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basics of autotransformer
|
|
| bhargava 2007-12-16, 9:25 pm |
| 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?
| |
| Salmon Egg 2007-12-17, 3:25 am |
| On 12/16/07 7:16 PM, in article
dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
"bhargava" <bhargava.prasad@gmail.com> 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.
Bill
| |
|
|
|
|
"bhargava" <bhargava.prasad@gmail.com> wrote in message
news:dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com...
> 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?
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
| |
|
| In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
> On 12/16/07 7:16 PM, in article
> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
> "bhargava" <bhargava.prasad@gmail.com> wrote:
>
>
> 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
| |
| Don Kelly 2007-12-18, 3:25 am |
| ----------------------------
"krw" <krw@att.bizzzzz> wrote in message
news:MPG.21d0338f138a95829896a5@news.individual.net...
> In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
> alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
> 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.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
| |
| phil-news-nospam@ipal.net 2007-12-18, 3:25 am |
| On Mon, 17 Dec 2007 20:38:05 -0500 krw <krw@att.bizzzzz> wrote:
| In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
| alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
|> On 12/16/07 7:16 PM, in article
|> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
|> "bhargava" <bhargava.prasad@gmail.com> 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.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-12-17-2314@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Salmon Egg 2007-12-18, 3:25 am |
| On 12/17/07 8:52 PM, in article GmI9j.800$pr3.209@pd7urf1no, "Don Kelly"
<dhky@shaw.ca> 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.
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
| |
| phil-news-nospam@ipal.net 2007-12-18, 9:25 am |
| On Tue, 18 Dec 2007 04:52:54 GMT Don Kelly <dhky@shaw.ca> wrote:
| ----------------------------
| "krw" <krw@att.bizzzzz> wrote in message
| news:MPG.21d0338f138a95829896a5@news.individual.net...
|> In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
|> alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
|>> On 12/16/07 7:16 PM, in article
|>> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
|>> "bhargava" <bhargava.prasad@gmail.com> 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?
|>
|> --
|> 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.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-12-18-0709@ipal.net |
|------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2007-12-18, 9:25 am |
| On Tue, 18 Dec 2007 07:12:00 GMT Salmon Egg <SalmonEgg@sbcglobal.net> wrote:
| On 12/17/07 8:52 PM, in article GmI9j.800$pr3.209@pd7urf1no, "Don Kelly"
| <dhky@shaw.ca> 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.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-12-18-0710@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| In article <fk7l3u11bcu@news2.newsguy.com>,
alt.engineering.electrical, phil-news-nospam@ipal.net says...
> On Mon, 17 Dec 2007 20:38:05 -0500 krw <krw@att.bizzzzz> wrote:
> | In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
> | alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
> |> On 12/16/07 7:16 PM, in article
> |> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
> |> "bhargava" <bhargava.prasad@gmail.com> 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?
> 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.
--
Keith
| |
|
| In article <GmI9j.800$pr3.209@pd7urf1no>,
alt.engineering.electrical, dhky@shaw.ca says...
> ----------------------------
> "krw" <krw@att.bizzzzz> wrote in message
> news:MPG.21d0338f138a95829896a5@news.individual.net...
>
> 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?
Which prompted my question.
> With an autotransformer, if the turns ratio is small, the efficiency, size
> and cost
> are better than that of a 2-winding transformer.
Yes, but without a tap... ;-)
> 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.
Didn't know that, but it makes sense. Thanks!
--
Keith
| |
| phil-news-nospam@ipal.net 2007-12-18, 9:25 pm |
| On Tue, 18 Dec 2007 18:32:41 -0500 krw <krw@att.bizzzzz> wrote:
| In article <fk7l3u11bcu@news2.newsguy.com>,
| alt.engineering.electrical, phil-news-nospam@ipal.net says...
|> On Mon, 17 Dec 2007 20:38:05 -0500 krw <krw@att.bizzzzz> wrote:
|> | In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
|> | alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
|> |> On 12/16/07 7:16 PM, in article
|> |> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
|> |> "bhargava" <bhargava.prasad@gmail.com> 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.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-12-18-2056@ipal.net |
|------------------------------------/-------------------------------------|
| |
|
| In article <fka1cj013ll@news3.newsguy.com>,
alt.engineering.electrical, phil-news-nospam@ipal.net says...
> On Tue, 18 Dec 2007 18:32:41 -0500 krw <krw@att.bizzzzz> wrote:
> | In article <fk7l3u11bcu@news2.newsguy.com>,
> | alt.engineering.electrical, phil-news-nospam@ipal.net says...
> |> On Mon, 17 Dec 2007 20:38:05 -0500 krw <krw@att.bizzzzz> wrote:
> |> | In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
> |> | alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
> |> |> On 12/16/07 7:16 PM, in article
> |> |> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
> |> |> "bhargava" <bhargava.prasad@gmail.com> 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.
> |> 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.
--
Keith
| |
| VWWall 2007-12-19, 3:25 am |
| Salmon Egg wrote:
> On 12/17/07 8:52 PM, in article GmI9j.800$pr3.209@pd7urf1no, "Don Kelly"
> <dhky@shaw.ca> wrote:
>
>
> 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.
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!
