AB Contact Relay premature failure 3

[darkstorm]

Here is a wierd one.

I have a AB contact relay with a 5.4 tp 27 A overload driving a 15HP motor drawing 9-10amps in both directions.
Overload set at 17A motor FLA 23.
Coil voltage is 120VAC
23A 15hp 3ph contactor

This contactor works fine for 2 mos and then the coil burns up and contactor is burned shut.

The duty cycle on this is not exceeded, not even close.
Amp draw-ok
Meger Motor-good
Change contactor and Overload- several times
Check for erratic chatter and arching, -none
Check for heat after over running the contactor- no heat

I have another setup-identical- giving no problems.

Can anyone shed some light as to why this could potentially be happening.

thanks

 

[itsmoked]

Yep, yep, yep.

One of two things is occurring. Take your pick!

1) This thing is mounted in a shaking environment like a rail car with the contactor mounted in the wrong plane being mechanically cycled by external forces.

2) Far more likely. You are experiencing relay chatter. Your case is a prime example. The relay's coil voltage is being dropped because the relay is running off one of the motor legs. The facility gets heavily loaded and the motor is started. The current draw drops the plant/circuit voltage dramatically. So dramatically that the relay drops out during a start. This means the contacts try to to interrupt LRA! The arc that forms reduces the current to a level that the coil pulls back in. This cycle can continue until A) the motor actually starts. B)The relay is blown off the wall. C)The contacts weld shut. D) The coil melts open because it hovers in its inrush area of operation for more than 1 second.

Solutions are run the the coil from a different leg or supply. Or run the coil via a DC supply with capacitors in it, to help keep the relay sealed until the motor is up to speed. Or fix the wiring problem that is causing the motor circuit brown outs.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jraef]

He said it is a 120V coil, and it is a 15HP motor rated at 23FLA, which would indicate a 460V motor, so it's not likely he is pulling coil voltage from one leg.

What we don't know is if the coil is failing first, or the contactor is welding first. Are the contacts truly welded, or is the coil swollen around the armature and holding the contacts closed? It may be difficult to tell, but try to disassemble it to get a close look.

Coil failures, if that's what it is, stem from 2 main sources; Excess duty cycle as itsmoked mentioned above, and power problems such as high or low voltage, or mis-matched frequency. What is controlling your coil? Any chance there is a relay race as itsmoked mentioned in your control circuit? Sometimes they are not obvious because they only happen under specific circumstances, so unless you happen to be there when it happens, you don't see it. One way to find out is to wire a cycle counter in parallel with the coil so that every time the coil is energized, the counter is incremented. If you are thinking it is getting, for example, 100 start commands per day, but the counter counts 1000, there's your problem.

Also double check that your coil is rated for the same frequency as your supply. 120V coils are going to be almost universally 60Hz, but are you in a 60Hz part of the world? Next, is there any chance your voltage is changing dramatically on this circuit? I know you said you have an identical setup nearby, but if you have a bad connection somewhere, you could be seeing a severe voltage drop under some conditions (again, as mentioned by itsmoked).

 

[waross]

With a 120 volt coil and a 480 volt supply you probably have a control transfomer fed from 2 legs of the 480 volts.
As per itsmoked's suggestion, check for excess voltage drop on the 480 volt supply.
Then check the control transformer. It may be undersized, it may be failing, it may have a bad connection in the wiring.
You may want to check your wiring to the motor for an intermitent short. I have seen a couple of instances when closing on a fault melted one of the contact faces. The contactor opened as the metal was cooling and left the movable contact welded at an angle. One side was welded and the other side was open. In one case this resulted in the contactor being prevented from completing its travel with the resulting coil burnout.
Closing on a moderate fault may also lead to the "Machine-gunning" that itsmoked mentions.
respectfully

 

[darkstorm]

Thanks for all your replies.
I went back into the 'box' of contactors and started pulling some apart.
The coil seems to be ok.

The contacts themselves are burned and on some, the armature is swollen which prevents the contact to open and close freely.

Seems to be a lot of heat generating through the contact when its on then when it releases, the plastic which houses each separate contact is burned eventually bringing the contact closer to close position when it should be open.

If the motor checked out ok, what am I missing........

 

[dpc]

Have you measured the control voltage at the coil while it is energized and again when it is de-energized?

 

[darkstorm]

Coil voltage is good, do you think that it may be 'lazy coil' trying to pull in the contactor and severely arching and chattering the contacts and melting them??

looking at these contactors more carefully, the only damage seems to be localised around the 3 phases, not the coil as previously mentioned. The terminals have signs of scorching and discolouration.

When I monitor this contactor its behaving normal.

 

[jraef]

Ghosts...

In all seriousness though, I come back to your control circuit. It sounds as if there is some sort of chatter (relay race) going on. You will occasionally come across a defective coil, i.e. a broken shading coil or a loose wire inside, but if you have replaced the entire contactor and it still happens, that effectively eliminates coil defects because multiple defective coils woul;d be a one in a million shot.

So since the load and normal duty cycle are not excessive, phantom chattering is the prime remaining source for the trouble. If it is acting normally when you check on it, the chattering is happening at some other time. You need some way of recording it, as I suggested earlier.

Another very remote possibility is load harmonics. They can sometimes cause excessive heating in the connections. Is this starter on the load side of a VFD by any chance? That's about the only way to get enough harmonics to do damage such as you are describing.

 

[bklauba]

darkstorm,

I agree with jraef to focus on the control circuit. Any device thru which passes the control signal current is suspect, such as a NC contact block on the stop pushbutton, etc.

It could be fine when cold (when you're hanging around), but if corroded, can increase in resistance after passing current. Then you start getting less than full hold in the contactor coil, and arcing. This explains the increasing popularity of gold-flashed contact blocks in control circuits passing low current (well less than 1 amp).

BK