The varistor function and selection of AC contactor

Today, some customers ask the editor, how to choose the varistor, what varistor is used at the ends of the coil of the AC contactor, adhering to the principle of serving the customers, I will give you the choice of the varistor under the popular science.

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

Why is the varistor connected in parallel across the coil of the AC contactor? In order for the contacts of the contactor to be attracted, there must be sufficient current in the coil. When the contactor is de-energized, a self-induced electromotive force is generated. Since the coil is instantaneously de-energized, the current change rate is large, so the self-inductance electromotive force is also large. Instantaneous high voltages can break through electronic components that exceed the withstand voltage, even exceeding the withstand voltage of the contactor coil itself, causing damage to the coil. Therefore, in order to protect the electronic components in the line, the current shock of the absorption coil is lost, and the power factor is increased. The varistor should be connected in parallel with the coil of the AC contactor.

The characteristic of a varistor is that when the voltage across it is below a certain value, it is almost non-conductive, which is equivalent to an open circuit; and when the voltage above 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 the open state after the power is turned off. Selecting the breakdown voltage of the varistor to be greater than the energization voltage of the coil prevents the varistor from having a current when the coil is normally energized.

Therefore, the customer's PLC output is connected to the AC220V AC contactor. A fuse is connected in series in the line in front of the varistor, so that when the line is over-voltage, the varistor breaks down, causing the fuse to blow. The varistor that should be used at this time is 430V~470V. And if it is used for lightning protection, it must be connected between the line and the ground, and also a fuse in series. When lightning strikes, the voltage of the lightning is much higher than the line voltage, exceeding the threshold of the varistor, the varistor is turned on, discharges to the earth, and avoids damage to the electrical equipment in the line. At this time, the varistor is selected from 470V to 510V.