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

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

• A. Current relay
• B. Thermal overload
• C. Fuse
• D. Pressure switch

Answer: (A)

In a single-phase fractional-horsepower compressor, the start winding is used only to get the motor turning. Once the motor reaches speed, that extra winding should be removed to prevent unnecessary current draw and overheating. A current relay is designed for exactly this task: it senses the current in the motor circuit and automatically disconnects the start winding when the motor is running smoothly. When the motor starts, the current is high enough to energize the relay, which keeps the start winding engaged (often along with a start capacitor). As the rotor speeds up and load conditions settle, the current in the circuit drops. The relay then releases and opens the circuit to the start winding, effectively removing it from the circuit. This provides a reliable, automatic way to stop using the start winding at the right moment. Thermal overload devices protect the motor from overheating, but they don’t sequence the start winding. A fuse offers protection against overcurrent but isn’t used to disconnect the start winding during startup. A pressure switch governs refrigerant pressure and overall compressor operation, not the electrical start sequence.

In a single-phase fractional-horsepower compressor, the start winding is used only to get the motor turning. Once the motor reaches speed, that extra winding should be removed to prevent unnecessary current draw and overheating. A current relay is designed for exactly this task: it senses the current in the motor circuit and automatically disconnects the start winding when the motor is running smoothly.

When the motor starts, the current is high enough to energize the relay, which keeps the start winding engaged (often along with a start capacitor). As the rotor speeds up and load conditions settle, the current in the circuit drops. The relay then releases and opens the circuit to the start winding, effectively removing it from the circuit. This provides a reliable, automatic way to stop using the start winding at the right moment.

Thermal overload devices protect the motor from overheating, but they don’t sequence the start winding. A fuse offers protection against overcurrent but isn’t used to disconnect the start winding during startup. A pressure switch governs refrigerant pressure and overall compressor operation, not the electrical start sequence.