AC MACHINES 1

AC MACHINES

1. Silicon content in iron lamination's is kept within 5% as it
a) increases hysteresis loss
b) increases the cost
c) makes the material brittle
d) reduces the curie point.
ans. c

2. 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
ans. d

3. Which of the following will improve the mutual coupling between primary and secondary circuits?
a) Transformer oil of high breakdown voltage
b) High reluctance magnetic core
c) Winding material of high resistivity
d) low reluctance magnetic core
ans. d

4. In order to reduce the hysteresis loss
a) core may be laminated.
b) silicon steel may be used as the core material
c) core may be constructed with any permanent magnet material such as Alnico.
d) core may be impregnated with varnish.
ans. b

5. Why is the core of the transformer built up of lamination?
a) To reduce eddy current loss.
b) For the convenience of fabrication.
c) No specific advantage.
d) For increasing the permeability.
ans. a

6. When are the eddy-current losses in a transformer reduced?
a) If laminations are thick.
b) If the number of turns in the primary winding is reduced.
c) If the number of turns in the secondary winding is reduced.
d) If laminations are thin.
ans.d

7. The primary and secondary winding's are interlaced for
a) easiness of coil making
b) reduced leakage
c) reduced cost
d) uniform heating.
ans. b

8. When a transformer winding suffers a short-circuit, the adjoining turns of the same winding experience
a) an attractive force
b) a repulsive force
c) no force
d) all the above
ans. a

9. Under short-circuit condition, the windings of a transformer having 5% impedance will experience a radial force
a) 5 times of full load value
b) 20 times of full load value
c) 25 times of full load value
d) 400 times of full load value
ans. d

10. The function of oil in a transformer is
a) to provide insulation and cooling.
b) to provide protection against lightning
c) to provide protection against short circuits.
d) to provide lubrication.
ans. a

AC MACHINES 2

AC MACHINES

1. Conservator ( for a transformer) is used

a) to take up the expansion of oil due to temperature rise.
b) to act as an oil storage
c) for better cooling of transformer
d) none of the above

ans. a

2. 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 the transformer with oil insulating materials.
d) Oxidation of transformer oil.

ans. d

3. A Buchholz relay is used for

a) protection of transformer against all internal faults.
b) protection of transformer against external faults.
c) protection of transformer against both internal and external faults.
d) protection of induction motors.

ans. a

4. Which is the arrangement of windings in a core type single phase transformer?

a) Half lv inside and half hv outside on each core limb.
b) Lv on one core limb and hv on the other limb.
c) Sandwiched lv and hv discs on each core limb.
d) Half hv side and half hv outside on each core limb.

ans. a

5. Addition of tubes to the transformer tank improves heat dissipation capacity because of

a) additional cooling surface
b) additional dissipation by radiation only.
c) additional dissipation by convention only
d) additional dissipation by radiation and convention both.

ans. a

6. Transformer is rated in kVA instead of kW because

a) load pf is often not known.
b) kVA is fixed where kW depends on load pf.
c) total transformer loss depends on the volt-amperes.
d) it has become customary.

ans. c

7. Which three-phase connection can be used in a transformer to introduce a phase difference of 30 degrees between its output and corresponding input line voltages?

a) Star-Star
b) Star-Delta
c) Delta-Delta
d) Delta-ZigZag

ans. b

8. A 3-phase transformer has a rating of 20 MVA,220 kV(star)- 33kV(delta) with leakage reactance of 12%. The transformer reactance (in ohms) referred to each phase of lv delta connected side is

a) 23.5
b) 19.6
c) 18.5
d) 8.7

ans. b

9. The percentage resistance and percentage reactance of a 10kVA, 400V/200V,3-phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%., the maximum possible percentage efficiency of the transformer is

a) 98.32
b) 97.25
c) 96.85
d) 96.12

ans. b

10. A 3-phase,delta-star transformer is supplied at 6000 V on the delta connected side.The terminal voltage on the secondary side when supplying full-load at 0.8 lagging power factor at 415V. The equivalent resistance and reactance drops for the transformer are 1% and  5% respectively. The turns ratio of the transformer is

a) 14
b) 24
c) 42
d) 20

ans. b

AC MACHINES 3

AC MACHINES

1.The supply to one terminal of delta-wye connected 3-phase core type transformer which is on no-load fails. Assuming magnetic circuit symmetry, voltages on the secondary side will be
a) 230,230,115
b) 230,115,115
c) 345, 115,115
d) 345,0,345
ans. b

