Final answer:
The centrifugal switch in a resistance split-phase motor opens the contacts when the motor reaches about 75% to 80% of its rated speed, disconnecting the start winding and ensuring it operates efficiently.
Step-by-step explanation:
The centrifugal switch in a resistance split-phase motor is designed to disconnect the start winding from the circuit once the motor reaches about 75% to 80% of its rated speed. This switch opens the contacts due to the force generated by the rotating part of the switch, often referred to as the centrifugal mechanism, as the motor achieves sufficient angular velocity. It ensures that the start winding is only in use during the startup phase to initiate the necessary torque and is not needed during normal operation, as the running winding suffices.
The principle of back electromotive force (emf) explains that when the motor starts, the back emf is zero and the motor draws maximum current. As the motor speeds up, the back emf increases, reducing the voltage across the coil and the current drawn. This effect is similar when centrifugal switch action occurs to disconnect the start winding and reduce current draw, contributing to the efficient operation of the motor.