Electrical Machines Test Questions - Set 7

1)   The advantages of star-delta starter over other types of starter is

a. Cheapest of all
b. Maintenance free
c. Both (a) & (b)
d. None of these
Answer  Explanation 

ANSWER: Both (a) & (b)

Explanation:
No explanation is available for this question!


2)   When rotor resistance starter is used with induction motor then

a. Only starting current is limited
b. Only starting torque is limited
c. Both starting current and starting torque are limited
d. Neither starting current nor starting torque is limited
Answer  Explanation 

ANSWER: Both starting current and starting torque are limited

Explanation:
No explanation is available for this question!


3)   Direct online starter also called D.O.L. starter is used for motors having capacity

a. Less than 5 h.p.
b. Less than 10 h.p.
c. Greater than 10 h.p.
d. For any capacity motor
Answer  Explanation 

ANSWER: Less than 5 h.p.

Explanation:
No explanation is available for this question!


4)   The NO contact and NC contact of D.O.L. starter is normally

a. Open, closed
b. Closed, open
c. Open, open
d. Closed, closed
Answer  Explanation 

ANSWER: Open, closed

Explanation:
No explanation is available for this question!


5)   A machine without commutator, providing an ac emf to the external circuit is called as

a. D.C. generator
b. Alternator
c. Synchronous motor
d. Transformer
Answer  Explanation 

ANSWER: Alternator

Explanation:
No explanation is available for this question!


6)   Practically, most of the alternators prefer which type of construction?

a. Rotating field type
b. Rotating armature type
c. Both are equally important
d. None of these
Answer  Explanation 

ANSWER: Rotating field type

Explanation:
No explanation is available for this question!


7)   In synchronous machine poles are made up of thick steel laminations. In salient pole type rotor

a. Poles are projected out from the surface of the rotor
b. The unslotted portions of the cylinder itself acts as the poles
c. Both (a) and (b)
d. None of these
Answer  Explanation 

ANSWER: Poles are projected out from the surface of the rotor

Explanation:
No explanation is available for this question!


8)   Salient pole type rotors are generally used for the range of 125 rpm to 500 rpm. The limiting factor for the size of the rotor is the

a. Rotating field type construction
b. Rotating armature type construction
c. Centrifugal force acting on the rotating member
d. All of these
Answer  Explanation 

ANSWER: Centrifugal force acting on the rotating member

Explanation:
No explanation is available for this question!


9)   The prime movers used to drive projected pole type alternator is

a. Water turbines
b. I.C. engines
c. Both (a) and (b)
d. None of these
Answer  Explanation 

ANSWER: Both (a) and (b)

Explanation:
No explanation is available for this question!


10)   Separate damper winding is necessary in

a. Smooth cylindrical type rotor
b. Salient pole type rotor
c. Both (a) and (b)
d. None of these
Answer  Explanation 

ANSWER: Salient pole type rotor

Explanation:
No explanation is available for this question!


11)   Generally, in alternator divided winding rotor is preferred to a conventional winding rotor, because it gives

a. Better damping
b. Less harmonics
c. More efficiency
d. None of these
Answer  Explanation 

ANSWER: Better damping

Explanation:
No explanation is available for this question!


12)   The air gap in a salient pole machine is

a. Maximum at the center of poles
b. Least at the center of poles and increases while moving away from the center
c. Maximum at the center and decreases while moving away from the center
d. Equally distributed
Answer  Explanation 

ANSWER: Least at the center of poles and increases while moving away from the center

Explanation:
No explanation is available for this question!


13)   The diameter and axial length of rotors of smooth cylindrical type rotors are

a. Small diameter, small axial length
b. Large diameter, large axial length
c. Large diameter, small axial length
d. Small diameter, large axial length
Answer  Explanation 

ANSWER: Small diameter, large axial length

Explanation:
No explanation is available for this question!


14)   Which type of rotor is generally used for high speed alternators or turbo alternators?

a. Salient pole type
b. Projected pole type
c. Smooth cylindrical type
d. Both (a) & (b)
Answer  Explanation 

ANSWER: Smooth cylindrical type

Explanation:
No explanation is available for this question!


15)   The phasor addition of stator and rotor mmfs is possible in a smooth cylindrical synchronous generator because

a. Mmfs of stator and rotor are stationary with respect to each other
b. Mmf of stator is stationary and that of rotor is rotating
c. Mmf of stator is rotating and that of rotor is stationary
d. None of these
Answer  Explanation 

ANSWER: Mmfs of stator and rotor are stationary with respect to each other

Explanation:
No explanation is available for this question!


