Thermal Overload Relays

Thermal overload relays sense motor current by converting this current to heat in a resistance element. The heat generated is used to open a normally closed contact in series with a starter coil, causing the motor to be disconnected from the line.

Thermal overload relays are very effective in providing motor-running overcurrent protection. 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 (if the motor and the 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.

Schneider Electric brand offers two basic types of thermal overload relays: melting alloy and bimetallic. In some types, the bimetallic overload relay is available in both noncompensated and ambient-temperature compensated versions. In both melting alloy and bimetallic types, single-element and three-element overload relays are available.