electrical fault causes torque which damages load? 6

[electricpete]

Does anyone have any experience with electrical fault in a motor or motor terminals causing damage to the load (presumablyl due to transient torques)?

I previously described an event that I believe fell in this category here thread237-67490

Also the theory of transient torques during an electrical fault was discussed a little in that thread.

We have just had another experience that I beleive may be attributable to a similar cause . When I get a chance I will post the details in here. In the mean time, any response to the question posted in bold above?


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[itsmoked]

I don't know of any though I would believe it possible. I have seen it with generators. I saw a prime mover break a connecting rod due to an electrical fault.

 

[Marke]

Certainly seen plenty of damage caused by auto reclose or open transition switching.

Best regards

Mark Empson

http://www.lmphotonics.com

 

[stevenal]

A line to line fault causes high levels of negative sequence currents to flow. These components have reverse phase rotation. Until the fault clears, there will be an equal component of reverse torque.

 

[ScottyUK]

Not sure how it would translate to a motor, but loss of field or abnormally weak field in a synchronous generator can lead to pole slipping which in turn causes huge transient torque on the shaft. Shaft fracture, prime mover damage - especially to reciprocating engines, winding movement and wedge loosening are all possible outcomes.

Perhaps a synchronous motor with a would experience a similar effect given a dynamic load? To be honest I've never really considered it before now.

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[motorspert]

The same applies to motors as generators - the initial fault current will cause a transient torque in the rotor. Peak vaues of between 7-10xFLT can occur. On a large drive the mechanical equipment maker will ask the mnotor vendor for this information and check the shaft system. On smaller, more standard drives they will use their experience and design in safety factors.

The biggest torques occur in generators when synchronising out of phase, although we have come across broken gear teeth in a few cases.

 

[LionelHutz]

We had an customer with an issue with the non-reverse rachet in a US Electric motors vertical shaft pump motor. There was some type of electrical event at the site and it destroyed the rachet in the motor and caused damage to the soft-starter controlling it.

 

[ScottyUK]

Hi motorspert,

Not sure if I agree about an out-of-step synch being worse than pole slip. The pole slip typically occurs under heavily loaded conditions with a weak field, so the prime mover is applying an accelerating torque near rated torque. As the pole slip occurs the machine accelerates then decellerates almost instantaneously as it recovers synchronous conditions. I was lead to believe that the torsional stress on the shaft is worse in this situation than an out-of-step synch where the machine is pulled into synch because the grid alone pushing or pulling the machine into synch, without the massive contribution of the prime mover adding to the problem. I'm open to discussion on this - I had it explained to me and the analysis seemed reasonable at the time. It's such an uncommon fault that I haven't studied it in great detail.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[davidbeach]

ScottyUK, as I understand pole slipping (almost entirely from a generator perspective though) is that once the machine starts to slip poles it won't pull back into synchronism, but will just continue to slip more poles, faster, until separated from the line and allowed to recover. I think that the likelihood of slipping a single pole and then resynchronizing on its own is rather remote.

 

[ccjersey]

I think what you describe, "slipping more poles, faster" could create a harmonic condition in the shaft which would be worse than one shock.

 

[ScottyUK]

Thanks David,

I agree with your logic - I've only really discussed this at any length with one or two people who both have a lot of experience in generator behaviour (ex-CEGB research staff). Their opinion was that it was possible to recover given a sufficiently fast and powerful AVR. They have each forgotten more than I know about generators, so I don't often challenge them. Perhaps I should!

Most texts I've read seem to be very light on the pole slip condition, saying little more than 'it can happen under XYZ conditions and it is not a good state to be in'. If you know of any good published studies or research on it I'd appreciate the details. I'm sure GEC, Parsons, the CEGB, etc must have studied this in depth at some point in time and published the results.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[davidbeach]

Pole slipping, as I understand it, isn't something that an AVR can help much with; a really fast governor maybe, but the problem is with the time constants of the prime mover/generator assembly. The good old equal area problem. I've done numeric integration simulations in excel where it nearly didn't slip a pole so that first pole takes a long time to slip, but once it does everything does to heck in a hand basket quite quickly thereafter. Every generator has a critical reclosing time, it that is exceeded you won't resynchronize on reclose but will have to leave the breaker open and go through the full synchronization process.

I guess the situations for motors would be similar, but rather than accelerating during the fault the motor would be decelerating due to the load and the trick would be enough area left under the curve for it to re accelerate without slipping a pole. The higher up the power transfer curve the motor operates, the more likely it would be to pull out of step during a transient loss of voltage.

What does in the shaft is that during pole slipping, the electric machine alternates between motor and generator every 180 electrical degrees, passing through maximum power transfer in both directions. The shaft becomes a torsion spring connecting two energy sources, both fighting each other. Run away, run very fast.

 

[edison123]

pete,

This happened in a rubber mixing unit driven by a DC motor. The slow speed rubber mixer (heavy ss cylinder), running a very low speed (after multiple stages of gearing), broke and simultaneously (as heard by the operator), the DC motor armature flashed over. The on-line event recorder for the motor saw a huge rise in the armature current just prior to trip. The question was (and still is), which came first, the chicken or the egg ?


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[ScottyUK]

Thanks again David! Perhaps it has been my interpretation of what the oldtimers were saying - that it tried to pull back into synch causing the torsion in the shaft as it goes from motoring to generating as you describe, but does not actually pull back into synch. Probably my misunderstanding more than their error.



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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[rmw]

As a mechanical guy reading this electrical stuff, I have to read carefully to try to comprehend it. (If I can)

What I do know is that I have experience with pumps out there where the impellers are threaded onto the pump shaft where any reverse rotation, be it a 'bump check' for rotation in the wrong direction, or any electrical anomaly that would act as such (torsional shaft reactions) the impeller would want to unthread due to either direction or inertia, jacking it into casing parts of the pump where the clearances are already close, and where there is no tolerance for backing off of the threads to any extent.

That would definitely qualify as the load being damaged by just the right kind of an electrical fault.

rmw