Induction machine_Objectives

1.If two induction motors A and B are identical except that the air-gap of motor ‘A’ is 50% greater than that of motor ‘B’ then

a. The no-load power factor of A will be better than that of B
b. The no-load power factor of A will be poorer than that of B
c. The core losses of A will be more than those of B
d. The operating flux of A will be smaller than that of B

2.Which of the following statements regarding skewing of motor bars in a squirrel-cage induction motor are correct?
1. It prevents cogging.
2. It produces more uniform torque.
3. It increases starting torque.
4. It reduces motor ‘hum’ during its
operations
Select the correct answer using the codes given below:
a. 2,3 and 4
b. 1,2, and 3
c. 1, 2 and 4
d. 1, 3 and 4


3.The rotor power output of a 3-phase induction motor is 15kW and the corresponding slip is 4%. The rotor copper loss will be

a. 600 W
b. 625 W
c. 650W
d. 700 W

4.A 3-phase wound rotor induction motor, when stated with load connected to its shaft, was found to start but settee down at about half synchronous speed. If the rotor winding as well as the stator winding were star connected, then the cause of the malfunctioning could be attributed to

a. One of the stator phase windings being short-circuited
b. One of the supply fuses being blown
c. One of the rotor phases being open-circuited
d. Two of the rotor phases being open-circuited


5.In the case of a converter- inverter speed control arrangement for an induction motor operating with v/f constant and with negligible stator impedance

a. The maximum torque is independent of frequency
b. The maximum torque is proportional to frequency
c. The slip at maximum torque is proportional to frequency
d. The starting torque is proportional to frequency

6.A 3-phase induction machine draws active power ‘P’ and reactive power ‘Q’ from the grid. If it is operated as a generator, P’ and Q will respectively be
a. positive and negative
b. negative and negative
c. positive and positive
d. negative and positive

7.The deep bar rotor of an induction motor is considered equivalent to a double cage rotor for the purposes of starting and performance. At the time of starting, the current distribution in deep bar rotor slot will be as shown in



8.A 3-phase induction motor fed f a 3-phase voltage controller is suitable for driving loads whose torque is

a. constant irrespective of speed
b. inversely proportional to the square of speed
c. directly proportional to the square of speed
d. inversely proportional to speed



9. A 3-phase induction motor is used as an adjustable speed drive from zero to 2.0 per unit speed using a variable frequency inverter. In the speed range of 1.0 to. 2.0
per unit, in order to ensure satisfactory operation. Which one of the following sets of quantities is to be maintained approximately constant?

a. Voltage and power
b. Flux and torque
c. Voltage and torque
d. Flux and power


10. A 3-phase slip-ring induction motor has chopper controlled resistance in the rotor circuit. Its power factor and efficiency can be characterized respectively as

a. good and poor
b. poor and good
c. good and good
d. poor and poor


11.A 3-phase slip-ring induction motor when started picks up speed but runs stably at about half the normal speed. This is because of

a. unbalance in the supply voltages
b. non-sinusoidal nature of the supply voltage
c. stator circuit asymmetry
d. rotor circuit asymmetry


12. The phenomenon of crawling in a 3-phase cage induction motor may be due to

a. unbalance supply voltages
b. 7th space harmonic of air-gap field
c. 7th time harmonic of voltage wave
d. 5th space harmonic


13. Zero sequence impedance of a 3-phase cage induction motor is due to

a. stator circuit 3rd time harmonic
b. stator circuit 3rd space harmonic
c. rotor circuit 3rd time harmonic
d. rotor circuit 3rd space harmonic


14.Consider the following statements:
Star-delta starter is used in 3-phase induction motor because it
1. prevents heating of the motor windings.
2. ensures permissible minimum starting current.
3. is regulated by electricity authorities.
4. ensures smooth run-up to full speed.

Of these statements :
a. 1, 2 and 3 are correct
b. 2, 3 and 4 are correct
c. 1, 3 and 4 are correct
d. 1 and 2 are correct

15.Match List I with List II and select the correct answer:
List I
A. Single-phase inductor motor
B. 3-phase slip ring induction motor
C. 3-phase squirrel cage induction motor

List II
1. Rotor resistance starting
2. Not self-starting
3. Auto-transformer starting

Codes;
---A B C
a. 2 1 3
b. 1 2 3
c. 2 3 1
d. 3 1 2

16.A 3-phase induction motor is driving a content torque load of 1 Pu at 5% slip. It has a maximum torque of 2 Pu at 10% slip. Torque-speed variation in stable zone is assumed to be linear. For the stable operation of the motor, the minimum pu supply voltage is approximately

a. 0.25
b. 0.50
c. 0.707
d. 0.80

17.100. As compared to DOL starting, a cage induction motor with star-delta starting shall have

a. more starting torque
b. more starting time
c. reduced starting current
d. smoother acceleration


18. The speed of a 3-phase induction motor is controlled from 1 pu to 2 pu using a variable frequency inverter. The equivalent circuit parameter most likely to vary is

a. stator leakage inductance
b. rotor leakage inductance
c. magnetizing inductance
d. core loss resistance

19.‘Crawling’ is an induction motor is due to

a. time harmonics in supply
b. slip ring rotor
c. space harmonics produced y winding currents
d. insufficient starting torque

20.The mmf produced by the current of a 3-phase induction motor

a. rotates at the speed of rotor in the air gap
b. is standstill with respect to stator mmf
c. rotates at slip speed with respect to stator mmf
d. rotates at synchronous speed with respect to rotor


