VFD Troubleshooting

GregPLC

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Join Date
Oct 2002
Location
Milwaukee, WI
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I am not an electrician, but like all of us, are sometimes required to be one when troubleshooting a VFD. Does anyone know of a good reference, website, or other learning tool for troubleshooting VFD's at the 101 level with a scope? We have a very nice scope, but not a lot of knowledge applying it in the higher voltage applications for troubleshooting. Where's a good starting point?

TIA,

Greg
 
And isolate the ground lug on the power cord - a the prong to two prong adaptor, otherwise you could blown up your scope
 
I have found that a scope is not very helpful in diagnosing drives. Usually either every leg has an output or it doesn't. You can check this with a good DMM. The signal is so digitized it just looks like garbage on a scope.
 
Forget the scope or any other test equipment on the motor leads.

Use the keypad to diagnose drive output problems. The signals are so fast and complex on the motor leads that very little useful information can be gained from them in the typical field situation.

And, as always, and as mentioned above, watch out for safety issues. And be aware that the DC bus in the drive can store dangerous levels of energy for several minutes after power down.
 
VFDs

The scope isnt needed in the Allen Bradley, Mitz, or Toshiba drives. The installation manual with the maintenance manual are all that are needed with a good DVM. The front panel has many trouble shooting points covered. It will tell you most of what you need. There are over 100 parameters to look at. If you cant get it going, the maintenance manual gives you DVM points to help isolate where the problem is. The Allen Bradley Plus 2 manual shows how to remove parts and test the resistance of the multijunction transistors. It supplies the values that are normal and what is considered "damaged". Hope this helps:) Bruce. Have a good weekend!
 
DickDV said:
Forget the scope or any other test equipment on the motor leads.
......
And, as always, and as mentioned above, watch out for safety issues. And be aware that the DC bus in the drive can store dangerous levels of energy for several minutes after power down.

Dick! I'm suprised!

One of the most valuable quick-checks of VFD's is to use a decent clamp-on ammeter just to verify that the output phase currents are balanced. True, has to be a decent clamp on, but as part of a general check (at PM times), output imbalances can be pretty good predictors of upcoming failures.

Monitoring input current's is also a wise idea, but not quite as good a predictor.

On another note, Fluke has a very nice new meter out, the 87-V series, that works very well, even with PWM AC Drives... It can measure both RMS AC output, no matter what the PWM waveform is, as well as the effective sinusoidal frequency. Not very useful to diagnose a drive failure, but useful for general information.
 
rdrast, my comment is based on over 15 years experience watching field test equipment give false readings and field people misintrepret the reading when they get some. This is not to say that there is any problem with the field people themselves.

While your point about measuring current balance in the motor leads (error and all!) is valid, I find that so few problems develop that way that I had forgotten it.

My main point is, for most field troubleshooting situations, use the drive keypad and stay off the drive output leads. A good digital drive can tell you so much about itself and the motor that I find little use for any test equipment except for those ugly cases where the keypad doesn't power up.

Now, having said that, it is also probably significant that I rarely encounter failed drives and do very little work with drives that have actually failed. Almost all of my field work is related to application problems and the approach as well as the tools are different for that kind of work.

That's where I'm coming from when I made that statement.
 
Understood Dick, My comments come from working in a split company. I'm in applications engineering, but we also have about a dozen field service engineers that do nothing but troubleshoot and repair problem drives, as well as regular contract PM's at many sites.

The input/output 'under load' current tests have consistently been an accurate predictor of a soon-to-show up problem in a drive. (The most accurate predictor of a soon-to-show up problem anywhere is a decent thermal imaging scan).
 
I agree. The only time I used a scope on a VFD was to check the noise on the incoming power lines. It ended up showing the 3 phase voltage was not consistent which fed an older allen bradley drive.

The drive was stoping and giving strange errors. The 3phase 220 volt (not 208V and not 240V) feeding the drive was bizarre. The plant power was delta with no neutral. It feed a step down transformer that was delta/wye. The secondardy wye was not bonded what-so-ever as the drive did not need a neutral. So voltage was all over the place feeding that poor drive.

A month later after above was repaired, had the same symptoms on same drive. Took compressed air/elbow grease and cleaned massive amounts of dirt/junk off of heat sinks.

Drive still works today/never gives errors/is cleaned regularly. Also tightened every single terminal.

Could have just used a voltage meter rather than scope.

Johnny
 
Thanks for the input. I'm very aware of the existing data from the drive keypad, DMM, and other basic check techniques. I'm rather good with a scope and have lots of experience with one from my electronics days. Just thought I could do some further tests that were usefull with our scope. I guess not so much extreme faults, but troubleshooting those ocassional unexplained trips, more advanced tuning....a little beyond the basic stuff.


TIA,

Greg
 
VFD troubles

I have to agree with Dick, in the long run, 'learn' what the VFD Drive Display is telling you. I also agree a 'quick' Amp-Probe check of the outputs won't hurt anything. AB & Toshiba's displays are great in trouble shQQting.
 

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