Single Phase or Polyphase?

[mivey]

Aren?t you saying there are two voltages in different directions available from a single winding?

I am saying I have two sinusoidal inputs that are 180 degrees out of phase. These are two signals that are really & truly shifted by 1/120 of a second in time. Real voltages. Not some mathematical model or manipulation of numbers. I really started one sinusoidal signal, waited 1/120 of a second and started another.

Really. Honest. No kidding. I promise that is what I did. I'll sign an affidavit. The two voltages are really 180 degrees out of phase.

I watched the first signal on the scope. It started off at the positive peak value and when the clock read 1/120th of a second, I instantly triggered the second voltage. At that point, the first voltage was at a negative peak and the second voltage started at a positive peak. They have been cycling that way ever since.

Aren?t you saying, here, that a single winding can have current flowing in different directions, regardless of the voltage direction?

If there is a neutral it can. Step through the time domain. No matter which direction you pick for current, you will find times where one current is positive and one current is negative.

You could also write out the two current equations and see that it is true.

This contradicts the source I quote that applied Lenz?s law.

No, it does not. Care to explain how you think it does? I'm guessing you did not follow up on winnie's posts where he explained that the currents can be out of phase. If they are out of phase, they will not cross the zero axis at the same time and you will have one positive current and one negative current in the secondary coil, regardless of which direction you are calling positive.

 

[mivey]

...They have been cycling that way ever since.

Just went and looked at the terminals to double-check. I took some quick instantaneous voltage readings from both sets of terminals. On the first transformer I got 1_pu from X1 to X4 and at the same instant I read -1_pu from X1 to X4 on the other transformer. The time-displaced phases are still running fine.

The point is, the secondary could be either type system. The fact that the single-phase system with a center-tap looks just like the two-phase system is neat and all that, but the truth of the matter is: in this case we can't tell any difference between the systems so either one can be represented by either circuit.

 

[jim dungar]

I am saying I have two sinusoidal inputs that are 180 degrees out of phase.

Two out of phase voltages from a single winding, is only possible if you use the neutral as your reference. What you are doing is a trick of math because it requires a very specific assumption and it does not occur if any other assumption is made. For you to have two truly independent voltage directions, you need two independent secondary windings. A single winding with a center tap simply creates two in-phase series connected voltages.

Using almost gross simplifications.
Farady's Law says a magnetic flux creates a voltage, the direction of the flux dictates the direction of the voltage. Lenz's Law says that current flows to oppose the change in the flux. With a single winding primary there is a single magnetic flux. With a single magnetic flux there is a single voltage direction induced. With a single voltage direction, there is a single current direction. And yes, while the current can be 'out of phase' with its voltage by +/-90? its relative direction is due to the voltage that created it.

I was hoping to get past single winding transformers (several pages ago in fact) but, it looks like that is not going to be possible. I found my old reference books, that confirm the method I was taught, so I am at peace for the present time.

 

[gar]

100319-1133 EST

Jim:

Use the primary as your reference voltage and connect this to channel A of an oscilloscope. A single terminal does not define a phase reference. If using voltage as the reference, then two terminals with a voltage difference are required to supply the reference.

With the scope channel A as the reference also synchronize the scope sweep to it. Sync so the left edge is 0 V and a positive slope.

Make the secondary consist of two independent coils of equal output amplitude. Use channel B to measure the secondary voltages. You will get outputs that are in phase with the input or out by 180 deg depending upon the polarity of connection. Obviously there is a slight phase shift from the 0 or 180, but theoretically there is none.

Thus, there are two output phases possible, 0 or 180.

If these coils are connected together one way, then the result is a single phase output, another way is a two phase output where one connection point is your previously referenced center tap. Use the scope to look at these different phases.

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[jim dungar]

Make the secondary consist of two independent coils of equal output amplitude.

This is the key, two independent coils.

I have consistently talked about a single coil, center tapped.
A single coil cannot have two voltages induced in different directions.

Apples and oranges.

 

[gar]

100319-1421 EST

jim:

What is the difference between one coil with a center tap brought out and two coils wound on the same core and connected so that their outputs add?

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[LarryFine]

If these coils are connected together one way, then the result is a single phase output, another way is a two phase output where one connection point is your previously referenced center tap. Use the scope to look at these different phases.

I would place leads on A and N, and then move them to N and B; they'd appear to be in phase. By keeping one lead on the N (CT), you're flipping the measuring polarity.

What you're discussing is the difference between subtracting a positive number and adding a negative number. Mathematically the same.

 

[LarryFine]

What is the difference between one coil with a center tap brought out and two coils wound on the same core and connected so that their outputs add?

As long as they're on the same core bobbin, they're the same to me.

As a matter of fact, some center-tapped secondaries are wound as two parallel wires (bi-filar) connected in series, so both halves have the same pohycical characteristics (distance from core, etc.)

