Utility High Leg, Open Delta Phase Converter, and a Motor That Can't Use Either

[winnie]

Why not just Buck the Highleg?

You can't have balanced L-L and L-N voltage simultaneously on a delta.

If you just put a buck transformer on the high leg, you could get 120V L-N on all three legs. But this would also change the L-L voltages, and you'd be left with a 240V, 170V, 170V system rather than 240V all around.

Jonathan

 

[xformer]

You can't have balanced L-L and L-N voltage simultaneously on a delta.

If you just put a buck transformer on the high leg, you could get 120V L-N on all three legs. But this would also change the L-L voltages, and you'd be left with a 240V, 170V, 170V system rather than 240V all around.

Jonathan

IMHO The bucking would need to occur at the load. This would be a design issue with the equipment and not a supply issue.

 

[kwired]

Thanks for the more in depth explanation. This converter had 267V phase to ground, significantly higher than the incoming from utility...

That can be fine tuned to some extent by changing value of connected capacitors within the rotary phase converter. It can still vary depending on loading conditions though. That is with a basic RPC. There are some out there with more complex controls on them to monitor and adjust to put out a more balanced voltage. They still are going to have a high leg though, even if the idler motor is wye connected, if you draw out the connections including the supply source, you have that supply source across two ends of the wye with a mid point ground on it, the neutral of the wye is left floating or it would result in high fault current if it were grounded. The derived third phase is the remaining motor winding and if your drawing is accurately drawn to vector distance and correct angles you will still have 1.732 times the phase to phase voltage from that high leg to the grounded point, which is part of the supply circuit and not the neutral of the wye connected motor windings.

 

[Jraef]

I assumed the same but the technician said that it specifically stated no high leg for motor.

This is the result of an Italian engineer not understanding what a "high leg" really means, and/or someone explained it to them wrong. A 230V 3 phase motor doesn't USE anything that it L-N or L-G, it only uses L to L, and on a "high leg" delta, it is still 240V L-L on all legs. It is only "high" with reference to ground.

CONTROLS might care, but if the machine is built with a 400Y230V motor capability, it's likely that the controls are NOT using 120V, they are using an auto-ranging SMPS to provide 24VDC for the controls, that way the same machi9ne design works in any configuration.

Now, the "245V max" may be true, in that the motor is designed as 230V 50Hz, and IEC motors are somewhat "unforgiving" when it comes to voltage tolerance. Some of them are +-5%, whereas a NEM<A motor is +-10%. So a 230V motor +5% puts it at 241V, but because the V/Hz ratio is lower because we are 60Hz, they are likely making a little more allowance.

I wouldn't put any more thought into this and just use a 7.5kVA 230D to 230Y133 Drive Isolation Transformer as Jim Dungar said a while ago, like an Acme DTFA72S*, should be under $2k for the transformer, maybe a couple of hundred for fusing and appurtenances. It has taps to adjust the voltage too, solves all problems.

* Any brand is fine, I just grabbed that because I happen to have a catalog here.

 

[hillbilly1]

Yeah thats what I'd quote along with getting a quote from the POCO to either close the delta or if they are a POCO thats getting rid of delta's I might even ask them for a cost esitmate on providing a 220Y/127 service at the customers expense as neither of those involve any changes on the customer side.
I'd rather throw the $$ at the POCO for a transformer upgrade as the customer side transformer losses are forever on the monthly bill.
If they did do a customer owned 240Y/139 it might be worth considering a 240Y/139 panelboard as other equipment gets upgraded it will also probably want that.


I doubt that, the EU's Low Voltage Directive (LVD) (2014/35/EU) state for their nominal 230V supply, the permitted voltage variation is +10% and -6%. This means any equipment made for the common market should accept a voltage range from 207V - 253V.
IEC 60038 allows +/- 10% voltage variation under normal conditions.
EN 50160 further defines normal as 95% of the time and does not have a limit for the other 5%.

Most of the time they use open delta’s because they don’t have to run the third line in a wye system. If it’s a rural area without the third line, and they just need a cheap way to get three phase out there. So in the OP’s situation, they may also need to run the third line some distance, along with the added transformer.

 

[Little Bill]

like an Acme DTFA72S*,

I know Wiley E. Coyote loved those!

 

[C4ctus99]

The customer decided to revert the machine back to 480V input and we will get a 240D-480Y transformer to step up and balance everything.

As a side note, the senior tech who could answer my questions is apparently on vacation and customer did not want to wait till he got back to resolve the issue. This has been a good learning experience for me however so I will not complain too much. The techs are all American that I have been communicating with.

Thanks for the help y'all!

Jonathan