Unbalanced open delta loads

[kclark8799]

I have a 15hp 3phase 230 v. pump. Voltage is good phase to phase but amps are 16 , 34 , and 34. On stand by generator everything is great at 34 amps. Power company replaced everything but the pole with no change. Even followed the primary when crossed. Any ideas.

 

[zbang]

What are the pump's L-L volts on utility power? What's the service configuration and voltages?

 

[ptonsparky]

You can help balance the the load for motors on open delta by rotating the phases. Move A to B, B to C and C to A. Repeat until the imbalance is the least.
If the problem moves with the change in leads, its the motor, otherwise the POCO.


You are sure the service is not Corner Ground?

 

[MTW]

Likely your open delta voltage balance is dropping under load. With the generator the balance remains steady, but utility open delta it is usually comprized of a larger split phase transformer and a smaller wild leg transformer to make up the other two phases voltages.

Are the transformer sizes on thr pole labeled with their size? What was the size of your generator that gave balanced conditions during running?

 

[LarryFine]

On stand by generator everything is great at 34 amps.

You are sure the service is not Corner Ground?

Sounds like a good possibility here.

 

[hillbilly1]

You can help balance the the load for motors on open delta by rotating the phases. Move A to B, B to C and C to A. Repeat until the imbalance is the least.
If the problem moves with the change in leads, its the motor, otherwise the POCO.


You are sure the service is not Corner Ground?

That sounds likely, parallel path on the grounded phase, causing the reading on that leg to be half, because the current is flowing two different ways.

 

[hillbilly1]

I have a 15hp 3phase 230 v. pump. Voltage is good phase to phase but amps are 16 , 34 , and 34. On stand by generator everything is great at 34 amps. Power company replaced everything but the pole with no change. Even followed the primary when crossed. Any ideas.

Check phase to ground voltages. If one phase is zero volts to ground, you probably have a corner grounded delta.

 

[kclark8799]

Phase to neutral are 124, 214, 124.

 

[zbang]

Sure looks like a 240/120v 3ph 4w "high-leg delta" configuration.
a) the generator is probably wired as 208/120v wye, check that
b) which of the motor leads with the low current corresponds to which power leg/voltage?
c) this starts to look like a power company problem, but rotate the motor leads as suggested in post #2 and see if the low current stays with the motor lead or the power leg.

 

[hillbilly1]

Phase to neutral are 124, 214, 124.

What about to ground? They may not have bonded the neutral at the service. Definitely strange!

 

[jim dungar]

Phase to neutral are 124, 214, 124.

Adjectives are important.

To me an open delta means you have a 240V 3-phase 3-wire system feed by two equal size transformers.

You have a center tapped open delta which gives you a 240/120V 3-phase 4-wire system fed from two transformers, which are likely different sizes.

 

[kclark8799]

Sure looks like a 240/120v 3ph 4w "high-leg delta" configuration.
a) the generator is probably wired as 208/120v wye, check that
b) which of the motor leads with the low current corresponds to which power leg/voltage?
c) this starts to look like a power company problem, but rotate the motor leads as suggested in post #2 and see if the low current stays with the motor lead or the power leg.

We did swap around the pump leads and the low leag stayed on A phase. No change. Now the power company changed the wiring on the pots and it followed no the low amp leg is B phase.

 

[zbang]

...are they going to do anything about it?....

 

[kclark8799]

They replaced everything but the pole. FPL engineers are straight out of college 25 with no experience. They are done unless I can prove to them differently.

 

[ptonsparky]

Phase to neutral are 124, 214, 124.

These are voltages while under motor load or static no load?

Are you tripping overloads? What brought this issue to your attention?

 

[Joethemechanic]

What are the voltages at the service with the pump running under load? There has to be high impedance on the high leg somewhere. It's only a 15 HP motor, I can't imagine they undersized the pole pig that much. Are there other customers on that primary circuit?

 

[hillbilly1]

With the amperage being half of the other two, I keep coming back to there being a parallel path somewhere. With the amperage’s being equal on the generator, that rules out the motor. I would think there would be some overheating issues if the amperage is actually half on utility. That’s why I think checking voltage to ground is necessary. The center tap on the transformer may be connected to the neutral, but it may not be bonded or grounded, and a fault on one phase to ground would not trip the ocp or transformer fuses.

 

[Joethemechanic]

Primary is an open wye isn't it?

 

[zbang]

Hmm, maybe take the motor out of the equation-
find yourself three 8kw 240v heaters, wire them in delta, and connect to the motor disconnect. When that's energized, if you don't get even current readings, there's a problem either in you premises wiring or in the service itself. Take voltage and current readings up the line until you get to the service disconnect should lead to the problem. Or even move the dummy load from the motor disconnect to another 3-phase breaker somewhere else in the system.

If the issue continues all the way up to the service disconnect, call the PoCo back.

Get lots of voltage (L-L, L-N, and L-G) and current readings, the only extra cost is your time.

 

[hillbilly1]

Hmm, maybe take the motor out of the equation-
find yourself three 8kw 240v heaters, wire them in delta, and connect to the motor disconnect. When that's energized, if you don't get even current readings, there's a problem either in you premises wiring or in the service itself. Take voltage and current readings up the line until you get to the service disconnect should lead to the problem. Or even move the dummy load from the motor disconnect to another 3-phase breaker somewhere else in the system.

If the issue continues all the way up to the service disconnect, call the PoCo back.

Get lots of voltage (L-L, L-N, and L-G) and current readings, the only extra cost is your time.

And N-G.