220-440 3 phase open delta?

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No need for that first factor. A transformer is just as efficient either way

As I recall, it was something like 60 or 66% power. Common on secure sites - police, army... delta-delta allowing a lost line in or out and still run ok.

Should work just fine. I do full 3-phase like that - Using 3 large and 3 low voltage ones that subtract some voltage. (IIRC).

The inputs can be delta or wye the output in open delta or inverted V. Martin

Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member.

Grant Erw> Suppose I have two single phase transformers, call them A and B, each

Not sure, but I think one noticeable difference will be an increased no-load power loss when running the transformer in reverse. Don Foreman or Bob Swinney may be able to help you with numbers.

Best wishes,

Chris

Thanx for the ref., Chris. I am drawing a blank as to "numbers". As a WAG, suppose it to function the same as any other 3-phase open delta. However, the transformers and load will become part of an RPC network. Accordingly, "tweaking" for voltage balance of the new network would be in order.

Bob Swinney

L1 to X1A L2 to X2A & X1B L3 to X2B X1 to H1A X2 to H2A & H1B X3 to H2B

The connection is relatively ineffcient where three-phase loads predominate since it has only 86.6% of the rating of the two units making up the three-phase bank. It also has only 57.7% of the three-phase rating of a closed delta-delta bank of three units.

Don

Thanks, Don! Would it be common or desirable to ground any point in this connection scheme? I read that it's good to ground something in a transformer if only to drain off static electricity. But you can find a lot of BS on the Internet too.

Grant

On floating delta ckts in plants, the procedure is to run three light bulbs to ground. If a short to ground develops in any equipment, one light bulb will burn a lot brighter than the others, indication a ground fault. This is an old system but it will work fine. Newer systems are all electronic and wired directly into the main breaker, but they cost thousands of dollars.

John

Grant,

I really can't speak to grounding, as I'm not that familiar with the NEC. My background is as a high voltage troubleshooter. But I would think all that you would need is a green safety ground to all your transformers and equipment.

Also I don't know if it would make a difference if your manufactured phase is L2, as opposed to L1 or L3. In any case, I think "tweaking" like Robert Swinney suggested would be in order.

Don

Efficiency should be about the same in either direction.

The 57% derating factor might be right, but I don't see where it comes from. Seems to me it would be 67%. The vector sum of the two xfmr voltages is the same as a third xfmr of proper phase would have if it were present, so all of the phase voltages are the same either way. In one case 3 xfmrs carry the total (symmetrical) load while in the other case 2 do, so the two-xfmr setup can handle 2/3 (67%) of what a three xfmr setup can.

I've assumed a symmetrical balanced load. If ratings must accomodate unbalanced loads then the derate factor would be different.