Motor current imbalance

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thumbcove

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We have a new tank farm pump installation with three 40 hp pump motors that are rated at 230/460 volt 3 phase, 1780 RPM, 90/45 amps. We are using soft starts. The voltage with no motors running is phase to ground 288, 289, and 289 and phase to phase 500, 500, 504. With one motor running the voltage is 283, 284, and 285 and phase to phase 492, 492, and 495. The current is 50, 52, and 46. Rpm is 1790. The other two motors are close to the same except lower amps due to lower head pressure at the time. I rotated the conductors on the load side of the starter and saw no change. I see no difference in the readings on the line side and load side of the starter.
We originally set the overloads at 45 but had to set them higher to keep the motors running.
Motor manufacturer doesn't have much for ideas on why we have this current imbalance. Motors are cool running, sound good and move product very well.
Should we be concerned?
 
It sounds like you are right on the edge. 10% between phases is as much as I would ever want to go and I like to stay at 115% of nameplate for overloads. Manufactures can be known to put more pump than the motor can handle figuring that nobody will max out the pump.

If your amps are lower on the other pumps it's because you have more head pressure, not less.
 
My spreadsheet says to investigate a current imbalance at 5%, you're at 6.7%.

Minor detail though, the pump is overloaded. Restrict the flow by closing a valve partially until the total current drops to the nameplate FLA.

A pump designed to run at 1780 RPM will require more HP at 1790. Add in what Dave said about motors being undersized to begin with and you have your current problem. (Pun not intended, but it works.)
 
In addition to what has been said so far, it seems a bit odd that a motor loaded slightly above nameplate HP would be running faster than NP RPM.
 
Would the higher voltage explain the rpm but not the high current. The motor manufacturer says it is likely putting out 45 hp. They also stated that the impeller may need to be trimmed down some. What might explain the current imbalance and is it a problem?
 
thumbcove said:
Would the higher voltage explain the rpm but not the high current.
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Higher voltage would explain higher current. Despite the old saw that higher voltage equals less amps once you get past too low of a voltage, which is a different problem, you do get higher amps with higher voltage. You also get high amps on an impeller pump when you are moving too much liquid, aka not enough back pressure.

RPM is limited by AC frequency and physical characteristics of the motor.

The motor manufacturer says it is likely putting out 45 hp. They also stated that the impeller may need to be trimmed down some.
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Choking down the output side of the pipe would do the same thing as trimming the impeller, either way you are moving less liquid.

What might explain the current imbalance and is it a problem?
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A bad connection, load imbalance on the whole system, bad winding in the motor, you name it. Have you tried restricting the flow, like ptonsparky suggested and getting the amps within nameplate?

I wouldn't want a motor running that high for an extended period.
 
ActionDave said:
... Choking down the output side of the pipe would do the same thing as trimming the impeller, either way you are moving less liquid.
...
Click to expand...
Well, not exactly the same thing, but yes moving less liquid. Throttling an output valve may well increase pressure whereas trimming the impeller would decrease the dynamic head.
 
ActionDave said:
Higher voltage would explain higher current. Despite the old saw that higher voltage equals less amps once you get past too low of a voltage, which is a different problem, you do get higher amps with higher voltage. You also get high amps on an impeller pump when you are moving too much liquid, aka not enough back pressure.

RPM is limited by AC frequency and physical characteristics of the motor.


Choking down the output side of the pipe would do the same thing as trimming the impeller, either way you are moving less liquid.


A bad connection, load imbalance on the whole system, bad winding in the motor, you name it. Have you tried restricting the flow, like ptonsparky suggested and getting the amps within nameplate?

I wouldn't want a motor running that high for an extended period.
Click to expand...

The other two pumps also show this imbalance in current. We were only able to pump product in a 50 foot circle at the time but had readings of 30.5, 32, and 28 on the gasoline pump motor and 44.9, 44.5, and 39 on the ULSD 1 and very close tho the same readings on voltage. I am not sure it is a good idea to throttle back flow with the gate valves. These are new pump motor packages. New building, utility tranformer, all new. I assume if we were really moving product with these two pumps the current would be very close to and above nameplate also.
 
thumbcove said:
The other two pumps also show this imbalance in current. We were only able to pump product in a 50 foot circle at the time but had readings of 30.5, 32, and 28 on the gasoline pump motor and 44.9, 44.5, and 39 on the ULSD 1 and very close tho the same readings on voltage. I am not sure it is a good idea to throttle back flow with the gate valves. These are new pump motor packages. New building, utility tranformer, all new. I assume if we were really moving product with these two pumps the current would be very close to and above nameplate also.
Click to expand...

Not that it helps very often, did you have engineering? Who supplied the pump package?
 
