Free Electrical Machines MCQs with Answers

• A. lagging power factor only
• B. leading power factor only
• C. unity power factor only
• D. lagging, leading and unity power factors

Explanation: Changing the field excitation changes the reactive current: under-excitation gives lagging power factor, correct excitation gives unity…

• A. 2
• B. 3
• C. 4
• D. 6

Explanation: Electrical angle equals pole pairs multiplied by mechanical angle. A four-pole motor has two pole pairs, so its mechanical load angle is…

• A. zero power factor leading
• B. unity power factor
• C. 0.707 power factor lagging
• D. 0.707 power factor leading

Explanation: For a given mechanical load, the armature current is minimum when the motor operates at unity power factor.

• A. the synchronous speed
• B. less than synchronous speed
• C. more than synchronous speed
• D. none of the above

Explanation: A synchronous motor locks its rotor to the rotating stator field, so its steady-state speed is exactly synchronous speed.

• A. Periodic variation of load
• B. Over-excitation
• C. Over-loading for long periods
• D. Small and constant load

Explanation: Hunting is the oscillation of the rotor about its equilibrium position when the torque or load changes periodically.

• A. power factor as well as armature current will decrease
• B. power factor as well as armature current will increase
• C. power factor will increase but armature current will decrease
• D. power factor will decrease and armature current will increase

Explanation: With normal excitation initially giving approximately unity power factor, increasing the load increases the active armature-current…

• A. the increased load has to take more current
• B. the rotor by shifting its phase backward causes motor to take more current
• C. the back e.m.f. decreases causing an increase in motor current
• D. the rotor strengthens the rotating field casuing more motor current

Explanation: When load increases, the rotor falls back by a larger load angle relative to the rotating field.

• A. zero
• B. 45°
• C. 90°
• D. 120°Physics

Explanation: For a synchronous motor, the power-angle relation is approximately proportional to sin δ.

• A. pony motor
• B. D.C. compound motor
• C. providing damper winding
• D. any of the aboveIndustrial Materials & Equipment

Explanation: A synchronous motor is not self-starting, so it may be brought near synchronous speed by a pony motor or started as an induction motor…

• A. small induction motor
• B. D.C. series motor
• C. D.C. shunt motor
• D. double winding A.C./D.C. motor

Explanation: A pony motor is a small auxiliary induction motor used to accelerate a synchronous motor close to synchronous speed before its field is…

• A. under-loaded
• B. over-loaded
• C. under-excited
• D. over-excited

Explanation: An over-excited synchronous motor draws leading current from the supply.

• A. alternator type machines
• B. induction type machines
• C. salient pole type machines
• D. smooth cylindrical type machinesIndustrial Materials & Equipment

Explanation: Most synchronous motors operate at relatively low or medium speeds, where salient-pole rotors are economical and provide the required…

• A. d.c. excitation only
• B. speed of the motor
• C. load on the motor
• D. both the speed and rotor flux

Explanation: At constant supply frequency, synchronous speed is fixed, so the stator back-emf magnitude is controlled mainly by the rotor's DC…

• A. mild steel
• B. chrome steel
• C. alnico
• D. stainless steelElectricity

Explanation: Mild steel provides adequate mechanical strength and is commonly used for the shaft of a synchronous motor.

• 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…