SOLUTION

The centrifugal switch is connected in series with the starting winding. The primary function of the starting winding is to produce rotating flux in conjunction with the main winding at the start. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross-field effect. The starting winding now has no function to perform and is removed from the circuit by a centrifugally operated switch.

Centrifugal switch working

With the rotor at standstill, the pressure of the spring on the bakelite piece keeps the contacts PP’ closed. The starting winding circuit which is connected across the supply terminals through the centrifugal switch terminals PQ remains closed. The auxiliary winding flux helps the motor to start. As the motor picks up speed, centrifugal force acts on parts a and b. These two parts are pushed backward. They in turn push against the spring pressure, the cup-shaped bakelite piece forward. The spring pressure is such that at about 75 percent of the synchronous speed the centrifugal switch contact points PP’ open, thereby cutting out the auxiliary winding from the supply.