The bimetallic Overload Relay

Overload protection of the relay is accomplished with the use of a bimetal strip. The bimetallic overload relay consists of a small heater element wired in series with the motor and a bimetal strip that can be used as a trip level. A bimetal strip is made up of two dissimilar metals permanently joined together. The two metals have different thermal expansion characteristics, so the bimetal bends at a given rate when heated.

Under normal operating conditions the heat generated by the heater element will be insufficient to cause the bimetal strip to bend enough to trip the overload relay.

Under overload conditions a current rises and heat also rises. The hotter the bimetal becomes, the more it bends. In an overload condition the heat generated from the heater will cause the bimetal strip to bend until the mechanism is tripped, stopping the motor.


Bimetallic overload relays are available in manual reset and automatically reset.

Once the tripping action has taken place, the bimetal strip cools and reshapes itself, automatically resetting the circuit. If the cause of the overload still exists, the motor restarts, and the motor will trip and reset again. This cycle will repeat and eventually the motor will burn out due to the accumulated heat from the repeated inrush and overload current. Care must be exercised in the selection of this type of overload relay. More important is the possibility of danger to personnel. The unexpected restarting of a machine may find the operator or maintenance man in a hazardous situation as he attempts to find out why machine has stopped.

Different heaters give different trip points. In addition, most bimetallic overload relays are adjustable over a range of 85% to 115% of the nominal heater rating.

Ambient Compensated Overload Relay

Ambient compensated overload relays are designed for the situation when motor is at a constant ambient temperature and controller is located separately in a varying ambient temperature. In certain applications, such a submersible pump, the motor may be installed in a location having a constant ambient temperature. The motor control, along with the overload relay, may be installed in a location with varying ambient temperature. The trip point of the overload relay will vary with the temperature of the surrounding air as well as current flowing through the motor. This can lend to premature and nuisance tripping. To compensate for temperature variations an ambient compensated overload relay is used.

Ambient compensated overload relays are designed to overcome this problem. A compensated bimetal strip is used along with a primary bimetal strips. As the ambient temperature changes, both bimetal strips will bend equally and the overload relay will not trip the motor. However, current flow through the motor and heater element will affect the primary bimetal strip. In event of an overload condition the primary bimetal strip will engage the trip unit.