Saw tooth UPS output

[raithrovers1]

I have a colleague who is working on a three phase UPS (MGE Comet). He attached his scope to the output and he is getting 480VAC phase to phase and a perfect sine wave which is as expected. He is reporting that when he goes from phase to neutral he gets a 277V saw-tooth. I do not have any pictures from his scope to post yet. This makes no sense to me as a PWM Inverter produces a PWM square wave which is then filtered to create a sine wave. If the sine wave is there for phase to phase then the PWM section of the UPS is working correctly. Any ideas?????

UPS Service Manager

http://www.nolanpower.com

 

[Skogsgurra]

The voltage drop across the diodes in the mains rectifier adds to the sine. There is not much rationale doing that measurement. Especially not if you think that what you see is a problem. It is not.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

Gunnar, wasn't there a very similar question roughly a year ago? I've tried searching for it but can't find it. Perhaps my recollection is incorrect.

 

[raithrovers1]

VE1BLL

You have just confirmed that I am definately going senile! I just checked my threads and I did already post this problem last year. Everything blends into a blurr after a week or so!!!!!!

UPS Service Manager

http://www.nolanpower.com

 

[VEBill]

Ah, here it is (from August 2012):

http://www.eng-tips.com/viewthread.cfm?qid=328620

 

[IRstuff]

Sounds like problem not resolved. Did the manufacturer come back with anything? Is your cohort using a differential probe; I'm guessing he should be, but...

TTFN
faq731-376

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[Skogsgurra]

OK, remember that now. I misinterpreted the question this time. Thought you were talking about L-GND, but you say L-N.
So, it is probably third tone injection this time also. No need to worry, I would say.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[raithrovers1]

IRstuff: Yes he should be using a 10x probe as that is the equipment that we gave him! Also- manufacturer very unhelpful as usual.
Gunnar: I was referring to L-GND as the output of this UPS should have the neutral grounded.

UPS Service Manager

http://www.nolanpower.com

 

[IRstuff]

I'm not sure that you've allayed my question. Is he definitely using a differential-mode probe set? Or is he doing a Ch2 - Ch1? If the latter, where did he put the grounding clip?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Skogsgurra]

Obviously a 10:1 probe with ground clip to GND. No diff-probe and no Y1-Y2. The 480 V measurement sounds a bit dangerous without a diff-probe. But if battery-driven scope - OK.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

Nothing is obvious, nor should assumed. In my dumber days, it was "obvious" that chassis was ground, and connecting that chassis to scope ground nearly took out my eye from the exploding part.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Skogsgurra]

he should be using a 10x probe as that is the equipment that we gave him

That is obvious, isn't it?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[IRstuff]

But, what kind of 10x probe. And even then, it's "should" not "is"

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[Skogsgurra]

Splitting hairs? Do we need to?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.