In melting alloy overload relay, the motor current pass through a small heater winding. Under overload conditions, the heater coil heats the eutectic alloy tube. The heat melts the alloy, freeing the ratchet wheel and allowing it to turn. This action opens the normally closed contacts in the overload relay. The magnitude of the current and the length of time it is present determine the amount of heat registered in the heater coil.
Melting alloy overload relays must be reset by deliberated hand operation after they trip. A reset button is usually mounted on the cover of overload relay.
The heating units of melting alloy overload relay are interchangeable. The heater units are rated in amperes and are selected on the basis of motor full load current, not horsepower.
One of the fundamental application differences between the bimetallic and melting alloy overload relay is that bimetallic overload relay is available in ambient compensated forms. Only 5-10% of all installations require ambient compensation. Ambient compensated overload relays are designed for applications where the motor is in a constant ambient and the controller is in a varying ambient. Applications where the controller and motor are in the same varying ambient do not require compensation. Also, applications where the controller is at a constant temperature and motor is in a varying ambient do not require compensation. In all cases, the melting alloy or bimetal units should be sized according to the motor full load ampere (FLA).
The fundamental difference between bimetallic and melting alloy overload relay is the design and operation of the trip mechanism. Because 90-95% of all applications do not require ambient compensation, the choice of bimetallic or melting alloy overload relay is usually a matter of user preference, and not an absolute application requirement. In these cases, bimetallic or melting alloy overload relay works equally well.
