Rpc phantom leg voltage change

[Plum Creek]

After moving my rpc enclosure from a temporary spot in the shop to a permanent wall mount, I now have 220v on the phantom leg.
My rpc is basically WNY components built into a static converter running a 15hp Baldlor idler motor. It worked well enough , 118v on 2 legs and 126v on the phantom leg. Until..
This year I added a 3 ph cb panel and a motor controller for the compressor and the 3 ph floater line.
Full disclosure, this is my fault. I needed to move the components in the rpc enclosure to the opposite sides to match feed side of the power and wanted to tidy things up a bit. I wasn’t careful enough and ended up leaving a wire on the wrong side of the 1 phase power contractors. It ended up welding the power contractors closed. I replaced both the 1 and the 3ph contractors. Everything seems to work normally but now there is 220v on the phantom rather than the previous 126.
Obviously the phases are combining somewhere.

Switching legs of the idler circuit and the high 220v stays on the phantom side.

Removing all the idler wires and energizing the rpc circuit results in approximately 118v on all three legs

Is it possible that the potentiometer is permanently fixed at a point high enough to yield 220v?

Sorry I don’t have the schematics or pics here today I can add them tomorrow.
h

 

[Nutfarmer]

Are you measuring voltage between each leg with a three phase load? If you measure each leg to ground the readings can be all over the place and meaningless . The driven three phase motor only cares what the voltage between the windings are

 

[Plum Creek]

I was measuring between each leg and ground with the idler running

 

[Canus]

You need to measure from leg to leg not leg to ground. i.e. leg 1 to leg 2, leg 2 to 3 and 1 to 3. Each should be around 220.

 

[Beckerkumm]

Three phase 240 delta will have the manufactured leg measuring something at 208 or higher to ground . Dave

 

[Plum Creek]

Thanks guys!!
I sounds like its time for a trip back to the basics for me. Its been a while since I have looked at the parameters and my mind doesn't hang onto things as well as it used to.
The leg to leg voltages are a bit high, 258, 244, and 239, but thats not under load. I won't know the actual numbers until I get the compressor wired.
Thanks again, h

 

[Eddyde]

The leg to leg voltages are a bit high, 258, 244, and 239, but thats not under load. I won't know the actual numbers until I get the compressor wired.
Thanks again, h

A 10% difference between any two legs is considered acceptable, so you are okay even without the load. If you want to get it better you can fine tune your RPC by swapping out the run capacitors with different values.

 

[mksj]

RPC leg to Leg voltages should be checked under load for balance, balancing unloaded is not very relevant. Be mindful that the wild/generated leg should not be attached to transformers/control circuits, should be used primarily for connection to the motor(s).

 

[Plum Creek]

I may try to fine tune it later after the various loads are installed. I did check the RPC leads with a clip on ammeter and found that one leg was pulling 19a while the other was less than 4a. We will see how it looks after load.
Thanks MKSJ, I didn't know that. That could be why motor controller relay isn't closing. Ill swap leads and see if that will do the trick.

 

[Winegrower]

Be mindful that the wild/generated leg should not be attached to transformers/control circuits, should be used primarily for connection to the motor(s).

This is a really important point that probably bears reemphasizing. The schematics I've looked at, for lathe, surface grinder and bandsaw, use only two lines in for power to the controls. You can trace this back and figure out which line should only go to the generated leg.

Umm...I taught myself this out of surprise and initial panic when things didn't work as expected.