Can a delta-delta transformer be backfed

[Slipped Eccentric]

Hello all. Trying to get a new lathe up and running. Lathe is 440v so I need to step up the 240v that I have (coming from a rotary phase converter). The transformer that I have looks like a delta-delta step down, can it be backfed as a step up? I've found tons of information on delta-wye transformers but almost nothing on this configuration. I do plan on getting a proper transformer in the future but need to get back to making parts quickly to pay for it.

Thanks again, Justin.

[Will's 2882]

I have done that many times. The transformer can be hooked up low to high and high to low voltage. It will work just fine that way. You may need to adjust the out put voltage with the taps if it is not right.

[Slipped Eccentric]

Thank you. Does anything special need to be done with grounds like the corner grounding needed in a delta-wye transformer?

[choprboy]

It depends... and would pretty hard for anyone to give you an answer without more specific information.

A delta-wye transformer would generally not be corner grounded on the input (delta) side. The wye side, or basically any output configuration with a derived neutral, would require grounding on the output side. For a delta-delta, you have several options: corner grounding, center-tap grounding, zig-zag grounding, and no grounding. Generally, if you are using the output delta for distribution to other devices, it would require either grounding or a ground fault detection system (in the case of no grounding). There are exceptions permitting ungrounded/unmonitored systems in dedicated industrial/machinery where only authorized/qualified personal would have access. (There is a whole slew of different required/not required scenarios for special types of systems). NEC article 250 has the specifics.

https://c03.apogee.net/contentplayer/?c ... ht&id=1366
https://www.ecmag.com/section/codes-sta ... not-ground

[John Hasler]

You can backfeed it but as Will's 2882 says check the output voltage and adjust taps if needed. You needn't ground the output as long as it is dedicated to this one machine and not feeding a distribution system. Personally, I'd rather have it ungrounded than have two 480 V high legs. Best to bolt the transformer to the machine. Run the 480 in metal conduit.

You can add ground fault detection by connecting identical 400 V pilot lamps from each phase to machine ground. Normally they will all glow dimly. When one phase shorts to ground that lamp will go out and the others will go to full brightness. This will also provide high resistance grounding. Each phase will be 277 volts above ground with fault current limited by the lamps.

[Slipped Eccentric]

Pardon my ignorance, but the transformer needs to be grounded back through the converter to my bonded subpanel, correct? Wouldn't running the wiring to the lathe in metal conduit just continue the ground circuit? Due to size limitations, I can't mount the transformer on the lathe itself.

As for the rest of the wiring, power from the rotary converter will be fed into a load center with one circuit feeding a mill and another feeding the lathe. There will be a disconnect from the load center that I will use to kill power to the transformer. This is my first jump to three phase so just want to make sure all my ducks are in a row. Thank you all again.

[John Hasler]

Slipped Eccentric writes:
> ...the transformer needs to be grounded back through the converter to my bonded subpanel, correct?

The transformer frame does, yes. You must *not* ground any of the primary conductors at the transformer.
That's all taken care of at the rotary converter and the panel, which is hopefully wired correctly.

> Wouldn't running the wiring to the lathe in metal conduit just continue the ground circuit?

It will continue the ground *connection* and ground the machine to your building ground, which is fine (run a ground wire as well). There is no ground *circuit* until something goes wrong.

The transformer secondary is a seperately derived system. What you do on the primary side is irrelevant to it.
If you don't bond any of the secondary conductors to ground it is an ungrounded system
(which, as we've explained above, is not necessarily bad).

> Due to size limitations, I can't mount the transformer on the lathe itself.

That's ok. Just don't use it for anything but that one machine and label it.

[Slipped Eccentric]

That makes much more sense, thank you for clarifying. I do know the difference between ground connection and circuit, brain fart on my part.

[Slipped Eccentric]

Thank you for the help, the lathe is wired up and running great.

Justin