Electrical Engineering - transformer as current limiting inductor

Author

transformer as current limiting inductor

phil-news-nospam@ipal.net

2007-05-06, 9:25 pm

Given a transformer of known specification normally associated with a
transformer (e.g. voltage of winding, volt-amps capacity, impedance
percentage), is it possible to go from that to figure the inductance
to determine the absolute impedance and thus the maximum current it
would pass when wired as an inductor? It would seem to me that this
should be possible, because given these specifications, the maximum
current when used as a transformer is known. But I don't know the
relationship between that and open circuit inductance. Also, there
are a couple possible ways to do the wiring: use one of the windings
or use all of them in series with the appropriate connection order.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-05-06-2049@ipal.net |
|------------------------------------/-------------------------------------|

Salmon Egg

2007-05-07, 3:25 am

On 5/6/07 6:56 PM, in article f1m10v114l8@news5.newsguy.com,
"phil-news-nospam@ipal.net" <phil-news-nospam@ipal.net> wrote:

> Given a transformer of known specification normally associated with a
> transformer (e.g. voltage of winding, volt-amps capacity, impedance
> percentage), is it possible to go from that to figure the inductance
> to determine the absolute impedance and thus the maximum current it
> would pass when wired as an inductor? It would seem to me that this
> should be possible, because given these specifications, the maximum
> current when used as a transformer is known. But I don't know the
> relationship between that and open circuit inductance. Also, there
> are a couple possible ways to do the wiring: use one of the windings
> or use all of them in series with the appropriate connection order.

You are talking about the first things you learn in a course on ac electric
machines. Essentially, you can represent a transformer by its equivalent
circuit. It a three terminal network. A tee of three inductors does a decent
job. From the hot input, there is a primary leakage inductor connected to a
secondary leakage inductor to a hot output terminal. From the junction of
these inductors, there is another magnetizing inductor connection going to
the common neutral of the network.

From this network, you should be able to figure out how to determine the
inductor values from measurements at the terminals using shorts and opens on
primary and secondary. You do have to throw in compensating factors for any
step up and step down.

Bill
-- Fermez le Bush--about two years to go.

operator jay

2007-05-07, 8:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:f1m10v114l8@news5.newsguy.com...
> Given a transformer of known specification normally associated with
> a
> transformer (e.g. voltage of winding, volt-amps capacity, impedance
> percentage), is it possible to go from that to figure the inductance
> to determine the absolute impedance and thus the maximum current it
> would pass when wired as an inductor? It would seem to me that this
> should be possible, because given these specifications, the maximum
> current when used as a transformer is known. But I don't know the
> relationship between that and open circuit inductance. Also, there
> are a couple possible ways to do the wiring: use one of the windings
> or use all of them in series with the appropriate connection order.
>
> --
> |---------------------------------------/----------------------------------|
> | Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address
> below |
> | first name lower case at ipal.net /
> spamtrap-2007-05-06-2049@ipal.net |
> |------------------------------------/-------------------------------------|

Short circuit current equals full load current divided by p.u.
impedance (4.7%IZ = 0.047). Same as for a utility transformer when
you are determining available fault current at the secondary based on
transformer rating and percent impedance. Note that 'current' is
about the same as 'power' in p.u. because voltage is 1 p.u.
(|S|=V*I=1*I=I).

If you have multiple impedances in series, add the let through kVA
like you would add admittances. If the utility txf is 1 MVA with
4%IZ, secondary available kVA with infinite primary is 25000kVA. If
your 600D:600Y txf is 1 MVA at 6%IZ, it's max kVA let through is 16667
kVA. 1/(1/25000+1/16667)=10000kVA is the total available at the
600:600 secondary. Here the 4% and 6% add up nice to 10% (and agree
with 1MVA/10%=10000kVA) but that only works because I happened to use
the same txf rating (1 MVA) thereby giving the p.u. a common base.

John Gilmer

2007-05-07, 8:25 pm

<phil-news-nospam@ipal.net> wrote in message
news:f1m10v114l8@news5.newsguy.com...
> Given a transformer of known specification normally associated with a
> transformer (e.g. voltage of winding, volt-amps capacity, impedance
> percentage), is it possible to go from that to figure the inductance
> to determine the absolute impedance and thus the maximum current it
> would pass when wired as an inductor?

Sure.

But remember how a transformer "works."

Without a load but with normal voltage on the primary, the current is a
small fraction of any expected load current. That "excitation" current
would be the current used with the input voltage to calculate the impedance.

It you expect a transformer to "drop" significantly voltage than it's
rating, it will "saturate" and the current will only be limited by the
winding resistance.

