All Free Electrical Engineering MCQs with Answers

• A. the power factor will be low
• B. windage losses will be more
• C. bearing friction will reduce
• D. copper loss will reduce In an induction motor

Explanation: A larger air gap increases the magnetizing current required to establish flux, making the induction motor draw more reactive current and…

• A. rotor current
• B. rotor power factor
• C. rotor e.m.f.
• D. shaft diameter

Explanation: Induction-motor torque is proportional to rotor current, rotor emf, and rotor power factor, but shaft diameter is only a mechanical design…

• A. Air motor
• B. Induction motor
• C. D.C. shunt motor
• D. Synchronous motor

Explanation: An air motor is preferred in explosive atmospheres because it has no electrical sparking contacts or windings that could ignite gas.

• A. graphite
• B. grease
• C. mineral oil
• D. molasses

Explanation: Grease is normally packed into ball bearings because it remains in place and provides both lubrication and sealing against dust.

• A. leakage reactance
• B. power factor on short circuit
• C. short-circuit current under rated voltage
• D. all of the above

Explanation: The blocked-rotor test gives the equivalent leakage impedance and hence leakage reactance, directly gives the short-circuit power factor…

• A. noload test
• B. blocked rotor test
• C. stator resistance test
• D. all of the above

Explanation: The no-load test supplies the magnetizing and core-loss data, the blocked-rotor test supplies series impedance data, and the…

• A. to reduce the magnetic hum and locking tendency of the rotor
• B. to increase the tensile strength of the rotor bars
• C. to ensure easy fabrication
• D. none of the above

Explanation: Skewing the rotor slots reduces magnetic cogging, locking and objectionable humming by preventing the rotor teeth from aligning…

• A. at the unit value of slip
• B. at the zero value of slip
• C. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
• D. at the value of the slip which makes the rotor reactance half of the rotor

Explanation: Maximum induction-motor torque occurs when the effective rotor reactance equals rotor resistance, so sX₂ = R₂.

• A. The slip of the motor will be 5%
• B. The rotor will not run
• C. The rotor will run at very high speed
• D. The torque produced will be very large

Explanation: Zero relative speed means zero slip, so no rotor EMF or rotor current is induced and no electromagnetic torque is produced.

• A. external inductance to the rotor
• B. external resistance to the rotor
• C. external capacitance to the rotor
• D. both resistance and inductance to rotor

Explanation: Adding external rotor resistance increases the starting torque of a slip-ring motor by moving the maximum-torque condition toward starting…

• A. 0.01
• B. 0.02
• C. 0.03
• D. 0.04

Explanation: Synchronous speed is Ns = 120f/P = 120(50)/6 = 1000 rpm. Therefore, slip = (1000 − 960)/1000 = 0.04, or 4%.

• A. stator resistance test
• B. no-load test only
• C. short-circuit test only
• D. noue of the above

Explanation: The short-circuit, or blocked-rotor, test supplies known voltage, current and power values used to establish the circle-diagram scale.

• A. open-circuit test only
• B. short-circuit test only
• C. stator resistance test
• D. none of the above

Explanation: The blocked-rotor, or short-circuit, test permits comparison of the applied stator voltage with the induced rotor voltage at standstill…

• A. power factor at starting is high as compared to that while running
• B. power factor at starting is low as compared to that while running
• C. power factor at starting in the same as that while running

Explanation: At starting, rotor frequency is maximum and rotor leakage reactance is significant, so the motor draws a highly inductive current with low…

• A. improve starting torque
• B. reduce copper losses
• C. improve efficiency
• D. improve power factor

Explanation: Deep rotor bars produce a strong skin effect during starting, increasing the effective rotor resistance.

• A. Block rotor test only
• B. No load test only
• C. Block rotor test and no-load test
• D. Block rotor test, no-load test and stator resistance test

Explanation: The no-load test provides the shunt-branch and rotational-loss data, while the blocked-rotor test provides series impedance data.

• A. pulling the motor directly on line without a starter
• B. locking of rotor due to harmonics
• C. starting the motor on load which is more than the rated load
• D. interchanging two supply phases for quick stopping

Explanation: Plugging is dynamic braking by interchanging any two supply phases, which reverses the rotating field while the rotor is still moving.

• A. windage losses
• B. copper losses
• C. transformation ratio
• D. power scale of circle diagram

Explanation: In the blocked-rotor test the motor is stationary, so windage loss is zero and cannot be measured from this test.

• A. twice the full load torque
• B. 1.5 times the full load torque
• C. equal to full load torque

Explanation: A conventional squirrel-cage induction motor is commonly specified as having a starting torque of about 1.5 times its full-load torque in…

• A. double cage rotor
• B. wound rotor
• C. short-circuited rotor
• D. any of the above

Explanation: A slip-ring motor uses a wound rotor whose three-phase winding is connected to slip rings and external resistance.