# 480V High leg?



## just the cowboy (Sep 4, 2013)

*What are phase to phase voltage*

What are phase to phase voltage??

Sounds like a ungrounded system with a phase faulted to grounded in the middle.


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## MDShunk (Jan 7, 2007)

Eh, voltages are pretty much in line with what I'd expect from a center-tapped 480 delta system. Nominally, you'd expect 240 Phase-neutral on the two outsides legs, and about 420 phase-to neutral on the high leg. Your nominal voltages are probably running a tad high. No big deal, in my opinion.

You injector pump overload tripping out is another matter. Your phase to phase voltages are really all it cares about. I doubt anything in your setup is using the neutral, so ignore your P-N measurements. Your injector pump might have mechanical issues and it's righteously tripping out. Unattended installations are a "special treat" to troubleshoot.


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## wknoxfarms (Jun 23, 2016)

L1-l2 500v
L1-l3 520v
L2-l3 560v


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## MDShunk (Jan 7, 2007)

wknoxfarms said:


> L1-l2 500v
> L1-l3 520v
> L2-l3 560v


Yeah, pretty much what I expected based on your phase to ground measurements. Why do they have you tapped up so hot? Lot of summer voltage sag in that area when the pivots are needed most? The disparity in your measurements concerns me a little, but I'm at a loss to explain it at the moment. It's on the PoCo's end to sort out.


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## wknoxfarms (Jun 23, 2016)

I found out the pivot is fed by single phase 240 and then to a phase converter and then boost transformer. So depending on if its a multi tap transformer i could retap and get the voltage in line. But has anyone seen where a phase converter would cause that much imbalance?


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## JRaef (Mar 23, 2009)

wknoxfarms said:


> I found out the pivot is fed by single phase 240 and then to a phase converter and then boost transformer. So depending on if its a multi tap transformer i could retap and get the voltage in line. But has anyone seen where a phase converter would cause that much imbalance?


Yes, it's the nature of the beast with a phase converter. If it is a rotary phase converter (RPC) and you you had the time and everything was stable, you could futz around with the capacitors on the manufactured leg to get the voltage reading a little closer. But that's something even seasoned veterans have difficulty with and if you get something wrong, it can get very expensive very fast. If it's a static phase converter, don't waste your time, they are fugly to the power system. 

What's likely happening is that your somewhat extreme voltage imbalance is causing a severe current imbalance: a voltage imbalance of just 2% results in a current imbalance of over 10%, your voltage imbalance is over 5% so I would suspect your current imbalance is well over 25%, maybe 30%. What that means is that the motor will heat up a lot more than it would normally, even if the highest leg current is still within the nameplate limits. Some types of overloads will detect that current imbalance and trip to keep the motor from being damaged. If you can't correct the voltage imbalance, you could get a bigger injection pump motor than you need, only so that it can handle the added heating effect from being fed unbalanced voltage.


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## wknoxfarms (Jun 23, 2016)

Thanks all for your thoughts, things are starting to add up here. I didnt realize that small of a voltage imbalance led to that high of a current imbalance. It makes sense now that i think about it. 

It is a RPC and a pretty old one at that so i wouldnt doubt the capacitors aren't in top shape. Im thinking a VFD might be the way to go for phase conversion in the future as there isnt much for total motor load to run the pivot itself. If i remember right when using a VFD for phase conversion you derate by 50%?


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## JRaef (Mar 23, 2009)

wknoxfarms said:


> Thanks all for your thoughts, things are starting to add up here. I didnt realize that small of a voltage imbalance led to that high of a current imbalance. It makes sense now that i think about it.
> 
> It is a RPC and a pretty old one at that so i wouldnt doubt the capacitors aren't in top shape. Im thinking a VFD might be the way to go for phase conversion in the future as there isnt much for total motor load to run the pivot itself. If i remember right when using a VFD for phase conversion you derate by 50%?


Generally, yes. Even though technically the simple current relationship is the sq. rt. of 3 (1.732X), you need extra capacitance to deal with the added DC bus ripple from having a single phase supply, so doubling the size of the drive per the motor nameplate amps gets you that added capacitance. On smaller VFDs that don't have DC bus chokes in them (usually under 15HP and most Asian brands), the capacitors will also run very hot, so you must either de-rate the operating temperature to about 30C (86F), or use a de-rate on the drive sizing of 65%; ie use a 30A drive for a 10A motor. You never said what the HP size is.

Also though, not all 480V drives can be fed with single phase. Technically it's always possible, but some mfrs use a Phase Loss trip circuit on the input that cannot be defeated, so they would not work for this. To make sure, read the manual of whatever drive you want to use and see if it monitors for _Input_ Phase Loss and if it does, dig through to see if you can turn that feature off. If not, find a different one.


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