Aluminium bobbin for choke. Is this a common practice?

[John Crighton]

Hello All,
How common are Aluminium bobbins for 50/60 Hz
chokes and inductors? This is the first one I have seen.

I was at a ham radio field day recently and picked
up a heavy duty coil which I thought might make
a good choke for small home made rectifier unit
for a small welder. I will add the iron core when I
find some laminations.

The winding is aluminium strip 0.7 inches by 0.1inches.
There are two strips of aluminium making up the 0.1 inch
thickness. The strips are covered with some sort of
fabric tape hand wound over the two strips.
The whole winding is taped and painted so I can't see
how many turns.

The hole in the bobbin measures just over 4 inches by 1 inch.
The bobbin is 3.3 inches wide.

I didn't notice, since the whole coil assembly is covered
in thick grey paint, but the bobbin is made of aluminium.
I just assumed the bobbin would have been made of
some sort of insulating material such as fibre glass or
paxolin. An aluminium bobbin was a surprise to me.
Is this a common practice to make heavy duty robust
bobbins for low voltage applications out of Aluminium?

Regards,
John Crighton
Sydney

 

[Schweinkolben]

It is difficult to get a good idea of your actual problem without photos or
diagrams. Unless I am missing something, aluminum bobbins for coils are
going to be trouble. They are shorted turns. Thus, you choc will turn into a
transformer with the number of turns wound on the bobbin being the number of
primary turns. You will have a single secondary turn that is shorted. The
coupling coefficient may be small, but it is a transformer nevertheless.

Bill

It doesn't sound like a choke to me.
Al is a such a pain to machine or cast, ($$) plastic is widely used.
need a pix.

 

[daestrom]

It is difficult to get a good idea of your actual problem without photos
or
diagrams. Unless I am missing something, aluminum bobbins for coils are
going to be trouble. They are shorted turns. Thus, you choc will turn into
a
transformer with the number of turns wound on the bobbin being the number
of
primary turns. You will have a single secondary turn that is shorted. The
coupling coefficient may be small, but it is a transformer nevertheless.

I agree. The only way an Al bobbin would work is if it has a slot cut down
the length of the hole and out the end faces to break the 'shorted turns'.
The OP didn't mention such a slot though. As is often the case, "a picture
is worth a thousand words".

daestrom

 

[--]

Hello All,
How common are Aluminium bobbins for 50/60 Hz
chokes and inductors? This is the first one I have seen.

I was at a ham radio field day recently and picked
up a heavy duty coil which I thought might make
a good choke for small home made rectifier unit
for a small welder. I will add the iron core when I
find some laminations.

The winding is aluminium strip 0.7 inches by 0.1inches.
There are two strips of aluminium making up the 0.1 inch
thickness. The strips are covered with some sort of
fabric tape hand wound over the two strips.
The whole winding is taped and painted so I can't see
how many turns.

The hole in the bobbin measures just over 4 inches by 1 inch.
The bobbin is 3.3 inches wide.

I didn't notice, since the whole coil assembly is covered
in thick grey paint, but the bobbin is made of aluminium.
I just assumed the bobbin would have been made of
some sort of insulating material such as fibre glass or
paxolin. An aluminium bobbin was a surprise to me.
Is this a common practice to make heavy duty robust
bobbins for low voltage applications out of Aluminium?

Regards,
John Crighton
Sydney

The coil appears to be a rather neat bit of creative engineering - better
and lower cost.

as to aluminum as the conductor and as the core

conductor -

1) The conductor in an inductor can be any material, since it is the
orientation of the current, not the wire material, that does the work. An
aluminum winding will work.

2) And if you wish to get the oxide deep enough, I think it aluminum oxide
is a fairly decent insulator. Aluminum welding machines use a high frequency
low current high voltage rider on the AC/DC power to punch through the
aluminum oxide for the low voltage power flow. It would have to be fairly
deep oxide, I think, architectural grade or better anodizing. The tape
would keep it from scratching both in handling and thermally.

