# 3 phase current imbalance



## WORDSMITH07 (May 7, 2016)

I'm having trouble with a small local utility ( Athens, AL ) in reference to my supply voltages on a 120/208/120 system.
Recent addition of two 3 phase machines and I thought I would verify current draws on both. Utilities state they are within tolerances but I disagree.

No load voltage
A 122.5
B 213.3
C 122.1

No Load to phase
A-B 241,5
A-C 245.1
B-C 248.2

Loaded voltage 
A-B 238
A-C 244
B-C 246

Currents machine 1
A 6.9
B 8.37
C 12.2


So I change leads on machine 1 below contactor to check motor windings and current stays high on C leg. Move to right to keep rotation same

A 6.78
B 8.21
C 10.02


Currents machine 2
A 7.8
B 8.2
C 10.2

Wire runs same length in conduit, #8 THHN 

What would you do?
Engineer visited (twice) and said he would get back. Now won't answer calls.

He disavows any possibility of harmonic issues...


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## telsa (May 22, 2015)

The trade expression for that Service is "Center-Tapped Delta" -- 240/120.

And your Poco seems to be delivering the correct voltage.


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## micromind (Aug 11, 2007)

Is the current imbalance causing a problem? If not and the highest leg is within specs, I wouldn't worry too much about it. 

If it is causing trouble, I would do a fall of potential test from a point closest to the service entrance to the machine terminals. This will prove whether or not there's excessive voltage drop in the wiring system. 

If it's good, then I'd call the POCO and have them monitor voltage on their side. 

Rolling the phases (moving each one to the right or left) is always a good idea, as you have seen. 

Also, is the load motors only or motors and controls? 

If possible, check ground current at the machine. More than once I have seen 120 controls connected phase to ground because there was no neutral or control transformer present.

P.S. Delta systems (like yours) have far fewer issues with harmonics that Y systems.


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## AK_sparky (Aug 13, 2013)

Voltages all look good to me. What is the actual problem you are having?


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## chrisfnl (Sep 13, 2010)

Are you sure the connections are good? 

I've seen minor corrosion in some bolted connections sufficient to cause single phasing in a motor. 

Not enough to notice with a regular ohm meter, but enough to cause voltage drop sufficient to cause the single phasing.


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## wildleg (Apr 12, 2009)

he's not returning your calls because you are wasting his time.

if you feel that the imbalances are a problem, or they don't meet a
specific industry standard, what standard is it that you are using that
they don't meet ? and what problems are you having that you have not
stated ?


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## JRaef (Mar 23, 2009)

I agree, you are incorrect about the supply voltage, there is no such thing as 120-208-120. You have a 240/120 3 phase 4 wire system where it is 240 phase to phase, and one winding is center tapped to give you 120 to Neutral from phases A and C only, then you will read 208 from C to neutral. 

Your voltages are balanced, so it is not a voltage imbalance that is causing your current imbalance. As chrisfnl suggested, it is more likely a connection issue, or something else in the motor circuit such as a severely burned contact on a motor starter, a conductor leaking to ground, something like that. 

You DO need to fix it though. A current imbalance causes what is called "negative sequence current" to flow in the rotor, in opposition to the normal rotation. That means your rotor is fighting itself to create torque, so for a given load, your motor is heating up disproportionately and can be damaged beyond repair without ever tripping the overload relay. We generally talk about the need for balanced voltages to avoid this, but only because a small voltage imbalance will cause a much larger current imbalance. It's really the current imbalance that is doing the damage.


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## wildleg (Apr 12, 2009)

you haven't spent much time describing these machines.

do they have single phase loads as well as 3 phase ?


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## AK_sparky (Aug 13, 2013)

wildleg said:


> you haven't spent much time describing these machines.
> 
> do they have single phase loads as well as 3 phase ?


My thoughts exactly. If the only significant load is a motor, then ya, you have problematic current imbalance. Did the machines come with any sort of commissioning report stating what the original current values were?


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## AK_sparky (Aug 13, 2013)

And still to be answered: Are you actually having any problems? If everything is in within limits and the machine is running properly, what's the problem?


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## WORDSMITH07 (May 7, 2016)

Thanks for all the input.
Machine 1 is a wood planer and the loaded figures previously given are with motor running with no wood being run through. The currents will increase dramatically under work load.
It is a small wood shop addition to plant with 10 single phase machines of 5 hp and less. The only 120 v single phase load is 8 amps for lighting and I changed it from A to C phase to see if that would help balance voltages in any way and it did not.
From what I have been told, drawing almost twice the current on one leg as another is indeed a problem. 

The contractors on machine one is closed and not available locally so I will swap contactor/relays tomorrow to see if one might be bad if high current "follows" contactor.
Yes the control voltage is part of the load.
Wiring is 55 feet of # 8 in EMT with additional #10 earth ground.
Utility is using two transformers (1 smaller for 208) and six businesses are serviced. Only one uses 3 phase and it is a diner.
There is another woodshop with single phase power but running a rotary phase convertor and owner stated the electrician had much difficulty balancing the voltages in its setup. I know very little about RPC's so I could take nothing from that.
Machine 2 has a single Square D magnetic starter and I will also inspect the contacts on it, but it was no surprise that it also had a higher draw on leg C.
Both are used machines and all windings both machines read awful close on fluke digital VOM.
Again, thanks for the help....


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## RePhase277 (Feb 5, 2008)

But what are the control voltages? Does any machine have a neutral? If so, you need to make sure that what ever the neutral is used for is not powered from the high-level phase.

What you have is an open delta with a high leg. Open deltas do have stability issues compared to closed deltas, but I don't think that's your problem here. You may just have a motor winding shorting to ground. 

Put your ammeter on the ground wire and see if any current is flowing on it. Also disconnect the ground and see if the phase currents get closer to balance.


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## telsa (May 22, 2015)

WORDSMITH07 said:


> Thanks for all the input.
> Machine 1 is a wood planer and the loaded figures previously given are with motor running with no wood being run through. The currents will increase dramatically under work load.
> It is a small wood shop addition to plant with 10 single phase machines of 5 hp and less. The only 120 v single phase load is 8 amps for lighting and I changed it from A to C phase to see if that would help balance voltages in any way and it did not.
> From what I have been told, drawing almost twice the current on one leg as another is indeed a problem.
> ...


You are describing the CLASSIC open delta 3-phase scheme.

The third phase is provided by a much smaller transformer. ( aka 'a stinger.' )

The main load is expected to draw from the big boy a center-tapped 240/120 phase transformer.

Everything about your situation is normal -- except the odd current imbalances.

I leave it to others to jump in.

My commercial and industrial experience has been -- almost entirely -- with Wye Services.

Sorry.


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