How can inductance related effects change AC resistance?

[G Patel]

Hello,

I am well aware of both the proximity effect and skin effect but never
really understood how these inductance/magnetic related effects are
used to subtract away from DC resistance to get the AC resistance of
conductors.

Is this purely a way to model it (easy way out) or am I missing
something? Are there more "accurate" models that treat these phenomena
for their true inductive/magnetic properties?

[I just can't wrap my mind around the fact that AC inductance is
changing AC resistance]

Email+NG responses will be appreciated.

 

[John Larkin]

Hello,

I am well aware of both the proximity effect and skin effect but never
really understood how these inductance/magnetic related effects are
used to subtract away from DC resistance to get the AC resistance of
conductors.

Is this purely a way to model it (easy way out) or am I missing
something? Are there more "accurate" models that treat these phenomena
for their true inductive/magnetic properties?

[I just can't wrap my mind around the fact that AC inductance is
changing AC resistance]

Email+NG responses will be appreciated.

Skin effect and proximity effect both steer AC current away from
regions of the wire. In the case of skin effect, magnetic fields near
the center of the wire oppose current trying to travel down the
center. So the current is crowded into the outer regions of the wire,
reducing the effective cross-section. Less copper, more resistance,
just as if you'd drilled out the wire, turning it into a thin-walled
tube.

John

 

[G Patel]

Hello,

I am well aware of both the proximity effect and skin effect but never
really understood how these inductance/magnetic related effects are
used to subtract away from DC resistance to get the AC resistance of
conductors.

Is this purely a way to model it (easy way out) or am I missing
something? Are there more "accurate" models that treat these phenomena
for their true inductive/magnetic properties?

[I just can't wrap my mind around the fact that AC inductance is
changing AC resistance]

Email+NG responses will be appreciated.

Skin effect and proximity effect both steer AC current away from
regions of the wire. In the case of skin effect, magnetic fields near
the center of the wire oppose current trying to travel down the
center. So the current is crowded into the outer regions of the wire,
reducing the effective cross-section. Less copper, more resistance,
just as if you'd drilled out the wire, turning it into a thin-walled
tube.

John

What I'm wondering is whether we are just using the REDUCED RESISTANCE
model to simplify the issue (instead of dealing with the inductance
head on).

Consider the following:


Say there is a series circuit like this...

---(100K resistor)---(1nH inductor)---(130K resistor)----

Obviously the 1nH inductor is of very little significance until we hit
very high frequencies... say we're operating at 10GHz... we can
consider the IMPEDANCE of the inductor as +j10 or we can get lazy and
just ignore the complex nature of the beast and treat it as a 10 ohm
resistor.


Is this similar to the way we treat Proximity and Skin effects or is
the "resistance reduction" model a more legitimate case for these
inductive phenomena?

 

[G Patel]

Hello,

I am well aware of both the proximity effect and skin effect but never
really understood how these inductance/magnetic related effects are
used to subtract away from DC resistance to get the AC resistance of
conductors.

Is this purely a way to model it (easy way out) or am I missing
something? Are there more "accurate" models that treat these phenomena
for their true inductive/magnetic properties?

[I just can't wrap my mind around the fact that AC inductance is
changing AC resistance]

Email+NG responses will be appreciated.

Skin effect and proximity effect both steer AC current away from
regions of the wire. In the case of skin effect, magnetic fields near
the center of the wire oppose current trying to travel down the
center. So the current is crowded into the outer regions of the wire,
reducing the effective cross-section. Less copper, more resistance,
just as if you'd drilled out the wire, turning it into a thin-walled
tube.

John

What I'm wondering is whether we are just using the REDUCED RESISTANCE
model to simplify the issue (instead of dealing with the inductance
head on).

Consider the following:


Say there is a series circuit like this...

---(100K resistor)---(1nH inductor)---(130K resistor)----

Obviously the 1nH inductor is of very little significance until we hit
very high frequencies... say we're operating at 10GHz... we can
consider the IMPEDANCE of the inductor as +j10 or we can get lazy and
just ignore the complex nature of the beast and treat it as a 10 ohm
resistor.

