The Thermal Overload Relay: A Bimetal That Heats Like the Motor

A motor can be overloaded in two very different ways: a short circuit, which a fuse or breaker must clear at once, and a steady overload or a stalled rotor, which cooks the windings over seconds or minutes. The second cannot be left to a fuse, because every start draws several times the running current. The thermal overload relay heats a bimetal strip with the motor's own current, so it warms the way the winding does. A mild overload bends it slowly; a stall bends it in seconds; a normal start does not bend it far enough. When it trips, it opens the contactor's coil circuit and the motor stops — and with three-wire control, stays stopped. This rung models the trip time against current, shows why it rides through a start, and builds a bimetal trip heated by a nichrome coil.