Re: Any experts on 3 phase power issues?
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Thanks Brett, one other data point, my single phase loads, (lights, pool pump part of the day, and 5 Ton heat pump are maybe 30A and I have a 600A service.?
There is maybe 0.1V drop when those are running.? 600A service has fat wires.
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From: [email protected] < [email protected]>
On Behalf Of Brett Wissel
Sent: Monday, July 26, 2021 2:26 PM
To: [email protected]
Subject: Re: [FOG] Any experts on 3 phase power issues?
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Joe,?
Okay, between your reply and Mark Thomas's earlier reply, my next thoughts are that the voltage differential is likely the transformer impedance?correction lugs. Chances are they made some adjustment when you first got attention from the
electric co, and now needs to correct it on the other side. That would be the result of 2 transformers matching each other but the 3rd phase being separately transformed down way back at the last step-down leg in the distribution line.
After that, I'd be inclined to agree with Mark Thomas's remarks that the unbalanced single-phase loads would be causing the difference since the single phase is a direct pull off the one leg, and shouldn't be regarded as a malfunction.
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Brett,
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No question, I have open delta. Two new transformers
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The utility measures Leg ¡°A¡± to neutral and Leg ¡°B to neutral at the transformer and at the service entrance to so if there is meaningful voltage drop differences.? They also measured
leg to leg and Leg ¡°C¡± to ground.? They did this with a Fluke RMS multimeter.?? I understand 3 phase power and motors, I am an electrical engineer.? Measured current is determined by voltage, and impedance and impedance is resistance, capacitance, and inductance.?
What I am seeking to understand is if the voltages are all in the correct phase relationship ie 120 degrees from each other, and the leg to leg voltages are perfect, what causes the current to be low on one leg, the same percentage for three different 3 phase
motors and a VFD?? Also if it were in my shop, then I would not see the current differences change day to day.
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I have open delta. L1-N=121.9V? L2-N=121.9V? L1-L2=240.1V L2-L3=240.1V L3-L1=240.1V?? L3-N=231.4V
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I measure current inside my panel after the breaker for each motor. The utility measures at the service with includes all loads. We¡¯ve done experiments where I turn off all loads except
for each to be measured.
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They installed the data logger inside the large commercial style service entrance.? Left if for 4 days.? Goal was see if the voltage phase relationships were stable and they were. I was
not running any 3 phase loads at that time as the current on leg ¡°B¡± was so low they feared it would harm a motor.? Agree the current at the service entrance is not where to measure for balance.? In the experiments where I isolated loads, I tuned all breakers
off except for the load we were measuring and they used the power quality meter to measure. We also added single phase loads to see the impact they had.
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The phase relationships for voltage was perfect for all three legs but the phase shift for
current was different on each leg but not a big difference.? Just a couple of degrees difference.
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Joe,
I'm a little puzzled, and have a couple comments and questions:
1) If you have two transformers, you have Open Delta -- zero question about that.? This was common in rural areas.? It's cheaper because 2 transformers cost less than 3 transformers.? This configuration has some downsides, but is not necessarily your problem.
2) You cite voltage measurements A-N(eutral), B-N, and C-N.? This is absolutely irrelevant.? Three phase motors do not get a Neutral.? So you are measuring something that your motors do not and cannot ever see.? ?The only voltages a motor sees are A-B, B-C,
and C-A.? ??
3) If you have a high-leg delta, it is impossible that A-N, B-N, and C-N are all 120v (plus/minus couple percent).? Something is wrong here.
4) Exactly how, and exactly where, are you measuring current?? You said you stopped using all motors, but you also said that you are measuring current.? Do you mean you did in fact start
and run each motor, and used an ammeter and measured current on all three legs?
5) Where is the data logger and what is running during the periods you are referring to that the data logger records inconsistent current?? Note that it is expected? that current will not be the same on all legs when you have a mix of loads.? If you have lights,
air conditioning, and one three phase motor running and you measure current upstream of the panel, it's guaranteed it will not be balanced, and this is not a problem.??
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Brett Wissel
Saint Louis Restoration
1831 S Kingshighway Blvd (at Shaw Blvd)
St Louis, MO 63110
314.772.2167
brett@...
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Joe Jensen