2. 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 the core.
ans. d

3. The magnetizing current in a transformer is rich in
a) 3rd harmonic
b) 5th harmonic
c) 7th harmonic
d) 13th harmonic
ans. a

4. Which of the following conditions is/are necessary for the parallel operation of two transformers?
a) percentage impedance should be the same
b) polarities of both secondaries should be the same.
c) voltage ratings should be the same.
d) All the above
ans. d

5. Two transformers of identical voltages but of different capacities are operating in parallel, for satisfactory load sharing
a) impedance's must be equal
b) per unit impedance's must be equal
c) per unit impedance's and X/R ratio must be equal
d) impedance's and X/R ratio must be equal.
ans.c

6. Two transformers are to be operated in parallel such that they share the load in proportion to their kVA ratings. The rating of the first transformer is 500 kVA and its pu leakage impedance is 0.05 pu. If the rating of the second transformer is 250 kVA, its pu leakage impedance is
a) 0.20
b) 0.10
c) 0.05
d) 0.025
ans. b

7. An auto-transformer having a transformation ratio of 0.8 supplies a load of 10 kW. The power is transformed inductively from the primary to secondary is
a) 10 kW
b) 8 kW
c) 2 kW
d) zero
ans. c

8. The coils are to be connected to obtain a single-phase,400V/1000V, auto-transformer to drive a load of 10kva. Which of the given options should be exercised to realize the required auto-transformer?
a) Connect A and D; Common B
b) Connect B and D; Common C
c) Connect A and C; Common B
d) Connect A and C; Common D.
ans. a

9. In the auto-transformer obtained in question 48, the current in each coil is
a) Coil-1 is 25 A and Coil-2 is 10A
b) Coil-1 is 10 A and Coil-2 is 25A
c) Coil-1 is 10 A and Coil-2 is 15A
d) Coil-1 is 15 A and Coil-2 is 10A
ans.d

10. Constant current transformer is of ____ type.
a) core
b) shell
c) core or shell
d) None of these
ans. b

AC MACHINES 4

AC MACHINES

1. Tappings of a transformer are provided

a) at the phase end of the lv side.
b) at the phase end of hv side.
c) at the neutral end of hv side.
d) at the middle of hv side.

ans. d

2. Tappings are usually provided on the HV winding of a transformer because it

a) is easily accessible physically
b) has to handle low currents
c) has a large number of turns which allows smoother variation of voltage
d) all of the above

ans. d

3.In a constant voltage transformer (CVT), the output voltage remains constant due to

a) capacitor
b) input inductor
c) saturation
d) tapped winding's

ans. c

4. The rating of transformer _____ when the supply frequency is increased

a) remains unchanged
b) decreases
c) increases
d) None of these

ans. c

5. Distributed winding and short chording employed in ac machines will result in

a) increase in emf and reduction in harmonics
b) reduction in emf and increase in harmonics
c) increase in both emf and harmonics
d) reduction in both emf and harmonics

ans. d

6. It is desirable to eliminate the 5th harmonic from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of

a) 30 degrees
b) 36 degrees
c) 72 degrees
d) 18 degrees

ans. b

7. A 4-pole, 3-phase, double layer winding is housed in a 36 slot stator for an ac machine with 60 degrees phase spread. Coil span is 7 slot pitches. The number of slots in which top and bottom layers belong to different phases is

a) 24
b) 18
c) 12
d) 0

ans. a

8. The armature of a single-phase alternator is completely wound with T-single turn coils distributed uniformly. The induced voltage in each turn is 2V (RMS). The emf of the whole winding is

a) 2 T volts
b) 1.11 T volts
c) 1.414 T volts
d) 1.273 T volts

ans. d

9. A 3-phase alternator is wound with a 60-degree phase-spread armature winding and develops 300 kVA. If the armature is reconnected utilizing all the coils for single-phase operation with a phase spread of 180 degrees, the new rating of the machine is

a) 100 kVA
b) 200 kVA
c) 250 kVA
d) 300 kVA

ans. b

10. The phase sequence of the three-phase alternator will reverse if

a) the field current is reversed keeping the direction of rotation the same.
b) the field current remains the same but eh direction of rotation is reversed.
c) the field current is reversed and the number of poles is doubled.
d) the number of poles is doubled without reversing the field current.

