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Why can any two phases be connected together? 
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#37
Jan1810, 08:04 PM

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#38
Jan1810, 09:10 PM

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#39
Jan1810, 09:13 PM

P: 754

There is such a thing as 2phase (2 voltages that are 90 degrees out of phase from each other), but let's not get into that here. 


#40
Jan1810, 09:22 PM

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#41
Jan1810, 09:24 PM

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#42
Jan1810, 09:29 PM

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I hardly ever hear anyone refer to it as split phase. Split phase to me is a type of electric motor that has a high resistance starting winding that is taken out of circuit by a centrifugal switch.



#43
Jan1810, 09:36 PM

P: 27

Anyone know where I can find a diagram of how the load completes the 3phase circuit? On the secondary side, nothing is going to happen until a load draws current right? But how does this complete the loop?



#44
Jan1810, 09:53 PM

P: 2,499

I thought the link in post #17 did a pretty good job. Just imagine that each coil peaks in voltage at a different time. Current is drawn by each coil at a different time also.



#45
Jan1810, 10:00 PM

P: 27

So the coils here prevent a direct short? 


#46
Jan1810, 10:15 PM

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#47
Jan1810, 10:17 PM

P: 4,513

zgozvrm, I don't know how to draw diagrams as you do. Your phase diagram was very well rendered. How do you do it?



#48
Jan1810, 11:11 PM

P: 754

Now, let's take a 2nd transformer that is identical to the 1st one, except that it has a "centertapped" secondary (let's label this lead as X0). This is a single coil with a lead attached to each end (X1 and X2) and one attached to the center of the coil (X0). There would be 1/2 the number of turns in the secondary coil as there are in the primary (just as in the 1st xfmr). The center tap will have half that number of turns (or 1/4 the number of turns in the primary) on either side (between X0 and X1 and between X0 and X2). If you were to apply the same AC voltage to the primary coil and measure the voltage from one of the end leads, say X1 to the center tap X0 (ignoring X2 for now), you would in effect have a xfmr with a turns ratio of 4:1. Again, the induced secondary voltage will rise when the primary voltage rises (and viceversa), so it, too, is in phase with the primary. Agreed? No matter how many times a single secondary coil is tapped, a voltage measured from any 2 leads connected to different points on the coil will rise and fall with the inducing primary voltage. This supports why we correctly refer to 240/120V systems as "split phase" rather than "2phase" ... there are not 2 different phases, but rather a single phase that has been split in two parts. To repeat an earlier example, I can make a Dcell battery look as though it supplies negative 1.5 volts merely by reversing the leads of my voltmeter. This is what we're doing with the scope. So, in effect, what we end up with is a single primary coil and 2 secondary coils connected endtoend. Imagine then there are actually 2 separate secondary coils, each with leads connected at both ends (only). The secondary induced voltages will both rise and fall in time with the source (primary) voltage. These voltages are both in phase with the primary voltage and, therefore in phase with each other. Using vectors to illustrate: We know that the two 120V voltages of splitphase 240/120V add up to 240V. So if we take the voltage between X0 and X1, and assume its angle to be 0 degrees (it's not in reference to anything, so we can choose any angle), we would have a vector of length 120 pointing directly to the right. Now, take the voltage between X0 and X2 and let's assume that it is 180 degrees out of phase from the first voltage. We would then have a vector of length 120 pointing directly to the left. I you add these 2 vectors, you can see that they would cancel each other out. Alternatively, if we have 2 vectors both of length 120 pointing in the same direction and add them together, we would have a resultant vector of length 240 pointing in the same direction as the original 2 vectors. Remember, too, that splitphase power (3wire 240/120V power) is considered single phase, not 2phase (you said it, too). 


#49
Jan1810, 11:13 PM

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#50
Jan1810, 11:24 PM

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#51
Jan1810, 11:54 PM

P: 754

This is where the confusion starts for most people. Although it has become accepted, it is confusing to call the 3 leads coming off a 3phase transformer, "phases" (forget neutral and/or ground connections, for now). It is generally clearer to call those wires "legs": Leg A, leg B, & leg C. Measuring between 2 legs, you will see different phases. For example, you might find that the voltage measured from leg A to leg B is 240 volts. You will find that the voltage measured from leg B to leg C is also 240 volts, but that it lags the first voltage by 120 degrees (i.e. it is out of phase by 120 degrees). And, you will also find that a 3rd voltage can be seen by measuring from leg C to leg A and that it lags the 2nd voltage by another 120 degrees (240 degrees from the first voltage). Remember that it takes two reference points to have a voltage; you can't say that a single wire coming off a transformer has a certain voltage (voltage is the difference in electrical pressure between 2 points). It must be measured in reference to another point. Also, it takes 2 or more voltage measurements to be able to find a phase angle between them. (So it is generally incorrect to give a single voltage measurement a phase angle). For simplicity, we usually we label the first of 2 or more measurement taken to be at 0 degrees. (In fact, we could use any number.) So, if we call any (or all) of the three secondary wires coming off a 3phase transformer (whether delta or wye) a "phase," what exactly does that mean? That single wire, by itself is useless and has no voltage, so it cannot be compared to a voltage and, therefore it has no phase angle. When you measure the voltage between any 2 legs, you are measuring across one of 3 coils, each giving a voltage at a different phase angle in relation to the other two. 


#52
Jan1910, 07:31 AM

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PF Gold
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http://www.animations.physics.unsw.e...ors.html#three Edit: Oops. I thought you were asking about foos images. 


#53
Jan1910, 11:22 AM

P: 754

Here is another way to illustrate my point (hopefully the pictures will help):
If I have four DCell batteries connected in series like this: and measure them with a voltmeter, I get 6 volts. As you can see the batteries are all "in phase" with each other. That is, the direction of their polarity is consistent. Now, if I reverse the meter leads like this: the meter reads 6 volts. Did the voltage of the batteries change? No! I'm just looking at them differently; they are all still "in phase" with each other. Now,if I measure from one end of the seriesconnected batteries to the middle connection (the centertap) like this: I now get a reading of 3 volts. And, if I measure from the middle connection to the other end of the seriesconnected batteries like this: I still get a reading of 3 volts. However, if I now move my red (positive) probe back to the other end of the seriesconnected batteries, leaving the black (negative) probe at the middle connection like this: I get a reading of 3 volts. This is similar to how most people view a 240/120V 3wire singlephase connection. The phasing (or, in the case of the DC batteries, the polarity) of any part of the system never reverses, only the way we choose to look at it does. Just as the upper 2 batteries in my diagrams are always "in phase" with the lower 2 batteries, one half of a coil is always in phase with its other half. 


#54
Jan1910, 12:33 PM

P: 2,499

zgozvrm, I get it. I've always 'gotten it'. I just don't understand why you insist that a center tapped winding cannot be considered to have each end of the coil 180 degrees out of phase with each other when the voltage is referenced to the center tap. But somehow, you claim that having two completely seperate windings that it is ok to call them 180 degrees out of phase with each other when the scope is hooked accordingly. So then do you feel that you can no longer call them 180 degrees out of phase if you were to hook the seperate windings together end to end? This essentially would become a single center tapped winding. Many power transformers are configured like this. To me it looks like this: I stand facing North. I can say for fact that the sun gets up to my right and sets to my left. Then I stand facing South and I say that now the sun gets up to my left and sets to my right. You argue that I cannot say that because I'm just facing the wrong way. I no longer wish to argue this specific point. However, I would still like your opinion to the question I asked about manufacturing a new 'phase' or 'leg' or whatever you prefer to call it.



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