DHANALAKSHMI SRINIVASAN INSTITUTE OF RESEARCH AND TECHNOLOGY SIRUVACHUR

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1 DHANALAKSHMI SRINIVASAN INSTITUTE OF RESEARCH AND TECHNOLOGY SIRUVACHUR DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING QUESTION BANK (PART B) YEAR/SEMESTER :III/V SUB CODE/SUB NAME : EE2302/ELECTRICAL MACHINES II `

This is also called projected pole type as all the poles are projected out from the surface of the rotor. 2.

2 UNIT I 1. (a) (i)explain in detail, with a neat sketch the constructional details of an alternator. Stator: Rotor: 1.stator is a stationary armature. 2.This consists of a core and the slots Salient pole type: 1.This is also called projected pole type as all the poles are projected out from the surface of the rotor. 2.Preferred for low speed alternators ranging from 125 r.p.m to 500 r.p.m. Smooth cylindrical type: 1.This is also called as non salient type on non projected pole type of rotor. 2.The poles are not projecting out 3. Preferred for high speed alternator are called turbo alternators.

3 2.Explain voltage regulation calculation of an alternator ZPF method. 1.To determine the regulation using Potier reactance 2.Phasor diagram Phasor diagram Complete phasor diagram under excitation Potier reactance : X Lph = l(rs) or l(bc)/(i aph )FL Ώ 3. From the first principles of and alternator, derive its emf equation. E.M.F.per turn=2*(emf//conductor)=2*(2fφ) = 4fΦ volts Average Eph= Tph*4fΦ Kf = RMS/Average=1.11 RMS value of Eph = Kf* Averagvalue Eph= 4.44 fφtph volts.

4 (b) Using slip test, explain the procedure of determining X d and X q 1.The method used to determine the X d andx q, the direct and quadrature axis reactance is called slip test. 2.Circuitdiagram 3.Waveform Xd=Maximum voltage/minimum current= (Vt) line(at minimum Ia)/X q 4.With neat sketch describe the principle of operation of salient pole alternator. Principle of Operation: 5. Elaborate the discussion on capability curve with its boundaries of synchronous Machine. Note : Circuit diagram of capability curve for an alternator. 1.If synchronous generator is supplying power at constant frequency to a load whose power factor is constant then curve showing variation of field current versus armature current when constant power factor load is varied is called compounding curve of alternator 2.constant power factor load is varied in called compounding cure for alternator. 1.Position 1 &3:conductor and flux parallel so EMF zero 2.Position 2&4:flux cuts the conductor, EMF induced is maximum. 3.The capability curves for the synchronous generator specifies the bounds within which it can operate safely. 4.The field current should not exceed its limiting value as it may cause rotor heating. 6. Discuss about the parallel operation of two alternators with identical speed/load characteristics. i. Help in operating the alternator near its full load capacity so that efficiency will also be better..

5 ii. The operating cost reduced. iii. economical and optimum utility of the alternator. continuity and reliability Diagram of load sharing between two alternators. 7.The following open circuit and short circuit test are made on a 6000V, star connected, 2 pole, 60Hz turbine driven alternator. If I= 125A, at open circuit terminal voltage=8200v. With armature slot circuited I= 125A. IL=800 A(line). At the rated load and upf armature loss is 1.5% of output. Determine the percentage regulation at rated load and 0.8 p.f. I aph = I L V ph = V L / Z s = V oc /I sc, X s = Reg = (E 1ph ) 2 /Vph *100 Ans : Explain armature reaction in three phase alternator. 1.The flux produced by the armature is called armature flux. 2.Second is produced by the field winding called main flux. 3.Power factor : 1.Zero leading power factor (with phasor diagram) 2.Zero lagging power factor(with phasor diagram 3.Unity power factor(with phasor diagram UNIT II 1.Draw the equivalent circuit and phasor diagram of a synchronous motor

6 V T = I a R a + j I a X L + ji a X ax +E f E f = V T - I a Z s 2.Explain V- curves of synchronous motor. Inverted V curves T Excitation varied from low to high value Ia decrease and it is very less at unity PF. 3.What are the starting methods of synchronous motors?

