Split Phase Motor MCQ :: Single Phase Induction Motor Question and Answer

1. A Split phase motor, also called ______ start motor.

  1. Reactance
  2. Resistance
  3. Inductance
  4. Any of the above

Answer.2. Resistance

Explanation: A resistance-start motor is a form of the split-phase motor having a resistance connected in series with the starting (sometimes called auxiliary) winding.

2. The outer frame and stator core of a split-phase motor is similar to _____

  1. 3-phase Induction Motor
  2. DC Series Motor
  3. DC compound Motor
  4. Synchronous Motor

Answer.1. 3-phase Induction motor

Explanation: The outer frame and stator core of a split-phase motor are similar to the outer frame and stator core of a 3-phase induction motor.

3. In a split-phase motor the auxiliary winding is also called ______

  1. Running winding
  2. Starting winding
  3. Either running and  starting winding
  4. None of the above

Answer.2. Starting winding

Explanation: The split-phase motor is provided with an auxiliary stator winding called starting winding in addition to the main winding.

4. The auxiliary and main winding are placed in

  1. Stator slots
  2. Rotor Slots
  3. Either stator or Rotor
  4. Any of the above

Answer.1. Stator Slots

Explanation: The split Phase motor is provided with an auxiliary stator winding called starting winding in addition to the main winding. These windings are placed in the stator slots. These two windings, held in place in the slots of a laminated steel core. The windings are made up of insulated coils. The coils are placed so that they are 90 electrical degrees apart. One winding is the main (running) winding. The other winding is the auxiliary (starting) winding.

The running winding is made of a heavy insulated copper wire. It is located at the bottom of the stator slots. Small wire size is used in the starting winding. This winding is placed near the top of the slots above the running winding.

split-phase motor

5. The rotor of the split-phase motor is the same as that of ____

  1. DC motor
  2. DC Generator
  3. Synchronous Motor
  4. Squirrel Cage Induction Motor

Answer.4. Squirrel Cage Induction Motor

Explanation: The rotor of the resistance-start, induction-run motor is the same as that of a three-phase, squirrel-cage induction motor. The rotor contains no windings, brushes, slip rings, or commutator where faults may develop. Fans are provided with a rotor to keep the temperature of the windings at a reasonable level.

6. In a split-phase induction motor, the two stator windings _______.

  1. Have equal R / XL ratio
  2. Draw only the in-phase currents
  3. Are mutually displaced by 90° electrically
  4. Draw equal currents

Answer.3. Are Mutually Displaced by 90° electrically

Explanation:- In a split-phase induction motor, the two stator windings are mutually displaced by 90° electrically. One is main winding and the other is auxiliary winding. The current of the main winding (lM) lags the current of the starting winding (IS) by nearly 90 electrical degrees. When a current passes through each of these windings, the resulting pulsating field-effect gives rise to a rotating field around the inside of the stator core. If only one stator winding is energized, an alternating field, rather than a rotating field, is formed. If the rotor is at rest, this field induces an alternating current in the rotor winding.

The motor resembles a two-phase induction motor because the currents of the windings are approximately 90° electrical degrees out of phase with each other. However, a single-phase source supplies the motor.

A two-phase system is produced by having an alternator with two sets of coils wound 90° apart. The voltages of a two-phase system are, therefore, 90° out of phase with each other. These two out-of-phase voltages can be used to produce a rotating magnetic field in a manner similar to that of producing a rotating magnetic field with the voltages of a three-phase system. Since there have to be two voltages, or currents, out of phase with each other to produce a rotating magnetic field, split-phase motors use two separate windings to create a phase difference between the currents in each of these windings. These motors literally split one phase and produce a second phase, hence the name split-phase motor.

7. The main winding of the split-phase motor has _____ and  _______.

  1. Low Inductance and High Resistance
  2. High Resistance and Low Inductance
  3. High resistance and High inductance
  4. Low resistance and Low Inductance

Answer.1. Low Resistance and High inductance

Explanation:- The main winding of the split-phase motor has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance. The main winding has many turns of large diameter wire wound in the bottom of the stator slots to get high reactance, which is an inductive circuit. The auxiliary Winding has a few turns of small diameter wire wound to get high resistance, which is a resistive circuit. Therefore, the current in the auxiliary winding leads the current in the main winding, causing a rotating field.

