Motor Drive Current Imbalance 1

[rdrago]

A 20 HP, 3 phase, 480 volt motor is connected to a motor drive. The measured current (Fluke 43B) at the input to the drive is as follows:
Phase 1- 2.3A
Phase 2- 5.4A
Phase 3- 5.3A

Does anyone have any idea why the Phase 1 current at the input to the drive is so low? The motor and drive seem to be functioning ok. Because the load is not balanced, my Fluke 43B power measurements may be invalid. Can someone help?

rdrago

 

[buzzp]

I am assuming the current to the motor is more balanced than that going into the drive. Were these measurements taken individually with your unit or did you only have to place it on one conductor? I read, briefly, the spec sheet and it seems they assume a balanced load and give you all the info you need. Below is the link to your units spec sheet in case someone wants to see it. Not knowing how you arrived at these measurements I would measure the currents with a different RMS meter and see if you get the same results before deciding there may be some issues.

http://www.fluke.com/Download/ElectricalPower/43b_spex.pdf

 

[skiier]

A drive is a rectifier. It receives voltage and filters it thru electronics to your motor so that you can vary speed and frequency and load, etc. How it does it do it? A black box? Who cares? What u should be looking at is the load side of the drive and seeing if all your line currents are balanced.
R all ur fault indicators/lites ok? Can this unit be used on a single phase system as well? Dual volatge/dual phase circuitry?
Fuse it according to manufacturer's specs and relax. If it aint broke don't fix it.
Oh and correct me if I misunderstand but how can a 3 phase motor load be unbalanced? We are just talking about 1 motor connected to this drive? Do you mean to say that the load varies?

Just a coincidence? The current measurements you recorded -if measured at the motor - would indicate a dual voltage motor connected for low voltage with an open winding. If the motor was lightly loaded an open winding might go unnoticed and the phase currents would become unbalanced.This is what I thought when I first read the thread. Anyone else see it?

 

[jOmega]

Since the Fluke 43B has a scope mode, you might want to take a look at the input voltage waveform, phase-to-phase.
You might also want to record the voltage harmonics on each phase.

I suspect that you will find that the sine wave line voltage isn't pure..i.e., not a pure sine wave, and that there is some distortion on it.

I also suspect that the voltage peak amplitude phase-to phase isn't equal.

The front end of the VFD is a three-phase full wave diode bridge...with a capacitive filter or with a dc link reactor, an inductor input capacitive filter.

Ergo, the bus caps charge to the peak of the AC line waveform. The caps draw current from the line when the load or inverter side draws current from the caps, they are recharged from the AC line thru the rectifier diodes, when the rectified voltage is greater than the voltage on the bus caps. If the AC line sine wave isn't of equal amplitude, then the bus caps will charge from the the legs with the higher peak voltage before pulling current from the legs with the lower peak voltage. ERGO, the current draw will be unequal.

So, use the instrument to do some more investigation.

At the reported current values, it would seem that the 20 hp VFD / motor are running unloaded. That's not very much current... more like motor no-load value...

 

[rdrago]

Thanks for the responses.

To buzzp: I did not measure the current on the load side of the drive because access was not easy. I will measure tomorrow. Today, I measured each phase current on the drive input individually using the 43B. Thanks for link.

To skiier: I am trying to get a handle on how to measure power in a drive/motor combination in order to justify purchasing drives to install on fan motors. See the thread "Power in 3 Phase Motors". I was advised to measure the power input to the drive because this is what the utility is billing. Also, the 20 HP motor is loaded and running at frequency of 43HZ. There is only 1 motor connected to the drive. Based on your comments, I will check the motor wiring tomorrow.

To jOmega: I did a voltage balance check between phases and it turned out good (0.12%). However, the phase to ground voltages were as follows:

Phase 1: 265.2V
Phase 2: 272.3V
Phase 3 273.2V

Note that phase 1 (phase to ground) is slightly lower. I don't know why.

Thanks again to all that answered this thread and my previous one. rdrago.

