In the last post I took the control circuit through to the contactor coil for the purpose of fault finding. What we need to look at briefly is the operation of the contactor. As you saw in the sketch of the control circuit, we followed the circuit through to the coil. What happens once we have power to the coil is also important for you to understand.
The coils can be compared to an electro magnet. It has 3 components namely the coil, fixed core and moving core. The moving core is also attached to the different sets of contacts. These are the main contacts and the auxiliary contacts. The main contacts are the ones where you would have your power connected to the line side and let’s say your electric motor to the load side. These contacts are rated to carry relatively high current and it is important that you use the correct rating contactor that matches the rating of the motor
When the coils is powered or energised, it creates an electromagnet within the fixed core which will then attract the moving part of the core onto it. With the moving part being pulled against the fixed part, the contacts move with it, and subsequently closes (or opens in the case of the normally closed auxiliary contact).
This is most likely the simplest way of explaining the operation without getting too deep into it. Suffice to say, if you have power at the coil and the contactor does not operate, you will most likely find that you have a faulty coil.
Now there is one more test I do before turning power back on. I test the mechanical operation of the coil by pushing it in with a screwdriver from the front. Please note: NEVER ATTEMPT THIS WITH POWER CONNECTED. Once I have established that it moves freely, I will test the continuity of each contact by connecting one lead from my tester to the line side of each contact and the other to the load side of the same contact one at a time. With the leads connected to contact 1, I will push the contactor in and check to make sure there is continuity. Do the same with all 3 the contacts
There is a good reason for doing this. The contactor may have arced and welded a contact or all contacts. If it welded one contact, it means you will have power to one set of the motor winding the minute power is turned on. Should all 3 contacts have welded in, the motor will most likely start the minute you turn power on. Can you see how dangerous that could be?
If you can get your hands on a contactor, pull it apart and you will see how it works having read the above. Till next time, stay safe and make sure you love what you do!