Delta mains service wiring to a Wye Transformer

Howdy All,

I've done a lot of reading and I can't seem to find the precise answer I'm looking for. I'm also working with something I don't understand well so please forgive my questions if they seem stupid.

I'm powering a 380v machine from my shop 3 phase service, which is 240 Delta high leg. The machine requires a 5 wire connection, but my delta service only has 4 (3 hot plus ground). My understanding is that a wye transformer can give me the neutral I need.

If my above understanding is correct, here is my question: The only suitable transformers I can find (220>380) are wye-wye transformers. Can I rewire the primary side to delta? Any details as to how this can be done? I know Delta-Wye transformers exist but the two suitable transformers I have access to right now are both wye-wye. Does rewiring a wye-wye transformer to delta-wye affect the transformation ratio?

Many thanks for your thoughts.

Kind Regards,

Don

You can connect a wye transformer to a delta supply. Leave the wye primary neutral unconnected, you don't need it on the primary side and it would conflict with the supply neutral. You will need to ground the secondary neutral. There is no conversion needed on the transformer. A wye service only has four wires. What does the fifth one do?

Bill

Leaving the primary neutral disconnected on a wye winding would not be advisable if you have an unbalanced secondary load. If you have an unbalanced secondary load a floating primary neutral will give unbalanced secondary voltages, both phase to neutral and phase to phase. (Line to neutral and line to line for you 60Hz types). The degree of voltage imbalance depends, amongst other factors, on the degree of load imbalance.

In a wye wye double wound transformer the winding on each leg of the transformer is only good for the line to neutral voltage. If you rewire a 220/380 wye wye transformer (and assuming you get all the winding polarities right) for delta wye the primary will be rated for 220/1.732 = 127 Volts line to line. Doesn't work for your application.

I'm assuming the 5 wire supply you mention is Euro-speak, meaning three phases + neutral + earth.

Since it requires a neutral, it must be assumed there is an imbalance.

But how MUCH? Probably not a lot, but the machine specs and/or schematic will tell.

"Imbalance" sounds bad, but is not. As long as no winding is called on to provide more current than it is capable of, there is no issue with some imbalance. So if the transformer is sized to the load maximum current draw, you can pretty much forget about the imbalance problem for this sort of application. .

Thank you for the replies.

@9100 The machine is German. Most european 3 phase service is 5 wires (3 hot, 1 neutral, and ground).

@GM5735 Yes, correct. The 5 wires are three phases plus neutral plus ground (earth)

The neutral is required for the single phase loads in the machine (it has multiple transformers in the control cabinet) including a DC feedmotor, machine light, and DRO.

The transfomers I have lined up are 14 and 15 kva, which are oversized for the calculated requirements of the machine (11 kva).

I remain uncertain...can anyone enlighten me further?

dgcope said:

Thank you for the replies.

@9100 The machine is German. Most european 3 phase service is 5 wires (3 hot, 1 neutral, and ground).

@GM5735 Yes, correct. The 5 wires are three phases plus neutral plus ground (earth)

The neutral is required for the single phase loads in the machine (it has multiple transformers in the control cabinet) including a DC feedmotor, machine light, and DRO.

The transfomers I have lined up are 14 and 15 kva, which are oversized for the calculated requirements of the machine (11 kva).

I remain uncertain...can anyone enlighten me further?

Click to expand...

One wonders ... if you might be going about all this "the hard way".

If we knew more ABOUT those single-phase loads, they might be served just fine if coalesced, severed from the primary power input source, and then either;

-fed the needed single phase off a separate breaker from your load center..

- and/or via a smaller, single-phase, multi-tap transformer fed off that load center OR across any two legs of the primary feed, within the machine's own envelope and environment.

There may even already be provision for such a re-arrangement. 1-P in Europe is not ordinarily our North America/Japan 100-120 VAC "split phase" anyway. It is usually 230-250 VAC, one (3-P) leg to Neutral.

IOW - I/we do not (yet) know if the aggregate 1-P load justifies the larger transformer to re-derive a Neutral for the entire environment.

2CW

Hopefully you have fully investigated what the machine needs EXACTLY, because although you can fix the voltage issue with a transformer, you cannot fix the frequency issue. 380V in Germany is going to be 50Hz, power in North America is 60Hz. If you have AC motors that run Across-the-Line, they will run 20% faster than they are designed for. If they have VFDs on them, or they are all DC motors, then none of this matters.

@thermite Thank you. Yes, I've given some thought to internally rewiring the control cabinet to accommodate for the lack of a neutral. I had hoped to avoid it however, as getting the transformer wiring correct seemed more straightforward to me.

@Jraef Thank you, yes I'm well aware of the frequency issue, and I my case this will not be a problem.

dgcope said:

@thermite Thank you. Yes, I've given some thought to internally rewiring the control cabinet to accommodate for the lack of a neutral. I had hoped to avoid it however, as getting the transformer wiring correct seemed more straightforward to me.

Click to expand...

That needed doing in any case. It COULD be all that was wanted, especially with the actually more common Delta-in, Wye-out transformers.

I want three of those by the time I am done. One for each of the two P-P, another for my RPC.

"Slight" but worthwhile reduction in 'lectrocution risk if max potential above ground is but 125 VAC or so.

Mind - no panacea, no complacency!

I still have 350 V or so "stick and fry" Dee Cee (INSIDE the 10EE) ..and the usual 240-and-a-bit (245/246 at the mains is pretty stable) line-to-line.... 1-P OR 3-P ... to be careful of EVERYWHERE.