Induction Motors- PF low on Phase B 1

[Slagathor]

We have a Stormwater Pump Station that is exhibiting strange behavior.
*(3) 150 890 RPM 3/60/480V motors. FLA = 186
*We are at the far end of the power dist run
*Pump/Motor 1 by itself runs OK. 130-140A. 483V
*Add Pump/Motor 2, and the amps of Motor2 rise more than pump load indicates they should. 160+A. 475V
*Add Pump/Motor 3 so all Pumps/Motors are running and the Amps on Pump 3 really shoot up...to over 200A! 465V
*None of the motors get hot, at least to the feel. Unfortunately our Engineer did not specify TStats on the motor
*We do not think it is a pump problem. The pumps were factory tested.
*All electrical connections have been triple checked.
*When we check the power quality (Fluke 1732), Phase B PF shows low. Phase A and C are close to motor nameplate. Phase B is always 2.5% to 4% lower. This occurs on all motors / combinations.

We can not use the station as Pump 2, and especially Pump 3 go high on amps over time when they are all run.

Is our phase B out of phase with the rest? Call the power company?

We are baffled.

 

[RRaghunath]

In a low voltage distribution system, it is common to have some unbalance in phase voltages, especially if the supply cables / over head lines are long and with no transposition.
The problem in your case seems to be long cables causing voltage drop and thus higher currents in the motors.
The voltage available at the motor terminals is within +/-5% of rated. So, it should be safe for the motors.
Motors may run a little hotter than supposed to be, for the given ambient temperature. But that is within motor design tolerances and should be OK, provided the frequency stays at rated value.
Better for the long run will be to run a few extra cables.

 

[waross]

I suspect a power quality issue.
Possibly "B" phase of a voltage regulator bank somewhere back up on the distribution feeder.
If the Amps go high on all three phases, it may be a pump issue.
Normally, with greater flow, we would expect the current on each pump to decrease.
If more flow is causing a current increase you may be operating outside of the normal operating range of the pump curves.
If the Amps go high on "B" phase only, this is a very strong indication of a failed voltage regulator.
More pumps online means more voltage drop.
The voltage regulators (one per phase) raise the voltage on "A" and "C" phases but not on "B" phase.
The motors act as induction generators and try to raise the voltage on "B" phase.
(It may be a failure on two phases.)
Check your voltages with one, two and three pumps running.
Voltage checks should be made on all three phases and be from line to neutral and from line to line.
Line to neutral unbalances do not cause a problem with the motor. The motor has no neutral connection.
Line to neutral unbalances do indicate a problem.
Line to neutral unbalances cause a line to line unbalance and unequal line to line phase shifts.
Both of these issues cause problems with motors.
Please check your voltages and let us know what you find.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

Does the "problem" with high current on 3 only appear when all pump are running, but not when 3 alone is running? If so that fits the scenarios above.

I agree it would be nice to get a better picture of the voltages to check for voltage balance.

Also you listed some currents... do you have current readings for all three phases?


=====================================
(2B)+(2B)' ?

 

[edison123]

Roll the phases and see if the PF also follows them.

Also, motor nameplate PF is for the entire 3 phase system, not individual phases.

Muthu

http://www.edison.co.in

 

[jraef]

They will likely deny it to the death, but your utility might be using capacitors to try to raise voltage drop issues and one of their caps has failed, so under load that phase gets worse as Bill said.

I had a similar issue and the utility denied it repeatedly. The site owner and I drove along their distribution lines and found the capacitor bank, took a picture of it and sent it to them with the GPS coordinates. Only then did they admit that they were using caps... and yes, one had blown but they didn’t know it (because “there were no caps”).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[LionelHutz]

What is the pump configuration? All 3 pumping into a common header?

What happens with pump 2 and pump 3 by themselves?

The amps on the pump 1 and pump 2 motors doesn't rise when all 3 pumps are running and the current on the pump 3 motor is high?

 

[Slagathor]

Thanks for all the good input. Sorry I have not been able to follow up.

Here is link to the motor data, and the test data I logged on the Fluke:
Link

Pump/Motor 1 is the least sensitive.
Pump/Motor 2 is subject to moderate amp rise
Pump/Motor 3 has the largest amp rise. When pumps 1,2,3 are all run, the amps on Pump 3 go way over nameplate. At the same time, 1 is reasonable, and 2 is moderately bad.

I did log data on pump 3 alone. I did not log data on pump 3 when running all pumps. But from my notes, when running all 3 pumps, Pump 3 exceeded 205A, while Pump 1 was at 165, and Pump 2 was at 180.

LionelHutz: The pumps are NOT connected to a common header. Each pump has its own discharge to to a drainage canal. So turning another pump on should not change the hydraulic/mechanical loading of a pump already running. Each pump will run on its own system head curve. So in other words, the amps draws ACT LIKE the pumps are on a common header, but they are not!

RRaghunath: The current rise is way out of proportion to the voltage drop. Going from one pump to 3 pumps drops the voltage about 3%. The current rises 30%.

waross: These are high Nss pumps (low head, high flow), so yes, you are correct. As we pump against more head, the flow drops, and amps rise. But the amps are rising way way more than could be explained by the pumps moving back left on their curves.