21. A 3-phase slip-ring induction motor is wound for 4 poles on stator and 6 poles on
rotor. When 3-phase balanced voltage source at 50 Hz is applied to the motor, it will run at :

a. 1500rpm
b. 1000rpm
c. 750 rpm
d. zero speed


22. If the rotor power factor of a 3-phase induction motor is 0.866, the spatial displacement between the stator magnetic field and the rotor magnetic field will be

a. 30°
b. 90°
c. 120°
d. 150°


23. The stator of a 6-pole, 3-phase induction motor is fed from a 3phase, 50 Hz supply which contains a pronounced fifth time harmonic. The speed of the fifth space harmonic field produced by the fifth time harmonic in the stator supply will be

a. 200 rpm
b. 1500 rpm
c. 1000 rpm
d. 5000 rpm


24. A 6-pole, 50 Hz wound rotor induction motor when supplied at the rated voltage and frequency with slip-ring opencircuited, developed a voltage of 100 V between any wo rings. If the rotor is driven by an external means at 1000 rpm opposite to the direction of stator field, the frequency of voltage across slip-rings will be

a. zero
b. 50 Hz
c. 100 Hz
d. 200 Hz


25. In a 3-phase squirrel cage induction motor, skewing of rotor slots reduces

a. parasitic torque and noise but increases pullout torque
b. parasitic torque and noise but increases starting torque
c. noise but increases pullout torque and parasitic torque
d. parasitic torque, noise, pullout torque and starting torque


26.The advantage of the double squirrel-cage induction motor over single cage rotor is that its

a. Efficiency is higher
b. Power factor is higher
c. Slip is larger
d. Starting current is lower


27. An induction motor when started on load does not accelerate up to full speed but runs at 1/7th) of the rated speed. The motor is said to be

a. Locking
b. Pluming
c. Crawling
d. Cogging


28.The rotor power output of 3-phase induction motor is 15 kW The rotor copper losses at a s1p of 4% will be

a. 600W
b. 625W
c. 650W
d. 700W


29. Skewing of the rotor in a three-phase squirrel-cage induction motor reduces

a. Noise, parasitic torque, starting torque and pullout torque
b. Noise and parasitic torque, but increases starting torque and pullout torque.
c. Noise and pullout torque, but increases parasitic torque and starting torque
d. Noise, parasitic torque and starting torque, but increases pullout torque

30.The starting current of a 3 phase induction motor is 5 times the rated current, while the rated slip is 4%. The ratio of starting torque to full-load torque is

a. 0.6
b. 0.8
c. 1.0
d. 1.2

Synchronous machine_objectives

1.A 3-phase alternator with high rotational inertia and negligible armature resistance was delivering power to an isolated load when the armature terminals got short circuited. After a while, the mechanical power input to the shaft of the machine is terminated at time t0. The short-circuit current Is circulating in the armature will very till the machine comes to rest at time ts. Which one of the curves shown in the following figures best reflects this variation?


2.When a synchronous motor is running at synchronous speed, the damper winding
produces
a. Damping torque
b. Eddy current torque
c. Torques aiding the developed to torque
d. No torque

3.For a given developed power, a synchronous motor operating from a constant voltage and constant frequency supply, will draw the minimum and maximum armature currents, Imin and Imax respectively, corresponding to
a. Imm at unity pf, but Imax at zero pf
b. Imax at unity pf, but Imin at zero pf
c. Both Imin and Imax at unity pf
d. Both Imin and Imax at zero pf

4.Match list-I (Type of winding) with list-Il (Feature) and select the correct answer
using the codes given below the Lists:
List I
A. Fractional slot windings
B. Chorded and distributed windings
C. Damper windings
D. Single-phase windings
List II
1. Only two- third of the pole pitch is occupied by the winding
2. Effects of slot harmonics are reduced or even eliminated
3. The effects of phase belt harmonics are reduced
4. Additional asynchro nous torque capability in projected pole motors
---A B C D
a. 2 3 4 1
b. 2 3 1 4
c. 3 2 1 4
d. 3 2 4 1


5.While conducting a “slip’ test for the determination of direct and quadratureaxis synchronous reactance’s Xd and Xq of a salient pole synhronous machine, the rotor of the machine is run with a slip’s’ and stator supply frequency ‘f’. The frequency of 1. Voltage induced across open field terminals.
2. Envelope of the armature terminal voltage,
3. Envelope of the armature current and
4. Armature current will be respectively

a. sf, sf, sf and f
b. sf, f, sf and f
c. f, sf, f and sf
d. f, (1– s) f, (2 – s) f and sf

6.If the dimensions of all the parts of a synchronous generator, and the number of field and armature turns are doubled, then the generated voltage will change by a factor of
a. 1
b. 2
c. 4
d. 8


7.Consider the following statements
The effects of slot harmonics are
1. skewing the slots
2. fractional slot winding.
3. short - chorded winding
4. distributed winding.
Of these statements
a. 1 and 2 are correct
b. 2, 3 and 4 correct
c. 1, 3 and 4 are correct
d. 1 and 3 are correct

8.Which one of the following methods would give a higher than actual value of regulation of an alternator?
a. ZPE method
b. MMF method
c. EMF method
d. ASA method


9.A turbo alternator set feeds power to a 3- phase constant-voltage,constant frequency bus. If the steam supply to the set is cut off, then the set will
a. continue to run at rated sped in the same direction
b. continue to run at a reduced speed in the same direction
c. run at rated speed in the reverse direction
d. come to a stop


