Thermal Overload Relay
A thermal overload relay is an electromechanical motor protection device that trips when motor current remains above a set value long enough to create an overheating risk, opening the contactor control circuit to disconnect the motor before winding damage occurs. It operates on the principle of thermal expansion (I²R heating), using a bimetallic strip that bends when heated by excessive current, mechanically activating the trip mechanism. It provides time-delay protection mimicking motor heating characteristics but cannot protect against short-circuits.
Mounted in series with the motor contactor on the shop floor, it is adjusted to the motor's rated current (ampere setting) to match the motor's thermal profile. During operation, it continuously monitors sustained overload current; if the current exceeds the threshold (e.g., due to a jammed conveyer or belt tension), the bimetallic strip heats, bends, and trips the relay, de-energizing the contactor and stopping the motor. After tripping, it requires a cooling-off period before automatic reset or manual reset, preventing immediate re-start while the motor cools. It also protects against phase failure, phase loss, and current imbalance.
Why does a thermal overload relay trip slower at lower overcurrents?
It mimics the motor's thermal time constant; lower overcurrents generate heat slower, so the bimetallic strip takes longer to reach the trip temperature, matching the motor's actual heating curve.
How does phase loss trip a thermal overload relay?
In a 3-phase motor, phase loss causes the remaining phases to draw excessive current to maintain torque, generating heat that triggers the bimetallic strip.
Can a thermal overload relay protect against voltage surges?
No; it responds only to current-induced heating, not voltage fluctuations; voltage protection requires separate devices like surge suppressors.