I recently purchased a used 75kva 208v delta to 208v Y transformer and have some concerns about some strange results. We would like to use this transformer to create separate (isolated) service between audio and lighting systems.
I wired up the ground and then wired the H1, H2 and H3 terminals to the three hots coming out of my buildings 3 phase 208v service. I disregarded the neutral as its not used in delta. I didn't think the phase order would matter, but I tripped the breakers when I turned it on. After swapping phases the transformer came to life with a lovely buzz like they always do.
I took out my meter and got some strange results. I was happy with my 3 hot to hot results (between 208v-210v) and hot to neutral results (between 121v-122v), but For some reason I have 7 volts between ground and neutral. I also got strange results between hots and ground. X1 reads 110volts while X2 and X3 both read between 120-122volts. I could bond the ground and neutral, but it kind of defeats the purpose of why I bought the transformer.
Lastly, I metered the current draw with no load. Black phase (H3)reads 14.32amps, red phase (H1) reads 11.08amps and blue (H2) reads 9.47amps. I would think they would all be the same.
In summary, I've never hooked up a transformer before so I don't know if the results I got are to be expected. Seems like something is wrong on the H1/X1 phase. Kinda feel like I was sold a lemon. Thoughts?
Best Answer
The phase order should not matter and should not trip the phases. You've connected the transformer correctly and discarding the neutral on the primary side is fine. Just to be sure, your MEN is not an open or high leg delta?
The difference in the amp draw can be caused by two things: - failure in one of the windings - unbalance on the primary side
I would suggest to take the following actions: - run an insulation test on the transformer - verify if the unbalance is still within the limits
Also the 7 volts on the neutral will be caused by the phase unbalance.