10.An ideal synchronous motor has no starting torque because the
a. rotor is made up of salient poles
b. relative velocity between the stator and the rotor mmf’s is zero
c. relative velocity between stator and rotor mmf’s is not zero
d. rotor winding is highly reactive


11.A synchronous motor is operating on noload at unity power factor. If the field current is increased, the power factor will become
a. leading and the current will decrease
b. lagging and the current will increase
c. lagging and the current will decrease
d. leading and the current will increase

12.A synchronous motor with negligible armature resistance runs at a load angle of 20° at the rated frequency. If supply frequency is increased by 10%, keeping other parameters constant, the new load angle will be
a. 16°
b. 18°
c. 20°
d. 22°


13.A synchronous motor is running from an infinite bus of voltage Vt in steady-stat, at about 50% of its rated load with a powerangle , between Vt and Ef (induced voltage). The load is suddenly decreased to 25%. Ef attains its new steady-state powerangle with Vt by initially,
a. falling behind and making a complete rotation
b. advancing and making a complete rotation
c. falling behind, followed by oscillation about
d. advancing, followed by oscillation about


14.The two-reaction phasor diagram of a salient-pole synchronous generator is considered, where Ia = armature current per phase, X1 = armature leakage reactance per phase, Xd = direct-axis synchronous reactance, Xq = quadrature-axis synchronous reactance and V = generator terminal voltage, taken as reference. The voltage in phase with the direct-axis will be the phasor-sum of
a. V + Iara + IaX1
b. V + Iara + IaXd
c. V + Iara + IaXq
d. V + Iara + Ia(Xd + Xq)

15.Consider the following statements : The synchronous generators used in thermal power stations have
1. cylindrical rotors
2. slip rings and brushes
3. laminated rotor
4. stator slots in multiples of 6
5. 3-phase star connected stator windings
Which of these statements are correct?
a. 1, 2, 3 and 4
b. 1, 2, 3 and 5
c. 3, 4 and 5
d. 1, 2, 4 and 5

16.Slip test is performed to determine
a. Slip
b. direct axis reactance and quadrature axis
c. positive sequence reactance and negative sequence reactance
d. sub-transient reactance


17.Match List I (methods of full-load regulation of 25 MVA alternator at 0.8 Power factor lagging) with list II (% regulation):
List I
A. emf
B. mmf
C. ZPF
List II
1. 13%
2. 18%
3. 32%

---A B C
a. 3 1 2
b. 1 3 2
c. 2 3 1
d. 1 2 3

18.The phasor diagram of a synchronous machine connected to an infinite bus is shown in figure The machine is acting as a


a. generator and operating at a lagging power factor
b. generator and operating at a leading power factor
c. motor and operating at a leading power factor
d. motor and operating at a lagging power factor

19.An induction motor and synchronous motor are connected to a common feeder line. To operate the feeder line at unity pf, the synchronous motor should be
a. under-excited
b. over-excited
c. normally excited
d. disconnected from the common terminals


20.Consider the following statements regarding the design of distributed armature winding in a 3-phase alternator :
1. It reduces the phase belt harmonics
2. It increases utilisation of armature iron and copper
3. It increases rigidity and mechanical strength of the winding
4. It reduces copper in the overhang of the winding
Which of these statements are correct?
a. 1, 3 and 4
b. 2, 3 and 4
c. 1, 2 and 3
d. 1, 2 and 4


21.Match List I (Power factor) with list II (Armature reaction of an alternator) and
select the correct answer :

List I
A. UPF
B. ZPF lagging
C. ZPF leading
D. Intermediate Lagging

List II
1. Fully demagnetising
2. Fully magnetising
3. Cross - magnetising
4. Partly cross-magnetising and
5. partly demagnetising

---A B C D
a. 3 1 4 2
b. 3 1 2 4
c. 1 3 2 4
d. 1 3 4 2

22.In a synchronous machine, damper windings are used to
a. help in starting as a motor
b. run it as an induction motor
c. help in starting as a motor and to reduce hunting
d. increase efficiency

23.Match List I with list II and select the correct answer:

List I
Flux due to
A. currents in damper windings
B. currents in field windings
C. armature currents with unity internal pf
D. armature currents with zero pf

List II
1. Direct axis synchronous reactance
2. Quadrature axis synchronous reactance
3. Transient reactance
4. Sub-transient reactance

---A B C D
a. 4 3 1 2
b. 3 4 2 1
c. 3 4 1 2
d. 4 3 2 1

24.In the ‘V’ curve shown in the figure for a synchronous motor, the parameter of x and y coordinates are, respectively

a. Armature current and field current
b. Power factor and field current
c. Armature current and torque
d. Torque and field current

25.Match List I (Equivalent circuit parameter) with List II (Values) for a 50 MVA three phase alternator arid select the correct answer:

List I
A. Armature resistance
B. Synchronous reactance
C. Leakage reactance

List II
1. 1 Pu
2. 0.1 Pu
3. 0.01 pu

---A B C
a. 1 2 3
b. 3 1 2
c. 3 2 1
d. 1 3 2


26. Synchronous condenser means
a. A synchronous motor with capacitor connected across stator terminals to improve pf
b. A synchronous motor operating at full load with leading pf
c. An over-excited synchronous motor partially supplying mechanical load, and also improving. Pf of the system to which it is connected
d. An over-excited synchronous motor operating at no-load with leading pf used in large power stations for improvement of pf


27. Match List I with List II aid select the correct answer:

List I (Tests)
A. No-load and blocked rotor test
B. Sumpner’s test
C. Swinburne’s test

