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Old 07-15-2010, 10:47 AM   #1
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Multiwire Diagram


Good morning forum

I received a message from one of our members about making one of my multi-wire diagrams better representative of the relationship to the transformer. So certainly I am interested in doing that. So I'm going to show it here and lets take a look at it and make some improvements if necessary.

For some background the arrows were drawn in (different colors) to indicate which end of the transformer the ungrounded leg represented and to indicate that voltage and current are changing directions with the wave cycle.
The junction with the -5 and +10 was a simple way to imagine that a cancellation occurs leaving a net of 5 amps on the shared neutral. I can't say for sure why I put the arrow on the shared neutral ... been too long ago...

Anyway lets see if we can make the diagram less confusing and technically more correct or more representative of a multiwire branch circuit.

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Old 07-15-2010, 12:46 PM   #2
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The peak value of 120 vrms is more like 170 v, if that's what the half-sine pulses in your diagram are meant to represent.
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Old 07-15-2010, 01:06 PM   #3
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How about this:

1. Omit the plusses and minuses.

2. For the top line, show the 10 amps with the arrow going to the right.

3. For the bottom line show the 5 amps with the arrow going to the left.

4. In the middle show both a 10 amps with the arrow going to the right and a 5 amps with the arrow going to the left and also a third caption "net 5 amps" in parentheses with an arrow going to the left.

5. Top right show 10 amps with arrow pointing down.

6. Bottom right show 5 amps with arrow also pointing down.

7. Leave the 120 volts captions at the half sine wave humps but have the vertical arrow inside touch the horizontal line but leave a noticeable gap between the other pointed end and the hump.

8. At left, have the vertical arrows with single heads both pointing up.
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Multiwire Diagram-multi2.jpg  
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Last edited by AllanJ; 07-15-2010 at 01:43 PM.
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Old 07-15-2010, 02:01 PM   #4
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Ok we are getting very good suggestions. Bear in mind this drawing in its original beginnings was made in response to questions asked by a homeowner sometime ago. So it was drawn not so much for technical correctness but to get the general idea across to a homeowner.

When I drew this a few years ago I debated on whether or not to even show the half cycle relationship as it may not really be needed. As Yoyizit mentions the peak is more like 170 volts with an rms more like 120. I drew the 120 volts as a peak in order to not get too technical and to simplify the drawing to test values the homeowner would get with a voltage tester.

The idea in explaining a multiwire to a homeowner (DIY) is not to see if they can see the physics but to make sure they can see the need for opposite legs to prevent them from overloading a shared neutral wire.

Nonetheless I do think since the drawing needs to represent a multiwire and that the relationship with the transformer could be better depicted.

So we will let this gather some more input then I will redo the drawing with some of the changes suggested. In the end we will certainly improve the drawing and then I'll put the "new" "improved" version in the stickey drawings.
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Old 07-15-2010, 02:07 PM   #5
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AlanJ

Just saw your modification of the drawing. I think it is indeed an improvement so lets see if we get more suggestions. Then I'll repost the drawing with credit to all those who contributed. All royalties to me of course....
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Old 07-16-2010, 09:18 AM   #6
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If no more suggestions the drawing posted by AlanJ will be the new improved version and it is also very close to the same suggestions (changes) that were wanted by A7eCorsair. If we want to look at a 240 volt split single phase wave diagram I believe this (below) would be a fair representation with each hot leg offset by 180 degrees. The + and - are added merely for indicating that at any point in time there is an opposite current and voltage with relation to the neutral. If I were to super impose both current and voltage waves for Alan and A7eCosairs suggestions it would be quite obvious why the neutral cancellation occurs leaving a net 5 amps on the neutral.

