Free Electrical Machines MCQs with Answers

• A. start and then stop
• B. start and run slowly
• C. start and run at rated speed
• D. not start at all

Explanation: The capacitor produces the phase lead needed to create starting torque in a capacitor-start motor.

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

Explanation: Mixies require a compact motor with high speed and substantial starting torque, which is supplied by the universal motor.

• A. disconnects auxiliary winding of the motor
• B. disconnects main winding of the motor
• C. reconnects the main winding the motor
• D. reconnects the auxiliary winding of the motor

Explanation: The auxiliary winding is needed mainly for starting, so the centrifugal switch disconnects it after the motor reaches a suitable speed.

• A. at the end connections
• B. at the end terminals
• C. anywhere on the winding inside a slot
• D. at the slot edge where coil enters or comes out of the slot

Explanation: Mechanical vibration and electromagnetic forces can damage insulation where a coil enters or leaves a slot, making the slot edge the most…

• A. 100 uF
• B. 200 uF
• C. 300 uF
• D. 400 uF

Explanation: Fractional-horsepower capacitor-start motors commonly use a relatively large starting capacitor, often around 200 to 300 microfarads.

• A. is opposite to that of brush shift
• B. is the same as that of brush shift
• C. is independent of brush shift

Explanation: In a repulsion motor, shifting the brushes changes the direction of the armature current and hence the torque.

• A. run for about 1 minute
• B. run for about 5 minutes
• C. picked up about 50 to 70 per cent of rated speed
• D. picked up about 10 to 25 per cent of rated speed

Explanation: The starting winding is needed only during acceleration, so the centrifugal switch opens it after the rotor reaches roughly 50 to 70…

• A. inductor
• B. capacitor
• C. resistor
• D. shading coils

Explanation: Shading coils are short-circuited copper rings placed around part of each pole.

• A. Universal motor
• B. Shaded pole type motor
• C. Capacitor start motor
• D. Capacitor start and run motor

Explanation: A universal motor operates on either AC or DC and provides both very high starting torque and high speed.

• A. Capacitor run motor
• B. Shaded pole motor
• C. Capacitor start motor
• D. Split phase motor

Explanation: The running capacitor supplies the phase-shifted current continuously and improves the motor's power factor during operation.

• A. a single turn of heavy wire which is in parallel with running winding
• B. a single turn of heavy copper wire which is short-circuited and carries only induced current
• C. a multilayer fine gauge copper wire in parallel with running winding
• D. none of the above

Explanation: A shading coil is a single-turn thick copper ring that is short-circuited, so its current is produced by electromagnetic induction.

• A. more than i.he rated torque
• B. rated torque
• C. less than the rated torque
• D. zero

Explanation: A single-phase induction motor has two equal, oppositely rotating field components at standstill, whose torques cancel each other.

• A. the starting winding is connected through a centrifugal switch
• B. the running winding is connected through a centrifugal switch
• C. both starting and running windings are connected through a centrifugal switch
• D. centrifugal switch is used to control supply voltage

Explanation: In a split-phase motor, the centrifugal switch disconnects the high-resistance starting winding after the motor accelerates.

• A. the motor will not start
• B. the motor will run
• C. the motor will run in reverse direction
• D. the motor will run in the same direction at reduced r.p.m

Explanation: Short-circuiting the capacitor removes the phase difference between the main and auxiliary winding currents, so the starting torque is…

• A. improve power factor
• B. increase overload capacity
• C. reduce fluctuations in torque
• D. to improve torque

Explanation: The running capacitor remains in circuit and provides the required phase shift for the auxiliary winding, improving the motor's running…

• A. high resistance and low inductance
• B. low resistance and high inductance
• C. high resistance as well as high inductance
• D. low resistance as well as low inductiance

Explanation: The running winding is designed with low resistance and relatively high inductance, giving it a better power factor during normal…

• A. current in the starting winding leads the voltage
• B. current in the starting winding lags the voltage
• C. current in the starting winding is in phase with voltage in running winding
• D. none of the above

Explanation: The capacitor in series with the starting winding makes its current lead the applied voltage, producing the phase difference needed for…

• A. 10°
• B. 30°
• C. 60°
• D. 90°

Explanation: A capacitor-start motor is designed to produce a large phase difference between the starting and running winding currents, approaching…

• A. Capacitor start motor
• B. Capacitor run motor
• C. Split phase motor
• D. Shaded pole motor

Explanation: The capacitor-start motor has relatively high starting torque because its capacitor produces a large phase displacement between the two…

• A. increases the load torque
• B. decreases the starting torque
• C. increases the starting torque
• D. none of the above

Explanation: Adding resistance in the stator causes a voltage drop, so the motor receives a lower voltage and its starting torque decreases…