Magnetic path length of regular old bar core inductor?

I've scoured the interwebz for information on inductor design. I want
to construct an inductor by basically wrapping wire around a round bar
of iron. My problem? What value do I use for magnet path length in
my calculations? I assume since the flield will be exiting the bar
ends and wrapping around in air back to the opposite pole it would not
be as simple as the length of the bar.

Every design example I've run across assumes a closed box section core
or torroid. What ever happened to a simple bar core inductor?

Any useful web links or information you could provide would be useful.

Thank you,

Gerb

 

[christofire]

<Gerbermultit00l@gmail.com> wrote in message
news:fbcd434d-28b5-41fe-b55f-ea14f689e726@g17g2000prg.googlegroups.com...

I've scoured the interwebz for information on inductor design. I want
to construct an inductor by basically wrapping wire around a round bar
of iron. My problem? What value do I use for magnet path length in
my calculations? I assume since the flield will be exiting the bar
ends and wrapping around in air back to the opposite pole it would not
be as simple as the length of the bar.

Every design example I've run across assumes a closed box section core
or torroid. What ever happened to a simple bar core inductor?

Any useful web links or information you could provide would be useful.

Thank you,

Gerb

This is explained in books such as 'Electromagnetics' by John Kraus. A
library could be a good bet - perhaps a bit more certain and thoroughly
explained than the opinion-net.

Chris

 

[Jon Slaughter]

<Gerbermultit00l@gmail.com> wrote in message
news:fbcd434d-28b5-41fe-b55f-ea14f689e726@g17g2000prg.googlegroups.com...

I've scoured the interwebz for information on inductor design. I want
to construct an inductor by basically wrapping wire around a round bar
of iron. My problem? What value do I use for magnet path length in
my calculations? I assume since the flield will be exiting the bar
ends and wrapping around in air back to the opposite pole it would not
be as simple as the length of the bar.

Every design example I've run across assumes a closed box section core
or torroid. What ever happened to a simple bar core inductor?

Any useful web links or information you could provide would be useful.

Thank you,

Gerb

http://en.wikipedia.org/wiki/Reluctance

 

[christofire]

<Gerbermultit00l@gmail.com> wrote in message
news:5cad1fd0-eb88-43c3-a4b9-ce98f300cc7d@z18g2000prn.googlegroups.com...
On Oct 7, 5:07 pm, "Jon Slaughter" <Jon_Slaugh...@Hotmail.com> wrote:

Gerbermultit...@gmail.com> wrote in message

news:fbcd434d-28b5-41fe-b55f-ea14f689e726@g17g2000prg.googlegroups.com...

I've scoured the interwebz for information on inductor design. I want
to construct an inductor by basically wrapping wire around a round bar
of iron. My problem? What value do I use for magnet path length in
my calculations? I assume since the flield will be exiting the bar
ends and wrapping around in air back to the opposite pole it would not
be as simple as the length of the bar.

Every design example I've run across assumes a closed box section core
or torroid. What ever happened to a simple bar core inductor?

Any useful web links or information you could provide would be useful.

Thank you,

Gerb

http://en.wikipedia.org/wiki/Reluctance

Thank you for the informative reading materials. However, I still did
not find what I am looking for. However, I think I have my answer if
my assumptions are correct.

May I assume that I will not gain much benefit of using a core by
winding wire around a solid flat bar core that is open at each end?
Am I correct in assuming this essentially will be equivalent to an air
core inductor since the magnetic path is not closed? I've read that
some improvement in inductance occurs by wrapping wire around a bar
but nothing quantifies this or offers a means to calculate.

Thank you,

Gerb


.... sorry for the repetition but my recommendation remains to a library and
read one of the many decent books on electromagnetics.

The answer for a solenoid much longer than it is wide is L = (mu of core) x
(number of turns)^2 x (cross-section area of solenoid) / (length of
solenoid) according to Kraus and Fleisch, Electromagnetics with
applications, ISBN 0-07-116429-4, Page 91

.... which is proportional to the permeability of the core, but if you want
or care to know why then this will require a small amount of study. If you
accept answers handed to you on a plate from sources of unknown quality then
you can probably expect to be in error at least half the time, so don't
trust what I've written, read the book.

Chris

 

On Oct 7, 5:07 pm, "Jon Slaughter" <Jon_Slaugh...@Hotmail.com> wrote:

Gerbermultit...@gmail.com> wrote in message

news:fbcd434d-28b5-41fe-b55f-ea14f689e726@g17g2000prg.googlegroups.com...

I've scoured the interwebz for information on inductor design.  I want
to construct an inductor by basically wrapping wire around a round bar
of iron.  My problem?  What value do I use for magnet path length in
my calculations?  I assume since the flield will be exiting the bar
ends and wrapping around in air back to the opposite pole it would not
be as simple as the length of the bar.

Every design example I've run across assumes a closed box section core
or torroid.  What ever happened to a simple bar core inductor?

Any useful web links or information you could provide would be useful.

Thank you,

Gerb

http://en.wikipedia.org/wiki/Reluctance

Thank you for the informative reading materials. However, I still did
not find what I am looking for. However, I think I have my answer if
my assumptions are correct.

