Heating, Ventilation, and Air Conditioning (HVAC ) Electrical Practice Test

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

1. Thermal overload relay

2. Capacitor

3. Current relay

4. Contactor

The correct answer is: Current relay

The commonly used device that disconnects the start winding after a single-phase fractional horsepower compressor motor starts is the current relay. This relay monitors the current flowing to the motor during startup. When the motor reaches a certain speed and the current drops to a predetermined level, the current relay opens the circuit to the start winding, effectively disconnecting it. This action is crucial because it prevents the start winding from overheating and being damaged; it's designed to operate for a short period only when the motor is starting. The operation of a current relay is typically based on the principle of electromagnetic induction, where the relay's coil reacts to the decreasing current as the motor accelerates. Once the motor runs at full speed, the current sufficiently drops, causing the relay to operate and disconnect the start winding, allowing the motor to run efficiently on the run winding alone. In contrast, other devices such as thermal overload relays are used primarily for protecting motors from overheating during operation and do not specifically serve to disconnect the start winding. Capacitors are used in conjunction with start windings to provide an initial boost of energy needed to start the motor but do not perform the disconnection function. Contactors are utilized to switch electrical circuits on and off but are not specifically designed for the purpose