Motor Contactor

Hi ya all, I have a quick question.

One of my customers has an operation with a 230V 30Amp contactor, the coil is wired in series with a number of pressure switches and through the output of a Moeller Easy Micro PLC, when the pressure switches hit the setting, thier contacts open, the Motor Contacts are suppose to drop out. Frequently, when the pressure switches open the coil circuit, the contact remains closed and the motor continues to run. This then overloads the machine and they have a huge mess to dig out. I supect that they are overloading the motor circuit. Would this cause the contacts to remain closed even when the contactor coil is deenergized? Too much current causing the contacts to remain closed. Even when the Easy output open the contacts sometimes remain closed.

Thanks in advance for any help.

Willpower

First guess is a worn or defective contactor.

I supect that they are overloading the motor circuit.

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Doesn't the contactor have overload protection?

Hi,

Try to replace the contactor, it contacts are sticking could be dangerous.

Rudi

Sounds like welded contacts.

If the motor is overloaded, then why doesnt the motor protection trip out ?
There IS a motor overload protection, right ?

If the overload protections consists of a thermal (bimetallic) trip unit that has an auxiliary contact in series with the contactor coil, then it will be ineffective if the contactor has a tendency to hang.

Yes there is overload protection, but I am guessing that they have up sized the fuses to prevent shutdowns. I am going to check all of this today, thanks for the help so far.

Like the others have posted, the contacts could be welded OR the contactor's moving parts are physically binding, preventing the contact from breaking for long enough to cause a mess. Replace the contactor and see if that doesn't fix the problem.

Overload vs. Overcurrent protection

After your note:

Yes there is overload protection, but I am guessing that they have up sized the fuses to prevent shutdowns.

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I feel obligated to point out the overload protection and over-current protection are two different things.

Fuses and circuit breakers are designed to protect against overcurrent (often described as instantaneous over-current) caused by short circuits where the CURRENT spikes to abnormally high levels rather instantaneously.

On the other hand "Heaters" and thermal overload devices are designed to protect against sustained OVER LOADS where a motor is being asked to work too hard, for too long. When this happens, devices and wiring heats up and causes bad things like breakdowns in wire and motor insulation.

For the situation you describe, an over load is what you are describing. Most likely, you will find a set of three "heaters" wired in on the secondary side of your motor contactor. These heaters are suppose to be sized to the motor but, as most of us have seen one time or another, you might find that they have been replaced with ones that are too big or remove (jumpered-out) all together.

Whatever you find with your original contactor problem, do make sure you get your heaters and fuses properly sized.

Steve

There were, and still may, be contactors that must be mounted vertically or upright. Some were made for remote pumping applications (and other uses) where the line voltage drops when the motor starts, and the coil would drop out. To compensate, GE and others would use very weak springs. This is probably not what you have, but a remote possibility.

regards.....casey

Just out of curiousity, what brand contactor do you have?

Watch out for poor workmanship

This is a whole different thing though but watch out.
This happened yesterday, while I first switch on the AC supply to a console panel. There are relays in there in 3 different enclosures: metal sealed, plastic sealed and and plastic dust cover. I did not noticed the problem immediately. The whole row of relays in plastic dust cover have faulty coils and contacts that are jammed up. This causes a few short circuits. It would have caused a lot of problems and damages if not for the good use of fuses. 25 fuses had burnt from that one fault.
Do take note of the workmanship of components sometimes they may cause a lot of trouble.
I do not understand why some companies wanted to use over-rated fuses and risk jeopardizing the safty of the equipment(s).

regards
Sherine T.

I do not understand why some companies wanted to use over-rated fuses and risk jeopardizing the safty of the equipment(s).

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Sad to say but its not always the company that does that. Maintenance people have a tendency to "up" fuses when something consistently blows fuses and they dont see an apparent cause. Its also possible that the fuses and/or heaters have been replaced with the wrong size...for whatever reason....lack of knowledge, right size not in stock...it happens especially when management says make the machine run.

