Split Capacitor Motor: What is it (And How Does it Work)?

What is a Permanent Split Capacitor Motor?

A permanent split capacitor motor (also known as a single value capacitor motor or PSC motor) is defined as a type of split-phase induction motor in which the capacitor is connected permanently.

A split capacitor motor is an AC motor. It is a type of single-phase induction motor. Similar to other AC motors, a split capacitor motor consists of a stator and a cage-type rotor.

In a split capacitor motor, a capacitor is permanently connected to the circuit in both starting and running conditions. Therefore, this motor is known permanent split capacitor motor.

As the capacitor always remains in the circuit, this motor does not require a centrifugal switch to connect and disconnect the capacitor.

This motor produces uniform torque. Because the auxiliary winding is always connected in a circuit, this motor operates in the same way as a balanced two-phase motor.

How Does a Permanent Split Capacitor Motor Work?

A permanent split capacitor motor is a single-phase AC motor. Hence, we need a single-phase AC supply to run this motor.

Two windings are connected in stator;

  • Main winding
  • Auxiliary winding (Starting winding)

The single-phase power supply is given to the main winding. And secondary winding is connected via a capacitor C.

The connection diagram of this motor is given in the figure below.

connection diagram of split capacitor motor
Connection Diagram of Split Capacitor Motor

A capacitor is connected in auxiliary winding. Hence, the auxiliary winding is highly capacitive, and the main winding is highly inductive.

Therefore, it creates a 90˚ electrical angle between main winding and auxiliary winding.

When a single-phase supply is given to the main winding, Im current will flow through it. Due to the capacitor, a short delay occurs in auxiliary winding. And Ia current will flow through the auxiliary winding after delay.

The current waveform passes through the main winding and auxiliary winding, as shown in the figure below.

waveform of main and auxiliary winding
Waveform of Main and Auxiliary Winding

The currents that pass through the stator produce a rotating magnetic field. This field generates torque in the rotor, and the rotor starts rotating.

In auxiliary winding, a counter EMF is produced as the speed of the motor increases. And this EMF will limit the current passes through the auxiliary winding.

When the motor operates at rated speed, a very small current passes through the auxiliary winding. Hence, auxiliary winding (starting winding) will not be overheated.

When the load increases, the speed reduces slightly. A decrease in rotor speed causes a reduction in counter EMF. And it creates a significant potential difference between counter EMF and applied voltage.

The large potential difference causes an increase in the current of auxiliary winding, which will cause an increase in torque of the rotor. Hence, the motor tries to operate at a constant speed under varying load conditions.

Permanent Split Capacitor Motor Speed Control

The permanent split capacitor motor can be used for variable speed applications. The speed of the motor varies by varying input voltage.

An autotransformer is used to get variable input voltage.

But, for low voltage conditions, the starting torque developed by the motor is very low. And the speed is sensitive to voltage changes.

The second method to control the speed is by controlling the frequency of input supply. As the frequency increases, speed increases and vice versa.

In this method, the AC supply is converted into a DC supply with the help of a controlled rectifier. And again, DC supply is converted into AC supply with the required frequency.

For example, the AC supply is converted into a 40 Hz frequency. The motor rotates at a slower speed. Similarly, if the AC supply is converted to 60 Hz, the motor speed will increase. By this method, the speed of a motor can be controlled in the range of 20 to 110% of full-load speed.

Advantages

The advantages of a permanent split capacitor motor are as listed below.

  • This motor does not require a centrifugal switch. It also reduces the maintenance of the motor.
  • The efficiency of a motor is high.
  • The capacitor is permanently connected to the circuit. Hence, this motor has the advantage of a higher power factor.
  • The pull-out torque of this motor is high.

Disadvantages

The disadvantages of a permanent split capacitor are as listed below.

  • In this motor, a capacitor is used for continuous running. Hence, an electrolytic capacitor cannot be used in this motor. For continuous running applications, paper-spaced oil-filled type capacitors are required. And this type of capacitor is costly and larger.
  • A single value capacitor has low starting torque.

Applications of Permanent Split Capacitor Motor

The applications of permanent split capacitor motors are as listed below.

  • This motor is used to drive refrigerator compressors.
  • It is used in office machinery.
  • It is used in heaters and air-conditioners for fans.
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