basics of autotransformer

An autotransformer has only one, usually tapped, winding. There is no separate primary and secondary. This saves on the copper and iron required to build. Some output current flows conductively from input to output. Such construction greatly lowers the cost of the transformer.

Bill

Looking at the schematic symbol for the device explains all:

A good resource you should know:

I can explain single phase operation: Typically refereed to as buck/boost in the field, I tend to reserve the term autotransformer for the three phase versions of the same concept. The feed for the primary windings of the transformer continue on to the load after being manipulated by the secondary windings. With a standard 10:1 ratio transformer that would be used for control power, i.e.: 240 volts down to

24, or 120 volts down to 12, you could boost 208 volts up to a more tolerable range for machine sensitive to a 230 volt feed. The voltage would be subtractive if passed through the secondary windings in reverse.

The application is limited though, generally to about a 20% manipulation of the original voltage.

The advantage is reduced size, (since the windings so not carry the entire load), & cost if you are not purchasing a special order item.. ______________________ ____________/ ___________________ 208 + 20.8 = 229 volts

208 volts____________P ][ S \__/ 10:! ratio general purpose transformer

You can obtain good information at the Acme transformer website. evo

Just a question... Is the winding ever *not* tapped? What would be the purpose?

You could have a 1:1 or untapped autotransformer but that would be useless because it wouldn't do anything useful that a direct connection without the autotransformer would accomplish - so -why pay for something that does nothing useful, has losses, costs money and complicates the circuit?

With an autotransformer, if the turns ratio is small, the efficiency, size and cost are better than that of a 2-winding transformer. A 1000/200V transformer rated 10kVA can be connected as a 1000/1200KVA transformer rated 60KVA with the same losses. physically it is a 10KVA transformer and if it's efficiency at rated load as a 2 winding transformer is 97% the losses would be 300 watts. Now, as an autotransformer it's losses are still 300 watts and the efficiency is roughly 99.5%. A corresponding 60KVA 1000/1200V two winding transformer with 97% efficiency at rated load would have losses of about 1800 watts and would be about 3 times as big and expensive.

However, this size/efficiency advantage becomes negligable for larger turns ratios so that a 1.5 to 1 ratio (or 1:1.5) is about the practical limit for most applications.

On Mon, 17 Dec 2007 20:38:05 -0500 krw wrote: | In article , | alt.engineering.electrical, snipped-for-privacy@sbcglobal.net says... |> On 12/16/07 7:16 PM, in article |> snipped-for-privacy@d21g2000prf.googlegroups.com, |> "bhargava" wrote: |> |> > Its been qouted in one of the earlier discusions that autotransformer |> > doesn't provide isolation. Why so? |> > Also, what is the basic difference between autotranformer and a |> > regular one? |> |> An autotransformer has only one, usually tapped, winding. There is no |> separate primary and secondary. This saves on the copper and iron required |> to build. Some output current flows conductively from input to output. Such |> construction greatly lowers the cost of the transformer. |> | Just a question... Is the winding ever *not* tapped? What would | be the purpose?

I would think _you_ would know about autotransformers.

It would technically not be tapped if it were 2 windings that can be wired in either a buck or boost configuration. But the electrical effect is the equivalent of a tapped winding, anyway. You just get the flexibility of wiring it either way, and the flexibility of having the lower voltage winding rated for a proportionally higher current to maximize the rating in a buck/boost purpose.

There are situations when you do not want to change voltage but do want isolation. Thus, and isolation transformer often is just a primary and a secondary winding relying upon the insulation to keep the secondary from connecting conductively to the primary.

One example where that may be wanted is when you are troubleshooting circuitry such as phase control circuitry with an oscilloscope. In the old days, before dual trace scopes were common, using a scope probe to measure waveforms between two points with a probe that had a ground lead could be a problem. With an isolation transformer, such a probe could be used between any two points in the circuit providing voltage ratings were not exceeded.

Bill

|>> > Its been qouted in one of the earlier discusions that autotransformer |>> > doesn't provide isolation. Why so? |>> > Also, what is the basic difference between autotranformer and a |>> > regular one? |>>

|>> An autotransformer has only one, usually tapped, winding. There is no |>> separate primary and secondary. This saves on the copper and iron |>> required |>> to build. Some output current flows conductively from input to output. |>> Such |>> construction greatly lowers the cost of the transformer. |>>

|> Just a question... Is the winding ever *not* tapped? What would |> be the purpose? |>

