×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Voltage and current imbalance on VFD output

Voltage and current imbalance on VFD output

Voltage and current imbalance on VFD output

(OP)
We are developing a dynamometer stand for durability testing of a hybrid vehicle driveline, which includes on-board PWM inverter and PM motor. Can anybody recommend a commercially available monitoring system which would detect voltage and current imbalances on inverter output? We can not use any inverter internal signals and must use an external monitoring provisions.

RE: Voltage and current imbalance on VFD output

DC clamps with a grounded shield and usually also some low pass filtering work well for current.

For voltage(s), I use a low pass filter and a differential probe. Set the low pass filter frequency to 1 Hz. That will give you the V/Hz number directly and any change will be readily visible.

NI's LabView is relatively easy to build the monitoring functions in. I'm afraid that is the nearest a standard equipment you can get. Flexible and easy to extend when you need to.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Voltage and current imbalance on VFD output

(OP)
Thank you, Gunnar. This seems to be a good idea to try unless something like Baker Dynamic Motor Monitor exists (which would handle complex waveforms).

RE: Voltage and current imbalance on VFD output

Why not ask Baker for a DMM? smile

Are you interested in the complex waveforms? I try to avoid them when checking motors and drives. But, of course, I need to look at them in detail when checking switching patterns and "modulator", which can be anything these days, mostly SW.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Voltage and current imbalance on VFD output

If you are looking for a hand held instrument, a Fluke 123 scope meter is a nice choice. I have used one for years before losing it to a thief. Not cheap, but they do a good job on VFD outputs.

This is a good reference piece on the subject, albeit slightly biased.
http://assets.fluke.com/AppNotes/ElectricalPower/GO416b_u.pdf


"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: Eng-Tips.com Forum Policies  

RE: Voltage and current imbalance on VFD output

jraef, I also use a Fluke 123 and like it.  Except, it has a tendency to display drive output voltage pulses in pairs.  I know the pulses are not actually paired up like that but I've never found a way to get rid of the double "image".  Have you seen that and do you have a fix?

Also, have you used a current probe with yours?  I don't have one and have often wondered whether it would be worth the investment.

RE: Voltage and current imbalance on VFD output

Dick,
Yes I used a current probe, but not the one from Fluke. A friend of mine is the rep for AEMC and they make most of the current probes sold by everyone else, so I used a flexible one he got for me as a "sample" wink
http://www.aemc.com/products/html/moreinfo.asp?id=2010202&dbname=products
They aren't cheap though, I have to warn you.

Huh, funny that I never noticed (or paid attention to) the double image. Wish I had mine back so I could look at it again sad


"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: Eng-Tips.com Forum Policies  

RE: Voltage and current imbalance on VFD output

A good trick from old days is to convert the three phase signals (current as well as filtered voltage) into a rotating two-phase 90° system and display on the scope as x-y (don't know if moders DSO still support this mode). You need two OPs to do this.

If everything is perfect you will see a circle otherwise quite deviations quite easy to analyse. Many older papers on PWM AC motor control use this kind of display.

RE: Voltage and current imbalance on VFD output

Thinking a little more about the OP, it seems to me that voltage imbalance on the drive output would be extremely unlikely, unless there are serious IGBT failures developing.

Current imbalance is much more likely and probably reflects imbalanced impedances in the leads and motor.

RE: Voltage and current imbalance on VFD output

(OP)
DickDV,

You brought an interesting point. The voltage and current imbalance measurement was a part of customer requirements to provide an early indication of potential e-motor problems during durability testing. We might limit our design to current imbalance monitoring only. I tend to agree that voltage imbalance would indicate inverter rather than motor problems.

RE: Voltage and current imbalance on VFD output

thinker, that is exactly my thought, as well.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources