By John Papiewski – Updated Mar 24, 2022
Single‑phase AC motors struggle to generate the torque required to move from a dead stop. A centrifugal switch supplies the missing boost at startup and then disengages once the motor reaches operating speed, ensuring efficient operation.
A centrifugal switch is mounted on the motor shaft and remains closed while the motor is idle. When the motor is powered on, the closed switch energizes a capacitor and an auxiliary winding, producing a temporary surge of torque that propels the motor past its initial inertia.
As the motor accelerates, centrifugal force acts on a weighted cam. When the force overcomes a calibrated spring, the switch opens, breaking the boost circuit. The motor continues running on its main winding alone, operating at full efficiency.
Household and many commercial systems supply only single‑phase AC, unlike the three‑phase power used in heavy industry. While three‑phase motors deliver strong starting torque inherently, single‑phase motors rely on the centrifugal switch’s temporary boost. Removing the boost once speed is reached prevents unnecessary power draw and heat.
Calibrated weights determine the rpm at which the switch opens. A heavier weight opens the circuit at a lower rpm, while a lighter weight requires a higher speed. Typical operating ranges are from 500 to 10,000 rpm, depending on the motor’s design and application.
When the motor stops, a spring returns the switch to its closed position, ready for the next start cycle.
