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

What type of relay is used to automatically disconnect an AC motor?

1. Solid-state relay

2. Contactor

3. Current relay

4. Time-delay relay

The correct answer is: Current relay

The current relay is specifically designed to automatically disconnect an AC motor based on the current flowing through it. This type of relay operates by sensing the current and will open the circuit if the current exceeds or falls below a predetermined threshold. It is typically used to protect motors from overload conditions, ensuring that they do not run under potentially damaging situations. By interrupting the power supply to the motor when unsafe current levels are detected, a current relay helps prevent motor burnout and prolongs the lifespan of the equipment. In contrast, while solid-state relays are effective for switching applications, they do not typically include current sensing capabilities. Contactors are primarily used to control the power supply to electrical loads but do not inherently disconnect based on current conditions; their primary function is to act as a switch. Time-delay relays can delay the operation or disconnection of a circuit for a set period but do not monitor current levels to provide overload protection. Thus, the current relay is the appropriate choice for automatic disconnection based on current flow in an AC motor application.