Introduce The Working Principle Of Thermal Relay

The heating element in the thermal relay that generates thermal effects should be connected in series to the motor circuit, so that the thermal relay can directly reflect the overload current of the motor. The sensing element of the thermal relay generally uses a bimetallic sheet. The so-called bimetallic sheet is formed by mechanical rolling of two metal sheets with different linear expansion coefficients. The one with a large expansion coefficient is called an active layer, and the one with a small expansion coefficient is called a passive layer. The bimetallic sheet generates linear expansion after being heated. Since the linear expansion coefficients of the two layers of metal are different and the two layers of metal are closely attached to each other, the bimetallic sheet bends toward the passive layer side, resulting from the bending of the bimetallic sheet. The mechanical force drives the contact action. The common JR36 uses this principle.
There are four heating methods for bimetallic sheets, namely direct heating, indirect heating, composite heating and current transformer heating. The direct heating type uses a bimetal sheet as a heating element and allows current to pass directly through it; the indirect heating type heating element is made of resistance wire or tape, wound around the bimetal sheet and insulated from the bimetal sheet; the composite heating type is between the above Between the two methods; the heating element of the current transformer heating type is not directly connected in series to the motor circuit, but is connected to the secondary side of the current transformer. This method is mostly used in situations where the motor current is relatively large to reduce heat generation. component current.