How does a typical residential utility transformer work?

[rattus]

Forget to reply to this one:

Forget to reply to this one:

But you are not correct for the second. Again from IEEE 100 (definition of phasor):

A complex number expressing the magnitude and phase of a time-varying quantity. Unless otherwise specified, it is used only within the context of steady-state alternating linear systems

The phrase, “phase of a time-varying quantity”, is generally translated as (oh-oh, I’m doing it again) as, “rotation”.

carl

Carl, you are still misinterpreting the definition; the quantity is "time-varying", but the definition does not mention rotation, and the examples do not include any variables. A "phasor" provides only the RMS magnitude and phase angle--both constants. e.g.,

V = 120Vrms x exp(jphi) where "phi" is the phase angle.

This is used in steady state analysis with impedance.

A rotating phasor includes the radian frequency and an independent variable, e.g.,

v = 170 x exp(j(wt + phi))

Think about it.

 

[rattus]

Three sine waves

Three sine waves

They are not really out of phase by 180 degrees...you are just measuring for the the center of the wave to each end....there is only one sine wave and it cannot be out of phase with itself.
Don

Done there are three--V1n, V2n, and V12. V1n and V2n are nominal inverses of each other, but the magnitudes may differ someone.

V12 = V1n - V2n.

My Tek scope says they are out of phase, so they are. If it looks like a duck...

It would make no difference if these voltages came from two transformers or whatever.

 

[don_resqcapt19]

My Tek scope says they are out of phase, so they are. If it looks like a duck...

If you put the scope on the two ends there is only one sine wave. Choosing to put the scope on multiple points does not change this. You are measuring a single sine wave with multiple points of reference. That does not create additional sine waves. I see this the same as two dry cells in series. You have 3 volts end to end and 1.5 from each end to the center. There is only one 3 volt source, but by measuring it at multiple points it can appear to be a plus 1.5 volt source, a minus 1.5 volt source and a 3 volt source.
Don

 

[winnie]

Oh lordy lordy lordy!

Okay, I fall squarely into the camp that says 'it is reasonable to say that the two legs of a single phase system are 180 degrees out of phase relative to the neutral'.

This does not mean to me that multiple taps on a single transformer coil mean that we have a 'polyphase' system. Two AC voltages, 180 degrees out of phase cannot produce a rotating field.

This does not mean that there is a time lag between one side of the transformer and the other side. You have a single coil, a single phase, and no time difference between the outputs, no matter how you flip your scope leads.

However an inverted sine wave looks just like a 180 degree phase shifted sine wave, and is _indistinguishable_ from such. The _equivalence_ of inversion and 180 degree phase shift is regularly used in rotating machines: consider a standard 2 pole stator winding, where the A' coil ends sit on the stator between the B and C coil end. 'A inverted' sits on the stator between 'B' at 120 degrees and 'C' at 240 degrees. In this context A inverted is _clearly_ associated with a 180 degree phase angle.

-Jon

 

[rattus]

Exactly:

Exactly:

If you put the scope on the two ends there is only one sine wave. Choosing to put the scope on multiple points does not change this. You are measuring a single sine wave with multiple points of reference. That does not create additional sine waves. I see this the same as two dry cells in series. You have 3 volts end to end and 1.5 from each end to the center. There is only one 3 volt source, but by measuring it at multiple points it can appear to be a plus 1.5 volt source, a minus 1.5 volt source and a 3 volt source.
Don

Don, if you have two dry cells, you do have two sources, but that really doesn't matter. However we measure these voltages, we take note of their polarities, and that is determined by our choice of reference, that is all we are doing.

If I had a 1.5V bus, and a -1.5V bus, I would measure each of them relative to "ground" with no thought about their origin. If I measure the voltage on L1 and L2 I would do the same. Like the batteries, V1n and V2n are nominal negatives of each other and can be considered to be separate sources even if they come from one transformer or two. We could trace the circuits back to the generator and claim everything is in phase.

When we say that V2n is 180 degrees out from V1n, we are describing a voltage measured in a particular way. This does not magnically create another phase voltage though which seems to be the hangup.

Summary:

If

V1n = 120V @ 0

Then

V2n = 120V @ 180

And

Vn2 = 120V @ 0

That is all. We don't care whether we have one source or two or three!

