Introduction to Circuit and Motor Protection – MOTOR PROTECTION (4)

The bimetal overload relay operates similarly to the eutectic melting alloy, except that the bimetal overload relay uses a bimetal strip made of two bonded metals that expand with heat (see Figure 35).

This heat is the result of overload current created by the heater element and then transferred to the bimetal element. When an overload condition is detected, the bimetal element will bend and open the normally closed overload contacts (see Figure 36). After cooling off from the heat created by the current, the bimetal may return to its original position automatically or manually,

depending on the assembly. The automatic resetting feature of these overloads is restricted according to Article 430-43 of the National Electric Code because of the potential for injury and equipment damage due to an automatic restart of a motor (see Figure 37).


The bimetal overload relays will disconnect the motor from the circuit as soon as the heater coils
detect enough current to disengage the normally closed contacts (see Figure 38). Therefore, it is
important for the heater elements to be properly sized.

The solid-state overload relay in a motor circuit is represented by the symbol shown in Figure 39. This overload relay provides the same action as the heaters. The overload relay detects overcurrents by sensing the AC current magnetically from the motor leads passing through its current loops. This overload assembly provides an automatic reset, producing the additional circuitry necessary to provide for the manual reset. One advantage that this overload assembly provides is that it is unaffected by ambient temperature as heater coils are, thus eliminating the problem of tripping during hot weather.

to be continued……

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