7 1.Using Ponymotors a.rotor brought to Ns using pony motor 2.Using Damper winding a.additional winding in the slots are shorted,at start like Squirrel cage IM it runs at subsychronous speed. b.when DC supply given it rotates at synchronous speed. 3.Using small DC machine a.large synchronous motor couples with small dc M/C to make the machine to run at synchronous A 3000 V, 3 phase synchronous motor running at 1500 r.p.m. has its excitation kept constant corresponding to no load terminal voltage at 3000V. determine the power input, power factor and torque developed for an armature current of 250 A, if the synchronous reactance is 5 ohm per phase and armature resistance is neglected. Hint : Z s =R a +jx s T g Pin N 60 2 s E V E 2V E cos bph ph Rph ph Rph P in Tg x P in 3V I L cos L 5.Explain effect of changing field current excitation at constant load. Or Describe the operation of synchronous motor working at constant load with variable excitation. Excitation systems are usually 125 V up to ratings of 50kW.

A large flywheel is used to carry the exciter through short periods of severely reduced system voltage. 6.Give Brief notes on Brushless Excitation System: A.

8 A.Conventional dc shunt generator mounted on the shaft of the synchronous machine furnishes the field excitation B.An induction motor is used instead of a synchronous motor because in a severe system disturbance a synchronous motor may pullout of synchronism with the system. C.A large flywheel is used to carry the exciter through short periods of severely reduced system voltage. 6.Give Brief notes on Brushless Excitation System: A.The brushless excitation system eliminates the usual commutator, collector rings, and brushes. The B.permanent magnet pilot exciter has a stationary armature and a ro- tating permanent magnetic field. C. The output of the ac exciter is rectified by diodes and delivered to the field of the turbo generator. D.Used extensively in the much smaller generators employed in aircraft applications where reduced atmospheric pressure intensifies problems of brush deterioration. 7.Draw and explain the phasor diagram of a synchronous motor operating at lagging and leading power factor E is the peak induced voltage, e E cos(ω t) d m If the damper coil inductance is neglected, the damperbar current can be approximated b

9 i I cos(ω t) d m 8.Explain Hunting of synchronous motor. angle. A.Rotor pole axis falls back with respect to stator,the angle by which rotor retards called load B.Sudden removal/application of load is called swinging/hunting. C.Due to Larger fluctuation in current motor may come out of synchronism D.Motor subjected to large electrical stress. 9Give brief notes on Synchronous condenser. A. This leading power factor can be exaggerated by removing the mechanical load and over exciting the field of the synchronous motor. Such a device is known as a synchronous condenser. B. A synchronous condenser is operated between a motor and a generator It can compensate either a leading or lagging power factor.

synchronous condensers to absorb or produce reactive power on a transient basis stabilizes the power grid against short circuits and other transient fault conditions.

Explain the principle of operation of 3-phase induction motor and explain how the rotating magnetic field is produced by three phase currents. electromagnetic induction.

10 synchronous condensers to absorb or produce reactive power on a transient basis stabilizes the power grid against short circuits and other transient fault conditions. Transient sags and dips of milliseconds duration are stabilized UNIT III 1. Explain the principle of operation of 3-phase induction motor and explain how the rotating magnetic field is produced by three phase currents. electromagnetic induction. When a three phase supply is given to the three phase stator winding, a rotating magnetic field of constant magnitude is produced Working principle A.Now at this instant rotor is stationary and stator flux R.M.F. is rotating it cuts the rotor conductor and EMF get induced. B.If rotor is shorted then current flows,according to Lenz lawit opposes the cause C.Force experienced by each conductor and starts to rotate.. INTERACTION OF FLUXES

