Motor surge testing

[Aleman]

I would appreciate any comments on the surge testing of motors. I have never done this myself and the test equipment is not cheap. I am also frustrated with my current test methods
which are a megger and a meter. I have had a few motors fail recently with VFD phase to phase faults. With the megger I could not see any problem with the motor, with both phase to
phase and phase to ground insulation testing. But the drives picked it right up, making the drive essentially a piece of test equipment. But when these situations come up and we cannot
verify a bad motor, we then have to verify the VFD also...it wastes a lot of time. It looks like surge testers run from $2K to $15K or more. The selling point here is putting critical motors
on a periodic testing schedule. Thoughts anyone?

As for the meggers, how high should the reading be to call it good?

Thanks

 

[petersonra]

I would appreciate any comments on the surge testing of motors. I have never done this myself and the test equipment is not cheap. I am also frustrated with my current test methods
which are a megger and a meter. I have had a few motors fail recently with VFD phase to phase faults. With the megger I could not see any problem with the motor, with both phase to
phase and phase to ground insulation testing. But the drives picked it right up, making the drive essentially a piece of test equipment. But when these situations come up and we cannot
verify a bad motor, we then have to verify the VFD also...it wastes a lot of time. It looks like surge testers run from $2K to $15K or more. The selling point here is putting critical motors
on a periodic testing schedule. Thoughts anyone?

As for the meggers, how high should the reading be to call it good?

Thanks

Are you sure the motors are bad and not the wiring or something else?

 

[big john]

We do surge testing as part of our standard motor PM package, and I have definitely had it find failures that a megger could not locate.

For one thing, selecting surge test voltage follows the same rules as a hipot test: 2 x Nameplate + 1kV This allows you to find failures that a lower voltage insulation-resistance test won't uncover, but because the surge is a alternating-current it also places far less stress on service-aged insulation than a hipot test would.

Because the surge test results also depend on the inductance of each winding, it will find failures other than simple ground-faults: Turn-to-turn failures and in some rare cases even severe problems within the rotor.

While a megger isn't a perfect test by any means, I'm a little surprised that it has repeatedly failed to find winding-to-winding failures. You should be testing these motors at a minimum of 1kVDC for 1 minute and should see minimum values of 100M, and a DAR of 1.5-2 or better.

What are you seeing? Also, if this is a common problem, do you have load-reactors on these drives?

 

[Aleman]

Are you sure the motors are bad and not the wiring or something else?

They are bad, but to prove that we had to spin other motors on the same drive and wiring. That's what frustrates me.

 

[Aleman]

We do surge testing as part of our standard motor PM package, and I have definitely had it find failures that a megger could not locate.


While a megger isn't a perfect test by any means, I'm a little surprised that it has repeatedly failed to find winding-to-winding failures. You should be testing these motors at a minimum of 1kVDC for 1 minute and should see minimum values of 100M, and a DAR of 1.5-2 or better.

What are you seeing? Also, if this is a common problem, do you have load-reactors on these drives?

It looks like I am not megging these correctly. Not holding for 1 minute and letting readings below 50M go by. One problem I have here is I really don't trust the megger I have.
The readings seem to be all over the place at times. It is possible it's doing it's job but I am failing to believe what it's telling me. Just the same we have had motors that are above 100M
winding to winding and they still fault the drives.

We don't have many load reactors, there are a few but the new stuff we are building comes straight off the drives. Is it possible we are damaging our motors because of that? I would say
that the recent failures are on 50-80' runs, shielded SO cable in tray.

What are you using for your surge testing?

 

[big john]

...One problem I have here is I really don't trust the megger I have. The readings seem to be all over the place at times....

That's a big red flag. Insulation resistance should plot as a very smooth gradually increasing curve. If you have sharp dips and spikes that's a concern.

You also need to be aware that megging these in a cool damp environment will often absorb condensation and result in much worse readings than if they've been tested warm and dry.

If you're comparing trends, all the readings need to be corrected to a common temperature. IEEE specs 40?C.

What, specifically, are you seeing during your tests questionable?

...We don't have many load reactors, there are a few but the new stuff we are building comes straight off the drives. Is it possible we are damaging our motors because of that...?

I'm not a drive guy, but it seems possible to me that you're getting resonant ringing causing high voltage to puncture the winding insulation. If that is what is happening, a reactor could be the solution.

...What are you using for your surge testing?

We've got Baker Winding Analyzers. They've been bought up by SKF.

 

[Aleman]

That's a big red flag. Insulation resistance should plot as a very smooth gradually increasing curve. If you have sharp dips and spikes that's a concern.

You also need to be aware that megging these in a cool damp environment will often absorb condensation and result in much worse readings than if they've been tested warm and dry.

If you're comparing trends, all the readings need to be corrected to a common temperature. IEEE specs 40?C.