ans. b

AC MACHINES 5

AC MACHINES

61. A 500 MW 3-phase Y-connected synchronous generator has a rated voltage of 21.5 kV at 0.85 pf. The line current when operating at full load rated conditions will be
a) 13.43 kA
b) 15.79 kA
c) 23.25 kA
d) 27.36 kA
ans. b

62. A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is
a) magnetizing
b) demagnetizing
c) cross-magnetizing
d) ineffective
ans. b

63. A field excitation of 20 A in a certain alternator results in an armature current of 400A in short circuit and a terminal voltage of 200V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200 A is
a) 1 V
b) 10 V
c) 100 V
d) 1000 V
ans. d

64. A 100 kVA ,415V(line) , star-connected synchronous machine generates rated open circuit voltage of 415 V at a field current of 15 A.The short-circuit armature current at a field current of 10 A is equal to the rated armature current. The per unit saturated synchronous reactance is
a) 1.731
b) 1.5
c) 0.666
d) 0.577
ans. a

65. A single-phase ,2000 V alternator has armature resistance and reactance of 0.8 ohms and 4.94 ohms respectively. The voltage regulation of the alternator at 100 A load at 0.8 leading power-factor is
a) 7 %
b) -8.9 %
c) 14 %
d) 0 %
ans. b

66. Which one of the following is not correct with respect of synchronous machines?
a) In salient pole machines , the direct-axis 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 field current required to produce the rated voltage on the open circuit to the rated armature current.
d) The V-curve of a synchronous motor represents the variation in armature current with field excitation at a given output power.
ans. c

67.Higher synchronous reactance is preferred in the present day alternators because one can have
a) reduced sub transient current
b) reduced harmonic currents
c) reduced transient currents
d) higher voltage regulation with the load.
ans. c

68. A stand-alone engine-driven synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nullify the inductive current. For this operating condition
a) the field current and fuel input have to be reduced
b) the field current and fuel input have to be increased
c) the field current has to be increased and fuel input left unaltered
d) the field current has to be reduced and fuel input left unaltered.
ans. d

69. A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be
a) unity
b) leading
c) lagging
d) dependent on machine parameters
ans. b

70. A synchronous machine connected to a power system grid bus-bar is operating as a generator. To make the machine operate as a motor, the
a) direction of rotation is to be reversed
b) phase-sequence is to be changed
c) field excitation is to be decreased.
d) mechanical input is to be less than the losses at the shaft.
ans. d

AC MACHINES 6

AC MACHINES

1.  A synchronous generator connected to an infinite bus is over-excited. Considering only the reactive power, from the point of view of the system, the machine acts as
a) capacitor
b) inductor
c) resistor
d) none of the above
ans. a

2. A synchronous motor operates at 0.8 pf lagging. If the field current of the motor is continuously increased
a) the power factor decreases up to a certain value of the field current and thereafter it increases
b) the armature current increases up to a certain value of the field current and thereafter it decreases
c) the power factor increases up to a certain value of the field current and thereafter it decreases
d) the armature current decreases up to a certain value of filed current and thereafter it increases
e) both c and d
ans. e

3. A cylindrical rotor synchronous motor is switched on the dc supply with its field winding's shorted on themselves. It will
a) not start
b) start but not run at synchronous speed
c) start as an induction motor and then run as synchronous motor
d) start and run as a synchronous motor
ans. b

4. Two 3-phase Y-connected alternators are to be paralleled to a set of common busbars. The armature has a per phase synchronous reactance of 1.7 ohm and negligible armature resistance. The line voltage of the first machine is adjusted to 3300 V and that of the second machine is adjusted to 3200 V. The machine voltages are in phase at eh instant they are paralleled. Under this condition, the synchronizing current per phase will be
a) 16.98 A
b) 29.41 A
c) 33.96 A
d) 58.82 A
ans. a

5. A power station consists of two synchronous generators A and B ratings of 250 MVA and 500 MVA with inertia 1.6 pu and 1 pu respectively on their own base MVA ratings. The equivalent per unit inertia constant for the system on 100 MVA  common base is
a) 2.6
b) 0.615
c) 1.625
d) 9.0
ans. d

