emf when the coil is short pitch to the result emf when the coil is full pitched. It is always less than one. where α = Angle of short pitch 1.The wave form of induced emf will be improved i.e. the wave form willbe very close with perfect (ideal sine wave). 2.The harmonic contents of the induced emf reduces 3.As a length required for armature winding decreases, the copper material will be saved, hence it is economical. 4.As the high frequency harmonics are illuminated, Hysteresis & eddy current losses will be reduced and this increases the efficiency Distribution Factor (Kd): Similar to full pitch coils, concentrated winding is also rare in practice. Attempt made to use all the slots available under a pole for the winding which makes the nature of the induced emf more sinusoidal. Such a winding is called distributed winding. Consider 18 slots 2 pole alternator. So slots per pole i.e. n = 9. m = Slots per pole per phase = 3 β = 180°/9 = 20° Let E = Induced emf per coil and there are 3 coils per phase. In concentrated type, all the coil sides will be placed in one slot under a pole. So induce' e,m.f. in all the' coils will achieve maxima and minima at the same time i.e. all of them will be in phase. Hence resultant emf after connecting coils in series will be algebraic sum of all the emf's as all are in phase. As against this, in distributed type, coil sides will be distributed, one each in the 3 slots per phase available under a pole as shown in the below figure. Though the magnitude of emf in each coil will be same as 'E', as each slot contributes phase difference of β° i.e. 20° in this case, there will exist a phase difference of β° with respect to each other as shown in the above figure(b). Hence resultant emf will be phasor sum of all of them as shown in the figure to the left side. So due to distributed winding resultant emf decreases. Key Point: The factor by which there is a reduction in the e.mf. due to the distribution of coilsis called distribution factor denoted as Kd. The distribution factor is defined as the ratio of the resultant emf when coils are distributed to the resultant emf when coils are concentrated. It is always less than one. where m = Slots per pole per phase β = Slot angle = 180°/n n = Slots per pole When β is very small and m is large then the total phase spread is mβ. The phasor sum of coil emf's now become the chord AB of a circle as shown in the figure to the left. Keypoint: The angle (mβ/2) in the denominator must be in radians. Generalised expression for EMF equation of an Alternator: Considering full pitch, concentrated winding, Eph = 4.44 f ΦTph volts But due to short pitch, distributed winding used in practice, this Eph will reduce factors Kc and Kd. So generalised expression for the derivation of emf equation of Synchronous generator or Alternator can be written as Eph = 4.44 Kc Kd f ΦTph volts For full pitch coil, Kc = 1 For concentrated winding Kd = 1 Key Point: For short pitch and distributed winding Kc and Kd are always less than unity. Derive the relationship between Ns and f of alternator. One complete cycle of voltage is generated in an armature coil when a pair of field poles passes over the coil, the number of cycles generated in one revolution of the rotor will be equal to the number of pairs of field poles. Define each of following terms of alternator 3.4 Alternator loading: Factors affecting the terminal voltage of alternator The factors affecting terminal voltage of alternator: 1) Load current 2) Armature resistance per phase 3) Leakage reactance per phase 4) Armature reaction reactance per phase 5) Excitation (field current) 6) Speed 7) Load power factor OR when load power factor is unity or lagging, the terminal voltage drops with increase in load, when the load power factor is leading, the terminal voltage increase with increase in load. 3.5 Armature resistance and leakage reactance drops. Armature reaction at various power factors and synchronous impedance. In case of an alternator, we supply electricity to pole to produce magnetic field and output power is taken from the armature. Due to relative motion between field and armature, the conductor of armatures cut the flux of magnetic field and hence there would be changing flux linkage with these armature conductors. According to Faraday’s law of electromagnetic induction there would be an emf induced in the armature. Thus, as soon as the load is connected with armature terminals, there is a current flowing in the armature coil. As soon as current starts flowing through the armature conductor there is one reverse effect of this current on the main field flux of the alternator (or synchronous generator). This reverse effect is referred as armature reaction in alternator or synchronous generator. In other words, the effect of armature (stator) flux on the flux produced by the rotor field poles is called armature reaction. It has two undesirable effects, either it distorts the main field, or it reduces the main field flux or both. They deteriorate the performance of the machine. When the field gets distorted, it is known as a cross magnetizing effect. And when the field flux gets reduced, it is known as the demagnetizing effect. Due to armature reaction, flux is reduced or distorted, the net emf induced is also affected and hence the performance of the machine degrades. Armature Reaction in Alternator In an alternator like all other synchronous machines, the effect of armature reaction depends on the power factor i.e the phase relationship between the terminal voltage and armature current. Reactive power (lagging) is the magnetic field energy, so if the generator supplies a lagging load, this implies that it is supplying magnetic energy to the load. Since this power comes from excitation of synchronous machine, the net reactive power gets reduced in the generator. Hence, the armature reaction is demagnetizing. Similarly, the armature reaction has magnetizing effect when the generator supplies a leading load (as leading load takes the leading VAR) and in return gives lagging VAR (magnetic energy) to the generator. In case of purely resistive load, the armature reaction is cross magnetizing only. The armature reaction of alternator or synchronous generator, depends upon the phase angle between, stator armature current and induced voltage across the armature winding of alternator. The phase difference between these two quantities, i.e. Armature current and voltage may vary from – 90o to + 90o If this angle is θ, then, To understand actual effect of this angle on armature reaction of alternator, we will consider three standard cases, 1. When θ = 0 2. When θ = 90o 3. When θ = – 90o Armature Reaction of Alternator at Unity Power Factor At unity power factor, the angle between armature current I and induced emf E, is zero. That means, armature current and induced emf are in same phase. But we know theoretically that emf induced in the armature is due to changing main field flux, linked with the armature conductor. As the field is excited by DC, the main field flux is constant in respect to field magnets, but it would be alternating in respect of armature as there is a relative motion between field and armature in the alternator. If main field flux of the alternator in respect of armature can be represented as Then induced emf E across the armature is proportional to, dφf/dt. Hence, from these above equations (1) and (2) it is clear that the angle between, φf and induced emf E will be 90o . Now, armature flux φa is proportional to armature current I. Hence, armature flux φa is in phase with armature current I. Again at unity electrical power factor I and E are in same phase. So, at unity power factor, φa is phase with E. So at this condition, armature flux is in phase with induced emf E and field flux is in quadrature with E. Hence, armature flux φa is in quadrature with main field flux φf. As this two fluxes are perpendicular to each other, the armature reaction of the alternator at unity power factor is purely distorting or cross-magnetising type. As the armature flux pushes the main field flux perpendicularly, distribution of main field flux under a pole face does not remain uniformly distributed. The flux density under the trailing pole tips increases somewhat while under the leading pole tips it decreases. Armature Reaction of Alternator at Lagging Zero Power Factor At lagging zero electrical power factor, the armature current lags by 90o to induced emf in the armature. As the emf induced in the armature coil due to main field flux thus the emf leads the main field flux by 90o . From equation (1) we get, the field flux, Hence, at ωt = 0, E is maximum and φf is zero. At ωt = 90o , E is zero and φf has maximum value. At ωt = 180o , E is maximum and φf zero. At ωt = 270o , E is zero and φf has negative maximum value. Here, φf got maximum value 90o before E. Hence φf leads E by 90o . Now, armature current I is proportional to armature flux φa, and I lags E by 90o . Hence, φa lags E by 90o . So, it can be concluded that, field flux φf leads E by 90o . Therefore, armature flux and field flux act directly opposite to each other. Thus, armature reaction of the alternator at lagging zero power factor is a purely demagnetising type. That means, armature flux directly weakens main field flux. Armature Reaction of Alternator at Leading Power Factor At leading power factor condition, armature current “I” leads induced emf E by an angle 90o . Again, we have shown just, field flux φf leads, induced emf E by 90o . Again, armature flux φa is proportional to armature current I. Hence, φa is in phase with I. Hence, armature flux φa also leads E, by 90o as I leads E by 90o . As in this case both armature flux and field flux lead, induced emf E by 90o , it can be said, field flux and armature flux are in the same direction. Hence, the resultant flux is simply arithmetic sum of field flux and armature flux. Hence, at last, it can be said that armature reaction of alternator due to a purely leading electrical power factor is the magnetizing type. Nature of Armature Reaction 1. The armature reaction flux is constant in magnitude and rotates at synchronous speed. 