## #99 ideal transformer - again

0.0.17
closed-invalid
Richard C
None
5
2014-07-28
2013-06-07
Thomas Bechteler
No

Dear Qucs-Team!

It seems that there is a lack of profound knowledge in electronics.
I still believe that there is a bug in the (ideal) transformer.
At the end of this message I cited the last statement about the ideal transformer.

This statement is wrong!

The ideal transformer is defined by the governing equations

turn ratio N:=N2/N1

u2 = N * u1
i1 = -N * i2

These simple equations can be looked up in
1) Leon O. Chua: Linear and Nonlinear Circuits. McGraw Hill, 1987.
2) Jiri Vlach: Computer Methods for Circuit Analysis and Design. VNR, 1994.
and in many other books.

Due to the definition above, the ideal transformer does NOT
act as a short circuit, not even for dc!
A REAL transformer acts as a short circuit for dc only!
For ac signals it acts not as a short circuit, as practice shows us.
Even on the cited webpage (http://qucs.sourceforge.net/tech/node48.html)
it is stated "And in matrix representation this is for DC and for AC simulation".

The current which flows into, e.g., port 1 is only defined by the resistance
connected to port 2 (and, for the real transformer, some current due to the
stray field of the transformer).

The ideal transformer is not only a perfect current transformer,
but ALSO a perfect voltage transformer!

The equations which can be found on the cited webpage
(http://qucs.sourceforge.net/tech/node48.html)
scattering parameters. In my humble opinion, this was didactically
not accomplished.

As a result, there is still a bug in the ideal transformer!!!

Cordially

Thomas Bechteler

=====start citation============================================================

Having investigated this, I think there is not a bug after all, and instead there is a misunderstanding of what the ideal transformer component actually does.

The ideal transformer acts as a perfect current transformer, the current in the primary is mutiplied on the secondary. If you just put a voltage source across the terminals, it acts like a short circuit, so there is infinite current. At the same time in this case, there is no resistance, and therefore no voltage drop across the tranformer inputs, hence the output voltage is also zero.

http://qucs.sourceforge.net/tech/node48.html

The ideal transformer component does not reflect a real physical transformer device, a real device must have winding resistance and inductance, and only works with AC voltages. An ideal transformer can transform DC current, which is physically impossible in the real world (maybe some exotic superconducting device I'm not aware of).

What changed in Qucs 0.0.15 -> 0.0.16 as far as I can tell was that the behaviour was actually corrected for DC simulation.

=====end citation============================================================

## Discussion

• Richard C
2013-06-08

Thomas, ok, lets make this simpler, could you outline how the mna
equations shown in the link I gave to the qucs docs are incorrect, as it
is these equations exactly which are implemented in qucs.

The ideal transformer does act as a perfect voltage transformer but what
is the voltage u1 across the winding when the winding has no resistance
or inductance?

Richard

• Dear Richard,

all right, I studied a little bit on the MNA,
since I was not very firm in this topic.

I checked the equations on the webpage
http://qucs.sourceforge.net/tech/node48.html
and they are correct. I didn't mean that
they are incorrect. There is no clear explanation
how to come from Eq. (9.26) to the S-parameters.
(Maybe there was no such intention?)
The link between Fig. 9.2 and the S-parameters
is very dangerous. In Fig. 9.2 the numbers are
terminal numbers and not port numbers! Actually,
the shown ideal transformer is a two-port where
terminals 1 and 4 make up port 1, and terminals 2 and 3
make up port 2. But that is another story ...

Back to Eq.(9.26): While the matrix itself is
correct, the rightest side seems not correct to me.
The currents are all set to zero which is not
the case when the ideal transformer is connected
to a circuit including sources. In Section 5.3.13
(Ideal Transformer), on the webpage
http://www.analog-electronics.eu/analog-electronics/modified-nodal-analysis/modified-nodal-analysis.xhtml
the current vector is not a zero vector as it
should be for the general case. Maybe this is
a flaw implemented in QUCS.

Remark 1:
In the literature, the currents I_t and T \times I_t
are shown all into the transformer. In Fig. 9.2 current
I_t is shown flowing out of terminal 1. This is,
of course, possible since any designation can be
chosen arbitrarly. Only the signs in the matrix
change accordingly.

Remark 2:
I want to mention again that in QUCS the ideal transformer
does not work anymore in the TRANSIENT analysis.
But is does work in the AC analysis.
By the way, the mutual inductor in QUCS works as it
should be in both, the TRANSIENT analysis and the
AC analysis. This is now the work around for me.

