Q. A large textile plant purchased several looms from Europe, which operate on 380V, 3-phase. The only distribution voltage available is 480V, 3-phase. The plant requested advice on the most economical way to obtain 380V for these loads. The electrical service to these looms is from a 3-wire busway. The service to the plant is 480/277V, 3-phase, grounded wye.
I determined an autotransformer connected in open delta (no neutral) would be the most cost-effective arrangement. However, local jurisdiction told me this connection violates Sec. 210-9 of the NEC. I see how an autotransformer in an ungrounded wye system would cause problems, but why would an open (or closed) delta autotransformer create problems? Can anyone explain this to me? —N.K.
A. N.K. didn’t mention the circuit frequency for the looms. The frequency in UK is 50 Hz—in France it’s 331/3 Hz. These parameters were adapted in the channel tunnel projects a few years back. At one time, there were other variations in power sources in Europe. Perhaps they still exist.
The proposed solution is technically reasonable, but without details of the 380V, 3-phase system, it’s difficult to be sure. We once had a situation requiring application for 50 Hz in our 60 Hz arrangement. Our solution was to use a standard 60 Hz generator running at reduced speed to provide 50 Hz power. It worked. Perhaps N.K. could apply a similar solution to his situation. —B.B.B.
A. I recommend using a 3-phase transformer with a 480V delta primary and a 380Y/220V secondary. These are available for 60 Hz applications. As far as compliance with Sec. 210-9 of the NEC, the application would fall under one of the exceptions, since the grounded circuit conductor isn’t needed for supplying the loads. However, I assume the exceptions don’t list the specific voltage of 380V, since it’s rare in the U.S. —M.R.P.
A. N.K.’s reason for using an open-delta connected autotransformer scheme to change 480V, 3-phase to 380V or 400V, 3-phase is not appropriate. For example, the maximum kVA loading can’t be more than 58% of the bank’s total nameplate rating. If you exceed this limit, the transformers will overheat. Also, the current loading on the 3-phase, 480V primary system will be unbalanced on a per-phase basis. And, the 380V to 400V, 3-phase line-to-neutral voltages won’t be symmetrical, because the neutral “zero” point is determined by the original 480V supply. Because he would have to use two “custom” single-phase autotransformers, each rated 480V, with a tap at 380V or 400V, his plan would cost the same as a single “special” 3-phase transformer rated 480V delta to 380V or 400V wye. This latter type of transformer is usually obtainable from the larger manufacturers.
For either arrangement, N.K. would need primary and secondary overcurrent protection in accordance with the NEC. The European Union Electrical power utilization format is now 400VY/230V, 3-phase—it was 380VY/220V, 3-phase. This parameter is comparable to the U.S. practice of using 460V-rated equipment on a 480V supply. It is also important to verify these looms will correctly operate on 60 Hz power. Some minimally designed European electrical utilization systems will not work on other than 50 Hz. —F.M.P.