11 2. A 50 HP, 6pole, 50 Hz, slip ring IM runs at 960 r.p.m on full load with a rotor current of 40 A. Allow 300 W for copper loss in S. C and 1200 W for mechanical loses, find R2 per phase of the 3- phase rotor. Soln: N s = S = P m -Mechanical loss = P out P m = P 2 -P c R 2 I 2 = P c Ans :R 2 = Ω 3. (i) Derive an expression for the torque of an induction motor and obtain the condition for maximum torque. T Φ I 2r cos Φ 2r (1) Φ E 1 T se 2 2R 2 / R (sx2 ) 2 N-m Condition for maximum torque: dt/ds = 0 S = Maximum torque, T m = Nm 4. Discuss the different power stages of an induction motor with losses. The various power losses in an induction motor can be classified as, (i)constant losses (ii) variable losses. (i)constant losses: A. core losses (Iron losses reduced by laminations),hysterisis losses reduced by using silicon steel and mechanical losses. ii) Variable losses: This include the copper losses in stator and rotor winding due to current flowing in the winding. As current changes as load changes, these losses are said to be variable losses.

Rotor copper loss = 3 I 2r 2 R2 5. An 18.65 kw, 4 pole, 50 Hz, 3 phase induction motor has friction and windage losses of 2.5% of the output. Full load slip is 4%. Find for full load.

12 Rotor copper loss = 3 I 2r 2 R2 5. An kw, 4 pole, 50 Hz, 3 phase induction motor has friction and windage losses of 2.5% of the output. Full load slip is 4%. Find for full load. 1) rotor copper loss, 2) rotor input 3) shafttorque 4) the gross electromagnetic torque. Solution: a) Rotor Cu loss/rotor gross output = s/1-s, b) Rotor input P 2 = rotor cu loss/s c) Tsh = 9.55 Pout /N: d) Gross torque Tg = 9.55 Pm/N 6. Write a brief note on induction generator. To run the induction machine as a generator, its slip must be less than zero i.e. negative. The negative slip indicates that the rotor is running at a speed above the synchronous speed. 7. Write a brief note on double cage rotor induction motor. A. Upper cage : High resistance,less leakage flux linkage,low impedance B. Lower cage : Low resistance,more linkage,high impedance C.More I flows in upper cage,high starting torque is achieved at low starting current. 8. Explain Torque-Slip characteristics of 3 phase induction motor.

13 i) Low slip region: T s R 2 / R (s X 2 ) 2 s is very small. T sr 2 / R 2 2 s At N = Ns, s = 0 hence T = 0. As no torque is generated at N = Ns, motor stops if it tries to achieve the synchronous speed. Torque increases linearly in this region, of low slipvalues. High slip region: In this region, slip is high T sr 2 / (sx 2 ) 2 1/s Where R 2 and X 2 are constants 9.Develop the equivalent circuit for a 3 phase induction motor. Ī o = Ī c + Ī m I 2r = Rotor current in running condition = E 2r /Z 2r = se 2 / R (sx2 ) R 2 /s = R 2 + R 2 (1/s 1) = R 2 + R 2 (1-s/s) So the variable rotor resistance R 2 /s has two parts, 1.Rotor resistance R 2 itself which represents copper loss. 2.R 2 (1-s)/s which represents load resistance R L. So it is electrical equivalent of mechanical load on the motor. 10. Explain the no-load and block rotor test and derive the expressions for finding the losses and efficiency of a 3 phase induction motor. No load test: In this test, the motor is made to run without any load

14 W o = 3 V o I o cosφ o cosφ o = W o / 3 V o I o I c = I o cosφ o I m = I o sin Φ o Blocked rotor test: W o = Fixed loss In this test, the rotor is locked and it is not allowed to rotate. Now W sc = 3 V sc I sc cosφ sc Cos Φ sc = W sc / 3V sc I sc W sc = 3 (I sc ) 2 R 1e R 1e = W sc /3 (I sc ) 2 X 1e = Z 1e 2 R 2 1e W sc = Stator copper loss + Rotor copper loss I SN = (V L /V sc ) I sc W SN = (I SN /I sc ) 2 W sc UNIT-IV 1. Explain the speed control of three phase induction motor by pole changing. The stator poles can be changed by following methods: 1. Consequent poles method 2. Multiple stator winding method 3. Pole amplitude modulation method. Consequent Poles Method: In this method, connections of the stator winding are changed with the help of simple switching. Due to this, the number of stator poles gets changed in the ratio 2:1. the Multiple Stator Winding Method: A.Ttwo separate stator winging are placed in the stator core.

