Phase Angles in a Single Phase System:

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jcormack said:
So if you want, the Voltages are 180 out of phase, the currents are 180 out of phase (that is why no neutral current in a balanced load)........but the POWER is in phase.........
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I like it.

In a polyphase system, when one phase goes to zero power, the other phases are still delivering power.

In a balanced polyphase system, the total power delivered remains constant as the various phases cycle from zero to full power.

In a single phase system, power goes to zero at least twice each cycle.

-Jon
 
Measure the voltage of a battery with your DVM. Say it measures +12V. Swap the leads and it measures -12V. If we invert a sinusoid, we have in effect shifted it by 180 degrees.
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Rattus what did you change in the battery?
Nothing the current in your meter is just flowing the other direction.

The batteries pos. is still pos. and the neg. is still neg.:rolleyes:
 
Trifling Point:

Trifling Point:

winnie said:
If you didn't have a potential difference, you would have nothing to measure.

The end terminals of the single winding have a potential difference with each other.

If you swap the leads of your scope, the _measured polarity_ of that potential difference will change. Of course the physical reality will not change; you still have a single coil with a single potential difference between its terminals. In this sense, there is absolutely no time displacement between the two measured waveforms. They are simply inverted versions of each other.

_However_ the is no measurable difference between an inverted sine wave and a sine wave with a 180 degree phase shift. They are absolutely equivalent. Therefore, if it is more convenient for a particular application to call the inverted sine wave a sine wave with a 180 degeee phase shift, then do so. They are equivalent and interchangeable.

If the input to the transformer is _not_ a sine function (and the input is never truly and exactly a sine function, if only because it has finite duration), then an inversion _is_ distinguishable from a phase shift.

-Jon
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Winnie,

If the waveform is not sinusoidal, we can only speak of the phase shift of the fundamental, and that, in effect, is what we are doing.
 
Jcormack....thank you for your succinct argument. I have always maintained that inverting a sinusoid "appeared" to be a 180-degree phase shift, but was yes indeed, only an inversion. Your power observation seems to be as strong a position as I've seen.

Unfortunately, I lost the "ability (read: willingness) to post twelve-paragraph arguments 20 times per day over the internet" war.

Now all we need is published work defininga polyphase system as having power always greater than zero...
 
Nothing

Nothing

ronaldrc said:
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Rattus what did you change in the battery?
Nothing the current in your meter is just flowing the other direction.

The batteries pos. is still pos. and the neg. is still neg.:rolleyes:
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Ronald,

Never said I changed anything internal to the battery. No need to. It is a matter of the reference we choose to use. Just describing the voltage in a different way. That is what this is all about.

BTW, is the voltage of a flashlight battery +1.5V or -1.5V?
 
120/240 transformer is a polyphase transformer.You can have three terminals; 1, 2 and 3. One being one end of the coil, 3 being the other and 2 is a tap in the middle of the winding.
Voltage between 1-2 will be half of 1-3 and in phase.
When one establishes terminal 2 as the Zero "0" reference by connecting it to earth, grounding it, 1-2 and 2-3 will be identical halfs of the volatge measured between 1-3.
Now do a mental excercise. (Come on, humor me, it'll not hurt ;-)

Imagine the three points along a stick. Hold the stick at point 1 and rotate it 360 degrees. If you would have a pencil attached at points 2 and 3 and would drag a sheet of paper under the rotating stick and pencils, you would create the sinewaves you see on the oscilloscope.

Now hold the stick at point 2 and repeat the excercise.

No further explanation is necessary.

(Hint 1: the stationary point is always the earthed, grounded point.
Hint 2: point 2 is chosen as the "common", so each part of the winding can be equally and fully loaded with single phase loads otherwise 2-3 would only carry the higher voltage load and be limited by the load on 1-2)
 
Constant Power:

Constant Power:

bcorbin said:
Jcormack....thank you for your succinct argument. I have always maintained that inverting a sinusoid "appeared" to be a 180-degree phase shift, but was yes indeed, only an inversion. Your power observation seems to be as strong a position as I've seen.

Unfortunately, I lost the "ability (read: willingness) to post twelve-paragraph arguments 20 times per day over the internet" war.

Now all we need is published work defininga polyphase system as having power always greater than zero...
Click to expand...

I started a thread on this subject a couple of years ago. We proved that a the power in a balanced, 3-ph system is constant. Here is a reference:

"instantaneous value of three-phase power to be independent of time."