List II (Machines)
1. Transformer
2. Induction motor
3. Synchronous motor
4. dc motor

---A B C
a. 1 4 2
b. 2 1 4
c. 3 4 2
d. 3 1 4


28. Match List 1 with List II and select the correct answer:

List I (Regulation method)
A. Synchronous impedance method
B. Mmf method
C. Zero power factor (ZPF) method
D. American standard Association method

List II (Relevant phasor)
1. emf phasor
2. Predominantly mmf phasor
3. Both emf and mmf phasors
4. emf and mmf phasors including saturation

---A B C D
a. 1 2 3 4
b. 1 2 4 3
c. 2 1 3 4
d. 2 1 4 3

29. In a cylindrical rotor synchronous machine, the phasor addition of stator and
rotor mmfs is possib1 because
a. Two mmfs are rotating in opposite direction
b. Two mmfs are rotating in same direction at different speeds
c. Two mmfs are stationary with respect to each other
d. One mmf is stationary and the other mmf is rotating

30.The ‘synchronous - impedance method’ of finding the voltage regulation by a cylindrical rotor alternator is generally considered
a. a pessimistic method because saturation is not considered V
b. an optimistic method because saturation is not considered
c. a fairly accurate method even if power factor is not taken into account while
determining synchronous impedance
d. a fairly accurate method when power factor is taken into account while determining synchronous impedance

31.Generally the no-load losses of an electrical machine is represented in its equivalent circuit by a
a. parallel resistance with a low value
b. series resistance with a low value
c. parallel resistance with a high value
d. series resistance with a high value


32.The power factor of a synchronous motor
a. improves with increase in excitation and may even become leading at higher excitations
b. decreases with increase in excitation
c. is independent of its excitation
d. increases with loading for a given excitation


33.When the excitation of normally operating unloaded salient-pole synchronous motor suddenly gets disconnected, it continues to run as a
a. Schrage motor
b. Spherical motor
c. Switched- reluctance motor
d. Variable - reluctance motor


34.A 6-pole, 3-phase alternator running at 1000 rpm supplies to an 8-pole, 3-phase induction motor which has a rotor current of frequency 2 Hz. The speed at which the motor operates is
a. 1000 rpm
b. 960 rpm
c. 750 rpm
d. 720 rpm


35.For maximum current during ‘Slip Test’ on a synchronous machine, the armature and aligns along
a. d-axis
b. q-axis
c. 45° to d-axis
d. 45° to q-axis


36.Match List I with List II and select the correct answer:

List I (Name of test)
A. Open circuit and short circuit tests
B. Open circuit and zero power factor tests
C. Slip test
D. Maximum lagging current test

List II (Result)
1. Leakage reactance
2. Direct axis synchronous reactance
3. Quadrature axis synchronous reactance
4. Ratio, of direct axis synchronous reactance to quadrature axis synchronous reactance

Codes;
---A B C D
a. 1 2 4 3
b. 1 2 3 4
c. 2 1 4 3
d. 2 1 3 4


37.A 3-phase 50 MVA 10 kV generator has a reactance of 0.2 ohm per phase. Hence the per-unit value of the reactance on. a base of 100 MVA 25 kV will be
a. 1.25
b. 0.625
c. 0.032
d. 0.32


38. The results of a ‘Slip Test’ for determining direct-axis (Xd) and quadrature-axis (Xq) reactance’s of a star-connected salient - pole alternator are given below
phase values: Vmax =108V; Vmin = 96V,Imax =12A, Imin = 10A. Hence the two reactance will be

a. Xd = 10.8 ohms and Xq = 8 ohms
b. Xd = 9 ohms and Xq = 9.6 ohms
c. Xd = 9.6 ohms and Xq = 9 ohms
d. Xd = 8 ohms and Xq = 10.8 ohms

39.If the field of a synchronous motor is under excited, the power factor will be
a. Lagging
b. Leading
c. Unity
d. More than unity


40.Which of the following graphs represents the speed-torque characteristic of a synchronous motor?



41.Match List I (Machine Components) with List II (Functions) and select the correct
answer :

List I
A. Damper Winding
B. Interpole Winding
C. End Ring
D. Conservator

List II
1. Used in synchronous motor to provide starting torque
2. Used in d.c. generator to neutralize the reactance voltage and cross magnetizing effect of armature reaction
3. Used to short circuit rotor bars of cagetype induction motor
4. Used to protect transformers from damage when oil expands due to rise in temperature

---A B C D
a. 2 1 3 4
b. 1 2 4 3
c. 2 1 4 3
d. 1 2 3 4

42.The synchronous reactance is the
a. Reactance due to armature reaction of the machine
b. Reactance due to leakage flux.
c. Combined reactance due to leakage flux and armature reaction
d. Reactance either due to armature reaction or leakage flux


43. Match List I (Type of AC Winding) with List II (Feature of AC Windings) and
select the correct answer :

List I
A. Single Layer
B. Double Layer
C. Full Pitch
D. Chorded

List II
1. Coil span and pole pitch are. equal
2. Coil span is shorter than pole pitch
3. Number of slots is equal to number of coils
4. Number of slots is twice the number of coils

---A B C D
a. 3 4 1 2
b. 4 3 2 1
c. 3 4 2 1
d. 4 3 1 2

44. Match List I (Machines) with List II (Tests) and select the correct answer :

List I
A. Transformer V
B. DC Motor
C. Alternator
D. Induction Motor

List II
1. Slip Test
2. Blocked Rotor Test
3. Sumpner’s Test
4. Swinburne’s Test

---A B C D
a. 3 4 1 2
b. 4 3 2 1
c. 3 4 2 1
d. 4 3 1 2


45.The leakage reactance of a three-phase alternator determined by performing
a. Open circuit and zero power factor tests
b. Zero power factor and slip tests
c. Open- and short-circuit tests
d. Short-circuit and slip tests

