IAM - 22523 Notes On Unit 2 Single Phase Induction Motor Industrial Ac Machines

IAM - 22523 Notes On Unit 2 Single Phase Induction Motor Industrial Ac Machines Download Link ⬇️

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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.