Free Electrical Machines MCQs with Answers

• A. flux density
• B. horse power rating
• C. speed
• D. all of the above

Explanation: For a given power, increasing allowable flux density or speed permits a smaller machine.

• A. Windage loss
• B. Copper losses
• C. Any of the above
• D. None of the above

Explanation: Windage loss is approximately constant when the motor runs at constant synchronous speed, so it does not significantly change with load.

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

Explanation: For a fixed load, both under-excitation and over-excitation move the motor away from its minimum-current condition, so armature current…

• A. number of poles
• B. flux density
• C. rotor speed
• D. rotor excitation
• E. none of the above

Explanation: The synchronous motor’s back emf is produced by the rotor field, so it varies primarily with rotor excitation or field current.

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

Explanation: An underexcited synchronous motor has insufficient rotor field, so it draws a lagging armature current to supply the required magnetising…

• A. reduce noise level
• B. reduce the eddy currents
• C. provide starting torque only
• D. prevent hunting and provide the starting torque
• E. none of the above

Explanation: Damper windings act like an induction-motor cage during starting and provide damping when the rotor oscillates around synchronism.

• A. the motor is said to be fully loaded
• B. the torque generated is maximum
• C. the excitation is said to be zero per cent
• D. the excitation is said to be hundred per cent

Explanation: With negligible armature impedance drop at no load, an induced back e.m.f.

• A. 3000 r.p.m.
• B. 1500 r.p.m.
• C. 750 r.p.m.
• D. none of the aboveTry CAD Software

Explanation: Synchronous speed is Ns = 120f/P. Halving both frequency and pole number leaves their ratio unchanged, so the speed remains 1500 r.p.m.…

• A. armature current and field current
• B. power factor and speed
• C. field current and speed
• D. field current and power factorTry Prep Courses

Explanation: A V-curve shows how armature current varies with field current at a constant load.

• A. By changing the supply frequency
• B. By interchanging any two phases
• C. By changing the applied voltage
• D. By changing the loadLearn Quantum Physics

Explanation: A synchronous motor runs at Ns = 120f/P, so changing the supply frequency changes its fixed speed.

• A. the supply voltage and the back e.m.f.
• B. magnetising current and back e.m.f.
• C. the rotating stator flux and rotor poles
• D. none of the above

Explanation: The torque angle is the angular displacement between the rotating stator flux and the rotor field axis or rotor poles.

• A. the motor will stop
• B. the motor continue to run in the same direction
• C. the winding of the motor will burn
• D. the motor will run in the reverse direction
• E. none of the above

Explanation: Reversing field current reverses rotor magnetic polarity but does not change the stator phase sequence, which determines the…

• A. vector sum of Eb and V
• B. arithmetic sum of Eb and V
• C. arithmetic difference of Eb and V
• D. vector difference of Eh and V

Explanation: The voltage across the synchronous impedance is obtained from the phasor difference between the applied voltage and the back e.m.f., not…

• A. stator frame
• B. rotor shaft
• C. pole faces
• D. none of the above

Explanation: Damper bars are embedded in the pole faces of a salient-pole rotor. They provide induction-motor starting action and suppress rotor…

• A. supplied with unbalanced voltage
• B. under-loaded
• C. over-loaded
• D. none of the above

Explanation: Negative phase-sequence currents arise from unbalanced three-phase voltages or currents and produce a reverse-rotating field.

• A. High field current
• B. Low short circuit ratio
• C. High core losses
• D. Low field current

Explanation: A synchronous motor may fail to pull into synchronism when its field excitation is too low to provide sufficient synchronizing torque.

• A. d.c. shunt motor
• B. d.c. series motor
• C. d.c. compound motor
• D. induction motor
• E. synchronous motorSelect Industrial Relays

Explanation: A rotary converter has a commutator and slip rings, so its AC-side machine can operate as a synchronous motor while producing DC output.

• A. D.C. motor
• B. Reluctance motor
• C. Universal motor
• D. Synchronous motor
• E. Induction motor

Explanation: In a synchronous motor, the stator's rotating magnetic field and the rotor's magnetic field rotate together at synchronous speed.

• A. speed of the motor is reduced
• B. power factor is decreased
• C. Eb (back e.m.f.) becomes less than V (applied voltage)
• D. Er (net resultant voltage) in armature is increased
• E. none of the above

Explanation: Increasing load increases the torque angle, which increases the resultant voltage across the synchronous impedance and hence the armature…

• A. will become more
• B. will become less
• C. will remain unchanged
• D. none of the above.

Explanation: A leading power factor indicates overexcitation; increasing field current makes the current still more leading and increases its phase…