Correction: +j10 ==> +j20pi
10 ohm ==> 20pi ohm

 

[Ban]

Correction: +j10 ==> +j20pi
10 ohm ==> 20pi ohm

Well with 10GHz we do not use 100k resistors in the signal path, but maybe a
10 ohm resistor. Now be your j10 in series with it and what would be the
combined resistance? (14.14 ohms), and what would it be for 1GHz ? (10.05
ohms). Can you see the difference between a resistor and inductor or
capacitor.
Try to solve it for j62.83. The combined value is sqrt(X^2 + Y^2).

 

[John Woodgate]

I read in sci.electronics.design that G Patel <[email protected]>
wrote (in <[email protected]>) about
'How can inductance related effects change AC resistance?', on Fri, 22
Apr 2005:

I am well aware of both the proximity effect and skin effect but never
really understood how these inductance/magnetic related effects are
used to subtract away from DC resistance to get the AC resistance of
conductors.

They don't subtract, they ADD. Both effects act to reduce the
cross-sectional area of the wire that is available for carrying current.

 

[G Patel]

Well with 10GHz we do not use 100k resistors in the signal path, but maybe a
10 ohm resistor.

I gave you a hypothetical circuit with ideal components, so why are you
correcting me? There is a reason why I made the resistors very large.

Now be your j10 in series with it and what would be the
combined resistance? (14.14 ohms), and what would it be for 1GHz ? (10.05
ohms).

Wrong. The combined resistance in both cases is unaffected. Inductive
Reactance is orthogonal to Resistance, you can't add it like this. On
the other hand, the IMPEDANCE is a vector sum of the two.

Can you see the difference between a resistor and inductor or
capacitor?

Yes. There is a HUGE difference.

Try to solve it for j62.83. The combined value is sqrt(X^2 + Y^2).

Hmmm.......

Thanks for your insights.

 

[Pig Bladder]

Well with 10GHz we do not use 100k resistors in the signal path, but maybe
a 10 ohm resistor. Now be your j10 in series with it and what would be the
combined resistance? (14.14 ohms), and what would it be for 1GHz ? (10.05
ohms). Can you see the difference between a resistor and inductor or
capacitor.
Try to solve it for j62.83. The combined value is sqrt(X^2 + Y^2).

I think Jack the Giant Killer needs to take a course in Beanstalk Climbing.

 

[Ban]

I gave you a hypothetical circuit with ideal components, so why are
you correcting me? There is a reason why I made the resistors very
large.

Well if the resistors are large, they dominate and there is no significance.
Why do you place 2 resistors and the inductance in series? Is there a reason
not to add the resistors to 230k?

Wrong. The combined resistance in both cases is unaffected. Inductive
Reactance is orthogonal to Resistance, you can't add it like this.
On the other hand, the IMPEDANCE is a vector sum of the two.

You observed that well, it is Z impedance, and the vector is of the
calculated length. the current through it would be inverse to the scalar
value. If you have 230k the current is identical with or without the 1nH
even at 10GHz, with a 10 Ohms resistor that you wanted to take instead of
the j10 your Z will be measurable higher, the increase is several orders
higher.

What do you want to proof here, I do not understand what is your point? Why
did you make the resistors so large?

 

[Genome]

Hello,

I am well aware of both the proximity effect and skin effect but never
really understood how these inductance/magnetic related effects are
used to subtract away from DC resistance to get the AC resistance of
conductors.

Is this purely a way to model it (easy way out) or am I missing
something? Are there more "accurate" models that treat these phenomena
for their true inductive/magnetic properties?

[I just can't wrap my mind around the fact that AC inductance is
changing AC resistance]

Email+NG responses will be appreciated.

Being aware and understanding are, as you suggest, two different things.

Since I don't understand either I will put on my Barry hat explain things to
you.

I think you should ignore inductance, this is about resistance and magnetic
fields. Plus inductance is not changing AC resistance.

First one, as someone else has mentioned, is that the two effects increase
the AC resistance of your wire. To do the sums you have to look at the
current waveform and consider its AC and DC components. Then apply those
components to the AC and DC resistance of the wire to get the power losses.

Now, skin effect arises because the AC current in the wire due to the AC
electerons causes a varying magnetic field that forces the AC electerons to
the outside of the wire. It does not affect the DC electerons. It has
something to do with refractive indexes.

It was Nimitz who figured it out and he got a boat named after him.

