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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?

Input Wiring Wiring diagram

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    \$\begingroup\$ Consulting a licensed electrician is probably the best plan. \$\endgroup\$
    – user2943160
    Commented Jul 6, 2016 at 2:48
  • \$\begingroup\$ Where'd you get a 208V delta from? 208V service is just about always wye -- which means you have the transformer wired wrong, so no wonder your results are fubar :P \$\endgroup\$
    – ThreePhaseEel
    Commented Jul 6, 2016 at 3:23
  • \$\begingroup\$ As someone with 20 years of experiance providing generators and temporary electrical systems for special events, I am not a noob to electrical wiring. I'd just like to know if my results are typical or not. I also think it's important for anyone else installing a transformer to know what typical results are. Many "licensed" electricians have never installed a transformer like this before. \$\endgroup\$
    – Dan
    Commented Jul 6, 2016 at 3:27
  • \$\begingroup\$ The transformer is 208 delta to 208 Y. My buildings service is 208 Y. Am I correct that you just disregard the neutral when hooking up in delta fashion? We've rented a similar transformer before and that's how it is connected. \$\endgroup\$
    – Dan
    Commented Jul 6, 2016 at 3:33
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    \$\begingroup\$ 7 volts between ground and neutral - why are you surprised - there is no connection other than capacitive between primary and secondary so something round about 0V +/- X volts is to be expected. maybe you have left out some information somewhere? \$\endgroup\$
    – Andy aka
    Commented Jul 6, 2016 at 7:08

1 Answer 1

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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.

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  • \$\begingroup\$ This is great. Thanks for the info. Building’s service does not have a high leg. The voltage was tested and was between 208-210volts across all three phases. Something else must be wrong with my building’s service because I just hooked the transformer up to one of our 60kw generators and it powers up with any phase order. It’s just plain weird. \$\endgroup\$
    – Dan
    Commented Jul 6, 2016 at 17:58
  • \$\begingroup\$ I also ran the same battery of tests once the transformer was powered by the generator. The idle current draw was similar to the original results, H3-16amps, H2-10amps and H1-12amps. The neutral/ground was 7volts, X3 to ground was 111volts, X2 to ground was 118volts and X1 to ground was 122volts. All three secondary phases were 122volts to neutral. \$\endgroup\$
    – Dan
    Commented Jul 6, 2016 at 17:59
  • \$\begingroup\$ Unbalance will probably the cause then. So the phase angles are not 120 degrees. Or one of the windings in the transformer is off. Can you measure the resistance of each winding? \$\endgroup\$
    – rfkortekaas
    Commented Jul 6, 2016 at 18:02
  • \$\begingroup\$ Unfortunately I don't have a meter sensitive enough to test the resistance. Any workaround? \$\endgroup\$
    – Dan
    Commented Jul 6, 2016 at 20:05
  • \$\begingroup\$ Can you measure the voltage between the jumpers and ground? \$\endgroup\$
    – rfkortekaas
    Commented Jul 7, 2016 at 18:26

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