3PH Motor Current and Voltage Unbalance 10%?

[Grizzly]

Hi Guys.
I have allways assumed that 10% is acceptable.

However having read the attached, I am now not so sure!

Grizzly

[install monkey]

isnt that 10% based on all phases having the same voltage

[chemi-cool]

From what I have learnt, this page is fine, when currant unbalanced is more then 5%, you better check the motor bearings for exesive wear.

[install monkey]

engineeringtoolbox
Voltage Imbalance and Derating Factor in Polyphase MotorsThe efficiency of an electrical polyphase motor decreases with increased voltage imbalance.

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Electrical motors are designed to operate with high efficiency within some variation in the electric power supplied.

• voltage variation should not exceed +/- 10%
• frequency variation should not exceed +/- 5%
• combined arithmetic sum of voltage and frequency variation should not exceed 10%
• voltage imbalance between phases should not exceed 1%

Voltage Imbalance %	Derating factor
0	1.00
0.5	1.00
1.5	0.97
2.0	0.95
2.5	0.93
3.0	0.89
3.5	0.85
4.0	0.82
4.5	0.78
5.0	0.76

[goshen]

Hi
as install monkey posted ,over the years these numbers have proved to be correct ,what is important is how you messure these values ,on some trane units they use voltage transformers that tend to give problematic readings after a few years of use,so in any case of suspicion for voltage or current imbalance it is crucial to use good tools ,these days that most large motors are fed via vsd ,you dont have to worry about grid imbalance .most vfds alert you when these values are approached.
interesting related articles:
http://www.leconsystem.com/Articles/Technical paper 1.pdf
http://www.elongo.com/pdfs/voltages.PDF
http://www.pge.com/includes/docs/pdfs/mybusiness/customerservice/energystatus/powerquality/voltage_unbalance_rev2.pdf

[Grizzly]

Goshen
Thanks for the input, I have saved the links for a little light reading later.
However the last 2 links are for the same document?

IM and Chemi.
Thanks also guys.
Cheers Grizzly

[Magoo]

Hi Grizzly.
avery good post, thanks Ta

[Grizzly]

Hi Grizzly.
avery good post, thanks Ta

A pleasure my friend!
Not to busy keeping things going then?

Grizzly.

[goshen]

Hi grizzly
oops !!!
best regards

[Yuri B.]

Some 2% imbalance in voltage of a lightly loaded induction motor can lead to imbalance in current of 40%. As the load increases the current imbalance decreases.

[chilliwilly]

I've yet to see a symetrical/balanced power reading at the terminals of a motor or a dist board for that matter. Except when the power is supplied from an invertor or semi online or fully online UPS, even when its fed from a local private generator supplying a building with no UPS protection. The diverse demand on each phase will cause varying voltage readings due to single phase loads fluctuating. And if the motor windings aren't as symetrical in value as you would expect them to be or one or more windings having a low insultion reading, will cause a similiar effect.

This is one of the reasons why you sometimes see overloads set higher than the motor rating. And sometimes bypassed altogether due to the windings being passed the stage of needing overload protection due to their condition, and just litteraly being run into the ground until they slow right down and stop and turn into heaters. And just rely on the fuses to protect the feeder cable and allowing the windings to partially melt and run into the bottom of the casing.

An interesting link though Griz. It looks like its referring to a North American system although obviously the theory will apply on any system. If only theory and practice harmonised each other though eh.

[engrp]

good day, fellow members. we also do encounter substantial voltage and current differences, and the references in this post were very informative. thanks to all members/moderators for sharing the above information/references. i always presumed that there was nothing i could do with the voltage unbalances, prior to the posts i read here. thanks again