The optimum starting and running performance can be achieved by using two capacitors. By a combination of the motor-run and motor-start capacitors connected in parallel shown in Fig. 3a. a large starting and running torque can be obtained. When a motor is speeded up to approximately 75% of the synchronous speed the start capacitor is disconnected by a centrifugal switch. The start capacitor is large in value and is of the AC electrolytic type. The motor-run capacitor which is always connected will preserve the rotating field of the stator and improve torque at operating speed.

Typical value of these capacitors for a 0.5 hp motor are:

• motor-start capacitor is 300F
• motor-run capacitor is 40F

A good performance, high power factor and a high power is achieved in a motor of a small dimension.

A typical torque-speed characteristic is shown in Fig. 3b.

Fig. 3a

Fig. 3b

Capacitor start and run induction motors