8. The starting winding of the split-phase motor has _____ and ______.

  1. Low Inductance and High Resistance
  2. High Resistance and Low Inductance
  3. High resistance and High inductance
  4. Low resistance and Low Inductance

Answer.2. High Resistance and Low Inductance

Explanation:- The main winding of the split-phase motor has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance. The main winding has many turns of large diameter wire wound in the bottom of the stator slots to get high reactance, which is an inductive circuit. The auxiliary Winding has a few turns of small diameter wire wound to get high resistance, which is a resistive circuit. Therefore, the current in the auxiliary winding leads the current in the main winding, causing a rotating field.

9. The centrifugal switch disconnects the starting winding after the rotor reaches _____ of rated speed.

  1. 50%
  2. 100%
  3. 75%
  4. 30%

Answer.3. 75%

Explanation:- The centrifugal switch disconnects the starting winding after the rotor reaches 75% of the rated speed. The starting winding is designed to operate across the line voltage for Just three or four seconds as the motor accelerates to the rated speed. Therefore, the starting winding must be disconnected from the line by the centrifugal switch as soon as the motor accelerates to three-quarters of the rated speed. If the motor operates on its starting winding for more than sixty seconds, the winding may char or burn out.

10. If the split-phase motor does not start, but a low humming sound is present, then the probable reason is

  1. Stator winding is open
  2. Rotor winding is open
  3. Either rotor and stator winding is open
  4. None of the above

Answer.3. Either rotor and stator winding is open

Explanation:- If the motor does not start, but a low humming sound is present, then one winding is open. The centrifugal switch should be checked to determine whether its mechanism is faulty or whether the switch contacts are pitted.

11. To reverse the direction of rotation of the split-phase motor, interchange the leads of _____

  1. Starting winding
  2. Running winding
  3. Both starting and running winding
  4. Either Starting and Running winding

Answer.4. Either starting and Running winding

Explanation:- To reverse the rotation of the split-phase motor, simply interchange the leads of either the starting winding or the running winding, preferably the starting winding. Interchanging these leads reverses the direction of rotation of the stator field (and the rotor).

12. In a split-phase motor the centrifugal switch fails to operate the most probable reason is

  1. Mechanical load is too great
  2. Terminal voltage is too low
  3. Both 1 and 2
  4. None of the above

Answer.3. Both 1 and 2

Explanation:- If the mechanical load is too great when a split-phase motor is started, or if the terminal voltage is too low, then the motor may fail to reach the speed required to operate the centrifugal switch.

13. The starting winding and running winding of the split-phase motor is connected in

  1. Series
  2. Parallel
  3. Can be connected in both way
  4. None of the above

Answer.2. Parallel

Explanation:- The starting winding and running winding of the split-phase motor are connected in parallel. First, both windings run on the same voltage, and parallel branches have different currents based on impedance, but voltage is constant across all branches.

Second, the auxiliary windings are typically switched off at operating speed, meaning when the motor gets up to speed, the speed switch opens up the auxiliary winding circuit and de-energizes the windings because they’re no longer necessary; if they were in series, that switch would shut off the whole motor.

Third, the auxiliary winding needs to have a different phase angle than the main windings in order to create the rotating magnetic field and turn the motor, and as stated above, parallel branches have different currents based on impedance, and the additional capacitive portion of the auxiliary branch’s impedance is what puts the auxiliary winding at a different phase angle; if they were connected in series, the capacitor would affect the total current and there would be no phase angle difference between the windings, so the rotating magnetic field would not be formed.

14. The starting torque of the split-phase motor is ____ full load torque.

  1. 5 times
  2. 2 times
  3. 10 times
  4. Equal

Answer.2. 2 times

Explanation:- The current in the two windings is not equal as a result the rotating field is not uniform. So, the starting torque is small, of the order of 1.5 to 2 times the start, running torque.

15. The power factor of Split Phase motor is about

  1. 0.1 – 0.3
  2. 0.8 – 0-9
  3. 0.5 to 0.6
  4. 1

Answer.3. 0.5 to 0.6

Explanation:- The starting torque of the split-phase motor is about twice the full load torque. The current at the start is about 6 to 8 times. The speed falls with an increase in load by only about 5% to 7% otherwise it is a constant speed motor. It has lower p.f. and lesser efficiency. P.f. is about 0.6 and efficiency is also about 60%.

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