15 B.Thus giving supply to one of the two winding and using switching arrangement, two speeds can be achieved. Pole Amplitude Modulation Method: The basic principle of this method is the modulation of two sinusoidally varying m.m.f waves, with different number of poles. f (Ө) = F sin (P/2) P 1 = P P M and P 2 = P + P M (P±P M /P) (2π/3)r = (1±P M /P) (2π/3)r 1/3 [1 - P M /P] = 1/3 [1-2/8] = ¼ = n/r satisfied 1/3[1+P M /P] = 1/3 [1+2/8] = 10/24 n/r not satisfied P 1 = P P M Discuss various starting methods of squirrel cage induction motors. Stator resistance starter Autotransformer starter Star-delta starter Rotor resistance starter Direct on line starter Stator Resistance Starter: A. Three resistances are added in series with each phase of the stator winding, B. Due to this, large voltage gets dropped across the resistances. Hence a reduced voltage gets applied to the stator which reduces the high starting current. Auto transformer starter A three phase star connected autotransformer can be used to reduce the voltage applied to the stator. Such a starter is called auto transformer starter.

16 Star-Delta Starter: 1.This is the cheapest starter of all and hence used very commonly for the induction motors. It uses triple pole double throw switch(tpdts). The switch connects the stator winding in star at start. 2. Hence per phase voltage gets reduced by the factor 1/ 3. Due to this reduced voltage, the starting current us limited. Rotor Resistance Starter: 1.. This addition of resistance in rotor to limit the high starting current 2. As motor gathers speed. The resistance gradually cut off. The operation may be manual or automatic. 3. Explain the following methods of speed control scheme. i) Cascaded connection. ii) Slip power recovery scheme. Cascaded connection: A. concatenation or tandom operation

B. The stator of the main motor is connected to the three phase supply. While the supply of the auxiliary motor is derived at a slip frequency from the slip rings of main motor.

17 B. The stator of the main motor is connected to the three phase supply. While the supply of the auxiliary motor is derived at a slip frequency from the slip rings of main motor. This is called cascading of the motors. C. If torque produced by both act in the same direction, cascading is called cumulative cascading. If torque produced by opposite direction, cascading is called differential cascading. Slip power recovery scheme: 1. To recover the slip power from the rotor and fed back to the supply using static devices. Thus controlling the speed of the induction motor. 2.Speed control below synchronous speed only. This method provides a constant torque control and its main limitation is lower power factor at reduced speeds. 3.The main advantages of this method re high efficiency and low cost. I. 4.Discuss the various speed control of 3phase induction motor from rotor side. Kramer system Scherbius system Kramer System: 1. The speed of the set is controlled by varying the field of the d.c. motor with the rheostat R. when the field resistance is changed, the back e.m.f. of motor changes. 2.Thus the d.c. voltage at the commutator changes. This changes the d.c. voltage on the d.c. of a rotary converter. Now rotary converter has a fixed ratio between its a.c. side and d.c. side voltages. Thus voltage on its a.c. side also changes.

18 Scherbius System: 1.This method requires as auxiliary 3 phase or 6 phase a.c. commutator machine which is called scherbius machine. 2.Scherbius machine is not directly connected to the main motor, whose speed is to be controlled. UNIT V 1. i) Explain the operation of a single phase induction motor using double revolving theory. Double revolving theory consider two components of stator flux, each having magnitude half of maximum magnitude of stator flux. i.e, (φ 1m /2). And rotating in opposite direction φ f is forward component, φ b is backward component Figure 1 shows the stator flux and its two components φ f and φ b. Both components are rotating and gets cut by rotor conductors. Due to cutting of flux, emf gets induced in rotors and circulates rotor current, produces rotor flux. Opposite torque cancelled,net torque is zero.