E = voltage
I = current
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Old 07-16-2010, 10:33 AM   #7
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I disagree with the arrow for load B pointing to the left. To me that makes it look like the current is flowing out on the neutral and returning on the hot. I would think the arrows for both loads would point away from the tranny and point toward the tranny for the neutral. Open to other opinions.
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Old 07-16-2010, 11:50 AM   #8
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Quote:
Originally Posted by Stubbie View Post
Good morning forum
Rather than reinvent the wheel the dot notation for transformers was invented to solve the current flow confusion.

I can't find a good link and it is difficult to explain but obvious when you see it.

For a center tapped 240v sec. like this, and in this case, there'd be a dot at the top of the top winding and a dot just below the neutral tap. It means that, at any arbitrarily chosen instant the current is leaving or entering the dotted terminals.

It tells you that the two halves of the sec. winding are phased to add voltages rather than to cancel out voltages.

http://www.google.com/search?client=...UTF-8&oe=UTF-8

If you included the 'former primary winding and it had a dot at the top of the primary winding it means that the current enters/leaves the top of the primary winding and it leaves/enters all the secondary windings at their dots.

Last edited by Yoyizit; 07-16-2010 at 12:04 PM.
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Old 07-16-2010, 12:03 PM   #9
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Yoyizit

I've actually never heard of dot notation .. so I learned something or maybe my memory is failing me again....

Jim

I'll answer your question but I thought I would wait for the members AlanJ or A7eCorsair to answer ... if not look back in latter and I'll have something posted.
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Old 07-16-2010, 01:30 PM   #10
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Quote:
Originally Posted by Jim Port View Post
I disagree with the arrow for load B pointing to the left. To me that makes it look like the current is flowing out on the neutral and returning on the hot. I would think the arrows for both loads would point away from the tranny and point toward the tranny for the neutral. Open to other opinions.
technically it is flowing out on the neutral. You know, AC current, it flows back and forth.

the image is a representation of an instantaneous event. If you notice on B leg, the voltage is negative and on the A leg it is positive. The direction of current flow would be accurate for this instant in time.
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Old 07-16-2010, 02:00 PM   #11
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Nap is correct

The drawing is showing an instantaneous moment on the half cycle. As A7eCorsair pointed out to me current must be flowing in the same direction thru both loads regardless of what point in time.. This is what was confusing about the original diagram and I agreed ... even though my intention was not a specific current direction with the arrows in the beginning.
The drawing as it stands now is a better representation with current direction in mind.
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Old 07-16-2010, 10:11 PM   #12
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Dot notation for transformers only designates whether the secondary waveform is in phase or out of phase with the primary.
The top diagram is what is referred to as a dot to dot transformer.
This has meaning in radio electronics but I can't recall the exact application.
A standard transformer has phase inversion.
http://www.allaboutcircuits.com/vol_2/chpt_9/4.html

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Old 07-16-2010, 10:14 PM   #13
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Quote:
Originally Posted by Stubbie View Post
Nap is correct

The drawing is showing an instantaneous moment on the half cycle. As A7eCorsair pointed out to me current must be flowing in the same direction thru both loads regardless of what point in time.. This is what was confusing about the original diagram and I agreed ... even though my intention was not a specific current direction with the arrows in the beginning.
The drawing as it stands now is a better representation with current direction in mind.
OK, why not just show the 5 amps flowing to the left, this is what is really happening.. Kirchoff would like that
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Old 07-16-2010, 10:17 PM   #14
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Quote:
Originally Posted by a7ecorsair View Post
OK, why not just show the 5 amps flowing to the left, this is what is really happening.. Kirchoff would like that
he did note that in text.
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Old 07-16-2010, 10:28 PM   #15
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Using a DC analog meter and a battery you can correctly dot any 'frmr.

If a meter on a secondary winding swings positive when the battery is connected to the primary then the battery positive and the meter positive both go to dotted terminals.
Multiple sec. windings are handled the same way.

Disconnect the meter before you disconnect the battery. The neg. voltage spike from a collapsing mag. field is theoretically infinite.

Piece o' cake.

Last edited by Yoyizit; 07-16-2010 at 10:30 PM.
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