Why does electricity behave the way it does in a center tapped, split phase transformer?

Thread Starter

ARutledge

Joined Jul 21, 2022
6
Hi, I've been having trouble fully grasping how electrical current behaves on a center tapped, split phase transformer.

1658418360984.png

What I know is, as shown in the image above, the secondary winding inside a transformer is a single wire coiled at the center, with the the two ends sent out to the service panel for a building. There is also another wire that is grounded and 'tapped' to the center of the secondary winding. We call this the 'Neutral' wire, and it is the zero reference point for the 2 lines.
In the USA, the potential difference between 'Line 1' and 'Line 2' is 240v (When one line is pushing 120v+ referenced to ground the other line is pulling 120v- referenced to ground). The potential difference between either line and the neutral is 120v (Either line is pushing or pulling 120v referenced to the grounded neutral "0v")

- If the neutral wire is considered to be 0v because it is connected to ground, then why isn't the entire wire at 0v because the coil is directly connected to ground and neutral? What makes the lines 'Hot' and the neutral 'Not' when they are all connected together and grounded on the same side of a load?

- If the neutral wire wasn't grounded, would that make it a 60v hot wire? Like in the 240v circuit, would ungrounding the neutral create a 120v split phase? (60v+ on one side and 60v- on the other?)
 

MaxHeadRoom

Joined Jul 18, 2013
25,630
The fact the CT is connected to earth GND is inconsequential as to how the circuit functions, the GND is there purely as a safety reference procedure.
There would be/is no difference to the circuit performance without the GND reference.
This means that if each 120v side were feeding a 120v outlet, each with a 15a load, and each circuit shared the same neutral return conductor, there would exist zero current in the neutral.
 
Last edited:

BobTPH

Joined Jun 5, 2013
5,467
Th neutral wire is 0V because we define it to be. Voltages are not absolute, they are relative, so, when stating a voltage we have to have a reference point.

The connection to earth ground has nothing to do with it. If disconnected, we can still call it 0V.

I don’t know where you are getting 60V. There is no connection that is half way between the neutral and either “hot”.
 

crutschow

Joined Mar 14, 2008
30,438
If the neutral wire is considered to be 0v because it is connected to ground, then why isn't the entire wire at 0v because the coil is directly connected to ground and neutral? What makes the lines 'Hot' and the neutral 'Not' when they are all connected together and grounded on the same side of a load?
All voltages are referenced to some point.
There is no such thing as an isolated single voltage.

But I'm not sure I understand you question.

The neutral wire is connected to "ground" for safety purposes.
Otherwise it has no effect on the relative voltages between the windings, and no effect on the load voltage.
If you measure the voltage between one of the "hot" wires and "neutral" the voltage is the same no matter what neutral is connected to.

The "Hot" line is called hot because it is not at ground potential as the neutral wire is.
If you grounded one of the hot wires, than the neutral would become "hot".
If the neutral wire wasn't grounded, would that make it a 60v hot wire? Like in the 240v circuit, would ungrounding the neutral create a 120v split phase? (60v+ on one side and 60v- on the other?)
60V relative to what?
If no wire is grounded, than the voltage to earth is undefined.
 

Thread Starter

ARutledge

Joined Jul 21, 2022
6
Th neutral wire is 0V because we define it to be. Voltages are not absolute, they are relative, so, when stating a voltage we have to have a reference point.

The connection to earth ground has nothing to do with it. If disconnected, we can still call it 0V.

I don’t know where you are getting 60V. There is no connection that is half way between the neutral and either “hot”.
So let me try to rephrase. Let’s say I have a washing machine connected between the Line 1 and Line 2 for 240v. If I touch either line and my body is grounded, I would get a 120v shock. If I held both hot wires in either hand I would get a 240v shock.

Now say I have a receptacle connected between Line 1 and Neutral. If I touch Line 1, and my body is grounded, I would get a 120v shock. If I held Line 1 and Neutral in either hand I would get a 120v shock. If I touched the Neutral I get no shock (in an ideal circuit).

