Phasor Diagram

Capacitor Start Capacitor Run Motor

The Capacitor Start Capacitor Run Motor has a cage rotor, and its stator has two windings known as Main and Auxiliary Windings. The two windings are displace at 90 degrees in space. There are two capacitors in this method one is use at the time of the starting which call as starting capacitor. The other one is use for continuous running of the motor which call as RUN capacitor.

So this motor is name as Capacitor Start Capacitor Run Motor. This motor is also call as Two Value Capacitor Motor. Connection diagram of the Two valve Capacitor Motor is below

Capacitor Start and capacitor run motor
Capacitor Start and capacitor run motor

There are two capacitors in this motor represent by CS and CR. At the starting, the two capacitors are connect in parallel. The Capacitor Cs is the Starting capacitor is short time rate. It is almost electrolytic. A large amount of current id required to obtain the starting torque. Therefore, the value of the capacitive reactance X should be low in the starting winding. Since, XA = 1/2πfCA, the value of the starting capacitor should be large.

The rated line current is smaller than the starting current at the normal operating condition of the motor. Hence, the value of the capacitive reactance should be large. Since, XR = 1/2πfCR, the value of the run capacitor should be small

As the motor reaches the synchronous speed, the starting capacitor Cs is disconnect from the circuit by a centrifugal switch Sc. The capacitor CR is connect permanently in the circuit and thus it is call as RUN Capacitor. The run capacitor is long time rated and is made up of oil filled paper.

The figure below shows the Phasor Diagram of the Capacitor Start Capacitor Run Motor.

Phasor Diagram
Phasor Diagram

Fig(a) shows the phasor diagram when at the starting both the capacitor are in the circuit and ϕ > 90⁰. Fig (b) shows the phasor when the starting capacitor is disconnect, and ϕ becomes equal to 90⁰.

This type of motor is quiet and smooth running. They have higher efficiency than the motors that run on the main windings only. They are use for loads of higher inertia requiring frequent starts where the maximum pull-out torque and efficiency required are higher. The Two Value Capacitor Motors is use in pumping equipment, refrigeration, air compressors, etc.

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