Free Electrical Machines MCQs with Answers

• A. its power factor goes on decreasing
• B. its power factor remains constant
• C. its power factor goes on increasing even after full load
• D. its power factor goes on increasing up to full load and then it falls again

Explanation: As load increases, the motor draws more active current and its power factor improves up to approximately full load.

• A. speed control is required
• B. frequent starting, stopping and reversing is required
• C. high starting torque is needed
• D. all above features are required

Explanation: A slip-ring motor permits external rotor resistance, giving high starting torque and useful speed control, including controlled starting…

• A. stator slots are equal to rotor slots
• B. stator slots are exact multiple of rotor slots
• C. stator slots are not exact multiple of rotor slots
• D. none of the above

Explanation: Stator and rotor slot numbers are selected so that they are not exact multiples, which helps prevent magnetic locking, or cogging, and…

• A. will continue running burning its one phase
• B. will continue running burning its two phase
• C. will stop and carry heavy current causing permanent damage to its winding
• D. will continue running without any harm to the winding

Explanation: Opening one fuse in a heavily loaded delta motor causes single-phasing and an unbalanced current distribution.

• A. will continue running burning its one phase
• B. will continue running burning its two phases
• C. will stop and carry heavy current causing permanent damage to its winding
• D. will continue running without any harm to the winding

Explanation: A lightly or normally loaded delta motor may continue rotating after one fuse opens, but the remaining phase currents become unequal.

• A. bigger
• B. smaller
• C. same
• D. any of the above

Explanation: For the same horsepower, a high-speed motor develops the required power at lower torque.

• A. magnetic field and rotor rotate in opposite direction
• B. rotor speed is less than the synchronous speed of the field and are in the same direction
• C. rotor speed is more than the synchronous speed of the field and are in the same direction
• D. none of the above

Explanation: Slip is defined as s = (N_s − N_r)/N_s, so it becomes negative when the rotor runs faster than the rotating magnetic field.

• A. humming
• B. hunting
• C. crawling
• D. cogging

Explanation: Running at about one-seventh of normal speed is characteristic of crawling, caused by the interaction of harmonic rotating fields with the…

• A. proper ventilation
• B. using DOL starter
• C. auto-transformer starter
• D. having number of rotor slots more or less than the number of stator slots (not equal)

Explanation: Cogging occurs when stator and rotor teeth have equal or suitably related slot numbers and magnetically lock together.

• A. 0.2 lagging
• B. 0.2 leading
• C. 0.5 leading
• D. unity

Explanation: At no load, an induction motor still draws a substantial magnetizing current, which is highly reactive, while its useful-load current is…

• A. high loads
• B. low voltage supply
• C. improper design of machine
• D. harmonics developed in the motor

Explanation: Crawling is produced by harmonic components of the air-gap flux, especially the seventh harmonic, which can create a low-speed torque near…

• A. changing supply frequency
• B. changing number of poles
• C. changing winding resistance
• D. reducing supply voltage

Explanation: A squirrel-cage rotor has permanently short-circuited bars, so its rotor resistance cannot be changed externally as in a wound-rotor…

• A. low horsepower motors
• B. variable speed motors
• C. high horsepower motors
• D. high speed motors

Explanation: DOL starting draws a large inrush current, which can cause excessive voltage drop in the supply network.

• A. zero
• B. more than the number of poles in stator
• C. less than number of poles in stator
• D. equal to number of poles in stator

Explanation: The rotor current is induced by the stator’s rotating magnetic field, so the rotor magnetic field has the same number of poles as the…

• 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…