VFD Motor - High No-load current

[edison123]

Motor specs - GE 440 V, 100 HP, 169 Amps, 350 RPM, 60 Hz.

Now running by VFD.

No-load volts : 188 V

No-load current : 123 A

No-load frequency : 23 Hz

Why is the no-load current nearly 73% of FLC ? I know that for 23 Hz, the voltage must be 169 V. But will a 11% volt increase change the no-load current so drastically ?

I am investigating why the drive is outputting 188 V isntead of 169 V at 23 Hz.

 

[Skogsgurra]

Hello Edison!

An idling motor has a very low PF and consumes mostly magnetizing current. If FL PF is, say .8 (a common assumption), then magnetizing current is around 60 % of FLA, i.e. 101 A.

Increasing voltage above rated V/Hz usually takes magnetizing current up more than the voltage increase, mainly because there are some saturation effects starting to show. So, I do not think that a 22 A increase from 101 to 123 is surprising at all when you increase voltage to 11 % above rated V/Hz.

My PF assumption is probably very conservative. Your motor seems to be a 20 pole motor and motors with many poles are known to have a rather low PF. So, the current seems to be surprisingly low, actually. Or, shall we say, just right

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[edison123]

Hi Gunnar.

This is a special motor made by GE as part of a vertical machining center with the face-plate fixed at the bottom. The nameplate does not mention the pf. This motor has now come to me for rewind after a winding failure (this is the 3rd such machine) and the above nos. were given by the client as recorded before it failed.

The other two motors I had rewound had only 40% no-load current (around 70 Amps) at 370 V, 50 Hz. So, why would a 40% jump to 73% with just 11% volt rise ?

My question was also about the drive, which doesn't seem to follow the V/Hz dictum. I have asked the client for the drive input parameters and am yet to get a reply.

 

[DickDV]

No-load current for a low-speed motor like this is common. The current value looks about right to me.

As to the higher than expected voltage, the motor nameplate is 440V at 60hz. I would first check to see if someone programmed the drive with 480V at 60hz. That along would explain it.

Once again, it is essential when setting up a drive to provide the correct nameplate voltage, even if it is a bit off standard.

 

[electricpete]

fwiw, it looks high to me.
Nameplate V/hz = 440/60 = 7.33
Actual V/hz = 8.17 (11% above nameplate).

Let's say no load current for this slow speed motor at nameplate voltage and freq is 50% FLA (I don't think it would be higher than that based on review of our similar slow speed motors). That would be 84.5

Linear increase of 11% would put you to 94%.

jraef correctly points out that linearity is not assured. But consider that a slow speed motor is typically designed to run at very LOW flux densities under full load nameplate conditions. That means well below the knee of the magnetization curve. Just a little bit higher flux density at no load. So I wouldn't think 11% more is going to push you far far beyond the knee to cause such a dramatic increase. It would be more credible if we were talking 2-pole motor. That's just my thought.

Just as a matter of curiosity it would be interesting to put a scope on there to see if the waveform shows evidence of dramatic saturation or perhaps harmonics that may be contributing (even though we know most of vfd volt harmonics do not show up in motor current).

Here is one datapoint for slow speed motor. We have 3500hp 13.2kv 324 rpm motor. Full load amps is 153. No load amps (at nameplate voltage and frequency) is 77 - around half.


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[electricpete]

I should clarify, I'm not saying you should be alarmed. Just my best guess like yours is that the current would have been lower under these conditions. But certainly might be off in some of my assumptions and this motor might just act differently.

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[edison123]

Thanks Dick and pete.

I agree that the drive parameters would have to be wrongly fed for that bad V/Hz. After rewind, I will test in my shop at various voltages to see what happens to the no-load current.