ELECTRICAL MACHINE
1) The overload release protects the motor against
a. Over voltage
b. Over current
c. Over load
d. All of these
ANSWER: Over load
2) 3 - point starter is used to start the
a. Series motor
b. Shunt motor
c. Compound motor
d. Only (b) and (c)
ANSWER: Only (b) and (c)
3) Slip of an induction motor increases with
a. increase in current and decrease in torque
b. increase in current and torque
c. decrease in current and torque
d. decrease in current and increase in torque
ANSWER: increase in current and torque
4) Efficiency of a transformer is maximum at
a. Leading power factor
b. Lagging power factor
c. Unity power factor
d. None of these
ANSWER: Unity power factor
5) The non salient pole type synchronous generator is driven by
a. Hydraulic turbines
b. Diesel engines
c. Steam engines
d. All of these
ANSWER: Steam engines
6) The braking torque of induction type single - phase energy meter is
a. Directly proportional to the flux
b. Inversely proportional to the flux
c. Directly proportional to the square of the flux
d. Inversely proportional to the square of the flux
ANSWER: Directly proportional to the square of the flux
7) The field winding of dc series motor consists of few turns of
a. Thin wire with low resistance
b. Thick wire with low resistance
c. Thin wire with high resistance
d. Thick wire with high resistance
ANSWER: Thick wire with low resistance
8) When the motor runs on no load, then
a. Back emf is almost equal to applied voltage
b. Back emf will be greater than applied voltage
c. Back emf will be less than applied voltage
d. None of these
ANSWER: Back emf is almost equal to applied voltage
9) If the number of conductors and speed of a lap wound generator is
doubled then the generated emf will be
a. Remains same
b. Twice of the former
c. Four times of former emf
d. Half of the former emf
ANSWER: Four times of former emf
10) While connecting three phase transformer in star-star connection, the
terminals of secondaries are wrongly joined. Then the mutual phase angle and
magnitude of line to line voltages of secondaries will
a. Equal to 120 degree and equal as before
b. Not equal to 120 degree and different
c. Equal to 120 degree and different
d. None of these
ANSWER: Not equal to 120 degree and different
11) Small iron core transformers used in certain frequency range is also called
audio frequency transformer. The audio frequency range is
a. 20 t0 2000 Hz
b. 20 to 20000 Hz
c. 2 to 2000 Hz
d. 200 to 20000 Hz
ANSWER: 20 to 20000 Hz
12) For a two winding transformer if VA rating is given by (VA)tw and K is the
transformation ratio of an autotransformer then the VA rating of the
autotransformer is given by
a. (K/(K-1)) * (VA)tw
b. (K/(1-K)) * (VA)tw
c. ((1-K)/K) * (VA)tw
d. ((K-1)/K) * (VA)tw
ANSWER: (K/(K-1)) * (VA)tw
13) When a resistance is to be transferred from primary to secondary side of
transformer with the turns ratio as k, it must be
a. Multiplied by k ∧ 2
b. Multiplied by k
c. Divided by k
d. Divided by k ∧ 2
14) Back emf in case of synchronous motor depends on the
a. Excitation given to the field winding
b. Speed
c. Both (a) and (b)
d. None of the above
ANSWER: Excitation given to the field winding
15) If two mechanically coupled alternators deliver power at 50 Hz and
60 Hz respectively, then the highest speed of alternators will be
a. 1200 rpm
b. 1500 rpm
c. 600 rpm
d. 300 rpm
ANSWER: 600 rpm
16) What happens if a stationary alternator is connected to live bus
bar?
a. It can result in short circuit
b. A large emf will be induced by alternator
c. Both (a) and (b)
d. None of these
ANSWER: It can result in short circuit
17) Whether the given autotransformer is step up or step down, its VA
rating is always
a. Greater than the two winding transformer
b. Equal to the two winding transformer
c. lesser than the two winding transformer
d. cannot say
ANSWER: Greater than the two winding transformer
18) The most commonly used connections for power systems as a step
- up and step - down transformers are
a. Star - delta, star - star
b. Delta - star, star - delta
c. Star - star, delta - delta
d. Star - delta, delta - star
ANSWER: Star - delta, delta - star
19) D.C. shunt motors are commonly used in
a. Cranes
b. Electric traction
c. Elevators
d. Lathe machines
ANSWER: Lathe machines
20) Due to overdamping, the instrument will become
a. Slow
b. Lethargic
c. Fast
d. Both (a) and (b)
ANSWER: Both (a) and (b)
21) For a single phase no load transformer, which among the following
losses will be minimum?