Why is that? If the neutral wasn’t grounded wouldn’t I get a 60v shock from touching it? (Half of the 120v circuit like touching either line in the washing machine example?)

Or am I way off?
 

Thread Starter

ARutledge

Joined Jul 21, 2022
6
The "Hot" line is called hot because it is not at ground potential as the neutral wire is.
If you grounded one of the hot wires, than the neutral would become "hot".
60V relative to what?
If no wire is grounded, than the voltage to earth is undefined.
If the neutral is center tapped to the winding of the hot wire at the transformer, then how is it considered 0v and the hots considered 120v if it’s all connected together? And if it’s because grounding the neutral puts it at ground potential, then why aren’t the lines also at ground potential, since it’s all connected together?
 

BobTPH

Joined Jun 5, 2013
5,467
If none the 3 wires is grounded, but you are grounded, there is no path from any of the wires to ground, so you can touch any one of them without having a current flow through you.

You say all three wires are connected together. Where did you get that idea? If all three were connected together, there would be no electrical power possible between them!
 

MaxHeadRoom

Joined Jul 18, 2013
25,630
Imagine just a single ungrounded secondary 120v , we would measure 120v between each conductor regardless which we decided was 0v. In reality the voltage 'alternates' between the two. We just call it 'zero' as handy reference point.
In the 240v CT version, the CT is considered zero because each of the 120v conductors are 180° out of phase and cancel at that point.
 

crutschow

Joined Mar 14, 2008
30,438
If the neutral wasn’t grounded wouldn’t I get a 60v shock from touching it?
No.
If the transformer output is not grounded (left floating) then you would get no shock, since there is no path for the current to ground.
f the neutral is center tapped to the winding of the hot wire at the transformer, then how is it considered 0v and the hots considered 120v if it’s all connected together? And if it’s because grounding the neutral puts it at ground potential, then why aren’t the lines also at ground potential, since it’s all connected together?
Don't understand what you mean by "all connected together".
Neutral is connected to ground, but the other two transformer wires are not, as they are separated by the winding which generates the voltage.

That's like saying if one end of a battery is grounded, then the other side of the battery is also grounded (which it isn't, of course).
 

vu2nan

Joined Sep 11, 2014
262
1. The turns ratio of the split-phase transformer is such that, at the rated primary voltage, the secondary voltage is 240 V.

2. The secondary winding has a centre tap. With respect to the centre tap, the voltage at either end of the secondary winding is 120 V but out of phase by 180°. The wires connected to the end terminals of the secondary are called the 'lines' and designated as 'L1' and 'L2'.

3. The earthed wire connected to the secondary centre tap is called the 'neutral' and designated as 'N'.

4. The 3 wires L1, L2 and N constitute a 120V split phase system.

5. Without the neutral, the 2 wires L1 and L2 constitute a 240 V single phase system.

6. The potential difference between the neutral and earth is zero. Hence personnel, in contact with the earth and coming into contact with the neutral, will not experience an electric shock. However personnel, in contact with the earth and coming into contact with the lines, will experience a 120 V electric shock.
 

Thread Starter

ARutledge

Joined Jul 21, 2022
6
Don't understand what you mean by "all connected together".
Neutral is connected to ground, but the other two transformer wires are not, as they are separated by the winding which generates the voltage.
Because the neutral is tapped to the center of the winding. That's what I meant by "all connected together". If there is voltage on the coil at the transformer, and the neutral is connected to the center of that coil, why is there no voltage on that wire also? If we choose earth ground to be 0v reference, and neutral is at ground potential because it is connected to earth, why isn't the coil also at ground potential since neutral is tapped to it?

