Fractional motors 3 Phase unbalance
Fractional motors 3 Phase unbalance
(OP)
We received an italian machine with 7 VFDs (40HP)each one equipped with a small 3 phase motor to operate the cooling fan (spec 460 VAC, .29A).After 5 hours of operation the overloads began tripping. Checked amperage (Di-log clamp meter) T1:.41A, T2:.27A, T3:.19A, Supply Voltage:L1-L2 490.2, L2-L3 491.4, L3-L1: 490.1.
What could it be the source of unbalance?
What could it be the source of unbalance?





RE: Fractional motors 3 Phase unbalance
RE: Fractional motors 3 Phase unbalance
http://www.franklinelect.com/Manual/AIM_29.htm
RE: Fractional motors 3 Phase unbalance
RE: Fractional motors 3 Phase unbalance
RE: Fractional motors 3 Phase unbalance
"If, on the three possible hookups, the leg farthest from the average stays on the same power lead, most of the unbalance is coming from the power source. However, if the reading farthest from average moves with the same motor lead, the primary source of unbalance is on the "motor side" of the starter."
If you do the test right at the motor and it follows the motor lead then the fault is the motor.
Doing that test beats having to remove the motor and take it to a shop and have them tell you "its not the motor"!
RE: Fractional motors 3 Phase unbalance
Average current does not appear above FLA. Perhaps you can double check if there remains a motor with balanced current to see if that is above FLA.
Is the motor being operated within nameplate parameters.... ambient temperature, continuous or intermittent rating, proper wiring (delta or wye), correct frequency?
Voltage is a little high but still below 110% (although maybe it is varying over time?)
It seems remotely possible that your drives might be putting voltage spikes onto your 480volt system?
You might want to troubleshoot a little by checking winding resistance, insulation resistance, and if you have access to test equipment... a surge comparison test.
I'm just guessing here. If you can contact motor manufacturere I'm sure he'll help you work through application considerations. He'll also probably be able to tell you what tests were done on the motors before they left the factory. If you can't narrow down the cause I would recommend having the motors torn down for evaluation.
RE: Fractional motors 3 Phase unbalance
FLUKE CLAMP(DI-LOG) VOLTAJE RESISTANCE
.299 .28 L1-L2 496.1 T1-T2 231.1
.270 .23 L1-L3 496.7 T1-T3 246.3
.311 .327 L2-L3 495.4 T2-T3 238
I will try next rotating the phases. Thanks a lot
RE: Fractional motors 3 Phase unbalance
RE: Fractional motors 3 Phase unbalance
RE: Fractional motors 3 Phase unbalance
Note that there are 2 different ways to report it:
“range”=(Max-Min)/average
“imbalance”=(max deviation from avg)/avg.
Typically Range~2x imbalance.
For your data of 231, 238, 246, we compute average = 238
range = (246-231)/238~ 6%
imbalance = (246-238) ~ 3%
Here are some references:
EPRI 108773V1 (Large Electric Motor Preventive Maintenance Guidelines) recommends 5% range (~2.5% imbalance)
IEEE 1290 (Nuclear Motor Operated Valves) 5% range (~2.5% imbalance)
EPRI 1000897 (Large Motor Rewind Spec) recommends a maximum 1% winding resistance "imbalance" (approx 2% range.)
EPRI 111195 (Nuclear MOV’s) – 2% range (~1% imbalance).
EPRI 107524 (Large Nuclear Motors) recommends 1% range (~0.5% imbalance)
EPRI 111196 (large motor rewind spec) – 1/2% range (~1/4% imbalance!).
I'll grant you that some of these specs apply to large motors. The MOV specs apply to small motors. Your data with 6% range and 3% imbalance does not meet any of these criteria.
RE: Fractional motors 3 Phase unbalance
It sounds like other locally connected loads don’t exhibit this problem. The “…throw the whole machine away?” comment was not warranted.
RE: Fractional motors 3 Phase unbalance
comment - my comments above imply the motor may be bad with possible shorted turns... but I should add that you must also consider the accuracy of the measurement. Is it repeatable? If not then contact resistance might be affecting it... use 4-wire resistance measurement.
question - is it just one fan that tripped it's overloads or several?
RE: Fractional motors 3 Phase unbalance
The winding resistance imbalance is the reason the motor currents are imbalanced.
RE: Fractional motors 3 Phase unbalance
RE: Fractional motors 3 Phase unbalance
RE: Fractional motors 3 Phase unbalance
Have you checked the sizing of the overloads? I would agree with others that these very small motors can have considerable variation in current per phase, especially running lightly loaded.
Is this happening in every VFD or only one?
dpc