Insulation and motors 'burning' up

[controlnovice]

Just recieved a call from one of our plants.

They've just reported, although they've had the problem for over 6 months, that they are burning up motors left and right.

No trips and no overloads (heaters) need reseting.

They've also described, on some of the motors, that the insulation is burned (crystalized) a total of 4-5 feet from the motor terminals.

They've also stated that two wall mounted motor starters have blown off the wall. And sometimes have found contactors melted together.

I'll be heading out there next week to investigate. It's a 480V Y system, operating around 100A. Voltage varies from 265 to 280V. Current is very unbalanced: 62 on Phase A, 78 on Phase C. Not sure on transformer size yet.

Any initial ideas on what to look for? This is an 'unmanned' plant. I've got too many to list: AIC, ground faults, wet insulation, harmonics, sizing of protection, cable sizing, etc....

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[dpc]

That's a wide range of problems - they might not all be directly related.

I would check the 480 V voltage balance and to verify that you haven't lost a phase somewhere. Motors really don't like unbalanced voltage. It is possible for motors to overheat and fail due to unbalanced voltage (and resultant negative sequence current)without operation of the overload relay.

Beyond that, you'll probably need to go take a look firsthand to find out what is really going on. You may need to check out the motors are they are torn down to see what you can see.

 

[jraef]

I'd be willing to bet it isn't one problem as well. Crystallized motor leads 4-5 ft. from the termination block sounds like a bad termination job, i.e. high resistance.

Welded contacts can be from severe voltage spikes, i.e. lightning hits or FPC capacitor failures, but also from foreign objects or crud getting into the movable parts, rapid cycling (chatter) and/or improper component sizing / duty cycle rating.

Motor insulation failures can be from causes all over the map as dpc says, but I too would strongly suspect voltage and subsequent current imbalances there, for the same reason he states. It's a good argument for a decent solid state Overload Relay that has current imbalance protection.

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[waross]

Hello controlnovice
The majority of welded contacts in motor starter size contactors in my experience have been the result of closing on a short circuit. A possible scenario is that a motor fails for whatever reason. The breaker and/or overload relay is reset (or inoperative) and an attempt is made to restart the motor. Then the contacts weld. The welded contacts and the "contactors blowing off the wall" are probably results of trying to start failed motors.
One of the things on my check list would be whether the overload protection is functioning on most of the motors.
respectfully

 

[electricpete]

I would check the ground of the transformer. Possibly a system intending to be grounded has inadvertantly become ungrounded. Most of the time the voltage remains balanced with respect to ground due to capacitance but there is a repetitive restrike phenomenon which can build up very high voltages to ground in response to a transient.

It doesn't really match all your symtpoms but as I have heard it described it can cause widespread problems.






















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[itsmoked]

Look for environmental changes. Exceptionally long period of rain or humidity. Water on the floor that wasn't there before. Something misting. Room ventilation newly blocked. Room fan failed/stopped/off. Roof leaking.

Keith Cress
Flamin Systems, Inc.- <

http://www.flaminsystems.com>

 

[Marke]

Hello controlnovice

To have the insulation of the feeders burnt back from the motor terminals sounds like in some cases there is an issue with the termination between the cable and the motor. If there is a problem, then there will be considerable heat generated. This will a) burn the insulation back on the feedrs and b) put additional heat into the motor which could accelerate motor aging and cause motor failure. This would not affect the overloads unless the motor was close to full load, in which case the reduced voltage on the motor terminals would cause a rise in the current and the overloads could then trip.

I'd check for aluminium cables with poor terminations, or even standard cables with poor terminations as one possible cause.

I always find that when I get on site and start to investigate, the reality is different from the reports, so keep an open mind.

Best regards,

Mark Empson

http://www.lmphotonics.com