Final answer:
The start winding of a split-phase motor is de-energized by a centrifugal switch, which operates when the motor reaches approximately 75% of its operational speed to prevent overheating.
Step-by-step explanation:
When a general duty split-phase motor reaches approximately 75% of its operating speed, the start winding is de-energized by a centrifugal switch. This action occurs in the motor's start-up phase, where the start windings are used to create an initial phase shift to start the motor. Once the motor reaches a sufficient speed, the centrifugal switch opens to disconnect the start windings to prevent them from overheating. It is a crucial component for the safe and efficient operation of split-phase motors.