Typical circuit diagram for thermal relay protection of electric motor - News - Global IC Trade Starts Here Free Products

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Thermal relays have been widely used due to their small size, simple structure and low cost. At the same time, there are the following shortcomings that are difficult to overcome: in the process of bending the bimetal, the heat transfer takes a long time. Therefore, the thermal relay cannot be used as short-circuit protection, but can only be used as overload protection. Short-circuit protection of the motor is usually carried out by means of a series of fuses in the power circuit; the thermal relay relies on the mechanical action formed by the mechanical structure to achieve shutdown protection, when the operating structure produces mechanical fatigue (aging), deformation of the machine When the action threshold is deviated from the set value, causing malfunction or protection failure; the ordinary thermal relay does not have the phase failure protection function.
A typical circuit for protecting a motor with a thermal relay is shown in the figure below:


Figure 1. Motor overload protection circuit composed of thermal relays FR1 is connected in series in the main circuit. The normally closed contact of FR1 is connected in series to the control loop. When an overload fault occurs, the FR1 control contact is disconnected. AC contactor KM1 When the coil loses power, KM1 breaks and acts as an overload shutdown protection.

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