What is the Best Way to Test a 3 Pole Contactor

[brenden80]

Excuse me if I sound inexperienced, but I have been looking for some type of documented method.

What is the Best Way to Test a 3 Pole Contactor?

Is there a set pattern to follow or is replacement/process of elimination the best form of troubleshooting? I know replacing with a known good contactor would rule it out but I am hoping for something more than that. Sometimes I would like to test a 3 pole contactor with the power on and sometimes I can shut the machine down.

[adam12]

The best way to test a 3 pole contactor is ....  very carefully. 

Actually, testing a 3 pole contactor is not hard at all. A contactor is an electrically controlled switch, it's also referred to as a relay. They are used for switching a power circuit on and off with a separate voltage supply. The design of the contactor enables it to be opened and closed repeatedly with minimal damage on the high current contacts.



First... Testing the contacts for failure

In a perfect world, these contacts last forever. This is directly related to design and the load on the contacts, as well as the frequency at which they are opened and closed under load.

So these higher current contacts can fail in an open position, burnt or melted as a result of the contactor being under- sized or as a result of a shorted load creating a high current situation. Power down the system on which you are working, remember to follow local laws regarding lockout/tagout where required. it is your responsibility to use safe work practices.

With power verified off, tag and remove the power leads from the high current side of the contactor. Essentially you'll be disconnecting all the wires on the 3 pole contacts. If you can manually operate the contactor with your fingers or a hand tool, you can test the contactors with an ohmmeter. High ohms nearing infinity when the contactor is open, low ohms or shorted to 0 when you manually engage the contacts.

Again, this is done with the power off. This should give you the answer you've been looking for... if not, continue testing your 3 pole contactor, this time under power....

Reconnect the feeding side of the contacts and keep the load side disconnected. Safely energize the coil control circuit and check for the appropriate voltage on the load terminals. Adjust your multimeter for volts. Are all three phases showing up when the contactor is energized?

By this time you would have determined if one or more of the high current contacts have failed. On the older obsolete contactors, the contacts may actually be exposed and you can visibly see the damage. Newer models may not be so easy to disassemble.

Other problems you may encounter on a 3 pole contactor, include mechanical failure such as broken or missing return springs, even a worn frame of the contactor. Most of the time a visual inspection is enough to find problems like these.
You'll see more failures occur due to excessive heat or after the service life of the contactor has been exceeded.

Testing the contactor coil is another story, again not too awful.

More commonly, a failed coil can be the source of an inoperable contactor. This is typically the root cause when a 3 pole contactor fails to energize event though voltage is being supplied to the coil.

It is tested in either of two ways. First, in a power-on scenario, a check of the voltage at the coil terminals should give an instant answer as to whether the coil is being electrically energized.

On contactors with overload resets, you will have to check the overload circuit for failure or an open circuit if you do not read the appropriate voltage directly from the coil terminals. It can be a tripped overload preventing the coil from being energized.

Of course you should verify the voltage with the label or nameplate on the coil also. This  ensures that the proper voltage is being applied.

In a power-off situation, you can check for coil continuity using an ohmmeter. Be sure to safely power down the circuit (or the entire machine), select a low ohm setting on your multi-meter and check directly on the coil's terminals.

An open circuit (or very high resistance) signifies an open coil; a moderate resistance would indicate that the coil is capable of conducting electricity. A reading of 0 ohms, however, could indicate a short circuit of the coil but this is more of a rarity since energizing a shorted coil usually amounts to excessive current and heating of the coil until it fails.

Testing 3 pole contactors is not entirely simple nor is it too difficult either, yet even a seasoned maintenance technician can get confused every now and then. I think these are some good basic rules for testing.

[3rdshiftguy]

How to test the contacts of a 3 pole contactor...

Here's another great test for a 3 pole contactor. You can actually test the contacts to see if they are beginning to fail. This 3 pole contactor test comes in handy when you've got an intermittent problem with motor overloading an you already tested the motor.

This test will be done under power.

• Apply power to the circuit or motor and enerize the 3 pole contactor.
• Put your voltmeter on the AC volts setting (DC Volts if it applies to your application)
• Put one meter lad on the feed side of the contactor (top side in most cases) and the other lead on the load side (bottom in most cases). Be sure both leads are on the same leg L1, L2, or L3
• Theoretically, with the contactor energized you should get 0 Volts. Test all 3 poles individually and verify something close to 0V. Obviously, if there is any large amount of voltage on one of these legs, you have found a problem.
• If you fail to find substantial voltage on any leg, switch your meter to milli-volt range and perfom the test again on each leg. Any notable difference in millivolts can be evidence of a contact beginning to corrode, decay, or beak down electrically..

Hope that helps...

[Cheller]

Finding fault in a contactor is best done when it's de-energized but I have to admit, the contactor test with power applied is interesting if it's done safely.

I would prefer to do the power-on tests with a digital multimeter, with your gloves on, of course.