Is it safe to backfeed a delta to wye. I have a 480 VAC wye input and I want a 440 VAC delta output. What type of issue (ground) can I expect if I move forward with this. An alternative I have in mind it to use a buck boost--> autotransform which I beleive will also work.
Using an isolation transformer to buck the voltage for a piece of equipment is expensive as compared to a buck transformer. Usually the buck boost transformer only needs to be 10% of the current rating. Transformers work both directions so Wye to Delta does work. I just realized you also asked about grounding, The secondary of an isolation trasformer (not the medical kind of isolation) will result in an ungrounded secondary and ground detection lights would also be required. Use a buck transformer so the ground works.
Forgive me, I am an electrical engineer with little experience in grounding issues. I will be using the backfeed, I have no say in that. I have had my circuit approved. What are ground detection lights?
Phil, Why do you need a Delta output? If you need to drop the voltage, use your correction taps on the delta side and feed the transformer from the delta side. If you are trying to get a sitiuation where the first fault is free, in other words the first fault does not cause a machine or the whole plant to shut down, then perhaps you should look in to a high resistance grounding system, which will come with its own fault detection system.
Phillip since a delta connected transformer has no ground when a leg does get grounded no over current device will trip but the enclosure will have a live conductor connected to it. If a second phase should ground the resulting connection would be a phase to phase fault and the resulting damage would also be greater. As you know the stress on the insulation would be phase to phase instead of phase to ground. So 440 volts instead of 256. A ground detection light is a neon bulb connected from each phase to ground rated at the phase to phase voltage. When a ground fault occurs the bulb connected to the phase that is faulted goes out and the other 2 get a little brighter. This obviously lets you continue working but without the indicator no one starts looking for the fault. Do a damage curve at 256 volts and one at 440 volts to see the incident energy available. I had inserted a comment here regarding the posters experience and in retrospect I was out of line. Philip I appologise if my comments were taken in offence. This forum is for electrical professionals and the sharing of our experiences is why we are here. I am sure you will get the experience you seek and this is a good place to seek it.
Backfeed Delta to wye
Posted - 04/07/2009 : 07:35:32
