Standard for Defining Shaft Voltage of Voltage Source VFD Motors 1

[jimmy2times]

Is there a standard (either IEC or NEMA / IEEE ) that defines the maximum shaft voltage when fed from non-sinusoidal supplies as when motor supplied by VSI drive?

I have looked at IEC 60034-25, AC electrical machines used in power drive systems - Application guide. This doesn't actually define a limit for what is/isn't acceptable. I don't have access to NEMA MG-1 but articles I've read on line suggest this is lacking information in this regard and should be updated (not sure if that has happened).

So I would be interested to know if anyone is aware if a standard defines this?

I had AEGIS rep in to attend our ship where we have some fire pumps (150kW) and we have two side by side feeding into a sea water fire ring main, one is VFD motor, other is DoL. Motors identical.

Attached are the shaft voltages of both.

Prior to measurements being taken I asked what would be acceptable voltage. AEGIS rep advised 8Vpk-pk being an upper limit as a guideline.

So we measured 0.1V for the DoL drive and varying on the VFD drive, think the highest seen was 9.2V pk-pk briefly but I didn't get to capture that as a photo of his meter. Attached shows it being 7.8V.

He demonstrated the application of the shaft grounding microfiber device by dabbing it on the shaft, this seemed to reduce voltage by 60-70%.

AEGIS agent suggested it wasn't probably an issue but caveated that it was only a snap shot in time (couple of minutes taking measurements) and for low installation cost of the ring, he would recommend fitting one regardless.

Reading another part of IEC standard 60034 (part 17 Cage induction motors when fed from converters—Application guide) it discusses shaft voltage of 500mV being an upper limit for sinusoidal voltage fed motors. It then goes on to discuss issues related to Voltage Source VFDs but doesn't set a shaft voltage limit as such, but seems to infer it should be below 500mV....extract here

"Depending on the switching frequency, on the rise time of the pulse and on the motor
rating, the shaft voltage contains peaks of high frequency, possibly in excess of 10 V
which may cause the puncture of the lubrication film of the bearings. Tests showed that
the current flow may be sustained by the components of operating frequency and its third
harmonic, even when the amplitude of the low frequency shaft voltage is less than 500 mV
(peak). Therefore, a motor with low shaft voltage at sinusoidal supply is advantageous to
avoid circulating currents at converter supply."

 

[jimmy2times]

sorry forgot to include DoL measurement, see attached.

 

[jraef]

I don't know of any standards as to what is "acceptable", partly because it's difficult to define that term in the first place. One of our members, Skogsgura (Gunnar) has a very detailed paper on this subject, it's something he has studied extensively. Unfortunately he no longer puts the full detailed version on his website, but I'm sure if you ask him, he will share it with you. He has a shortened version of it here:

http://www.gke.org/index.php?page=bearing-predicor

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Skogsgurra]

Yes, Jeff. I haven't had time (read: lack knowledge) to do anything about the site for quite a while.
A rather elaborated article on the subject is here:

https://issuu.com/rivista_ab/docs/ee2_2017

Re the measurements: The 500 mV limit is an old recommendation from long before VFD:s came into use. It is mainly for turbo generators (long rotor and running at 3000 or 3600 RPM). It was also supposed to be measured with a moving iron voltmeter.

You cannot use an instrument like that to measure fast signals in the us range. Also, you should use 2 or 5 us/div to study those voltages with a scope. Milliseconds will not say much.

Gunnar Englund

http://www.gke.org

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

 

[jimmy2times]

Thanks Jeff/Gunnar