Free Electrical Machines MCQs with Answers

• A. through resistance
• B. through reactances
• C. through capacitors
• D. solidly

Explanation: AC series motors have poor commutation because of reactance, so resistance is commonly included in the armature-coil connections or leads…

• A. slip rings
• B. commutator
• C. both A. and (b)
• D. none of the above

Explanation: A repulsion motor requires a commutator and brush arrangement to produce the repulsion torque.

• A. Repulsion motor
• B. Repulsion induction motor
• C. Repulsion start induction run motor
• D. None of the above

Explanation: A repulsion induction motor combines separate repulsion and induction arrangements, giving it two distinct windings for these operating…

• A. centrifugal switch
• B. capacitor
• C. commutator
• D. all of the above

Explanation: A shaded-pole motor has no centrifugal switch, capacitor, or commutator because its starting torque comes from the shaded coil and delayed…

• A. disconnect motor from the supply till it stops then reconnect it to supply with reversed connection of main or auxiliary winding
• B. disconnect motor from supply and immediately reconnect it to supply with reversed connections of the main winding
• C. reverse the direction of connection of the auxiliary winding and after motor comes to rest then connect auxiliary winding to the supply
• D. reverse the direction of connections of the auxiliary winding and immediately connect it to supply

Explanation: Reversing a capacitor-start motor while it is rotating can cause severe current and mechanical stress, so it must first be disconnected…

• A. become unstable
• B. draw excessive armature current and may burn out
• C. fall out of synchronism and come to stand still
• D. run as induction motor

Explanation: When the load exceeds the reluctance motor's pull-out torque, it loses synchronism and its squirrel-cage component allows it to continue…

• A. hard and annealed bearings
• B. ball or roller bearings
• C. soft and porous bearings
• D. plain or sleeve bearings

Explanation: Fractional-horsepower motors commonly use plain or sleeve bearings because they are inexpensive, compact, and adequate for moderate loads.

• A. open armature winding
• B. shorted armature winding
• C. shorted field winding"
• D. high commutator mica
• E. all of the above

Explanation: Brush sparking in a universal motor can result from armature winding faults, field winding faults, or high commutator mica that prevents…

• A. It is similar to reluctance motor
• B. It is basically an induction motor and not a synchronous one
• C. So far as its basic working principle is concerned, it is similar to shaded pole motor
• D. the air-gap between rotor and salient poles is non- uniform

Explanation: A reluctance-start motor is fundamentally an induction motor with a non-uniform air gap and salient rotor, and its starting action…

• A. run faster
• B. spark at light loads
• C. draw excessive current and overheat
• D. run slower

Explanation: The starting winding is designed for short-time operation; if it remains connected, it draws excessive current and overheats.

• A. motor will not come up to speed
• B. motor will not carry the load
• C. current drawn by the motor will be excessively high
• D. electrolytic capacitor will, in all probability, suffer break down

Explanation: If the switch fails to open, the electrolytic starting capacitor remains connected during running, where its alternating-current duty can…

• A. It is a reversing motor
• B. It is preferred to permanent-split single-value capacitor motor where frequent reversals are required
• C. It has low starting as well as rushing currents
• D. It has high starting torque

Explanation: A two-value capacitor motor provides high starting torque with a large starting capacitor and efficient running with a smaller capacitor.

• A. worn bearings
• B. short-circuit in the winding
• C. open-circuit in the winding
• D. none of the above

Explanation: Worn bearings increase friction and can prevent the rotor from reaching its normal speed.

• A. it is comparatively cheaper
• B. it has almost non-destructible capacitor
• C. it has low starting as well as running currents at relatively high power factor
• D. it is quiet in operation

Explanation: A two-value capacitor motor uses a large capacitor for starting and a smaller capacitor for running, giving high starting torque with…

• A. Hysteresis motor
• B. Reluctance motor
• C. Capacitor-run motor
• D. Universal motor

Explanation: A hysteresis motor runs smoothly and quietly at nearly constant speed, making it suitable for tape-recorders, clocks, and other timing…

• A. magnetic relay
• B. thermal relay
• C. centrifugal switch
• D. none of the above

Explanation: Domestic refrigerator compressors commonly use a magnetic current relay to disconnect the starting winding after the motor accelerates.

• A. 30 to 40
• B. 70 to 80
• C. 80 to 90
• D. 100

Explanation: The centrifugal switch normally opens when the motor reaches approximately 70 to 80 percent of synchronous speed, removing the auxiliary…

• A. Induction motor
• B. Three-phase series motor
• C. Schrage motor
• D. Hysteresis motor

Explanation: A three-phase series motor has a characteristic suitable for applications in which the load torque rises with speed, such as traction-type…

• A. Hysteresis motor
• B. Schrage motor
• C. Universal motor
• D. Reluctance motor

Explanation: A Schrage motor can be designed to operate with approximately unity power factor while also providing speed control.

• A. brush axis is 45° electrical to field axis
• B. brush axis coincides with the field axis
• C. brush axis is 90° electrical to field axis
• D. both B. and (c)Electricity

Explanation: Repulsion torque varies with the sine of twice the angle between the brush and field axes, so it becomes zero when the axes coincide or…