Capacitor Start Capacitor Run Induction Motor

Capacitor Run Induction Motor-Representational Image 

We know that single phase induction motor is not self starting due to the absence of rotating magnetic field at starting. In order to produce rotating field there must be some phase difference. In capacitor start capacitor run induction motor we are using two winding, one main winding and other auxiliary winding. For producing the phase difference two capacitors are used since we know that in capacitor the current leads the applied voltage by some angle. The capacitor serves to shift the phase on one of the windings so that the voltage across the winding is at 90° from the other winding, thus making the capacitor run motor a truly two-phase machine at its rated load.

Capacitor start capacitor run induction motor

Instead of one if two capacitors are used in the auxiliary winding of the motor, one of them for starting and other for running, optimum starting and running performances can be obtained. Figure shows the schematic  diagram of the motor. The capacitor Cs is cut out when the motor reaches about 75% of the synchronous speed by centrifugal device. The capacitor C and auxiliary winding A retained in the circuit for running condition. A motor starting capacitor may be a double-anode non-polar electrolytic capacitor which could be two + to + (or - to -) series connected polarized electrolytic capacitors. Such AC rated electrolytic capacitors have such high losses that they can only be used for intermittent duty (1 second on, 60 seconds off) like motor starting. A capacitor for motor running must not be of electrolytic construction, but a lower loss polymer type.

Torque- Speed Characteristics of Capacitor Start Capacitor Run Induction Motor

Capacitor-start/capacitor-run motors have moderate-to-high starting torque compared to other types of single-phase motors. Starting torque generally ranges from 200%-350% of normal full-load torque.

Torque- Speed Characteristic of Capacitor Start Capacitor Run Induction Motor

Their main advantage over the capacitor-start motor is their lower starting-torque that results in lower starting current.
Because of the lower starting current, they are generally used for most single-phase applications between 3-and-10 horsepower  due to their lower starting current. Common applications include: larger single-phase compressors, pumps, grinders, conveyors, and larger single-phase air-conditioning compressors.