Electrical Machines Expected MCQ PDF 10 For Coal India Limited (CIL) MT Exam 2017

1)   In a 20 kVA,
2200 / 220 volts transformer iron and copper losses are 300 and 400
watts respectively. Its efficiency at half load and unity power factor
is

a. 95.11%
b. 96.15%
c. 97.77%
d. 98%

ANSWER: 96.15%

2)   A 500 kVA
single phase transformer has 90% efficiency at both half load and full
load at unity power factor. Then iron losses will be

a. 12.55 KW
b. 13.55 kW
c. 16.55 kW
d. 18.55 KW

ANSWER: 18.55 KW

3)   While performing a circuit test on transformer at 30v, 30 Hz it gives Φ1 as lagging power factor. If the same test is performed at 30V, 50Hz, then

a. Φ1 > Φ2
b. Φ1 < Φ2
c. Φ1 = Φ2
d. None of these

ANSWER: Φ1 < Φ2

4)   If the
magnitude of leakage reactance is equal to the resistance of both
primary and secondary of a single phase transformer. Then the input
power factor is

a. 1 / √2
b. 1
c. √2
d. zero

ANSWER: 1 / √2

5)   Open circuit test is to be performed on transformer to find

a. Constant losses
b. Copper losses
c. Both (a) and (b)
d. None of these

ANSWER: Constant losses

6)   If the transformer is step – up transformer then for performing short circuit test meter is connected to the

a. H.V. side
b. L.V. side
c. Any side
d. Test cannot be performed on step up t / f

ANSWER: H.V. side

7)   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

8)   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

9)   While
conducting the sumpner’s test, the primaries of the two identical
transformers are connected in parallel across the supply. The
secondary’s are connected in

a. Parallel
b. Series opposition
c. Both in series and parallel
d. Not connected at all

ANSWER: Series opposition

10)   In sumpner’s test, the net voltage in the local circuit of secondary’s is

a. Equal to zero
b. Twice of the induced emf in each secondary’s
c. Equal to the induced emf in each secondary’s
d. Square of the induced emf in each secondary’s

ANSWER: Equal to zero

11)   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

12)   If W1 is the
input power measured by the wattmeter connected across the primaries
and W2 is the measurement of wattmeter connected in secondary’s while
performing sumpner’s test then iron losses and copper losses is equal to

a. W1, W2
b. W1 / 2, 2 * W2
c. 2 * W1, W2 / 2
d. W1 / 2, W2 / 2

ANSWER: W1 / 2, W2 / 2

13)   For parallel
operation of transformers, the most satisfactory condition is achieved
when the load shared by the transformers is in exact proportion with
their

a. Efficiency
b. Ratings
c. Impedances
d. None of these

ANSWER: Ratings

14)   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

15)   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

16)   If the
transformers have different kVA ratings, then for parallel operation the
equivalent impedances should be

a. Directly proportional to individual kVA rating
b. Inversely proportional to individual kVA rating
c. Inversely proportional to the square of the individual kVA rating
d. Directly proportional to the square of individual kVA rating

ANSWER: Inversely proportional to individual kVA rating

17)   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

18)   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

19)   A 3 phase
induction motor in a short circuit current is equal 3 times of the full
load current. If the full load slip is 2%, then the starting torque as a
percentage of full load torque is

a. 6% of full load torque
b. 18% of full load torque
c. 36% of full load torque
d. None of these

ANSWER: 18% of full load torque

20)   In textile
machines, to avoid breaking of threads, soft starters are used. The
commonly used method in soft starter is

a. Direct online method
b. Star-delta method
c. Thyristor voltage controller method
d. Thyristor current controller method

ANSWER: Thyristor voltage controller method

21)   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

22)   The consequent pole method used for controlling the number of poles can be used for

a. Only squirrel cage type motors
b. Only slip ring induction motor
c. For both squirrel cage and slip ring induction motor
d. None of these

ANSWER: Only squirrel cage type motors

23)   The best suited method for smooth speed control by controlling number of poles is

a. Consequent poles method
b. Multiple stator winding method
c. Pole amplitude modulation method
d. None of these

ANSWER: Pole amplitude modulation method

24)   In pole
amplitude modulation method for controlling number of poles,
practically, the wave used for modulation is

a. Square wave
b. Triangular wave
c. Saw tooth wave
d. Sinusoidal wave

ANSWER: Square wave

25)   In cumulatively cascade method for speed controlling, if PA is the number of poles of main motor and PB is the number of poles of auxiliary method. Then the speed of the rotor B is given by

a. 120f / PA + PB
b. 120f / PA – PB
c. 120f / PA
d. 120f / PB

ANSWER: 120f / PA + PB

26)   In cascade control method, the set cannot be operated if

a. PA > PB
b. PA < PB
c. PA = PB
d. None of these

ANSWER: PA = PB

27)   Kramer system for controlling the speed of 3 phase induction motor is mostly used for motors of

a. Above 4000 kW
b. Below 4000 kW
c. Below 3000 kW
d. None of these

ANSWER: Above 4000 kW

28)   The disadvantages of scherbius system used for speed controlling in 3 phase induction motor is

a. It cannot be used for slip ring induction motors
b. It cannot be used for squirrel cage induction motors
c. It can be used for large induction motors
d. None of these

