Electrical Machine I-Objective Questions

1) In an alternator, voltage drops occurs in

a) armature resistance only

b) armature resistance and leakage reactance

c) armature resistance, leakage reactance and armature reaction

d) armature resistance, leakage reactance, armature reaction and earth connections.

2)The power factor of an alternator depends on
a) Load
b) Speed of rotor
c) Core losses
d) Armature losses.

3) The number of electrical degrees passed through in one revolution of a six pole synchronous alternator is
a) 360
b) 720
c) 1080
d) 2160

4) When an alternator is running on no load the power supplied by the prime mover is mainly consumed
a) to meet iron losses
b) to meet copper losses
c) to meet all no load losses
d) to produce induced emf in armature winding.

5) Dampers in a large generator
a) increase stability
b) reduce voltage fluctuations
c) reduce frequency fluctuations
d) none

6) An alternator is rated for 75 kW at 0.8 power factor. It means that
a) alternator has 4 poles
b) alternator can supply 75 kW at 0.8 power factor
c) alternator can supply power only to loads having power factor 0.8 only
d) the peak efficiency of alternator occurs only at 75 kW load having 0.8 lagging power factor.

7) A synchronous motor can be started by

a) pony motor

b) D.C. compound motor 

c) providing damper winding

d) any of the above

8) The working of a synchronous motor is similar to
a) gear train arrangement
b) transmission of mechanical power by shaft
c) distribution transformer
d) turbine

9) If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is
a) 2
b) 3
c) 4
d) 6

10) The speed regulation of a synchronous motor is always
a) 1%
b) 0.5%
c) positive
d) zero

11) Synchronizing power of a synchronous machine is
a) directly proportional to the synchronous reactance
b) inversely proportional to the synchronous reactance
c) equal to the synchronous reactance
d) none of the above

12) The standard full-load power factor ratings for synchronous motors are
a) zero or 0.8 leading
b) unity or 0.8 lagging
c) unity or 0.8 leading d) unity or zero

13) Regarding skewing of motor bars in a squirrel cage induction motor ,which statement is true
a) it prevents cogging
b) it increases starting torque
c) it produces more uniform torque
d) it reduces motor ‘hum’ during its operation

14) The magnetizing current drawn by transformers and induction motors is same the cause of their ________ power factor
a) leading
b) lagging
c) unity
d) zero

15) At a slip of 4%,Maximum possible speed of a 3-phase squirrel cage induction motor is
a) 2880rpm
b) 3000rpm
c) 1500rpm
d) 1440rpm

16) Mmf produced by the current of a 3-phase induction motor
a) rotates at the speed of rotor in the air gap
b) is stand still with respect to stator mmf
c) rotates at sleep speed with respect to stator mmf
d) rotates at synchronous speed with respect to rotor

17) In a 3-phase squirrel cage induction motor ,skewing of rotor slots reduces
a) parasitic torque and noises but increases pull-out torque
b) parasitic torque and noises but increases starting torque
c) noise but increases pullout torque and parasitic torque
d) parasitic torque ,noise pullout torque and starting torque

18) Approximate value of the efficiency of a three phase induction motor running at a slip ‘s’ is given as
a) 1/(1-s)
b) s/(1+s)
c) (1-s)/(1+s)
d) s/(1-s)

19) In case the air gap in induction motor is increased
a) the magnetizing current of the rotor will decrease
b) the power factor will decrease
c) speed of the motor will increase
d) the windage loss of the motor will increase

20) In an induction motor ,on-no load the slip is generally
a) less than 1%
b) 1.5%
c) 2%
d) 4%

21) A double squirrel- cage induction motor has
a) two rotors moving in opposite direction
b) two parallel winding in stator
c) two parallel winding in rotor
d) two series winding in stator

22) Star-Delta starting of motor is not possible in case of
a) single phase motor
b) variable speed motor
c) low horse power motor
d) high speed motor

23) Determine approximately the starting torque of an induction motor in terms of full load torque when started by autotransformer with 50% tapping. The short-circuit current of the motor at normal voltage is 5 times the full load current and the full load slip is 4%
a) 9.5 of TFL
b) 18% of TFL
c) 25% of TFL
d) 40% of TFL

24) In a slip-ring induction motor ,resistance is connected in rotor phase
a) to limit the starting torque
b) to limit the starting current
c) to limit the starting current and to increase the starting torque
d) none of the above

25) A hysteresis motor
a) is not a self-starting motor.
b) is a constant speed motor.
c) needs dc excitation.
d) cannot be run in reverse speed.

26) In a stepper motor the angular displacement
a) can be precisely controlled.
b) it cannot be readily interfaced with microcomputer based controller.
c) the angular displacement cannot be precisely controlled.
d) it cannot be used for positioning of work tables and tools in NC machines

27) A ceiling fan uses
a) split-phase motor.
b) capacitor start and capacitor run motor.
c) universal motor.
d) capacitor start motor.

28) A 3 stack stepper motor with 12 numbers of rotor teeth has a step angle of------
a) 120
b) 80
c) 240
d) 100

29) The speed-torque characteristics of a DC series motor are approximately similar to
those of the _________motor.
a) universal
b) synchronous
c) DC shunt
d) two-phase

30) Reduction in the capacitance of a capacitor-start motor, results in reduced
a) Noise.
b) Speed.
c) Starting torque.
d) Armature reaction



Answer Keys:
1) c 2) a 3) c 4) c 5) a 6) b 7) d 8) b 9) b 10) d 11) b 12) c
13) b 14) b 15) a 16) b 17) d 18) c 19) b 20) a 21) c 22) a 23) c
24) b 25) b 26) a 27) d 28) d 29) a 30) c