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ddahl84 11-09-2012 07:57 PM

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If the setup works from the pic attached wouldn't it work to just use line from plug 1 to a ssr then line from plug 2 to a contactor?
Attachment 83113


whoaru99 11-11-2012 04:49 PM

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No. It is not two phase and those two legs are not 180 deg out of phase.

Perhaps the transformer drawing I marked up (showing just a half cycle to keep it less cluttered) will clarify it. It's clear the two 120V legs are in phase. They have to be because it can't go both ways simultaneously as would be the case in the lower illustration. If that even worked, the neutral would see the sum of the currents not the difference which is further evidence to support the single, split phase illustrated at the top, not the 180 degrees out of phase myth.

It's all about the transformer windings in series, nothing about phase in this case.

If you want to run that 2-phase thing higher up the flag pole I suggest trying it on a dedicated electrical forum such as forums.mikeholt.com.


ajdelange 11-11-2012 10:46 PM

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Quote:
Originally Posted by whoaru99 View Post
It is not two phase power and the legs are not 180 out of phase.
It is indeed as we shall see in a moment.

Quote:
Originally Posted by whoaru99 View Post
The explaination with the oscilloscope is a common one, albeit flawed, because both leads have to be moved, not just one.
No. That's not true. Reread what I wrote. The low side of the scope is connected to the common point between the two phases. If the probe is connected to one phase of the system you will see a wave form in phase with the primary. If you then move only the probe lead, leaving the low lead in place, to the other phase you will see a waveform 180 ° out of phase with the primary.

But a picture is worth a thousand words. You'll have to look closely to pick up some of the details but they are all there. The yellow cable is connected to a 4 prong 120/240 biphase system (right of the panel in the brewery). This cable makes both phases available in, respectively, cable marked with a black stripe and unmarked. There is a plug in a striped (but the stripe is barely visible at the right edge) and another in an un striped line. As you can see, though again you have to look closely, a red aligator clip is hooked to the gold (hot) terminal in one phase and a red one to the gold terminal on the plug connected to the other phase. You will also see a white wire connected to the silver (neutral) plug on the striped phase. No need to run this wire again from the unstriped as the neutral is common to both - it is the neutral of every circuit connected to this panel. It is connected to the center tap of the transformer on the pole.The white wire runs to the low wire of oscilloscope channel B (gray probe) and is jumpered (yellow) to the low wire of the other (A, red) oscilloscope channel. Thus both oscilloscope channels are measuring relative to the neutral. The oscilloscope shows two waveforms of identical amplitude but opposite (180 °) phase.


Quote:
Originally Posted by whoaru99 View Post
Using the center tap as reference is the reason that oscilloscope example is flawed.
You now have solid visual evidence that the oscilloscope example is not flawed. If you have an oscilloscope you can repeat what I did (just in case you think I photoshopped this - and I did, I used unsharp mask to make the picture a little clearer) and you will get exactly the same result which is, very plainly, two phases in a 180° relationship to one another.

Quote:
Originally Posted by whoaru99 View Post
This is akin to putting the black lead of a multimeter on the positive battery post and the red lead on the negative post, the proclaiming the battery polarity has miraculously reversed because the multimeter shows negative voltage.
No, it is akin to connecting two batteries in series and hooking the black lead of a voltmeter to the common point then measuring the two voltages at the other ends of the two batteries. You would get, with 1.5 volt batteries, + 1.5 and - 1.5 and, of course 3 across the outside.

So you've now had detailed theoretical explanations from an electrical engineer, multiple examples and visual evidence. Plus the 'step back from the trees to see the forest' concept which is that if you are right electrical engineers, the professors that taught them, GE, Westinghouse and Siemens and all the textbooks have all been wrong from the days of Steinmetz and Tesla (who invented 3 phase power) to the present.

If you can't take in what your eyes show you then I fear there isn't much else I can do for you.

whoaru99 11-11-2012 10:58 PM

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It is not two phase power with two legs 180 degrees out of phase.

That connection of the o-scope doesn't hold water for this purpose. Neutral isn't the correct reference in this regard. Use the waveform math on that scope and add those two together. Do you get 240V or zero?

You have to use an end point for reference because it's the only place you can get 120 AND 240, and as you see below they are in phase and that the voltage on CH B is 2x that of CH A because it's taken at the correct reference for a 120/240V circuit (disclaimer, I'm using a small step down transformer from the line but net result is the same, just lower voltage).

I do like your newer ScopeMeter though, but for my uses this old dog still hunts.


ajdelange 11-12-2012 02:24 AM

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Quote:
Originally Posted by whoaru99 View Post
It is not two phase power with two legs 180 degrees out of phase.
Well, yes it is. To quote from the summary at http://www.allaboutcircuits.com/vol_2/chpt_10/2.html (at which you ought to have a look)

"A split-phase power system is one where there are two voltage sources, 180o phase-shifted from each other, powering a two series-connected loads. The advantage of this is the ability to have lower conductor currents while maintaining low load voltages for safety reasons."

