Current balance percentages.

[Paul583]

Hi, when placing a clamp meter across each phase and measuring the current, what is the permissible difference expected between the phases, 2%, 5%, 10%?

 

[Skogsgurra]

Five percent is quite high. You may have a problem with grid symmetry or motor windings if you have that much. At 10 percent something is very, very wrong.

Two percent is probably not to worry about. Especially if your grid shows a simmilarly bad symmetry.

If your grid is perfectly symmetrical, a two percent difference between line currents can be an indication that something is wrong. Especially in larger machines. But probably OK.

Usually two percent is within normal tolerances. Make sure that you keep the clamp centered around the conductor or you may get an additional reading error of one or two percents.

 

[motorsdirect]

Also, make sure that you measure the current with a clamp meter away from the motor, or you could well be suffering induced readings..
jeff

http://www.motors-direct.co.uk

 

[RonShap]

Where are you measuring the imbalance? Is this on a panelboard or service, where single phase loads exist?

 

[busbar]

There has been much discussion in electrical topics at this site. Search on “current imbalance” and “current unbalance.”

 

[Paul583]

Thank you all. I just needed a refresher for this and other things that I haven't done in quite a while.

 

[electricpete]

http://www.joliet-equipment.com/voltage_unbalance.htm

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[buzzp]

Technically, unbalance and percent difference are two different things. Unbalance (NEMA definition) is
max deviation from average/average * 100% = unbalance (uses three numbers).
Percent difference (used on only two numbers) is (#1-#2)/#2.
Maybe this is already understood and the terminology was just used wrong.

 

[electricpete]

Buzz - it's a good point about the terminology. It is defined below the figure in the link above.

I have seen higher unbalances at low load than at high load.

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[sabap]

Buzz is right. NEMA limits are related to % unbalance. 1-2 % unbalance in voltage is the limit. I think in NEMA its 1 % and in some other standard , dont remember the name, it is allowed upto 2 %.

But for current 9-10 % is the limit. For motors 1 % voltage unbalance will normally result in 9-10 % current unbalance.

System voltage and winding resistances of motor should be further checked to see the root cause. System voltage can be brought within limits by readjusting / balancing single phase loads in the system.

 

[electricpete]

The graph linked is from NEMA. Below 1% voltage unbalance, no derating required. Between 1% and 5% derating is given by the graph. Above 5% continued operation is not recommended.

Good point sahab to clarify those numbers are voltage unbalances.




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[busbar]

 
ANSI Standard C84.1-1995 Annex D — Field surveys and statistics indicate that:

(1) Each motor rating is associated with a unique optimal range of voltage unbalance
(2) These ranges vary from 0–2.5 percent to 0–4.0 percent voltage unbalance with the average at approximately 0–3.0 percent
(3) Approximately 98 percent of the electric supply systems surveyed are within the 0–3.0 percent voltage-unbalance range, with 66 percent at 0–1.0 percent or less

Electric supply systems should be designed and operated to limit the maximum voltage unbalance to 3 percent when measured at the electric-utility revenue meter under no-load conditions.

For 3ø induction motors, current imbalance with respect to voltage imbalance is proportional to:

Motor Locked-Rotor Current / Motor Full-Load Current [a factor of roughly 4-7]