IAM - 22523 Notes On Unit 3 Three Phase Alternator Industrial Ac Machines

 IAM - 22523 Notes On Unit 3 Three Phase Alternator Industrial Ac Machines Download Link ⬇️

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. 

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