All Free Electrical Engineering MCQs with Answers

• A. supplied with unbalanced voltage
• B. under-loaded
• C. over-loaded
• D. none of the above

Explanation: Negative phase-sequence currents arise from unbalanced three-phase voltages or currents and produce a reverse-rotating field.

• A. High field current
• B. Low short circuit ratio
• C. High core losses
• D. Low field current

Explanation: A synchronous motor may fail to pull into synchronism when its field excitation is too low to provide sufficient synchronizing torque.

• A. d.c. shunt motor
• B. d.c. series motor
• C. d.c. compound motor
• D. induction motor
• E. synchronous motorSelect Industrial Relays

Explanation: A rotary converter has a commutator and slip rings, so its AC-side machine can operate as a synchronous motor while producing DC output.

• A. D.C. motor
• B. Reluctance motor
• C. Universal motor
• D. Synchronous motor
• E. Induction motor

Explanation: In a synchronous motor, the stator's rotating magnetic field and the rotor's magnetic field rotate together at synchronous speed.

• A. speed of the motor is reduced
• B. power factor is decreased
• C. Eb (back e.m.f.) becomes less than V (applied voltage)
• D. Er (net resultant voltage) in armature is increased
• E. none of the above

Explanation: Increasing load increases the torque angle, which increases the resultant voltage across the synchronous impedance and hence the armature…

• A. will become more
• B. will become less
• C. will remain unchanged
• D. none of the above.

Explanation: A leading power factor indicates overexcitation; increasing field current makes the current still more leading and increases its phase…

• A. decreases with increase in its excitation
• B. increases with increase in its excitation
• C. remains unaffected with increase in excitation
• D. any of the above

Explanation: Increasing excitation raises the internal generated emf and the synchronizing power, improving the machine's steady-state stability limit.

• A. increase in both armature current and power factor angle
• B. increase in back e.m.f. but decrease in armature current
• C. increase in both armature current and power factor which is lagging
• D. increase in torque angle but decrease in back e.m.f.

Explanation: For constant load, reducing excitation lowers the back emf, so a larger armature current is required; the motor becomes underexcited and…

• A. induction motor torque in field winding
• B. induction motor torque in damper winding
• C. eddy current and hysteresis torque in pole faces
• D. reHetance motor torque due to saliency of the rotor
• E. all of the above methods

Explanation: During induction starting, current in the damper and field windings produces induction torque, while pole-face losses can add eddy-current…

• A. remain same
• B. go down
• C. improve
• D. none of the above

Explanation: With fixed excitation and load, reducing the terminal voltage makes an underexcited motor draw relatively less lagging reactive current…

• A. D.C. shunt motor
• B. D.C. series motor
• C. A.C. induction motor
• D. A.C. synchronous motor

Explanation: An AC synchronous motor runs at exactly synchronous speed, making it suitable for an electric clock where constant speed and accurate…

• A. the armature current is maximum
• B. the armature current is minimum
• C. the armature current is zero
• D. none of the above

Explanation: The synchronous motor has unity power factor at the minimum point of its V-curve, where the armature current is minimum.

• A. one-third
• B. twice
• C. thrice
• D. six times
• E. nine times

Explanation: For a synchronous motor, torque is proportional to sin of the torque angle.

• A. a higher value of stability limit
• B. a small value of inherent regulation
• C. a higher synchronizing power which makes the machine less sensitive to load variations
• D. all of the above

Explanation: A large air gap reduces synchronous reactance, which raises the stability limit and synchronizing power while improving voltage…

• A. voltage rating
• B. current rating
• C. power factor
• D. speed

Explanation: Machine heating is strongly determined by current because copper loss varies as I²R.

• A. Stator flux
• B. Pull in torque
• C. Both A. and (b)
• D. None of the above

Explanation: Increasing the applied voltage increases stator flux and also increases pull-in torque, which is approximately proportional to the square…

• A. carbon or graphite
• B. brass or steel
• C. silver or gold
• D. copper or aluminium

Explanation: Slip rings require good mechanical strength, wear resistance, and adequate conductivity, so brass or steel is commonly used depending on…

• A. synchronous motor has no slip
• B. stator supply is not required to produce magnetic field
• C. mechanical load on the rotor remains constant
• D. synchronous motor has large airgap

Explanation: The synchronous motor obtains its rotor magnetic field from DC excitation, so the stator does not need to supply the full magnetizing…

• A. not-self starting
• B. self-starting
• C. essentially self-starting
• D. none of the above

Explanation: A synchronous motor has no starting torque when connected directly to a fixed-frequency supply because its rotor cannot immediately lock…

• A. direcly proportional to the synchronous reactance
• B. inversely proportional to the synchronous reactance
• C. equal to the synchronous reactance
• D. none of the above

Explanation: Synchronizing power is the restoring power produced for a small change in torque angle, and it is proportional to the inverse of…