Adding transformer to a 3ph rotary system

[Tom Kitta]

So I got a small transformer - 10kva. Problem is that as with most transformers on auction it is a step down transformer - Delta - Wye with high side voltages at between 624 - 600 - 572 and low side wye at 208 and 120 (leg to neutral). Its an isolation transformer.

Which brings me to few questions that I was unable to find answers to (at all despite a lot of web searching). First question:

1. Is it at all possible to add more taps vs current 3? Is it hard - I assume I could just scrape off some varnish and connect to one of the wires. I want to decrease voltage.
2. What would be the effect of changing the wye input side to a delta - or as some pp say un-ground wye i.e. connect X1-X2-X3 back to X1?
3. I am assuming that the transformer will balance the uneven input voltage from the rotary - i.e. 240-240-270 input - is this correct?

Main problem is that connecting this backwards to 240-240-270 supply from the rotary and even using the 572V tap the result voltage would be very high. The band saw has 550V motor, so 600V would still be OK but 660V is pushing it (not to mention most isolation is rated to 600V).

Also it would be great if I could reduce voltage even more - to something around 500V or even less as I have a surface grinder that is 440V motors (someone rewound 550V motors to 440V).

The 550V band-saw does not start with 240V - even the contactors in the magnetic switch are too weak to turn stuff on & motor is under powered it will not spin even with gearing in neutral.

If anyone wonders it is a German horizontal saw 15" for metal.

I also know this has been mentioned online a lot but not quite my setup.

 

[JohnW]

It is possible to tap into the windings to reduce voltage. I've done that a couple of times with single phase transformers in the past to get a slightly lower secondary voltage. You only have access to the outermost winding though. Are the three phases wound consistently so you can modify each winding in the same way? You would lose any sort of approval the transformer had by modifying it like that.