It is incorrect to say that V1n and V2n are in phase. It is correct to say that V1n and Vn2 are in phase.

 

[kingpb]

Its a good thing NASA uses Vdc, otherwise engineers would never agree and we could have never reached the moon.

Could it be possible that your saying the same thing, just looking at it from different perspectives?

 

[rattus]

Not really:

Not really:

Its a good thing NASA uses Vdc, otherwise engineers would never agree and we could have never reached the moon.

Could it be possible that your saying the same thing, just looking at it from different perspectives?

Not really. Don and others claim that since V1 and V2 come from the same secondary, they have to be in phase.

I claim that if I measure V1 and V2 relative to a common reference--say neutral--one voltage is the inverse of the other, that is, 180 degrees out. I think we now agree on that fact.

I further claim that the fact that these voltages derive from the same secondary is immaterial. That has absolutely no effect on any analysis I may perform. I am free to define my voltages anyway I wish, and it is a convenient and common practice to use the neutral as a common reference.

What if I obtained V1 and V2 from two alternators running off the same shaft? Would that make any difference? Fact is, when we measure these voltages at the panel or at an outlet, we can't determine their source from the measurements.

I further claim that most if not all electricians in the field measure V1 and V2 in this matter, that is they measure V1n and V2n without thinking about it. If their DVMs also measured phase angles, they might be surprised.

 

[winnie]

As stated above, I agree that V2n and V1n have a 180 degree phase difference.

However for the purpose of generating a rotating magnetic field, I claim that they are 'in phase'.

Discussing this topic, I mentioned the concept of a 'delay line', which could be used to generate an arbitrary phase angle between its input and its output.

I believe that we would agree that for single frequency sinusoidal signals, a delay of 360 degrees (or any N * 360 degrees) results in the output matching the input, and thus an output that is 'in phase' with the input.

When _magnitude_ is all that matters, a delay of 180 degrees is 'maximally out of phase'. An example of this would be interference patterns generated by waves. When two waves are 180 degrees out of phase, they 'destructively interfere' and the result is zero amplitude. So in this particular context, 360 degrees is 'in phase' and anything else is 'out of phase'.

However for the purpose of generating a rotating magnetic field, 'maximally out of phase' happens with a delay of _90_ degrees. To get rotation from a pair (or more) of sinusoids, you need some sense of orthogonality, of one wave going up while the other wave is not moving at all. If you were to take the two signals and plot the Lissajous curve ( http://en.wikipedia.org/wiki/Lissajous_curve ) then at a delay of 90 degrees you would have a circle.

A 180 degree phase difference has no orthogonality. For any N * 180 degree phase delay, the Lissajous curve is a straight line, and you couldn't use a phase delay of 180 degrees to generate rotation.

Thus my claim that while an inversion gives a 180 degree phase difference, a signal and its inverted value are 'single phase' in the context of things such as polyphase rotating fields.

-Jon

 

[don_resqcapt19]

rattus,

Not really. Don and others claim that since V1 and V2 come from the same secondary, they have to be in phase.

If those two voltages are out of phase, how can it be single phase power??
Don

 

[ronaldrc]

Rattus

You know as good as the rest of us that there is no Phase angle change and nothing is inverted.It is because you are getting both your readings from the neutral.Putting the common on the neutral and measuring from there to Line #1 and then Line #2. they are not 180 degrees out.

You should be using Line #1 as your common and then measure to the neutral for one reading and then to Line #2 for the second reading.

And the only change between the two will be amplitude of the voltage.

The neutral just happens to be a means of using the circuit for a balance circuit this way we can have multiwire circuits and a lower potential grounding point. And the NEC and electrical field uses this to their advantage.

It does not create a phase shift.

 

[cschmid]

This is just to get it so I can read entire thread..lazy mans way..

 

[ronaldrc]

Rattus

You know as good as the rest of us that there is no Phase angle change and nothing is inverted.It is because you are getting both your readings from the neutral.Putting the common on the neutral and measuring from there to Line #1 and then Line #2. they are not 180 degrees out.

You should be using Line #1 as your common and then measure to the neutral for one reading and then to Line #2 for the second reading.