2. The armature reaction is cross magnetising when the generator supplies a load at unity power factor. 3. When the generator supplies a load at lagging power factor the armature reaction is partly demagnetising and partly cross-magnetising. 4. When the generator supplies a load at leading power factor the armature reaction is partly magnetising and partly cross-magnetising. 5. Armature flux acts independently of main field flux. The effect of armature reaction depends upon power factor the load: Waveforms showing the effect of armature flux: 1. Armature reaction in alternators for Unity Power factor 2. Armature reaction in alternators for Zero Power factor Lagging Load: 3. Armature reaction in alternators for Zero Power factor Leading Load: 3.6 Voltage regulation: direct loading and synchronous impedance methods. Voltage Regulation of Alternator: It is defined as the rise in voltage when full load is removed, keeping excitation & speed of alternator constant, expressed as percentage of rated terminal voltage is called “Voltage regulation”. OR It is defined as the ratio of sudden rise or fall in voltage when the load is removed suddenly to the rated terminal voltage, keeping speed & excitation of alternator constant. Following factors on which voltage regulation depends: 1.Armature resistance per phase: As armature resistance increases IaRa drop increases, which make voltage regulation poor. 2. Armature Leakage flux: If leakage flux is more, the leakage reactance XL increases which increases IaXL drop. Hence regulation becomes poor. 3. Magnitude of load current: If load current increases IaRa and IaXL drop increases and armature reaction effect also increases. Therefore terminals voltage drops which makes regulation poor. 4. Load Power factor :i) For lagging power factor the effect of armature reaction is demagnetizing and therefore the main flux reduces, considerably which causes poor regulation. ii)For unity P.f, the effect of armature reaction is cross magnetizing, therefore distortion in main flux will be resulted & hence regulation is comparatively less. iii) For leading P.f, the effect of armature reaction is strong magnetizing therefore main flux will be more stronger and so terminal voltage actually increases which gives negative regulation. Here are some methods below used to determine voltage regulation of an alternator. 1.Direct Loading method 2.Synchronous Impedance method or E.M.F method 3.Ampere-turns method or M.M.F method 4.Zero Power Factor method or Potier triangle method 5.A.S.A modified form of M.M.F method 6.Two Reaction Theory [Blondel's Theory] 1. Voltage Regulation of alternator by Direct Loading method: The below figure shown is three phase alternator on which Direct Loading test is conducted. A three phase load is connected to star connected armature with the help of TPST [Triple Pole Single Throw] switch. By using an external D.C supply, the field winding is excited. A rheostat is connected in series with the field winding, to control the flux i.e. current in the field winding. In the below figure, the prime mover shown is used to drive the alternator at Synchronous speed. Direct loading method Procedure: By using prime mover, the alternator is driven at Synchronous speed [Ns]. Now Eph is proportional to phi After giving the D.C supply to the field winding, the field current is adjusted in order to adjust flux so that rated voltage appears across the terminals. This is observed on a voltmeter connected across the terminals. Next load is connected by using the TPST switch. The load is increased in steps so that ammeter reads rated current. This is the full condition of the alternator. Again adjust the voltage to its rated value by field excitation using the rheostat. Then disconnect the entire load by opening TPST switch, keeping the speed and field excitation constant. As load is disconnected there will be no armature current and associated drops. Now the voltmeter shows a reading which is the actual value of internally induced e.m.f called no load terminal voltage. Next convert both the readings into phase values. The voltage on full load is Vph and voltage when load is thrown off is Eph. So voltage regulation of alternator or synchronous generator can be determined by using the formula %Reg = [Eph - Vph]/Vph * 100 The voltmeter is connected between two line terminals to measure o.c voltage of the alternator. For the purpose of excitation, a DC supply is connected field winding. A rheostat is also connected in series with DC supply which is used to vary the field current i.e field excitation. The value of regulation of alternator or synchronous generator obtained by this Direct loading method is accurate because a particular load at required power factor is actually connected to note down the readings. 2. Voltage regulation of synchronous generator using EMF method: Generally, we use this Synchronous Impedance Method for highspeed Alternators or synchronous generator. This method is also known as EMF method. Before calculating the voltage regulation we need to calculate the following data. 1. Armature Resistance per phase [Ra] 2. Open Circuit characteristics which are a graph between open circuit voltage [Voc] and field current. 3. Short circuit characteristics which is a graph between short circuit current [Isc] and field current. The circuit diagram to perform this O.C test and S.C test is given below. The alternator or synchronous generator is coupled with the prime mover to drive alternator at synchronous speed. The armature of the alternator or synchronous generator is connected to TPST switch. The three terminals of the switch are short circuited by an ammeter. Voltage Regulation Of Alternator Using Synchronous Impedance Method 1. O.C test Procedure: 1) By using the prime mover start the alternator or synchronous generator and adjust its speed to the Synchronous speed. 2) Note that rheostat should be in maximum position and switch on the D.C supply. 3) The T.P.S.T. switch should be kept open in the armature circuit. 4) Field current is varied from its min. value to the rated value using the rheostat. So now flux increases, which leads to increase in the induced e.m.f. The voltmeter now the actual line value of open circuit voltage. For various values of field currents, voltmeter readings are noted in a table. Now plot a graph between o.c phase voltage and field current. The graph obtained is called o.c.c . 2. S.C test Procedure: 1) After the o.c test, the field rheostat should be kept at max. Position, reducing field current to min. value. 2) Now the T.P.S.T switch is closed. 3) The armature gets short circuited because ammeter has negligible resistance. Now increase the field excitation is increased gradually till full load current is obtained through armature windings. This is observed on the ammeter connected in the armature circuit. Tabulate the values of field current and armature current values obtained. 4) Now plot a graph between s.c armature current and field current. The graph obtained is called S.C.C. The S.C.C. is a straight line passing through origin but o.c.c resembles a B.H curve of a magnetic material. Voltage Regulation Of Alternator Using Synchronous Impedance Method Voltage Regulation of synchronous generator Calculations: The above is the Voltage regulation of alternator or synchronous generator formula using Synchronous Impedance Method or EMF Method. Numerical: A 3 phase star connected alternator is rated at 1500 kVA, 13.5 kV. The armature resistance and synchronous reactance are 1.4 ohm and 25 ohm respectively per phase. Calculate percentage voltage regulation for a load 1200 kW at 0.8 leading pf. A 3-phase, star connected alternator rated at 1600 kVA 13500 V; The armature resistance and synchronous reactance are 1.5 ohms and 30 ohms respectively per phase -calculate percentage voltage regulation for a load of 1280 kW at a power factor:(i) 0.8 leading (ii) unity Calculate the value of pitch factor for a 3 phase winding of a 4 pole alternator having 36 slots and the coil is spread from 1stslot up to 7thslot. A 3-phase, 6 pole, star connected alternator revolves at 1000 r.p.m. The stator has 90 slots and 8 conductors per slot. The flux per pole is 0.05 wb (sinusoidally distributed) calculate the value of phase voltage and line voltage generated by the machine, if the winding factor is 0.96. 

