Explore topic-wise MCQs in Electrical Machines.

This section includes 51 Mcqs, each offering curated multiple-choice questions to sharpen your Electrical Machines knowledge and support exam preparation. Choose a topic below to get started.

1.

A slip-ring induction motor is expected to be started by adding extra resistance in the rotor circuit. The benefit that is derived by adding extra resistance in the rotor circuit in comparison to the rotor being shorted is

A. The starting torque would be higher.
B. The power factor at start will be lower.
C. The starting current is higher.
D. The losses at starting would be lower.
Answer» B. The power factor at start will be lower.
Discussion
2.

In a constant \(V/f\) induction motor drive, the slip at the maximum torque

A. is directly proportional to the synchronous speed.
B. remains constant with respect to the synchronous speed.
C. has an inverse relation with the synchronous speed.
D. has no relation with the synchronous speed.
Answer» D. has no relation with the synchronous speed.
Discussion
3.

A 6 – pole, 50 Hz, 3-phase induction motor is running at 950 rpm and has rotor copper loss of 5 kW. It rotor input is:

A. 10 kW
B. 95 kW
C. 100 kW
D. 5.3kW
Answer» D. 5.3kW
Discussion
4.

A three-phase induction motor is running at 4% slip. If the input to the rotor is 1,000 W, then the mechanical power developed by the motor will be _______.

A. 960 W
B. 9,600 W
C. 96 W
D. 0.96 W
Answer» B. 9,600 W
Discussion
5.

A 3-phase induction motor has a starting torque of 200 N-m when switched on-directly to supply. If an auto-transformer with 50% tapping is used for starting, the starting torque would be

A. 400 N-m
B. 200 N-m
C. 100 N-m
D. 50 N-m
Answer» E.
Discussion
6.

Full load current of an Induction motor is 20 Ampere. The no-load current may be expected to be

A. 25 Ampere to 28 Ampere
B. 15 Ampere to 18 Ampere
C. 11 Ampere to 14 Ampere
D. 6 Ampere to 10 Ampere
Answer» E.
Discussion
7.

A 3-phase, 4 pole, 400 V, 50 Hz, squirrel-cage induction motor is operating at a slip of 0.02. The speed of the rotor flux in mechanical rad/sec sensed by stationary observer, is closest to

A. 1500
B. 1470
C. 157
D. 154
Answer» D. 154
Discussion
8.

Generally, Crawling in squirrel cage induction motors is the tendency to run stably at speeds as low as

A. One-third of their synchronous speed
B. One-fifth of their synchronous speed
C. One-seventh of their synchronous speed
D. One-ninth of their synchronous speed
Answer» D. One-ninth of their synchronous speed
Discussion
9.

For a 3-phase induction motor, what fraction/multiple of supply voltage is required for a direct-on-line starting method such that starting current is limited to 5 times the full-load current and motor develops 1.5 times full-load torque at starting time?

A. 1.632
B. 1.226
C. 0.816
D. 0.456
Answer» C. 0.816
Discussion
10.

In star/delta starting of a 3-phase induction motor, the starting voltage is reduced to:

A. 3 times the normal voltage
B. \(\frac{1}{3}\) times of normal voltage
C. \(\frac{1}{{\sqrt 3 }}\) of normal voltage
D. \(\sqrt 3 \) times of normal voltage
Answer» D. \(\sqrt 3 \) times of normal voltage
Discussion
11.

A 3 phase, 4 pole, 400 V, 10 kW slip ring induction motor has rotor resistance = 0.16 Ω per phase and stator resistor of 0.27 Ω per phase. The voltage across the slip rings at standstill is 141 V per phase. The motor develops a torque of 62.4 Nm at a slip of 0.08 and the rotor current is 6 A. What is the rotor current if a voltage is injected to the rotor so that the motor runs of slip s = 0.02 and develops same torque?

A. 6 A
B. 5 A
C. 4 A
D. 3 A
Answer» E.
Discussion
12.

A 4 pole induction machine is working as an induction generator. The generator supply frequency is 60 Hz. The rotor current frequency is 5 Hz. The mechanical speed of the rotor in RPM is

A. 1350
B. 1650
C. 1950
D. 2250
Answer» D. 2250
Discussion
13.

Double cage induction motors are used for

A. High starting torque
B. Better speed control
C. High running torque
D. None of the above
Answer» B. Better speed control
Discussion
14.

Assertion A: Copper Losses are small in Squirrel cage induction motors.Reason R: Overhang is less and the space factor is better in Squirrel cage induction motors.

A. R is correct but A is not correct
B. A is correct but R is not correct
C. A is correct and R is the suitable reason for it
D. A is correct but R is not a suitable reason for it
Answer» D. A is correct but R is not a suitable reason for it
Discussion
15.

