The capacitor start motors suffer from the disadvantage that they are not easily reversible due to the centrifugal switch connected in the auxiliary winding. The motor cannot be instantaneously reversed by simple control as the auxiliary winding remains disconnected till the motor comes near to zero speed. However, by proper design of the control circuit the motor can be made instantly reversible. This is accomplished by using an electromagnetic relay along with a special two contact centrifugal switch as shown in Figure.
The circuit shown in Figure is for a small hoist using a capacitor start motor. The upper and lower limits of travel are controlled by two limit switches viz. LSU and LSD.
When UP-push button is pressed, contactor U gets energized. Its contacts U1 and U2 energies the main winding. Closing of its contact U3 causes relay R to get energized through the centrifugal switch contacts A—B. After the energization of relay R auxiliary winding gets energized through capacitor C, the centrifugal switch, and the closed contact R2 of relay R.
Figure: Control circuit for a small hoist using a capacitor start |