 Unit 4 Synchronous motors  

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4.1 Principle of working [operation, significance of load angle. 4.2 Torques: starting torque, running torque, pull in torque, pull out torque. 4.3 Synchronous motor on load with constant excitation (numerical), effect of excitation at constant load (numerical). 4.4 V-Curves and Inverted V -Curves. 4.5 Hunting and Phase swinging. 4.6 Methods of Starting of Synchronous Motor. 4.7 Losses in synchronous motors and efficiency (no numericals). 4.8 Applications areas 4.1 Principle of working [operation, significance of load angle. Construction of Synchronous Motor: Construction front, synchronous motor consists of the stator is a stationary part and rotor is a rotating part. The stator consists of a core and the slots to hold the armature winding similar to Synchronous generator. Construction of Synchronous Motor Usually, its construction is almost similar to that of a 3 phase induction motor, except the fact that here we supply DC to the rotor, the reason of which we shall explain later. Now, let us first go through the basic construction of this type of motor. From the above picture, it is clear that how do we design this type of machine. We apply three phase supply to the stator and DC supply to the rotor. Main Features of Synchronous Motors 1. Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized. 2. The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition This motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement Stator The armature winding will be insulated by the varnish or paper. The stator’s core is made of silicon stamping with the laminated assembly which helps us to reduce the eddy current losses and hysteresis loss. Rotor Rotor poles are projected type and they are mounted on the shaft of the motor. The rotor winding will be connected to a DC source with the help of slip rings. Also, the speed of the motor is purely depending on the input supply frequency and poles. Working Principle of a Synchronous Motor The stator and the rotor are the two main parts of the synchronous motor. The stator is the stationary part of the motor and rotor is their rotating part. The stator is excited by the three-phase supply, and the rotor is excited by the DC supply. The term excitation means the magnetic field induces in the stator and rotor of the motor. The main aim of the excitation is to convert the stator and rotor into an electromagnet. The three-phase supply induces the north and south pole on the stator. The three-phase supply is sinusoidal. The polarity (positive and negative) of their wave changes after every half cycle and because of this reason the north and south pole also varies. Thus, we can say that the rotating magnetic field develops on the stator. The magnetic field develops on the rotor because of the DC supply. The polarity of the DC supply becomes fixed, and thus the stationary magnetic field develops on the rotor. The term stationary means their north and south pole remains fixed. The speed at which the rotating magnetic field rotates is known as the synchronous speed. The synchronous speed of the motor depends on the frequency of the supply and the number of poles of the motor. NS = 120f/P When the opposite pole of the stator and rotor face each other, the force of attraction occurs between them. The attraction force develops the torque in the anti-clockwise direction. The torque is the kind of force which moves the object in the rotation. Thus, the poles of rotor dragged towards the poles of the stator. After every half cycle, the pole on the stator is reversed. The position of the rotor remains same because of the inertia. The inertia is the tendency of an object to remain fixed in one position. When the like pole of the stator and rotor face each other, the force of repulsion occurs between them and the torque develops in the clockwise direction. Let understand this with the help of the diagram. For simplicity, consider the motor has two poles. In the below figure, the opposite pole of the stator and rotor face each other. So the attraction force develops between them. After the half cycle, the poles on the stator reverse. The same pole of the stator and rotor face each other, and the force of repulsion develops between them. The non-unidirectional torque pulsates the rotor only in one place and because of this reason the synchronous motor is not self-starting. For starting the motor, the rotor is rotated by some external means. Thus, the polarity of the rotor also changed along with the stator. The pole of the stator and rotor interlock each other and the unidirectional torque induces in the motor. The rotor starts rotating at the speed of the rotating magnetic field, or we can say at synchronous speed. The speed of the motor is fixed, and the motor continuously rotates at the synchronous speed. What is Load Angle: Load angle is nothing but an angle different between stator axis and rotor pole axis of the synchronous motor. For ideal motor, the load angle is zero since the rotor poles aligned with stator poles, but in practice, this is not possible. The motor has both mechanical and electrical losses, hence load angle is always present in the synchronous motor. Load angle Load angle can be calculated by using the below-mentioned formula, Refer above, the induced torque is directly proportional to the load angle δ. It is denoted by δ It is also called a power angle, torque angle and coupling angle. Note: Synchronous motor is a constant speed motor which speed does not vary with respect to load. At No-load: Fig 1.1 synchronous motor No load and Load Characteristics Look at fig 1.1, The phase voltage V and back emf Eb has some slight difference. This slight difference is called the load angle. At no-load condition, the load angle is very small since the losses in the motor are less. At load condition: While raising the load in the synchronous motor, increases the load angle even though the magnitude of the V and Ebph is same. Look at fig 1.1 the angle difference between V and Eb increases with respect to armature current Ia. Significance of load angle: 1. The Torque produced in the synchronous motor is purely depending on the load angle (sin δ). It is measured by the degree in electrical. 2. Increasing in load angle indicate the decreasing magnetic locking between the rotor poles and stator poles. 3. When δ = 90 deg. The motor reaches to the full load torque. If you increase the load further, the motor losses the synchronism. Such a torque is called pull out torque. 4.2 Torques: starting torque, running torque, pull in torque, pull out torque. Starting torque  It is torque developed by the synchronous motor when name plate voltage is applied to its armature winding. It is also called as breakdown torque.  It may as low as 10% in the case of centrifugal pump. Running torque  It is torque developed by the synchronous motor under running condition. It is determined by horse power and speed of the motor. P = Tω Pull in torque  A synchronous motor is started as induction motor during starting condition and its speed below 2 to 5% of the synchronous speed.  When the excitation ( or DC supply ) to the field winding is applied, the motor pull into synchronism resulting stator and rotor magnetic field rotates at same speed.  The amount of torque requires for synchronous motor to pull into synchronism is called as pull in torque. Pull out torque  It is maximum torque which the synchronous motor can develop without pulling out of step.  When the synchronous motor is loaded, the rotor falls back by some angle  is called as load angle.  The stator and rotor magnetic rotates at synchronous speed in spite of some load is applied on the rotor.  The synchronous motor developed maximum torque when the rotor falls back by angle 900 .  When the load increases beyond its maximum rating, the rotor steps out of synchronism and synchronous motor stop. Starting Torque: The torque is being developed at the starting time of the motor. It is also called as breakaway torque. The starting torque of the synchronous motor is purely depending on the method of starting the motor. Running Torque: The full load torque of the motor is called running torque. The running torque is defending on the motor specifications. Pull-in Torque: Let we assume the synchronous motor is started and the speed is nearer to the synchronous speed, during that time the stator pulls the rotor into synchronism, that torque is called pull-in torque. Pull out Torque: Let we assume the motor is running at the maximum torque, beyond that slight increase in load causes the motor pulls out the synchronism, that maximum torque is called pull out torque. The pull out torque will be three to four-time of the full load torque of the motor. Torque Vs Load Angle Refer to fig 1.1 of torque vs speed characteristic of the Synchronous motor. At load angle 90 degree the motor produces the maximum torque. Further increasing the loads, the magnetic locking between the stator and rotor become weak. Then the motor stops. Therefore, the maximum torque is produced by the motor without loss of synchronism is called pull out Torque. 