How does a typical residential utility transformer work?

[BretHeilig]

Does anyone know of a thorough, reliable article that explains this? I understand how transformers work in general, but I don't quite get how a single high-voltage wire goes in, and two low-voltage wires, 180 degrees out of phase with a neutral center tap, come out (on top of that, I'm sure there are plenty of different configurations out there - the one I describe is how I believe it works in Hawaii). Thanks very much for any help.

 

[stickboy1375]

I don't have all the theory behind it, but I love pictures...

 

[augie47]

how

how

stickboy's diagram should show you. note that along with that 1 high voltage wire, there is another attached to the primary....it may be a grounded conductor, but there is a second conductor.

 

[don_resqcapt19]

but I don't quite get how a single high-voltage wire goes in

There will always be two conductors supplying the primary side of the transformer. In many cases there will be a single "high voltage" conductor and a grounded conductor. The grounded conductor is common with the grounded conductor, so it may appear that there is a only a single conductor supplying the primary.
Don

 

[BretHeilig]

Hey guys, thanks very much. The grounded conductor, that's a big piece of the puzzle there. I see I must have missed an article on Howstuffworks, I'll check that out again. I need to understand the theory because I have to teach a class on home wiring basics in a couple of weeks, and I want to be able to field the inevitable questions about why this all works the way it does.

 

[rattus]

Very Basic Theory:

Very Basic Theory:

Does anyone know of a thorough, reliable article that explains this? I understand how transformers work in general, but I don't quite get how a single high-voltage wire goes in, and two low-voltage wires, 180 degrees out of phase with a neutral center tap, come out (on top of that, I'm sure there are plenty of different configurations out there - the one I describe is how I believe it works in Hawaii). Thanks very much for any help.

It is like this in an IDEAL transformer.

A magnetizing current flows in the primary winding, and this current is limited by the primary inductance--more turns, more inductance.

This current causes a magnetic flux in the transformer core, and this flux, like the primary current is constantly changing.

This changing flux intersects both the primary and secondary coils, and this induces a voltage in each coil. These voltages are proportional to the number of turns in each coil. In the primary the voltage is called the "back emf" and is equal and opposite to the primary voltage. Then the secondary EMF is computed as,

Vs = Vp x Ns/Np

 

[jim dungar]

Does anyone know of a thorough, reliable article that explains this? I understand how transformers work in general, but I don't quite get how a single high-voltage wire goes in, and two low-voltage wires, 180 degrees out of phase with a neutral center tap, come out (on top of that, I'm sure there are plenty of different configurations out there - the one I describe is how I believe it works in Hawaii). Thanks very much for any help.

The legs are not actually 180 degrees apart. They appear to be 180 degrees apart only when measured using the neutral as a reference point.

If you look at the diagram, posted by stickboy, you will see a single winding on the primary which creates a single magnetic flux which in turn creates a single output of 240V when measured between the two outside legs of the secondary. Adding the center tapped 120V neutral point does not magically create a different "phase".

 

[rattus]

Here we go again:

Here we go again:

The legs are not actually 180 degrees apart. They appear to be 180 degrees apart only when measured using the neutral as a reference point.

If you look at the diagram, posted by stickboy, you will see a single winding on the primary which creates a single magnetic flux which in turn creates a single output of 240V when measured between the two outside legs of the secondary. Adding the center tapped 120V neutral point does not magically create a different "phase".

Jim, since L1 and L2 are measured relative to the neutral, V1 and V2 are indeed 180 degrees apart. However, as you imply, this does not make this a 2-phase system.

It is a matter of where you place your test leads. That is,

V1n and V2n are out of phase.
V1n and Vn2 are in phase.

 

[eric stromberg]

The legs are not actually 180 degrees apart. They appear to be 180 degrees apart only when measured using the neutral as a reference point.

If you look at the diagram, posted by stickboy, you will see a single winding on the primary which creates a single magnetic flux which in turn creates a single output of 240V when measured between the two outside legs of the secondary. Adding the center tapped 120V neutral point does not magically create a different "phase".

Thank you very much Jim. When i get a graphics program i'm going to show this. The phases are, in fact, perfectly in phase with each other. How else could one wire a dual secondary transformer, with the windings in parallel with each other?

I like to explain the turns of a transformer winding like batteries in a flashlight. If there are 4 batteries in the flashlight, and you decided to pick the middle of the batteries as the "Zero" reference, then, going one direction the voltage would be positive at the end; going the other direction, the voltage would be negative at the end. In reality, the batteries are all pointing the same direction.

 

[jim dungar]

Jim, since L1 and L2 are measured relative to the neutral

Rattus, this is true only IF you use the neutral as the reference. While this is normal practice it is not a requirement. And it does not change the physics of a single core transformer.

It is a matter of where you place your test leads. That is,

V1n and V2n are out of phase.
V1n and Vn2 are in phase.

This is one of those facts that is often mis-used causing easily accepted myths. There are very few reasons to use this fact except to simply the reason that the two 120V L-N currents add to zero, but this then creates the problem of explaining how the two 120V L-N voltages add to 240V.

 

[rattus]

Rattus, this is true only IF you use the neutral as the reference. While this is normal practice it is not a requirement. And it does not change the physics of a single core transformer.

