All Free Electrical Engineering MCQs with Answers

• A. changing the load
• B. changing the supply voltage
• C. changing frequency
• D. using brakes

Explanation: Synchronous speed is given by Ns = 120f/P, so changing the supply frequency changes the motor's fixed operating speed.

• A. 30°
• B. 60°
• C. 90°
• D. 180°

Explanation: Power transfer varies as P = Pmax sin δ, where δ is the coupling or torque angle. Its maximum value occurs when sin δ = 1, giving δ = 90°.

• A. Universal motor
• B. Synchronous motor
• C. Induction motor
• D. Reluctance motorElectromechanical Devices

Explanation: In a synchronous motor, the rotor magnetic field locks with the stator's rotating field, so both rotate at synchronous speed.

• A. 60 to 70%
• B. 75 to 80%
• C. 85 to 95%
• D. 99 to 99.5%

Explanation: A properly designed synchronous motor generally has high efficiency because it has no slip-related rotor copper loss, so its usual…

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

Explanation: The armature current is minimum near unity power factor at normal excitation.

• A. inductor motor torque
• B. induction generator torque
• C. synchronous motor torque
• D. d.c. motor toque
• E. none of the aboveElectromechanical Devices

Explanation: When the rotor moves faster than synchronous speed, the damper bars experience negative slip and produce a braking torque opposing the…

• A. connected to D.C. supply
• B. short-circuited by low resistance
• C. kept open-circuited
• D. none of the above

Explanation: During induction starting, the field winding is not supplied with DC; it is closed through a low-impedance path to prevent dangerously…

• A. of reducing number of slip rings on the rotor
• B. armature is associated with large power as compared to the field circuits
• C. of difficulty of providing high voltage insulation on rotor
• D. all of the above reasons

Explanation: The armature is placed on the stator because it carries the larger power, permits easier high-voltage insulation, and avoids carrying…

• A. stator supply is relieved of responsibility of producing magnetic field
• B. mechanical load on the motor can be adjusted
• C. synchronous motor runs at synchronous speed
• D. synchronous motor has large air gap

Explanation: The synchronous motor has a separate DC field, so the stator supply does not have to provide all the magnetizing current required to…

• A. 10 per cent
• B. 6 per cent
• C. 4 per cent
• D. 2. per cent
• E. zero

Explanation: In steady operation, a synchronous motor remains locked to the rotating field and runs at synchronous speed.

• A. rotor excitation
• B. maximum value of coupling angle
• C. direction of rotation
• D. supply voltage

Explanation: The maximum power is proportional to the supply voltage and rotor excitation voltage and occurs at a coupling angle of 90 degrees.

• A. decrease both armature current and power factor
• B. decrease armature current but increase power factor
• C. increase armature current but decrease power factor
• D. increase both its armature current and power factor

Explanation: Increasing mechanical load requires a larger torque angle, which increases the real component and total magnitude of armature current.

• A. load on the motor
• B. d.c. excitation only
• C. both the speed and rotor flux
• D. none of the above

Explanation: At constant supply frequency, synchronous speed is fixed, so the back e.m.f.

• A. windage friction
• B. variable load
• C. variable frequency
• D. variable supply voltage

Explanation: Hunting is oscillation of the rotor about its synchronous position and is commonly triggered by changes in load, supply voltage, or…

• A. no change of power factor
• B. lagging power factor with over-excitation
• C. leading power factor with under-excitation
• D. leading power factor with over-excitation

Explanation: With load and speed fixed, increasing excitation above the unity-power-factor value makes the synchronous motor over-excited.

• A. 45°
• B. 60°
• C. 90°
• D. 120°Engineering & Technology

Explanation: For a cylindrical-rotor synchronous motor, power varies approximately as sin δ, where δ is the load angle.

• A. has large values for low excitation i niy
• B. has large values for high excitation only
• C. has large values for low and high excitation
• D. any of the above

Explanation: The synchronous motor has a V-curve: armature current is minimum at unity power factor and rises for both under-excitation and…

• A. d.c. series machines
• B. d.c. shunt machines
• C. d.c. compound machines
• D. any of the above

Explanation: Traditional synchronous-machine exciters are generally d.c. shunt generators because they provide a controllable, nearly constant field…

• A. one hour
• B. one minute
• C. one second
• D. none of the above

Explanation: A sudden short-circuit test records rapidly changing transient and subtransient currents, so it is kept very brief to prevent excessive…

• A. D.C. series motor
• B. synchronous motor
• C. Squirrel cage induction motor
• D. Wound round induction motor

Explanation: A synchronous motor produces no average starting torque because its stator field rotates past the stationary rotor too rapidly to…