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Centrifugal switch open when motor attain
10% speed
30% speed
50% speed
70% speed
70% speed
A centrifugal switch is an electromechanical device used in single-phase induction motors to disconnect the auxiliary (starting) winding once the motor reaches a specific operating speed. It is calibrated to open when the rotor attains approximately 70% to 80% of its rated synchronous speed to prevent burnout of the starting winding.
A centrifugal switch is an electromechanical device used in single-phase induction motors to disconnect the auxiliary (starting) winding once the motor reaches a specific operating speed. It is calibrated to open when the rotor attains approximately 70% to 80% of its rated synchronous speed to prevent burnout of the starting winding.
NsтАЛ=P120fтАЛ тАФ Synchronous speed calculation where f is frequency and P is number of poles
NswitchтАЛтЙИ0.70├ЧNsтАЛ тАФ Estimated opening speed of the centrifugal switch
As the motor gains speed, the centrifugal force acts on the governor weights attached to the switch assembly. At approximately 70% of the synchronous speed, these weights overcome the force of the spring and push the moving contact away from the fixed contact. This effectively de-energizes the starting capacitor or the auxiliary winding, allowing the motor to continue running solely on the main running winding.
The centrifugal switch is placed in series with the auxiliary winding.
If the switch fails to open, the starting winding will draw excessive current and burn out.
It is typically used in Split-phase and Capacitor-start induction motors.
The 70% threshold ensures the motor has developed sufficient torque to reach full load speed on the main winding alone.
Simple and rugged construction.
Effective at isolating high-current starting windings.
Mechanical wear and tear over time.
Prone to contact pitting/arcing if operated frequently.
Single-phase induction motors
Air compressors and pump starters
Standard industrial practice dictates the switch closes at approximately 20-30% speed upon startup and opens at 70-80% speed during acceleration.
Options A, B, and C are incorrect because the motor would not have sufficient centrifugal force to actuate the spring-loaded mechanism at these lower speeds.
D is correct тАФ The centrifugal switch is designed to disconnect the auxiliary winding at approximately 70% of the motor's synchronous speed.
Always remember that in single-phase machines, the starting winding is only needed for initial torque and must be disconnected quickly to prevent insulation failure due to overheating.