On Sun, 7 Dec 2003 19:13:36 +0000, John Woodgate
wrote:

Low-frequency iron-cored coils are quite another matter; the
inductance varies with frequency, voltage, temperature, previous history
and the state of the tide on Europa.

I assume that you are referring to DC biased iron cores (without an
air gap) or some high permeability ferrites with a strong DC bias
current. These do indeed show a variation of inductance depending on
the DC bias current.

Paul OH3LWR

Bill Turner writes:

On Sun, 07 Dec 2003 20:00:37 GMT, John Popelish
wrote:

The inductive component of the impedance remains
essentially constant through resonance. What is non ideal about the
inductor is that it does not exhibit just inductance, but a parallel
combination if inductance and capacitance. Ignoring the capacitance
and calling the effect variable inductance is just not as accurate a
way to describe what is going on.

__________________________________________________ _______

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

No, you are neglecting the phase. The two cases would have very
different phase shifts (the current would be out of phase with the
applied voltage, by different amounts), depending on whether you were
below, at, or above resonance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

Yes it can. This is what a network analyser or impedance bridge does,
(as I understand it, I've never actually had to use either!).

At low frequencies the black box would be inductive. The current would
lag the voltage. At resonance the voltage would be in phase with the
current (the black box would appear resistive). At high frequencies
the current would lead the voltage. It would appear capacitive.


--

John Devereux

Bill Turner writes:

On Sun, 07 Dec 2003 20:00:37 GMT, John Popelish
wrote:

The inductive component of the impedance remains
essentially constant through resonance. What is non ideal about the
inductor is that it does not exhibit just inductance, but a parallel
combination if inductance and capacitance. Ignoring the capacitance
and calling the effect variable inductance is just not as accurate a
way to describe what is going on.

__________________________________________________ _______

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

No, you are neglecting the phase. The two cases would have very
different phase shifts (the current would be out of phase with the
applied voltage, by different amounts), depending on whether you were
below, at, or above resonance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

Yes it can. This is what a network analyser or impedance bridge does,
(as I understand it, I've never actually had to use either!).

At low frequencies the black box would be inductive. The current would
lag the voltage. At resonance the voltage would be in phase with the
current (the black box would appear resistive). At high frequencies
the current would lead the voltage. It would appear capacitive.


--

John Devereux

Bill Turner wrote:

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

Not if I can measure both the magnitude and phase relationship of the
device.

If I can only measure the magnitude of impedance at one frequency, I
can't even tell if the device is predominately inductive, capacitive
or resistive. So it would be a bit silly to call that magnitude an
inductive impedance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

Only because you are willing to confuse complex impedance with
inductive reactance.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

You are projecting your limitations onto others.

--
John Popelish

Bill Turner wrote:

Your point is well taken, but look at it this way:

Say I give you a black box containing an inductor with two terminals on
the box. If I have you measure the inductance at one and only one
frequency, there is no way for you to know whether it is an inductor
operating well below its self-resonance point, or an inductor operating
near its self-resonance point. To the outside world, at ONE frequency,
they appear identical; same reactance, same inductance.

Not if I can measure both the magnitude and phase relationship of the
device.

If I can only measure the magnitude of impedance at one frequency, I
can't even tell if the device is predominately inductive, capacitive
or resistive. So it would be a bit silly to call that magnitude an
inductive impedance.

And yet, at some other (lower) frequency, they will measure quite
differently. This is the basis for my observation that inductance does
indeed vary with frequency, based on the parasitic capacitance present
in all inductors.

Only because you are willing to confuse complex impedance with
inductive reactance.

And yes, if you can factor out the self-capacitance, then the inductance
would indeed be constant with frequency. The problem is, no one has
ever figured out how to do that with an actual coil. It can't be done.

You are projecting your limitations onto others.

--
John Popelish

On Sun, 07 Dec 2003 11:10:46 -0800, Bill Turner
wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

_________________________________________________ ________

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

How could you possibly define it otherwise?

I don't understand this all-important formula you keep quoting. Kindly
explain what "sub" is and clearly re-state the formua in unambiguous
terms.

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Sun, 07 Dec 2003 11:10:46 -0800, Bill Turner
wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

_________________________________________________ ________

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

How could you possibly define it otherwise?

I don't understand this all-important formula you keep quoting. Kindly
explain what "sub" is and clearly re-state the formua in unambiguous
terms.

--

"I expect history will be kind to me, since I intend to write it."
- Winston Churchill

On Sun, 07 Dec 2003 16:16:26 -0800, Bill Turner
wrote:

On Sun, 07 Dec 2003 21:35:22 GMT, John Popelish
wrote:

You are projecting your limitations onto others.

_________________________________________________ ________

I do have one limitation: I don't take insults from people I'm trying
to have a discussion with.

Bye.

--
Bill, W6WRT


Hello John, Hello Bill,
c'mon chaps, kiss and make up.
This sort of thing happens all the time.
Someone asks an innocent question and later on down
the discussion, two highly respected fellows fall out.
So sad, because readers like me and others, who are
trying to learn something, miss out when the discussion
stops because of a silly personal remark. What a pity! :-(
Regards,
John Crighton
Sydney

On Sun, 07 Dec 2003 16:16:26 -0800, Bill Turner
wrote:

On Sun, 07 Dec 2003 21:35:22 GMT, John Popelish
wrote:

You are projecting your limitations onto others.

_________________________________________________ ________

I do have one limitation: I don't take insults from people I'm trying
to have a discussion with.

Bye.

--
Bill, W6WRT


Hello John, Hello Bill,
c'mon chaps, kiss and make up.
This sort of thing happens all the time.
Someone asks an innocent question and later on down
the discussion, two highly respected fellows fall out.
So sad, because readers like me and others, who are
trying to learn something, miss out when the discussion
stops because of a silly personal remark. What a pity! :-(
Regards,
John Crighton
Sydney

Bill Turner wrote:

On 07 Dec 2003 18:25:51 GMT, (Avery Fineman) wrote:

Write on the whiteboard 100 times: Inductance does not change
with frequency...reactance changes with frequency.

__________________________________________________ _______

Not true. Inductance and reactance are related by the formula
XsubL = 2 pi F L. If XsubL has changed, then so has the inductance, and
vice versa.

Say what???? You have two variables that satisfy the equation:
XsubL and F
The equation does not mean that L varies!!!!!!!!!!!





How could you possibly define it otherwise?

--
Bill, W6WRT