Varistor Function and Selection of AC Contactor (I)

First of all, let's understand what is a varistor: It is a resistive device with nonlinear volt-ampere characteristics. It is mainly used for voltage clamping when the circuit is subjected to overvoltage and absorbs excess current to protect sensitive devices. Good linearity, large flow capacity, normal leakage current, low residual pressure level, quick response, and no freewheeling. Widely used in power systems, surge suppressors, security systems, motor protection, automotive electronics systems, Household appliances.

Why is the varistor connected across the coil of the AC contactor? To make the contacts of the contactor close, there must be enough current in the coil. When the contactor is powered off, self-induction electromotive force is generated. Since the coil is momentarily de-energized, the rate of current change is large, so the self-induction electromotive force is also large. Instantaneous high voltage will breakdown more than withstand voltage electronic devices, even beyond the pressure of the contactor coil itself to damage the coil. Therefore, in order to protect the electronic components in the circuit, the current shock of the absorption coil at the moment of power loss increases the power factor, and the varistor must be connected in parallel across the coil of the AC contactor.