Actually, when speaking about an ideal transformer,
there is no winding, coil, inductor or resistor to be
considered. The ideal transformer is only defined by
mathematical equations. So it works perfectly well for
dc or ac, in the time domain or in the frequency domain.
Unfortunately or not, the schematic symbol for the
ideal transformer is depicted as two inductors.
This may mislead some people that there are indeed
inductors. However, there are no inductors!

Please, find attached to this message a pdf-file
which describes two examples for the MNA with an
ideal transformer. I hope that I did not introduce
any mistake and that I illuminated the topic from a
different point of view.

Cheers

Thomas

Attachments

• Richard C
2013-06-10

So, I went back and tried the schematic in the original bug report (very simple, but I've also attached it). This seems to work fine in the development version of Qucs. You're right about it not being a short-circuit, because the impedance is propogated from the other side.

Thomas, could you try the development snapshot to confirm this works for you?

Attachments

• Richard C
2013-06-10

This works fine on the dev version compiled on Mint Linux 14 I should add, to be specific.

• qucs 0.0.17 compilation fails because of

componentdialog.moc.cpp:15:34: fatal error: private/qucomextra_p.h: No such file or directory

I guess it is a problem with qt, as usual ... I tried to fix it,
but couldn't succeed up to now. Maybe OpenSuSE (12.1) always causes
qt-problems ... or I wait until an rpm for SuSE is available ... lets see ...

Thomas

• Frans
2013-06-10

<html>
<meta content="text/html; charset=ISO-8859-1" http-equiv="Content-Type">
<body bgcolor="#FFFFFF" text="#000000">
Have you installed libqt4-devel?

On 06/10/2013 09:41 PM, Thomas Bechteler wrote:

qucs 0.0.17 compilation fails because of

componentdialog.moc.cpp:15:34: fatal error:
private/qucomextra_p.h: No such file or directory

I guess it is a problem with qt, as usual ... I tried to fix
it,

but couldn't succeed up to now. Maybe OpenSuSE (12.1) always
causes

qt-problems ... or I wait until an rpm for SuSE is available
... lets see ...

Thomas

[bugs:#99]
ideal transformer - again

Status: open

Created: Fri Jun 07, 2013 09:57 PM UTC by
Thomas Bechteler

Last Updated: Mon Jun 10, 2013 06:02 PM UTC

Owner: Richard C

Dear Qucs-Team!

It seems that there is a lack of profound knowledge in
electronics.

I still believe that there is a bug in the (ideal)
transformer.

At the end of this message I cited the last statement about
the ideal transformer.

This statement is wrong!

The ideal transformer is defined by the governing equations

turn ratio N:=N2/N1

u2 = N * u1

i1 = -N * i2

These simple equations can be looked up in

1) Leon O. Chua: Linear and Nonlinear Circuits. McGraw Hill,
1987.

2) Jiri Vlach: Computer Methods for Circuit Analysis and
Design. VNR, 1994.

and in many other books.

Due to the definition above, the ideal transformer does NOT

act as a short circuit, not even for dc!

A REAL transformer acts as a short circuit for dc only!

For ac signals it acts not as a short circuit, as practice
shows us.

Even on the cited webpage
(http://qucs.sourceforge.net/tech/node48.html)

it is stated "And in matrix representation this is for DC and
for AC simulation".

The current which flows into, e.g., port 1 is only defined by
the resistance

connected to port 2 (and, for the real transformer, some
current due to the

stray field of the transformer).

The ideal transformer is not only a perfect current
transformer,

but ALSO a perfect voltage transformer!

The equations which can be found on the cited webpage

(http://qucs.sourceforge.net/tech/node48.html)

scattering parameters. In my humble opinion, this was
didactically

not accomplished.

As a result, there is still a bug in the ideal transformer!!!

Cordially

Thomas Bechteler

=====start
citation============================================================

Having investigated this, I think there is not a bug after
all, and instead there is a misunderstanding of what the ideal
transformer component actually does.

The ideal transformer acts as a perfect current transformer,
the current in the primary is mutiplied on the secondary. If
you just put a voltage source across the terminals, it acts
like a short circuit, so there is infinite current. At the
same time in this case, there is no resistance, and therefore
no voltage drop across the tranformer inputs, hence the output
voltage is also zero.

http://qucs.sourceforge.net/tech/node48.html

The ideal transformer component does not reflect a real
physical transformer device, a real device must have winding
resistance and inductance, and only works with AC voltages. An
ideal transformer can transform DC current, which is
physically impossible in the real world (maybe some exotic
superconducting device I'm not aware of).

