Free Electrical Machines MCQs with Answers

• 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…

• A. remains same as the original value
• B. decreases to half the original value
• C. tends to becomes zero
• D. increases to two times the original value

Explanation: Synchronous speed is Ns = 120f/P, so at unchanged frequency, halving the number of poles doubles the rotor speed.

• A. more
• B. less
• C. equal
• D. twice

Explanation: With excitation fixed, increasing load changes the armature-current magnitude more gradually than it changes the power factor, so the…

• A. when supply voltage fluctuates
• B. when load varies
• C. when power factor is unity
• D. motor is under loaded

Explanation: Hunting, or rotor oscillation about its steady torque angle, is commonly caused by sudden changes in mechanical load.

• A. 1%
• B. 100%
• C. 0.5%
• D. zero

Explanation: A synchronous motor runs exactly at synchronous speed after it is synchronized, so its slip is zero.

• A. leading
• B. lagging
• C. unity
• D. zero

Explanation: An under-excited synchronous motor draws a lagging current to obtain the required magnetizing ampere-turns.

• A. maintaining constant excitation
• B. running the motor on leading power factors
• C. providing damper bars in the rotor pole faces
• D. oscillations cannot be damped

Explanation: Damper bars in the rotor pole faces develop damping torque when the rotor oscillates relative to the rotating field.