All Free Electrical Engineering MCQs with Answers

• A. rotor conductor ends are short-circuited through slip rings
• B. rotor conductors are short-circuited through end rings
• C. rotor conductors are kept open
• D. rotor conductors are connected to insulation

Explanation: In a squirrel-cage rotor, conducting bars are permanently joined at both ends by end rings, forming a closed electrical path.

• A. frequency
• B. rotor inductive reactance
• C. square of supply voltage
• D. all of the above

Explanation: Maximum torque is governed by the torque equation, which includes supply voltage, frequency through synchronous speed and reactance, and…

• A. self-starting with zero torque
• B. self-starting with high torque
• C. self-starting with low torque
• D. non-self starting

Explanation: A three-phase induction motor is self-starting because its stator produces a rotating magnetic field, but its starting torque is…

• A. the motor will run in reverse direction
• B. the motor will run at reduced speed
• C. the motor will not run
• D. the motor will burn

Explanation: Interchanging any two supply phases reverses the rotating magnetic field produced by the stator.

• A. squirrel-cage induction motors only
• B. slip ring induction motors only
• C. both A. and (b)
• D. none of the above

Explanation: A rotor rheostat must be connected to the rotor circuit, which is accessible only in a slip-ring induction motor.

• A. Squirrel-cage induction motor
• B. Slip-ring induction motor
• C. Both A. and (b)
• D. None of the above

Explanation: Rotor-side speed control is possible in a slip-ring motor because external resistance or injected voltage can be connected through its…

• A. slip
• B. rotor current
• C. running torque
• D. line voltage

Explanation: The induction-motor circle diagram is drawn on a current scale, so its diameter represents the corresponding rotor-current quantity.

• A. in series
• B. in parallel
• C. in series-parallel
• D. in parallel with stator

Explanation: The high-resistance starting cage and low-resistance running cage are both connected across the stator-induced rotor voltage.

• A. k/torque with direct switching
• B. K x torque with direct switching
• C. K2 x torque with direct switching
• D. k2/torque with direct switching

Explanation: If the autotransformer voltage ratio is k, motor starting current and torque are reduced, with starting torque becoming k² times the…

• A. star connected only
• B. delta connected only
• C. A. and B. both
• D. none of the above

Explanation: An autotransformer starter can reduce the applied starting voltage for a cage motor regardless of whether its normal stator connection is…

• A. low voltage supply
• B. high loads
• C. harmonics develped in the motor
• D. improper design of the machine
• E. none of the above

Explanation: Crawling is caused by harmonic torque, especially the seventh space harmonic, which can produce a low-speed torque near one-seventh of…

• A. By changing the number of stator poles
• B. Rotor rheostat control
• C. By operating two motors in cascade
• D. By injecting e.m.f. in the rotor circuit

Explanation: Changing the number of stator poles changes synchronous speed and is readily implemented in a squirrel-cage motor.

• A. D.C. compound motor
• B. D.C. series motor
• C. synchronous motor
• D. asynchronous motor

Explanation: An induction motor has no electrical connection to its rotor and therefore runs asynchronously, with a small slip below synchronous speed.

• A. zero frequency
• B. the same frequency as the slip frequency
• C. the same phase as the rotor e.m.f.
• D. high value for the satisfactory speed control

Explanation: The rotor e.m.f. exists at slip frequency, so an injected e.m.f. must have the same frequency to produce effective speed control.

• A. inversely proportional to (Vslip)
• B. directly proportional to (slip)2
• C. inversely proportional to slip
• D. directly proportional to slip

Explanation: In the normal low-slip operating region, rotor reactance is small compared with rotor resistance, making torque approximately directly…

• A. two rotors moving in oppsite direction
• B. two parallel windings in stator
• C. two parallel windings in rotor
• D. two series windings in stator

Explanation: A double squirrel-cage rotor contains two cages, or two rotor windings, placed one inside the other.

• A. single phase motors
• B. variable speed motors
• C. low horse power motors
• D. high speed motors

Explanation: Star-delta starting requires a three-phase motor with accessible stator winding terminals, so it cannot be used with a single-phase motor.

• A. low
• B. negligible
• C. same as full-load torque
• D. slightly more than full-load torque

Explanation: A conventional squirrel-cage motor has relatively low starting torque because its rotor resistance is low.

• A. Ns
• B. s.N,
• C. (l-s)Ns
• D. (Ns-l)s

Explanation: Slip is defined as s = (Ns − N)/Ns, so rearranging gives N = (1 − s)Ns. The motor runs below synchronous speed unless the slip is zero.

• A. the magnetising current of the rotor will decrease
• B. the power factor will decrease
• C. speed of motor will increase
• D. the windage losses will increase

Explanation: Increasing the air gap raises magnetic reluctance, so the motor draws a larger magnetising current and its power factor falls.