Voltage imbalance tripping 75 HP sumbersible.

[11bgrunt]

75HP water pump has a Submonitor connected. Owner has raised imbalance trip to 10% to keep motor on line. Utility voltage measurements have been hovering around 3%+ between any two legs at meter socket. This motor has run for 2 months without a trip caused by imbalance.
Served 480/277. When I checked control today with motor running, there was a 5c difference in lug temperatures at the main breaker. Amps were 84,88,92. Voltage at main in control was what I call 1%. 475,476,480.
I have not been able to get a no load voltage check. Does anyone think the temperature difference is great enough to trip control out on voltage imbalance? The ambient temperatures lately have been running 0c-30c. Lug temps at 125 amp main today were as high as 80c.
I checked across the breaker , line to load, and each leg measure 250 millivolts.
Motor tripped once Saturday and once Sunday. Has run for two days no problem.

 

[GoldDigger]

If one termination is hotter, it probably has a higher resistance and therefore a higher voltage drop across the termination.
Where is the monitor measuring the voltage?
And are you sure that it is not also monitoring current, since the imbalance there is +/- 5%?

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[ptonsparky]

The Submonitor monitors the current imbalance. They work. Your voltage appears to be good. Try rotating the load conductors until you find where the current is most evenly balanced. Move A to B, B to C, and C to A. Take current readings and repeat. That does seem to help on occasion and is or was in the Franklin Pump manual.

Maybe the voltage is good. One leg seems high. To late for me to do math.

 

[meternerd]

Not unusual for pump motors to have an impedance difference between the three phases. We set all of our phase monitors to 10% difference. Seems to be more of a problem with our deep submersibles. Most likely impedance from the wires going to the motor rather that the motor itself. Ditto on swapping wires as long as rotation stays the same. We as a POCO, try to supply voltages as balanced as possible, but line losses, single phase taps, etc. can cause voltage imbalances at the customer's location. Problem is, the high or low phases will vary depending on line loads, so what works one day may not work the next.

 

[11bgrunt]

Most in Engineering that I talk to, believe that submersibles are handicapped from the start because of hole depth, wire impedance and motor windings. I can agree with most of the logic.
Yesterday we attacked this account a little differently from an old lineman?s view.
Today we own tools that can measure the voltage on the primary conductor with reasonable accuracy and another that can identify the phasing on a wye/wye system easily from the primary through the transformers down the riser to the fuses and conductors by phase.
When I draw this information out, the trend is that one primary phase tends to run higher than the other two primary phases. I haven?t been given the chance to kill the load and get no load voltage checks. Engineering has asked that we install voltage/current recording equipment on the secondary to see if this is a utility or customer issue.
The technique used at this location with primary voltage checks is our first time gathering information this way.
Can the secondary load increase the voltage on the primary conductor?
Always interested in how others think.

 

[GoldDigger]

Handicapped from the start, and also relying on the fact that the motor is water cooled to run it close to its limit.
I do not see a way a three phase load can increase the voltage on one primary phase except in the case of an unbalanced line to neutral load on the primary combined with a POCO neutral problem..
If one primary line has high source impedance and the motor in question is acting as a phase converter to supply local single phase loads maybe?
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[electric_cal]

Interesting problem. I looked in the SubMonitor troubleshooting guide and under "unbalance" it lists three possible causes. Phase Loss, Unbalanced power supply, and Open delta transformer. Have you taken any voltage readings on the line and load side phases to ground? Can you possibly place a power monitor on the system to record the voltage and current?

 

[11bgrunt]

Interesting problem. I looked in the SubMonitor troubleshooting guide and under "unbalance" it lists three possible causes. Phase Loss, Unbalanced power supply, and Open delta transformer. Have you taken any voltage readings on the line and load side phases to ground? Can you possibly place a power monitor on the system to record the voltage and current?

Recorders were installed at fuse disconnect panel and a set in motor control. Voltages have been checked in every position at every location.
Motor dropped out about an hour after recorders were installed. Decision was made to leave in place until after the holiday. Maybe we will have info to lead us then.

 

[weressl]

The Submonitor monitors the current imbalance. They work. Your voltage appears to be good. Try rotating the load conductors until you find where the current is most evenly balanced. Move A to B, B to C, and C to A. Take current readings and repeat. That does seem to help on occasion and is or was in the Franklin Pump manual.

Maybe the voltage is good. One leg seems high. To late for me to do math.

Best suggestion: rotate.

The reading appear to be within the expectable range, submersibles are inherently more imbalanced than ordinary motors. Note the rule of I(Squared)t, meaning that the temperature will rise squared of the current, so clean and tighten your connectors, but temperature difference is based on the current. Conductors will also exhibit temperature difference in accordance with the current and weather the temp rise comes form a high resistance connection or the current difference can only be made if you start reading the temp away from the connection(conducted heat).