ECN Forum
Posted By: mustangelectric Single Phase X-fmr Feed - 11/01/04 09:20 PM
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).]
Posted By: sparky66wv Re: Single Phase X-fmr Feed - 11/01/04 09:42 PM
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!

[Linked Image]

[This message has been edited by sparky66wv (edited 11-01-2004).]
Posted By: SolarPowered Re: Single Phase X-fmr Feed - 11/01/04 10:58 PM
However, please wear safety glasses when spinning the batteries. [Linked Image]

[This message has been edited by SolarPowered (edited 11-01-2004).]
Posted By: mustangelectric Re: Single Phase X-fmr Feed - 11/02/04 12:28 AM
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
Posted By: Dave55 Re: Single Phase X-fmr Feed - 11/02/04 01:43 AM
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
Posted By: Dave55 Re: Single Phase X-fmr Feed - 11/02/04 01:44 AM
And cycles 60 times per second...(Hz).
50 Hz. in other parts of the world.

Dave
Posted By: electricman2 Re: Single Phase X-fmr Feed - 11/02/04 01:48 AM
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.
Posted By: winnie Re: Single Phase X-fmr Feed - 11/02/04 02:09 AM
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!)
Posted By: Dave55 Re: Single Phase X-fmr Feed - 11/02/04 02:15 AM
Awesome, winnie. Do I hear applause?

Thank you.

Dave
Posted By: mustangelectric Re: Single Phase X-fmr Feed - 11/02/04 02:32 AM
Hi,
Thanks very much for the excellent replies.

I think I understand it now.

-regards

Mustang
Posted By: JBD Re: Single Phase X-fmr Feed - 11/02/04 02:39 PM
Some confusion about single phase is caused by thinking that the term "phase" means number of wires (in particular hot wires). For any voltage to exist you need two measurement points not just one, therefore you need a single pair of (two) wires to feed the primary of a single phase transformer.

Instead of worrying about phase angles, vector additon, and such, think of "pairs of line/hot conductors".
Posted By: earlydean Re: Single Phase X-fmr Feed - 11/02/04 04:47 PM
I teach that each coil of a transformer's secondary produces a small voltage. Every coil in a transformer has the same small voltage. Each one of these small voltages can be thought of as a battery cell conected in series; they add up to the total voltage. If we tap different groups of these coils we get a voltage somewhere between just above zero and full voltage. By center-tapping the entire secondary coil we get 2 equal voltages, phase A and phase B.
It is possible to get a number of different voltages from a number of taps on the secondary of a transformer. Think Buck and Boost, or think of the old electronic power supply transformers that would supply several different voltages for several different needs in the old vacuum tube TVs.
Posted By: pauluk Re: Single Phase X-fmr Feed - 11/02/04 06:33 PM
Tapped transformer windings and how they are referenced to ground seems to cause a lot of confusion.

I sometimes try to the use the battery arrangement mentioned above to start the explanation, as many people seem to be able to visualize this more easily with the simple one-way flow of direct current.

Take those two 1.5-volt cells and wire them in series. You now have 3 volts between the outer ends, and 1.5 volts between either end and the center tap.

Nothing you do with a ground connection will change that, but what it does do is affect the voltage between each of those three legs and ground. For the following, let's call the free negative end X, the free positive end Y, and the center tap Z.

If you took a ground rod and connected it to X, then you have referenced the negative end of the battery to ground. Point X will obviously then be at zero volts, while Y will now be at +3 volts with respect to ground. Because Z is the center-tap between the two cells, it will be +1.5V with respect to ground.

Now take the ground connection off of X and move it to Y. The positive end of the battery is now at earth potential, so X will be at -3 volts and Z will be at -1.5V with respect to ground.

Note that in in both cases we still have 1.5V between X and Z, 1.5V between Y and Z, and 3V between X and Y. All that has changed is which point is the ground reference.

Now let's move the ground connection to Z, the center-tap. X-to-Z is still 1.5V, Y-to-Z is also still 1.5V, and X-to-Y remains at 3V. This time, however, because the center-tap is grounded, X is at -1.5V with respect to earth while Y is at +1.5V.

Now move from two simple cells to the two halves of transformer winding, and you get a similar arrangement, only this time we have alternating current so the polarities are constantly changing. With the center-tap grounded, the voltages on each end are 180-degrees out of phase, so when one hot leg is positive (with respect to ground) the other is negative, and vice versa (as Virgil put it "Spin the batteries around" [Linked Image]). The current is changing direction every 120th of a second, but the polarities at the opposite ends of the winding will always be opposite. That's why you get 120V from hot to neutral/ground, and 240V between hot legs.

If you took that transformer secondary and grounded one end instead of the center tap, then you would have the hot leg running from the center tap at 120V and the other hot leg at a full 240V to ground. But because the ground refernce is now at one end of the winding, the voltages on those hot legs will be in phase, so there would be only 120V between them
Posted By: Scott35 Re: Single Phase X-fmr Feed - 11/05/04 05:30 AM
Mustang;

Kind of a late response here, but have you checked out the 1Ø Transformer Schematics / Information pages at the Technical Reference Area?

Here are links to those pages:

1 Phase Trans. Schematics-Part 1

and

1 Phase Trans. Schematics-Part 2

Here are a few Schematics for basic references:

[Linked Image]

[Linked Image]

[Linked Image]

[Linked Image]

This information may be of further / additional assistance.

Scott35
Posted By: mustangelectric Re: Single Phase X-fmr Feed - 11/05/04 12:01 PM
Hi,
Thanks for the drawings. That helps a lot.

-regards

Mustang
© ECN Electrical Forums