# How does an auto-transformer work?

1. Apr 25, 2012

### Bararontok

Transformers work by using a primary inductor to convert electrical energy into low frequency radiation which will be absorbed by the secondary inductor that will convert this energy back into electrical energy. The output voltage is varied by changing the number of turns of the secondary inductor. But how does an auto-transformer work? Does it absorb the energy of the radiation it releases so that current can be sent to the load? Does it harness the principle of reverse current, so that when the magnetic field collapses, the generated current is sent to the load?

2. Apr 25, 2012

### Bararontok

If the conductors of a load are connected to both ends of the inductor, it will be as if the load is in parallel with the inductor and it will receive the full voltage from the source. But if one terminal of the load is tapped into a fraction of the full length of the inductor then will the untapped region transfer its energy magnetically to the tapped region? How is this possible? Will the current not just go directly to the load without any voltage reduction?

3. Apr 25, 2012

### gerbi

Damn, I've deleted my message.. :/ in attempt to add a schematic.

anyway.. check this.

4. Apr 25, 2012

### gerbi

Are we discussing real autotransformer ? If yes, then we have third winding which help us to change the relation of primary/secondary turns => change in voltage on output. Regulation winding can be trated as a part of primary or secondary winding - it operates in magnetic field ! This is not changing the principle of autotranformer working.

5. Apr 25, 2012

### Bararontok

And how exactly does the third winding change the voltage output of the auto-transformer?

6. Apr 25, 2012

### gerbi

Third (regulation) winding: wounded (on most occasions) as the outermost looking from the core. Assume it looks like a normal winding (mostly it is). It's tough to explain this completly how it works - sooo long story (TL:DR ;)).

How it works in a simple way ? Imaginge that You add/remove some turns in primary winding. Make it that simple. Explaining it completly in details will take long reading.

EDIT: One more thing I've forgotten: do not call regulation winding a third one. Third winding often refers to a delta winding wounded closer most to the core. Regulation winding is regulation winding.

Last edited: Apr 25, 2012
7. Apr 25, 2012

### Bararontok

It is acceptable to post the explanation even if it is long, just post images and diagrams to help clarify the explanation.

8. Apr 26, 2012

### gerbi

Hmm.. so be it, I have not much time now but ok.

Autotransformer looks like any other transformer for a first look. It's smaller than a transforme of the same rated power. Have no schematic I am allowed to post, and have no time to search for one in net so I'l try to explain it.

Typical power autotransformer works in grid as unit connecting high voltage networks (110/230 kV in example). The closer are the primary and secondary voltages the more you gain compared to a trasnformer of same rated power.

In general, it is construced just like a transformer (core, windings, insulation system, cooling, tank, tap changer and others). Main difference is how the windings are connected.

1. You have tetriary winding, it is wounded mainly as the one closest to the core. Connected as delta winding. It supports the main flux in core with higher harmonics, so the flux in core is much more like sinusoidal. It is not indicated on schematic I've uploaded.
2. Second from the core is the low voltage winding (secondary). It is the lower one (where I=I2-I1 current flows) on picture (a) in one of my previous posts.
3. Third one is the high voltage winding (current I1 flows thru it).
4. The last one is the regulation winding. Regulation winding is connected between end of primary winding and point A1 (it's only one of the possibilities, it can be placed elsewhere. For more details, please look in books). It has coils which can be connected in many configurations by device called tap changer (for more details http://www.reinhausen.com). It can be done under full load. Now, imagine You have 100 turns in primary winding and 20 turns in regulation winding. What are the possibilities of regulation ? For some types of tap changers You can have 100-20 to 100+20 turns (plus all sort of steps between them) because You treat primary and regulation winding connected in series as one winding (Imagine solenoid. It has 120 turns, all connected in series so the current value is the same in all turns. 20 turns can be connected as opposite or in the same direction to this 100). You change the turn ratio, You change the voltage output. I hope that is clear.

The main prinicple of AT working is the same as in transformer, no black magic here. You may ask, how it is that currents I+I1= I2 and flow to the output. I'd say this is done by proper connection of winding end terminals (another long story here).

Sorry, but this is complicated stuff. Discussing or explaining it in details will take long time which I don't have. That's why I'm sending You to books, when You'll rethink this and have some more detailed quetions we can talk.

EDIT: from profile of Your questions I see that You're not intrested in power auto transformers. Note that designs vary, small 'lab' AT's are made different than big power grid ones.

Last edited: Apr 26, 2012
9. Apr 26, 2012

### Staff: Mentor

Such confusion. I wondered where all this was going!
I didn't know a tap-changing transformer could be referred to as an auto-transformer

But thank you, gerbi, for all the trouble you've gone to in describing the tap-changer.

