A thermal device that opens its contacts when a motor heater coil uses too much current is a __________

A thermal device that opens its contacts when a motor heater coil uses too much current is a __________

• A. Thermal fuse
• B. Thermal switch
• C. Thermal relay
• D. Circuit breaker

Answer: (C)

Overload protection for motors commonly uses a thermal relay. Its sensing element includes a heater coil that carries the motor current. When the current is too high, the heater warms up and drives a bimetal strip inside the relay. The bending of that strip trips the relay contacts, opening the circuit to stop the motor and prevent overheating. This type of device is designed to reset after cooling (some automatically, some manually), which distinguishes it from a one-time thermal fuse. Thermal switches open or close based on temperature but aren’t driven by the motor current in the same integrated way, and circuit breakers trip due to overcurrent with magnetic or thermal action, not specifically through a heater element inside a relay.

Overload protection for motors commonly uses a thermal relay. Its sensing element includes a heater coil that carries the motor current. When the current is too high, the heater warms up and drives a bimetal strip inside the relay. The bending of that strip trips the relay contacts, opening the circuit to stop the motor and prevent overheating. This type of device is designed to reset after cooling (some automatically, some manually), which distinguishes it from a one-time thermal fuse. Thermal switches open or close based on temperature but aren’t driven by the motor current in the same integrated way, and circuit breakers trip due to overcurrent with magnetic or thermal action, not specifically through a heater element inside a relay.