4.3 Synchronous motor on load with constant excitation (numerical), effect of excitation at constant load (numerical). In a d.c. motor, the armature current Ia is determined by dividing the difference between V and Eb by the armature resistance Ra. Similarly, in a synchronous motor, the stator current (Ia) is determined by dividing voltage-phasor resultant (Er) between V and Eb by the synchronous impedance Zs. One of the most important features of a synchronous motor is that by changing the field excitation, it can be made to operate from lagging to leading power factor. Consider a synchronous motor having a fixed supply voltage and driving a constant mechanical load. Since the mechanical load as well as the speed is constant, the power input to the motor (=3 V*Ia *cos ф) is also constant. This means that the in-phase component Ia cos ф drawn from the supply will remain constant. If the field excitation is changed, back e.m.f Eb also changes. This results in the change of phase position of Ia w.r.t. V and synchronous motor for different values of field excitation. Note that extremities of current phasor Ia lie on the straight line AB. hence the power factor cos of the motor changes. Fig: shows the phasor diagram of the synchronous motor. (i) Under excitation The motor is said to be under-excited if the field excitation is such that Eb < V. Under such conditions, the current Ia lags behind V so that motor power factor is lagging as shown in Fig: (i). This can . be easily explained. Since Eb < V, the net voltage Er is decreased and turns clockwise. As angle ( δ δ = 90°) between Er and Ia is constant, therefore, phasor Ia also turns clockwise i.e., current Ia lags behind the supply voltage. Consequently, the motor has a lagging power factor. (ii) Normal excitation The motor is said to be normally excited if the field excitation is such that Eb = V. This is shown in Fig: 2.28 (ii). Note that the effect of increasing excitation (i.e., increasing Eb) is to turn the phasor Er and hence Ia in the anti-clockwise direction i.e., Ia phasor has come closer to phasor V. Therefore, p.f. increases though still lagging. Since input power (=3 V*Ia *cos ф) is unchanged, the stator current Ia must decrease with increase in p.f. Suppose the field excitation is increased until the current Ia is in phase with the applied voltage V, making the p.f. of the synchronous motor unity [See Fig: 2.28 (iii)]. For a given load, at unity p.f. the resultant Er and, therefore, Ia are minimum. (iii) Over excitation The motor is said to be overexcited if the field excitation is such that Eb > V. Under-such conditions, current Ia leads V and the motor power factor is leading as shown in Fig: 2.28 (iv). Note that Er and hence Ia further turn anti-clockwise from the normal excitation position. Consequently, Ia leads V. From the above discussion, it is concluded that if the synchronous motor is under-excited, it has a lagging power factor. As the excitation is increased, the power factor improves till it becomes unity at normal excitation. Under such conditions, the current drawn from the supply is minimum. If the excitation is further increased (i.e., over excitation), the motor power factor becomes leading. Note. The armature current (Ia) is minimum at unity p.f and increases as the power factor becomes poor, either leading or lagging. 4.4 V-Curves and Inverted V -Curves. The graphical representation of armature current Ia vs field current If is called V-curve since the final view looks like English letter V. At the same time the power factor vs field current is called inverted V-curve of a synchronous motor. Experimental setup of synchronous motor Just you connect, two three-phase wattmeters and an ammeter to the stator winding of the synchronous motor. Also, connect another ammeter to the field input supply. Continue this experiment, with different field current as well as various load conditions such as load, half load and full load. Here stator side connected ammeter gives you armature current Ia and field-side connected ammeter gives you field current If, V curve Ia Vs If: V curve Take If reading at x-axis and Ia reading at Y-axis. Connect all the reading points, we get a V-shaped curve for a synchronous motor is shown in Figure 1.1. At no-load condition, the real power supplied to the machine is zero since Ia=0, at unity power factor. When you increase the field current above that point, the line current increases with respect to applied field current, (Here the reactive current flows from the motor to source). in such case, the synchronous motor can be used as a synchronous capacitor. At load condition, the motor draws a small amount of real current at unity powe r factor. After plotting the graph, the same V shape is still maintained. V curve at load Inverted V-curve: Now take the reading of the power factor in the y-axis and field current in the x-axis and plot a graph. Look at the shape of the graph, it seems like inverted V. Inverted V curve Power factor vs If Frequently asked questions: What is the use of the V curve and Inverted V-curve? Just understand the characteristics of the synchronous motor both the curves are used. How to calculate the power factor of the synchronous motor? In modern days, we can use digital energy meter to calculate power factor, armature current, the power consumed by the motor etc, but to calculate the pf traditionally, we can use two wattmeter method with the help of below mentioned formula 4.5 Hunting and Phase swinging. Function of Damper Winding ( Effect of hunting in the Synchronous Motor )  When the synchronous motor is driven for variable load such as shears, punching press, compressors etc., its rotor falls back or advance by some angle  .  When the load on the motor increases, its rotor falls back by some angle .  Similarly when the load on the motor decreases, its rotor advances by some angle  .  The rotor falls back or advances according to nature of the load. The oscillation of rotor about its main axis is called as Hunting.  The rotor starts overshoot and pulled back due to variation of load.  The rotor starts oscillating about its new position corresponding to new load.  If the time period of the oscillation is equal to natural time period of the machine, the mechanical resonance set up.  The amplitude of these oscillations is built up such that machine may out of synchronism. Function of Damper winding  The rotor oscillation may dampen out by employing damper winding in the faces of field poles of the motor.  The damper winding is nothing but copper bar embedded in the rotor faces. The copper bars are short circuited at both ends.  The motion of rotor sets up eddy current in the damper winding.  The direction of this eddy current is such that it suppresses the rotor oscillations.  The damper winding does not prevent completely oscillation but reduce to some extent.  The function of the damper winding in the synchronous generator is to suppress the negative sequence field and to dampen oscillation whereas it is used to provide starting torque and reduce effect of hunting to some extent in the synchronous motor. Causes of hunting  Sudden change of load  Sudden change in field current  Variation of load torque Loss of Synchronism  Produces mechanical stress on the shaft  Cause temperature rise  Produces resonance condition Reduction of hunting  Using flywheel  Using damper winding 4.6 Methods of Starting of Synchronous Motor. Starting Methods of synchronous motor: 1. Reduced frequency method 2. External driver Method 3. Using Induction motor 4. Using a DC machine 5. Using damper winding 6. Slip ring assembly Variable Frequency or reduced frequency Method: As you know, the synchronous speed is directly proportional to the supply frequency. By reducing the supply frequency, we can reduce the speed of the synchronous motor’s stator magnetic fields. Hence by controlling the speed of the stator magnetic field, you can lock the stator and rotor magnet easily. This is the basic concept behind the variable frequency starting method. Generally, variable frequency drives are used to reduce the supply frequency. VFD consists of rectifier/converter and inverter module. These modules can be used to convert the constant input frequency to any desired output frequency from a fraction to full rated frequency. VFD – Starting of synchronous motor Also, such VFD units are also used to control the motor’s speed to a very low value for starting, and then raise it up to the desired operating speed for normal running. While operating a synchronous motor lower than the rated speed, its back emf is generated in the motor also lower than the normal. If the back emf is reduced in magnitude, then the applied voltage to the motor must be reduced as well in order to keep the stator current at safe levels. That’s why VFDs are designed to reduce the voltage along with its frequency. It means V/F ratio must be constant at any instant. Motor starting with the external driver circuits: The second way to starting of a synchronous motor is by coupling an external driver which help the synchronous motor to reach the full speed. Here the motor is connected like a generator in the power system. Once the motor is reached to full speed starting motor will be detached from the synchronous motor. The synchronous machine can be paralleled with its power system as a generator, and the starting motor can be detached from the shaft of the machine. Once the stating motor is disconnected means, the speed of the SM starts to fall down, the rotor magnetic field Br falls behind stator rotating magnetic field Bs. And the synchronous machine starts to act as a motor. Actually we have two kinds of starting motors such as 1. An induction motor or pony motor Pony Motor starting Here left side motor indicate – induction motor and right side motor indicate synchronous motor. A small (also called as a pony) Ac induction motor will be connected with the synchronous motor. The synchronous motor runs along with the Induction motor initially. If the motor reaches to rated RPM, the DC excitation will be turned on. If the synchronisms is established the induction motor will be decoupled from the synchronous motor. 