# Trouble shooting modern drives



## oliquir (Jan 13, 2011)

only power ic and capacitors are changeable in most vfds, all other parts are only repairable by manufacturer or drive specialist or just trown away.


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## MDShunk (Jan 7, 2007)

Couple thoughts.... VFD's are self diagnostic. I tend to believe the fault codes. It's so simple now. The fault code is a symptom of something messed up in the field, not in the VFD. Also, (although some may disagree), VFD's are disposable. When they fail, pitch them. Yes, even a 17,000 dollar 150 horse drive. Costs too much for a certified repair and it's going to be untrustworthy after that anyhow.


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## Cow (Jan 16, 2008)

My only recommendation would be to keep spare fans in stock for all the critical drives and blow all of them out every once in a while with a clean DRY air supply. I don't see the need for any specialized tools/meters. Like mentioned above, modern drives have pretty good fault menus to give you an indicator of any issues you may have.

If Jraef sees this thread, he might have some tips for you.


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## chrisfnl (Sep 13, 2010)

MDShunk said:


> Couple thoughts.... VFD's are self diagnostic. I tend to believe the fault codes. It's so simple now. The fault code is a symptom of something messed up in the field, not in the VFD. Also, (although some may disagree), VFD's are disposable. When they fail, pitch them. Yes, even a 17,000 dollar 150 horse drive. Costs too much for a certified repair and it's going to be untrustworthy after that anyhow.


150hp? That's cute

I should have been more specific, our larger drives are 4.9MW.

We've got loads of smaller drives for fans and pumps, I'm not worried about them, they're all redundant anyway.

I might be over thinking, I'm used to SCR drives from the 80s, same scale, just much older technology. 

The DCS seems like the only part I really need to worry about, the drives themselves seem to all replaced as complete modules.

I hadn't thought of the fans though, that was an excellent suggestion.


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## JRaef (Mar 23, 2009)

So you undoubtedly have Medium Voltage drives then. In that world, there are multiple types of technologies used, some of which are simpler than others. It might behoove you to learn and understand which topology your drives use.

Two main topology types: Voltage Source Inverters (VSI) and Current Source Inverters (CSI).

VSI has multiple sub-types: 2 level, 3 level, 3 level Neutral Point Clamped (NPC) and 3 level Cascaded H Bridge (CHB). With the CHB, the MV output is comprised of low voltage (usually 600V) power cells that are put together in series strings to create the MV output. Servicing those is a little easier because the entire power module can be removed and bypassed, then taken to the bench (although they often weight over 150#) where everything you work on is LV. But if you have one of the other topologies, that will not be true, so understanding what you have means understanding your risks and the tools you will need.

CSI inverters have 2 types, only one of which is still used, an SGCT PWM inverter. They are much simpler from a repair and troubleshooting standpoint and replacing power modules is a simple 5 minute process of pulling out a card with the power device on it using an open ended wrench and a Phillips screwdriver.

Because even the worst type is still very reliable, it's possible that you have older drives that may not provide the same full diagnostics that newer drives do, so you may need additional troubleshooting skills. Just be aware that in MOST cases, you will be REMOVING anything MV related to work on it, NOTHING can be worked on while energized!


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## KennyW (Aug 31, 2013)

A 4 channel scope with current clamps and recording ability can be handy. Sometimes when a drive is tripping, being able to log current on all three phases at high speed can really help diagnose problems. You can also use it to watch encoder channel waveforms, etc. It's one of those things, you don't need it often, but when you need it, its damned handy.


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