a. hysteresis losses
b. eddy current losses
c. copper losses
d. mechanical losses
ANSWER: copper losses
22) A 50 Hz, 3 phase induction motor with 4 pole stator is supplied by
mains, which contains a pronounced fifth time harmonic. The speed of
the fifth space harmonic will be
a. 100 rpm
b. 300 rpm
c. 600 rpm
d. 7500
ANSWER: 300 rpm
23) The rotational losses in d.c. machines is equal to the
a. Kinetic energy of armature
b. Half of the kinetic energy of armature
c. Square of the kinetic energy of armature
d. Rate of change of kinetic energy
ANSWER: Rate of change of kinetic energy
24) Differential compound motors are mainly used in
a. Drilling machines
b. Elevators
c. Electric traction
d. Not suitable for any practical application
ANSWER: Not suitable for any practical application
25) Which motor is not suitable for the application of centrifugal
pumps?
a. Shunt motor
b. Series motor
c. Cumulative compound motor
d. Differential compound motor
ANSWER: Series motor
26) In terms of slip, the actual speed (N) of the induction motor can be
expressed as
a. Ns (1-s)
b. Ns (s-1)
c. Ns / (1-s)
d. Ns / (s-1)
ANSWER: Ns (s-1)
27) In double delta transformation, a double delta refers to the case
where there are
two delta transformations in
a. Parallel
b. Series
c. Both series and parallel
d. Neither series nor parallel
ANSWER: Series
28) Small iron core transformers used in certain frequency range is
also called audio frequency transformer. The audio frequency range
is
a. 20 t0 2000 Hz
b. 20 to 20000 Hz
c. 2 to 2000 Hz
d. 200 to 20000 Hz
ANSWER: 20 to 20000 Hz
29) The constant current transformer are mainly used for
a. Supplying series connected street light
b. Personal computers
c. Arc welding
d. Electric toys
ANSWER: Supplying series connected street light
30) The constant voltage transformers are most commonly used in
a. Personal computers
b. Arc welding
c. Power distribution
d. Electric cars
ANSWER: Personal computers
31) For moment of inertia to be small in two phase servomotor, the
ratio of rotor
diameter to its length should be
a. Equal to 1
b. small
c. large
d. All of these
ANSWER: small
32) In a single phase induction motor, the motor speed on no load
is
a. Almost equal to its synchronous speed
b. Less than its synchronous speed
c. Greater than synchronous speed
d. None of these
ANSWER: Almost equal to its synchronous speed
33) The starting torque and power factor of shaded pole induction
motor
a. High, low
b. Low, high
c. Low, low
d. High, high
ANSWER: Low, low
34) In refrigerators the commonly used motors are
a. Split phase induction motors
b. Capacitor induction motors
c. Shaded pole induction motors
d. None of these
ANSWER: Capacitor induction motors
35) In capacitor start capacitor run induction motor, the capacitor
remains in
circuit
a. Only at start
b. Only during running condition
c. At start as well as during running condition
d. None of these
ANSWER: At start as well as during running condition
36) In washing machines, most commonly used motors are
a. Split phase induction motors
b. Slip ring induction motors
c. Capacitor start induction motors
d. Shaded pole induction motors
ANSWER: Split phase induction motors
37) Single phase induction motors are
a. Self starting
b. Not self starting
c. None of these
ANSWER: Not self starting
38) The method which can be used for the speed control of induction
motor from stator side is
a. V / f control
b. Controlling number of stator poles to control Ns
c. Adding rheostats in stator circuit
d. All of these
e. None of these
ANSWER: All of these
39) For parallel operation of transformers, the p.u. impedances of
transformer must be (based on their own kVA rating)
a. directly proportional to their ratings
b. inversely proportional to their ratings
c. equal
d. none of these
ANSWER: equal
40) For satisfactory parallel operation, the circulating current should
not exceed
a. 5% of normal rated current
b. 50% of normal rated current
c. 10% of normal rated current
d. 25% of normal rated current
ANSWER: 10% of normal rated current
41) For carrying out parallel operation, transformers should have same
X / R ratio in order to avoid
a. Leakage reactance
b. Insulation failure
c. Circulating current
d. Inrush current
ANSWER: Circulating current
42) Two transformers are operated in parallel, most essential condition
to be satisfied is
a. Polarities should be same
b. Leakage reactance should be equal
c. Turns ratios should be equal
d. Impedances should be in proportion to their kVA rating
ANSWER: Polarities should be same
43) While performing sumpner’s test on transformer, if the secondary
are not in series opposition then the meter will read
a. 0
b. Twice of the induced emf in each secondary’s
c. Equal to the induced emf in each secondary’s
d. Keep fluctuating
ANSWER: Twice of the induced emf in each secondary’s
44) Which test is also called as back to back test?
