Baker Testing of Motors - Destructive or Non Destructive?? 3

[gokulkrish2]

I searched for this topic on this forum and did find some topics discussing this. But none answered me straight if Baker testing is destructive or non destructive.

I do not have a problem with coil resistance test and Meg ohm test. I know they are not destructive as the voltage level do not exceed the rated voltage level.

But i think Hi pot and surge tests are destuctive even though Baker representative is trying to convince me that it is not. Hence i want to get some advice from pros like you people to guide me.

The situation in my company right now is that we are conducting an experiment which requires the motor to run at extreme overload levels. Atleast hoover around 1.15-1.25 times the rated. My maintenance and the Baker repres are suggesting that we do Baker tests every week on this particular motor to make sure if its failing on its insulation. I seem to have a problem with it on the high pot and surge testing.

As i mentioned we are already overloading this motor and so i do not want to introducing twice the rated voltage on its winding by our testing.

What do you guys think?

What is the right thing here to do??

 

[jraef]

SKF Baker makes a lot of very nice testing and monitoring equipment, I'm afraid "Baker testing" is a bit to vague for me to understand what you think might be destructive.

And if you ask me, I'd think you have a decent candidate for their on-line monitoring system. It is definitely non-destructive and is based primarily on monitoring multiple (100+) operating conditions to look for trends that can be used in predictive failure analysis. I've seen it in action, I was impressed.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
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[KllrWolf]

When you talk about running higher than rated, are you meaning over the service factor, or just the standard rating?

As a motor manufacturer, we do all those tests on every motor we build or rewind. Not sure how the baker test will help with checking for insulation degredation over a meg test.

Not sure if I would have the hi-pot test performed every time unless you are looking for borderline insulation.

Personally, I believe that the megger test would be sufficient if all you are looking for is insulation degredation. make sure to monitor the voltage and current as well. Keeping good records of all three will let you see what is normal and what is not.

Final thought, wouldn't just buying a larger motor be less expensive in the long run?

 

[electricpete]

But i think Hi pot and surge tests are destuctive even though Baker representative is trying to convince me that it is not

Yes, those tests are "potentially destructive". That means that your motor which appears to be working satisfactorily right now may not be working upon completion of the test. Therefore, you want to ensure you have suitable contingency plans available prior to performing the test (and don't do the test if you don't have acceptable contingencies for failure during test).

That doesn't mean these are necessarily bad tests. In many applications it is far preferable to fail during test than during service, so we would prefer to "fail" a marginal motor during a test, since it can buy us confidence that the passed motors will operate reliably.

It certainly is more probing than a megger test. There are plenty of motors that will pass megger and fail hi-pot...I've seen it happen many times. If you do a dc step test, the current is plotted vs voltage so you can check for non-linearity. This provides additional diagnostic info... motor may be judged bad or marginal from non-linear curve even if it doesn't "blow" during the test. Additionally, it MAY be possible to stop the test before damaging the winding based on plotting this non-linearity (not a guarantee).

Should only be done by qualified people. If you do the test you'll want to pay attention to the test voltage (lower levels are used for field testing of aged motors compared to testing of new or rewound motors.) You'll also want to pay attention to winding conditions and test conditions... not to be done on high humidity or wet windings - check IR/PI first. Not a great idea on dirty windings (if motor is in a shop, clean and bake before hi-pot).

I'm sure there's lots more that can be said. It is somewhat controversial subject with different approaches depending on preferences of the machine owners.

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[electricpete]

There is also a lot of philosophical thought you can put into surge testing.... will the machine see many surges during operation? If it is well protected during operation (based on system design, possibly including arresters), then that may lessen the desire to perform surge testing. (although some would argue it will detect mechanical degradation of turn insulation.... sometimes true).

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[edison123]

Surge testing is non-destructive in that an interturn fault may be initiated at certain high voltage level but it does not permanently puncture the turn insulation (since the kit does not have the energy to puncture the fault). When you reduce the test voltage level, you will see the turn fault goes away.

Muthu

http://www.edison.co.in

 

[electricpete]

Well, I guess there is some controversy regarding the conclusion of whether surge test is potentially destructive. Three references below...

