OK, last month I tried to use a 240-208 transformer to convert 480 to 400. Transformer complained and that poor old 480 circuit breaker just would not cooperate.
So, today I have a 240 to 480 delta to delta. I rewired the 480 side to Y by joining all the taps. Feeding 208 in the 240 side should have given me 416 volts... one would think. First try, the transformer made lots of noise the the wires were dancing in the conduit. Probably means something is wrong. So I disconnected the Y connection and just had three windings on the HV secondary. But I was getting 720 volts instead of 400. Hmmm.. OK, not understanding something here, but it is off by a factor of the square root of 3 so it is a three phase problem and I would have to break out a book about phasor diagrams to understand it. I did discover that if I connected all the outputs and left the taps floating it remained silent. If I connected the taps and left the outputs floating it grunted loudly. Don’t understand that either but I am sure it has something to so with phase relations. So, thinking that the transformation ratio changes by the square root of 3 when you go from delta to Y, tomorrow I am thinking of converting the primary to Y so we are Y-Y and hopefully the original ratio will re-appear. I will be feeding it from a 208 delta circuit. This will involving taking a small hack saw to those huge square copper windings on the primary side to disconnect them from each other and tie three ends together. So kinda kills the resale value of the transformer if it does not work. So far, no smoke, fire, arc flash or electrocution. I was using a fluke voltmeter on 720 volts and bare hands though..... If I make my wife a widow, please nominate me for a Darwin.