46.Four important parameters of alternators have comparatively larger or smaller values. In comparison to a steam turbine driven alternator, a hydraulically driven machine will have which one of the following combinations?
---Number of | Axial | Number | Operating
---armature | length of | of poles | speed
---conductors | conductors | |
a. Smaller | larger | smaller | higher
b. Larger | smaller | larger | lower
c. Larger | larger | smaller | lower
d. Smaller | smaller | larger | higher


46. Which one of the following is the primary reason for placing field on rotor in an alternator?
a. Small power .in field circuit
b. Insulation of high voltage is made easy on stator than on rotor
c. Large power in stator
d. Large current in the stator

47.Consider the following:
1. Supply voltage
2. Excitation current
3. Maximum value of load angle

The maximum power developed by a synchronous motor is a function of which of the bove?
a. 1 and 2
b. 1 and 3
c. 2 and 3
d. 1, 2 and 3


48.When a 3–phase alternator is suddenly short–circuited at its terminals, the initial value of the short–circuit current is limited by which one of the following ?
a. Subtransient reactance x”d
b. Transient reactance x’d
c. Synchronous reactance xs
d. Sum of x”d, x’d and xs


49.Which one of the following methods gives more accurate result for determination of voltage regulation of an alternator?
a. m.m.f. method
b. Synchronous impedance method
c. Potier triangle method
d. American Institution Standard method


50.Which one of the following statements is correct?

In a salient pole synchronous machine the air gap is
a. uniform under the whole pole shoe
b. least under the middle of the pole shoe and increases outwards
c. largest under the middle of the pole shoe and decreases outwards
d. least at one end of the pole shoe and increases to the maximum Value at the other end


51.Which one of the following is not a necessary condition to be satisfied for synchronizing an incoming alternator to an already operating alternator?
a. Same voltage magnitude
b. Same frequency
c. Same prime mover speed
d. Same phase sequence

52.A synchronous motor is operated from a bus voltage of 1.0 pu and is drawing 1.0 pu zero power factor leading current. Its synchronous reactance is 0.5 pu. What is the excitation emf of the motor?
a. 2.0
b. 1.5
c. 1.0
d. 0.5

53.In the measurement of Xd, Xq (in ohms),following data are obtained by the slip test on a salient pole machine:
Id max = 10 A Id min = 6.5 A
Vd max = 30 V Vd min = 25 V
Which one of the following is correct?
a. Xd = 3, Xq = 3.86
b. Xd = 4.615, Xq = 2.5
c. Xd = 3, Xq = 2.5
d. Xd = 4.61, Xq = 3.86


54. What are the conditions to be satisfied for alternator to be synchronized with an
incoming supply?
1. Equal voltage
2. Equal frequency
3. Same power rating
4. Same phase sequence

Select the correct answer using the code
given below:
a. 2, 3 and 4
b. 3 and 4
c. 1, 2 and 3
d. 1, 2 and 4

55.What is the value of the load angle when the power output of a salient pole synchronous generator is maximum?
a. 0°
b. 45°
c. 90°
d. None of the above

56.Which one of the following statements is not correct in respect of synchronous
machines ?
a. In salient pole machines, the directaxis synchronous reactance is greater than the quadrature axis synchronous reactance.
b. The damper bars help the motor to self-start.
c. Short circuit ratio is the ratio of the field current required to produce the rated voltage on open circuit to the rated armature current.
d. The V-curve of a synchronous motor represents the variation in the armature current with field excitation at a given output power.


57.In which one of the following is reluctance power developed?
a. Salient pole alternator
b. Non-salient pole alternator
c. Squirrel cage induction motor
d. Transformer


58.Which of the following conditions are to be satisfied for proper synchronization of alternators?
1. Equal terminal voltage
2. Same frequency
3. Same phase sequence
4. Same kVA rating
5. Same phase displacement

Select the correct answer using the code given below:
a. 1, 3, and 4 only
b. 1, 2, 4 and 5 only
c. 2, 3, 4 and 5 only
d. 1, 2, 3 and 5 only

59. If the excitation of a 3-phase alternator operating on infinite bus bars is changed, which one of the following shall alter?
a. Active power of machine
b. Reactive power of machine
c. Terminal voltage of machine
d. Frequency of machine

Tranformers_objectives

1.If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced to 50% (assuming) that the magnetic circuit remains unsaturated), the maximum core /flux density will
a. Change to three, times the original value
b. Change to 1.5 times the original value
c. Change to 0.5 times the original value
d. Remain the same as the original value

2.The low-voltage winding of a core-type transformer is subdivided into two equal halves, each of half the original width of the single winding with the high-voltage winding in between these two halves of low-voltage winding (instead of having usual construction of low-voltage winding adjacent to the core and surrounded b the high-voltage winding.) Such an interlacing of coils would make the combined primary and secondary leakage reactance (in terms of the primary) nearly
a. Twice
b. Equal
c. Half
d. One-fourth

3.Two 3-limb, 3-phase delta-star connected transformers are supplied from’ the same source. One of the transformers is of Dy 1and the other is of Dy 11 connection. The phase difference between the corresponding phase voltages of the two second Aries would be
a. 0°
b. 30°
c. 60°
d. 120°

