Starter Thermal Elements
Overload relays protect motors, controllers and branch-circuit conductors against excessive heating due to prolonged motor overcurrents. Protection of the motor, and the other branch-circuit components, from higher currents is a function of the branch-circuit fuses, circuit breakers or motor short-circuit protectors.
Thermal overload relays are very effective in providing motor running overcurrent protection. They will produce a shorter trip time at a higher current, similar to the way a motor will reach its temperature limit in a shorter time at a higher current. In a high ambient temperature, a thermal overload rely will trip at a lower current, allowing the motor to be used to its maximum capacity in its particular ambient temperature. Once tripped, the thermal overload relay will not reset until it has cooled, allowing the motor to cool before it can be re-started.
Schneider Electric melting alloy thermal overload relays allow the motor current to pass through a small heater winding. Under overload conditions, the heat causes a solder to melt causing a ratchet shell to spin free, opening the control circuit contacts. Bimetallic thermal overload relays utilize a U-shape bimetal strip associated with a current carrying heater coil. When an overload occurs, the heat will cause the bi-metal to deflect and operate a control circuit contact. Bimetallic overload relays are used where the controller is remote or difficult to reach. Melting alloy and bimetallic overload relays are designed to approximate the heat actually generated in the motor. As the motor temperature increases, so does the temperature of the thermal unit.
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