Bararontok, you've got here the answer to a question that you didn't ask!

Last edited by a moderator: Apr 17, 2017
10. Apr 27, 2012

### Bararontok

The thread originator is more interested in knowing how the auto-transformer uses the principle of wireless energy transfer if according to its description on Wikipedia it uses a single inductor with no electrical isolation from the source.

11. Apr 27, 2012

### gerbi

[STRIKE]what kind of auto transformer, post a connection schematic.[/STRIKE]

EDIT: [STRIKE]Is it wounded on toroidal core or polyphase column core ? I don't know what is Your understanding of this at all. At which level I should start explaining ? Total basics ?[/STRIKE]

I'll try one more time.

Below You have schematic how to 'make' autotransformer from trasnformer.

As You can see (a), this device works basing on Faraday's law of electromagnetic induction. Ampereturns of primary winding (A-X) create a flux in core (there is a core on which both windings are wounded). Current and voltage are transfered to secondary circuit totally wireless.

On (b) we can see, that it is possible to connect both windings at points, where their potentials (voltages) are exacly the same at all times (X-x and A1-a). It won't change the current flow or voltage distribution. It still works basing on Faraday's law but we have established a galvanic connection between primary and secondary circuits.

Now (c), if we'll replace primary and secondary windings with one (single inductor wounded on toroidal core in ex.) with the same turn ratio we can see that power is transfered via two ways:
1) Electric conduction - both windings are connected. Since there is voltage difference the current will flow from source to output.
2) Electromagnetic induction. We have electromagntic coupling. Current in primary (A-a1) is opposite to current in secondary (A1-x). Electromagnetic coupling means there is voltage induced and current will flow. Look at the current flow direction. Currents from primary and secondary part of THE SAME INDUCTOR adds in point a or A1 and flow to output.

In small 'lab' AT You can change the turn ratio. Output is connected to moving contact joint that can be replaced changing turn ratio. Turn ratios of windings are:
1) primray N1-N2
2) secondary N2
Voltage change can be calculated from: V1/V2 ~= N1/N2

In big power AT's it's different. We have no moving contact joints but regulation winding treated electromagnetically just as integral part of one of the windings. With use of tap changer we change turn ratio, like I've said few post's above.

Does this need any more explanation ?

Last edited: Apr 27, 2012
12. Apr 27, 2012

### Staff: Mentor

13. Feb 19, 2013

### Bararontok

Because the auto-transformer is a transformer, it has to run on alternating current, and because the inductor is receiving an alternating current, it generates inductive reactance which is the inductor's equivalent to electrical resistance, so tapping into a fraction of the total length of the inductor produces a voltage divider effect where the voltage at the output taps are reduced.

This question has been answered. Thank you for all the information that has been provided.

Last edited: Feb 19, 2013
14. Sep 29, 2013

### Bararontok

I have one last question about auto-transformers, how does the auto-transformer step-up voltage?

Below is a diagram of a single winding auto-transformer in step-up mode:

Could it be because of this:

Because of electromagnetic coupling, the oscillating magnetic field due to the AC current in the primary releases energy in the form of low frequency electromagnetic radiation which induces an oscillating magnetic flux in the core which then induces an alternating current in the secondary, and in the case of a core-less auto-transformer, the low frequency electromagnetic radiation directly induces an AC current into the secondary winding. Because both the metal core and core-less transformer have their secondary windings in series with their primary windings, a series-aiding configuration is made which causes the voltage of the primary winding to add to the voltage of the secondary winding, causing the overall voltage to increase, resulting in a step-up of voltage while the current decreases because of the increased number of series connected turns which increases electrical resistance and inductive reactance, in accordance with the law of power conservation which states that,

P1 = P2

V1*I1 = V2*I2

Last edited: Sep 29, 2013
15. Sep 29, 2013

### Staff: Mentor

A transformer winding has 110v across T turns, now many volts across 2T turns?

16. Sep 29, 2013

### jim hardy

Observe that bottom part of core carries both input current AND output current

N O got it - volts per turn is the flux.

Search on "Variac" - a brand of adjustable step down autotransformers.

17. Sep 29, 2013

### Bararontok

Would the correct answer be 220V?

18. Sep 30, 2013

### Staff: Mentor

Don't try to extend this thinking to a potentiometer, though. The autotransformer has the underlying principles of transformer action to make it work. A potentiometer is just two resistances, so a potentiometer can never "step up" a voltage, even though the schematics may bear a superficial resemblance.

19. Sep 30, 2013

### Bararontok

This is correct because a potentiometer is made up of a resistive material while an auto-transformer uses an inductor.