Thermal Overload Relay

In spite of being relatively simple and inexpensive, thermal overload relays are very effective in providing motor-running overcurrent protection. This is possible because the most vulnerable part of most motors is the winding insulation and this insulation is very susceptible to damage by excessively high temperature.

As a thermal model of a motor, the thermal overload relay produces a shorter trip time at a higher current, similar to the way a motor reaches its temperature limit in a shorter time at a higher current. Similarly, in a high ambient temperature, a thermal overload relay trips at a lower current or vice versa, allowing the motor to be used to its maximum capacity in its particular ambient temperature (as long as the motor and overload relay are in the same ambient).

Once tripped, the thermal overload relay will not reset until it has cooled, automatically allowing the motor to cool before it can be restarted.

NOTE: The overload relay must be used in conjunction with a contactor. The overload relay has no power contacts and cannot disconnect the motor by itself. The overload relay control-circuit contact must be wired in series with the contactor coil so the contactor will de-energize the circuit when an overload occurs.

Schneider Electric manufactures three styles of overload relays:

Melting alloy
Bimetallic
Solid-state

Melting alloy is ambient-sensitive and may require additional calculations.

Bimetallic and solid-state overload relays are both ambient-compensated and do not require special derating when in different ambients than the motor itself.

For additional information, see Solid-State Overload Relays.