# Electrical Machines Expected MCQ PDF 6 For Coal India Limited (CIL) MT Exam 2017

1)   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

2)   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

3)   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: Poles are projected out from the surface of the rotor

4)   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: Centrifugal force acting on the rotating member

5)   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

6)   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

7)   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

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

9)   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: Small diameter, large axial length

10)   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)

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

12)   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)

13)   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

14)   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

15)   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: Four times maxima and four times zero

16)   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: 180 degree electrical of induced emf

17)   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

18)   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

19)   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

20)   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: Double layer, short pitched and distributed type

21)   Due to short pitching, the induced emf gets

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

22)   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

23)   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: sin(m β/2) / msin(β / 2)

24)   The generalized expression for emf equation of an alternator is

a. Eph = 4.44f φ Tph
b. Eph = 4.44 KcKdf φ E
c. Eph = 4.44 KcKdfφTph
d. Eph =4 KcKdf φ Tph

25)   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: Both are less than unity

26)   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

27)   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

28)   For reducing the slot harmonics in an alternator, the length of air gap is

a. Reduced
b. Increased
c. Not related to harmonics
d. None of these

29)   To minimize the harmonics from the voltage waveform, the suitable steps is / are

a. Using distributed type of winding instead of concentrated type
b. Using fractional slot windings
c. Skewing the pole face
d. All of these
e. None of these

30)   If x is the order of harmonic component, then the general expression for distribution factor is

a. xsin(β / 2m) / msin(β / 2x)
b. sin(x β /m) / xmsin(m β / 2)
c. sin(xm β / 2) / msin (x β / 2)
d. xmsin(β / 2) /sin(mx β)

 ANSWER: sin(xm β / 2) / msin (x β / 2)

31)   Generally the effective resistance of alternator under a.c. condition is taken as

a. Less than 1.25 times the d.c. resistance
b. 1.25 to 1.75 times the d.c. resistance
c. Greater than 1.75 times the d.c. resistance
d. None of these

 ANSWER: 1.25 to 1.75 times the d.c. resistance

32)   Salient pole alternators are best suitable for the application of

a. Hydro electric power plant
b. Thermal power plant
c. Tidal power plant
d. Nuclear power plant

33)   When the
voltage across any two terminals of an armature winding of star
connected alternator, then the armature resistance per phase is

a. Half of the resistance observed across any two line terminals
b. 3/2 of the resistance observed across any two line terminals
c. 2/3 of the resistance observed across any two line terminals
d. Same as the resistance observed across any two line terminals

 ANSWER: Half of the resistance observed across any two line terminals

34)   In the given figure of delta connected alternator, armature resistance per phase (Ra) is equal to

a. 1/2 of RRY
b. 2/3 of RRY
c. 3/2 of RRY
d. RRY

35)   The leakage flux in the armature of alternator makes the armature winding

a. Inductive in nature
b. Capacitive in nature
c. Resistive in nature
d. All of these

36)   Out of two
alternators which are sharing an inductive load equally, if excitation
of one alternator is increased then

a. Other alternator will deliver less current
b. Other alternator will deliver more current
c. Both will deliver equal current
d. None of these

 ANSWER: Other alternator will deliver less current

37)   The effect of armature flux in an alternator depends on

a. Magnitude of current flowing through the armature winding
b. Nature of power factor of the load connected to the alternator
c. Both (a) and (b)
d. None of these

38)   If ∅f is the main flux produced by the field winding of alternator is responsible for producing Eph, then

a. Eph leads ∅f by 90 degree
b. Eph leads ∅f by 45 degree
c. Eph lags ∅f by 45 degree
d. Eph lags ∅f by 90 degree

 ANSWER: Eph lags ∅f by 90 degree

connected to the alternator are having zero leading power factor and
zero lagging power factor, then the nature of load in each case will be

a. Inductive, capacitive
b. Inductive, inductive
c. Capacitive, capacitive
d. Capacitive, inductive

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