a. Retardation test
b. Sumpner’s test
c. Field test
d. Voltage drop test
ANSWER: Sumpner’s test
45) If short circuit test is performed on the transformer with constant
rated voltage and increased frequency, then the short circuit current and
power factor
a. Both will increase
b. Both will decrease
c. Increase, decrease
d. Decrease, remains constant
ANSWER: Both will decrease
46) If the power factor of a transformer increases, then its efficiency
will
a. Increases
b. Decreases
c. Remains constant
d. Not related to each other
ANSWER: Increases
47) What happens to the hysteresis losses and eddy current losses, if
the frequency of a transformer is increased keeping the supply voltage
constant?
a. Both will decrease
b. Hysteresis losses will increase and eddy current losses will decrease
c. Hysteresis losses will remain same and eddy current losses will decrease
d. Hysteresis losses will decrease and eddy current losses will remain same
ANSWER: Hysteresis losses will decrease and eddy current losses will remain same
48) For a transformer, no load primary current ( I0 ) has two
components, magnetizing component and active components. The
magnetizing component is given by
a. I0 cos Φ0
b. I0 sin Φ0
c. I0 cot Φ0
d. I0 tan Φ0
ANSWER: I0 sin Φ0
49) If the power factor of load is near unity and leading, the full load voltage regulation transformer will be
a. Zero
b. One
c. 0.5
d. 0.707
ANSWER: Zero
50) A 5kV / 250V, 50Hz single phase core type transformer with average flux in the core The number of turns in the primary winding is
a. 273
b. 278
c. 280
d. 287
ANSWER: 287
51) For the calculation of induced emf in a transformer the flux used in the
equation is in its
a. RMS value
b. Average value
c. Maximum value
d. Double of its maximum value
ANSWER: Maximum value
52) In a transformer tank, grounding is given to protect it from lightening stoke. Grounding usually installed near
a. Bottom of the tank
b. Top of the tank
c. In the middle of the tank
d. Anywhere on the tank
ANSWER: Bottom of the tank
53) In a transformer sludging is principally due to
a. Moisture present in oil
b. Dust particles present inside the tank
c. Decomposition of oil
d. Vaporization of oil
ANSWER: Decomposition of oil
54) For transformers ratings are always given in kVA, not in kW because
a. It is conventional
b. Power factor of load is not known
c. For higher efficiency
d. To make calculation simple
ANSWER: Power factor of load is not known
55) What happens if dc supply is given to the transformer?