"inductors tend to have "softer" iron (in the magnetic sense) that requires
much more ampere turns to saturate.

phil-news-nospam@ipal.net

2007-05-08, 3:25 am

On Mon, 07 May 2007 06:28:41 GMT Salmon Egg <salmonegg@sbcglobal.net> wrote:
| On 5/6/07 6:56 PM, in article f1m10v114l8@news5.newsguy.com,
| "phil-news-nospam@ipal.net" <phil-news-nospam@ipal.net> wrote:
|
|> Given a transformer of known specification normally associated with a
|> transformer (e.g. voltage of winding, volt-amps capacity, impedance
|> percentage), is it possible to go from that to figure the inductance
|> to determine the absolute impedance and thus the maximum current it
|> would pass when wired as an inductor? It would seem to me that this
|> should be possible, because given these specifications, the maximum
|> current when used as a transformer is known. But I don't know the
|> relationship between that and open circuit inductance. Also, there
|> are a couple possible ways to do the wiring: use one of the windings
|> or use all of them in series with the appropriate connection order.
|
| You are talking about the first things you learn in a course on ac electric
| machines. Essentially, you can represent a transformer by its equivalent
| circuit. It a three terminal network. A tee of three inductors does a decent
| job. From the hot input, there is a primary leakage inductor connected to a
| secondary leakage inductor to a hot output terminal. From the junction of
| these inductors, there is another magnetizing inductor connection going to
| the common neutral of the network.
|
| From this network, you should be able to figure out how to determine the
| inductor values from measurements at the terminals using shorts and opens on
| primary and secondary. You do have to throw in compensating factors for any
| step up and step down.

How do you "measure" it without an actual transformer? If I described a
specific transformer, could you tell me the equivalent Henries its primary
and secondary windings have?

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-05-07-2129@ipal.net |
|------------------------------------/-------------------------------------|

Don Kelly

2007-05-08, 3:25 am

----------------------------
<phil-news-nospam@ipal.net> wrote in message
news:f1ond8821ti@news2.newsguy.com...
> On Mon, 07 May 2007 06:28:41 GMT Salmon Egg <salmonegg@sbcglobal.net>
> wrote:
> | On 5/6/07 6:56 PM, in article f1m10v114l8@news5.newsguy.com,
> | "phil-news-nospam@ipal.net" <phil-news-nospam@ipal.net> wrote:
> |
> |> Given a transformer of known specification normally associated with a
> |> transformer (e.g. voltage of winding, volt-amps capacity, impedance
> |> percentage), is it possible to go from that to figure the inductance
> |> to determine the absolute impedance and thus the maximum current it
> |> would pass when wired as an inductor? It would seem to me that this
> |> should be possible, because given these specifications, the maximum
> |> current when used as a transformer is known. But I don't know the
> |> relationship between that and open circuit inductance. Also, there
> |> are a couple possible ways to do the wiring: use one of the windings
> |> or use all of them in series with the appropriate connection order.
> |
> | You are talking about the first things you learn in a course on ac
> electric
> | machines. Essentially, you can represent a transformer by its equivalent
> | circuit. It a three terminal network. A tee of three inductors does a
> decent
> | job. From the hot input, there is a primary leakage inductor connected
> to a
> | secondary leakage inductor to a hot output terminal. From the junction
> of
> | these inductors, there is another magnetizing inductor connection going
> to
> | the common neutral of the network.
> |
> | From this network, you should be able to figure out how to determine the
> | inductor values from measurements at the terminals using shorts and
> opens on
> | primary and secondary. You do have to throw in compensating factors for
> any
> | step up and step down.
>
> How do you "measure" it without an actual transformer? If I described a
> specific transformer, could you tell me the equivalent Henries its primary
> and secondary windings have?
>
> --
> |---------------------------------------/----------------------------------|
> | Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below
> |
> | first name lower case at ipal.net / spamtrap-2007-05-07-2129@ipal.net
> |
> |------------------------------------/-------------------------------------|

You generally have a handle on the % Impedance which is the short circuit
impedance as a % of the rated V/rated I
A 100/1000V 10KVA transformer will typically be about 5% so that with the
1000V winding shorted the impedance seen at the 100V winding will be 0.05
(100)^2/(10000) =0.05 ohms
If the 100V winding is shorted the impedance seen from the 1000Vwinding will
be 100 times this or 50 ohms. In either case the current at rated voltage
will be such as to let magic smoke out so you will be limited to rated
current on whichever winding you energise- unless you like replacing stinky
burnt out transformers.
If one winding is open then the current at rated voltage on the other
winding will be in the order of 2-3% of rated current for that winding (say
2 to 3A for the 100V winding at 100V and 0.2 to 0.3A at 1000V on the high
Voltage winding. If you try to get more current at a higher voltage- you
will but the current waveform will be distorted and excessive core heating
will occur because of saturation.

--

Don Kelly dhky@shawcross.ca
remove the X to answer

Salmon Egg

2007-05-08, 3:25 am

On 5/7/07 7:31 PM, in article f1ond8821ti@news2.newsguy.com,
"phil-news-nospam@ipal.net" <phil-news-nospam@ipal.net> wrote:

> How do you "measure" it without an actual transformer? If I described a
> specific transformer, could you tell me the equivalent Henries its primary
> and secondary windings have?

You measure it the same way you measure resistance of a resistor without
having a resistor.

Bill
-- Fermez le Bush--about two years to go.

phil-news-nospam@ipal.net

2007-05-12, 9:25 pm

On Tue, 08 May 2007 06:48:52 GMT Salmon Egg <salmonegg@sbcglobal.net> wrote:
| On 5/7/07 7:31 PM, in article f1ond8821ti@news2.newsguy.com,
| "phil-news-nospam@ipal.net" <phil-news-nospam@ipal.net> wrote:
|
|> How do you "measure" it without an actual transformer? If I described a
|> specific transformer, could you tell me the equivalent Henries its primary
|> and secondary windings have?
|
| You measure it the same way you measure resistance of a resistor without
| having a resistor.

I've long since forgotten the color band codes. Transformers have them,
too?

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2007-05-12-2049@ipal.net |
|------------------------------------/-------------------------------------|

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