Free Electrical Machines MCQs with Answers

• 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…

• 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…

• 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…

• 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…

• 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…

• 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.

• 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.