4.In a transformer fed from a fundamental frequency voltage source, the source of harmonics is the
a. Overload
b. Poor insulation
c. Iron loss
d. Saturation of core

5.A 40k VA transformer has a core loss of 400W and. a full-load copper loss of 800W. The proportion of full-load at maximum efficiency is
a. 50%
b. 62.3%
c. 70.7%
d. 100%

6.A single-phase transformer has a. rating of 15 k VA, 600/120V. It is reconnected as an auto- transformer to supply at 720 V from a 600 V primary source. The maximum load it can supply is
a. 90 kVA
b. 18 kVA
c. 15 kVA
d. 12 kVA

7. Two single-phase transformers with turns ratios 1 and 2 respectively are connected to a 3-phase supply on the primary side as shown in the figure. The voltmeter V2, will read


a. 100 V
b. 173 V
c. 200 V
d. 265 V
8. In a 3-phase transformer shown in the figure, the phase displacement of secondary line voltages with corresponding primary line voltages will be


a. Zero
b. 300 lag
c. 30° lead
d. 180°


9. Three 3-phase transformers each rated at 75 MVA, 132 kV/1 kV, have the following different methods of cooling:
1. Self oil cooled
2. Forced oil cooled.
3. Forced air cooled.
The correct sequence in ascending order in terms of the weights of these transformers is
a. 1,2,3
b. 2, 3, 1
c. 3, 1, 2
d. 3, 2, 1

10. A single-phase induction regulator is a constant input transformer to obtain smooth variation of the output voltage by varying the
a. ratio of turns between primary and secondary windings
b. frequency
c. flux density in the core
d. angle between the magnetic axes of the primary and secondary windings

11. An auto-transformer having a transformation ratio of 0.8 supplies a load of 10 kW. The power transferred inductively from the primary to the secondary is
a. 10 kW
b. 8 k W
c. 2kW
d. Zero

12. Full-load voltage regulation of a power transformer is zero, when power factor of the load is near
a. unity and leading
b. zero and leading
c. zero and lagging
d. unity and lagging

13. In a single-phase transformer, the magnitude of leakage reactance is twice that of resistance of both primary and secondary. With secondary short-circuited, the input power factor is
a. 1/ 2
b. 1/ 5
c. 2/ 5
d. 1/3

14. If a sinusoidal voltage source is connected to a power transformer, its no-load current would be
a. sinusoidal and lagging the voltage by 90°
b. sinusoidal and lagging the voltage by less than 90°
c. rich in third harmonic and its fundamental would lag the voltage by 90°
d. rich in third harmonic and its fundamental would lag the voltage by less than 90°

15. The zero power factor characteristic for the Potier’s diagram can be obtained by loading the alternator using
a. lamp load
b. synchronous motor
c. water load
d. dc motor

16. If all other requirements for parallel operation of transformer are fulfilled, which one of following pairs of three phase transformers, with given VECTOR GROUPS, can be operated in parallel?
a. Y d 1 and Yy 0
b. Y d 1 and Dy 11
c. Dd 6 and Dy 1
d. Dd 0 and Dy 11

17. When short-circuit test on a transformer is performed at 25 V, 50 Hz, the drawn current ‘I1’ is at a lagging p.f. angle of Φ1. If the test is performed at 25 V, 25 Hz and the drawn current “I2’ is at a lagging p.f. angle of Φ2, then
a. I2 > I1 and Φ2 < Φ1
b. I2 > I1 and Φ2< Φ1
c. I2 < I1 and Φ2 > Φ1
d. I2 < I1 and Φ2 > Φ1


18. A 10 kVA, 400 V/ 200 V single-phase transformer with a resistance of 3% and a reactance of 6% is supplying a current of 50 A to a resistive load. The voltage across the load is
a. 194 V
b. 196 V
c. 198 V
d. 390 V

19. A 3-phase alternator is connected to a Dd transformer. The hysteresis and eddy current losses of the transformer are respectively 300 W and 400 W. If the speed of the alternator is reduced by 10%, then the hysteresis and eddy current losses of the transformer will be respectively
a. 228 W and 262.44 W
b. 243 W and 324 W
c. 243 W and 360 W
d. 270 W and 400 W

20. Match List I with List II and select the correct answer:
List I
A. Shell type with wound core
B. Core type with core of laminated sheets
C. Shell type with laminated core
D. Core type with wound core
List II



A B C D
a. 4 3 1 2
b. 4 2 1 3
c. 1 2 4 3
d. 1 3 4 2

21.In an autotransformer of voltage ratio V1 / V2 and V1 > V2, the fraction of power transferred inductively is
a.V1/(V1+V2 )

b.V2/V1

c.(V1-V2)/(V1+V2 )

d.(V1-V2)/V1



22. The windings of a Q kVA, V1 / V2 volts, three phase Dd core type transformer are reconnected to work as a single-phase transformer, the power rating and the maximum voltage ratio of the new configuration will be respectively
a. Q/3 kVA and V1/ V2 volts
b. 3Q kVA and 3 V1 / 3 V2 volts
c. 2Q kVA and 3 V1 / 3 V2 volts
d. 2/3Q kVA and 2 V1 / 2 V2 volts


23. Which one of the following relays has the capability of anticipating the possible major fault in a transformer?
a. Over current relay
b. Differential relay
c. Buchholz relay
d. Over fluxing relay

24. A single-phase transformer when supplied from 220 V, 50 Hz has eddy current loss of 50 W. If the transformer is connected to a voltage of 330V, 50 Hz, the eddy current loss will be
a. 168.75 W
b. 112.5 W
c. 75W
d. 50W

