Voltage/Current Imbalance

[Electromatic]

I have a pumping station whose pumps are not running smoothly. We think it's mostly a mechanical/plumbing issue, but while investigating, I found what I consider a significant voltage and current imbalance with a motor running. The motor is 25HP, and the system is a high-leg 240V.
With the motor running, I have:
AB 240V
BC 237V
AC 235V

A 51.5A
B 53.5A
C 44.0A

Oddly, I get similar results with both an old motor and a newly-installed one. Also, there is a backup generator with slightly better voltage balance (238, 238, 241), and running off it, the motor pulls 51A, 53A, 46A.
Is there something I should look for to correct this, or is this a good as can be expected?
TIA

 

[ron]

If you turn the loads off, is the voltage provided by the utility "off-balance" They are generally permitted + or - 10%

 

[Electromatic]

With no major load (no motors, just a couple of lights and small items) I have:
AB 245V
BC 242V
AC 238V

A-gnd 118V
B-gnd 212V
C-gnd 121V

I'm fine with the service being +/-10% from a nominal 240V, but the imbalance concerns me. It's about 11% current imbalance at the motor. However, I don't know that the utility is going to give much thought about their 1.5% voltage imbalance.

 

[LarryFine]

With no major load (no motors, just a couple of lights and small items) I have:
AB 245V
BC 242V
AC 238V

A-gnd 118V
B-gnd 212V
C-gnd 121V

So, you have a high-leg delta. They're never balanced because they're never equally loaded.

 

[Electromatic]

The amperage readings I've listed are from the load side of the motor starter. Should the motor alone be drawing such uneven currents?

 

[Jraef]

A voltage imbalance is absolutely going to be a cause of a current imbalance. Maybe not the only one, but if you HAVE a voltage imbalance, you WILL therefore have a current imbalance.

You have a roughly 3% voltage imbalance, and a roughly 20% current imbalance. Most of that current imbalance is likely attributable to that voltage imbalance. The ratio of the affect of V-I to I-I is not linear, it’s almost logarithmic. So a slight change in the V-I will have a larger effect on the I-I.

Utilities are “allowed” up to a 2% voltage imbalance (per ANSI specs, but not enforceable). They may be persuaded to address this, at least to get to the 2% mark. That may bring your current imbalance down to around 10% (just a guess). But it needs attention. That much of a current imbalance will result in disproportionate heating of the motor that may not be within the protective band of a basic overload relay. So you could conceivably smoke the motor without it ever tripping on overload.

One tactic to try to get better balance of the current is to “roll” your conductors to the motor. So at the motor starter load terminals, you move A to B, B to C, and C to A. You are not changing the rotation sequence, you are just changing which line phase is feeding which motor winding. This can have the effect of having the natural small differences in the impedance of the different windings helping to lessen the effects of the V-I to then lessen the I-I. It’s worth a try as it only costs time. You can do it twice by the way to see if it changes anything (you CAN do it a third to line but you end up where you started ). Even a small improvement in the I-I will help prevent premature motor winding failure.

 

[LarryFine]

The amperage readings I've listed are from the load side of the motor starter. Should the motor alone be drawing such uneven currents?

Presuming no motor defects, the currents will be a result of applied voltages.

You can try "rolling" the phases to see if you can minimize the imbalances.

 

[Electromatic]

Thanks, all. I knew the voltage imbalance was leading to the current imbalance and that it's not a linear relationship. I mostly just wanted a sanity check that something more screwy wasn't going on. I doubt I'll bother with the utility unless something gets worse. I may try rolling the phases, though.
Thanks again!

 

[Jraef]

Presuming no motor defects, the currents will be a result of applied voltages.

You can try "rolling" the phases to see if you can minimize the imbalances.

lol. Me: 4 paragraphs to say the same thing Larry said in two short sentences…

 

[LarryFine]

lol. Me: 4 paragraphs to say the same thing Larry said in two short sentences…

I've noticed that happens often, and I actually enjoy it. I try to give concise answers. I believe it makes the point easier to grasp. Sometimes, I'll say "Didn't I just say that?" That's part of the fun of being here.

 

[ptonsparky]

Rolling usually helps, unless you were already at the best combination.

You may want to check the other motors as well

 

[Hv&Lv]

Have you contacted the utility?
Call and tell them what's going on and you want to make sure they don’t have a regulator that's stopped working. It happens more often than one would believe, and only takes a service man 5 minutes to check.

 

[ptonsparky]

So, you have a high-leg delta. They're never balanced because they're never equally loaded.

We've had single motor only installations with imbalance on high-leg systems. Often enough that we started 'rolling' as a typical procedure at start up.