6. A 3-phase,4-pole, squirrel cage induction motor to
a) increase the strength of rotor bars.
b) reduce the magnetic hum and locking tendency of the rotor
c) economize on the copper to be used.
d) provide ease of fabrication.
ans. b

7. The rotor of 3-phase, 5 kW, 400 V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no-load steady-state speed when this motor is connected to 400 V, 50 Hz supply is
a) 1500 rpm
b) 500 rpm
c) 0 rpm
d) 1000 rpm
ans. c

8. The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with a 3-phase sinusoidal voltage having phase sequence A-B-C. For counterclockwise rotation of the motor, the phase sequence of the power supply should be
a) B-C-A
b) C-A-B
c) A-C-B
d) B-C-A and C-A-B
ans. b

9. If a 400V,50Hz star-connected, 3-phase, squirrel cage induction motor is operated from a 400V, 75Hz supply, the torque that the motor can now provide while drawing rated current from the supply.
a) reduces
b) increases
c) remains the same
d) increases or reduces depending upon the rotor resistance.
ans. a

10. A 3-phase squirrel cage induction motor has a starting torque of 150% and a maximum torque of 300% with respect to rated voltage and rated frequency. Neglect the stator resistance and rotational losses. The value of slip for maximum torque is
a) 13.48%
b) 16.42%
c) 18.92 %
d) 26.79%
ans. d

AC MACHINES 7

AC MACHINES

1.The power input to a 415 V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 49 kW. The stator losses are 1 kW and friction and windage losses total 2 kW. the efficiency of the motor is
a) 92.5%
b) 92%
c) 90%
d) 88%
ans. c

2. A 400 V, 15 kW, 4-pole, 50 Hz, Y-connected induction motor has a full-load slip of 4%. The output torque of the machine at full load is
a) 1.66 Nm
b) 95.5 N
c) 99.47 Nm
d) 624.73 Nm
ans. c

3. A 400 V,50 Hz, 30 hp, a 3-phase induction motor is drawing 50 A current at 0.8 power factor lagging. The stator and rotor copper losses are 1.5 kW and 900 W respectively. The friction and windage losses of 1050 W and the core losses are 1200 W. The air-gap power of the motor will be
a) 23.06 kW
b) 24.11 kW
c) 25.01 kW
d) 26.21 kW
ans. c

4. No-load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. This intersection point yields
a) core loss
b) stator copper loss
c) stray load loss
d) friction and windage loss
ans. d


5.What is the shunt resistance component in the equivalent circuit obtained by a no-load test of an induction motor representative of?
a) windage and frictional losses only
b) core losses only
c) core, windage, and frictional losses
d) copper losses
ans. b

6. The synchronous speed for the seventh space harmonic MMF wave of a 3-phase,8-pole,50 Hz induction machine is
a) 107.14 rpm in forward direction
b) 107.14 rpm in reverse direction
c) 5250 rpm in forward direction
d) 5250 rpm in reverse direction
ans.a

7. In an induction motor, if the air-gap is increased
a) speed will reduce
b) efficiency will improve
c) power factor will be lowered
d) breakdown torque will reduce
ans. c

8. Unbalanced supply voltage is given to a 3-phase, delta-connected induction motor will cause
a) zero sequence currents
b) less heating of the motor
c) negative sequence component current
d) all the above
ans. c

9. An induction motor is fed from a balanced three-phase supply at rated voltage and frequency through a bank of three single-phase transformers connected in delta-delta. One unit of the bank develops a fault and is removed. Then
a) single phasing will occur and the machine fails to start
b) single phasing will not occur but the motor terminal voltages will become unbalanced and the machine can be loaded to the extent of 57.7% of its rating
c) the machine can be loaded to the extent of 57.7% of its rating with balanced supply at its terminals
d) the machine can be loaded to the extent of 66 2/3% of its rating with balanced supply at its terminals
ans. a

10. An induction motor having full-load torque of 60 Nm when delta connected develops a starting torque of 120 Nm. For the same supply voltage, if the motor is changed to star-connection, the starting torque developed will be
a) 40 Nm
b) 60 Nm
c) 90 Nm
d) 120 Nm
ans. a

AC MACHINES 9

AC MACHINES

1. A single phase induction motor with only the main winding excited would exhibit the following response at synchronous speed
a) rotor current is zero
b) rotor current is non-zero and is at slip frequency
c) forward and backward totaling fields are equal
d) forward rotating field is more than the backward rotating field.
ans. d

