All Free Electrical Engineering MCQs with Answers
Every Electrical Engineering question in the bank, across all chapters, each with the correct answer and a written explanation. Free and unlimited, with no account needed.
3,143 questions · page 49 of 158
- A. varies with power factor
- B. varies with speed
- C. varies with the load
- D. remains constant at all loads
Explanation: With constant d.c. field excitation, the rotor field flux of a synchronous motor is essentially fixed as load changes; load mainly changes…
Correct answer: remains constant at all loads- A. zero
- B. one
- C. two
- D. infinityIndustrial Materials & Equipment
Explanation: A synchronous motor has no starting torque when connected directly to the supply because its rotor cannot immediately follow the rotating…
Correct answer: zero963. The induced e.m.f. in a synchronous motor working on leading power factor will be_____________?
- A. more than the supply voltage
- B. less than the supply voltage
- C. equal to the supply voltage
- D. None of these
Explanation: For a synchronous motor at leading power factor, the reactance-drop phasor causes the internal induced emf to exceed the terminal supply…
Correct answer: more than the supply voltage- A. Phase to phase winding resistance
- B. Stator winding to earthed frame
- C. Rotor winding to earthed shaft
- D. All of the above
Explanation: An insulation-resistance test checks insulation between phases and between windings and grounded metal parts, including the stator frame…
Correct answer: All of the above- A. increases as the load increases
- B. decreases as the load decreases
- C. always remains constant
- D. none of the above
Explanation: A synchronous motor runs at synchronous speed, determined by supply frequency and the number of poles, not by load.
Correct answer: always remains constant- A. augments it directly
- B. directly opposes it
- C. cross-magnetises it
- D. none of the above
Explanation: At zero power factor lagging, the armature reaction of an alternator is purely demagnetising.
Correct answer: directly opposes it- A. applied voltage of the motor
- B. power factor of power drawn by the motor
- C. motor speed
- D. none of the these
- E. all of the these
Explanation: Changing DC field excitation changes the reactive current drawn by the motor and therefore its power factor.
Correct answer: power factor of power drawn by the motor- A. zero degree
- B. two degrees
- C. five degrees
- D. ten degrees
Explanation: The rotor load angle in a normally loaded synchronous motor is small because the rotor remains nearly aligned with the rotating stator…
Correct answer: five degrees- A. its power factor may be varied at will
- B. its speed is independent of supply frequency
- C. its speed may be controlled more easily
- D. none of the above
Explanation: A synchronous motor can operate underexcited, at unity power factor, or overexcited, allowing its power factor to be adjusted by changing…
Correct answer: its power factor may be varied at will- A. synchronizing
- B. obtuse
- C. torque
- D. power factor
Explanation: The angular displacement between the rotating stator field and the rotor pole axis is called the torque angle, also known as the load or…
Correct answer: torque- A. direct current
- B. alternating current
- C. no current
- D. all of the above
Explanation: The rotor field winding of a synchronous motor is supplied with direct current through slip rings.
Correct answer: direct current- A. voltage booster
- B. phase advancer
- C. noise generator
- D. mechanical synchronizer
Explanation: A synchronous motor operating at leading power factor supplies leading reactive power to the system, acting as a phase advancer.
Correct answer: phase advancer- A. zero or 0.8 leading
- B. unity or 0.8 lagging
- C. unity or 0.8 leading
- D. unity or zero
Explanation: Synchronous motors are commonly rated for unity power factor or 0.8 leading power factor.
Correct answer: unity or 0.8 leading- A. d.c. source
- B. armature input
- C. motor input
- D. supply lines
Explanation: Rotor copper loss is the I²R loss in the field winding, and this winding receives its excitation from a separate DC source or exciter.
Correct answer: d.c. source- A. supply voltage fluctuates
- B. load in motor varies
- C. excitation winding gets disconnected
- D. supply voltage frequency changes
Explanation: If the excitation winding is disconnected, the rotor field collapses and the motor loses synchronism, so it cannot continue normal…
Correct answer: excitation winding gets disconnected- A. 1%
- B. 0.5%
- C. positive
- D. zero
Explanation: A synchronous motor runs at synchronous speed, determined only by supply frequency and the number of poles, so its speed does not change…
Correct answer: zero- A. Windage loss
- B. Bearing friction loss
- C. Copper loss
- D. Core loss
Explanation: Armature copper loss varies with load because it is proportional to the square of armature current, I²R.
Correct answer: Copper loss- A. armature current and power factor
- B. terminal voltage and load factor
- C. power factor and field current
- D. field current and armature current
Explanation: A V-curve shows how armature current changes with field current at a fixed load.
Correct answer: field current and armature current- A. leading current
- B. lagging current
- C. both A. and (b)
- D. none of the above
Explanation: Increasing field excitation beyond the unity-power-factor point makes a synchronous motor over-excited, causing it to draw leading…
Correct answer: leading current- A. the force required to accelerate the rotor to the synchronous speed in an instant is absent
- B. the starting device to accelerate the rotor to near synchronous speed is absent
- C. a rotating magnetic field does not have enough poles
- D. the rotating magnetic field is produced by only 50 Hz frequency currents
Explanation: At standstill, the stator’s rotating field produces torque that reverses rapidly relative to the stationary rotor, giving no average…
Correct answer: the force required to accelerate the rotor to the synchronous speed in an instant is absent