Capacitor-start Capacitor-run Induction Motor

Capacitor-start Capacitor-run Induction Motor

The capacitor-start capacitor-run induction motor is similar to the capacitor start induction motor. The only difference is that the starting winding and capacitor are connected to the circuit permanently.

There are many advantages of connecting capacitors permanently in the circuit. These advantages are;

  • Improve over-load capacity
  • High power factor
  • High efficiency
  • Quiet operation

These advantages are desirable for small power electrical drives in commercial, industrial, and household applications.

According to the number of capacitors used in a motor, this motor is classified into two types;

  • Single-value capacitor run motor
  • Two-value capacitor run motor

Single-value Capacitor Run Motor

In this type of motor, one running winding and one starting winding are connected as shown in the below connection diagram.

Single-value Capacitor Run Motor
Single-value Capacitor Run Motor

A capacitor is connected in series with the starting winding and remains permanently in the circuit. Hence, this motor is also known as a permanent split capacitor run motor.

There is no need to remove the capacitor during running condition. Therefore, the centrifugal switch is not required in this motor.

A single capacitor is used for starting and running. So, the value of a capacitor is chosen for optimum starting and optimum running conditions.

The optimum value of a capacitor is 2 to 20 μF capacitance. Because of continuous duty running, these capacitors are made of more expensive oil or pyranol-insulated foil-paper capacitors.

The value of a capacitor decides the starting torque. Low capacitance results in small starting torque (about 50-100 percent of rated torque).

One advantage of this motor is that the running and starting winding is identical. Because of this advantage, it can be easily reversed by an external switch.

One winding serves as running winding and the other winding serves as starting winding for one direction of rotation.

Now, the winding purpose is changed for reverse rotation (previously starting winding becomes running winding and previously running winding becomes starting winding).

Such a type of motor is used for devices that must be moved back and forth very frequently like rheostats, induction regulations, furnace controls, arc welding, and valves.

Two value Capacitor Run Motor

In this type of motor, two capacitors are used. To achieve high starting torque, a high capacitance capacitor is connected in series with starting winding.

For running conditions, a lower value capacitor is connected with the help of a centrifugal switch.

Two value capacitances can be obtained by using two capacitors connected parallel at starting and then switching out one for low-valued capacitors.

And also, it can be obtained by using a step-up auto-transformer in conjunction with one capacitor so that the effective capacitance value is increased for starting purposes.

The connection diagram of two value capacitors is shown in the figure below.

Two value Capacitor Run Motor
Two value Capacitor Run Motor

In this figure, capacitor-B is an electrolytic capacitor of high capacity (short duty) and capacitor-A is an oil capacitor of low value (continuous duty).

The capacitor of starting capacitor is 10 to 15 times higher than the running capacitor.

At the time of starting, the centrifugal switch is open. In this condition, both capacitors are connected in a circuit. Therefore, their combined capacitance is the sum of their individual capacitances.

When the motor reached 75% of full-load speed, the switch opens, and only the running capacitor remains in the circuit.

So, we can get optimum starting and running performance in this motor.

The advantages of two value Capacitor-start Capacitor-run induction motor are;

  • Ability to start with a heavy load
  • Quiet operation
  • Higher efficiency and power factor
  • Ability to develop 25% overload capacity

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