Higher starting torque can be optioned if a start winding is provided with a starting capacitor in series with it. The phase of the current in the "starting branch" is determined largely by the capacitor, and this allows for the high starting torque typical of this method of starting.

  Once the motor is up to speed, it can operate with the main, or "running" winding alone; and the starting winding can be disconnected. This is usually accomplished by means of a centrifugal switch or some similar mechanism.

  The illustration shows both the circuit configuration and typical torque vs. speed curves for a capacitor-start motor system. Note that once the motor reaches about 75% of rated speed the starting winding can be disconnected, and the motor operated on the run winding alone.

  The rating of both the starting winding and the capacitor can be designed with the consideration that they will be engaged for only a short time. Typical capacitor value for a 0.5hp motor is about 300mF. The voltage developed across the capacitor (which is in series with the large inductance of the starting winding) is typically greater than the line voltage, and this must be an important element in the design of the system.

Fig. 1b

Fig. 1a

Capacitor-start induction motor