Purpose of Centrifugal Switch in Induction Motor

In the common Induction Motor which are used in drill presses, furnaces, table saws, pumps, grinders, washers and dryers, normally have a Centrifugal Switch. Centrifugal Switch is required together with an additional winding to start the motor.

An induction motor has no copper windings in the turning rotor part; windings are only in the fixed stator part. As in two and three phase induction motors, the additional phases of the stator windings induce magnetic poles in the rotor, and they “slip” behind the excitation and because of this the motor rotates. Thus two and three phase induction motor self starts, and reach a speed determined by the design of the motor.

But the common single phase induction motor can’t start by itself rather will just sit still and loudly hum for about 30 seconds and then burn the insulation off the windings. Therefore we have to get it started, and that’s where Centrifugal Switch and the extra winding come into picture. With an extra winding, the motor happily starts by itself. But this must be switched off before it reaches full speed else the start winding will burn up because auxiliary winding is just designed for a few seconds of operation.

In the figure below we can see “P” the centrifugal device on the rotor shaft that will push the electrical switch “S” to switch the start winding off when the motor starts. For getting more starting torque, a capacitor is added to shift the phase a little extra.


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