Steve Courtney
Member
Tomorrow first thing, I'll report back our findings. Thank you!
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High current on the high leg
- Thread starter Steve Courtney
- Start date
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Tomorrow first thing, I'll report back our findings. Thank you!
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kwired
Electron manager
- Location
- NE Nebraska
Is the AC units the only load using the third phase (high leg)?
How much other load is there on the 120/240 "lighting" pot?
I still think it may be possible the lighting pot maybe is heavily loaded dragging it's voltage down some, but the stinger pot is not really that heavily loaded for it's rating and isn't being dragged down and that is why there is such an imbalance
You seem to have near 250 volts from L2 to each other line, normal high leg voltage if line to line voltages is 250 would be about 216-217 volts getting close to your measured 220, but if the lighting pot is being dragged down to 238 then 118-119 to ground on those would be expected.
You can't expect current balance in a three phase motor if input voltages aren't balanced.
Add: then after prolonged usage at unbalanced currents, the condition of the motor possibly deteriorates and the unbalance gets even worse.
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JFletcher
Senior Member
- Location
- Williamsburg, VA
That thought crossed my mind too, with either a lot of kitchen equipment running or site lights at night.
The 1ph motor would account for some imbalance but not what the OP has. Blowing compressor motors over the years as the OP said seems to me to be a power problem (unless they are all underwired...).
I fail to see how a high leg voltage of even 240V to g/n would matter when all of the line to line voltages are right on the money. If I had to take a wag, I'd say there is an overload on the larger pot with single phase loads.
A power logger may be more helpful than instantaneous readings.
On an aside, I saw a Carrier 64Ton unit (2 compressors) blow 3 compressors in less than 2 years due to a leak in the system. HVAC guy recharges it, more oil, which eventually formed a slug that hydraulically locked the compressors, and metal carnage ensued. Nothing to do with the electrical at all.
Steve Courtney
Member
Here is our recorded voltage and amperage readings after a new compressor install on unit 1.
The following was provided by our HVAC Group:
June 6, 2017
86 degrees outside
PM heavy load
*Single phase condenser fan motor moved to L1 and L3
L1 to ground- 118.7 volts
L2 to ground- 216.9 volts
L3 to ground- 119.2 volts
L1-L2- 245 volts
L1-L3- 239 volts
L2-L3- 240 volts
Amps
L1- 21.2
L2- 27.5
L-3 20.2
*FULL LOAD AMPS OF THE COMPRESSOR IS 25
***The refrigerant charge was left at 80% of the total charge in order to keep the high leg at 27.5. The end result is the equipment is not running at full capacity. We know that as the outside temperature increases the amperage will increase across the board.
Trane says “any amperage unbalance is unacceptable, and the nameplate FLA can never be exceeded, but the voltage can be unbalanced by 2%”. In the real world, most motor experts say that anything below 10% amperage unbalance is acceptable.
Check my math and let me know.
Average amps = (21.2+27.5+20.2) /3= 22.97
(High reading 27.5 minus average 22.97) /average 22.97 x 100 = 19.7% unbalanced
If this is correct, we have to get the unbalance between 0 and 10% with a full refrigerant charge.
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The following was provided by our HVAC Group:
June 6, 2017
86 degrees outside
PM heavy load
*Single phase condenser fan motor moved to L1 and L3
L1 to ground- 118.7 volts
L2 to ground- 216.9 volts
L3 to ground- 119.2 volts
L1-L2- 245 volts
L1-L3- 239 volts
L2-L3- 240 volts
Amps
L1- 21.2
L2- 27.5
L-3 20.2
*FULL LOAD AMPS OF THE COMPRESSOR IS 25
***The refrigerant charge was left at 80% of the total charge in order to keep the high leg at 27.5. The end result is the equipment is not running at full capacity. We know that as the outside temperature increases the amperage will increase across the board.
Trane says “any amperage unbalance is unacceptable, and the nameplate FLA can never be exceeded, but the voltage can be unbalanced by 2%”. In the real world, most motor experts say that anything below 10% amperage unbalance is acceptable.
Check my math and let me know.
Average amps = (21.2+27.5+20.2) /3= 22.97
(High reading 27.5 minus average 22.97) /average 22.97 x 100 = 19.7% unbalanced
If this is correct, we have to get the unbalance between 0 and 10% with a full refrigerant charge.
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Steve Courtney
Member
The condenser fan motor as well as all of the other single phase loads were attached to L1 & L3
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drcampbell
Senior Member
- Location
- The Motor City, Michigan USA
- Occupation
- Registered Professional Engineer
Have you tried measuring the voltage & current at the compressor terminals?
kwired
Electron manager
- Location
- NE Nebraska
Gave those numbers some more thought. I think I mentioned before, or at least something to effect that if line to line voltages were all 250 volts then A and C to neutral would be 125 and B to neutral would be 216.5.
Now draw voltage down from other loads in the facility between A and C so that you have 238 between them and ~118-119 to neutral but leave load on B minimal enough you still have 250 from B-C. Drawing out that half of the delta with 119 on one side, 250 for hypotenuse leaves the other side at 219.86, that is why the high leg voltage is as high as it is.
What doe service voltage readings look like, both at minimal demand and at peak demand?
What is kVA rating of the transformers in the bank and how much load is connected - especially at moderate to high demand periods?
I still am convinced the size of the "lighting pot" possibly needs increased, or maybe if the stinger pot has the ability to change supply tap to lower output voltage so it better matches the loaded lighting pot. If POCO can't/won't do anything maybe an autotransformer in the high leg to better balance voltages.
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