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[Mike Sokol]

I think I would bond the shells together so they stay at the same potential-a neon light connected between them and ground would be a super safe way just in case they became energized through a fault.  Should be safe enough as long as the whole contraption is GFCI protected.

I guess I assumed you would not always have a bonded neutral to work with-if you wire and demo a true delta.

I'm mostly interested in demonstrating the types of 3-phase configuration that technicians could find in an entrance panel. I've never seen a floating delta entrance panel in the field, but maybe I wasn't looking hard enough. I've seen tons of 4-wire WYE and 4-wire High-Leg Delta, and an occasional 3-wire Grounded-Leg Delta, but the beauty of this open transformer architecture on my "white board" is that it would be pretty simple to wire up anything strange into the demo. I've read about Open-Delta 3-phase and understand its limitations, but never encountered one. But there's a first time for everything, I suppose....

[Stephen Swaffer]

I'm mostly interested in demonstrating the types of 3-phase configuration that technicians could find in an entrance panel. I've never seen a floating delta entrance panel in the field, but maybe I wasn't looking hard enough. I've seen tons of 4-wire WYE and 4-wire High-Leg Delta, and an occasional 3-wire Grounded-Leg Delta, but the beauty of this open transformer architecture on my "white board" is that it would be pretty simple to wire up anything strange into the demo. I've read about Open-Delta 3-phase and understand its limitations, but never encountered one. But there's a first time for everything, I suppose....

Hopefully they would never encounter it.  Just seems like every time I say "never"-and if I had a nickel for every time tech support or OEM techs told me "We've never seen that before"...

[Frank Koenig]

I've read about Open-Delta 3-phase and understand its limitations, but never encountered one.

I was under the (maybe mistaken) impression that most of the high-leg delta you see is also open delta. I base this on the transformers I see on the poles in all the light industrial areas around here. There's always a big pot with 3 secondary wires coming out and a little pot next to it with 2. I assume the big pot is 240 V center tapped with the center tap grounded and providing the neutral, while the little pot is just 240 V. Each primary is connected across one of two different phases and the POCO rotates which two as they go down the street to balance things out. Is this right or just an elaborate product of my imagination? Thanks.

The white board looks like a great idea, BTW.

Happy New Year to you all.

--Frank

[Mike Sokol]

I was under the (maybe mistaken) impression that most of the high-leg delta you see is also open delta. I base this on the transformers I see on the poles in all the light industrial areas around here. There's always a big pot with 3 secondary wires coming out and a little pot next to it with 2. I assume the big pot is 240 V center tapped with the center tap grounded and providing the neutral, while the little pot is just 240 V. Each primary is connected across one of two different phases and the POCO rotates which two as they go down the street to balance things out. Is this right or just an elaborate product of my imagination? Thanks.

The white board looks like a great idea, BTW.

Happy New Year to you all.

--Frank

I can imagine how this could work with reduced capacity on the 120/240-volt split transformer, but I'm mostly looking at power inside of the building and ignoring the transformers on the outside. Guess I'll start a new hobby.

A little Googling found this reference:

Open delta high leg

I have seen multiple instances of High leg implemented with two transformers in open delta configuration, where the neutral is the center tap of one of the transformers. I am trying to describe this and do not understand the objection that has twice been made.

I just found a good diagram of this, as the last one on the page: http://www.bmillerengineering.com/elecsys.htm and also Figure 10-18 in this book http://books.google.com/books?id=8cT8D9_WP7MC&pg=PA251&lpg=PA251#v=onepage&q&f=false BillHart93 —Preceding undated comment added 20:41, 17 October 2010 (UTC).

    The proposed edit said something like "one of the three transformers may be omitted" which is confusing to the reader since the object described in teh schematic diagram is called "a transformer". This three-phase transfomrer could be made out of a set of three individiual single-phase transformers, in which case an open delta would further allow only two transfomrers to be used. But was this in fact done? The point of high-leg delta was to provide for a small 120 V load in an otherwise 3 phase 240 volt distribution network, which would have been primarily 3 phase loads, warranting a 3-transformer bank (or a three-phase transformer). The citation in the proposed reference warnsa bout high-leg connections and shows an open-delta connection, but doesn't show an open-delta high-leg. --Wtshymanski (talk) 21:52, 17 October 2010 (UTC)

Diagrams in both links show open delta high leg with two transformers. As pointed out the first link needs to be scrolled to the bottom of its page. The second one points out Figure 10-18. Maybe you didn't look at all the figures?