|> -- |> Keith | | You could have a 1:1 or untapped autotransformer but that would be useless | because it wouldn't do anything useful that a direct connection without the | autotransformer would accomplish - so -why pay for something that does | nothing useful, has losses, costs money and complicates the circuit? | | With an autotransformer, if the turns ratio is small, the efficiency, size | and cost | are better than that of a 2-winding transformer. | A 1000/200V transformer rated 10kVA can be connected as a 1000/1200KVA | transformer rated 60KVA with the same losses. physically it is a 10KVA | transformer and if it's efficiency at rated load as a 2 winding transformer | is 97% the losses would be 300 watts. Now, as an autotransformer it's losses | are still 300 watts and the efficiency is roughly 99.5%. | A corresponding 60KVA 1000/1200V two winding transformer with 97% efficiency | at rated load would have losses of about 1800 watts and would be about 3 | times as big and expensive. | | However, this size/efficiency advantage becomes negligable for larger turns | ratios so that a 1.5 to 1 ratio (or 1:1.5) is about the practical limit for | most applications.

So I should use a regular isolation transformer to drop 480 down to 120/240.

On Tue, 18 Dec 2007 07:12:00 GMT Salmon Egg wrote: | On 12/17/07 8:52 PM, in article GmI9j.800$pr3.209@pd7urf1no, "Don Kelly" | wrote: | |> You could have a 1:1 or untapped autotransformer but that would be useless |> because it wouldn't do anything useful that a direct connection without the |> autotransformer would accomplish - so -why pay for something that does |> nothing useful, has losses, costs money and complicates the circuit? | | There are situations when you do not want to change voltage but do want | isolation. Thus, and isolation transformer often is just a primary and a | secondary winding relying upon the insulation to keep the secondary from | connecting conductively to the primary.

But that would not be an autotransformer. His point is, why have anything there at all if: it doesn't change the voltage -AND- it doesn't provide any isolation.

Why me?

Ok, I didn't consider separate windings as being an auto- transformer.

Which prompted my question.

Yes, but without a tap... ;-)

Didn't know that, but it makes sense. Thanks!

On Tue, 18 Dec 2007 18:32:41 -0500 krw wrote: | In article , | alt.engineering.electrical, snipped-for-privacy@ipal.net says... |> On Mon, 17 Dec 2007 20:38:05 -0500 krw wrote: |> | In article , |> | alt.engineering.electrical, snipped-for-privacy@sbcglobal.net says... |> |> On 12/16/07 7:16 PM, in article |> |> snipped-for-privacy@d21g2000prf.googlegroups.com, |> |> "bhargava" wrote: |> |> |> |> > Its been qouted in one of the earlier discusions that autotransformer |> |> > doesn't provide isolation. Why so? |> |> > Also, what is the basic difference between autotranformer and a |> |> > regular one? |> |> |> |> An autotransformer has only one, usually tapped, winding. There is no |> |> separate primary and secondary. This saves on the copper and iron required |> |> to build. Some output current flows conductively from input to output. Such |> |> construction greatly lowers the cost of the transformer. |> |> |> | Just a question... Is the winding ever *not* tapped? What would |> | be the purpose? |> |> I would think _you_ would know about autotransformers. | | Why me?

Not you alone. But you do seem to know lots of things around here, so this might be one of those things.

I could be wrong.

|> It would technically not be tapped if it were 2 windings that can be wired |> in either a buck or boost configuration. But the electrical effect is the |> equivalent of a tapped winding, anyway. You just get the flexibility of |> wiring it either way, and the flexibility of having the lower voltage |> winding rated for a proportionally higher current to maximize the rating |> in a buck/boost purpose. | | Ok, I didn't consider separate windings as being an auto- | transformer.

But it can behave as one if wired the right way.

I'm not a power engineer (my father and two brothers did that). I do things microprocessor.

Sure, I see that. ...just hadn't considered it (two windings in series buck/boost) an auto-transformer, as it surely is.

On a test bench where you are working on small appliances, an isolation transformer is a good safety precaution. I use one with a Variac, which is a variable tap auto-transformer feeding its primary. That way there is no current path to ground through your body from either side of the line power, and you can vary the voltage for testing.

An easy way to make a good, cheap, high power, isolation transformer is to use two power transformers from a defunct microwave oven. Strip off the high voltage windings on both, and connect the two magnetron filament windings together. May not be the most efficient design, but has excelleent isolation!

Certainly a 1:1 transformer for isolation is very useful but it is not classified as an autotransformer. I have used such in the situation that you mention. The alternative was hard on the 'scope.

When one speaks of an autotransformer, one is considering primary and secondary windings that are conductively connected so "autotransformer" and "isolation" don't go together.

Now any two winding transformer may have taps on one of the windings -a typical pole pig is an example and higher power transformers often have fairly complex arrangements to allow "on load' tap changing. These are not classified as autotransformers even though one winding, by itself may look like an autotransformer . (eg. 2300V/220, 225,230,235,240V depending on the secondary tap.) but is not used as an autotransformer.