What changed in Qucs 0.0.15 -> 0.0.16 as far as I can tell
was that the behaviour was actually corrected for DC
simulation.

=====end
citation============================================================

Sent from sourceforge.net because you indicated interest in https://sourceforge.net/p/qucs/bugs/99/

To unsubscribe from further messages, please visit https://sourceforge.net/auth/subscriptions/

</body>
</html>

#### Related

• Yes, libqt4-devel was installed.
Other qt4-related development packages
which I installed didn't change the situation ...

By the way, the directory /private only exist
in the qt3-path

Thomas

• Stefan Jahn
2013-06-13

I think my email didn't reach anyone...

Hello,

thank you for the explanation :)

The change from 0.0.15 to 0.0.16 was introduced since some
users expected the ideal transformer to do something different
in the time domain.

Actually, both trafo and strafo where just designed for AC domain.
They transfer ideally voltages and impedances.

In the DC steady state domain, since it is ideal element, there is a
short at input (only L) -> thus also V=0 on output.

Now, when using the thing in time domain, L for ideal trafo
is not defined: L --> 0 we still have a short as in DC. For
L --> infinite we get a transfer as in AC domain, BUT: also
a transfer for DC signals... (which is not physical)

Conclusion: depending on assumptions it will do different things.
My suggestion: Use mutual inductance models in transient simulations
instead of the ideal trafo models.

BR, Stefan.

On Fri, June 7, 2013 5:57 pm, Richard C wrote:

Having investigated this, I think there is not a bug after all, and
instead there is a misunderstanding of what the ideal transformer
component actually does.

The ideal transformer acts as a perfect current transformer, the current
in the primary is mutiplied on the secondary. If you just put a voltage
source across the terminals, it acts like a short circuit, so there is
infinite current. At the same time in this case, there is no resistance,
and therefore no voltage drop across the tranformer inputs, hence the
output voltage is also zero.

http://qucs.sourceforge.net/tech/node48.html

The ideal transformer component does not reflect a real physical
transformer device, a real device must have winding resistance and
inductance, and only works with AC voltages. An ideal transformer can
transform DC current, which is physically impossible in the real world
(maybe some exotic superconducting device I'm not aware of).

What changed in Qucs 0.0.15 -> 0.0.16 as far as I can tell was that the
behaviour was actually corrected for DC simulation.

[bugs:#83] ideal transformer

Status: open
Created: Mon Feb 20, 2012 01:30 PM UTC by Thomas Bechteler
Last Updated: Fri Jun 07, 2013 01:29 PM UTC
Owner: Richard C

Hi QUCS developer!

The ideal transformers (both under "lumped components") in version
0.0.15 work.
However, they do not work in version 0.0.16!

Thomas

#### Related

• Richard C
2013-06-13

Stefan, thanks for clearing this up. I think my understanding was not
quite wrong then after all, as really the behaviour in transient sims is
undefined.

Perhaps we should we consider issuing a warning in transient sims in
this case to this effect?

My conclusion though is that, whatever happens, there is no bug, or at
least nothing that holds up a release.

Richard

On 13/06/2013 07:31, Stefan Jahn wrote:

I think my email didn't reach anyone...

Hello,

thank you for the explanation :)

The change from 0.0.15 to 0.0.16 was introduced since some
users expected the ideal transformer to do something different
in the time domain.

Actually, both trafo /and/ strafo where just designed for AC domain.
They transfer ideally voltages and impedances.

In the DC steady state domain, since it is ideal element, there is a
short at input (only L) -> thus also V=0 on output.

Now, when using the thing in time domain, L for ideal trafo
is not defined: L --> 0 we still have a short as in DC. For
L --> infinite we get a transfer as in AC domain, BUT: also
a transfer for DC signals... (which is not physical)

Conclusion: depending on assumptions it will do different things.
My suggestion: Use mutual inductance models in transient simulations
instead of the ideal trafo models.

BR, Stefan.

• Frans
2013-06-14

• Description has changed:

Diff:

--- old
+++ new
@@ -1,4 +1,3 @@
-
Dear Qucs-Team!

It seems that there is a lack of profound knowledge in electronics.

• status: open --> closed-invalid

• Dear Qucs-Team,

maybe not calling it "ideal transformer".
Some different name indicating that only
ac simulation works ...

But still a pity that there is no ideal transformer!!!

cheers

Thomas

• Frans
2013-06-14

I am closing this bug and add a feature request in stead, for adding a warning for transient and dc simulations.