Motor Lead Failures

Loss of Synch field or a cracked bar in the ammortisseur windings were going to be my suggestions. I once say the shorting rings completely ripped off a 6.6kv 4000hp synch motor. It was caused by an impatient electrician closing the VCB at 1000rpm because the synchronising relay was playing up. Obviously the phase angle wasn't correct. I would chart the synch field current with something fast like a DAQpad. Is the spike on one phase or all three?

What do you mean by 'lead failure'?? Overheating, breaking, shorting?? And at the same place, ??

A momentary spike of 'amps' is likely to be accompanied by a spike of 'volts'.

The amps spike should not cause the cable to overheat - but the volts spike could damage the insulation. Alternatively you might have vibration problems and loose terminals.

Is your motor used for power factor correction??. You might have some tremporary very high voltages generated by resonating circuits. Or there could be a 'earth' fault via the motor leads as a return path.

Or the leads might be too small anyway.

The leads are burned or melted off close to but not at the lug that in each case has been crimped with a hydraulic crimping tool. I do not suspect bad connections as our electricians are very competent and very experienced. The failures have been on different phazes and one time at the star point.This is an old installation with relay based motor protection. Unfortunately I do not have something like a Multilynn that might give better information. The amp reading I see is from a CT on one phase.

Also the compressor this motor drives has a very consistant load that could not change suddenly.

I suspect the crimp job. Go for an upgrade (2x overkill) at that point. It is necesary that no connection creats the problem before you can describe anything else. Once a commector begins to heat up it gets instantly worse. Your first description should have said the lead connections are failing not the leads failed.

Hydrax

The connection lugs are the long barrel type that allow for two crimping points.

After a failure the leads are melted off about a 1/2 inch down the lead from the end of the lug.

It sounds like differential expansion and contraction causing the compression joint to loosen.

I don't mean loose in the 'wobbly' sense but loose in the mechanical sense causing a slight rise in resistance and a hot spot. The effect is cumulative - and soon rapid.

Is there any corrosion?

There are no signs of corrosion at the connections.

We have used the same tools and lugs on many other applications and never seen this type of failure. After a failure there is quite a mess in the connection box, so in each case all leads are dis-connected the box is cleaned and then all leads are re-connected. The odds of it being bad connections 4 times on the same machine doesn't seem plausible.

Our best guess right now is the motor is somehow dropping out of synchronous.

That is what we suspect but we do not know how this could be happening. Has anyone experienced this problem.

you are seeing failure do to heat, expansion and conraction, increase the lead gauge, fill the connection with solder(silver not lead) berfore crimping and please tell us you are using copper leads and not aluminum.

When I first read your problem I had no Idea on what could cause it but have since remembered an obscure fault that seems to fit. What was happening was a 200 amp multi strand cable was failing near the crimped connector like yours is. The fault turned out to be caused by very minor nicking of the cable where the insulation was removed. I don't understand the mechanism but suspect it has something to do with eddy currents. The nicking doesn't need to be great and in my case was almost to small to see with the naked eye. I am pretty sure NASA have seen this problem too because I have seen them using a laser to strip the insulation back when they apply crimp lugs to cables. If your cables are failing close to where the insulation ends then this could be your problem.

Masu,

Are you thinking Corona damage, very plausible. It will be made much worse by anything that creates fast rise time spikes or high frequency from other sources. Correct stress relief cable termination methods are absolutely essential, companies such as ABB and 3M make the gear and have good technical manuals for the techniques. Specialists from ABB are likely to have seen similar faults.

As for the actual fault (beyond termination issues), without decent fault monitoring equipment it can only be a guess. Companies such as tech rentals have the gear to look at the problem. Looking at the incoming wave form and the excitation current is essential. If the VARS regulator has gone crazy the results can be devastating.

Emjay4119,

I doubt if it would be corona damage as the cables I was referring to that were failing only carried 2.2V and 5 .0V at 150A and 200A ,but I could be wrong. Unfortunately I didn't pay attention at the time to the mechanism behind the failure, something I now regret. It would be interesting to know it the failures Altennant is referring to are near or at the point the insulation has been cut.

The only "rotor bar" in a synchronous motor is in the amortisseur winding, which is only used during starting. If you are starting under load (doubtful on a synchronous machine application) you may do damage to the lead connections just because of the high current flow at startup. If the amortisseur winding does have a broken rotor bar, it would manifest and a harmonic during starting, which theoretically could also account for some extra heating IF you had a long acceleration time and a high start/stop duty cycle. Again, these conditions are not typical for synchronous machine applications, but only you can answer if they apply to your situation.

If you slip a pole (pull out of synchronicity), your synch motor attemps to operate as an induction motor. But the slip is extermely high and causes very high currents to flow. Usually it damages the motor windings before it damages the leads because the windings are NOT designed for this to happen. That is why you must have pull-out protection in your synch motor protection scheme. If you don't, and you are slipping a pole, you could be damaging a lot of things. Maybe the motor leads are just what you can see from the outside.

You could also have a bad or damaged salient pole in the motor. Again, that manifests as a harmonic and means a lot of current flowing. If it is a Y would motor, that would show up in the Y point as well, something you indicated had happened in the past.

Given your observation of momentary current spikes, and assuming these happened when running, not starting, I would guess first that you are slipping a pole, which is often load related; i.e. not enough motor for the load or some abnormal load spike. If it were a bad salient pole, the condition would exist relatively constantly.

The primary purpose of the Amortisseur winding is to prevent hunting. It sometimes doubles as a squirrel cage for starting. The preferred method of starting especially large synch motors is via a pony motor for instance using one of the swing generators as a motor powered by a static package. The motor and mains are then synchronised in terms of speed, voltage and phase before the VCB is allowed to close. A cyclic load with damaged amortisseur windings can certainly cause current spikes and in extreme cases loss of synchronism. A damaged salient pole (intermittent open of some shorted turns) as posted above is also a plausible explanation. The simplest test for this is to use an AC drop test. The start and finish of each pole should be accessed for voltage drop measurements. If they cannot be exposed, then a gauss meter in the air gap should show a variation. A cracked bar on the other hand can be a pig to find. To keep the fault finding simple this order of events may help.

1. Check the brush grade is correct. That is not only the number, but check the manufacturer hasn't substituted a "New Improved" grade. I once saw several slip rings severely damaged after service and it wasn't until we exhaustively eliminated every other imaginable cause the brush supplier fessed up.

2. Thoroughly test all the field supply wiring. A ducter and a megger will be very useful.

3. Know what type of field exciter you have. It could be a simple device or it could be a full blown VARS regulator or something in between. Then find out exactly how it should perform and prove it is performing to spec.

4. Complete an AC drop test on the rotor as described.

5. Using either ultrasonics or high current and a gauss meter test the bars.

As a footnote: Motors that are DOL started via their amortisseur windings are more prone to rotor bar failure and or salient pole failures due to stress.