ANSWER: It cannot be used for squirrel cage induction motors

29)   Single phase induction motors are

a. Self starting
b. Not self starting
c. None of these
d. All of these

ANSWER: Not self starting

30)   According to
double revolving field theory, any alternating quantity can be resolved
into two rotating components. The directions and magnitude of these
components are

a. Same directions, each having magnitude equal to the maximum magnitude of alternating current
b. Opposite directions, each having magnitude equal to the maximum magnitude of alternating current
c. Opposite directions, each having magnitude half of the maximum magnitude of alternating current
d. Same directions, each having magnitude half of the maximum magnitude of alternating current

ANSWER: Opposite directions, each having magnitude half of the maximum magnitude of alternating current

31)   The spit phase induction motor has

a. Low starting current and high starting torque
b. Moderate starting current and moderate starting torque
c. Low starting current and moderate starting torque
d. Moderate starting current and low starting torque

ANSWER: Low starting current and moderate starting torque

32)   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

33)   Split phase induction motors are available in the range of

a. 1/2 to 1/10 kW
b. 1/20 to 1/2 kW
c. 1/10 to 1/20 kW
d. None of these

ANSWER: 1/20 to 1/2 kW

34)   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

35)   The capacitor connected permanently in capacitor run motor

a. Improves power factor
b. Lowers power factor
c. Does not affect power factor

ANSWER: Improves power factor

36)   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

37)   In shaded
pole induction motor, direction of rotation of magnetic field is from
non shaded pole to shaded pole

a. Shaded pole to non shaded pole
b. Non shaded pole to shaded pole
c. None of these

ANSWER: Non shaded pole to shaded pole

38)   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

39)   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

40)   The torque
equation of single phase induction motor contains a constant term
superimposed over by a pulsating torque. The pulsating frequency is

a. Equal to the supply frequency
b. Twice of the supply frequency
c. Half of the supply frequency
d. None of these

ANSWER: Twice of the supply frequency

41)   The stator
of split phase induction motor has two windings, main winding and
auxiliary winding. These windings are displaced in space by

a. 30 electrical degrees
b. 90 electrical degrees
c. 120 electrical degrees
d. 180 electrical degrees

ANSWER: 90 electrical degrees

42)   The flux under the unshaded pole of shaded pole induction motor

a. Is in equal phase of flux under the shaded pole
b. Lags the flux under the shaded pole
c. Leads the flux under the shaded pole
d. None of these

ANSWER: Leads the flux under the shaded pole

43)   A two phase
servomotor is commonly used in feedback control system to drive the
loads and as sensors to measure

a. Speed of controlled element
b. Position of controlled element
c. Both (a) and (b)
d. None of these

ANSWER: Both (a) and (b)

44)   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

45)   A load of
110kW at 0.8 power factor is to be shared by two single phase
transformer with equal turns have impedances equal to (0.4 + j 0.2) and
(0.6 + j 0.8) with respect to secondary. How will they share the load if
they are operating in parallel?

a. 80kW, 30kW
b. 82.55kW, 28.45kW
c. 75.55kW, 24.45kW
d. 85.45kW, 24.55kW

ANSWER: 85.45kW, 24.55kW

46)   If two
transformers are operated in parallel with voltage ratio of transformer 1
slightly greater than that of 2. E1 and E2 are the induced emf of
transformer 1 and 2, and Z1 and Z2 are the impedances then the
circulating current is given by

a. (E1-E2) / (Z1+Z2)
b. (E1+E2) / (Z1+Z2)
c. (Z1-Z2) / (E1-E2)
d. (Z1-Z2) / (E1+E2)

ANSWER: (E1-E2) / (Z1+Z2)

47)   For the same excitation voltage and winding currents, the autotransformer gives

a. Less output than two winding transformer
b. Equal to the output of two winding transformer
c. Half of the output of two winding transformer
d. More than the output of the two winding transformer

ANSWER: More than the output of the two winding transformer

48)   If K is the transformation ratio of step up autotransformer and Wtw is the total weight of copper in two winding transformer. Then the saving of copper is given by

a. K Wtw
b. Wtw/ K
c. Wtw/ K ∧ 2
d. K2 Wtw

ANSWER: Wtw/ K

49)   In a step down autotransformer, if the transformation ratio increases then the saving of copper

a. Increases
b. Decreases
c. Remains same
d. None of these

ANSWER: Increases

50)   If the
transformation ratio of an autotransformer is given by V2 / V1 then the
ratio of power transferred to that of input power is given by

a. (V2-V1) / V1
b. (V2+V1) / V1
c. (V1-V2) / V1
d. V2 / V1

ANSWER: V2 / V1

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Electrical Machines Expected MCQ PDF 9 For Coal India Limited (CIL) MT Exam 2017

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