CSLB C-20 HVAC ContractorControls, Electrical and SafetyMedium
A split-phase induction motor fails to reach running speed and hums loudly without starting. A technician suspects the centrifugal switch. What is the normal function of this switch during motor operation?
- AIt reduces voltage to the run winding during startup
- BIt disconnects the start winding once the motor reaches roughly 75% of synchronous speed
- CIt reverses motor rotation for reversible applications
- DIt regulates motor speed continuously like a variable frequency drive
Show answer & explanationAnswer & explanation
Correct answer: B. It disconnects the start winding once the motor reaches roughly 75% of synchronous speed
The centrifugal switch disconnects the start winding once the motor spins up to roughly 75% of synchronous speed, since the start winding is only needed to create the phase shift for initial torque. If the switch fails to open or close properly, the motor may hum without starting or overheat from the start winding staying energized.
Why the other options are wrong
- A. Voltage reduction to the run winding isn't the switch's function.
- C. Reversing rotation requires swapping winding leads, not the centrifugal switch.
- D. Centrifugal switches are on/off devices, not continuous speed regulators.
Centrifugal Switch
A mechanical switch in split-phase motors that disconnects the start winding once the motor reaches about 75% of synchronous speed.
- Located inside motor housing
- Failure to open causes start winding burnout
- Failure to close prevents motor from starting
- Common failure point in older split-phase motors
Memory trick: Start winding gets the boost, then the switch kicks it loose at 75%.