I can get shocked by either wire in a 240v circuit to a washing machine. The wires have a potential difference of 240v to each other. If I choose earth ground to be my relative 0v reference, then one wire is 120v+ the other is 120v- with respect to ground. Now if I look at Line 1 and Neutral, and the neutral wasn't grounded, I see the exact same kind of circuit as the washing machine, only now the coil at the transformer is half the total size. So in that circuit, if the wires have a 120v difference from each other wouldn't that mean that if one wire is 60v+ then the other wire is 60v-?

I'm 100% certain that I am wrong in how I'm thinking of this. I'm really trying to explain my thought process well enough for someone to show me where I'm misunderstanding.
 

MaxHeadRoom

Joined Jul 18, 2013
25,630
As I mentioned, the centre tap we just refer to as 0v, but this is just a reference point term, the 120v voltage alternates between the two and as a pair they are identical. and each produce 120v WRT to, or with ref to each other.
 

MrChips

Joined Oct 2, 2009
26,790
Read over what others have already posted. I am only repeating the same message using my own words.

There is no voltage at any single point.
Voltage is a measurement between two points, i.e. point-A and point-B.

If the center tap is not connected then you only have one connection point, i.e. point-A. There is no point-B.
There is no measurable voltage.
 

crutschow

Joined Mar 14, 2008
30,438
Because the neutral is tapped to the center of the winding. That's what I meant by "all connected together".
But the neutral is just a tap on the winding.
They may be "all connected together" since it's all one winding, but that doesn't mean the voltage potentials are all the same.
You need to understand that the transformer magnetic field generates a voltage in the winding coil of 240V form end-to-end.
Tapping this coil at the middle just means that now there is 120V from the tap to each end, along with the 240V at the ends.
Don't see why that's hard to understand.
if the wires have a 120v difference from each other wouldn't that mean that if one wire is 60v+ then the other wire is 60v-?
Sorry but I don't understand your math or thought process.
120V is 120V is 120V.
What "wires" are you referring to?
Why is one wire suddenly 60V?
How does 120V change to 60V?
 

BobTPH

Joined Jun 5, 2013
5,467
He is looking more and more like a troll. Ignoring our responses and then posing the same question over and over again.
 

BobTPH

Joined Jun 5, 2013
5,467
So in that circuit, if the wires have a 120v difference from each other wouldn't that mean that if one wire is 60v+ then the other wire is 60v-?
If no part of the circuit is connected to earth, then we cannot state anything about a single voltage. We can state the difference between any two points, but not any voltage with respect to ground, which is what you are insisting on doing. We know that the difference is 120V. But that does not means one wire is at -60 and the other is at +60. It could be one is 0 and the other is 120, or one is at 1260 and the other is at 1380. Since there is no connection to ground, no current can flow between a grounded you and any of the wires.
 

shortbus

Joined Sep 30, 2009
9,525
In the USA, the potential difference between 'Line 1' and 'Line 2' is 240v (When one line is pushing 120v+ referenced to ground the other line is pulling 120v- referenced to ground). The potential difference between either line and the neutral is 120v (Either line is pushing or pulling 120v referenced to the grounded neutral "0v")
If that is how your thinking about AC voltage that is where the problem is. In the case of the transformer you pictured, both wires marked "line 1 and Line2" are outputs, neither one of them pulls anything.
 

Thread Starter

ARutledge

Joined Jul 21, 2022
6
If that is how your thinking about AC voltage that is where the problem is. In the case of the transformer you pictured, both wires marked "line 1 and Line2" are outputs, neither one of them pulls anything.
I guess I’m trying to picture the alternating flow of electrons. I’ve heard it described as a “push pull” behavior many times. Maybe it’s a description I’ve taken too literally. But thank you for showing where I’m messing up, guess I need to rewind and relearn the fundamentals.
 

nsaspook

Joined Aug 27, 2009
10,104
I guess I’m trying to picture the alternating flow of electrons. I’ve heard it described as a “push pull” behavior many times. Maybe it’s a description I’ve taken too literally. But thank you for showing where I’m messing up, guess I need to rewind and relearn the fundamentals.
Electrons again. :( Forget electrons.
 
Top