30 KVA 208/480 transformer

[Wyo721]

Ok I have a situation where I need a 480 25 HP VFD and there is only 208 available, could I use a 30 KVA transformer and wired 208/480 instead of 480/208

Second question I have 100 KVA that is an extra could I protect it at 30 KVA and use it to step up the voltage??

Using the lower rating for sizing the feed and the secondary

 

[kwired]

Watch out if you are intending to back feed what is otherwise a step down transformer. The high voltage side is likely delta connected which means you can only corner ground it and will not have a 277 volt neutral point.

That may not be a problem - until you look at your VFD which likely has internal surge protection that is designed for 277 volts to ground nominal.

You can find drives that can run on corner ground, but most off the shelf drives want to be on a 480/277 system.

 

[iceworm]

Ok I have a situation where I need a 480 25 HP VFD and there is only 208 available, could I use a 30 KVA transformer and wired 208/480 instead of 480/208

Second question I have 100 KVA that is an extra could I protect it at 30 KVA and use it to step up the voltage??

Using the lower rating for sizing the feed and the secondary

W - Not sufficient information to give a good answer. There are maybe 100 posts on here about back feeding transformers. Might consider a search.

Here is a start:

Q: Is the 30kva a 480D/208Y. If so you will have problems on inrush.
As for th 30kva being big enough - I'd suggest checking the VFD manual to see what the max current draw is. Comapare to xfm protection. Check if xfm voltage drop is excessive at max current draw.

Q2: This 100kva extra xfm, is it a 480D/208Y. If it is you got major problems on inrush. If the feeder is sized for a 30kva, I'd guess it is unlikely you will ever get the primary feeder to close with out tripping.

ice