2. DC machine: The higher rating synchronous motor will be started with the DC machines only. Initia lly the DC machine act as a motor which drives the synchronous motor from 0 speed to rated speed. Hence the synchronism will be established. At the same time, the DC machine acts as a DC exciter and which provide DC supply to the synchronous motor’s rotor. Using Damper winding: The lower rating synchronous motor’s rotor field winding is designed along with the separate copper bars and whose end is short-circuited with the help of end rings. This additional winding is called damper winding and it is the most popular methods of starting synchronous motor. Synchronous motor rotor with damper winding Damper winding of the synchronous motor rotor Consider you turn on the three-phase voltage to the stator of the synchronous motor, the motor starts rotating at sub synchronous speed. Here synchronous act as an induction motor. If you turned on the field supply means, the stator pulls the rotor to its synchronous speed and the rotor start rotating at synchronous speed. During synchronous speed, the relative motor between the stator rotating magnetic field and damper winding is zero, hence the synchronous motor cannot produce induced emf in the damper winding. The damper winding is active at the time of stating only, after reaching synchronous speed, the damper winding act as dummy winding. The major disadvantage of this starting method of a synchronous motor is, the motor draws high starting current like an induction motor. Hence different types of starting method such as star-delta starter, autotransformer starter need to be employed. Slip ring Induction motor: In above starting method, the damper winding/induction motor does not provide the high starting torque to the synchronous motor, that’s why slip ring assembly is introduced in the field winding of the synchronous motor’s rotor. Here the end winding of the damper bar will be connected to the resistance circuit. The rotor acts as a slip ring rotors of the induction motor. as you know the rotor resistance is directly proportional to the starting torque. At starting high resistance circuit will be added and the resistance will be cut off with respect to the speed. When the motor reaches to sub-synchronous speed, the resistance across the rotor will be reduced. You have to turn on the DC excitation at sub synchronous speed, then the stator pulls the rotor to synchronous speed. The motor rotates at synchronous speed continuously. By this method of starting, we can achieve high starting torque. However, you have to follow some certain procedure to avoid electrical and mechanical damage to the motor. How to start synchronous motor: Step1: Turn on the input three-phase voltage to the stator and the same produce RMF at synchronous speed. Step3: Start the external driver motors with the direction of the rotating magnetic field. Run the driver circuit to reach the rated speed. Step2: Now turn on the DC excitation to the rotor. Now you see, the stator pulls the rotor to synchronous speed. The motor runs at synchronous speed. Step4: Stop the driver circuit. 4.7 Losses in synchronous motors and efficiency (no numericals). Unlike the induction motor, the synchronous machine also has power input to the field windings. The power flow diagrams are discussed in more detail below, but first, we will consider the losses in the synchronous machine. FIGURE 1: Power flow for a synchronous motor. Types of Losses in Synchronous Machines The losses in the synchronous machine are similar to those of the transformer and other types of rotating machines. Like all electrical machines, synchronous machines have copper, steel, rotational, and stray losses. Whether the machine operates as a motor or as a generator, the losses can be summed as Ploss=Pscl+Prl+Pfw+Pcore+Pstray(2)Ploss=Pscl+Prl+Pfw+Pcore+Pstray(2) Where Pscl = the copper loss in the stator (armature) windings Pri = losses in the rotor, which includes the copper losses of the field windings, the losses in the excitation system, and copper losses in the damper windings Pfw = friction and windage losses Pcore = hysteresis and eddy current losses, primarily in the stator iron Pstray = stray losses not otherwise accounted for in the calculation of the other losses Copper Losses Copper losses are found in all the windings in the machine. By convention, they are computed using the DC resistance of the winding at 75°C. The actual resistance depends on the operating frequency and flux conditions. Any difference between the actual and computed copper loss is accounted for under the stray loss category. Brush losses for machines with slip rings are normally neglected and accounted for under the stray loss category. Mechanical Losses The mechanical or friction and windage losses are due to the friction in the bearings and the energy that is dissipated in turning the rotor through the air inside the machine. The rotational losses can be determined by driving the machine at rated speed with no load or excitation. Rotational losses are frequently lumped with core loss and determined at the same time. Open-Circuit or No-Load Core Losses The core loss due to hysteresis and eddy currents is measured at no load, and when combined with the mechanical losses, they constitute the no-load rotational loss. The difference between the measured and actual core loss is put in the stray loss category. Stray Loss Stray losses include the difference between the actual losses and their calculated values, as well as losses that are not specifically calculated, such as the brush contact loss. Power Flow in Machines Motor Power Flow Motors receive electrical power at the armature as their inputs and deliver mechanical power at the shaft of the machine. For the power flow diagram, shown in Figure 2, the electrical power is input at the left. The copper losses are subtracted, leaving the developed power in the middle. The developed power is represented in the equivalent circuit of the synchronous motor by the power in the controlled voltage source and is calculated from equation 3. From the developed power, we subtract the friction and windage, core, and stray losses to find the output mechanical power. 4.8 Applications areas Application of Synchronous Motors 1. Synchronous motor having no load connected to its shaft is used for power factor improvement. Owing to its characteristics to behave at any electrical power factor, it is used in power system in situations where static capacitors are expensive. 2. Synchronous motor finds application where operating speed is less (around 500 rpm) and high power is required. For power requirement from 35 kW to 2500 KW, the size, weight and cost of the corresponding three phase induction motor is very high. Hence these motors are preferably used. Ex- Reciprocating pump, compressor, rolling mills etc. Synchronous Motors Advantage: 1. The speed of the motor is constant irrespective of the load changes. 2. The overexcited synchronous motor [leading power factor] can be used as a reactive power generator, the same principle is used in the transmission and distribution. 3. The efficiency of the Synchronous motor is high. 4. They are mechanically stable irrespective of the air gap. 5. Can be operated at variable power factor. Synchronous Motors Disadvantages: 1. They are not self-starting motors 2. High cost and cost vs Kw is higher than the three-phase IM. i.e Siemens make 2HP 3 Phase IM costs around Rs. 5,000 but the same capacity motor in SM will be 20,000 rupees, almost 4 times of the IM. 3. Field winding needs a DC external source. 4. Slip rings are connected the excitation circuit with the rotor circuit. It causes additional maintenance cost. 5. Hunting 6. If the load torque is higher than the motoring torque means, the motor comes to standstill position, again we need to restart the motor. Synchronous Motor is Not Self Starting: Why synchronous motor is not self-starting? Do you know the real reason? Do know, actually the net torque developed in the motor is zero at starting of the motor, that’s why the motor cannot rotate the rotor shaft at the time of starting. Let see why the torque is zero. For simple understanding, we take a single electrical cycle of 50Hz. Here, Bs is the magnetic field developed in the stator and it is rotating magnetic field. Br is the magnetic field developed in the rotor and it is a steady magnetic field. Tind is the Torque developed in the motor. Let we consider complete cycle with a different instance. Fig 1.1 Input voltage At t=0: Fig 1.2 The stator and rotor Position at t=0 Refer the picture 1.2, at t=0, the stator magnetic field Bs and rotor magnetic field Br is alleged with each other. They are magnetically locked. The torque developer in the AC machine is Hence the net torque produced in the motor is zero Tst=0 At t=1/200 s: Fig 1.3 The stator and rotor Position at t=1/200 s Refer to fig 1.1 and 1.3 At this time t=1/200 s, the rotor slightly moves but the stator magnetic field now points to the left. By the induced-torque equation, the torque produced in the rotor will be the counter-clockwise direction. At t=1/100 s: Fig 1.4 The stator and rotor Position at t=1/100 s At this instance (t=1/100 s), the rotor magnetic field Br and stator magnetic field Bs pointing in opposite directions, and hence the torque produced in the rotor is zero. t=3/200 s: Fig 1.5 The stator and rotor Position at t=3/200 s The stator magnetic field Bs now points to the right, and the resulting torque is clockwise. Hence the rotor shaft moves clockwise direction. t = 1/50 s: Fig 1.5 The stator and rotor Position at t=1/50 s Br and Bs are lined up, hence the torque produced in the rotor become zero. You observe that, during a complete cycle, first the torque produced in the rotor shaft is counterclockwise and second is clockwise. Therefore, the net torque developed in the rotor is zero. Meanwhile due to the rotor slight movement causes vibration in the motor for each electrical cycle and finally overheats. Now you understand that how the net torque developed in the motor is zero. Due to this zero torque, why synchronous motor cannot start itself.