A three-phase induction motor is running at 5 percent slip with certain load. If at this load the power input to the rotor is 2000 W, then the mechanical power developed by the motor is:

A. 2000 W
B. 2100 W
C. 1700 W
D. 1900 W
Answer» E.
Discussion
16.

A 500 HP, 440 V, 3-phase, 50 Hz induction motor runs at 950 r.p.m. when on full load with a synchronous speed of 1000 r.p.m. For this condition, the frequency of the rotor current will be

A. 4.0 Hz
B. 3.5 Hz
C. 2.5 Hz
D. 2.0 Hz
Answer» D. 2.0 Hz
Discussion
17.

A 3-phase induction motor has a starting torque of 100% and a maximum torque of 200% of the full load torque. The slip at maximum torque will be

A. 0.268
B. 0.579
C. 0.707
D. 0.07
Answer» B. 0.579
Discussion
18.

In an induction motor, the phenomenon of magnetic locking between the stator and the rotor teeth is called ______.

A. Hunting
B. Skewing
C. Cogging
D. Crawling
Answer» D. Crawling
Discussion
19.

A 3 phase, 50 Hz, six pole induction motor has a rotor resistance of 0.1 Ω and reactance of 0.5 Ω. Neglect the voltage drop in stator and assume that the rotor resistance is constant. Given that the full load slip is 5%, the ratio if maximum torque to full load torque is approximately equal to

A. 2.5
B. 1.125
C. 2.125
D. 2.34
Answer» D. 2.34
Discussion
20.

A three-phase slip ring induction motor develops full load torque at a slip of 0.04 when the slip rings are short circuited. If the rotor resistance is increased 3 times by inserting external resistance in the rotor circuit, then the slip will be:

A. 0.04
B. 0.01
C. 0.12
D. 0.06
Answer» D. 0.06
Discussion
21.

In forward regeneration mode of a three-phase induction motor drive, the value of slip 's' is:

A. negative
B. infinite
C. 1
D. 0
Answer» B. infinite
Discussion
22.

A wound rotor induction motor runs with a slip of 0.05 when developing full load torque. Its rotor resistance is 0.45 Ω per phase. If an external resistance of 0.50 Ω per phase is connected across the slip rings, what is the slip for full torque?

A. 0.03
B. 0.06
C. 0.09
D. 0.1
Answer» E.
Discussion
23.

In V/f control of induction motors, the ratio of V/f is boosted during

A. below rated frequency
B. at half rated voltage
C. below 5 Hertz
D. above rated frequency
Answer» D. above rated frequency
Discussion
24.

If a 4 –pole induction motor has a speed of 1800 rpm, then the frequency at which the motor is operating is:

A. 60 Hz
B. 40 Hz
C. 30 Hz
D. 50 Hz
Answer» B. 40 Hz
Discussion
25.

In a double squirrel cage motor, outer cage is made of high resistance metal bars primarily for the purpose of increasing its ______.

A. Starting torque
B. Speed regulation
C. Efficiency
D. Starting current
Answer» B. Speed regulation
Discussion
26.

A three-phase squirrel cage induction motor is operating at ‘dT/ds = 0’. The corresponding torque is:

A. Minimum
B. 1/2 (maximum)
C. Maximum
D. 1/4 (maximum)
Answer» D. 1/4 (maximum)
Discussion
27.

In a three phase slip ring induction motor if rotor resistance is changed then

A. the maximum torque limit changes
B. the speed at which maximum torque occurs changes
C. the iron loss changes
D. the speed remains constant but torque changes
Answer» C. the iron loss changes
Discussion
28.

A 4-pole induction motor (main) and a 6-pole motor (auxiliary) are connected in cumulative cascade. Frequency in the secondary winding of the auxiliary motor is observed to be 1 Hz. For a supply frequency of 50 Hz the speed of the cascade set is:

A. 1485 rpm
B. 990 rpm
C. 608 rpm
D. 588 rpm
Answer» E.
Discussion
29.

A direct on-line starter is used for starting motors up to

A. 5 hp
B. 10 hp
C. 20 hp
D. 40 hp
Answer» B. 10 hp
Discussion
30.

Directions: It consists of two statements, one labelled as the ‘Statement (I)’ and the other as ‘Statement (II). Examine these two statements carefully and select the answer using the codes given below:Statement (I): The speed control of induction motor by pole changing is suitable for cage motors only.Statement (II): The cage rotor automatically develops number of poles equal to the poles of stator winding.

A. Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)
B. Both Statement (I) and Statement (II) are individually true but Statement (II) is not the correct explanation of Statement (I)
C. Statement (I) is true but Statement (II) is false
D. Statement (I) is false but Statement (II) is true
Answer» B. Both Statement (I) and Statement (II) are individually true but Statement (II) is not the correct explanation of Statement (I)
Discussion
31.

A 3-phase, induction motor operating at a slip of 5% develops 20 kW rotor power output. What is the corresponding rotor copper loss in this operating condition?

A. 750 W
B. 900 W
C. 1050 W
D. 1200 W
Answer» D. 1200 W
Discussion
32.

An 4-pole 50 Hz induction motor with rotor resistance of 0.2 Ω produces a maximum torque of 10 N-m at 1200 rpm. At maximum torque, what will be rotor reactance and the difference between synchronous and asynchronous speeds (slip) respectively?

