Electronic – How does a center tapped transformer work

groundtransformervoltage

I'm having trouble understanding the electric principles that allow a center tapped transformer to work. Lets say we're working with 240/120V. I understand if you connect a voltmeter across the top and bottom wires, you'll get 240V, and 120V between the center tap and other two wires. This is where I'm having trouble. The center tap is essentially a grounded neutral wire if my understanding is correct. 

--------------A
~
~
~
--------------G
~
~
~
--------------B

Here is my secondary transformer. So i measure between A and G and get 120V because A is 120V more than G which is 0. So now when i measure between G and B i still get 120V telling me G is 120V more than B, or B is 120V less than G. This is the part that confuses me. How can my center tapped wire G which is grounded be at a higher voltage than B when it is supposed to be 0? Thinking about it the other way still confuses me as well; how can B be at a 120V less than G, what would that mean? Can someone explain the theory behind why/how this works?

Best Answer

With DC it's obvious.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Two 9 V batteries series connected but with various grounding options.

How can B be at a 120V less than G, what would that mean? Can someone explain the theory behind why/how this works?

You are forgetting that your meter only measures magnitude of an AC voltage. There is no sign so saying that B is less than or greater than ground doesn't make much sense.

schematic

simulate this circuit

Figure 2. For AC we have the same situation as with DC but with no sign - just the voltage magnitudes.

When working with AC, therefore, referencing to ground doesn't tell you what the voltage is between two non-grounded points. Let's look at a high-leg delta transformer arrangement which should be familiar to you in North America (but always looks so unbalanced to us Europeans).

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Figure 2. This transformer arrangement gives the possibility of obtaining single phases at 120 V, 240 V or 208 V plus the possibility of 3-phase 240 V. Image source: Wikimedia Commons.

If you stick one probe of your meter into the neutral (earth) terminal and measure around the circuit with the other you will get:

L1 - N:   120 V
L2 - N:   120 V
L3 - N:   208 V

These are all correct but without the knowledge of the transformer configuration there is nothing to tell you that these have three different phase angles. If we then take some more readings we get a better picture:

L1 - L2:  240 V
L2 - L3:  240 V
L3 - L1:  240 V

With three points and three same voltage measurements between any two of them the only solution is an equilateral triangle - a three-phase supply. Note again that all the voltages are "positive" as we are only reading magnitude.

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