2. A 230 V, 50 Hz, 4 pole, single-phase induction motor is rotating in the clockwise direction at a speed of 1425 rpm. If the rotor resistance at standstill is 7.8 Ohm, then the effective rotor resistance in the backward branch of the equivalent circuit will be
a) 2 ohm
b) 4 ohm
c) 78 ohm
d) 156 ohm
ans. b

3. For single phase capacitor start induction motor which of the following statements is valid?
a) The capacitor is used for power factor improvement
b) The direction of rotation can be changed by reversing the main winding terminals.
c) The direction of rotation cannot be changed.
d) The direction of rotation can be changed by interchanging the supply terminals.
ans. b

4. In a single-phase induction motor driving a fan load, the reason for having a high resistance rotor is to achieve
a) low starting torque
b) quick acceleration
c) high efficiency
d) a reduced size
ans. b

5. The type of single-phase induction motor having the highest power factor at full load is
a) shaded pole type
b) split-phase type
c) capacitor -start type
d) capacitor-run type.
ans. d

6. The torque-speed characteristics of a repulsion motor resembles which of the following dc motor characteristic?
a) separately excited
b) shunt
c) series
d) compound
ans. c

7. Ratio of the rotor reactance X to the rotor resistance R for a 2-phase servo motor
a) is equal to that of normal induction motor
b) is less than that of a normal induction motor
c) is greater than that of a normal induction motor
d) may be less or greater than that of a normal induction motor.
ans. b

8. In a stepper motor, the detent torque means
a) minimum of the static torque with the phase winding excited
b) maximum of the static torque with the phase winding excited
c) minimum of the static torque with the phase winding unexcited
d) may be less or greater than that of a normal induction motor.
ans. b

9. For a given stepper motor, the following torque has the highest numerical value
a) detent torque
b) pull-in torque
c) pull-out torque
d) Holding torque
ans. c

10. The following motor definitely has a permanent magnet rotor.
a) DC commutator motor
b) Brushless dc motor
c) Stepper motor
d) Reluctance motor
ans. c

AC MACHINES 8

AC MACHINES

91. The following starting method for an induction motor is inferior from the point of view of poor starting torque per ampere of the line current drawn
a) Direct-on-line starting
b) Auto transformer method of starting
c) series inductor method of starting
d) star-delta method of starting
ans. c

92. A 3-phase induction motor is started by direct-on-line switching at the rated voltage. If the starting current drawn is 6 times the full-load current, and the full -load slip is 4%, then the ratio of the starting developed torque to the full load torque is approximately equal to
a) 0.24
b) 1.44
c) 2.40
d) 6.00
ans. b

A three-phase squirrel cage induction motor has a starting current of seven times the full load current and a full load slip of 5%
93. If an autotransformer is used to reduced voltage starting to provide 1.5 per unit starting torque, the auto-transformer ratio (%) should be
a) 57.77 %
b) 72.56%
c) 78.25%
d) 81.33%
ans. c
94. If a star-delta starter is used to start this induction motor, the per-unit starting torque will be
a) 0.607
b) 0.816
c)1.225
d) 1.616
ans. b
95. If a starting torque of 0.5 per unit is required then the per unit starting current should be
a) 4.65
b) 3.75
c) 3.16
d) 2.13
ans. c

96. An ac induction motor is used for a speed control application. It is driven from an inverter with a constant v/f control. The motor nameplate details
V: 415 V
Ph: 3
f: 50 Hz
N: 2850 rpm
The motor is run with the inverter output frequency set at 40Hz, and with half the rated slip. The running speed of the motor is
a) 2400 rpm
b) 2280 rpm
c) 2340 rpm
d) 2790 rpm
ans.

