ECN Forum Forums General Topics of the Trade General Discussion Area Single Phase X-fmr Feed

Hi,
Can anyone give a decent explanation as to why a single phase 120/240 vac transformer can supply two legs from a single pahse distribution line?

I know it has something to do with phase angle and the generator..

I appreciate any replies.

-regards

Mustang

[This message has been edited by mustangelectric (edited 11-01-2004).]

It's all about the center tapped grounded conductor.

Secondary side:

120V----Neutral----120V
^-------240V-------^

Grounded in the center.

And on the primary side:

^-------4160V-------^
|
Grounded on one side.

Make sense?

Or:

Take two flashlight batteries with taps on each end and between the two.

Opposite ends gives 3.0VDC, but any end to the center gives 1.5VDC.

Now spin the batteries at 3600 RPM!



[This message has been edited by sparky66wv (edited 11-01-2004).]

However, please wear safety glasses when spinning the batteries.

[This message has been edited by SolarPowered (edited 11-01-2004).]

Hi,
Thanks for the replies. I always thought it was obtained due to the phase angle. I do not see how you can get two legs from a single phase 4160 or 7500V line by tapping the midpoint of the x-fmr... this is where the neutral is obtained is it not?

A phase of the distribution line has only one phase but it cycles through three different phase angles..correct?

A-B-C....90-180-360..

This cycles every second correct?

I am not too clear on your descriptions. I am sure your right but I cant see how you can get that by grounding the midpoint..

-regards

Mustang

I'm probably the worst one around to try to explain this, but it's three sine waves that start at different times. I'm sure a moderator can find a picture if it's not in the technical section. And my guess would be 0, 120, 240, 360(0) repeating with each rotation.

Dave

And cycles 60 times per second...(Hz).
50 Hz. in other parts of the world.

Dave

Dont know if this will help, but think of the transformer as two windings in series with the neutral conductor connected to one end of each winding. Each winding is 120V. From each of the other ends to neutral is 120V. The two windings in series will be 240V. Dont have any way to post a diagram.

Start by considering a step down transformer that produces 120V. The secondary has two terminals, and 120V AC measured between these two terminals.

Now set up a second transformer, with its primary in parallel with the first and its secondary in series with the first. The secondary of the second has 120V between its terminals, but there is now 240V between the first terminal of the first transformer and the second terminal of the second transformer.

You could add yet another transformer if you wish (still same phase), and add yet another 120V, so that between any of the secondaries on a particular transformer you would have 120V, but you would have 360V at the 'ends' of your string of transformers.

A center tapped secondary is essentially two transformers with parallel primaries but secondaries in series.

-Jon

P.S. I started writing a long essay, I've included what I wrote below, but I think the above makes the essential point.

Okay, lets go in sequence:

1) Voltage. Voltage is a measure of the work needed to move a charge (say a single electron) from one point to another. The greater the voltage, the greater the work needed to 'push the charge up-hill', or conversely, the greater the work that can be obtained by letting the charge 'fall down hill'. Voltage is _always_ measured between two points, like height. It is meaningless to say 'that mountain is 1000 feet high', you need to say 'that mountain is 1000 feet above the surrounding terrain', or 'that mountain is 1000 feet above sea level'. Likewise with voltage; it is always 'that electrode is 100V positive relative to that other electrode'.

2) Alternating current. Alternating current is a system where, given two conductors supplying a load, if you measure the voltage between the conductors you will find that it is continuously and periodically varying, starting at 0V, going to positive maximum, then dropping back to 0V, then negative maximum, and then back to 0 again. This variation is happening continuously and (hopefully) smoothly. If you were to draw a graph of voltage versus time, it should be a sine wave.

4) More than two wires. With three or more conductors, you have several different voltages that you can measure. Since voltage always is measured between pairs of points, you can have as many different voltages as there are pairs of wires. For example in a three wire system, there are three different voltages that you can measure A to B, B to C, and A to C.

4) Polyphase alternating current. A system of multiple conductor power delivery where if you measure the voltage between any pair of wires you have alternating current, where if you were to graph voltage versus time you would get a sine curve, and where the sine curves for symmetric pairs of conductors are equally displaced in time. So, for example, in a three phase three wire system, with three possible voltages, all of the sine curves have the same amplitude and frequency, but are displaced from each other by 1/3 of a cycle (ideally!)

Awesome, winnie. Do I hear applause?

Thank you.

Dave

Hi,
Thanks very much for the excellent replies.

I think I understand it now.

-regards

Mustang