The purpose of using high leg may sometimes be to provide for a small 3-phase load along with substantial single-phase loads, for which open delta works well. When the 3-phase load is large, all three are needed. I have seen more instances of open delta high-leg than with the full 3 transformers or 3-winding transformer.

[Stephen Swaffer]

Then you have really fun installs like the one in this area.  Open delta high leg with 2 42 space  panels side by side.  One wired as you would expect.  The other wired to allow every' space to be used for a single phase load.  Inspectors?  In Iowa?  In the early 70's??

And yes, I've had to tie bands in-as this is a former church whose auditorium was used for occasional CCM concerts.  At least they called an electrician familiar with the building-I eventually installed a 14-50 receptacle for them.

[Mike Sokol]

Hopefully they would never encounter it.  Just seems like every time I say "never"-and if I had a nickel for every time tech support or OEM techs told me "We've never seen that before"...

True, that....

[Mike Sokol]

Thought you would like to see how I'm generating 3-phase/120-degree test files while waiting for my 3-phase signal generator from China. I have an Allen & Heath ICE-16 interface which will play back up to 16 channels of wave files. I only need 3 channels for this generator so I went into Audacity and generated a mono 30 minute sine wave at 60 Hz. I copied and pasted it onto two more tracks, then slipped tracks 2 and 3 by 5.55 ms and 10.11 ms to create tracks that are phase shifted 120 and 240 degrees at 60 Hz. I added dead air at the beginning of the slipped tracks, then soloed each track and output them as separate mono 16-bit/44.1 files.

Now, don't go nuts on me about the phase thing because I realize that this is a time delay simulating a phase shift at a particular frequency, in this case 60 Hz. I would have to come up with different delay times at 50 Hz or whatever.   

I loaded these mono files on a USB thumbdrive in the appropriate folders, and the ICE-16 plays them from outputs 1, 2 and 3 in the proper 0/120/240 degree relationships. This works perfectly and is the easiest backup signal generator I could come up with until my real signal generator arrives. At least I can experiment with everything else and get my white board ready for action.

Yes, I'll post a few videos of this demonstration in operation showing measurements for the various flavors of 3-phase power. Thanks for all your suggestions so far (except for the 3-phase potato generator). You guys really are my brain trust.   

[Jonathan Johnson]

So here's what I'm thinking about doing to demonstrate wiring of various 3-phase configurations. I was going to mount this on a piece of plywood, but my wife gave me an old white-board yesterday, so I started laying out how to mount the transformers on it. Then I drew out where I could mount 5-way binding posts for various connections and test points. After I did that with dry-erase markers it hit me. Why don't I just mount the transformers and 5-way binding posts on the whiteboard and put a hinge on the bottom so it can lay flat for transport or an overhead video camera, and it can flip up at an angle for direct viewing and testing by the class. I can literally sketch the various diagrams for standard Delta, Grounded-Leg Delta, High-Leg Delta, 3-leg WYI, 2-leg WYI and whatever else comes to mind (open leg Delta, perhaps?) right on the white-board, then hook it up using colored banana plugs. I can get 5-way binding posts in Black, Red, Blue, Green and Yellow (no white that I can find, but maybe yellow is OK), and get banana plug jumpers in these colors as well.

Are you planning to fly with this? Let me know when. I want to see what happens at the TSA checkpoint. I'll bring popcorn. 

[Mike Sokol]

Are you planning to fly with this? Let me know when. I want to see what happens at the TSA checkpoint. I'll bring popcorn. 

Well, this will go in checked luggage along with printed copies of my Powerpoint presentation, but hopefully it will pass muster. 

Once I was flying back from a microphone manufacturer with a pair of side-address mics for review packed in my carry-on luggage. This was back in the day when you could watch the X-ray screen as your luggage went through. When the security guy stopped the belt and called for help I looked at the screen, and I've got to admit that side-address condenser mics look a lot like pipe bombs. More security showed up and I had to show them the loaner receipt for the microphones. Everyone calmed down a bit, but they still took my carry-on bag apart (carefully) to confirm they were mics and not bombs.

[Mike Sokol]

I just received notification from the USPS that the 3-phase signal generator has arrived at my local post office, so it might be delivered today or Monday. Just in time to mount it on the front of the white board along with the transformers. All my colored 5-way binding posts and banana jumpers are here as well, so tomorrow could be a build/test day. Too much fun!

Now I just need a 4-channel scope to show phase relationships.

 

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