25. In case of auto-transformers, which of the following statements are correct?
1. An auto-transformer requires less copper as compared to a conventional, 2-winding transformer of the same capacity.
2. An auto-transformer provides isolation between the primary and secondary windings.
3. An auto-transformer has less leakage reactance as compared to the conventional, 2-winding transformer of the same capacity.
Select the correct answer using .the codes given below:
a. 1, 2 and 3
b. 1 and 2
c. 1 and 3
d. 2 and 3

26. Match List I with List II and select the correct answer
List I (Transformer)
A. Power transformer
B. Auto transformer
C. Welding transformer
D. Isolation transformer
List II (Voltage ratio)
1. 230 V / 230 V
2. 220 V / 240V
3. 400 V / 100V
4. 132 k V / 11 kV
Codes;
A B C D
a. 4 2 3 1
b. 4 2 1 3
c. 2 4 1 3
d. 2 4 3 1

27. A two-winding transformer is used as an auto-transformer. The kVA rating of the auto-transformer compared to the twowinding transformer will be
a. 3 times
b. 2 times
c. 1.5 times
d. same

28. A 20 kVA, 2000/200 V, 1-phase transformer has nameplate leakage impedance of 8% Voltage required to be applied on the high - voltage side to circulate full - load current with the low voltage winding short-circuited will be
a. 16 V
b. 56.56 V
c. 160 V
d. 568.68 V

29. The full-load copper-Loss and iron-loss of a transformer are 6400 W and 5000 W respectively. The copper-loss and iron-loss at half load will be, respectively
a. 3200 W and 2500 W
b. 3200 W and 5200 W
c. 16O0 W and 1250 W
d. 1600 W and 5000 W

30. In a 100 kVA, 1100/220 V, 50 Hz single -phase trans former with 2000 turns on the high -voltage side, the open- circuit test result gives 220 V, 91 A, 5 kW on low voltage side. The core-loss component of current is, approximately
a. 9.1 A
b. 22.7 A
c. 45.0 A
d. 91 A

31. The use of higher flux density in the transformer design
a. Reduces the weight per kVA
b. Increases the weight per kVA
c. Has no relation with the weight of transformer
d. Increases the weight per kW

32. The function of oil in a transformer is to provide
a. Insulation and cooling
b. Protection against lightning
c. Protection against short circuit
d. Lubrication

33. A coil of 1000 turns is wound on a core. A current of 1 A flowing through the coil creates a core flux of 1 mWb. The energy stored in the magnetic field is
a. 0.25 J
b. 0.5 J
c. 1 J
d. 2 J

34. Consider the following statements relating to the constructional features of a large power transformer:
1. The conservator is used to maintain the level of oil in the transformer tank 2. The bushing is used to protect transformer insulation against lightning over-voltages
3. The Buchholz relay is an over current relay
4. Silica gel is used to absorb moisture.
Which of these statements are correct?
a. 1, 2, 3 and 4
b. 2 and 3
c. 1 and 4
d. 1, 2 and 4

35. Two transformers operating in parallel will share the load depending upon their
a. Ratings
b. Leakage reactance
c. Efficiency
d. Per unit impedance

36. A 1 kVa, 200/100 V, 50 Hz, single-phase transformer gave the following test results on 50 Hz :
OC (LV side) : 100 V, 20 watts
SC (HV side) : 5 A, 25 watts
It is assumed that no-load loss components are equally divided. The above tests were then conducted on the same transformer at 40 Hz.
Tests results were:
OC (HV):160 V, W1 watts.
SC (LV): 10 A, W2 watts neglecting skin effect, W1 and W2 will be
a. W1 = 16 watts, W2 = 25 watts
b. W1 = 25 watts, W2 = 31.25 watts
c. W1 = 20 watts, W2 = 20 watts
d. W1 = 14.4 watts, W2 = 25 watts

37.The diagram given below shows the connection of a four–wire delta bank for obtaining a 3–phase 4–wire distribution system. The secondary voltages between the terminals are as indicated, a. What is the voltage between the terminals b and d in the above system when the primary side is energized from an appropriate symmetrical 3–phase system?



38. If per unit impedances of two transformers connected in parallel are not equal, then which one of the following statements is correct?
a. The power factor of the two transformers will be different from that of the connected load
b. Transformers will get overloaded
c. Dead short circuit occurs
d. The transformer with higher per unit impedance will share more load

39. A coil of 1000 turns is wound on a core. A current of 1 A flowing through the coil creates a core flux of 1 mWb. What is the energy stored in the magnetic field?
a. 1/4J
b. 1/2J
c. 1 J
d. 2 J

40. Match List I (Type of Coil) with List II (Use of Coil) and select the correct answer using the code given below the lists:
List I
A. Sandwitch coils
B. Disc coils
C. Cross-over coils
D. Spiral type
List II
1. Low voltage coils for currents above 100 A
2. High voltage windings of small transformers
3. Cooling oil is in contact with each turn of the winding
4. Shell-type transformer core
Codes;
A B C D
a. 2 3 4 1
b. 4 1 2 3
c. 2 1 4 3
d. 4 3 2 1

41. Match List I (Test) with List II (Quantities) and select the correct answer using the code given below the lists:
List I
A. O C Test
B. S C Test
C. Sumpner’s Test
D. Load Test
List II
1. Copper loss and iron loss
2. Total losses
3. Iron loss
4. Copper loss
Codes;
A B C D
a. 3 4 1 2
b. 2 1 4 3
c. 3 1 4 2
d. 2 4 1 3
42. The exact equivalent circuit of a two winding transformer is given in the figure given above. For affecting simplification, the parallel magnetizing branch, consisting of RC and XQ is shifted to’ the left of the primary leakage impedance (r1 + jx1). This simplification introduces the inaccuracy, in the neglect of:



a. Voltage-drop in ‘the primary impedance due to the secondary current
b. Voltage-drop in the primary impedance due to the exciting current
c. Voltage-drop in the secondary impedance due to the exciting current
d. Reduction in values of RC and XQ of the exciting circuit.

