Working principle of thermal relay

Thermal relay is a very important part of the relay family, and the production is more frequently used, which is of great significance. This paper will introduce the working principle of the so-called thermal overload relay

The heating element that produces thermal effect in the heat relay should be connected to the motor circuit, so that the thermal relay can directly reflect the current of the motor. The sensing element of a thermal relay is usually bimetallic. The so-called bimetallic strip is the combination of two pieces of metal with different expansion coefficients. The expansion coefficient is called the active layer, and the expansion coefficient is called the passive layer. Bimetallic strip is heated to produce linear expansion, as a result of the two layers of metal linear expansion coefficient is different, and two layers of metal and closely fit together, as a result, the bimetallic strip to passive layer side bending, mechanical force produced by the bimetallic strip to bend to lead contact.


There are four heating modes of bimetallic plates, namely direct heating, indirect heating, composite heating type and current transformer. Direct heating is to use the bimetal sheet as a heating element to let the current pass directly through it; The heating elements of indirect heating are made of resistance wire or tape, which is wrapped in a bimetallic strip and insulated with bimetallic plates. The complex heating is between the above two modes; Current transformer type heating element is not directly heated concatenated in motor circuit, but in the current transformer secondary side, this way is much used in the motor current is larger, in order to reduce the current through heating elements.


A thermal relay with broken phase protection

One of the main reasons for the burning of the three-phase asynchronous motor is that one of the three phase motor’s wiring is loosened or the fuse of a phase fuse is fused. If thermal relay protection of motor is Y connection, when a phase power line, the other two phase current is increased a lot, because the line current is equal to the phase current, and flow through the motor winding current overheat relay the same proportion of current is added, so the ordinary two phase or three phase thermal relay to protect. If the motor is delta connection, phase failure occurs, due to the phase current of the motor and the line current, and flow through the motor winding current overheat the current relay the increase ratio is not the same, the power of the motor and hot components in series into line, according to the rated current of the motor line current setting, namely setting value is bigger. When the fault line current reaches the rated current, the fault current of the current larger phase winding inside the motor winding will exceed the rated phase current, and the risk of overheating and burning will be high. Therefore, it is necessary to adopt a thermal relay with broken phase protection.


The thermal relay with broken phase protection is a differential mechanism based on the normal thermal relay and compares the three current.