-------------------- Didn't ou noticed that I hedged my bets? 1.5:"1 is about.."

You may find it better- it depends on what is out there and the relative costs. If you found a 480V/120 transformer with a center tap on the HV then you could use it as an auto transformer 480/240 auto and 480/120 2 winding (at the same time) as long as all windings are within rated current. If you wanted 480/240/120V then a two winding transformer with a secondary center tapped would be likely to be a better option. If you wanted 120/12V then a 2 winding transformer will be the better choice. In theory there would be a little difference in favour of the autotransformer but when you put a value on isolation, then the 2 winding unit wins. If you are building a 500/765KV 1000MVA transformer, there is a hell of an incentive to use an autotransformer - typically 500Y/765Y and a tertiary , say 13KV delta for a 3 phase unit. The delta provides a path for 3rd harmonic currents and may be used for local loads.

An example of an autotransformer that is used with a wide range of ratios is a Variac. Care must be taken to stay within current limits when the ratio is high or the magic smoke leaks out.

On Tue, 18 Dec 2007 23:06:06 -0500 krw wrote: | In article , | alt.engineering.electrical, snipped-for-privacy@ipal.net says... |> On Tue, 18 Dec 2007 18:32:41 -0500 krw wrote: |> | In article , |> | alt.engineering.electrical, snipped-for-privacy@ipal.net says... |> |> On Mon, 17 Dec 2007 20:38:05 -0500 krw wrote: |> |> | In article , |> |> | alt.engineering.electrical, snipped-for-privacy@sbcglobal.net says... |> |> |> On 12/16/07 7:16 PM, in article |> |> |> snipped-for-privacy@d21g2000prf.googlegroups.com, |> |> |> "bhargava" wrote: |> |> |> |> |> |> > Its been qouted in one of the earlier discusions that autotransformer |> |> |> > doesn't provide isolation. Why so? |> |> |> > Also, what is the basic difference between autotranformer and a |> |> |> > regular one? |> |> |> |> |> |> An autotransformer has only one, usually tapped, winding. There is no |> |> |> separate primary and secondary. This saves on the copper and iron required |> |> |> to build. Some output current flows conductively from input to output. Such |> |> |> construction greatly lowers the cost of the transformer. |> |> |> |> |> | Just a question... Is the winding ever *not* tapped? What would |> |> | be the purpose? |> |> |> |> I would think _you_ would know about autotransformers. |> | |> | Why me? |> |> Not you alone. But you do seem to know lots of things around here, so this |> might be one of those things. |> |> I could be wrong. | | I'm not a power engineer (my father and two brothers did that). I | do things microprocessor.

I'm not a power engineer, either. I do software (Linux distributions). But I learned electronics and electricity all along the way, and continue to learn when I get the chance. I think power stuff is fun (so you might want to keep at a great physical distance when I roll out the pole pig). I'm also a ham radio operator (and thus, know some RF stuff, too).

|> |> It would technically not be tapped if it were 2 windings that can be wired |> |> in either a buck or boost configuration. But the electrical effect is the |> |> equivalent of a tapped winding, anyway. You just get the flexibility of |> |> wiring it either way, and the flexibility of having the lower voltage |> |> winding rated for a proportionally higher current to maximize the rating |> |> in a buck/boost purpose. |> | |> | Ok, I didn't consider separate windings as being an auto- |> | transformer. |> |> But it can behave as one if wired the right way. | | Sure, I see that. ...just hadn't considered it (two windings in | series buck/boost) an auto-transformer, as it surely is.

It's more interesting when you do buck/boost cross-phased between different phases in a three phase system. There are more wiring combinations and a lot of crazy voltages are possible. I once wrote a program to calculate a great many possibilities.

This whole autotransformer is being made too complicated. To simplify it break it down int a multiwinding transformer, and treat it as such. That is, cut the winding wires wherever a lead comes out and remove all the leads so to speak. You are now left with a bunch of separate windings. The emf induced in each one is the number of turns times the flux in the core which is usually the same for each winding. Now connect the windings the way they are going to be whether an auto transformer or not. Bring leads out to loads corresponding to the way the transformer gets connected. Now solve the combination circuit and magnetic equations equations. Use the Kirchhoff laws for the electricity and the equivalent for the magnetism. All transformers will work the same way. From the currents, and knowledge of the construction, you can tell how much current flows in each winding and thereby determine how big the wire should be.

For an autotransformer, the connection is usually simple so the solution of the equations is easy.

Bill

Just to point out a fact that I have designed several 1:1 ratio autotransformers in the past. So yes they are out there.

Their principle use is as a phase shift transformer, generally at mains voltage they help to give 12 pulse rectification if one bridge is connected direct to the mains.

Euclid