What, specifically, are you seeing during your tests questionable? I'm not a drive guy, but it seems possible to me that you're getting resonant ringing causing high voltage to puncture the winding insulation. If that is what is happening, a reactor could be the solution. We've got Baker Winding Analyzers. They've been bought up by SKF.

It's possible the last motor in question got wet as there was an open hole in the peckerhead and the area gets a lot of water spray. Definitely a damp environment. But when we looked at it, it was dry
inside the peckerhead. I noted that when I held down the test button the initial reading would hold and then start to drop to as low as 20M. I don't know if that was a real reading or not. We just got in a new Fluke
megger and I will look at it again tomorrow with that unit. I have some resistors laying around and am going to make up a dummy jig to try the megger on to see how it does on a straight known resistance.

I did some reading up and it seems that with runs under 100' we probably don't need the load reactors. This is from a Rockwell paper I found.

I saw the Baker units and they are not cheap but look great. But it sounds like the megger might be working ok and the operator (myself) is not.

 

[big john]

...I noted that when I held down the test button the initial reading would hold and then start to drop to as low as 20M....

Increasing current on a constant voltage is a failed test and there's a very good chance you're looking at damaged insulation.

Even on contaminated insulation without a fault I would expect nothing worse than a perfectly flat reading of constant leakage current.

Ideally you want to see a significant rise in insulation resistance with time. Here's a good basic tutorial.

 

[milldrone]

I did some reading up and it seems that with runs under 100' we probably don't need the load reactors. This is from a Rockwell paper I found.

This PDF from Rockwell has the distances based on motor type (insulation resistance) and VFD series as well as motor size and switching frequency. Some of the distances are as short as 25 feet.

http://www.google.com/url?sa=t&rct=...=5e7P8Q4juT82hb-vJztSiA&bvm=bv.86475890,d.cGU

 

[Aleman]

Thanks for the links guys. The megger link I had already found. I hadn't seen this particular Rockwell paper and it does indicate that we should use reactors on our runs.
I did find an article that gave a general rule of using a reactor on runs over 100'. Thought it was Rockwell but I went back and it was from this article which is a generalization on
line/load reactors and not specific to AB drives which we use.

http://ecmweb.com/design/line-reactors-and-vfds

In any case I will keep poking around and am still interested in surge testing. But it looks like my megger was telling me the truth to begin with. What gets me is that I didn't see
any phase to phase readings below 20Mohms, but the drive picked up the error so fast the motor didn't spin at all. Instant fault. Chances are if this motor was on a starter it would
be running fine right now.

 

[Jraef]

As big john mentioned, a megger has more difficulty picking up the subtlety of a turn-to-turn winding insulation failure, and that is exactly the kind of thing you can expect from a VFD operating a motor not designed to be run from a VFD.

As to the distance, the only SAFE distance if you know nothing else is now considered to be 25 feet. Years ago it started out higher, but as the technology inside of VFDs has become smaller/cheaper/faster, the distance at which the standing waves can be created has become shorter and shorter. There are specific mitigation techniques that can be used by VFD mfrs even involving "double secret probation" methods of firing the transistors, which have allowed for longer distances again on some. That document from AB is showing TESTED distances with the specific drives listed and the specific motors / filter combinations used. The reason some are much shorter than others is because as a general rule, the smaller and cheaper the VFD, the less ability the mfr has to employ those "double secret probation" techniques. Small cheap drives all now use what are called "IPMs" (Intelligent Power Modules) that are a prepackaged set of diodes, transistors and firing circuits in a single potted unit made by the power device mfrs. When using those IPMs, the firing controls are already done as part of the package and are optimized for producing as little heat as possible, which helps make them smaller. But the VFD mfr then no longer has the option of tweaking the way the transistors are fired to mitigate the standing wave generation. Combined with the latest MOSFET IGBTs that have a turn-on time that is 1000 times faster than the previous generation, the small cheap drives are actually MORE likely to cause it now. Bottom line, I am recommending to everyone that for 10HP and below, you should be AT LEAST using a load reactor if your circuit length is over 25ft and you are not using VFD shielded cable, regardless of brand.

 

[Aleman]

The drives on the latest failures have all been Allen Bradley 4M's for what it's worth. Going to revisit this today at work. Thanks for the thoughts!

 

[Aleman]

I went back and looked at the motor in question today. Also tried my megger out on a fixed 50 Mohms worth of resistors. Megger worked good and the motor read good,
maxed out the range on the megger on all phases and to ground. Now I believe it just got damp inside and after sitting a couple of weeks it looks good. We are going
to hook it up to the same drive and see if it runs. I bet it will. So now I know what to look for and thanks for the help and suggestions. Not sure if a surge tester is worth
the money for us. It would great to have one but the management is unconvinced.