You can argue that this is something (in fact the first thing) I found on the web searching for 'polyphase neutral' but I have never seen anything published nor, up until this point in my life, met anyone that contradicts what it says.

Quote:
Originally Posted by whoaru99 View Post
That connection of the o-scope doesn't hold water for this purpose. Neutral isn't the correct reference in this regard. Use the waveform math on that scope and add those two together. Do you get 240V or zero?
I get 240 because I add them in the proper phase as happens when the transformer coils are connected correctly. One channel is connected from L2 to neutral and the other is connected from L1 to neutral so they are 180 ° out of phase. L2 is thus negative with respect to the neutral when L1 is positive WRT it but that means that the neutral is positive with respect to L1. Thus we are connected - + - + just like the batteries I mentioned in an earlier post and we get summation of the voltages. Take your transformer drawing (upper one) put in a load and do Kirchoff's law around the loop. Bottom to neutral is plus 120 V (bottom is negative with respect to neutral). Neutral to top is 120 V. They are connected in series positive to negative. The voltages add.

But at least I now know what the source of your confusion is. You don't understand that the reference is always taken at the neutral point in a Y connected system and perhaps don't understand that a biphase system is a degenerate Y connected 3 phase system.

Quote:
Originally Posted by whoaru99 View Post
You have to use an end point for reference because it's the only place you can get 120 AND 240, and as you see below they are in phase and that the voltage on CH B is 2x that of CH A because it's taken at the correct reference for a 120/240V circuit (disclaimer, I'm using a small step down transformer from the line but net result is the same, just lower voltage).
Not so. Using the center as the reference my scope picture clearly shows two 120 volt circuits. If I connect them in series I get 240V across the outside.

But the essence of your problem is the reference point and as I noted earlier this is arbitrary and mentioned that in a Y connected system the neutral is the only sensible reference (which is why it is used as such). So lets suppose we are in an industrial or office building fed with 3 phase Y connected power as they often are. It is common to have both 120V and 208 V circuits (for lights, floor polishers etc.). A given panel has two (of the three) phases connected to its breaker bars and the neutral connected to the neutral bar. It's a biphase system with the phases 120° apart instead of 180°. If we did my experiment or yours (I did notice that you changed the photograph from one with 2 different references to one with a common reference so I'm assuming the later is the one you want to convey) we would get a picture like the one below. I would get the green and blue traces. You would get one or the other and the black trace. If I moved the reference line on one of my channels to the the opposite phase I would get the black trace as well. Note that 100 here on the voltage scale represents 100% of either rms or peak voltage.

Now in this picture with the references connected my way the two phases are not 180° apart and connected your way the one phase and the sum are not in phase. How would you argue here? You couldn't say this is not a polyphase system and that the two phases not 120° apart because they clearly are. You cannot say the 120V and 240V circuits are in phase because they clearly aren't. The only difference between this 120° arrangement and the 180° one is the phase angle is 180 ° and that happens to be the phase angle that brings the phase to neutral voltage into alignment with the phase to phase voltage.

Now in a delta connected system there is no neutral and you have to pick a phase or phases (in a 2 channel scope hookup) as the reference. So perhaps that's where the misunderstanding lies i.e. you are thinking of a split-phase system as being more like a degenerate delta connected system when it is actually more like a degenerate Y connected one.


Quote:
Originally Posted by whoaru99 View Post
I do like your newer ScopeMeter though, but for my uses this old dog still hunts.
Fluke does make nice stuff.

whoaru99 11-12-2012 12:59 PM

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Quote:
Originally Posted by ajdelange View Post
I'm not so concerned about what you call it but more with failure to understand that there is a 180° phase shift between the hots on 120V circuits connected to different sides of your panel. If the two sides were in phase then an outlet connected between them would show 0 volts.
No.

There is no 180 phase shift. Your own oscilloscope proves that using the waveform math because if you sum those waves you put up the result is zero volts. Yet, we know for sure that if you grab those two wires it's not zero volts there, it's 240V. The only reason it looks like they're 180 out is because you're using the wrong reference point. A parlor trick, for lack of better terms.

One would think an EE would know that two waves of equal amplitude and frequency, when 180 out, will cancel not sum. Since we know the waves aren't canceled when both legs are used, we know the waves cannot be 180 out of phase.

Here are your two waves 180 out of phase (CH A and CH B), and their sum sitting right as zero (trace 4).

All the spin, fancy dancing, and deflection doesn't change the fact that waves 180 out don't add, they cancel. Therefore, the two legs cannot be 180 out because they definitely don't cancel. Further, the point about being two phase in this case is wrong. I grant you that two phase power could exist and may well exist in the wild (120/208 - 120V L-N, 208V L-L), but what we're discussing isn't that nor is that common household distribution.

Frankly, I think the whole notion of two waves in this regard (120/240 split voltage) is even flawed. It's the same wave, you're just looking at half of it on the left and half on the right from the midpoint. No out of phase involved because it's just plain old, simple, single phase. All the talking point trying to apply polyphase theory are non sequitur.


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