Capacitor Motor MCQ :: Single phase Motor Questions and Answers

Single Phase Motor / December 13, 2021

1. Capacitor Motor is similar to the ______.

  1. Split Phase Motor
  2. Synchronous Motor
  3. DC Motor
  4. Compound Motor

Answer.1. Split Phase Motor

Explanation: The capacitor-start motor is essentially the same as a split-phase motor where the two windings are out of phase with each other, providing the necessary torque for starting the motor to rotate. The addition of the capacitor in series with the start winding adds to the initial torque and causes the current in that winding to lead the running winding phase voltage, obtaining a greater angle of displacement between the starting and running windings.

2. The type of capacitor used in the Capacitor start motor is

  1. Mica Capacitor
  2. Paper Capacitor
  3. Film Capacitor
  4. Electrolytic Capacitor

Answer.4. Electrolytic Capacitor

Explanation: In a capacitor start motor, the value of the capacitor is quite large, not possible with oil-filled or another type in reasonable size or economically feasible. Values can range normally anywhere from 40 mfd to 220 mfd, or even higher.

Electrolytic capacitors are cheaper and small enough for this application. However, losses in these capacitors are comparably quite high, and they get hot quickly. So they can remain in the circuit for a very short time, typically under 3 seconds, and need about 3 minutes gap before it can be reconnected. The capacitor-start motor has a low reactance electrolytic capacitor in series with the start winding. The capacitor value ranges from 150 to 180 μfd. To keep it at a reasonable size, an electrolytic type made for intermediate duty is used. It is usually mounted in a metal casing located on top of the motor.

3. In capacitor start single-phase motors

  1. Current in the starting winding leads the voltage
  2. Current in the starting winding legs the voltage
  3. Current in the starting winding is in phase with voltage in running winding
  4. None of the above

Answer.1. Current in the starting winding leads the voltage

Explanation: A capacitor motor has a capacitor in the auxiliary winding of the split-phase motor. Because of the capacitor, the starting (or auxiliary) Winding current leads the main winding current approximately by 90° in time.

CAPACITOR MOTOR

On starting, the switch is closed, placing the capacitor in series with the auxiliary winding. The capacitor is of such a value that the auxiliary winding is effectively a resistive capacitive circuit in which the current leads the line voltage by approximately 45°. The main winding has enough inductance to cause the current to lag the line voltage by approximately 45°. The two field currents are therefore approximately 90° out of phase. Consequently, the fields generated are also at an angle of 90°. The result is a revolving field that is sufficient to start the rotor turning.

4. In a capacitor start motor, the phase displacement between starting and running winding can be nearly

  1. 10°
  2. 30°
  3. 60°
  4. 90°

Answer.4. 90°

Explanation: The capacitor start motor has two windings similar to the split-phase motor generally spaced 90° (electrical) from each other.

5. A capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by an inductor of equivalent reactance value. It will

  1. Start and then stop
  2. Start and run slowly
  3. Start and run at rated speed
  4. Not start at all

Answer.2. Start and run Slowly

Explanation:-

  • If capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by an inductor of equivalent reactance value then small hp motor can start but large hp motor will not start,
  • The Single-phase Induction motor is not self-starting because of phase shift between the main winding and starting winding is practically zero and the current lags the voltage.

  • To make it self-start there should be sufficient phase displacement between current and voltage.

    capacitor start phase difference copy
    Capacitor Start Phase Difference
  • The start winding and run winding are already inductors, so adding inductance in series with the start winding does not produce much phase difference between the start and run windings hence the torque is not developed until there is some phase shift between these two windings.
  • The winding inductance creates a phase delay. It is only used on small motors because of its low efficiency, but it is smaller and more reliable than designs that require a start capacitor on a separate winding.
    Split phase phase difference copy
    Split phase difference
  • The capacitor is used in a single-phase Induction motor for creating a phase shift. The addition of a capacitor creates a phase shift of 90 degrees between the two windings which makes it self-starting.
  • The addition of an inductor for a high Hp motor won’t make the motor self-starting ( even if it starts it will rotate at very less speed) and inherently will lead to an increase in losses only. 

6. In a capacitor start, a single-phase motor when the capacitor is replaced by a resistance

  1. Motor will consume less power
  2. Motor will continue to run in the same direction
  3. Motor may get burn
  4. Both 2 & 3

Answer.4. Motor may get burn

Explanation:-

  • When the capacitor is replaced by a resistor it will behave as the split-phase resistance start the motor.
  • In a resistance split-phase induction motor, the starting winding has a relatively small number of tums of fine wire.
  • Consequently, its resistance is higher than that of the main winding.
  • The resistance starts motor has poor starting torque because the phase angle between the running winding and start winding current is 30 to 40 degrees.
  • Owing to the fine wire used on the starting winding. the current density is high and the winding heats up quickly.
  • If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in thermal relay.

7. A single-phase capacitor-start motor will take starting current nearly

  1. Four to six times the full load current
  2. Three times the full load current
  3. Two to four times the full load current
  4. Same as full load current

Answer.3. Two to four times the full load current

Explanation

A capacitor-start motor has a starting torque that ranges from 225 to 400 percent of its rated full-load running torque, or roughly 2.5 times greater than a resistance-start motor.

8. The capacitors used in single-phase capacitor motors have no

  1. Polarity Marking
  2. Voltage Rating
  3. Dielectric medium
  4. None of the above

Answer.1. Polarity Marking

Explanation:

In single-phase electric induction motors generally two separate sets of windings are used, Viz. main and auxiliary winding. A capacitor is used in series with auxiliary winding to create a phase difference between the currents of the two windings. This in turn creates a rotating magnetic held around the rotor, making it rotate.

Some motors use high-value capacitors in auxiliary windings only during the starting, which are disconnected through a centrifugal switch once the motor attains the required speed. These are mostly of electrolytic type and are called motor start capacitors. Capacitor-start capacitor-run motors are also commonly employed.

There are generally three different reasons why a capacitor is used with a ceiling fan or motor:

(a) The capacitor is used to give a phase shift and create a rotating magnetic field, so the motor will start turning more easily, against the inertia of the blades or load.

(b) It is sometimes used in speed control circuitry.

(c) The capacitor may be used for power factor correction.

(d) A capacitor-start induction motor has a non-polarized electrolytic capacitor in series with the auxiliary winding during starting, which is disconnected once the motor starts.

(e) A capacitor-start capacitor-run motor typically has a large non-polarized electrolytic capacitor in series with the auxiliary winding for starting, then a smaller non-electrolytic capacitor during running. An electrolytic capacitor has no polarity marking.

9. The direction of rotation of capacitor start motor can be changed by changing the connection of

  1. Running Winding
  2. Starting Winding
  3. Either Running or Starting Winding
  4. The direction of rotation can’t be reversed

Answer.3. Either Running or starting winding

Explanation:- The direction of rotation of the capacitor start motor can be changed by interchanging the connection of either starting or running winding.

10. Which of the following are the applications of capacitor start motor

  1. Refrigerator
  2. Compressor
  3. Pumps
  4. All of the above

Answer.4. All of the above

Explanation:-

Capacitor start motors are well suited for applications that require relatively frequent starting and hard-to-start loads, such as pumps, conveyors, and compressors used by refrigeration systems and air conditioners. They also drive machine tools that require single-phase power.

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