# uneven amp draw



## tmessner (Apr 1, 2013)

What is an acceptable level of uneven current draw for a 3 phase submersible motor? I have a customer with a 50 hp 480 volt submersible pump. It is pulling 62 amps on lines 1 and 2 and 72 amps on line 3. We swapped the lines around and the high current stayed on line 3. The mfg is blaming the power company. I think the motor has an issue. What is acceptable for an unbalance?


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## cuba_pete (Dec 8, 2011)

15% is pretty high in my book. Swapping the lines eliminates the motor, but not the connections, but you should see see pretty good heating for a connection dropping 10 amps!

Swapped at the motor, or at the panel?

The motor will have heating in the windings too with that much imbalance.

Are you using a drive for the motor?


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## oliquir (Jan 13, 2011)

if imbalance is always on line 3, it means the motor is ok, can you borrow an oscilloscope or power meter to check line quality (sinus form, harmonics,...)


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## tmessner (Apr 1, 2013)

The motor is 50 ft down a well. It is across the line starting. I have not checked the power quality yet but that is my suspicion. I am hesitant to blame the poco as around here it seems that they take the rap for everything and they usually work with us very well.


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## just the cowboy (Sep 4, 2013)

Couple of questions.
What is the name plate information Amps and SF?
What are you calling Line 3, the L or T leads? 
Is the high current always on the same T lead?
Even the 63 amps on the other 2 legs is close to full load.

Cowboy


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## VELOCI3 (Aug 15, 2019)

All of the above and should check the wiring in the motor to make sure the leads are connected correctly for the supply voltage. 


Sent from my iPhone using Tapatalk


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## tmessner (Apr 1, 2013)

Line 3 is the L lead from the poco. The high current stays on the poco line 3 when the leads are rotated. On a submersible install the only info we are given is the motor hp. The data plates are not to be seen we we show up. Usually submersibles run on the side of load and sf.


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## gpop (May 14, 2018)

Voltage reading would be helpful


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## 460 Delta (May 9, 2018)

gpop said:


> Voltage reading would be helpful


A little investigation with an accurate volt meter is needed next.


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## Bird dog (Oct 27, 2015)

gpop said:


> Voltage reading would be helpful





460 Delta said:


> A little investigation with an accurate volt meter is needed next.


Maybe check voltage drop across the contactor contacts for L3 (maybe 1 or 2 volts is fine, the higher the voltage the higher the contact resistance).


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## Cow (Jan 16, 2008)

tmessner said:


> What is an acceptable level of uneven current draw for a 3 phase submersible motor? I have a customer with a 50 hp 480 volt submersible pump. It is pulling 62 amps on lines 1 and 2 and 72 amps on line 3. We swapped the lines around and the high current stayed on line 3. The mfg is blaming the power company. I think the motor has an issue. What is acceptable for an unbalance?


What was the service call that got you turned onto this problem?

Overload trip?

At the point you are at now, I usually get out a couple of meters to verify voltage rather than just relying on one meter. My Fluke 289 is usually the one I go with for the final voltage reading when it's all said and done though.

I've had submersibles over the years with pretty significant amperage differences, I've never had much luck getting to the bottom of the issue. I've rolled phases, verified contacts, done voltage checks. I think I had the power co. out once probably 10 years ago and they found a hot connection at the pole mounted transformer they fixed. All I know is what I was told a long time ago. A little bit of a voltage difference makes an exponential current difference. So even a few volts can make for a big current imbalance.


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## just the cowboy (Sep 4, 2013)

What are the resistance readings? Hard to tell most times but sometimes you can see a difference. 
I assume you megged it


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## tmessner (Apr 1, 2013)

just the cowboy said:


> What are the resistance readings? Hard to tell most times but sometimes you can see a difference.
> I assume you megged it


Not yet, that is to be the next step. After all of the responses I am leaning to the poco. 

I am not sure what the trigger was as I am not personally involved. I think the customer was curious about the load and checked the current draw on his own and then called us. I know the voltage was checked and I was told it was even but it may have been off a few volts. Normally I do not worry if I read 485, 485, and 489. In this case it may be a problem. I will post back when I know more.


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## gpop (May 14, 2018)

Poco might not be much help as they are allowed a percentage of voltage imbalance so i tend to call them last. 
If they are low on one leg they may be willing to bump up the nearest regulator even if they are with in limits but if you go off half cocked and blame them before doing meg/voltage tests they might be less then helpful.


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## tmessner (Apr 1, 2013)

agreed. I am on very good terms with them and I am not going to ruin that relationship.


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## micromind (Aug 11, 2007)

Cow said:


> A little bit of a voltage difference makes an exponential current difference. So even a few volts can make for a big current imbalance.


Especially with a high-strung motor like a hermetically sealed A/C compressor or a submersible pump motor.