 

[gar]

100319-1454 EST

The coils are wound per Larry's suggestion.

One coil is labeled X1 and X2. The phasing dot is beside X1. The other coil is labeled X3 and X4. Its phasing dot is beside X3.

If X2 and X4 are connected together, then there is essentially zero voltage difference between X1 and X3 and these two voltages relative to the common connection point are in phase, and X1 can be connected to X3 with no fireworks.

If X2 and X3 are connected together, then there is big voltage difference between X1 and X4, and these two voltages relative to the common connection point are out of phase by 180 deg. If X1 is connected to X4, then BIG fireworks, and a hot transformer with smoke and smell.

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[LarryFine]

. . . same pohycical characteristics . . .

Would you believe "physical"? :roll:

 

[jim dungar]

One coil is labeled X1 and X2. The phasing dot is beside X1. The other coil is labeled X3 and X4. Its phasing dot is beside X3.

The phasing dots indicate the direction of the voltage.
The industry standard convention for connection is X1->X23->x4. This means the direction of the voltage is from X1->X4. Therefore saying the voltage waveform of X23->X1 is 180? is nothing more than a trick of math (a simple inversion) because the transformer action is actually creating the voltage X1->X23. This inversion then requires us to make another inversion prior to adding the voltages X23-X1 and X23-X4 in order to get X1-X4.

 

[gar]

100320-0823 EST

jim:

This is not a trick of math. That is why in a previous post I started with two secondary coils. Labels such as X1, X2, etc. really mean nothing until their phasing is defined and that is the purpose of the DOT convention as a means of defining the phase.

In common usage it is true that X1 and X3 as one end of secondary coils are considered as having the same phase angle.

Go back to my two oscillator discussion. Let waveform 1 be the reference. As I adjust the phase of waveform 2 toward 180 deg is there some special transformation that at exactly 180 degrees of phase shift makes this not a 180 deg phase shift? Should there be such a discontinuity at exactly 180 deg? I do not think so.

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[jim dungar]

...the purpose of the DOT convention as a means of defining the phase.

In common usage it is true that X1 and X3 as one end of secondary coils are considered as having the same phase angle.

This is why I have tried to focus on how a single winding transformer operates. With a single winding there can only be one direction. Placing taps on a single winding can not change the directions of the resultant multiple voltages. Changing the reference point simply changes the math not the physics of the single winding transformer.

A transformer with a single winding primary and a two winding secondary may have different voltage directions, but as you have pointed out the industry convention is to identify and treat the windings as being 'in phase'.

 

[gar]

1000320-1201 EST

jim:

I can wind a single transformer secondary coil with an unbroken wire with a center tap and have the voltage of the two hot terminals relative to the center tap have the same phase, thus no voltage difference between the hot terminals, and those two hot terminals can be connected together without fire or smoke.

If two voltages are of the same shape, amplitude, frequency, and zero phase difference, then these voltages can be combined in parallel and the result is no circulating current. Any other phase angles, except N*2*Pi, will cause some circulating current.

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[jim dungar]

I can wind a single transformer secondary coil with an unbroken wire with a center tap and have the voltage of the two hot terminals relative to the center tap have the same phase, thus no voltage difference between the hot terminals, and those two hot terminals can be connected together without fire or smoke.

So, X1->X23 = X23->X4 and X1->X23->X4=0

Please describe a winding where the two ends of a single winding have the same polarity dots.

If you wind part of the single conductor in one direction, then stop and call this the center tap, then you wind the conductor in the other direction, you have two windings.

 

[gar]

100320-1901 EST

jim:

Take a long piece of wire bend it into a hairpin shape. Attach the center tap wire at the center of the hair pin. Next wind this on the bobbin. This is one coil with three connection points.

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[jim dungar]

Take a long piece of wire bend it into a hairpin shape. Attach the center tap wire at the center of the hair pin. Next wind this on the bobbin. This is one coil with three connection points.

Such an interesting idea.

A new view on a three winding secondary.

 

[LarryFine]

Take a long piece of wire bend it into a hairpin shape. Attach the center tap wire at the center of the hair pin. Next wind this on the bobbin. This is one coil with three connection points.

Now, that would be an out-of-phase pair of windings.

 

[gar]

100321-1116 EST

Larry:

They are in phase because you can connect the two end points together with no circulating current. The voltages are identical in amplitude and phase.

On a normal center tapped winding if you connect the two end points together there are real big problems because the two voltages are 180 degrees out of phase.

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[jim dungar]

They are in phase because you can connect the two end points together with no circulating current. The voltages are identical in amplitude and phase.

Are you sure there won't be any circulating current between these multiple windings (I haven't tried to evaluate this circuit)? I agree that the 'polarity' of your X1 and X4 can be identical, but even though you have a single conductor you do not have a single winding. Your conductor must double back on itself, otherwise its two ends would not be able to have the same polarity dots (the same relationship to one end of the primary winding).