All Free Electrical Engineering MCQs with Answers

• A. locked-rotor torque
• B. synchronous torque
• C. pull up torque
• D. reluctance torque

Explanation: Locked-rotor torque is the maximum torque available when the rotor is stationary, considering the possible rotor angular positions at…

• A. lower stability limit
• B. high stability limit
• C. good speed regulation
• D. good voltage regulation
• E. none of the above

Explanation: A low short-circuit ratio means a relatively high synchronous reactance, reducing the maximum power-transfer angle and hence the stability…

• A. pull-up torque
• B. pull-in torque
• C. pull-out torque
• D. none of the above

Explanation: Pull-in torque is the maximum constant load torque at which a synchronous motor can accelerate and lock into synchronism under the…

• A. a series motor
• B. an induction motor
• C. an alternator
• D. a rotary converter

Explanation: A synchronous motor has the same basic construction as an alternator: a three-phase stator armature and a DC-excited rotor field system.

• A. no-load and greatly over-excited fields
• B. no-load and under-excited fields
• C. normal load with minimum excitation
• D. normal load with zero excitation

Explanation: For power-factor correction, a synchronous motor is run lightly loaded or at no load with over-excitation, causing it to draw leading…

• A. Eddy current losses in the conductors
• B. Iron losses in the stator
• C. Copper losses in the slot portion of the conductors
• D. Windage losses
• E. None of the above

Explanation: Windage loss is caused by air friction from the rotating rotor and is not dissipated by the stator core surface.

• A. flux density
• B. horse power rating
• C. speed
• D. all of the above

Explanation: For a given power, increasing allowable flux density or speed permits a smaller machine.

• A. Windage loss
• B. Copper losses
• C. Any of the above
• D. None of the above

Explanation: Windage loss is approximately constant when the motor runs at constant synchronous speed, so it does not significantly change with load.

• A. high excitation only
• B. low excitation only
• C. both high and low excitation
• D. none of the above

Explanation: For a fixed load, both under-excitation and over-excitation move the motor away from its minimum-current condition, so armature current…

• A. number of poles
• B. flux density
• C. rotor speed
• D. rotor excitation
• E. none of the above

Explanation: The synchronous motor’s back emf is produced by the rotor field, so it varies primarily with rotor excitation or field current.

• A. zero
• B. unity
• C. lagging
• D. leading

Explanation: An underexcited synchronous motor has insufficient rotor field, so it draws a lagging armature current to supply the required magnetising…

• A. reduce noise level
• B. reduce the eddy currents
• C. provide starting torque only
• D. prevent hunting and provide the starting torque
• E. none of the above

Explanation: Damper windings act like an induction-motor cage during starting and provide damping when the rotor oscillates around synchronism.

• A. the motor is said to be fully loaded
• B. the torque generated is maximum
• C. the excitation is said to be zero per cent
• D. the excitation is said to be hundred per cent

Explanation: With negligible armature impedance drop at no load, an induced back e.m.f.

• A. 3000 r.p.m.
• B. 1500 r.p.m.
• C. 750 r.p.m.
• D. none of the aboveTry CAD Software

Explanation: Synchronous speed is Ns = 120f/P. Halving both frequency and pole number leaves their ratio unchanged, so the speed remains 1500 r.p.m.…

• A. armature current and field current
• B. power factor and speed
• C. field current and speed
• D. field current and power factorTry Prep Courses

Explanation: A V-curve shows how armature current varies with field current at a constant load.

• A. By changing the supply frequency
• B. By interchanging any two phases
• C. By changing the applied voltage
• D. By changing the loadLearn Quantum Physics

Explanation: A synchronous motor runs at Ns = 120f/P, so changing the supply frequency changes its fixed speed.

• A. the supply voltage and the back e.m.f.
• B. magnetising current and back e.m.f.
• C. the rotating stator flux and rotor poles
• D. none of the above

Explanation: The torque angle is the angular displacement between the rotating stator flux and the rotor field axis or rotor poles.

• A. the motor will stop
• B. the motor continue to run in the same direction
• C. the winding of the motor will burn
• D. the motor will run in the reverse direction
• E. none of the above

Explanation: Reversing field current reverses rotor magnetic polarity but does not change the stator phase sequence, which determines the…

• A. vector sum of Eb and V
• B. arithmetic sum of Eb and V
• C. arithmetic difference of Eb and V
• D. vector difference of Eh and V

Explanation: The voltage across the synchronous impedance is obtained from the phasor difference between the applied voltage and the back e.m.f., not…

• A. stator frame
• B. rotor shaft
• C. pole faces
• D. none of the above

Explanation: Damper bars are embedded in the pole faces of a salient-pole rotor. They provide induction-motor starting action and suppress rotor…