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Wierd voltage readings on a 240 volt plug


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44 replies to this topic

#21 Keinokuorma

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Posted 05 May 2008 - 12:05 PM

Wait... this may be calling for another unintelligible hand sketch accompanied with easily misunderstood writing that contains overly technical jargon... be quick to say NO if you don't want it...

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#22 certified tech group 51

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Posted 05 May 2008 - 04:38 PM

Ptrue = 1.73 E LINE  I LINE  COS   (power factor)


#23 Keinokuorma

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Posted 06 May 2008 - 03:46 AM

Hmm... on a 3 phase system,

E(Lx-Ly) = SQR(3)*E(Lx-N)

I was thinking of a special stepdown that has 3 split phase outputs... that layout could be wired to give 6x120V, 3x208V, or 3x240V feeds... and at least theoretically, if not all phases have a common center tap, it could be wired to produce abnormal voltages too... I'll try to figger a possible wiring...


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#24 certified tech group 51

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Posted 09 May 2008 - 02:41 AM

That formula is for a delta-connection....... and we all know, that delta connection provides an increase in current but no increase in voltage....A wye connection provides an increase in voltage but no increase in current.... We nad to learn all of this stuff in the Technical college, years ago........ Your brain is like a piece of cheese  in the rear of the drawer in the refer.   if you use the cheese regularly it stays good , but forgotten and when you want it, it is now moldy....    P.S  I had to take a peek in the book to get the statement correct.      USE IT OR LOSE IT   ( my headcheese)


#25 Keinokuorma

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Posted 09 May 2008 - 11:10 AM

Uh,

Hmm.

It's primarily the voltage fed to the load that increases when you go from wye to delta. In Ameedica, typically 120V => 208V, in Yurup, typically 230V => 400V.

The higher voltage yield allows to transfer more power by the same wire gauge, or the same magnitude of power with smaller gauge.

Note that these are rough nominal RMS values for the voltage output. Expect the RMS 120V sinusoid waveform to have peak and bottom values of +/- 170V!!! The RMS (Root Mean Square) value correlates with the efficiency factor of said waveform: Steady 120V DC would deliver the same power to a pure resistive load, as RMS 120V AC sinusoid voltage. Or, for the matter, if there was triangle, sawtooth or square AC wave fed to a load, equal RMS waveforms would have the same efficiency, but different peak values.

Efficiency factor (RMS/PEAK) for sinusoid wave is 1/SQR(2) or roughly 0,71. The peak value would be SQR(2) or roughly 1,41 times the RMS value.

Here's a graph with phase voltage vectors, N for the neutral or "equilibrium" point, origo, whatever... L1 is marked for the upstraight vector, the other two are 120 degrees off of it and each other. full cycle = 360 degrees, 1/3 turn = 120 degrees. So the black vectors  originating from the origo (how come???) depict the main voltage vectors. The blue vectors drawn between the main vectors depict the live-to-live voltage. The distance each vector covers in the graph is proportional to the voltage output between the vector's endpoints.

You could measure straight off with a ruler, and calculate that the blue vectors are roughly 1,73 or SQR(3) times as long as the black vectors.

Further, if you were to draw this graph to a piece of cardboard, cut it off along the edges of the triangle, and be able to suspend it exactly by the Neutral point, it would (in an ideal case) hang entirely flat and balanced. That point would be the gravitational equilibrium point of the triangular cardboard piece.

And you know what, this crappy sketch is actually drawn with a little help from the computer. :dude:

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#26 Budget Appliance Repair

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Posted 09 May 2008 - 10:58 PM

Someone called me today and explained this to me.  They saw the discussion here but wasn't a member so didn't think they could post.

I wish I had remembered to tell him that he could post in an open discussion without signing up as a member and asked him to come back and post his explaination here.

The jest of what I got and can remember, he said some industrial applications are wired with a wye-delta transformer, (I could be very wrong on that being what he called it), but he said you have 3 legs, A/B/C,  A-B/B-C/A-C will all be 240 volts, A-Neutral/C-Neutral will both be 120volts and B-Neutral will be 208volts, (he said they call this the stinger leg and in some applications they wire all lighting up to this leg to balance out the load.

He also said you have to do just as I did when wiring up the oven, know that you're dealing with this type of system and make sure if something in your application requires the the 120volt leg that it's wired to the A or C leg NOT B.

I guess the way the circuit box is configured any where in the box when you install a 2pole breaker you will be getting the A-B legs or B-C legs, the only way to get just the A-C legs is with a 3 pole breaker and not use the B leg tap, wasting a circuit spot in your breaker box, (he said in his explanation).

William Burk (Willie)
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Eureka, CA 95501

#27 Keinokuorma

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Posted 10 May 2008 - 11:10 AM

OK, starts soundin plausible. for those who aren't yet familiar with the issue, here's an awfully sloppy drawing to visualize the components of a 3 phase stepdown transformer.