 

Unit 5  Fractional Horsepower Motors  Industrial Ac Machines Download Link

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Unit5 Fractional Horsepower Motors (FHP) 5.1 Construction and working: Synchronous Reluctance Motor, Switched Reluctance Motor, BLDC Permanent Magnet Synchronous Motors, stepper motors, AC and DC servomotors. 5.2Torque speed characteristics of above motors. 5.3 Applications of above motors. 1 Construction and working of Synchronous Reluctance Motor Torque speed characteristics Construction of Reluctance Motor: We know about different types of synchronous motors, apart from all these motor works based on reluctance. So it is called Reluctance Motor. Here we will discuss construction and working principle of Reluctance Motor. The reluctance motor has basically two main parts called stator and rotor. the stator has a laminated construction, made up of stampings. The stampings are slotted on its periphery to carry the winding called stator winding. The stator carries only one winding. This is excited by single-phase a.c. supply. The laminated construction keeps iron losses to a minimum. The stampings are made up of material from silicon steel which minimises the hysteresis loss. The stator winding is wound for certain definite number of poles. The rotor has a particular shape. Due to its shape, the air gap between stator and rotor is not uniform.No d.c supply is given to the rotor. The rotor is free to rotate. The reluctance i.e., the resistance of the magnetic circuit depends on the air gap. More the air gap, more is the reluctance and vice-versa. Due to the variable air gap between stator and rotor, when the rotor rotates, reluctance between stator and rotor also changes. The stator and rotor are designed in such a manner that the variation of the inductance of the windings is sinusoidal with respect to the rotor position. The construction of Reluctance Motor is shown in figure(a) while the practical rotor of Reluctance Motor is shown in figure(b) below. Construction of reluctance motor Working Principle of Reluctance Motor: The stator consists of a Single Winding called main winding. But single winding cannot produce rotating magnetic field. So for production of rotating magnetic field, there must be at least two windings separated by the certain phase angle. Hence stator consists of an additional winding called auxiliary winding which consists of a capacitor in series with it. Thus there exists a phase difference between the currents carried by the two windings and corresponding fluxes. Such two fluxes react to produce the rotating magnetic field. The technique is called split phase technique of production of the rotating magnetic field. The speed of this field is the synchronous speed which is decided by the number of poles for which stator winding is wound. The rotor carries the short-circuited copper or aluminium bars and it acts as a squirrel-cage rotor of an induction motor. If an iron piece is placed in a magnetic field, it aligns itself in a minimum reluctance position and gets locked magnetically. Similarly, in the reluctance motor, rotor tries to align itself with the axis of rotating magnetic field in a minimum reluctance position. But due to rotor inertia, it is not possible when the rotor is standstill. So rotor starts rotating near synchronous speed as a squirrel cage induction motor. When the rotor speed is about synchronous, stator magnetic field pulls rotor into synchronism i.e. minimum reluctance position and keeps it magnetically locked. Then rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called the reluctance torque. Thus finally the reluctance motor runs as a synchronous motor. The resistance of the rotor must be very shall and the combined inertia of the rotor and the load should be small to run the motor as a synchronous motor. Torque - speed characteristics of Reluctance Motor: The torque speed characteristics are shown in below figure. The starting torque is highly dependent on the position of the rotor. Torque speed characteristics of reluctance motor Reluctance Motor Advantages: The reluctance motor has the following advantages, 1) No d.c. supply is necessary for the rotor. 2) Constant speed characteristics. 3) Robust construction. 4) Less maintenance. Reluctance Motor Disadvantages: The reluctance motor has following limitations, 1) Less efficiency 2) Poor power factor 3) Need of very low inertia rotor 4) Less capacity to drive the loads Applications of Reluctance Motor: Reluctance motor is used in  Signalling Devices  Control Apparatus  Automatic regulators  Recording Instruments  Clocks  All timing devices  Teleprinters  Gramophones 2 Construction and working of Switched Reluctance Motor Torque speed characteristics Construction of SRM In switched reluctance motor, the stator and rotor have projected pole made up of soft iron and silicon stampings. Silicon stamping is used to reduce hysteresis losses. Stator => Inward projection Rotor => Outward projection. The rotor does not have winding and stator only carries main field winding. Each winding in the stator is connected in series with the opposite poles to increase the MMF of the circuit. It is called phase winding. Refer to fig 1.1 AA’, BB’ and CC’. Learn More: How to Run Three Phase Motor on Single Phase Power Supply Linear SRM – Fig 1.1 Pole concern, the number of poles in the stator will be around 6 to 8 numbers. But the rotor carries less number of poles with respect to the stator. The rotor poles will be 4 to 8 numbers. By increasing the number of poles we can get a low angle of rotation from the motor. The rotor’s shaft is mounted with a position sensor. The position sensor is used to determine the position of the rotor by a control circuit. The control circuit always collects the information of the rotor position and based on that the controller gives the input to the motor. Block diagram of SRM Block Diagram of SRM Fig 1.2 The DC input is connected to the driver/converter circuit and the output is connected to the motor. The rotor sensor’s feedback wire is connected to the controller circuit and it provides the position of the rotor with reference to the reference axis. Finally, the controller collects all information and based on that, reference will be given to the stator. Also, the controller monitors the motor current to protect the motor from internal and external faults. Learn More: What is Synchronous Machine? The converter circuit: Also, note that the output of the controller is DC. And the output will be as shown in the figure 1.3. Switching Pulse of SRM Fig 1.3 Working Principle The working principle of switched reluctance motor is simple, let we take an iron piece. If we keep it in a magnetic field means, the iron piece will align with the minimum reluctance position and get locked magnetically. The same principle is followed in the switched reluctance motor. The minimum reluctance portion of the rotor tries to align itself with the stator magnetic field. Hence the reluctance torque is developed in the rotor. Switched Reluctance motor Working, types In our motor, let us consider the following notation for better understanding. Stator Poles: AA’ poles axis for A phase BB’ poles axis for B phase CC’ poles axis for C Phase Rotor poles: aa’ rotor poles axis for Position 1 bb’ rotor poles axis for position 2 Now the input is given to the A-phase, other B and C phase neither maximum nor minimum, then stator pole axis AA‘ and rotor pole axis aa‘ are in alignment. Ref picture Fig 1.4 Figure 1.4 indicates that the A-phase reached the minimum reluctance position. Advantage of SRM 1. It does not require an external ventilation system as the stator and rotor slots projected. The airflow maintained between the slots. 2. The rotor does not have winding since therefore no need keeps the carbon brush and slip ring assembly. 3. Since the absence of permanent magnet, such motors are available at a cheaper price. 4. Simple three or two-phase pulse generator is enough to drive the motor 5. The direction of the motor can be reversed by changing the phase sequence. 6. Self-starting and does not require external arrangements. 7. Starting torque can be very high without excessive inrush currents. 8. High Fault Tolerance 9. Phase losses do not affect motor operations. 