Proximity effect is about the integral around a loop of thing that has to
sum to zero. That means that if you have one somewhere then you need -one
somewhere else and they add up with what was already there so you get twice
as much as what you had before. Power is a square term so that's four times
as much which is really bad.

Bernoulli explained this one and made the Chinese really good at table
tennis.

All quite simple really.

DNA

 

[John Larkin]

Hello,

I am well aware of both the proximity effect and skin effect but never
really understood how these inductance/magnetic related effects are
used to subtract away from DC resistance to get the AC resistance of
conductors.

Is this purely a way to model it (easy way out) or am I missing
something? Are there more "accurate" models that treat these phenomena
for their true inductive/magnetic properties?

[I just can't wrap my mind around the fact that AC inductance is
changing AC resistance]

Email+NG responses will be appreciated.

Skin effect and proximity effect both steer AC current away from
regions of the wire. In the case of skin effect, magnetic fields near
the center of the wire oppose current trying to travel down the
center. So the current is crowded into the outer regions of the wire,
reducing the effective cross-section. Less copper, more resistance,
just as if you'd drilled out the wire, turning it into a thin-walled
tube.

John

What I'm wondering is whether we are just using the REDUCED RESISTANCE
model to simplify the issue (instead of dealing with the inductance
head on).

Consider the following:


Say there is a series circuit like this...

---(100K resistor)---(1nH inductor)---(130K resistor)----

Obviously the 1nH inductor is of very little significance until we hit
very high frequencies... say we're operating at 10GHz... we can
consider the IMPEDANCE of the inductor as +j10 or we can get lazy and
just ignore the complex nature of the beast and treat it as a 10 ohm
resistor.


Is this similar to the way we treat Proximity and Skin effects or is
the "resistance reduction" model a more legitimate case for these
inductive phenomena?

Skin effect is not a lazy way to treat inductance. The resistance
increase due to skin effect is real... "real" in the mathematical
sense, not imaginary. It's not reactance, and it causes resonant
circuits to lose Q and wires to get hot.

And wire resistance *increases* due to skin effect.

John

 

[John Larkin]

Bernoulli explained this one and made the Chinese really good at table
tennis.

I played table tennis once with Lim Chui, an engineer with Simmonds
Precision who was, at the time, something like #12 in the world. Every
one of his serves had so much spin (we call it "English") that they'd
hit my paddle and go off at 90 degrees; I don't think I ever
successfully returned a ball. If he hit you with the ball (which he
did, often) it left a bruise.

Hey, he's still playing!

http://tabletennis.about.com/library/natt/bl-natt-SO-12.htm

http://www.usatt.org/news/2003_usopen_seniors.shtml

http://www.usatt.org/news/images/chui_hlava.JPG


At least I'm a better circuit designer than he is.

John

 

[Genome]

I played table tennis once with Lim Chui, an engineer with Simmonds
Precision who was, at the time, something like #12 in the world. Every
one of his serves had so much spin (we call it "English") that they'd
hit my paddle and go off at 90 degrees; I don't think I ever
successfully returned a ball. If he hit you with the ball (which he
did, often) it left a bruise.

Hey, he's still playing!

http://tabletennis.about.com/library/natt/bl-natt-SO-12.htm

http://www.usatt.org/news/2003_usopen_seniors.shtml

http://www.usatt.org/news/images/chui_hlava.JPG


At least I'm a better circuit designer than he is.

John

You should poof.

I spent one of my student placements with Yorkshire Television and was given
the job of trying out some microwave stuff for a cricket charity do at
Headingly in Leeds.

Mike Brierley, The English captain, was about and someone stuck the ball in
my hand.......

First bowl, and the wickets tumble.

He did not look very happy.

DNA

 

[John Woodgate]

I read in sci.electronics.design that Genome <[email protected]>

I spent one of my student placements with Yorkshire Television and was
given the job of trying out some microwave stuff for a cricket charity
do at Headingly in Leeds.

Mike Brierley, The English captain, was about and someone stuck the
ball in my hand.......

First bowl, and the wickets tumble.

He did not look very happy.

He probably was not used to underarm leg spin.

 

[Genome]

I read in sci.electronics.design that Genome <[email protected]>


He probably was not used to underarm leg spin.

I am suitably offended.

It was a proper one.

DNA