16)   The prime movers which are used to drive non projected type rotors are

a. I.C. engines
b. Electric motors
c. Steam turbines
d. Both (b) & (c)
Answer  Explanation 

ANSWER: Both (b) & (c)

Explanation:
No explanation is available for this question!


17)   In the brushless excitation system of synchronous generator, the feedback of output voltage of alternator is taken and given to the

a. Rotating armature a.c. excitor
b. Diode rectifier
c. Main alternator field
d. Magnetic amplifier
Answer  Explanation 

ANSWER: Magnetic amplifier

Explanation:
No explanation is available for this question!


18)   When we plot the magnitude of the induced emf against the time, the nature of the induced emf will be

a. Sinusoidal
b. Square
c. Triangular
d. Parabolic
Answer  Explanation 

ANSWER: Sinusoidal

Explanation:
No explanation is available for this question!


19)   During the one complete revolution of the conductor in a 4 pole synchronous generator, induced emf will experience

a. Two times maxima and two times zero
b. Two times maxima and four times zero
c. Eight times maxima and eight times zero
d. Four times maxima and four times zero
Answer  Explanation 

ANSWER: Four times maxima and four times zero

Explanation:
No explanation is available for this question!


20)   In a synchronous generator, one pole is responsible for

a. 90 degree electrical of induced emf
b. 180 degree electrical of induced emf
c. 360 degree electrical of induced emf
d. 720 degree electrical of induced emf
Answer  Explanation 

ANSWER: 180 degree electrical of induced emf

Explanation:
No explanation is available for this question!


21)   If the rotor poles of a 6 pole synchronous generator shift by 8 mechanical degrees from no load to full load, then its torque angle will be

a. 12 degrees
b. 24 degrees
c. 48 degrees
d. 136 degrees
Answer  Explanation 

ANSWER: 24 degrees

Explanation:
No explanation is available for this question!


22)   If coil side in one slot is connected to a coil side in another slot which is one pole pitch distance away from first slot, the winding is said to be

a. Short pitched
b. Full pitched
c. Concentrated
d. Distributed
Answer  Explanation 

ANSWER: Full pitched

Explanation:
No explanation is available for this question!


23)   Among the following sentences which is/are true about short pitch coils

a. Less amount of copper is required
b. Eliminates high frequency harmonics
c. Minimizes eddy current and hysteresis losses
d. All of the above
Answer  Explanation 

ANSWER: All of the above

Explanation:
No explanation is available for this question!


24)   In practice, which combination of armature winding is preferred for the alternators?

a. Single layer, full pitched and concentrated type
b. Single layer, short pitched and distributed type
c. Double layer, full pitched and concentrated type
d. Double layer, short pitched and distributed type
Answer  Explanation 

ANSWER: Double layer, short pitched and distributed type

Explanation:
No explanation is available for this question!


25)   Due to short pitching, the induced emf gets

a. Reduced
b. Increased
c. Remains same
d. None of these
Answer  Explanation 

ANSWER: Reduced

Explanation:
No explanation is available for this question!


26)   In a 4 pole, 3 phase alternator, armature has 40 slots. It is using an armature winding which is short pitched by one slot. Its coil span factor is

a. 0.9
b. 0.9243
c. 0.9476
d. 0.9876
Answer  Explanation 

ANSWER: 0.9876

Explanation:
No explanation is available for this question!


27)   The factor by which there is reduction in the emf due to distribution of coils is called distribution factor. It is given by

a. sin (β / 2m) / msin(β / 2)
b. sin(β / m) / msin(mβ / 2)
c. sin(m β/2) / msin(β / 2)
d. msin(β / 2) / sin(m β / 2)
Answer  Explanation 

ANSWER: sin(m β/2) / msin(β / 2)

Explanation:
No explanation is available for this question!


28)   For a short pitch and distributed winding, the value of coil span factor and distributed factor

a. Both are less than unity
b. Less than unity, greater than unity
c. Greater then unity, less than unity
d. Both are greater than unity
Answer  Explanation 

ANSWER: Both are less than unity

Explanation:
No explanation is available for this question!


29)   An armature of a three phase, 8 pole alternators has 120 slots. The distribution factor is equal to

a. 0.897
b. 0.946
c. 0.957
d. 1
Answer  Explanation 

ANSWER: 0.957

Explanation:
No explanation is available for this question!


30)   A 3 phase, 50 Hz, 8 pole, star connected alternator has 120 slots having 10 conductors per slot. The flux per pole is 0.02 Wb and distributed sinusoidally. The induced emf per phase for full pitch winding is

a. 849.816 V
b. 956.782 V
c. 1046.812 V
d. None of these
Answer  Explanation 

ANSWER: 849.816 V

Explanation:
No explanation is available for this question!