Table 1—Maintenance tests...
Test: Surge Test
Test Precautions/Limitations: Potentially destructive should only be undertaken after integrity of insulation quality of groundwall has been verified. Use voltages according to industry standards or manufacturer’s recommendations.

12.12 STATOR SURGE COMPARISON TEST
None of the tests discussed above directly measure the integrity of the turn insulation in
form-wound or random-wound stator windings. The stator voltage surge test does this by applying
a relatively high voltage surge between the turns. This test is a hipot test for the turn
insulation and may fail the insulation, requiring a repair, coil replacement, or rewind................

If a turn puncture has occurred, it is not acceptable to ignore it and return the stator to service.
Once the first significant surge occurs in service, the punctured turn insulation will
break down again, allowing power-frequency currents to flow, rapidly leading to groundwall
failure.

Early Intervention - IMPULSE TESTING and TURN INSULATION DETERIORATION in electric motors, Frey, Weidgenburg, Wilson, IEEE IAS 2004]
Conclusion
Very extensive impulse testing of a 5-hp motor, followed
by multiple starts and stops under normal line conditions,
lead to the following conclusions:....

2) Weakened insulation, tested at voltage levels above
breakdown showed no additional signs of deterioration,
even after extreme repetitions of discharges in
the weak insulation. We were not able to deteriorate
weak insulation any further.
3) The weakened insulation could not be detected by dc
resistance readings, IR (megger) tests, or hipot measurements.
The only reliable test showing the weakness
of the insulation prior to a turn-turn short was
the impulse test.
4) The motor with the weak insulation was very capable
to run steady state. It also withstood multiple starts
and stops. After dozens of starts, one caused the motor
to have a turn-turn short.
5) It took less than 1 min of having a turn-turn short
and running under a normal load condition for the
motor to release large quantities of smoke, which
was caused by the internal currents in the shorted
loops. After opening the stator, we saw that 40 s of
turn-turn operation sufficed to carbonize the insulation
of one phase.

Electricpete’s comments: The first 2 references above (IEEE 1415 and Electrical Insulation for Rotating Machines) identify surge test as potentially destructive. The third reference above identifies that it is not. That conclusion is supported based on testing of ONE motor... which did in fact fail some number of starts after retrurned to service (the first two don't cite any testing). The last is also published published in an IEEE peer-reviewed vendor, but writtten by three authors employed by Baker. (I’m sure you could single out commercial biases in authors of the 1st two if you looked also).

I don’t make any claims to know a true answer. I do know that in all industry meetings I have gone to (EPRI LEMUG, Iris Motor Training, Jarsco Motor Training), surge test has always been considered potentially destructive. The only people I have ever heard suggestit was not destructive is Baker and PJ.

From a physics standpoint, the surge tester involves a capacitor which discharges into the winding. I would think that can provide enough energy to produce a carbon track. I have never heard of an insulation breakdown that does not produce a carbon track. It is easy to imagine that carbon track may not be enough to bridge the insulation under the very small normal operating volts per turn stress... but certainly I’d think it lowers the breakdown voltage for subsequent surges. If motor does not meet standard breakdown voltage, I have a hard time believing any motor owner would allow to put the motor back in service in any critical application (we certainly would not). If motor owner is not willing to accept the option to put the machine back in service than usually rewind or at least repair activities are imminent. I think caution is in order for such test.

Muthu if you have experiences to share on that question, I’d be interested to hear.


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[edison123]

pete

I normally test overhauled machines at 1.5 times rated voltage. In some cases, I have found that the surge test shows a fault at higher voltages but the fault goes away at lower voltages. This happens even if I repeat the test. If the higher voltage voltage had punctured the fault, then I should be seeing the fault at lower voltages also.

What do I do ? I strongly recommend a rewind in case of such faults (even if it goes away at lower voltages). Most of my clients will heed my advice. Others, due to various reasons, will take the risk and those machines have run for sufficiently long time. (At least they didn't fail immediately after overhaul - much to my relief ).

So based on these practical experiences, I would say surge test is not a potentially destructive test.


Muthu

http://www.edison.co.in

 

[electricpete]

Thanks Muthu, good info. lps.

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[gokulkrish2]

All,

Thank you so much for very useful insight you all have provided in this topic.