This question was also asked at the other forum and I also suggested looking at the contactor for binding, sticking etc. I also thought putting the switches into the PLC was a good idea. I am not fond of the idea of using switches etc inline with a plc output....kind of defeats the purpose of using a plc.

This is a situation where looking at the machine may offer the most answers.

fuses on a three phase motor???

I noticed that fuses were mentioned as overcurrent protection for the motor. Actually, all fuses will do is ensure that you burn the motor out completely when one phase opens and the others don't. A few months ago we refitted a panel with breakers for this reason. We had to learn the hard way.

As far as the contactor goes, I think everyone else has already stated the obvious: worn out, dirty, welded contacts... replace it and get proper overload and short circuit protection.

Technically circuit breakers arent any more effective against single phasing then fuses. Depending on the situation fuses may protect but usually they have to be rated at 125% of full load, this will only work if motors run at or near nameplate.

Any voltage detection system is fallible because if the motor is running it will generate voltage onto the third leg.

3 Phase monitors are a better solution to protect against single phasing if that is a concern.

Another thought about a fuse blowing causing a single phase conditions means there was a PROBLEM that caused an overcurrent condition which blew the fuse.

In many cases its not economically feasible to use breakers.

Here is another manual to assist in determining motor protection.
http://www.bussmann.com/library/docs/spd02/SPDSection11.pdf

More info that may be needed:
http://www.bussmann.com/apen/pubs/spd/

Is the motor driving an hydraulic pump, thus the pressure switches, I would look at relief valves and or dump valves not working.

If mechanical how about a clutch?

I never heard of any one "deadheading" a motor, but I suppose there may be a reason.

willpower100 said...
"...when the pressure switches hit the setting, thier contacts open, the Motor Contacts are suppose to drop out. Frequently, when the pressure switches open the coil circuit, the contact remains closed and the motor continues to run."

Then he said...
"Too much current causing the contacts to remain closed. Even when the Easy output open the contacts sometimes remain closed."

If the maintenance folks up-sized the over-loads then you might have fused the contacts simply because of higher motor currents... I don't think so... unless they have really over-sized the overloads.

I suspect that your mag is suffering from "failed shaded-pole chatter". If so, then the mag would go ON, OFF, ON, OFF,... "chatter".

If this is the case, then you should hear a distinctive buzz from the Mag Bucket. This will cause one or more of the contacts to OPEN, CLOSE, OPEN, CLOSE,... this leads to arcing and fusing.

All that is necessary to keep the motor running is if two of the phases are fused.

Now, be aware... "fused" does not necessarily mean "welded".

"Welded" is permanent. "Fused" might be temporary... all that is necessary to stop the motor is to open two of the phases.

When the Mag releases, a pair of contacts might be "fused". If the spring in the coil is strong enough, then one of the "fused" contacts might open. This will stop the motor.

A question occurs to me...

Does the motor ever stay on after the STOP button is pressed?
Is that the EASY Output that you are referring to?

If so, then what I said applies. If not, then... damn... that's a booger!

DO THE WHOLE THING

I do not know a lot about PLCs.

BUT I sure know motor control.

I would check operation of ALL components in this circuit. This one has too many chefs in the stew and sounds all screwed up.

Do you know for sure taht the pressure switches and other controls are really functioning properly?? A stuck control switch (or one full of salt water etc) could just as easily keep that motor running.

I would ensure I have the right size conductors and they are properly fused.

Properly sized contactor and properly mounted etc etc IAW mfr instruction.

Proper sized motor overload protection. AT LEAST two and preferably 3 in my mind - if 3 phase.

I would tear down the contactor and look for proper parts inside and proper functioning how much contact burn, easy movement, etc. Overhaul or replace if needed.

Shading coil can be checked fairly easy - disconnect motor energize contactor. Should not chatter or buzz real loud.

After all this run several sets of clamp on ammter readings startup 15 minutes and maybe an hour later. Then again the next day.

Dan Bentler