May I assume that I will not gain much benefit of using a core by
winding wire around a solid flat bar core that is open at each end?
Am I correct in assuming this essentially will be equivalent to an air
core inductor since the magnetic path is not closed? I've read that
some improvement in inductance occurs by wrapping wire around a bar
but nothing quantifies this or offers a means to calculate.

Thank you,

Gerb

 

[Jon Slaughter]

<Gerbermultit00l@gmail.com> wrote in message
news:5cad1fd0-eb88-43c3-a4b9-ce98f300cc7d@z18g2000prn.googlegroups.com...
On Oct 7, 5:07 pm, "Jon Slaughter" <Jon_Slaugh...@Hotmail.com> wrote:

Gerbermultit...@gmail.com> wrote in message

news:fbcd434d-28b5-41fe-b55f-ea14f689e726@g17g2000prg.googlegroups.com...

I've scoured the interwebz for information on inductor design. I want
to construct an inductor by basically wrapping wire around a round bar
of iron. My problem? What value do I use for magnet path length in
my calculations? I assume since the flield will be exiting the bar
ends and wrapping around in air back to the opposite pole it would not
be as simple as the length of the bar.

Every design example I've run across assumes a closed box section core
or torroid. What ever happened to a simple bar core inductor?

Any useful web links or information you could provide would be useful.

Thank you,

Gerb

http://en.wikipedia.org/wiki/Reluctance

Thank you for the informative reading materials. However, I still did
not find what I am looking for. However, I think I have my answer if
my assumptions are correct.

May I assume that I will not gain much benefit of using a core by
winding wire around a solid flat bar core that is open at each end?
Am I correct in assuming this essentially will be equivalent to an air
core inductor since the magnetic path is not closed? I've read that
some improvement in inductance occurs by wrapping wire around a bar
but nothing quantifies this or offers a means to calculate.

========

the magnetic path takes the path of least resistance. If you have an open
bar then the magnetic field will have to travel back through the air to
complete the closed path. If the bar is closed then it doesn't have to.

You need to add the reluctances up of each path to get the total reluctance.
Reluctance is equivalent to resistance in electricity.

The problem with a bar is to determine the path through the air. Suppose you
use a closed core but then remove just a small slice. In the slice it is air
so you compute it's reluctance and add it to the reluctance of the other.

i.e.,

R1 = (l - dx)/u0/ur1/A and R2 = dx/u0/ur2/A

Although A in R2 is an approximation since it will actually be slightly
larger since the field will fringe out a bit.


But with a straight bar you end with dx being large and uncertain along with
A not being known. The reason being is that the field will "spread out"
throughout the air.

So we have something like

R1 = L1/u0/ur1/A1 and R2 = L2/u0/ur2/A2

and the total reluctance is R1 + R2. Determining R2 theoretically is
difficult but you can easily measure it since R1 is known.

i.e.

R = R1 + R2 = mmf/phi

Now if your really serious you could solve Maxewells equations to get the
solution but seems like a lot of work than just measuring it. (I'd imagine
that someone had to have solved the thing before)

 

[John Larkin]

On Wed, 8 Oct 2008 15:56:04 +0100, "christofire"
<christofire@btinternet.com> wrote:

Gerbermultit00l@gmail.com> wrote in message
news:5cad1fd0-eb88-43c3-a4b9-ce98f300cc7d@z18g2000prn.googlegroups.com...
On Oct 7, 5:07 pm, "Jon Slaughter" <Jon_Slaugh...@Hotmail.com> wrote:
Gerbermultit...@gmail.com> wrote in message

news:fbcd434d-28b5-41fe-b55f-ea14f689e726@g17g2000prg.googlegroups.com...

I've scoured the interwebz for information on inductor design. I want
to construct an inductor by basically wrapping wire around a round bar
of iron. My problem? What value do I use for magnet path length in
my calculations? I assume since the flield will be exiting the bar
ends and wrapping around in air back to the opposite pole it would not
be as simple as the length of the bar.

Every design example I've run across assumes a closed box section core
or torroid. What ever happened to a simple bar core inductor?

Any useful web links or information you could provide would be useful.

Thank you,

Gerb

http://en.wikipedia.org/wiki/Reluctance

Thank you for the informative reading materials. However, I still did
not find what I am looking for. However, I think I have my answer if
my assumptions are correct.

May I assume that I will not gain much benefit of using a core by
winding wire around a solid flat bar core that is open at each end?
Am I correct in assuming this essentially will be equivalent to an air
core inductor since the magnetic path is not closed? I've read that
some improvement in inductance occurs by wrapping wire around a bar
but nothing quantifies this or offers a means to calculate.

Consider a long solenoid winding (ie, L/D is a big number). With no
core, the field from one turn fans out in space, and a tiny fraction
of that field passes through turns that are far away. But if you add
the core, the field from any given turn is forced to pass through all
the other turns. That will greatly increase the inductance.

http://www.elnamagnetics.com/library/rfant.pdf

There's an equation on page 7 and a corresponding graph. It looks like
L can be increased by values in the 100x to 1000x range for decent L/D
ratios.

John