How to prevent misoperation of ac contactor or relay?

- Sep 12, 2019-

How to prevent misoperation of ac contactor or relay?For example, relay misoperation of a relay cabinet.The fault phenomenon, the field switch disconnects the control line, the relay is still on, the output contact fails to be completely disconnected.The indicator light is not red and dark red, the relay in the wrong operation and buzzing sound.The above problem is misoperation of relay. How to solve it?

Because of the relay, the fault was not eliminated after replacement, so the problem of the relay itself was eliminated.After repeated inspection, it was found that the continuous charging and discharging of the distributed capacitance of the instrument signal cable caused the misoperation of the relay.

The situation is like this, the control room is far away from the field instrument, and the switch quantity of electrical appliances/equipment consumption is large, so the switch quantity signal is particularly complex, and many signals need to participate in interlock control.Therefore, in order to be safe and reliable, a relay cabinet is added between the control system and the field instrument, and the power supply is 220V for the field instrument or contact switch.Considering the signal cable distance and ac power supply, it is estimated that the distributed capacitance generated by ac on the signal cable causes the misoperation of the relay.

So when we talk about distributed capacitance, what is distributed capacitance?

A distribution of capacitance forms between isolated and parallel conductors in an ac circuit.Although the capacitance is small, its presence can cause sensitive components in the circuit.

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Troubleshooting

The voltage generated by capacitance distributed on both ends of relay leads to poor reset.Add drain resistance at both ends of the relay.Make the capacitor discharge current mostly pass through the discharge resistance, and make the current flow through the relay circuit small enough to reach the minimum absorption current value of the relay coil, so that the relay will not misoperate.

To solve the misoperation of ac contactor, a capacitor is connected to the discharge resistance in parallel at both ends of the ac contactor coil.This diverts the current from the distributed capacitance in the line.In this way, the contactor can avoid misoperation.