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## cuba_pete (Dec 8, 2011)

We have two new lift pumps going in in a few months. It’s contracted out, but I’m going to have to follow along and get a baseline. Should be fun...except for the chitty parts...but work’s work.


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## paulengr (Oct 8, 2017)

micromind said:


> Especially with a high-strung motor like a hermetically sealed A/C compressor or a submersible pump motor.



It’s ALL motors. A 1% voltage unbalance causes a 6/8% current unbalance.

There are arguments but the way you measure these things is average the currents (or violates). Then subtract the average from all three. Then multiply by 100 and divide by the average so you get the percent difference. Then take the largest reading, ignoring the signs. Sounds like a lot but takes about a minute to calculate.

By NEMA MG-1 (the motor standard) up to 1% current unbalance no issue. After that you derate the motor. There is a chart for that. Derate to 75% at 5% current unbalance and not allowed to go beyond this.

So POCO is allowed voltage unbalance but it’s basically nothing...under 1%.

The way you track these things down is measure BOTH voltage and current unbalance. Multiply voltage unbalance by


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## paulengr (Oct 8, 2017)

Darn...hot send. Multiply voltage unbalance by 7 to see how much it explains if what you are seeing. If you have voltage unbalance it always comes from upstream. If only current unbalance its downstream or in the motor. So check multiple spots.

I’ve found the issue caused by fuses (partial melt), fuse holders, VFDs, contactors, and often the utility especially one that tries to use wye-wye transformers everywhere. Delta eye transformers naturally tend to mix/blend currents and quell some voltage issues.

It’s also a sign of shorted turns among other things within the motor. Aside from the standard battery of offline tests (usually Baker) most motor shops run motors at least once and check current unbalance to catch things the Baker can’t find.


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## tmessner (Apr 1, 2013)

results of personal site visit: 50 hp 480 volt 3 phase Franklin submersible motor/pump
line side volts : l1-l2 l2-l3 l3-l1
494 487.5 491.8 about 1.3% difference
load side of starter 493 487.4 490.8

full load current l1 l2 l3
71.4 71.4 64.2 about 9.6% difference
The Franklin rep says no more than 5% at the most. He would prefer about 2%. There was 0 v drop from the line to load side of the starter. about .2 v drop from the line side of the main disconnect to the load side of the starter. I know the voltage will change depending on how much load is on the poco system at any given time.


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## JRaef (Mar 23, 2009)

tmessner said:


> results of personal site visit: 50 hp 480 volt 3 phase Franklin submersible motor/pump
> line side volts : l1-l2 l2-l3 l3-l1
> 494 487.5 491.8 about 1.3% difference
> load side of starter 493 487.4 490.8
> ...


Just a word of warning; in a past project I did with Franklin Pumps, they went to far as to void the warranty on the pumps if the current imbalance was more than 5%. That can be REALLY tough when you are anywhere near residential services, because the utility can't control how many residences are on-line at any given time, turning on A/C, cooking, doing laundry etc. with all single phase loads, which is what causes the voltage imbalance.


There are a few Solid State Soft Starters that advertise a "voltage rebalancing" feature, although that's not what it really does; they just measure the line and LOWER the other two phase voltages to make them equal. The problem with that is that you then cannot use a bypass contactor, so using them outdoors becomes all but impossible at higher HP sizes.


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## paulengr (Oct 8, 2017)

tmessner said:


> results of personal site visit: 50 hp 480 volt 3 phase Franklin submersible motor/pump
> line side volts : l1-l2 l2-l3 l3-l1
> 494 487.5 491.8 about 1.3% difference
> load side of starter 493 487.4 490.8
> ...



Let’s see. 493, 487.5, 492.8. Average is 491.1. Biggest difference is second phase which is 3.6 V lower or -0.7%. Not sure how you got 1.3% but the load side of the contactor is what the motor sees assuming no loose connections.

With 6-8% current unbalance per % voltage unbalance that’s 4.2-5.9% of the current unbalance.

So current average is 69.0 so current unbalance is 7.0%. Be careful to do these calculations exactly the way NEMA says to do them.

Better than half your current unbalance is from the voltage unbalance. Some is coming from somewhere else. Focusing on the voltage aspect using a VFD would fix it although not cheaply so you’d need other justifications for doing it. A second option is to use a 480:480 delta-wye isolation transformer or ask the utility for delta-wye. They probably won’t do it but it never hurts to ask. This type of transformer mixes voltage from two of the three phases so the output is more even. This should get you down well under 5%. The VFD can give you a power savings to offset some of the cost.


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## gpop (May 14, 2018)

Ask poco if they have voltage regulators in the area. If they do ask them if they can check them as you have a leg that's a little low. 

Last place i worked we owned the sub station so it wasn't a big deal to ask poco who we contracted for maintenance to calibrate the regulators. If a regulator failed, jammed, lost control or was accidentally left in manual it would cause all sorts of problems.


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