Pump motor issue.

[itsmoked]

I have a thread over in "Pumps".
thread407-280636


To sum it up there is a pump down a hole that has a 10hp motor on it.

The load is about exactly 10hp.

The power is open delta. Two transformers mounted on a pole directly next to this well are running ONLY this pump.

The voltage at the motor starter is 242V between all legs.
The wire is number 6AWG running about 50 feet.

The motor, (a 10hp Hitachi), has an FLA of 29A.

Here's the problem: The currents are running at 31A, 31A and 34.5A...

Scotty has posited that the motor is probably a 230V expecting a 10V drop down a bunch of 'just big enough' cable. In this case the motor is likely having about 241V at its terminals.

Could we be seeing some saturation adding these extra amps?

The other question is why this nasty current imbalance? Could this brand new motor be bad?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ccjersey]

I know of instances where the rubber "bearings" in a submersible pump swelled over time and eventually loaded the motor so badly it would no longer start, so I believe there is always some doubt as to what the load on such a motor is exactly. You can calculate it, but you have a hard time knowing the un-know-able conditions which may be affecting the load. Any chance of some recirculation in the well going on? I know some wells have a bleed fitting a few joints of pipe down the casing to drain out when the pump cycles off so air can be admitted and keep a pressure tank charged. Have had these fail and the pump be running trying to keep up with the leak.

On the current imbalance, I guess I am used to seeing this kind of thing since all our 3 phase is open delta. It helps when the motor is not fully loaded. I would expect pretty dramatically shortened life of one like yours which is already running in the service factor with imbalance to boot. I am also interested in the explanation of why it occurs.

Have you rolled the phases to see if you could influence the imbalance.

 

[rbulsara]

I quickly read thru the other thread.. not in detail.

Both ScottyUK and BigInch have plausible points, including chasing a non-problem.

Overvoltage effect can be easily checked if you have means to lower the voltage to say 230V and see it makes any difference.

As for the nameplate ratings, they are accurate (for sizing wires and protection etc) but never precise. The actual motor HP as designed and constructed could very well be 10.5 HP or 11 HP.

Some "imbalance" may have to do with open delta source, not sure. Unless there is a real problem like overheating, I would not worry too much. At the most you can certainly lower the current by reducing load, if it makes you sleep better at night.



Rafiq Bulsara

http://www.srengineersct.com

 

[itsmoked]

Thanks guys.


ccjersey; The wires were rolled with no change resulting.
No recirc is occurring.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The real world condition that must be taken into account with many open delta banks is the neutral loading and the resulting voltage drop on the primary neutral neutral conductor. This can cause a phase shift with resulting unbalance. However, in your case, the balanced voltages strongly suggest correct phase angles. The motor may be causing the current unbalance. Is this consistent with the results when you rolled the connections?
If the designer did as good a job of sizing the piping as was done with the electrical sizing, it may be that the pump needs a little more back pressure. I would try throttling the output a little.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Does "no change" mean the high current stayed with the same motor lead?

Do you know the rated voltage of the motor? A 200V motor designed for a 208V service will likely show a higher current when running around 240VAC supply.

Do you happen to have a suitable VFD you could try? It would make a simple task of trying different voltages.

You also never posted how willingly the owners of this system will spend money to fix the problem since it's not too likely that any fix can be done for free. The transformers just might have internal taps but that's about the only simple fix I can think of besides just throttling the output until the current drops.

 

[itsmoked]

Bill and Lionel:

Sorry but I can't tell you if the high current stayed with the same leg of the motor or the same leg of the source. Just that, "The currents measured the same after rolling the leads".

The motor is rated at 230V not 200V.

It was found that the valve at the well head had to be closed by 80% to drop the current to FLA. This seems excessive and becomes hard to explain to the customer.


------------------------

The present thinking is that since we are doing everything correctly voltage and wiring wise, and are below the flow the pump is rated at, we shouldn't be seeing a motor current overload of 12%.

Nor, can that large a difference in leg currents be explained easily. We are contemplating replacing the motor with either an identical unit or a different brand even though it's only been run 1/2hr total.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rbulsara]

It may be worthwhile to check the power factor too. It may not be what is rated for. Check the actual kW being consumed using a meter, rather than relying just on amps and voltages.

There could indeed be some mechanical issue causing extra losses.

Rafiq Bulsara

http://www.srengineersct.com

 

[zlatkodo]

Can you accurately measure ( no clamp meter ) the no-load current when the motor is connected to a balanced voltages ?
Zlatkodo

 

[waross]

A heads-up Keith. If the other loads on thew utility meter may be turned off for a few minutes, you may use the kH factor to measure the draw with the utility meter. It takes about two minutes or less. Yuor meter will have a number on the face called kH. This is the Watt hours consumed by each turn of the dial. I often record the time for 5 or 10 turns. Do the math. Note: Watt hours, not kilowatt hours.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[krbernier]

how much static head does the pump have?
i have a well site that is a constant problem in the spring that goes away during the summer months.

the cause was too much water in the hole allowing the pump too run off its curve.

once the water table dropped no more problems.'

i also tried throttling the discharge but ran into the same problem as you.

check the depth of the motor versus the static level of the well and then look at your pump curve at that amount of head.

 

[Compositepro]

Statements like the valve had to be closed by 80% should not be given much weight. Most valves are not designed for throttling and therefore will not introduce much resistance to flow until almost fully closed. Pressure drop across the valve would be more meaningful.

 

[itsmoked]

rbulsara; [red]"[/red]As for the nameplate ratings, they are accurate (for sizing wires and protection etc) but never precise. The actual motor HP as designed and constructed could very well be 10.5 HP or 11 HP.[red]"[/red]

Yes, that is sort of what's panning out.

Sub pump makers appear to be lying in their data sheets. The motors are 10hp because that's what they're designed as. The pump people are in competition so they continually ratchet up the flow developed by the pumps.

"Buy our pump because hooked to a 10hp motor you get more flow than the other guy."

This in turn has to raise the HP requirements. It drives the motors into their service factors. This has gone on to the point that virtually all subs run in the SF all the time at their curve peaks.

They seem to get away with it because the motors run in cooler water than designed for. And, even running in the SF they last just enough years to avoid psychotic customers.
Today a pump company engineer agreed that the BEP point on their curve was in-fact the SF value not the FLA value..


zlatkodo; Unable to make those measurements.


Bill; Thanks for reminding me about reading the POCO meter!! I'll have them check that out.

Good point krbernier! There's 30ft of head.
Does additional suction head add to the flow directly or just thru reduction of final head?

Compositepro; You're absolutely correct. The pipe could be grossly over sized so a valve would need to be mostly closed to even get near the pumping volume. It's a ball valve which is about as bad as you can get in throttling.

The correct answer would be flow. The flow has to drop from 90gpm to about 50gpm to drop the load to FLA.



Keith Cress
kcress -

http://www.flaminsystems.com