19 2. Explain the operation of shaded pole induction motor with neat diagram. projected poles. Each pole carries a copper band When the single phase a.c supply is given to stator winding due to shading provided to the poles rmf is generated. At t=t1,more I,more Flux and Large EMF At t =t 2, So dφ/dt = 0. Hence there is a little induced emf in the shading ring. Att=t 3,thecurrentandfluxdecreasing..

20 . 3.Explain the principle and operation of reluctance motor and state its applications. Note : Diagram of reluctance motor Thus the exits a phase difference between current carried by two winding and fluxes. Such two fluxes react to produce rmf called split phase technique. Minimum reluctance position and get locked magnetically. Rotor tries to align itself with the axis of rotating magnetic field in a minimum reluctance positions. minimum reluctance position and keps it magnetically locked. The rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called reluctance torque. Applications: a. Signaling devices b. Control apparatus c. Automatic regulators d. Recovery instruments e. Clock etc. 4.Explain with neat diagram the construction of single phase induction motor. Stator and Rotor. A stator winding is wound for contain definite number of poles means when excited by 1ф a.c supply stator produce the magnetic field In this type rotor consists of uninsulated cual laws, placed in the slots. The bars are permanently shorted at both the ends with the help of conducting rings called end rings. Resistance of rotor is very small. The air gap is uniform. Automatically adjusts itself for same number of poles as that of stator winding

21 5.Explain the construction and working principle of hysteresis motor. Construction: 1.Stator 2.Rotor Operation: Supply given,rmf produced between main and auxillay winding rotor poles get attracted towards the moving stator field poles. Thus rotor gets selected to hysteresis torque. The torque is constant at all speeds. relative motion between stator field and rotor vanishes. So the torque due to eddy current vanished. Only hysteresis torque is present which keeps rotor running at synchronous speed due to principle of magnetic locking. 6.Explain with a neat diagram the following types of induction motor. i) Split phase induction motor ii) Capacitor start run induction motor and also draw the slip torque characteristics. i) Split phase induction motor: stator winding called main winding. In addition auxiliary winding or starting winding. Auxiliary winding carries a series resistance such that its impedance is highly resistive in nature current I M lags voltage v by angle ф m. Due to this, the starting torque, which act only in one direction is produced.

22 Capacitor start induction run motor and also draw the slip torque characteristics: In series with the auxiliary winding the capacitor is connected.the capacitive circuit draws a loading current, i) Capacitor start motors ii)capacitor start capacitor run motors. Capacitor start motors: It is used to run the motor at a synchronous speed. When the speed is attend starting winding gets disconnected due to operation of centrifugal switch. Capacitor start capacitor run motor: In this type of motor there is no centrifugal switch and capacitor remains permanently in the circuit. This improves power factor.

23 7.Explain the working of stepper motor. Also give the application and types. Convert a train of input pulses. The average motor speed is proportional to the rate at which input pulse command is delivered.. The stepper motors are widely used in x-y plotters, floppy disk devices, machine tools, robotics, etc., Types of stepper motor: 1. Variable reluctance stepper motor. 2. Permanent magnet stepper motor 3. Hybrid stepper motor 8.The following data relates to tests as a 110V, 150kw, 50Hz, 6pole, single phase induction motor. No load test : 55V, 63W, 2.7A Blocked rotor test : 110V, 212W, 5.8A The stator winding resistance is 2.5Ω during the blocked rotor test, the starting winding is open. Determine the equivalent circuit parameter. Also find the core, friction and windage losses. Solution: N S = 120/PV sc Xeq = X 1 + X 2 Z eq = V Sc / I Sc 2X 1 = 2X 2 Req = R 1 + R 2 R eq = W Sc / (I Sc ) 2 X 1 = X 2 = Xeq / 2

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING QUESTION BANK 16EET41 SYNCHRONOUS AND INDUCTION MACHINES UNIT I SYNCHRONOUS GENERATOR 1. Why the stator core is laminated? 2. Define voltage regulation