Based on all the reply's i am intending to incline that Hipot and Surge testing is the end user's option. Having said that, on my particular situation, dont you all agree that having the hipot test and surge test done on this motor is like asking for trouble.

Keep in mind all we are doing to this motor is to overload it till its SF level. Maybe because its a crusher load, sometimes the current peaks beyond the SF level but our set point is at the SF level. We are continously monitoring the RTD'S and they seem to hoover around 230F which is below 54F on our trip point.

@Jraef: Thanks for suggesting the online monitoring system. Good to know that tools exists if need be. By baker testing i mean Hipot and surge testing...i know coil resistance and meg ohm are non destructive...

@Klrrwolf: I meant running over the SF ratings. It is a SF motor rated at 1.15 but my Multilin Pickup is set at 1.25 times. And i completely agree with you. Buying a bigger motor would have been the best way to go.. but these are the things which happen when you have a mechanical superintendant over the maintenance department who thinks he know Electrical Quite Well..

@Electripete: Thank you for the valueable references.. and a straight answer.

@Muthu: It makes complete sense to do all these tests for acceptance testing purposes.... but to do this every week in my book is just asking for trouble... Hope you will agree with this...

 

[Motorwinder]

DC Hi-pot and Baker surge test (DC also) is non-destructive.

AC hi-pot is highly destructive.

At least for my 40 years of motor winding. We go double the voltage +1000V x 1.6.

 

[edison123]

gokul

These tests (DC Hipot and surge) are not part of routine checks for machine windings. They are done during overhauls and rewinds. If someone is insisting on these tests on a weekly/monthly basis, tell them they are stupid.

IR check is more than enough.

Muthu

http://www.edison.co.in

 

[electricpete]

I would suggest DC hi-pot is potentially destructive. i.e. a motor that was running in service may no longer be runnable after you complete the test at industry standard levels in accordance with IEEE95.

Example:
We have had 13.2 kv machine operating fine.
Did IR / PI – fine.
Did DC step voltage test to 30kvdc, winding failed at 28kvdc.
No longer passed IR... something like 1 megaohms to ground.
The machine had something like 10 half-lapped layers of mica flake tape insulation with mylar binder.
We were able to find the location of the failure just outside the slot and unwind the tape at that location. The black track travelled circuumferentially along the tape back and forth between layers (not really through the tape... but still caused a failure).

I know this is not a unique experience. I’m pretty sure there are multiple references that will support the fact that dc hi-pot is potentially destructive.

DC Hi-pot...... is non-destructive.

Can you clarify what you mean by that?
Maybe you mean a winding “in good condition” will not be destroyed, and if the winding fails it deserved to fail?


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[electricpete]

AC hi-pot is highly destructive.

I agree ac hi-pot can do more damage, simply because it must supply a high current to the motor capacitance to ground, so current trip cannot be below a few hundred microamps.

In contrast dc step voltage test set trip setpoint can be down around 100 microamps. That limits the amount of damage, but still enough to render the machine unrunnable as in the example above.


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[electricpete]

My maintenance and the Baker repres are suggesting that we do Baker tests every week on this particular motor to make sure if its failing on its insulation. I seem to have a problem with it on the high pot and surge testing.

@Muthu: It makes complete sense to do all these tests for acceptance testing purposes.... but to do this every week in my book is just asking for trouble... Hope you will agree with this...

These tests (DC Hipot and surge) are not part of routine checks for machine windings. They are done during overhauls and rewinds. If someone is insisting on these tests on a weekly/monthly basis, tell them they are stupid

We have a very few motors that we choose to do in-place periodic dc hi-pot to levels near the bottom of the maintenance range identified by IEEE. I missed the weekly part. Our interval is in the range 2.5 – 6 years. I agree weekly surge test / hi-pot would seem... stupid.


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[Motorwinder]

Maybe I should've said "not as destructive as AC" I've had grounds while rewinding. DC leaves a tiny arc path, where AC leaves a big hole. But there's still an arc path, so that would be destructive.

Your µA are going to shoot up before it breaks down though.

Baker does demos, where they carry a small stator around to show their tests are not destructive.

The main reason weekly tests would be, to see if you're getting some breakdown turn to turn or phase to phase for trending.

You engies work it out