Three Phase Motor Voltage Measurment.

[broc028]

I have a three phase motor which is running a conveyor. I want to measure the phase voltage. When I measure between phase and ground the reading varies between 75V and 160V. Is this normal??

 

[waross]

It is not good. It implies an ungrounded system which is not a good idea. But, for an ungrounded system it is not unusual. If you have an impedance grounded system it may be an indication of trouble developing.

Bill
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"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Yes, if you are measuring to ground then you need to know the system configuration and what to expect.

If you want to measure phase voltage then measure phase to phase.

 

[controlsdude]

Phase to Phase = phaseToGround * square root of 3
example
480vac = 277vac *sqrt3

This is what your asking right?

If your measuring phase to ground on each phase there should only be slight difference of volts due to unbalanced loads or tap differences.

yea something is wrong

check your grounding from the main panel to the motor, are all wires tight under terminals, etc

 

[Zogzog]

"Phase to Phase = phaseToGround * square root of 3
example
480vac = 277vac *sqrt3"

in a grounded system, yes, that is true, but as waross mentioned his readings imply an ungrounded system.

 

[broc028]

Thanks guys.

I have had another look at it, and just to clarify in more detail.

When I measure between any one of the phases and ground at the motor terminals, I get a varying voltage.

The phase to phase voltage is a steady value. There is a VFD on the motor, but the frequency is not varying.

Does this still all point to a grounding problem?

 

[davidbeach]

Use the display on the VFD and forget your meter, it can't deal with the voltage waveform from the VFD to the motor. Next time it would also be nice if all relevant information were included, like you are trying to measure downstream of a VFD as none of the prior comments apply to that condition.

 

[broc028]

Sorry, wasnt aware that VFD affected voltage waveform.

I have a true power meter and I want to also measure kW and PF.I presume I should also forget about that because of the VFD?

 

[davidbeach]

Yep, use the VFD display.

 

[itsmoked]

Yep fahgettabout your handheld.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Although a true RMS meter should, by definition, be able to measure the VFD output accurately in real ;life that may not be the case. A true RMF meter will give an accurate indication on distorted 60 Hz waveforms but you cannot assume that it will be accurate over the wide range of frequencies found in the output of a VFD.
It may be interesting to measure the voltage to ground ahead of the VFD. It may be the same or it may be normal (277 V).
The capacitive leakage to ground is frequency dependent and the leakage may be greater on the output of the VFD with the high frequency components of the VFD output.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[controlsdude]

Ok its ungrounded system but there still is grounding of sheilds, motor frames, improper conduit installation of grounds, that can cause noise and might cause some variance in voltage.

Also, since its a VFD on an ungrounded system, did you take out the jumper on the VFD that grounds the MOV? This might be the issue now, but if the problem gets much worse it might blow the caps.

 

[Skogsgurra]

I see this misconception so often that I need to say something about it.

A meter showing RMS, true or not, does not show what you want to see when you measure voltage on a VFD driven motor.

It shows RMS of the voltage, yes. But what you want to measure is the RMS of the drive's fundamental frequency. And that is something completely different. You could do it using a frequency analyzer and look at the fundamental component.

Another way of doing ot is to use a low pass filter and measure the output of it. If you select f0 to be 1 Hz, you will get the V/Hz on the voltmeter. And that is actually what you want to know. If it equals motor rated voltage divided by motor rated frequency, then then everything is OK. In Europe, the ratio used to be 380 V/50 Hz or 7.6 V/Hz. It is now 400/50 = 8 V/Hz.

Have used this method to find drives that are running way off rated V/Hz value.

Gunnar Englund

http://www.gke.org

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

 

[krbernier]

just a note....a good quality dmm with a low pass filter built in such as a fluke 289 is perfectly acceptable to measure the voltage output of a vfd.

 

[rovineye]

I have used analog meters to check voltage line to ground on VFDs circuits when looking for ground faults. Consistent enough readings to trace grounds.

 

[ScottyUK]

krbernier,

The LPF on that meter is intended to block frequencies above 1kHz and appears from the sketch in the manual to have a corner frequency somewhere below 1kHz; Fluke doesn't state the filter rolloff rate or the break point. As with all tools, knowing the limitations and how it could affect the measurement is worthwhile. Older and larger drives will have significant components down in that part of the frequency range.


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If we learn from our mistakes I'm getting a great education!

 

[krbernier]

well i guess im a victim of a false advertising campaign from world renown fluke! according to this (attached) application note from fluke.

i maintain approxamately 150 vfd's mainly operating fluid pumps and have yet to see my voltage measurements differ from the vfd's built in display.

however i have nowhere near the education required to say that flukes engineers are lying.

 

[ScottyUK]

Who mentioned false advertising?

If you work on big drives, especially old big drives with a quasi-square output, or a current-source drive, then you will get some peculiar results.


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If we learn from our mistakes I'm getting a great education!

 

[krbernier]

generally 200-500 hp drives. but the oldest i have is probably 8 years old.

i said false advertising because fluke sold me on this rather expensive meter because it was specially designed for industrial use and fully capable of accurate voltage measurements on the output side of vfd's.

"As with all tools, knowing the limitations and how it could affect the measurement is worthwhile."

From fluke's own application note i thought i did know the limitations of the tool they sold me.

You informed me that their application note is false.
I am not an electrical engineer and have no way of knowing who is right on this issue. My gut tells me to go with Fluke

 

[ScottyUK]

You informed me that their application note is false.

Did I? You must be reading a different post to me. I seem to have said

"The LPF on that meter is intended to block frequencies above 1kHz and appears from the sketch in the manual to have a corner frequency somewhere below 1kHz; Fluke doesn't state the filter rolloff rate or the break point. [red]It's on page 20 of the manual if you want to look[/red] As with all tools, knowing the limitations and how it could affect the measurement is worthwhile. [red]I would have thought this would be obvious, but maybe not.[/red] Older and larger drives will have significant components down in that part of the frequency range.[red]I don't make the rules, I just get to obey them like the rest of us.[/red]"

Their spec says what their meter does, although it perhaps should have a bit more detail on what the filter is in terms of corner frequency and rolloff rate. The manual is available online; you don't have to buy it to read the spec. I have the same meter and I think it is a good instrument other than it eats batteries. It's not magical and it does have limitations. Whether those limitations affect what you are using it for is something you need to determine for yourself depending on what you're doing. If you ask the technical guys at Fluke for help I have found them very helpful on the occasions when I've spoken to them.


Caveat emptor.

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If we learn from our mistakes I'm getting a great education!