The two types of relays used to start a single-phase hermetic compressor are ______.

• A. Potential relay and current relay
• B. Thermal relay and pressure relay
• C. Magnetic relay and solid-state relay
• D. Pneumatic relay and vacuum relay

Answer: (A)

Starting a single-phase hermetic compressor requires a device that briefly energizes the start winding and then disconnects it once the motor is turning. The two common devices used for this are a potential (voltage-sensing) relay and a current (amperage-sensing) relay. A potential relay uses line voltage to energize the start winding and then drops it out when the motor reaches speed, letting only the run winding carry the load. A current relay works by sensing the motor current and disconnecting the start winding when the current pattern indicates the motor is up to speed. Other devices like thermal or pressure relays serve protection or control roles, not the primary starting function described here, and while other relay types exist, the typical pairing for starting a single-phase hermetic compressor is these two.