The T1-T3 wires are for the transfer network. the respective primary windings are the red, green and blue boxes drawn over the left side of the black contraption, which is the transformer core. These are wired in triangle, or "delta" as we already have, and in the future, will call the connection.

On the right side of the core there are six windings, or three pairs, labelled A to F. This layout is very versatile, explanation coming up.

Each pair with consecutive alphabet and matching color would be capable of working as either a split-phase pair, serving one home with the very common 2x120V/1x240V feed... or, as two individual 120V feeds if such service is ordered. Remember we have three such pairs in this example, we could wire three homes with split phase, two homes with 3-phase, or 6 homes with single phase if we wanted to.

So, joining the windings A+B, C+D or E+F by the center wires, and wiring these as a Neutral wire, would allow us to tap the free ends of each pair for three 2x120V split phase feeds.

Bridging the A, C, and E windings by the center wires, then B, D, and F windings the same way, would give us two 3x120/208 feeds.

Bridging the same pairs without grounding the center taps, but rather joining the free wires of A, C, and E and grounding that wire, then running the free wires of B, D, and F as L1-L3 to the home, would give us a 3x240V, 3 phase feed... very similar to the service that Finnish owners of single home buildings usually subscribe to.

And now, for the tricky part, and more vector graphs... Lo and Behold Below!

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#28 Keinokuorma

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Posted 10 May 2008 - 11:26 AM

So, here's what I found to be a plausible solution to our dilemma.

If the previously described transformer was wired by bridging all the center wires of the windings together, we would have a voltage vector layout like in the upper left corner of this, another really poor drawing.

The center point being the origo, each vector A-F depicts a 120V output. This layout could be taken down to 6x120V, 3x240/120V, 3x120/208V, 2x 3 phase 120/208 etc...

AND NOW COMES THE TRICKY PART.

If we bridge the A and B windings together by the center wires, then make this point the Neutral point... and then we bridge the E and C windings together WITHOUT GROUNDING THAT POINT.... and further, bridge the free wire of the E winding to the neutral point... TA-DA!

.... We have a 2x120V split phase feed.
.... We have a third live leg to the system, with 208V from the neutral point...
.... AND WE HAVE 240V FROM THAT POINT to each live leg of the split phase feed.

Further we could wire the D and F windings the same way, and tap them to the other side of the graph... we would now have a 416V feed too!

Now that I have thought up a plausible solution to this... I still don't understand why anyone would want to wire lighting to the 208V leg... they would still need to have lighting devices designed for 208V.

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#29 Budget Appliance Repair

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Posted 10 May 2008 - 10:14 PM

[user=10234]Keinokuorma[/user] wrote:

Now that I have thought up a plausible solution to this... I still don't understand why anyone would want to wire lighting to the 208V leg... they would still need to have lighting devices designed for 208V.

After thinking about it, that was the thing that kind of threw me off also?????

Wouldn't you be burning out lots of light bulbs quickly,  Maybe getting good and
bright lights for the time they worked, but for how long.

Unless? Very many and all on at the same time all the times so quite a large load
always on the lights maybe able to handle it and also ?balance the sysem load at the same time?



William Burk (Willie)
Willie's Budget Appliance Repair
Eureka, CA 95501

#30 Keinokuorma

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Posted 11 May 2008 - 03:04 AM

After sleeping the night and letting the old beer odor and crapulent feeling pass, I re-thought this even further.

The person who called you, said that it was an industrial setting. They might well have high power lighting fixtures that are designed for 208 or 240V hookup. Otherwise, wiring standard 120V lights to 208V would triple their power and burn them out in seconds.

With constant load resistance, when voltage is multiplied by SQR(3), current is multiplied too. Power being the product of the two, it will be multiplied by SQR(3)^2 (which equals 3).

It wouldn't be really sensible to try and load the 208V feed so that it would "eat off" the excess... would take a lot of current, and the system would be unstable... one bulb pops open, the others get brighter, and they start popping open at an exponential rate... no, it's probably 208V stuff.

But yes, this wiring will give all of that. Actually it doesn't even need more than one secondary winding on each pole on the transformer core... it only takes these three 120V feeds that are offseted by 120 degrees each... wiring all of them serially (accounting for polarity of course) and tapping one joint for Neutral, will give you just this.

This took a while to figure out, we don't need do too many wirings like this. The ultimate standard for us is 230V, usually now in the bunch of three... then we get the 400V feed easily off of that, and if some facility requires a different layout, it is accounted in the stepdown design. Some factories get a 3x690V feed for example, that will directly convert to 3x1200V by the delta wiring... and they can step it down as they wish for lights and other standard voltage machinery...

