Transformer center tap question elaborated?

Question

Many thanks to those who responded earlier but those answers were not quite addressing what I was looking for.

I am thinking of the case of a typical pole mounted single phase distribution xmfr with loads on both sides of the center tap. If the secondary of the transformer is center tapped and the other two leads from the secondary are going from positive to negative with the AC current, that means that the center tap is either more negative or more positive than either outside lead at any given time.

If at a given instant the top lead is positive, then the center tap is more negative than the top lead. The current should flow from the top lead to the center tap. However, isn't the center tap more positive than the bottom lead at that same instant? Wouldn't the current therefor flow out of it into the bottom lead? (assuming loads connected to both sides of the tap) Flowing inward from the top lead and outward to the bottom lead at the same time. How does that work?

Answer 1

In the real world, the center tap is tied to a ground stake at the bottom of the pole and becomes the neutral (white) wire that ties to the neutral bus bar inside the circuit breaker box. There is 240VAC across the top and bottom terminals. There are two sources for 120 VAC which are 180 degrees out of phase: one between the top terminal and neutral (center tap), the second between the bottom terminal and neutral (center tap).

In the special case where the two 120VAC circuits carry exactly the same load, no current flows in the neutral wire. The two equal loads are effectively in series across 240VAC. The power you purchase is the sum (not difference) of the two currents times 120VAC. This is, of course, arithmetically the same as the current in one of the legs times 240VAC.

In the case where the two 120VAC legs are NOT equally loaded, then an unmetered current equal to the difference between the currents in the two (upper and lower) legs flows through the neutral wire back to the center tap. You must pay for the sum of the upper and lower currents times 120VAC.

In the special case where only one 120VAC leg has a load, then the current in that leg and the current in the unmetered neutral wire are exactly equal. Again, you pay for the sum of the currents in both the upper and lower legs (one of which is zero) times 120VAC. While the neutral (center tap) is grounded at the base of the power pole, the green safety ground carries no current (unless there's a fault) and has its own ground stake where the power enters the building.

I didn't have a chance to read the first question, so I just shot-gunned the second question with answers. Hopefully, one of them hit your target.................

Answer 2

You are forgetting that the phases of the two outside taps are 180 degrees out of phase - while they cross zero at the same time, when one is at the positive peak, the other is at the negative peak.
To further confuse the issue, remember that in modern wiring there must be a neutral AND a safety ground and the neutral must be grounded at only one point - on the panel.
Good overview.
http://www.allaboutcircuits.com/vol_2/chpt_10/1.html

Answer 3

the center tap is at zero. zero is less than any positive voltage, and greater than any negative voltage. this is why the center tap can be grounded for 2-phase home home service. the top tap is used for one 120 volt leg, the bottom tap for a second 120 volt leg, both referenced to the CT or ground. to get 240 volts, you connect to both top and bottom taps.

Answer 4

You're absolutely right in your assumption. The center tap is just another current carrying wire. The total current is the difference between the two currents from the outer leads.

Answer 5

once you're speaking approximately window portion of the middle, the respond is none, by using fact the node can easily be located outdoors of the transformer window, and commonly is. in many circumstances, a center faucet is largely a loop of the winding cord extra out to a terminal. Re different solutions, the middle faucet isn't unavoidably 0 volts or impartial in any respect. it truly is, or the two end could be, it is your decision.