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Motors Burning Problem 6

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tlona

tlona

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Jun 1, 2010
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In the past year we have have had (2) 150hp 480V motor windings burn up. The motors are on dedicated circuits and each had a few hundred hours run time prior to burning up. The Motors are started by a VFD which is set up as a soft starter that ramps the motor to full speed in about 15 seconds. The motors drive a vertical 40ft multistage pump used to transfer 2oil between tanks. We have found no mechanicial issues with the pumps. The motors are about 400feet away from the VFDs and I do not believe there are load side reactors installed. The installation replaced existing 200HP motors and removed the 2-stage across the line starters. The existing wiring remained and is about 20years old but successfully meggered and was therefore reused. We have almost the same installation on 4 other motors with no problems. Forensics determined both failures were burnt windings and no problem with bearings. I am going to recommend the installation of line reactors and proper grounding to avoid bearing degradation. I am inquiring to see if there are any other recommendations or experience with this issue. Also curious why the VFD overload did not protect the motor. Thank-you
 
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200 hp motors were replaced with 150 hp motors? Perhaps the motors are now undersized for the load. Are the motors rated for inverter duty?

I agree with adding load reactors and grounding. Overload may not have caused the motors to burn. Based on the 400' distance, it's likely cause is short circuit due to winding insulation failure, and the load reactors should help. A motor rewind shop may be able to tell you the root cause.

xnuke
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Thank-you for the response. The application changed so the pumps were changed as well so the pumps and motors came as a package. The motors are rated for inverter duty. As far as overload I was curios why the VFDs did not trip on overload to prevent the damage. The shop is sending us a full report but stating that the windings failed and bearings were fine.
 
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Many things can damage winding insulation, including high voltage or rate of voltage change caused by long cable length with voltage reflections occurring, the likely culprit here without load reactors. Overloads don't see the voltage problem, they detect excessive current that lasts for a period of time to try to prevent insulation damage due to overheating.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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Thank-you for input. I will pursue that direction. Main confusion for me was that the manufacurer's literature only required line side DC chokes for various motor lead lengths. Nothing about installation of load side filters at various lengths and when speaking with them they say that model doesn't need one which I am skeptical.
 
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Have you actually determined if the failure was thermal or an insulation breakdown, or just that the winding failed?

Pursuing solutions to avoid an insulation breakdown when the failure was thermal likely won't be successful.
 
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Understood. We haven't received the detailed report so do not know the answer to that question.
Yes the manual calls out for DC choke
 
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There seems to be a serious mistake. A line side DC choke is an oxymoron, let slone a motor side DC choke. Either it is a translation problem or a DC motor. Still don't see how a DC choke can be applied to the line side. Do you have a reference to the manual? Or at least the drive and motor make and designation?

Gunnar Englund
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Half full - Half empty? I don't mind. It's what in it that counts.
 
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I believe when installed it is downstream of inverter. Ref Schneider electric Altivar ATV61. Model number ATV61HC11N4
 
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No, it is in the DC link. So it is correct that it is a DC choke.

There is a peculiarity with the Altivar of this type; yYou can switch the slip compensation OFF. If there's no slip compensation, you get quite a lot of slip and that heats the motor more than needed. Have you checked that?

The U/f ratio shall be what the motor nameplate says. Divide rated voltage by rated frequency and check that you are close to that value.

Gunnar Englund
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
 
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If someone is claiming a DC bus choke has any effect on the OUTPUT waveform and the creation of standing / reflected waves, they are woefully misinformed. Often times I find these sort of things to be the fantasy musings of sales people, not engineers. A DC bus inductor can have several benefits, but have nothing to do with what takes place AFTER the inverter section. SOME VFD mfrs have developed sophisticated inverter output firing algorithms that can and do AID in REDUCING the effects of reflected wave phenomena, but REDUCING and ELIMINATING are two very different value assessments. I'd venture to say that you are the victim of the sort of hyperbole used by marketing departments. NOBODY can make a credible claim of totally eliminating the risk. I have had several installations where there should NOT have been issues, yet there were. The phenomena are perplexing and dependents on a number of mitigating factors, many of which are beyond your control. We can recommend best practices to MITIGATE the risk as much as possible, but nobody should be claiming ZERO risk, at least not ethically. Does it happen? Yes, unfortunately...

Were it my money, I would dispense with a load reactor as they are something you add when you THINK there MIGHT be an issue and you just want there to be something in place as a precaution. In your case, you now KNOW there is an issue. I would opt instead for a "dv/dt filter", which is a load reactor PLUS a det of filtering components.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
 
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No they weren't selling the line choke but I found it odd in their literature for this particular model there is no guidance on and output filter only an input filter for longer motor leads. Contacted tech support who basically stated if there is no guidance on an output filter then guess you don't need one. The newer model the Altivar 71 manual discusses both a line choke and output dv/dt filter. Thank-you all for support as I pursue the dv/dt filter and also double check all of the drive settings.
 
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If you look in the ATV61 Installation Manual in the Wiring Recommendation Section, you will see a table showing recommendations for different motor cable lengths. For your drive, a "Motor Choke" is recommended for cable lengths more than 100m (328 ft.). Do as jraef recommended and install a dv/dt filter.
 
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Calling it a line side DC choke would depend on the definition of line side. If you define line side as before the capacitor bank or components feeding power into the capacitor bank then the choke would be line side.
 
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Great post here! Thank-you. The Altivar 61 Installation Manual throughout refers to the installation of a line side Choke. Questioned tech support about the table showing recommendations for different motor cable lengths requiring a "Motor Choke" whether they were referring to the line side or load side. They said line side.
 
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