 

[jbartos]

Suggestion: Could you post voltages that materialize upstream when the motor drive is disconnected?

 

[busbar]

Specs for the 43B show 50 amperes for the low range—putting measurements at ~10% or range. Out of habit I think I probably would have verified each motor-phase conductor was reasonably centered in the probe window to better assure current accuracy.

 

[rdrago]

Current measurements at the drive output are as follows:
Phase 1: 4.88A
Phase 2: 4.83A
Phase 3: 4.85A
As you can see, the motor load is balanced.

However, I am still wondering why the current at the input to the drive is so unbalanced. Readings are as follows:
Phase 1: 2.3A
Phase 2: 5.4A
Phase 3: 5.3A

The voltages at the input to the drive measured phase to ground in a 3 wire Y connected system are as follows:
Phase 1: 265.5V
Phase 2: 268.5V
Phase 3: 268.0V
Phase to ground should be 277.0 volts. The inverter is not pulling the voltage down. We may have a problem further upstream.

Can anyone comment on this situation?
rdrago

 

[buzzp]

I do not see a problem. If your concerned about the current draw on the line side of the drive, call the manufacturer and ask them if this is normal. I believe you will find it is normal. The motor does not care about L-N voltages, look at L-L readings and you will likely find these as balanced as your L-N readings.

 

[rdrago]

Buzzp: You are right. I called the manufacturer and he said that the motor may have unbalanced current on the line side if the drive is not running at rated load. It is no problem.
Thanks for the help.
rdrago

 

[busbar]

rdrago, on your question about what’s OK for 277-volt equipment on the North American continent, ANSI Std C84.1 lists acceptable limits over about 254-291 volts; sometimes 245-293 volts. Note that utilization equipment may not necessarily operate with the most efficiently over that range, but appliance manufacturers and electric utilities generally agree that the stated range will not cause damage. As noted by buzzp, 440-504V [or 424-508V] line-to-line are readings of greater interest, for loads like drives have no neutral connection.

 

[jbartos]

Suggestion: There might be a higher resistance or impedance connection somewhere causing the current in one line to be lower and voltage to ground to be lower.

 

[electromike]

If you are using a clamp current probe you should make sure to observe the direction that your CT faces. These devices are polarity sensitive. I have seen cases where the enclosure that measurments were taken had unequal access to the conductors so the tech would turn his probe to gain access to the cramped conductor thus causing errors in the current balance readings. Another thing to check if you have a dual trace oscilloscope is the phase relation of the current and voltage waveforms on each phase going into your drive.

 

[jbartos]

Suggestion: Agree with the previous posting that thorough troubleshooting is needed.

 

[panelman]

We use abb drives...they will display instantaneous power and somewhere in the parameters is a "total power" record as well

Mike

 

[rdrago]

Thank you all for the comments. It is good advice that is well taken. I was very careful with the amp probe to make sure that the polarity and centering were correct.

I have another question concerning the same set of power measurements from this same motor/drive combination. I will describe the circumstances:

I bought a Fluke 43B to measure the power. I consulted with a Fluke engineer who understood the meter to be assured that I connected the meter correctly to obtain valid power measurements. He said that even with the unbalanced current, the method of measurement that I using (single phase) would produce valid power measurements. I connected the meter to the input of the drive.

The power factor measured 0.27 (The DPF was 0.91)

Why is the power factor so low? I have a hard time believing that this is correct. Is it possible that the power factor is really this low or is something else going on?

Thanks,
rdrago

 

[jbartos]

Suggestion: An oscilloscope might be applied to investigate waveform quality or distortions.

 

[busbar]

Minor aside — Current measurements are affected by current probe pole-face contamination and misalignment.

 

[cbarn24050]

Hi rdrago, you don't seem to have a problem here at all. jomega has explained to you why your currents are unequal your power factor is bad for the same reason. Once you put some load on your motor things will seem more normal.

 

[Allphase]

http://www.drivesurvey.com/dcforum/DCForumID10/44.html