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Centrifugal switch used to disconnect
Main winding
Auxiliary winding
Both of above
None of above
Auxiliary winding
A centrifugal switch is an electromechanical device designed to disconnect the auxiliary (starting) winding once the motor reaches approximately 75% to 80% of its rated synchronous speed. Its primary function is to eliminate the starting circuit to prevent overheating and power losses, as the main winding is sufficient to maintain rotation once the motor is running.
A centrifugal switch is an electromechanical device designed to disconnect the auxiliary (starting) winding once the motor reaches approximately 75% to 80% of its rated synchronous speed. Its primary function is to eliminate the starting circuit to prevent overheating and power losses, as the main winding is sufficient to maintain rotation once the motor is running.
NrтАЛтЙИ0.75NsтАЛ тАФ The approximate speed at which the switch opens, where NrтАЛ is rotor speed and NsтАЛ is synchronous speed.
TstartтАЛтИЭImтАЛIaтАЛsin(╬╕) тАФ The starting torque relation, where ImтАЛ and IaтАЛ are main and auxiliary currents.
The centrifugal switch is mounted on the motor shaft and consists of a stationary contact and a rotating flyweight assembly. As the motor accelerates, the centrifugal force overcomes a spring's tension, moving the weights outward and causing the contact to snap open. This action breaks the circuit of the starting winding, ensuring the motor continues to operate solely on the running (main) winding.
Located on the rotor shaft to sense rotational speed.
Prevents burnout of the auxiliary winding, which is designed for intermittent duty.
Commonly used in Split-phase and Capacitor-start induction motors.
Essential for motors that require a phase-shifted auxiliary flux only during startup.
Simple and robust mechanical design
Effective protection against auxiliary winding burnout
Moving parts subject to mechanical wear
Failure (stuck closed) leads to auxiliary winding smoke/burnout
Failure (stuck open) prevents motor from starting
Split-phase induction motors
Capacitor-start induction motors
General purpose industrial fans and pumps
Standard switch settings typically disconnect between 70% and 85% of rated speed.
Option A is incorrect because the main winding must remain energized for the motor to produce running torque.
B is correct тАФ the centrifugal switch disconnects the auxiliary winding to prevent overheating after the motor attains sufficient speed.
If a motor hums but fails to reach speed, check the centrifugal switch; a permanently open switch prevents the required phase shift for starting torque.