This is one of those facts that is often mis-used causing easily accepted myths. There are very few reasons to use this fact except to simply the reason that the two 120V L-N currents add to zero, but this then creates the problem of explaining how the two 120V L-N voltages add to 240V.

Jim, We are using the neutral as a reference. By defining V1 and V2 as the voltages on L1 and L2, we imply a common reference point--the neutral! This is no different in principle from defining wye voltages relative to a neutral. In either case you SUBTRACT vectorially one voltage from another to get the L-L voltage.

To remove any doubt though, we can insert the "n" in the subscripts. Then you must agree though that V1n and V2n exhibit a 180 degree phase difference?

I see no reason not to use the neutral as a reference.

 

[LawnGuyLandSparky]

What boggles my mind is how a 7,200 volt wire wrapped around something and then bonded to ground doesn't short out.

 

[hillbilly]

What boggles my mind is how a 7,200 volt wire wrapped around something and then bonded to ground doesn't short out.

It really is shorted out, but (I think) that the resistance (impedance?) of the winding limits the current to less than the ampacity of the conductor.

If that's not correct, I'm sure that we both will find out shortly.

steve

 

[eric stromberg]

Then you must agree though that V1n and V2n exhibit a 180 degree phase difference?

I see no reason not to use the neutral as a reference.

They exhibit a "difference," for sure. But it is not a 'phase' difference because they are not out of phase. They are in phase with each other, ergo "single phase." The winding is a single 240 Volt winding. It cannot be 'out-of-phase' with itself. A see-saw is made of one piece of wood. Hinged in the middle, one side goes up while the other goes down. Is the wood "out-of-phase" with itself? No, it is simply a piece of wood. Do the two ends, when viewed independently appear to be 'out-of-phase'? Maybe they appear that way, but it is still one piece of wood.

 

[jim dungar]

Jim, We are using the neutral as a reference. By defining V1 and V2 as the voltages on L1 and L2, we imply a common reference point--the neutral! This is no different in principle from defining wye voltages relative to a neutral. In either case you SUBTRACT vectorially one voltage from another to get the L-L voltage.

To remove any doubt though, we can insert the "n" in the subscripts. Then you must agree though that V1n and V2n exhibit a 180 degree phase difference?

I see no reason not to use the neutral as a reference.

I am nay-saying the practice of saying that because of the 180? phase difference I1n +I2n = 0 and then saying V1n + V2n = 240V is confusing to many people.

Using the neutral as the only point of reference makes it difficult to explain the current divisions in a multi-wire circuit consisting of both unbalanced 120V loads and 240V loads.

 

[kingpb]

Ohh-noooooooooooooo.....here we go again! :grin:

 

[rattus]

I am nay-saying the practice of saying that because of the 180? phase difference I1n +I2n = 0 and then saying V1n + V2n = 240V is confusing to many people.

Using the neutral as the only point of reference makes it difficult to explain the current divisions in a multi-wire circuit consisting of both unbalanced 120V loads and 240V loads.

But Jim,

V1n + V2n = 0--not 240V
V1n + Vn2 = 240V
V1n - Vn2 = 0
V1n - V2n = 240V

It is important that the sigificance of these subscripts be understood, then there should be no confusion. As I have said, this is no difference in principle from a 3-ph wye. So, why do the 1-ph system differently?

Of course, in explaining this to newbies, you can define the voltages any way you wish. Just keep the subscripts straight.

 

[LawnGuyLandSparky]

I think it's time for another posting of the dis-kombobulaitor link!

 

[eric stromberg]

Rattus,
Think of two cars that start from a stop at the same time. The first one is accelerating faster than the second one. They are both moving forward, they are both accelerating. But, if you are sitting in the faster car looking back at the other one, you will see that the car behind you is dropping further behind. Now, for some reason, if you consider yourself to be travelling at "zero" miles per hour (after all, you are 'sitting,' right?) the car that is behind you actually appears to be travelling backwards.

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."

Mathematical subscripts are simply a means to describe something in the real world. The real world is not governed by the math, it is the other way around.

I think we are all really saying the same thing, but we have different contexts. What is actually happening inside the transformer is one context. What you see on a scope with the leads hooked up in a certain configuration is something entirely different.

 

[rattus]

Rattus,
Think of two cars that start from a stop at the same time. The first one is accelerating faster than the second one. They are both moving forward, they are both accelerating. But, if you are sitting in the faster car looking back at the other one, you will see that the car behind you is dropping further behind. Now, for some reason, if you consider yourself to be travelling at "zero" miles per hour (after all, you are 'sitting,' right?) the car that is behind you actually appears to be travelling backwards.

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."

Mathematical subscripts are simply a means to describe something in the real world. The real world is not governed by the math, it is the other way around.

I think we are all really saying the same thing, but we have different contexts. What is actually happening inside the transformer is one context. What you see on a scope with the leads hooked up in a certain configuration is something entirely different.

Eric, I know all that stuff. I am saying though that the voltages on L1 and L2 in a single phase system are defined as they are in a 3-ph wye--relative to the neutral. For example, we could say V1n = 120V @ 0 and V2n = 120V @ 180. We could then plot these two waveforms on a common axis and point out that the difference between them is the L-L voltage.

I am arguing with Jim because he said that V1n and V2n were in phase when in fact they are inverses of each other.