10.High torque/inertia ratio 11.High starting torque can be achieved. The disadvantage of Switched reluctance motor Creates Torque ripple at high-speed operation The external rotor position sensor is required. Noise level is high At a higher speed, the motor generates harmonics, to reduce this, we need to install larger size capacitors. Since the absence of Permanent Magnet, the motor has to designed to carry high input current. It increases the converter KVA requirement. Application of SRM Domestic appliances such as washing machines, vacuum cleaners, fans etc. 3 Construction and working of BLDC Motor Torque speed characteristics Working Principle and Operation of BLDC Motor BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves. When the stator coils are electrically switched by a supply source, it becomes electromagnet and starts producing the uniform field in the air gap. Though the source of supply is DC, switching makes to generate an AC voltage waveform with trapezoidal shape. Due to the force of interaction between electromagnet stator and permanent magnet rotor, the rotor continues to rotate. Consider the figure below in which motor stator is excited based on different switching states. With the switching of windings as High and Low signals, corresponding winding energized as North and South poles. The permanent magnet rotor with North and South poles align with stator poles causing motor to rotate. Observe that motor produces torque because of the development of attraction forces (when North-South or South-North alignment) and repulsion forces (when North-North or South-South alignment). By this way motor moves in a clockwise direction. Here, one might get a question that how we know which stator coil should be energized and when to do. This is because; the motor continuous rotation depends on the switching sequence around the coils. As discussed above that Hall sensors give shaft position feedback to the electronic controller unit. Based on this signal from sensor, the controller decides particular coils to energize. Hall-effect sensors generate Low and High level signals whenever rotor poles pass near to it. These signals determine the position of the shaft. Brushless DC Motor Drive As described above that the electronic controller circuit energizes appropriate motor winding by turning transistor or other solid state switches to rotate the motor continuously. The figure below shows the simple BLDC motor drive circuit which consists of MOSFET bridge (also called as inverter bridge), electronic controller, hall effect sensor and BLDC motor. Here, Hall-effect sensors are used for position and speed feedback. The electronic controller can be a microcontroller unit or microprocessor or DSP processor or FPGA unit or any other controller. This controller receives these signals, processes them and sends the control signals to the MOSFET driver circuit. In addition to the switching for a rated speed of the motor, additional electronic circuitry changes the motor speed based on required application. These speed control units are generally implemented with PID controllers to have precise control. It is also possible to produce four-quadrant operation from the motor whilst maintaining good efficiency throughout the speed variations using modern drives. Advantages of BLDC Motor BLDC motor has several advantages over conventional DC motors and some of these are  It has no mechanical commutator and associated problems  High efficiency due to the use of permanent magnet rotor  High speed of operation even in loaded and unloaded conditions due to the absence of brushes that limits the speed  Smaller motor geometry and lighter in weight than both brushed type DC and induction AC motors  Long life as no inspection and maintenance is required for commutator system  Higher dynamic response due to low inertia and carrying windings in the stator  Less electromagnetic interference  Quite operation (or low noise) due to absence of brushes Disadvantages of Brushless Motor  These motors are costly  Electronic controller required control this motor is expensive  Not much availability of many integrated electronic control solutions, especially for tiny BLDC motors  Requires complex drive circuitry  Need of additional sensors Applications of Brushless DC Motors (BLDC) Brushless DC Motors (BLDC) are used for a wide variety of application requirements such as varying loads, constant loads and positioning applications in the fields of industrial control, automotive, aviation, automation systems, health care equipments, etc. Some specific applications of BLDC motors are  Computer hard drives and DVD/CD players  Electric vehicles, hybrid vehicles, and electric bicycles  Industrial robots, CNC machine tools, and simple belt driven systems  Washing machines, compressors and dryers  Fans, pumps and blowers 4. Construction and working of Permanent Magnet Synchronous Motors and Torque speed characteristics Construction of Permanent Magnet Synchronous Motor (PMSM): The basic construction of PMSM is same as that of synchronous motor. The only difference lies with the rotor. Unlike synchronous motor, there is no filed winding on the rotor of PMSM. Field poles are created by using permanent magnet. These Permanent magnets are made up of high permeability and high coercivity materials like Samarium-Cobalt and Neodium-Iron-Boron. Neodium-Iron-Boron is mostly used due to its ease of availability and cost effectiveness. Theses permanent magnets are mounted on the rotor core. Based on the mounting arrangement of magnet on rotor core, Permanent Magnet Synchronous Motor (PMSM) can be categorized into two types: Surface Mounted PMSMs and Buried or interior PMSMs In Surface Mounted PMSM, permanent magnet is mounted on the rotor surface as shown in figure below. This type of PMSM is not robust and therefore not suited for high speed application. Since the permeability of magnet and air gap is almost same, therefore this type of construction provides a uniform air gap. Therefore, there is no reluctance torque present. Thus the dynamic performance of this motor is superior and hence used in high performance machine tool drives and robotics. In Interior or Buried PMSM, the permanent magnets are embedded into the rotor instead of mounting on the surface. This provides robustness and hence can be used in high speed applications. Due to presence of saliency, reluctance torque is present in this type of PMSM. Working Principle of Permanent Magnet Synchronous Motor (PMSM): The working principle of permanent magnet synchronous motor is same as that of synchronous motor. When three phase winding of stator is energized from 3 phase supply, rotating magnetic field is set up in the air gap. At synchronous speed, the rotor field poles locks with the rotating magnetic field to produce torque and hence rotor continues to rotate. As we know that synchronous motors are not self starting, PMSM needs to be started somehow. Since there is no winding on the rotor, induction windings for starting is not applicable for such motors and therefore variable frequency power supply for this purpose. Applications: Permanent Magnet Synchronous Motor can be used as an alternative for servo drives. It is widely used in various industrial application viz. robotics, traction, aerospace etc. 5. Construction and working of Stepper Motors and Torque speed characteristics: Construction: Stepper motor is made up of the stator and rotor. The rotor is the movable part which has no winding, brushes and a commutator. The stator is made up of multipole and multiphase winding, usually of three or four phases winding wound for a required number of poles decided by desired angular displacement per input pulses. Working: Stepper motor works on the principle of electromagnetism. The magnetic rotor shaft is surrounded by the electromagnetism stators. Rotor and stator have poles which may or may not be teethed depending upon the types of the stator. Whenever the stators have energized the rotor, it moves to align itself along with the stator. In this fashion, the stators are energized in the sequence at different poles to rotate the stepper motor. Due to the very good control of the speed, rotation, direction and angular position, these are of particular interest in industrial process control system, CNC machine, robotics, manufacturing automation system and instrumentation. Types of stepper motor: 1. Variable reluctance stepper motor. 2. Permanent magnet stepper motor. 3. Hybrid stepper motor. 1. Variable reluctance stepper motor: Variable reluctance stepper motor has the simple design with soft iron, nonmagnetic toothed rotor and wound electromagnetic stators. No attraction between the rotor and stator winding when the winding being energized since the rotor is not magnetised. When an opposite pair of winding has current switched to them, a magnetic field is produced with lines of force which pass from the stator poles through the nearest set of poles on the rotor. It gives the steps angle of 7.5 or 15 degrees. 2. Permanent magnet stepper motor: Permanent magnet stepper motor has a permanent magnet rotor that is axially magnetised. It means that it has alternating north and south poles parallel to the rotor shaft. Each pole is wound with a field winding, the coils on opposite pair of poles in series. Current is supplied from D.C source to the winding through switches. The rotor is a permanent magnet and thus when a pair of the stator poles has a current switched to it the rotor will move to line up with it. steps angle of this motor are 1.8, 7.5,15,30,34 and 90 degrees. 3. Hybrid stepper motor: The hybrid stepper motor is the combination of both permanent and variable reluctance motor. It has a permanent magnet, toothed rotor made up two sections or cup, which are opposite in polarity and whose teeth are offset to each other. The rotor set itself in the minimum reluctance position in response to a pair of stator coil energised .Step angle of this motor are 0.9, or 1.8 degrees. You can also watch this video to understand the different types and working of stepper motor. Advantages: 1. The rotation angle is proportional to the input pulses. 2. Full torque at standstill. 3. Very low-speed synchronous rotation is possible to achieve. 4. There are no brushes so it is reliable. 5. Speed is directly proportional to the frequency of the input as pulses; hence a wide range of rotational speed can be realized. 6. Low speed with high precision. Disadvantages: 1. No feedback system. 2. Low effitiency. 3. May produce more noise. 4. Difficult to operate at very high speed. 5. For the smooth move, micro stepping is required. Applications: 1. Factory automation. 2. Packaging. 3. Material handling. 4. Aerospace industry especially in avionics. 5. 3D pictures acquisition system. 6. Laser measurements. 7. Robotics. 