How to troubleshoot contactors?

How to Troubleshoot Contactors

Last Updated on March 6, 2021 by Krunal Shah

You are solving a breakdown and you find the power circuit and control circuit all correct. Scratching your head, you are thinking – what is wrong with the circuit, why the load does not start?

Have you faced such a situation while solving a breakdown as an Electrical maintenance engineer in a factory?

Well, I have faced such a situation. I would share my experience.

While working with Power Company as an Electrical Maintenance Engineer, I faced the problem of ‘Contactor chattering’.

If you do not know anything about contactors and its working principle check out this article ‘Contactors: History and Working Principle‘. I would suggest you to know about working principle of contactors before you start troubleshooting. That will troubleshooting easy.

It was a new Air Condition unit where we faced this problem. I am telling you what the problem was exactly. As soon as we start the air condition unit, the contactor that is for controlling the compressor, starts chattering.

Let me tell you, what is chattering. Contactor switches ON/OFF rapidly and make an unusual sound. This unusual sound is called chattering.

Often when something goes wrong with the system we simply doubt on the device first. Same thing we did. Somewhere at the back of my mind, I knew that the problem was not with a contactor, but still, my subordinate and I decided to check the contactor.

I would share tips to troubleshoot the contactor, the way we did.

Remember that, the tips, which I am going to share, is not only for the above-mentioned problem. You can follow it when you want to be sure, whether the problem is with contactor or not in any breakdown. You can also consider it as guidelines for troubleshooting of contactor.

How to Troubleshoot Contactors?

  • Check the voltage potential across the coil of the contactor.

Remove all the wires from L1, L2, L3 and T1, T2, T3 for safety point of view. Put the multimeter in AC voltage mode (as shown in the image below). Put one lead of multimeter at A1 and other at A2. ON the control circuit. If the voltage is near to the rated voltage of the coil, it means that there is no problem with the voltage coming to the coil.

Contactor Coil Voltage Testing


  • Further, check the coil, if rated voltage coming to the coil is ok.

Remove the wires from the coil. Put the multimeter knob in ohms mode. Place one lead at A1 and the other at A2 just as you did to check the voltage across the coil. If the multimeter shows infinity, it means that the coil is open. In case, it shows zero ohms, it means that the coil is short.

If the multimeter is showing some resistance (as in the picture below), it means that the coil is ok.

Contactor coil Testing

  • Check the contacts.

Set the multimeter knob in continuity mode. Put the red lead of multimeter in L1 and other black lead to T1. You must hear the sound.

Contactor Main Contacts Testing


On display, it should show zero or near to zero ohms, in this way also, you can verify (as shown in above picture).

Repeat the same procedure for L2 and T2, L3 and T3. You must get continuity indication. If you are not getting continuity for all three, either you need to replace a few parts or the entire device.

  • Check the voltage potential between the terminals.

Connect main wires. Switch ON the power circuit. Put the multimeter knob into the AC voltage mode. Check the voltage potential between L1 and L2, L2 and L3, L3 and L1. Note down the voltage readings. Now switch ON the control circuit associated with the contactor you are troubleshooting. Check the voltage potential between T1 and T2, T2 and T3, T3 and T1. Note down all the readings again.

Contactor Main Contacts Voltage Testing


The voltage potential between L1 L2 and T1 T2, L2 L3 and T2 T3, L3 L1 and T3 T1 should be the same.

Otherwise, either check for the loose connections, if any. Else, you need to replace the contactor.

So this is how, you can be sure that whether the problem is with contactor or not.

In our case (the problem which I shared earlier in this article), the problem was not with the contactor. It was with Low-Pressure switch, which was malfunctioning.

Thus, I would recommend first to check the control circuit and power circuit associated with the device thoroughly and then you check the device. This will reduce the time.


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