Motor Drive Current Imbalance
Motor Drive Current Imbalance
(OP)
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
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





RE: Motor Drive Current Imbalance
http://www.fluke.com/Download/ElectricalPower/43b_spex....
RE: Motor Drive Current Imbalance
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?
RE: Motor Drive Current Imbalance
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...
RE: Motor Drive Current Imbalance
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.
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
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.
RE: Motor Drive Current Imbalance
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
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
Thanks for the help.
rdrago
RE: Motor Drive Current Imbalance
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.
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
Mike
RE: Motor Drive Current Imbalance
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
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
Minor aside — Current measurements are affected by current probe pole-face contamination and misalignment.
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance
RE: Motor Drive Current Imbalance