[Kercnher and Corcoran, "AC Circuits", Wiley, 1951]

Now, what is the difference in a 180 degree phase and an inversion? The inversion is convenient way to obtain the phase shift. A phase meter would read 180 degrees, therefore it must be. If it looks like a duck,.....
 
rattus said:
We proved that a the power in a balanced, 3-ph system is constant. Here is a reference:

"instantaneous value of three-phase power to be independent of time."

[Kercnher and Corcoran, "AC Circuits", Wiley, 1951]
Click to expand...

To sum up:

1. The power in a polyphase system is never zero.
2. The power in a 120/240V system is zero twice per cycle.
3. Therefore, a 120/240V system can not be a polyphase system.
4. Therefore, one "phase" of said 120/240V "two-phase" system is merely the result of inverting the other, and not time-shifting, despite the identical appearance, because time shifting creates different phases, which we do not have.
5. KCL says that the sum of the currents either entering a node or leaving a node must equal zero.
5. Since both hots of a 120/240V system are thus in phase with each other, the magnitude of one must be negative in order to satisfy KCL at the neutral.
 
bcorbin said:
To sum up:

1. The power in a polyphase system is never zero.
2. The power in a 120/240V system is zero twice per cycle.
3. Therefore, a 120/240V system can not be a polyphase system.
4. Therefore, one "phase" of said 120/240V "two-phase" system is merely the result of inverting the other, and not time-shifting, despite the identical appearance, because time shifting creates different phases, which we do not have.
5. KCL says that the sum of the currents either entering a node or leaving a node must equal zero.
5. Since both hots of a 120/240V system are thus in phase with each other, the magnitude of one must be negative in order to satisfy KCL at the neutral.
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Who said that a 120/240V system is a polyphase system? We all know it is not. I have said as much many times in this very thread.

Wait a minute now, if the two hots are in phase, one cannot be the negative of the other, and the "magnitude" carries no sign.

Another diagram coming up!
 
bcorbin said:
4. Therefore, one "phase" of said 120/240V "two-phase" system is merely the result of inverting the other, and not time-shifting, despite the identical appearance, because time shifting creates different phases, which we do not have.
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This point is the crux of the 'disagreement'.

Take the statement 'time shifting creates different phases': what happens when I use a delay line to time shift by exactly one half cycle? Have I produced a second phase or not?

The output of an inversion of a sine wave is exactly the same as a time shift by exactly one half cycle. It doesn't matter if I have a center tapped transformer, or a delay line or a rotary phase converter with 180 degree outputs.

A time delay of exactly one have cycle fails your tests for a 'polyphase' system (the power goes to zero at least twice per cycle, you cannot produce a rotating field), and thus a time delay of one half cycle looks and quacks like a single phase system. But you have a real time delay there, and a real phase angle difference.

Having a phase shift does not necessarily mean that you have a polyphase system that can produce a rotating field. Power falling to zero twice per cycle does not necessarily mean that you do not have a phase shift.

-Jon
 
I am no implying you said 120/240V is a polyphase system. To the contrary, I am using your opinion to settle our old phasors and time-shifting vs. inverting argument. And I think I did pretty well.
 
rattus said:
...We proved that a the power in a balanced, 3-ph system is constant. Here is a reference:

"instantaneous value of three-phase power to be independent of time."

[Kercnher and Corcoran, "AC Circuits", Wiley, 1951]...
Click to expand...
Just curious. Did "we" prove that, or did we just take Kercnher's presentation as truth?

I curious cause the derivation is not difficult, just a trig algebra problem. Pretty straight fordward for resistive loads, a little messy for ICE or ELI loads.

carl
 
Yes, yes, yes:

Yes, yes, yes:

coulter said:
Just curious. Did "we" prove that, or did we just take Kercnher's presentation as truth?

I curious cause the derivation is not difficult, just a trig algebra problem. Pretty straight fordward for resistive loads, a little messy for ICE or ELI loads.

carl
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It is proved in the reference, charlieb proved it, and I proved it. You are next.
 
Not really!

Not really!

bcorbin said:
I am no implying you said 120/240V is a polyphase system. To the contrary, I am using your opinion to settle our old phasors and time-shifting vs. inverting argument. And I think I did pretty well.
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A phase shift, however obtained, is still a phase shift. There is no logical reason to distinguish between a 180 degree phase shift and an inversion.

Plus, you made contradictory statements. You said that the voltages on L1 and L2 were in phase, then you said that they were inverses of each other. Can't have it both ways.

You can't prove something that is not true! And what difference does it make if we call it a phase shift? It looks like a phase shift; it works in the circuit like a phase shift, ... it must be a phase shift.
 
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