VFD Blowing IGBTs

[Drivesrock]

Hi all.

Anyone got any experience with a motor causing the IGBTs of a drive to go short-circuit? I have a 690V, 3000Amp drive and approx 1800Amp motor on a propulsion application. Drive to Motor is within 5metres of each other.
I can't tell if it's the IGBT or it's internal anti-parallel diode but in frequency mode and just 1% reference the current goes from normal to over 2500A on the display (and its audible) after 3 seconds from start with a 10 second ramp. I had limited the current to 50% so the drive should have tried to limit. Checking the drive and an IGBT module is short-circuit. Again and its getting expensive!
Without the motor connected and no problems with the drive running with noting connected - after replacing the bad IGBT ofcourse.
I've had problems in the past with dynamic braking units blowing up due to highly inductive resistors (thick wire wound ones) until I fitted fast acting anti-parallel diodes across them.
I wonder if a motor fault can keep killing my drive. It has been running for 2 years.

 

[Skogsgurra]

With these sizes. It is a nuisance to keep changing things until it works again.

You need to measure what is happeneing instead. And then, it is not a voltmeter and a clamp you shall use, but a fast differential probe (or isolation amplifier) and a CT with voltage output plus a fast oscilloscop with enogh channels.

What you need to measure is Motor voltage and motor current. Remember that it takes some experience to measure the true voltage. There are edges with rise times in the 200 ns (0.2 us) order of magnitude and with peaks reaching up to 2.5 kV. So you need someone with the right equipment and the right experience to do the measurements. Sometimes, the drive manufacturer can help out, but not always. The motor manufacturer seldom understands what is needed to measure and how to do it. I have seen what they do and it is best described as wrong, wrong and wrong.

See attached recording for an example.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

BTW. If your boost isn't correctly set, you will have lots of current if in f-mode. The current limit should have acted, though.

Your problem might be in the current transducer or its auxiliary circuitry. At these low frequencies - 1 % of rated means half-period times in the seconds range - it can sometimes happen that the power supply cannot cope (not designed to deliver voltage for a second without ripple) and that will make Iact look very bad and produce the result you describe. And you will definitely hear the ripple in the motor current. Like you said you do.

What happens in other modes? Ever tried sensorless vector mode?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

Sorry. Need to come back for just one more thing:

Did this happen after three years without problems? Then it is well worth checking all components in the current measurement part of the inverter. Capacitors, and resistors are known to go bad sooner or later in some makes of large drives.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Drivesrock]

Hi Gunnar.

In sensorless vector the motor mag current is 800A and as soon as the drive starts it imediatley trips with a current inbalance fault and I see the current go through the roof. So I put the drive in Frequency mode.

There is more history to this drive.
10 months ago this problem started and the manufacturer performed all the tests you suggested and didn't catch anything. A motor repair shop (in situ) measured, measured, measured and found nothing untoward. After changing everything easily changeable - and I understand every single card, control cable and IGBT module with the CTs - the faults persisted and then stopped after changing the output reactors. That was after 4 months.
Two months ago, there was one IGBT short-circuit fault. Changed and OK 'till now.
There is another identical drive of this type onboard. No issues.

So I suspect that the motor is doing something. I hear from 2 sources of similar experiences of a motor condition that causes a transient that the drives can't cope with. Different manufacturers drives.

Is this possible?

 

[Skogsgurra]

Yes. Just about anything is possible. But such hearsays are mostly anecdotal or misinterpreted. You really need to have someone do the measurements. Guesssing will not solve the problem.

Or do the measurements yourself. It is not difficult. You just have to do everything right if you want answers and insights that you can work from.

I can give you directions if you mail me. Put 'EngTips' and 'DrivesRock' in the subject line.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Phaneas]

At least you should measure if you're 3 Motor Currents a symetric and if the current you see on your display has something to do with the current you measure.
Still wondering where all the current should come from if running in frequency mode and having no load on the motor. Or do you run the motor always with load?

 

[Drivesrock]

The motor is still coupled. If I leave the drive running at 1% the current shoots up to over 2500Amps after 3s and the drive makes plenty of vibration noise. This is not normal for this drive or any other that I work with - in Frequency Mode. If I leave it long enough it trips on current inbalance or overcurrent - which then means the IGBT has failed.
As the drive O/P current is generated by PWM a clip on ammeter won't measure properly.
I think that the current is flowing through a short-circuit IGBT from the DC+ to - and/or its partner in series for that leg of the phase.

 

[Phaneas]

If you have 1800A motor current I guess you are using 2 or 3 cables in parallel per motor phase. If you swapped cables after reconnecting you could have shorted out two phases.

 

[jraef]

This may be simplistic, but sometimes it's the simple things that bite us. I had a similar situation. Motor was checked, rechecked, checked again by the motor shop that had supplied it. Drive was rebuilt and failed again. After a repair shop rebuilt it a second time in situ, I went to do the commissioning. The motor shop had meggered the motor at the peckerhead, not at the VFD lead ends. I meggered at the lead ends and saw major leakage. I found that the motor lead cables went through a low point and the conduits were filled with water (no drain fitting, someone sealed the threads with plumber's dope). The water build-up may have been from years of condensation because nobody admitted to knowledge of any event that could have cause it, but nonetheless the cable insulation leakage was bad enough to allow dI/dt damage the IGBTs. It may have been leaking for years until it got bad enough and completely failed, they reported having a seen few unbalance trips every year in winter, didn't think it was relevant.

"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376