a. Transformer will work more efficiently
b. It will work less efficiently
c. It will work same as with ac supply
d. Not work at all
ANSWER: Not work at all
56) An over excited synchronous motor operating on no load condition is called
as
a. Synchronous capacitor
b. Synchronous condenser
c. Both (a) and (b)
d. None of these
ANSWER: Both (a) and (b)
57) In a synchronous motor, the phenomenon of hunting caused by the sudden application removal of load is prevented by
a. Using damper winding
b. Using a capacitor
c. Using an inductor
d. All of the above
ANSWER: Using damper winding
58) Hunting in a synchronous motor results in
a. Loss of synchronism
b. Mechanical and electrical stresses on motor
c. Large current drawn by motor
d. all of the above
ANSWER: all of the above
59) The value of load angle δ and the internal machine angle ∅ for maximum power synchronous motor is equal to
a. 0°electrical, 0° electrical
b. 0° electrical, 90° electrical
c. 90° electrical, 0° electrical
d. 90° electrical, 90° electrical
ANSWER: 90° electrical, 90° electrical
60)For critical excitation the power factor of the synchronous motor will be equal
to
a. 0
b. 1
c. 0.5 MΩ
d. None of the above
ANSWER: 1
61) When the excitation of a synchronous motor is changed keeping the load
constant then the
a. Power factor will change
b. Ia cos ∅ remains constant
c. Both (a) and (b)
d. None of these
ANSWER: Both (a) and (b)
62) The impedance angle or internal machine angle (°) of a synchronous motor
is equal to
a. tan-1Xs / Ra
b. tan-1 Rs / Xa
c. tan-1 Xs / Za
d. tan-1 Zs / Ra
ANSWER: tan-1Xs / Ra
63) In a synchronous motor the torque produced will be maximum when load angle a. 0° electrical
b. 90° electrical
c. 180° electrical
d. 360° electrical
ANSWER: 90° electrical
64) With increase in load, the speed of synchronous motor
a. Increases
b. Decreases
c. Remains constant
d. None of the above
ANSWER: Remains constant
65) The method used for starting synchronous motor is/are
a. Using pony motors
b. Using damper windings
c. As a slip ring induction motor
d. All of the above
ANSWER: All of the above
66) The increase in mechanical torque in case of alternator will
a. Increase the armature current and reduce the power factor
b. Increase the armature current and improve the power factor
c. Decrease the armature current and improve the power factor
d. Decrease the armature current and reduce the power factor
ANSWER: Increase the armature current and improve the power factor
67) The electrical power output in case of synchronous machine is
a. Directly proportional to the power angle
b. Inversely proportional to the power angle
c. Directly proportional to the square of power angle
d. None of these
ANSWER: Directly proportional to the power angle
68) Which of the following could be lamina-proximately the thickness of
laminations of a D.C. machine ?
(a) 0.005 mm
(b) 0.05 mm
(c) 0.5 m
(d) 5 m
Ans: c
69) In D.C. generators the pole shoes are fastened to the pole core by
rivets
counter sunk screws
brazing
welding
ANSWER : : counter sunk screws
70) In a four-pole D.C. machine
all the four poles are north poles
alternate poles are north and south
all the four poles are south poles
two north poles follow two south poles
ANSWER : : alternate poles are north and south
71) Use of an autotransformer is economical when its transformation
ratio is
a. Near unity
b. Much greater than unity
c. Much lesser than unity
d. None of these
ANSWER: Near unity
72) Short circuit ratio of an alternator is given by
a. If for rated short circuit current / I f for rated short circuit voltage
b. If for rated short circuit Voltage / If for rated short circuit current
c. If for rated open circuit Voltage / If for rated short circuit current
d. If for rated short circuit current / If for rated open circuit voltage
ANSWER: If for rated open circuit Voltage / If for rated short circuit current
73) For finding voltage regulation of an alternator, the method which
gives most accurate result is
a. E.M.F method
b. M.M.F. method
c. Z.P.F method
d. ASA modification of M.M.F. method
ANSWER: Z.P.F method
74) The percentage voltage regulation of an alternator is given by
a. {(Eph - Vph/ Eph) *100}
b. {(Vph - Eph / Eph)*100}
c. {(Eph - Vph / V ph)*100}
d. {(Vph - Eph / V ph)*100}
ANSWER: {(Eph - Vph / V ph)*100}
75) In an alternator, the voltage regulation will be positive when the
power factor of the load is
a. Leading
b. Unity
c. Lagging
d. Both ( b ) & ( c )
ANSWER: Both ( b ) & ( c )
76) The synchronous reactance of an alternator is equal to
a. Leakage reactance of armature
b. Armature reaction reactance
c. Sum of leakage reactance and armature reaction reactance
d. Difference of leakage reactance and armature reaction reactance
ANSWER: Sum of leakage reactance and armature reaction reactance
77) 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
78) 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
ANSWER: Hydro electric power plant
79) 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: 0.957
80) 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
81) 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)
82) Due to short pitching, the induced emf gets
a. Reduced
b. Increased
c. Remains same
d. None of these
ANSWER: Reduced
83) 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
84) 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: Sinusoidal
85) 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: Magnetic amplifier
86) 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: Smooth cylindrical type
87) 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
88) 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: Salient pole type rotor
89) 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: Rotating field type
90) 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: Alternator
91) 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
92) 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: Both (a) & (b)
93) An autotransformer starter is suitable for
a. Star connected induction motor
b. Delta connected induction motor
c. Both (a) & (b)
d. None of these
ANSWER: Both (a) & (b)
94) Applications of induction generators are
a. In railways for braking purpose
b. Wind mils
c. Drilling machines
d. Only (a) and (b)
ANSWER: Only (a) and (b)
95) Which of the following is not true about induction generator?