Smart $ said:
Well, not exactly the same thing, but yes moving less liquid. Throttling an output valve may well increase pressure whereas trimming the impeller would decrease the dynamic head.
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There is also the possibility that the pump impellers arenot identical anyway and that could account for the differences in load current observed though not the imbalance.
 
thumbcove said:
The other two pumps also show this imbalance in current. We were only able to pump product in a 50 foot circle at the time but had readings of 30.5, 32, and 28 on the gasoline pump motor and 44.9, 44.5, and 39 on the ULSD 1 and very close tho the same readings on voltage. I am not sure it is a good idea to throttle back flow with the gate valves. These are new pump motor packages. New building, utility tranformer, all new. I assume if we were really moving product with these two pumps the current would be very close to and above nameplate also.
Click to expand...

Gate valves are for isolation only, and are probably the worst type for attempting to throttle flow.
 
The only way I can see unequal current with equal voltage on multiple motors is if the phase angles are somehow not at a uniform 120 degree spacing. (Or maybe really serious harmonics on only one phase line?)
Hard to check without a scope.

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single core transformer or a bank comprised of single phase transformers?

If a bank, are they all identical, have all good interconnections both primary and secondary sides?

Have POCO check for voltage regulation issues?

Presuming this is utility supplied and not on site generation?
 
You have a 0.8% voltage imbalance, that will result in a current imbalance. It's difficult to assign a direct relationship as it is not linear, however your current imbalance looks to be a little more than I would expect to see with that low of a voltage imbalance. Here is a chart I found once a long time ago from EASA (the Electric Apparatus Service Assoc.) when I did some work on motor protection for a talk at one of their conventions years ago. It's for a small 230V motor, but you can get the almost logarithmic relationship to % voltage imbalance and % current increase:
2easa%20figure%201.jpg
Your 0.8% voltage imbalance falls just shy of midway between the 0.3% and the 2.3% columns on that chart, so a 6.7% current imbalance would not be unheard of I suppose.

A contributing factor might also be that your 460V rated motor is ALREADY close to saturation as the others have mentioned. You are not technically over the +10% limit (506V), but for sure you are consuming most of the margin of error you get from the standard NEMA MG-1 tolerance specifications. That high voltage will result in higher running current as mentioned. Here is a graphical relationship chart to give you an idea:
motorlife1.jpg
You can see from here that at +9% V, your current will be about 103% of normal, and your Efficiency will be lower than normal, meaning that much of the extra current is becoming heat in the motor, not doing work.

This by the way is one of the reasons why people specify a "Service Factor" in motors, to ADD some additional margin of error for things like this. So if your motors have a nameplate FLA of 45A and the Service factor of 1.15, you can safely increase the OL trip point to 51.75A, however, that depends on exactly how your specific OL relay is set up, you need to RTFM before making any decisions, and also whether or not your pump motors are in fact rated for 1.15SF, many pumps are not.
 
This is utility served from a (I think) 150KVA pad mount transformer. The motor SF is 1.00. The facility owner purchased these from Northwest Pump & Equip and Nidec Motor Corporation/US motors. The facility was engineered but I do not know who decided which pump/motor to purchase. The original plans called for 20 hp but early on it was decided to go with 40 hp.
I am very grateful for all the input. I just wish I new what do next.
 
thumbcove said:
This is utility served from a (I think) 150KVA pad mount transformer. The motor SF is 1.00. The facility owner purchased these from Northwest Pump & Equip and Nidec Motor Corporation/US motors. The facility was engineered but I do not know who decided which pump/motor to purchase. The original plans called for 20 hp but early on it was decided to go with 40 hp.
I am very grateful for all the input. I just wish I new what do next.
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Maybe nothing.
From your initial post:
"Motors are cool running, sound good and move product very well."
 
thumbcove said:
This is utility served from a (I think) 150KVA pad mount transformer. The motor SF is 1.00. The facility owner purchased these from Northwest Pump & Equip and Nidec Motor Corporation/US motors. The facility was engineered but I do not know who decided which pump/motor to purchase. The original plans called for 20 hp but early on it was decided to go with 40 hp.
I am very grateful for all the input. I just wish I new what do next.
Click to expand...

When Motor 1 lets the smoke out, replace it with a higher SF motor, or have a 50 HP unit ready to go in.

CYA and let the owners/bosses know, in writing, what is going on.
 
Interesting that the original plans call for 20HP, you upped it to 40HP and the 40HP is still almost overloaded. Sounds like a mechanical engineer has dropped the ball on the hydraulics here.
 
ptonsparky said:
When Motor 1 lets the smoke out, replace it with a higher SF motor, or have a 50 HP unit ready to go in.

CYA and let the owners/bosses know, in writing, what is going on.
Click to expand...

Agree.

I thought the pump and motor were a package, apparently not, or the OP has some parameters beyond its duty spec (high head pressure on input side of pump, slurry/thick/viscous liquid being pumped, etc).
 
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