The hospital I worked at some 10 years ago, had their kitchen remodeled a bit, got two large cooking vessels that replaced the old steam operated units... each had a 3x230V, 63A feed wired for them... the cable used for that was thicker than my wrist. Whew, the kettles alone were allocated more power than five single home buildings, and there were already two recently installed baking ovens and ancient but functional cooktops of the same scale. But then, that kitchen catered for the whole hospital, the home for the elderly, and two schools that I knew about.

EDIT: Just noticed that I'm constantly getting more and more typos like leaving characters out, swapping their positions, or striking the wrong keys altogether... this is probably my hardware getting old and cranky... let's hope that the snotware development team figures out a workaround. And, hopefully they will get the Virtual TAAMI module re-worked to be less unstable. Just waiting to see and feel what they have screwed up with Doors OS kernel 32.00 build 1 (due on May 21).

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#31 Electrician Dude

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Posted 13 May 2008 - 04:48 AM

Im the one who called Willie. I need to start by retracting a couple of statements.
First, The part about the lighting on the HIGH LEG may or may not be accurate. I remember hearing this from someone or maybe it came to me in A dream. I thought I had evidence to support this but came up with nothing. For now we will erase this from our minds. Second, after visualizing it in my mind it is also possible to pick just C-A with A two pole breaker. These are really mice nuts in the scheme of things. I just wanted to clear this up before I have everyone in the electrical world come down on me with vengeance. The important thing here is to realize that delta high leg configurations are out there.

2002 NEC Articles, 215.8, 230.56, 408.3(E)
Ugly's 2005 Pg. 13


#32 Keinokuorma

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Posted 13 May 2008 - 06:18 AM

Well, is it anything like the diagrams I came up with?

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#33 Electrician Dude

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Posted 13 May 2008 - 09:35 AM

This is the only one I have. But note A in this case is the high leg.

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#34 Keinokuorma

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Posted 13 May 2008 - 10:27 AM

I guess it is quite the same.

Looking at my latest drawing, the LB leg wouldn't be necessary, the transfromer would do with three secondary windings (one on each pole of the core) then a serial wiring, and tapping one joint for Neutral, would give such a setup, where you would have one leg with 120V to Neutral, one with 208V to Neutral, and 240V between the two live legs.

This would differ from standard 208 and 240V layouts, but not remarkably considering most applications, as long as the person who wires the thing up knows what they are doing.

Standard split phase has 180 degree phase shift between the live legs, the 208V twin phase has 120 degree shift... on this special layout, the 208V would not be balanced around the Neutral, but it would appear as a 208V to Neutral output, and it would sit at 90 degrees off the 120V phase... ... and then, the 240V output would be at 60 degree phase shift compared to the 120V...

A dryer or oven element, or a completely 240V cooktop or other similar device that doesn't use the Neutral connection, would not care if the 240V it gets is 60, 120 or 180 degrees off the phase of the 120V feed.

Anyway, such wiring would be very unusual back here, I haven't run to one that even resembles it a wee bit. Surely one who hasn't seen it before, would be stumped for a good while.

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#35 Electrician Dude

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Posted 13 May 2008 - 10:39 AM

I like this one better. I drawed it myself


#36 Electrician Dude

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Posted 13 May 2008 - 10:40 AM

oops!

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#37 Keinokuorma

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Posted 13 May 2008 - 10:49 AM

Why not. I see that you're wiring the secondary windings in a triangle fashion? I imagine that would require them to be 240V windings, and one of them to be center tapped for Neutral... I must think about this option.

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#38 Budget Appliance Repair

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Posted 13 May 2008 - 09:09 PM

Thinks for coming in a clearing that up for us.....

Sorry, I didn't call you back, I ment to call you today but got busy and didn't get home till 9pm tonight.

I was looking for the way to let you post with having to pay for the membership.  Sorry, no way to do that now I guess.

William Burk (Willie)
Willie's Budget Appliance Repair
Eureka, CA 95501

#39 Electrician Dude

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Posted 14 May 2008 - 03:00 AM

It is not A problem. I was going to pay for the membership anyway. This just sped the process up A little.



#40 slawomir

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Posted 20 May 2008 - 09:50 AM

It seems that you had there 240V 3-phase Delta service with High Leg.
It means that at the transformer three secondary coils are connected in delta, like a triangle, with L1 and L3 at the bottom and L2 (high leg) at the top (of course, that's the grapgic representation only). In the middle of the L1L3 coil neutral wire is connected. In all cases L-L voltage will be 240v and L1-neutral and L2-neutral will be a half of it i.e. 120 v, since it is tapped in the middle of the coil. Voltage between L2 and neutral will be about 208v.
It seems that these figures are quite close to those that you've measured, being slightly lower due to extensive network and voltage drop.





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