Motor current imbalance 1

[tommom]

A client of mine has a problem - his air-handling units are exhibiting a current imbalance of upwards of 15% on a 208 volt 3 phase system. He has only checked four of his six or seven units, and has not looked at his other equipment.

I don't suspect severe load imbalance on the system - there simply isn't enough single phase load I don't think.

I suggested two things: ask the utility co. to come check the voltage output from their (208 volt) transformer (because the facility is isolated and has a long exposed utility-owned primary overhead line), and check the client's capacitor bank (one bank for the facility) for a failed phase.

Are there any other things I should check? The client has not had any of the motors checked, but I thought it unlikely that four motors could have the same problem. I also don't think single-phasing could be the problem.

Any thoughts?

 

[thewellguy]

More than likely its due to the incoming power. The supply is most likey a open delta. In the Chicago area these power supplies are famous for current imbalance. Not much you can do other than talk to the power company or install a transformer as a line filter.

 

[tommom]

The utility transformer I believe is a three-phase padmount, so I'm pretty sure it's not an open delta. However, I agree that the utility is probably supplying unbalanced supply voltage.

Thanks for your post.

 

[dpc]

It would be best to also check the voltage at whatever panel or board is feeding the motors. Balanced voltage at the transformer doesn't always guarantee balanced voltage at the distribution panel.

Sounds like a voltage imbalance issue.

 

[VxA]

I'm not so sure a 15% current imbalance is all that severe (though I'd be interested in hearing why it might be considered a major issue and what detrimental affects it would cause). I have read that current imbalance is typicallu between 6 & 10 times greater than the voltage imbalance. So if you are seeing 15%, that would correspond to about 2.5% voltage imbalance, which is within NEMA MG-1 standards for motors.

Like the others, I would suspect the incoming power is the major contributor.

I'd like to end in a disclaimer here that these are mostly just my thoughts out loud. This is not my area of expertise, so don't take it as gospel by any means.

 

[waross]

I suggested two things: ask the utility co. to come check the voltage output from their (208 volt) transformer (because the facility is isolated and has a long exposed utility-owned primary overhead line),

Primary voltage unbalance on a long line is not uncommon especially if there are no voltage regulators or you are far from a voltage regulator.

and check the client's capacitor bank (one bank for the facility) for a failed phase.

I would check for failed capacitors first. Or blown capacitor fuses first.

I'm not so sure a 15% current imbalance is all that severe (though I'd be interested in hearing why it might be considered a major issue and what detrimental affects it would cause).

A current unbalance may be more severe with older 208 volt installations than with 240 volt installations.
Although 200 volt motors are becoming more common in new 208 volt installations, for many years 230 volt motors have been used on 208 volt systems. These motors have given good service and probably there are millions in use. However the rated voltage is already almost 10% low and the terminal voltage may be a little more than 10% low. The current is proportionally higher. The motor is already running hot and may have no reserve thermal capacity left for the extra heating caused by unbalanced currents. (The NEMA standard was once 15% voltage tolerance and 230 volt motors adapted well to 208 volts. With the newer 10% voltage tolerance limit, most new installations should be considering 200 volt rated motors for 208 volt systems.)
respectfully

 

[thewellguy]

In the industry that I work in the motor manufacter will void the warrenty if the current imbalance is more than 5% and the voltage 10%. I have been told that the excess imbalance causes additional heating in 3 phase motors and shortens their life, however I have never been able to find a chart or info that explains x imbalance shortens the life y amount.

 

[jraef]

thewellguy,
Something is wrong with your numbers. A voltage imbalance of just 2% will result in a current imbalance well in excess of 5%! Motor winding temperature rise is exponential to voltage imbalance,

% temp. rise = 2 x (% voltage imbalance)[sup]2[/sup]

A 10% voltage imbalance would cause a 200% temperature rise in the windings! I think maybe you meant a 1% voltage imbalance.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[dpc]

Just to be sure we're all taking about the same thing:

Calculate the % imbalance by first taking average of the current or voltages, then computing percentage imbalance based on the maximum deviation of any one phase from the average.

 

[thewellguy]

jraef,

I think dpc is right we are using different definitions. Per the Hitachi Submersible Motors installation manual,"...voltage must be plus minus 10 % of rated." "...Phase unbalance: + - 5%

 

[jraef]

They are refering to the overall voltage level being + - 10% of rating, not related to the unbalance. 5% voltage unbalance is a lot as well, I believe Franklin Pumps specifies 1% unbalance max. Maybe they meant 5% current unbalance, that may be tolerable for them because the motor is in water to keep it cool, but 5% voltage unbalance would mean probably 50+% current unbalance!

Bottom line, just make sure you aren't swapping voltage and current around when discussing unbalance percentages, the difference is very critical.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

I am surprised. I see motors running on horrible imbalances all the time. I never realized the seriousness of the practise.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[stevenal]

Motors need to be derated if run under unbalanced voltage conditions. See NEMA MG1. Briefly spoken of in ANSI C84.1 Annex D.

 

[jraef]

Keith,
The reason why you don't see a lot of problems is that a lot of motors are slightly oversized and have 1.15 Service factors, so they can take a little extra heating from imbalance. Still not a good idea to ignore it though.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376