And the only change between the two will be amplitude of the voltage.

The neutral just happens to be a means of using the circuit for a balance circuit this way we can have multiwire circuits and a lower potential grounding point. And the NEC and electrical field uses this to their advantage.

It does not create a phase shift.

Just wanted to add this the Electrical field is complex enough without this
added confusion.

 

[rattus]

Not the same thing:

Not the same thing:

rattus,

If those two voltages are out of phase, how can it be single phase power??
Don

Don,

You are confusing polyphase systems with phase angle shift. In a single phase system the poco uses a single, center tapped, transformer to deliver a single phase voltage. No question about that.

However, with the CT, I can define 3 voltages--L1/L2, L1/N, L2/N. Now I can measure these 3 voltages in 6 different ways just by swapping leads. Each time I swap leads, I see a 180 degree phase shift from my previous reading. That is all we are talking about, swapping leads.

 

[rattus]

Come on Ronald:

Come on Ronald:

Rattus

You know as good as the rest of us that there is no Phase angle change and nothing is inverted.It is because you are getting both your readings from the neutral.Putting the common on the neutral and measuring from there to Line #1 and then Line #2. they are not 180 degrees out.

You should be using Line #1 as your common and then measure to the neutral for one reading and then to Line #2 for the second reading.

And the only change between the two will be amplitude of the voltage.

The neutral just happens to be a means of using the circuit for a balance circuit this way we can have multiwire circuits and a lower potential grounding point. And the NEC and electrical field uses this to their advantage.

It does not create a phase shift.

Just wanted to add this the Electrical field is complex enough without this
added confusion.

Ronald,

Have I ever steered you wrong? You can measure anyway you wish, but if you swap leads you will see the 180 degree difference from your previous reading.

 

[ronaldrc]

Rattus

I am not trying to take nothing from you as far as your knowledge in
this field, myself and most look up to you.

But I think this is making something fairly simple in our field sound a lot more complex than what it is.

Just my feeling

 

[don_resqcapt19]

However, with the CT, I can define 3 voltages--L1/L2, L1/N, L2/N. Now I can measure these 3 voltages in 6 different ways just by swapping leads. Each time I swap leads, I see a 180 degree phase shift from my previous reading. That is all we are talking about, swapping leads.

Which has nothing to do with anything other than the fact that you have changed the reference point.
Don

 

[winnie]

Which has nothing to do with anything other than the fact that you have changed the reference point.
Don

Thus showing that changing the reference point can show a phase difference of 180 degrees, without in any way making a single phase system a polyphase system.

An inversion _cannot_ be distinguished from a 180 degree phase shift for a single frequency sine function, and if (for whatever reason) you need a 180 degree phase shift, you could get that by an inversion or an actual time delay, and the results would be interchangeable.

-Jon

 

[ronaldrc]

I would like to point out one more thing we are talking about the different polarities here.

From my understanding of what I have learned or was taught,being of opposite polarity is not the same thing as being out of phase.

And as far as phase angle I thought that was the starting time of another source of electrical generation in a 360 degree cycle.

 

[mull982]

Rattus,

There is a very fundamental difference between single and three phase systems. The three phase systems have a time displacement between phases. Ergo, the word "phase." Single phase systems only have a "single phase."

So when looking at a plot of three phase you will see three vectors all displaced by a given angle and looking a plot of single phase you will only see a single vector having magnitude and direction?

 

[jcormack]

Power

Power

As I stated in another thread...............

"I am not going to argue that the voltage sine wave measured from L1 to N and L2 to N are not 180 degrees apart when looking at amplitudes (zero crossings are the same), I was simply pointing out that the Current Sine wave is also a mirror. Thus the resulting power sine wave (V*I, I*I*R, whatever...) is the same sine wave (time, amplitude, direction) on Either L1 120 or L2 120, (for same load) and that the these match the phasing (time) of a 240 load off the same transformer. In a 3 Phase system, three single phase loads would have power waveforms that are shifted from each other in time.

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........."

You can't neglect power.....that is what moves the motor, lights the lights,etc. you have to use both voltage AND current to allow energy usage.

The Power in a 240/120 single phase system is in one (1) time domain (phase)