6. Construction and working of Servomotor and Torque speed characteristics AC Servomotor Construction of AC Servomotor We have already said in the beginning that an ac servomotor is regarded as a twophase induction motor. However, ac servomotors have some special design features which are not present in normal induction motor, thus it is said that two somewhat differs in construction. It is mainly composed of two major units, stator and rotor.  Stator: First have a look at the figure shown below, representing stator of ac servomotor: The stator of ac servo motor consists of two separate windings uniformly distributed and separated at 90°, in space. Out of the two windings, one is referred as main or fixed winding while the other one is called control winding. A constant ac signal as input is provided to the main winding of the stator. However, as the name suggests, the control winding is provided with the variable control voltage. This variable control voltage is obtained from the servo amplifier. It is to be noted here that to have a rotating magnetic field, the voltage applied to the control winding must be 90° out of phase w.r.t the input ac voltage.  Rotor: The rotor is generally of two types; one is squirrel cage type while the other is drag cup type. The squirrel cage type of rotor is shown below: In this type of rotor, the length is large while the diameter is small and is constructed with aluminium conductors thus weighs less. It is to be noted here that the torque-speed characteristics of a normal induction motor have both positive as well as negative slope regions that represent unstable and stable regions, respectively. However, ac servo motors are designed to possess high stability thus, its torqueslip characteristics must not have a positive slip region. Along with this the torque developed in the motor must reduce in a linear manner with speed. To achieve this the rotor circuit resistance should have a high value, with low inertia. Due to this reason, while constructing the rotor, the diameter to length ratio is kept smaller. The reduced air gaps between the aluminium bars in the squirrel cage motor facilitate a reduction in magnetizing current. Let us now see the representation of the drag cup type rotor: This type of rotor is different in construction from that of squirrel cage one. It consists of a laminated core of aluminium around which drag cup is present with certain air gaps on both the side. These drag cups are attached with a driving shaft that facilitates its operation. The two air gaps in both sides of the core lead to reducing the inertia thus is used in applications where there is a low power requirement. Working Principle of AC Servomotor The figure below represents the AC two-phase induction motor that uses the principle of servomechanism: Initially, a constant ac voltage is provided at the main winding of the stator of the ac servomotor. The other stator terminal of the servomotor is connected to the control transformer through the control winding. Due to the provided reference voltage, the shaft of the synchro generator rotates with a particular speed and attains a certain angular position. Also, the shaft of the control transformer has a certain specific angular position which is compared with the angular position of the shaft of the synchro generator. Further, the comparison of two angular positions provides the error signal. More specifically, the voltage levels of the corresponding shaft positions are compared which generates the error signal. This error signal corresponds to the voltage level present at the control transformer. This signal is then provided to the servo amplifier which generates variable control voltage. With this applied voltage, the rotor again attains a specific speed and starts rotation and sustains until the value of the error signal reaches 0, thereby attaining the desired position of the motor in the AC servomotors. Torque-Speed Characteristics The figure below represents the torque-speed characteristics of the two-phase induction motor We have already discussed that the motor must be designed in a way to provide linear torque-speed characteristics, in which the torque changes in a linear manner with the speed. However, as we have seen in the above figure that the torque-speed characteristics here are not actually linear. This is so because it depends on the ratio of reactance to resistance. The low value of the ratio of reactance to resistance implies that motor possesses high resistance and low reactance, in such case, the characteristics is more linear that high value of the ratio for reactance to resistance. Applications of AC Servomotors Due to the various advantages offered by the AC servomotors, these majorly finds applications in the instruments that operate on servomechanism, in position controlling devices, computers. Along with this these also find applications in tracking systems, machine tools and robotics machinery. DC Servomotors: In the case of DC servomotor, the applied dc input causes the motor to rotate and acquire the desired position at the specified angle. It is a closed-loop operation and uses position feedback to precisely adjust the position at the desired angle. The DC servomotor is further classified on the basis of control provided to it. Basically, controlling to the DC servomotor is either provided from the field side or from the armature side. And this further classifies the DC servomotor. Here in this article, we will discuss each type separately. Field Controlled DC Servomotor In this type of DC servomotor, the controlling is provided to the field winding. More specifically, we can say, the controlled signal received from the amplifier is fed to the field winding. Thus, it is named so. While the armature current is maintained at a constant value using a constant current source. The figure given below represents the field controlled DC servomotor: It is to be noted here that the field in this type of dc servomotor can be either electromagnetic type where a salient pole is present with a field winding wound around it and excitation to it is provided with dc current or a permanent magnet type. Basically, according to the general torque equation of DC motor, the torque is directly proportional to the product of field flux and the armature current. However, in this specific type, the armature current is kept constant thus, T α Φ We have already discussed the amplified error signal from the servoamplifier excites the field. And in this way, the excitation provided controls the torque i.e., the rotation of the motor. In case the value of the current source applied at the armature is quite large, then for a small change in field current, there will be a proportional change in the torque of the motor. It is to be noted here that the direction in which the shaft rotates can be changed according to the polarity of the field or the by using split field dc motor. When the direction of rotation is polarity dependent then this signifies that with the change in polarity of the field, the rotational direction also changes. However, sometimes by the use of split field dc motor, the rotational direction of the motor changes. This is so because as the name suggests in this type of motor, the winding is split into two parts, where one part is wounded in a clockwise direction while the other in an anticlockwise direction. In this case, the error signal is provided at the junction of the winding. As the two windings possess a magnetic field in the opposite direction. Thus, when the error signal is provided then one side of the magnetic field will dominate the other. So, the motor rotates in the direction of the winding of the dominating side according to the error signal. Here, the ratio of reactance to resistance is quite large thereby exhibiting the high value of the time constant. This implies that for quick changes in the control signals the response will be slower. Thus, is majorly used in small rated motors. However, here the power requirement is less as the motor is controlled by the field. Armature Controlled DC Servomotor In armature controlled dc servomotor, the controlling is provided at the armature. This means, here the signal from the servoamplifier is provided at the armature and constant current is provided at the field winding. The voltage from the servoamplifier, Va(t) with resistance Ra and inductance La is provided at the armature. And this input voltage at the armature controls the shaft. The figure below represents the arrangement of armature coupled dc servomotor: Here the constant field is provided using the permanent magnets and hence no field coils are required. In the field controlled dc servomotor, we have already discussed that torque is in direct proportion the field flux and the armature current. Thus, its operating principle is such that if the field flux is quite large then even with the small change in the armature current there will be a large change in torque. Thereby making the servomotor sensitive to armature current. It is to be noted here that in armature controlled dc servomotor, the sensitivity towards the field current should be low. As the armature controlled motor must not respond to the field current. It offers a small value of the time constant so there is a rapid change in the armature current of the motor with the change in the voltage applied at the armature. Thus, it provides a faster dynamic response where the direction of rotation changes with the change in polarity of the error signal. Characteristics of DC Servomotor The figure below shows the torque-speed characteristics of armature controlled dc servomotor: It is noteworthy here that, the above-shown characteristics are similar to the torque-speed characteristics of ac servomotor.