Transformer selection

[Davepeterson22]

I have a homeowner that bought a 480 3 phase CNC machine. The property has a 240 volt phase converter on site that I installed a couple of years ago. So what I need help with, is what transformer to get? I know that we need a 45kva 240/480v 3 phase step up transformer. But should we get a delta delta or a delta wye? Never have wired a corner grounded transformer and don't think those are not common anymore. I am leaning towards a 240/480-277v wye, then we have an established grounded conductor. Thoughts? Thanks!

 

[GoldDigger]

A wye on the equipment side will make it possible to use a grounded system with balanced phase to ground voltages. In some cases that is much more friendly to the equipment than ungrounded or corner grounded delta. (In particular VDF electronics.)

The primary side may be either delta or wye, but if it is wye and the secondary is delta you must NOT connect the center point of the wye to either ground or neutral.

Try to find a transformer that is specifically specified as a step up transformer or else is specified for use in either direction.
Using a step down only transformer in reverse to get step up can cause very high inrush currents and any voltage taps present may not work as designed, in terms of core saturation.

 

[Davepeterson22]

Thanks for the info! The transformer we need then is the delta/wye 240/480-277. My next question then is grounding? I will have 2 EGC coming into the transformer. What will tie to X0 on the secondary? This should be my list of terminals.
240v. 480/277v
H1 H2 H3. X1 X2 X3 XO. Ground

 

[ActionDave]

Thanks for the info! The transformer we need then is the delta/wye 240/480-277. My next question then is grounding? I will have 2 EGC coming into the transformer. What will tie to X0 on the secondary? This should be my list of terminals.
240v. 480/277v
H1 H2 H3. X1 X2 X3 XO. Ground

250.30 is your reference. You are correct that there will be an equipment ground coming in from both sides of the transformer, the XO can bond in the transformer or in the first disconnect and that will be where you make your ground rod connection. If you have access to the NEC handbook there are some helpful illustrations.