Which device is commonly used on a single-phase capacitor start fractional horsepower compressor motor to disconnect the start winding after the motor starts?

A current relay is the device typically used in single-phase capacitor start fractional horsepower compressor motors for disconnecting the start winding once the motor has reached a certain operational speed. The current relay works by monitoring the current flow in the circuit. When the motor starts, the current is initially high due to the inrush, and as it reaches its operating speed, the current decreases. At this point, the current relay opens the circuit to the start winding, effectively disconnecting it.

This is significant because the start winding is only necessary for starting the motor; once it is running efficiently, the run winding takes over the motor's operation. If the start winding were to remain engaged, it could lead to overheating and potential damage to the motor.

The other devices mentioned serve different purposes: magnetic switches are typically used for switching applications and might not operate based on current levels; contactors are designed for larger loads and may require a different control mechanism; and thermal overload devices protect against overheating but do not directly disconnect the start winding during normal operation.