a. Synchronization for induction generator is required
b. Induction generators are more suitable for high speeds
c. With the help of excitation supply and frequency, the voltage and frequency
of induction motor are controlled
d. The construction is rugged for rotating parts
ANSWER: Synchronization for induction generator is required
96) In a slip ring induction motor, the rotor resistance is
a. High during starting and low at normal operating speed
b. Low at starting and high at normal operating speed
c. Remains same during starting and normal operating speed
d. None of these
ANSWER: High during starting and low at normal operating speed
97) In the blocked rotor tests, slip is equal to
a. 0
b. 1
c. 0.5
d. 0.707
ANSWER: 1
98) The crawling and cogging is not predominant in
a. Slip ring induction motor
b. Squirrel cage induction motor
c. Both (a) and (b)
d. None of these
ANSWER: Slip ring induction motor
99) What happens if the number of stator slots in an induction motor is
equal to the number of rotor slots or integral multiple of rotor slots?
a. Motor will run at very high speed
b. Motor will run at very low speed
c. Motor will fail to start
d. Does not get effected
ANSWER: Motor will fail to start
100) Crawling is a phenomena mainly associated with
a. 3rd harmonic
b. 5th harmonic
c. 7th harmonic
d. 2nd harmonic
ANSWER: 7th harmonic
101) Cogging in an induction motor is caused
a. If the number of stator slots are integral multiple of rotor slots
b. If the motor is running at fraction of its rated speed
c. Due to fifth harmonic
d. All of these
ANSWER: If the number of stator slots are integral multiple of rotor slots
102) The air gap flux of induction motor does not contain
a. 2nd harmonics
b. 3rd harmonics
c. 5th harmonics
d. 7th harmonics
ANSWER: 3rd harmonics
103) Torque of an induction motor can sometimes be given in
synchronous - watt. One synchronous - watt is equal to
a. 60 / (2 Π Ns) N - m
b. (2 Π Ns) / 60 N - m
c. 120 / (2 Π Ns) N - m
d. (2 Π Ns) / 120 N - m
ANSWER: 60 / (2 Π Ns) N - m
104) For slip s of an induction motor, the ratio of rotor copper loss to
the gross mechanical output is
a. 1 / ( 1 - s)
b. 1 / s
c. ( 1 - s ) / s
d. s / ( 1 - s )
ANSWER: s / ( 1 - s )
105) If the rotor resistance of an induction motor is doubled, keeping
the other parameters constant, then the maximum torque of the
induction motor will become
a. Double
b. Halved
c. One fourth
d. Remains same
ANSWER: Remains same
106) When the slip of an induction machine is negative, it is running as
a
a. Motor
b. Generator
c. Synchronous machine
d. None of these
ANSWER: Generator
107) When an induction motor is loaded from no load to full load, its
speed and slip
a. Increases, decreases
b. Decreases, increases
c. Both increases
d. Both decreases
ANSWER: Decreases, increases
108) For maximum torque, the only parameter which controls the
torque is slip. The maximum torque is
a. directly proportional to the rotor resistance
b. inversely proportional to the rotor resistance
c. directly proportional to the slip times the rotor resistance
d. independent of the rotor resistance
ANSWER: independent of the rotor resistance
109) The torque produced in a three phase induction motor is
a. Directly proportional to the square of the induced emf in rotor
b. Directly proportional to the induced emf in rotor
c. Inversely proportional to the square of the induced emf in rotor
d. Inversely proportional to the induced emf in rotor
ANSWER: Directly proportional to the square of the induced emf in rotor
110) The rotor frequency of a three phase induction motor is
a. Slip times the supply frequency
b. 1 / slip times the supply frequency
c. Equal to the supply frequency
d. None of these
ANSWER: Slip times the supply frequency
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