Capacitor Start Capacitor Run Induction Motor

A Capacitor Start Capacitor Run Induction Motor is a single phase motor consists of a stator and a single-cage rotor. The stator has two windings i.e. main winding and an auxiliary winding. The auxiliary winding is also known as starting winding. In construction, these two windings are placed 90° apart in space. This motor has two capacitors i.e. Cs and Cr. This motor is also known as Two value capacitor motor. The Capacitor Start Induction Motor is shown in figure 1.

Fig. 1 Capacitor Start Capacitor Run Induction Motor

The capacitors Cs and Care connected parallelly during motor starting.

Cs = start capacitor

C = run capacitor

Rm = main winding resistance

Xm= main winding inductive reactance

Ra = series resistor connected in the auxiliary winding

Xa = auxiliary winding inductive reactance

S = centrifugal switch

Working of Capacitor Start Capacitor Run Induction Motor

When the stator windings are energised from a 1-phase supply, the main winding carries current Im and the starting winding carries current Ia. During starting, high torque is required. For high torque, a high current is required. In order to draw high current, the auxiliary winding capacitance reactance should be low. As capacitive reactance is given by

Xs = 1/(2πfCs)

To have low Xs, Cs should be large.

During normal working, the line current required is small. In order to draw low current, the auxiliary winding capacitance reactance should be large. As capacitive reactance is given by

Xr = 1/(2πfCr)

To have large Xr, Cr should be low.

The capacitors Cs and Care connected in parallel during motor starting. The value of the capacitor Cs is chosen such that current Im is made to lag current Ia by greater than 90°. Therefore, there is a time-phase difference (α) and 90° space difference between the two currents. These two currents produce a rotating magnetic field which starts the motor.

When the motor reaches speed about 70 to 80 % of synchronous speed, the capacitor Cs disconnects from the supply by centrifugally operated switch S. The capacitor Cr is permanently connected to the circuit.

The phasor diagram for this motor is shown in figure 2. During starting, the phase difference is greater than 90° (α>90°) as shown in fig. 2(a). When capacitor Cis disconnected, the phase difference is equal to 90°.

Fig. 2 phasor diagram of Capacitor Start Capacitor Run Induction Motor

The torque-speed characteristic of this motor is shown in figure 3.

Fig. 3 Torque-speed characteristic


1. The reversal of the direction of rotation of this motor is possible by reversing the line connections of either main winding or the auxiliary winding. This can be done when the motor is at standstill condition.

2. The starting torque is high.

3. The motor has very less noise. Also motor runs smoothly.

4. These Motors are costly.

5. They have higher efficiency than motors running on main winding alone.

6. Start Capacitor Cs is short-time rated and has electrolytic construction.

7. Run Capacitor Cis long-time rated and has oil filled paper construction.


The Capacitor Start Capacitor Run Induction Motor is suitable for higher inertia loads where frequent starts are required.

1. It is used to pumps, refrigerator, air conditioner compressors, etc.

2 thoughts on “Capacitor Start Capacitor Run Induction Motor”

  1. In a capacitor start capacitor run induction motor does the axillary winding current increase at the same rate as the main winding when the motor is loaded down?

    • If the motor’s direction is reversible i.e. it can be operated in either direction both axillary and main windings are of same resistance, reactance and turns. But if motor is designed to operate in one direction, axillary winding has less turns, high resistance and low inductive reactance.


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