--
Virg Wall, P.E.
| |
| Don Kelly 2007-12-19, 3:25 am |
| ----------------------------
"Salmon Egg" <SalmonEgg@sbcglobal.net> wrote in message
news:C38CB2BF.1306C%SalmonEgg@sbcglobal.net...
> On 12/17/07 8:52 PM, in article GmI9j.800$pr3.209@pd7urf1no, "Don Kelly"
> <dhky@shaw.ca> wrote:
>
>
> 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
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.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
>
| |
| Don Kelly 2007-12-19, 3:25 am |
| ----------------------------
<phil-news-nospam@ipal.net> wrote in message
news:fk8gri2cba@news1.newsguy.com...
> On Tue, 18 Dec 2007 04:52:54 GMT Don Kelly <dhky@shaw.ca> wrote:
> | ----------------------------
> | "krw" <krw@att.bizzzzz> wrote in message
> | news:MPG.21d0338f138a95829896a5@news.individual.net...
> |> In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
> |> alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
> |>> On 12/16/07 7:16 PM, in article
> |>> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
> |>> "bhargava" <bhargava.prasad@gmail.com> 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?
> |>
> |> --
> |> 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.
--------------------
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.
--
Don Kelly dhky@shawcross.ca
remove the X to answer
>
> --
> |---------------------------------------/----------------------------------|
> | Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below
> |
> | first name lower case at ipal.net / spamtrap-2007-12-18-0709@ipal.net
> |
> |------------------------------------/-------------------------------------|
| |
| phil-news-nospam@ipal.net 2007-12-21, 9:25 am |
| On Tue, 18 Dec 2007 23:06:06 -0500 krw <krw@att.bizzzzz> wrote:
| In article <fka1cj013ll@news3.newsguy.com>,
| alt.engineering.electrical, phil-news-nospam@ipal.net says...
|> On Tue, 18 Dec 2007 18:32:41 -0500 krw <krw@att.bizzzzz> wrote:
|> | In article <fk7l3u11bcu@news2.newsguy.com>,
|> | alt.engineering.electrical, phil-news-nospam@ipal.net says...
|> |> On Mon, 17 Dec 2007 20:38:05 -0500 krw <krw@att.bizzzzz> wrote:
|> |> | In article <C38B4044.1298B%SalmonEgg@sbcglobal.net>,
|> |> | alt.engineering.electrical, SalmonEgg@sbcglobal.net says...
|> |> |> On 12/16/07 7:16 PM, in article
|> |> |> dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com,
|> |> |> "bhargava" <bhargava.prasad@gmail.com> 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.
--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-12-21-0903@ipal.net |
|------------------------------------/-------------------------------------|
| |
| Salmon Egg 2007-12-21, 5:25 pm |
| 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
| |
| Euclid 2007-12-29, 9:25 pm |
| 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
On Mon, 17 Dec 2007 07:35:13 -0500, "evo" <evo@sc.rr.com> wrote:
>
>"bhargava" <bhargava.prasad@gmail.com> wrote in message
>news:dd654d05-e00c-4efc-aa56-a3f5773f72c8@d21g2000prf.googlegroups.com...
>
>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
>
| |
| Ben Miller 2008-01-15, 9:25 am |
| Salmon Egg wrote:
> On 12/17/07 8:52 PM, in article GmI9j.800$pr3.209@pd7urf1no, "Don
> Kelly" <dhky@shaw.ca> wrote:
>
>
> 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
I always thought you were supposed to cut off the ground pin on the scope
plug, and set the scope on a piece of cardboard for isolation!
(Oh yeah, and don't touch anything metal!)
--
Benjamin D Miller, PE
www.bmillerengineering.com
| |
| phil-news-nospam@ipal.net 2008-01-15, 1:25 pm |
| On Tue, 15 Jan 2008 09:14:09 -0600 Ben Miller <Ben@somewhere> wrote:
| Salmon Egg wrote:
|> On 12/17/07 8:52 PM, in article GmI9j.800$pr3.209@pd7urf1no, "Don
|> Kelly" <dhky@shaw.ca> 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.
|>
|> 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
|
| I always thought you were supposed to cut off the ground pin on the scope
| plug, and set the scope on a piece of cardboard for isolation!
| (Oh yeah, and don't touch anything metal!)
I've always wondered how I was supposed to safely examine the power current
waveform as sensed by the current transformer on service entrance wires
coming into the building. NOT!
Sometimes some extreme isolation is really called for. In the above example
my idea would be some kind of device serving as the current transformer that
is transmitting the waveform by some wireless means, such as via an LED.
Digital, or at least PWM, would be preferred to avoid any distortion issues.
--
-----------------------------------------------------------------------------
| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/ http://ka9wgn.ham.org/ |
-----------------------------------------------------------------------------
| |
| Don Kelly 2008-01-15, 9:25 pm |
| ----------------------------
"Ben Miller" <Ben@somewhere> wrote in message
news:13opje5c2uuge3c@corp.supernews.com...
> Salmon Egg wrote:
>
> I always thought you were supposed to cut off the ground pin on the scope
> plug, and set the scope on a piece of cardboard for isolation!
> (Oh yeah, and don't touch anything metal!)
>
> --
> Benjamin D Miller, PE
> www.bmillerengineering.com
---------
Call it laziness, I had immediate access to a 1:1 transformer so it was
easier (and I was where it was a good idea to play it by the book).
Don Kelly dhky@shawcross.ca
remove the X to answer
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