PLUG REVERSING OF CAPACITOR START MOTORS

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

When the motor reaches near normal speed, the centrifugal switch opens and the auxiliary winding gets connected to a high resistance R. Under this condition the auxiliary winding is effectively an open circuit and therefore the developed torque is only by the main winding (recall that this is what is required during normal operation of the motor). When the hoist will reach the upper limit the limit switch LSU will be actuated, U contactor will get de-energized, and relay R also drops out due to opening of contact U₃. If, to achieve instant reversal, the DOWN push button is pressed, contactor D would pick-up energizing the main and also the auxiliary winding through normally closed (NC) contact R₁ (as relay R will not pick-up because of centrifugal switch being open). The centrifugal switch would close when speed becomes zero, the relay R would then get energized. The auxiliary winding would also get energized through the centrifugal switch contact B. In the downward direction when motor again reaches its final speed the centrifugal switch would open thus connecting the auxiliary winding to the high resistance R.