Free Electrical Machines MCQs with Answers

• 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.

• A. reduce windage losses
• B. reduce eddy currents
• C. reduce accumulation of dirt and dust
• D. reduce magnetic hum

Explanation: Slightly skewed rotor slots reduce slot harmonics and the resulting magnetic pulsations, which minimises magnetic hum.

• A. 0.04%
• B. 0.4%
• C. 4%
• D. 14%

Explanation: Medium-sized induction motors commonly operate with a slip of a few percent, typically about 4%.

• A. Bearings
• B. Shaft
• C. Statorcore
• D. None of the above

Explanation: The stator core is built from laminated silicon-steel sheets because silicon steel has low hysteresis loss and laminations limit…

• A. three phase transformers
• B. compound generators
• C. D.C. series motors
• D. induction motors

Explanation: Cogging is the tendency of an induction motor to refuse starting when rotor and stator slot harmonics lock magnetically.

• A. two
• B. three
• C. four
• D. none

Explanation: A squirrel-cage rotor has conducting bars permanently short-circuited by end rings, so it has no slip rings.

• A. 60 to 90%
• B. 80 to 90%
• C. 95 to 98%
• D. 99%

Explanation: A practical induction motor commonly achieves about 80% to 90% efficiency, depending on its rating and load.