# 220-440 3 phase open delta?

Suppose I have two single phase transformers, call them A and B, each configured to step down 440 to 220. Ignoring any ground connections, and
defining the primary connections as H1A, H2A, H1B, H2B and the secondary connections as X1A, X2A, X1B, X2B, I am considering running both transformers backwards in open delta to step up my shop-made 220 3 phase power to 440.
I drew a rough sketch: www.tinyisland.com/images/temp/xformers.pdf
If I have my 3 220 power connections labeled L1, L2, L3, and my 3 440 load connections labeled X1, X2, X3, can anyone tell me how exactly to connect these transformers in open delta?
Also, I'm wondering how much to derate the power ratings. There are two factors, one is that I'm running the transformer backwards which normally loses some efficiency; the other is that the open-delta derates the power to 57% of the total kVA, right?
Thanks!
Grant Erwin
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Grant Erwin wrote:

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No need for that first factor. A transformer is just as efficient either way
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if it's 98% efficient stepping down, it's 98% efficient stepping up. (But a
small step down transformer will have extra turns on the low side to
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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. http://lufkinced.com /
Grant Erwin wrote:

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Grant Erwin wrote:

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
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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

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On 9 Apr 2007 22:42:17 -0700, "Christopher Tidy"

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.
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Grant Erwin wrote:

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
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Don Murray wrote:

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
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Grant Erwin wrote:

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
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Grant Erwin wrote:

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