My current troubleshooting suggestion is to swap motors between pumps 1 and 3, and test again. See if the problem follows the motors.

 

[FacEngrPE]

I wounder if when 2 or 3 motors are on line if circulating currents between the motors could exist? The single motor test data can't answer that question.

Fred

 

[waross]

It is obvious that you have unbalanced voltages.
You may have a failed voltage regulator or a failed capacitor somewhere on the distribution circuit or you may have an issue with a PF correction capacitor in your plant.
It is common for the current unbalance to be a much greater percentage than the voltage unbalance that is causing the current unbalance.
Unequal voltages indicate accompanying phase errors which add to the current unbalance.
Low "A" phase to ground voltage will often cause a high "B" phase to "C" phase voltage.

What are your phase to phase voltages with no motors running?
Do you have CTs on only two phases?
If so can you check for a voltage drop across the CTs and phase to phase on each side of the CTs?
Check very carefully for an open in a CT circuit.
This is best done on dead circuits.
The last time that I inadvertently got across an open CT I don't know what the voltage was because it destroyed my multi-meter.
It is a long shot but the CTs may be acting as wireless reactors and causing voltage drops and phase shifts.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

You may have a transformer on the wrong tap on one phase.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Slagathor]

Bill,

Phase C Voltage is high (B to C and A to C are low), but it is only high by about 2.5 Volts.
Phase C current is low relative to A,B.
Phase C voltage is high by about 0.5%, but phase C current is low by about 1.5-1.8% This I can understand.

But it is the rise of current in all three phases over time that is baffling, not the relationship of phases.
Also, why would pump/motor 3 AMPS rise so much, while Pump/motor 1 does not.

I will see if I can get voltage data with no equipment running. I suspect it will be 485-490V.

Thanks again....

 

[waross]

Do you have any idea how large the supply transformer is?
The lower the transformer impedance (in Ohms. not per unit) the greater the current unbalance for a given voltage difference.

But it is the rise of current in all three phases over time that is baffling, not the relationship of phases.
Also, why would pump/motor 3 AMPS rise so much, while Pump/motor 1 does not.

That answer will probably be found in the pump curves.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

I can't follow the data you posted. What is the pump 2 and Pump 3 current when run by themselves.

Current rising over time with nothing else changes makes little sense. You're going to have to figure out what else is changing as the current rises.

GE motors are usually decent, but you never know it could be a motor issue.

You mention "phase C current is low by about 1.5-1.8%", if you only have <2% current imbalance then that is quite good. To me, that isn't pointing to a PQ problem. Your voltage is a little low, but should still be OK.

 

[waross]

The current rising can be expected pumping rain water during a storm.
Pump one starts but there is more water coming in than one pump can handle, so pump two starts.
The water is coming in faster than two pumps can handle so pump three starts.
The water is coming in faster than three pumps can handle and the water level starts to rise.
As the water rises the differential head drops and the pumps deliver more volume and the current increases due to more work being done.
Pump curves 101.
Remedy?
1.- Live with it.
2.- throttle the discharges slightly.
3.- Trim the impellers slightly.

High Amps?
An old rue of thumb:
The current unbalance will be the square of the voltage unbalance.
2% voltage unbalance = 4% current unbalance.
3% voltage unbalance = 9% current unbalance.
4% voltage unbalance = 16% current unbalance.

Three possible causes for the voltage unbalance:
A transformer is on the wrong tap.
You are a long way from the last voltage regulator and there are no transitions in the line.
Failed capacitors in one phase of a capacitor bank.

The regular small steps in the voltages may be a voltage regulator with too small a differential setting.
They seem to match on all three phases.
That may be an indication that the voltage regulator is working on all three phases.

The first thing to check:
Is your local transformer on the same tap on all phases?


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[LionelHutz]

You have the current vs voltage imbalance relationship backwards in your numbers.

Rule of thumb doesn't matter much when you're at site since you should be measuring it. I wouldn't call any of the power quality "issues" significant or an actual problem if there is <2% current imbalance.

Yes, there are various possible causes of the rising current, but it makes little sense to speculate. Figure out what is actually causing it.

 

[waross]

Thanks Lionel.
Fixed it.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Slagathor]

No new info. We have not been able to run the station as it only can be run when there is stormwater. This makes troubleshooting sort of hard.

 

[waross]

This may be investigated without running the pumps:

Three possible causes for the voltage unbalance:
A transformer is on the wrong tap.
You are a long way from the last voltage regulator and there are no transitions in the line.
Failed capacitors in one phase of a capacitor bank.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Slagathor]

Well, we had some water, and came up with plan to isolate variables to troubleshoot this. The station has a back up Genset. We were going to run the pump station in the same manner, and record the same power data. But we could not get the Genset to operate. Apparently the staff has not been regularly test running the Genset to make sure it was functional. It was run when the Contractor finished the station testing, and it has never been run since! So of course as soon as we tried to start pumps...the whole system tripped.

So...no news till we have water again....probably Fall 2021.