Author Topic: Using an Oscilloscope diagnosing motor problems  (Read 590 times)

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Offline sparkyinakTopic starter

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Using an Oscilloscope diagnosing motor problems
« on: November 19, 2024, 08:05:40 pm »
I am looking for some assistance with using my scope for diagnosing electric motors as another tool in my bag sort of speak. Scopes are not normally considered as a field expedient tool however cost and size for scopes have come way down so the industry is on the cusp of new ways of doing things. The downside is there is not a whole lot of information out there.

I did find a couple of articles that discuss using a scope's Fast Fourier Transform (FFT) setting as a spectrum analyzer to measure background noise. However, the articles do not provide any useful information like recommended settings or have very little on what one should or should not see on their scope. Does anyone have some working experience using scopes on electric motors for detail analysis?

I am currently dealing with an intermittent problem with a large 200hp motor that I am pretty sure is bad and will likely need rewinding like 99% certain mainly through the process of elimination. The problem is the customer has been burned by another contractor for $15,000 dollars already and a full rewind will be about $20,000 already.     
"Good Judgement comes from experience. Experience comes from bad judgement..."-
 

Offline ptluis

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Re: Using an Oscilloscope diagnosing motor problems
« Reply #1 on: November 20, 2024, 12:19:38 am »
I am looking for some assistance with using my scope for diagnosing electric motors as another tool in my bag sort of speak. Scopes are not normally considered as a field expedient tool however cost and size for scopes have come way down so the industry is on the cusp of new ways of doing things. The downside is there is not a whole lot of information out there.

I did find a couple of articles that discuss using a scope's Fast Fourier Transform (FFT) setting as a spectrum analyzer to measure background noise. However, the articles do not provide any useful information like recommended settings or have very little on what one should or should not see on their scope. Does anyone have some working experience using scopes on electric motors for detail analysis?

I am currently dealing with an intermittent problem with a large 200hp motor that I am pretty sure is bad and will likely need rewinding like 99% certain mainly through the process of elimination. The problem is the customer has been burned by another contractor for $15,000 dollars already and a full rewind will be about $20,000 already.     

why don't you capture more cycles to check if it's a random or sequential problem?
 

Offline Brad O

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Re: Using an Oscilloscope diagnosing motor problems
« Reply #2 on: November 20, 2024, 04:29:09 pm »
Tektronix has a document on motor measurements with a scope (attached). It's written for an on-scope software option, but the connection diagrams and measurement descriptions may be helpful for you. It's fundamentally possible to make all these measurements without the software option.
What stands out to me is that your issue starts on a rising phase then appears on the falling phase before showing up briefly on the next rising phase. That is not what I would expect if a short were occurring, but I'm not a motor expert. Are the time differences between the error appearing on the different channels consistent? Or how consistent are the waveforms across multiple stalls? I also notice after the issue appears your voltage low level is higher than before, possibly indicating some extra impedance has appeared?
 
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Offline --Oz--

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Re: Using an Oscilloscope diagnosing motor problems
« Reply #3 on: November 25, 2024, 04:44:54 am »
Scope current probes give some good info.

Camera IR guns with display can be very helpful (not scope related).

Give more details on "intermittent" issue with this motor.
 


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