why did my motor contactor burn up 3

[whseme]

Looking for possible reasons why a single phase of the M-contactor in my fire pump controller burnt up. I would like to avoid the next $5,000 replacement if it fails again!!!!

It is a 480v, 3ph, 200hp automatic pressure controlled application supplied by Joslyn Clark. Supposedly was wired and operating correctly for 6 months. Fire alarm tester did his 6th monthly routine of shutting pump down after testing. Noticed the motor started back up after normal push button stop sequence. So he disconnected power and when reclosing disconnect, the contactor smoked and he ran. The controller should be handling all the start sequencing functions. Everything he did was supposedly by the book, but is there a possibility something operationally could have caused the failure?

One possibility raised so far by the equipment supplier is that of a bug getting in the contact. Is this even possible?

Other idea is that contact was damaged on initial bump test for rotation and progressively got worse until the complete failure. Not sure if I buy this answer either.

 

[Barry1961]

Just a couple of random thoughts.....

Don't assume the tester did everything "right". It would not be the first time someone stretched the truth to keep from losing their job.

I have run into probelms in the past where there would be a loose cable in a lug on a starter or disconnect. This happens after new installations when a cable is not properly seated "wiggled" when connected in box lugs. This usally happens with stiffer cable larger that #4.

Is the voltage at the contactor balanced?

Have fun!!!

Barry1961

 

[jraef]

There are a couple of issues to get clear. Did the starter drop-out and then reclose after the first Stop sequence was called? This is important because it could establish the point of failure. If it did drop out and then reclose, it is likely a control circuit problem, i.e. a stuck Start PB, a stuck seal-in contact etc.

If in fact the contactor NEVER dropped out, it could have been a welded contact or the armature jammed because of some foreign material etc (i.e. drilling debris from conduit installation). Welded contacts are a possibility from bump testing, however a properly selected contactor (NEMA design criteria) is supposed to be capable of that. The "bug" theory is dubious IMHO. It would have to be a very big bug to provide enough carbon mass to cause a flashover or weld, and most armatures on a 200HP contactor are strong enough to break human bones caught in them. An insect exoskeleton would be relatively insignificant. Tt could possibly leave enough debris on a magnet pole face to cause uneven pressure on the contacts, which might lead to eventual failure. You could tell easily by just looking at the magnet surfaces for bug splatter.

2nd issue to check: Was this a vacuum contactor? Joslyn has had trouble in the field with their 400A Low Voltage vacuum contactors jamming. Their Aux contact module was designed to move straight up and down, but the vacuum armature moves in an arc. Over time the stress on the operating cog causes it to jam and keep the main contacts from fully closing, eventually causing failure. Josly supposedly fixed that a while ago, but your starter may be older.

"Venditori de oleum-vipera non vigere excordis populi"

 

[whseme]

Barry1961,
Of course, the electricians claim they did everything perfectly well, too. And the manufacturers all supplied perfect equipment... I agree with the operator issue, but barring throwing the contactor in by using the manual handle, I'm not convinced that no matter what he does, should he be able to cause this problem.

Voltage on three phases is plus/minus 4 volts when looked at yesterday. Went through all same steps operator claimed to have taken without incident. Problem occurred 4 weeks ago, I'm now involved after the fact, trying to prevent the next one.

jraef,
To the best of my knowledge, starter dropped out when stop depressed, kicked back in when stop let go. Normal occurence if pressure sensed as too low. Can't verify that fact or not. My suspicion is a control problem, but trying to determine if something physical with the contactor or wiring could have caused it.

System purchased and installed in 2003. I believe it is a magnetic contactor. The part number is A77-357400A-4 series 5000. Not sophisticated enough to know for sure what type, but I will try to look this up. Would be great to have a known problem to go back to the manufacturer with.

Thanks for the quick responses.

 

[jraef]

The A77 designates that it was an air gap style contactor, although it is sized as a "definate purpose" device. This only means that it was specifically selected for that motor, not necessarilly per NEMA sizing. This eliminates the vacuum contactor issue.

If it opened then reclosed, you likely have a control circuit problem. I would look for a stuck contact somewhere as the likely culprit, the Start button being the prime suspect. Another possibility is that the pressure switch is faulty. It should have some kind of hysteresis, meaning that once it comes on it should require quite a bit of pressure build-up to turn off again. Without it, the contactor would be chattering on and off rapidly wich could be the source of your failure.

Sometimes there is a minimum run timer that keeps the pump running for a minimum amount of time when there is no hysteresis in the switch (the Joslyn controller appears to have this). If your operator encounterd that without realizing it was there, it could explain why he thought it was stuck on. In a panic mode he may have inadvertantly caused real damage to the contactor by opening the disconnect then reclosing it under load.

"Venditori de oleum-vipera non vigere excordis populi"

 

[unclesyd]

Whsemse,
Kinda anecdotal but might be lead.

Not knowing the electrical part of this story, but was very involved.
On a large rotary screw compressor 1000 HP drive which we had just overhauled the compressor and had a successful restart. After running about 5 minutes I heard a noise from the rotors and hit the E-Stop, the Amps went to 0 and the compressor was coasting down and at 200 RPM it restarted on its own. I had to hold the E-Stop down to get the machine stopped. Evidently someone had put the "ride thru" in the wrong place in the feed from the substation. This episode wiped out the starter circuit in the switch gear.

Several months ago I was privy to information that this had happened on a spare compressor that had been installed and running for 2 years. The wiring was supposedly a duplicate of the original wiring problems. The original problem, location of the "ride thru" had been corrected but the follow up with the prints was lacking.
Form the information I received in both cases was that there would have been no problem if the local run switch had been used instead of the E-Stop.

Some of the electrical folks might be able to elaborate on how the use of an E-Stop might be the culprit.

 

[jbartos]

Suggestion to whseme (Industrial) May 13, 2004 marked ///\\
To the best of my knowledge, starter dropped out when stop depressed, kicked back in when stop let go. Normal occurence if pressure sensed as too low.
///In some case, the contact of the starter that seals the starting push button or pilot device contact, becomes welded or stuck, the Stop push button or similar control circuit opening contact will function for a period it is pushed only. Once, it returns the sealing contact is closed and the motor starter starts again. Beside the sealing contact, a short can accomplish the similar malfunction. It is sufficient to go over the contactor starter control circuit.\\ Can't verify that fact or not. My suspicion is a control problem, but trying to determine if something physical with the contactor or wiring could have caused it.
///The description of the malfunction suggests that the control circuit is defective.\\

 

[whseme]

jraef, Please elaborate on /// In a panic mode he may have inadvertantly caused real damage to the contactor by opening the disconnect then reclosing it under load. \\\

My understanding of the circuit is that this is across the line starter. Disconnect feeds line power to m-contactor and control power to controller. Controller tells m-contactor to engage or disengage. No power(disconnect open) controller shuts off, contactor should open. Power back on(disconnect closed) contactor should only re-close if controller tells it to. Does the time between opens and closes of the contactor matter? If so, why wouldn't the controller be managing this to prevent too short of a time no matter how fast an operator may be cycling the disconnect? Does it matter if the motor/pump is still spinning when power is reapplied by m-contactor closing? If so, why? What other damage may have occurred? Not sure I understand the cause of potential damage.

 

[jraef]

A contactor coil draws a lot of current when first energized, sometimes as much as 25 x the seal-in power. Normally, a contactor closes the motor power circuit slightly after the coil inrush drops off. Opening the disconnect under load was probably no problem. In reclosing it, if the timer were still finishing it's sequence, i.e. a pneumatic timer that continued being held closed even without power, the contactor would have been re-closing immediately at the same time as power was being supplied, and that may have cause a rapid chatter because the motor inrush and resultant voltage drop was occurring just as the contactor coil needed the most available power. I have seen this many times. That chattering can quickly cause damage and welding of the contacts. Each opening of the contacts liquifies and vaporizes minute amounts of contact material on both sides of the contacts. Chattering intesifies that to the point of leaving enough melted material behind that it solidifies.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jbartos]

Suggestion to whseme (Industrial) May 14, 2004 marked ///\\My understanding of the circuit is that this is across the line starter. Disconnect feeds line power to m-contactor and control power to controller.
///Please, clarify if the 200HP, 480V, 3ph fire pump controller is star-delta started with a timer or DOL starter?\\\
Controller tells m-contactor to engage or disengage.
///Yes.\\ No power(disconnect open) controller shuts off, contactor should open.
///Agree\\ Power back on(disconnect closed) contactor should only re-close if controller tells it to.
///Yes.\\ Does the time between opens and closes of the contactor matter?
///Yes, it does. The rotating motor has a residual voltage that becomes out of synchronism with the power supply, which can cause the higher inrush current in comparison to the motor being started from the standstill.\\ If so, why wouldn't the controller be managing this to prevent too short of a time no matter how fast an operator may be cycling the disconnect?
///It depends on the control circuit. If it is not started by the push-button station, i.e. it is started by the pilot device contact that can be closed, the disconnect is then in the direct control of m-contactor, unless in a highly unlikely case, there was a timer to protect the motor on/off cycling. This is usually not the case in the fire protection system, where the equipment is design to function even if it may be distroyed.\\ Does it matter if the motor/pump is still spinning when power is reapplied by m-contactor closing?
///Yes, it does. This has been discussed more in detail in the Forum several times. Try to use Advanced Search function above for more answers.\\ If so, why?
///See jraef and my posting answers above.\\ What other damage may have occurred?
///Beside the controller contactor damage and the control circuit damage, the motor may have been thermally impacted too.\\ Not sure I understand the cause of potential damage.
///When it comes to damages after malfunctions, one can often understand their causes after thorough root cause analysis only.\\

 

[tarzan56007]

Just a thought but from a maintenance mans perspective you may have two problems, one as most agree would be a control circuit problem but the fact that one of the main contacts burned up would seem to me a possible loose connection anywhere from the bottom to the starter to the motor windings, I would triple check all of these and also meg-out
the motor windings to be sure there are not internal problems, and also a complete volt and amp draw check next time you run this pump.