Analysis on the Function and Selection of Varistor of AC Contactor



How to choose a varistor? What varistor should be used at both ends of the AC contactor’s coil, adhering to the principle of serving the majority of users, let’s show you how to choose a varistor for popular science.

How to choose a varistor? What varistor should be used at both ends of the AC contactor’s coil, adhering to the principle of serving the majority of users, let’s show you how to choose a varistor for popular science.

First of all, let’s understand what a varistor is: it is a resistance 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 current capacity, low normal leakage current, low residual voltage level, fast action response and no freewheeling. It is widely used in power supply systems, surge suppressors, security systems, motor protection, automotive Electronic systems, Household appliances, etc.

Analysis on the Function and Selection of Varistor of AC Contactor

Why is the varistor connected in parallel at both ends of 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 de-energized, self-induced electromotive force will be generated. Since the coil is instantly de-energized, the current change rate is very large, so the self-induced electromotive force is also very large. The instantaneous high voltage will break down Electronic devices that exceed the withstand voltage, and even exceed the withstand voltage of the contactor coil itself to damage the coil. Therefore, in order to protect the electronic components in the circuit, absorb the current impact at the moment of power failure of the coil and improve the power factor, a varistor should be connected in parallel at both ends of the coil of the AC contactor.

The characteristic of a varistor is that when the voltage across it is lower than a certain value, it is almost non-conductive, which is equivalent to an open circuit; and when the voltage on it exceeds a certain value, it is broken down, which is almost equivalent to a short circuit. Therefore, we choose the safe working current of the varistor to be greater than the current of the coil, so that the varistor can be restored to an open circuit state after the power is cut off. Choosing the breakdown voltage of the varistor to be greater than the energizing voltage of the coil can prevent the varistor from having current when the coil is normally energized.

Therefore, the customer’s PLC output is connected to the AC220V AC contactor, and a fuse needs to be connected in series in the line in front of the varistor, so that when the line is overvoltage, the varistor will break down and cause the fuse to blow. The varistor that should be selected at this time is 430V~470V. If it is used for lightning protection, it should be connected between the line and the ground, and a fuse should also be connected in series. When lightning strikes, the lightning voltage is much higher than the line voltage, and exceeds the threshold of the varistor. The varistor is turned on and discharges to the ground to avoid damage to the electrical appliances in the line. At this time, select 470V~510V for varistor.

The Links:   PM20CTM060 LB043WQ1-TD02