97. The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/F) constant. At a rated frequency of 50 Hz and a rated voltage of 400 V, its speed is 1440 rpm. find the speed at  30 Hz, if the load torque is constant.
a) 882 rpm
b) 864 rpm
c) 840 rpm
d) 828 rpm
ans. c

98. A 3-phase induction motor is driving a constant torque load at rated voltage and frequency. If both voltage and frequency are halved, the following statements relate to new conditions if stator resistance, leakage reactance, and core loss are ignored.
P. The difference between synchronous speed and actual speed remains the same
Q. The air-gap flux remains the same
R. The stator current remains the same
S. The PU slip remains the same
among the above, correct statements are
a) all
b) P ,Q,R
c) Q, R, S
d) P, S
ans. b

99. A 3-phase squirrel cage induction motor has full-load efficiency of 0.8 and maximum efficiency of 0.9. It is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is
a) less than 0.4
b) greater than 0.6
c) in the range of 0.8+- 0.1
d) none of the above
ans. c

100. A 3-phase slip-ring induction motor is fed from the rotor side with the stator winding short-circuited. The frequency of the currents in the short-circuited stator is
a) slip frequency
b) supply frequency
c) frequency corresponding to rotor speed
d) zero
ans. a

AC MACHINES 10

AC MACHINES

1.Which one of the following types of motors is most suitable for a computer printer drive?
a) Reluctance motor
b) Hysteresis motor
c) Shaded pole motor
d) Stepper motor
ans. d

2. A 3-phase, 3 stack, variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this step motor is
a) 3 degrees
b) 6 degrees
c) 9 degrees
d) 18 degrees
ans. b

AC MACHINES 11

AC MACHINES

11. The core flux of a practical transformer with a resistance load
a) is strictly constant  with load changes
b) increases linearly with load
c) increases as the square root of the load
d) decreases with the increase of load
ans: a

12. In a transformer, zero voltage regulation at full load is
a) not possible
b) possible at unity power factor load
c) possible at leading power factor load
d) possible at lagging power factor load
ans: c

13. A single-phase transformer has a maximum efficiency of 90% at full load and unity power factor. Efficiency at half load at the same power factor is
a) 86.7%
b) 88.26 %
c) 88.9%
d) 87.8%
ans: d

14. The main purpose of performing open-circuit on a transformer is to measure its
a) copper loss
b) core loss
c) total loss
d) insulation resistance
ans: b

15. The desirable properties of transformer core material are
a) low permeability and low hysteresis loss
b) high permeability and high hysteresis loss
c) high permeability and low hysteresis loss
d) low permeability and high hysteresis loss
ans: c

16. The all-day efficiency of a transformer depends primarily on
a) it's copper loss
b) the amount of load
c) duration of load
d) both b and c
ans: d

17. 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 the transformer
d) none of the above
ans: a


18. What does the use of higher flux density in the transformer design
a) Increase the weight per KVA
b) Decrease in weight per KVA
c) Reduced iron losses
d) Reduced copper losses
ans. b

19. 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.
ans. c

20. Grain-oriented laminated sheet steel in transformer reduces
a) copper loss
b) eddy current loss
c) hysteresis loss
d) none of the above
ans. c

AC MACHINES

1. The basic function of a transformer is to change
a) the level of the voltage.
b) the power level
c) the power factor
d) the frequency
ans: a

2. In an ordinary transformer which of the following does not change
a) voltage
b) current
c) frequency
d) all the above
ans: c

3. The efficiency of a power transformer is around
a) 50%
b) 60%
c) 80%
d) 95%
ans: d

4. An ideal transformer does not change
a) voltage
b) power
c) current
d) none of the above
ans: b

5. For an ideal transformer the winding's should have
a) maximum resistance on primary side and least resistance on secondary side
b) least resistance on primary side and maximum resistance on the secondary side
c) equal resistance on primary and secondary sides
d) no ohmic resistance on either side
ans: d

6. The core flux in transformer depends mainly on
a) supply voltage
b) supply voltage and frequency
c) supply voltage, frequency and load
d) supply voltage and load but independent of frequency
ans: b

7. A transformer steps up the voltage by a factor 100. The ratio of current in the primary to that in the secondary is
a) 1
b) 100
c) 0.01
d) 0.1
ans: b

8. If the applied voltage of a certain transformer is increased by 50% and the frequency is reduced 50% , 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
ans: a

9. If the supply frequency of a transformer is increased , keeping the supply voltage constant, then
a) both magnetizing component and core loss component of current will decrease
b) both magnetizing component and core loss component of current will increase
c) magnetizing component of current will increase while core loss component of current will decrease
d) magnetizing component of current will decrease while core loss component of current will increase.
ans: a

10. Primary winding of a transformer comprises of two identical winding's in parallel. If one winding's is removed , magnetizing current will be
a) halved
b) the same
c) double
d) increased four times
ans: c