A. 2 Ω and 0.1 p u
B. 1 Ω and 0.1 p u
C. 2 Ω and 0.2 p u
D. 1 Ω and 0.2 p u
Answer» E.
Discussion
33.

A 3-phase induction motor draws 50 kW from a 220 V, 50 Hz mains. The rotor emf makes 100 oscillations/minute. If the stator losses are 2 kW the rotor copper loss would be

A. 0.16 kW
B. 0.32 kW
C. 1.6 kW
D. 3.2 kW
Answer» D. 3.2 kW
Discussion
34.

In a constant V/f control of induction motor, the ratio V/f is maintained constant from 0 to base frequency, where V is the voltage applied to the motor at fundamental frequency f. Which of the following statements relating to low frequency operation of the motor is TRUE?

A. At low frequency, the stator flux increases from its rated value.
B. At low frequency, the stator flux decreases from its rated value.
C. At low frequency, the motor saturates.
D. At low frequency, the stator flux remains unchanged at its rated value.
Answer» C. At low frequency, the motor saturates.
Discussion
35.

During plugging of a three-phase induction motor:

A. Rotor circuit is opened
B. Phase sequence of the stator voltage is reversed
C. One phase of stator is short circuited
D. Dc source is connected to stator
Answer» C. One phase of stator is short circuited
Discussion
36.

In a single-phase, single-winding induction motor, let Ns be the synchronous speed and N be the rotor speed. What will be the rotor slip with respect to the backward rotating field?

A. \(\dfrac{N_s+N}{N}\)
B. \(\dfrac{N_s-N}{N_s}\)
C. \(\dfrac{N_s+N}{N_s}\)
D. \(\dfrac{N_s-N}{N}\)
Answer» D. \(\dfrac{N_s-N}{N}\)
Discussion
37.

For a given three phase induction motor, (stator output) – (gross rotor output) = ?

A. Windage and friction losses
B. Rotor Cu losses
C. Stator iron losses
D. Stator Cu losses
Answer» C. Stator iron losses
Discussion
38.

In a three-phase induction motor regenerative braking occurs when:

A. Load is lowered by a hoisting machine
B. Motor speed falls due to overload.
C. Load is raised by a hoisting machine
D. Number of poles is reduced in a pole changing machine
Answer» B. Motor speed falls due to overload.
Discussion
39.

If the slip in an induction motor is 0.25 and the synchronous speed is 600 rpm, the motor speed (in rpm) would be:

A. 450
B. 125
C. 750
D. 900
Answer» B. 125
Discussion
40.

During constant torque operation of a.c. electrical machines

A. supply voltage or frequency has to remain constant
B. speed has to remain constant
C. power has to remain constant
D. both supply voltage and supply frequency has to remain constant
Answer» E.
Discussion
41.

Formula for Number of Magnetic poles (P) in an Induction motor= ____

A. (180 × f)/N
B. (60 × f)/N
C. (360 × f)/N
D. (120 × f)/N
Answer» E.
Discussion
42.

For heavy loads, the relation between torque (T) and slip (S) in induction motor is given by ______.

A. \(T \propto \frac{S}{{1 - S}}\)
B. T ∝ S
C. T ∝ (1 - S)
D. \(T \propto \frac{1}{S}\)
Answer» E.
Discussion
43.

An 8-pole, 50 Hz, three-phase induction motor is running at 705 rpm and has a rotor copper loss of 5 kW. Its rotor input is

A. 5.06 kW
B. 0.3 kW
C. 100 kW
D. 83.33 kW
Answer» E.
Discussion
44.

Implementation of Volts/Hertz strategy for inverter-fed induction motor in open loop is used in

A. Low performance applications
B. High performance applications
C. Both (1) and (2)
D. None of these
Answer» B. High performance applications
Discussion
45.

A 3 phase induction motor runs at almost 1000 rpm at no load and 950 rpm at full load when supplied with power from a 50 Hz 3 phase line. The frequency of rotor voltage is:

A. 2 Hz
B. 2.5 Hz
C. 250 Hz
D. 2.25 Hz
Answer» C. 250 Hz
Discussion
46.

For an induction motor, operating at a slip ‘s’, the ratio of gross power output to the air gap power is equal to

A. (1 - s)2
B. (1 - s)
C. √(1 – s)
D. 1 - √s
Answer» C. √(1 – s)
Discussion
47.

For a 3-phase slip-ring induction motor, the electrical rotor losses are proportional to

A. Synchronous speed
B. Air gap power
C. Slip
D. None of the above
Answer» D. None of the above
Discussion
48.

Increasing the air-gap of a squirrel-cage induction motor would result in

A. Increase in no-load speed
B. Increase in full-load power-factor
C. Increase in magnetizing current
D. Maximum available torque
Answer» D. Maximum available torque
Discussion
49.

A three phase induction motor is operating at full load at the difference (slip) between synchronous and non-synchronous speeds s. During operation, if the phase sequence of the supply is reversed, what will be the difference (slip) between synchronous and asynchronous speed immediately after reversal?

A. 2 - s
B. 1 + s
C. 1 - s
D. s
Answer» B. 1 + s
Discussion
50.

In the image shown, select the symbol from the given option.

A. Star-Delta starter
B. Motor starter
C. DOL starter
D. Autotransformer starter
Answer» B. Motor starter
Discussion