43. A single-phase, 10 kVA, 2000/200 V, 50 Hz transformer is connected to form an auto transformer as shown in the figure given above. What are the values of V1and I2 respectively?



a. 2200 V, 55 A
b. 2200 V, 45 A
c. 2000 V, 45 A
d. 1800 V, 45 A

44. Three single phase 11000/220 V transformers are connected to form 3-phase transformer bank. High voltage side is connected in star, and low voltage side is in delta. What are the voltage ratings and turn ratio of 3-phase transformer?
a. 19052/220V, 50
b. 19052/220V, 50 √3
c. 11000/381 V, 50 √3
d. 11000/220 V, 50



45. If the wave form of the voltage impressed on the primary of a star- delta bank contains 5th harmonics, what are the wave forms of the resultant voltages of the primary and the secondary?
Primary Secondary
a. Peaked Peaked
b. Peaked Flat-topped
c. Flat-topped Peaked
d. Flat-topped Flat-topped

46. Two single phase transformers A and B are connected in parallel, observing all requirements of a parallel operation, except that the induced voltage Ea is slightly greater than Eb; Zea and Zeb being the equivalent impedances of A and B, both referred to the secondary side.



Under this operating condition with the primary bus-bars being energized, a circulating current will flow:
a. Only in the secondary windings of A and B
b. In both the primary and the secondary windings of A and B
c. In both the primary and the secondary windings of A and B, as well as in the primary side network
d. In the primary and the secondary windings of A and B, and boost the voltages on the secondary side of both A and B


47. Which three-phase connection can be used in a transformer to introduce a phase difference of 300 between its output and corresponding input line voltages?
a. Star-delta
b. Star-star
c. Delta-delta
d. Delta-zigzag

48. What is the efficiency of an autotransformer in comparison to that of a two winding transformer of the same rating?
a. Slightly less than that of a two-winding transformer
b. Same as that of a two-winding transformer
c. More than that of a two-winding transformer
d. As low as 1/5th of the efficiency of a two-winding transformer

49. A two-winding transformer is converted into an auto-transformer. If we apply additive polarity and subtractive polarity for the connections, then the secondary voltage is 2640 V and 2160 V, respectively. What is the ratio of primary to secondary voltage on the original transformer?
a. 66 : 54
b. 54 : 66
c. 10 : 1
d. 1 : 10

50. Percentage resistance and percentage reactance of a transformer are 1% and 4%, respectively. What is voltage regulation at power factor 0.8 lagging and 0.8 leading?
a. 2.4% and –0.8%, respectively
b. 3.2% and —1.6%, respectively
c. 3.2 % and —3.2%, respectively
d. 4.8% and 1.6%, respectively

51. If P1 and P2 are the iron and copper losses of a transformer at full load, and the maximum efficiency of the transformer is at 75% of the full load, then what is the ratio of P1 and P2?
a. 9/16
b. 10/16
c. 3/4
d. 3/16

52. If the iron core of a transformer is replaced by an air core, then the hysteresis losses in the transformer will
a. increase
b. decrease
c. remain unchanged
d. become zero

53. The equivalent circuit of a transformer has the leakage reactance X1, X '2 and the magnetizing reactance Xm. What is the relationship between their magnitudes?
a. X1 >> X'2 >> Xm
b. X1 << X'2 << Xm
c. X1≈ X2 >> Xm
d. X1≈ X'2 << Xm


54. If the voltage applied to a transformer primary is increased by keeping the V/f ration fixed, then the magnetizing current and the core loss will, respectively,
a. decrease and remain the same
b. increase and decrease
c. remains the same and remains the same
d. remains the same and increase

55. Sludge formation in transformer oil is due to which one of the following?
a. Ingress of dust particles and moisture in the oil.
b. Appearance of small fragments of paper, varnish, cotton and other organic materials in the oil
c. Chemical reaction of transformer oil with the insulating materials
d. Oxidation of transformer oil

56. A single-phase transformer rated for 220/440 V, 50 Hz. This frequency operation at rated voltage results in which one of the following?
a. Increases of both eddy-current and hysteresis losses
b. Reduction of both eddy-current and hysteresis losses
c. Reduction of hysteresis loss and increase in eddy-current loss
d. Increase of hysteresis loss and no change in the eddy-current loss

57. What is the load at which maximum efficiency occurs in case of a 100 kVA transformer with iron loss of 1 kW and full–load copper loss of 2 kW?
a. 100 kVA
b. 70.7 kVA
c. 50.5 kVA
d. 25.2 kVA

58. Cores of large power transformers are made from which one of the following?
a. Hot-rolled steel
b. Cold-rolled non-grain oriented steel
c. Cold-rolled grain oriented steel
d. Ferrite

59. A transformer has a percentage resistance of 2% and percentage reactance of 4%. What are its regulations at power factor 0.8 leading, respectively?
a. 4% and - 0.8%
b. 3.2% and – 1.6%
c. 1.6% and – 3.2 %
d. 4.8% and – 0.6%