I have been looking for a used 3KVA 3 phase transformer to step up the output of my rotary phase converter to 440V, no luck so far. Used single phase 220V/440V machine transformers are easier to find, so I was thinking about getting 3 identical 1KVA machine transformers and wiring them up as a 3 phase transformer instead. - I want to run 440V motors up to 3 hp for brief times. Thoughts?
It can be done with two single phase units, 480/240 x 240/120, in a open delta configuration. The 480V output windings, output ungrounded open delta, as shown in the drawing. To ground the output winding's, to make it safe to use and have a fault return path, you would ground the center of one of the 480V output windings, at the center tap, at the junction of H2 &H3.
Have a look at this post.
Clausing drill press 480 to 240?
This
will not yield a WYE 277/480, but a high leg open delta, 240V to ground from two legs, and 415V on the wild leg.
It's not useful for things like VFD's that want a symmetrical Wye 277/480V voltage to ground.
But is perfectly, suitable for basic 3Φ motor loads.
Small 1Φ transformers, less than 3KVA, usually have some compensation windings added to one side, to adjust to the proper output voltage. So you need to use units 3KVA or above to get the correct ratio, for the correct output voltage. Smaller units will throw your output voltage off, due to the compensation windings.
Two 1Φ 3KVA units would yield you 5.16KVA output. And you still need to add the correct overcurrent protection on both sides of the transformer bank.
If you needed a 277/480V output from Three 1Φ units, they would need a 277V HV coil, which is a lot rarer in the used market.
Three 480V Primary coil units, can be wired in closed delta, which would then need to be corner grounded, or center tapped grounded, yielding the same high leg 415V. Both of these output connections are not suitable for most VFD units. Operating any transformer bank without grounding the 480V output winding, would be a violation of the NEC, and an unwise decision.
SAF Ω