All Free Electrical Engineering MCQs with Answers

• A. 730 r.p.m.
• B. 960 r.p.m.
• C. 1440 r.p.m.
• D. 2880 r.p.m.

Explanation: For the same horsepower, the low-speed motor needs more poles and a larger active size to produce the required torque.

• A. stall after sometime
• B. stall immediately
• C. continue to run at lower speed without damage
• D. get heated and subsequently get damaged

Explanation: Induction-motor torque is approximately proportional to the square of applied voltage, so a 30% voltage reduction leaves only about 49% of…

• A. Ball bearings
• B. Cast iron bearings
• C. Bush bearings
• D. None of the above

Explanation: Small induction motors commonly use ball bearings to support the rotor with low friction and maintain alignment.

• A. D.C. series motor
• B. D.C. shunt motor
• C. universal motor
• D. none of the above

Explanation: An induction motor's speed falls only slightly as load increases, closely matching the nearly constant-speed characteristic of a DC shunt…

• A. it will run in reverse direction
• B. it will pick up very high speed and may go out of step
• C. motor takes five to seven times its full load current
• D. starting torque is very high

Explanation: At direct line starting, an induction motor has very low impedance because there is no back emf, so it draws about five to seven times its…

• A. in the opposite direction as the direction of the rotating field
• B. in the same direction as the direction of the field
• C. in any direction depending upon phase squence of supply

Explanation: The rotating stator field induces rotor currents whose interaction produces torque in the same direction as the rotating field.

• 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…