Free Electrical Machines MCQs with Answers

• 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.

• 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…

• 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…

• 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…

• 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.

• 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.

• 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.

• 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.

• 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…

• 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…

• 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.

• 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.

• 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…

• 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…

• A. V
• B. V312
• C. V2
• D. 1/V

Explanation: For a synchronous motor with constant excitation, the maximum or breakdown torque is proportional to the applied voltage, as shown by Tₘₐₓ…

• A. no slip-rings
• B. one slip-ring
• C. two slip-rings
• D. three slip-rings

Explanation: The DC field winding of a conventional three-phase synchronous motor is supplied through two slip rings, one for each polarity.

• A. not start
• B. run at 2/3 of synchronous speed
• C. run with excessive vibrations
• D. take less than the rated load

Explanation: A short-circuited phase creates severe current imbalance and prevents the motor from developing normal rotating-field conditions for…

• A. the direction of rotation is not fixed
• B. the direction of instantaneous torque reverses after half cycle
• C. startes cannot be used on these machines
• D. starting winding is not provided on the machines

Explanation: At standstill, the synchronous rotor cannot follow the rapidly reversing stator field.

• A. voltage rating
• B. dielectric medium
• C. polarity marking
• D. definite value

Explanation: The alternating voltage reverses polarity, so the capacitor must be non-polarized and has no polarity marking.

• A. spark excessively
• B. have poor efficiency
• C. have poor power factor
• D. all of the above

Explanation: A DC series motor on AC suffers from high reactance, poor power factor, reduced efficiency, and severe commutator sparking.