3) Theory says the extruded rectangular shape you can have from aluminum
would let you get more field density than the round that copper is limited
in drawn wire, and the coil would be more efficient.

core -

4) As I recall, aluminum is what is called paramagnetic (weakly magnetic).
Where iron has a lot of its electrons oriented so as to spin in one
direction over the other which then makes it magnetic, I think aluminum,
nickel, cobalt, and a couple others have a few more spinning on one
direction than the other, and they are called paramagnetic. (I would double
check it in the book, but my kid borrowed it, and he has the book with the
info in it over at the U)

5) plastic in a coil ain't necessarily air-equal plastic as much as it can
be various levels of ferrite imbedded in the plastic matrix- some probably
akin to the levels of magnetism of paramagnetic aluminum -
remember that there is more available to fill a core shunt than the yes
of iron and the no of air - including iron-aluminum "alloys".

putting a magnetic material in the core of a coil interferes with the
magnetic field in that it :
-changes the coil's inductance according to the current carried and actually
drops inductance to zero at core saturation, It probably takes a LOT of
field to saturate a paramagnetic core
-decreases it to zero at certain frequencies so as to allow passage of those
frequencies otherwise blocked by the coil,
-severely drops it in the proximity of the coil (so tesla coils, sensors,
etc. use non-magnetic/air cores) ,
adds distortion and harmonics,
reduces Q,
lowers efficiency,
reduces power handling,
gives you the same inductance with fewer turns,
lowers evil self-resonance,
lower interference from stray field pickup,
and
it increases density near the coil proper.

Aluminum in a core will interfere less than iron, iron either as ferrite
(oxidized) or as element. And andozied deep enough and not scratched, I am
pretty sure it is non-conducting. And it can be extruded into shapes whose
edges leave no gaps.

a fairly neat bit of engineering.

 

[Schweinkolben]

The coil appears to be a rather neat bit of creative engineering - better
and lower cost.

as to aluminum as the conductor and as the core

conductor -

1) The conductor in an inductor can be any material, since it is the
orientation of the current, not the wire material, that does the work. An
aluminum winding will work.

AL losses are higher than copper, and it is the material that conducts the
current.

2) And if you wish to get the oxide deep enough, I think it aluminum oxide
is a fairly decent insulator. Aluminum welding machines use a high frequency
low current high voltage rider on the AC/DC power to punch through the
aluminum oxide for the low voltage power flow. It would have to be fairly
deep oxide, I think, architectural grade or better anodizing. The tape
would keep it from scratching both in handling and thermally.

Deep oxide is brittle and causes fractures on AL wire. If you bend it
slightly, it breaks through the surface.

3) Theory says the extruded rectangular shape you can have from aluminum
would let you get more field density than the round that copper is limited
in drawn wire, and the coil would be more efficient.

Ditto Copper. However extruding AL is more difficult that Cu, Cu is more
malleable.

core -

4) As I recall, aluminum is what is called paramagnetic (weakly magnetic).

Weakly anti magnetic. It causes L to decrease as it is put further into the
core. (at high F)
At low Frequency it is "transparent" to magnetic fields.

Where iron has a lot of its electrons oriented so as to spin in one
direction over the other which then makes it magnetic, I think aluminum,
nickel, cobalt, and a couple others have a few more spinning on one

nickel is magnetic.


need a pix of the gizmo.

 

[John Crighton]

need a pix of the gizmo.

Hello All,
the best I can do for a picture is to refer you to this site
http://website.lineone.net/~dave.cushman/transformers.html

Half way down the web page there is a couple of pictures
under Coil Formers or Bobbins.

My coil former looks like the one on the left hand side.
On my former, the side cheeks are made of thin aluminium sheet.
The dark orange centre piece is again made of thin sheet
aluminium (not an extrusion) with folded up 3/4 inch tabs to
hold the cheeks in place. To stop the cheeks falling off the
edge of the centre dark orange section.

Just thinking about this coil. The fact that there was no
iron core or scratch marks on the paint, it probably was
never used or intended to be used for an iron core choke.
The bobbin would be a shorted turn like you guys said.
Not a good idea.
I think someone has made this coil up and then
realised, ooops! metal bobbin, scrap it!

When I find some laminations to fit the 4" by 1" slot
in the bobbin, I will experiment with the aluminium former
and without it just for fun.
All the hard work of winding the heavy conductor has
been done, so tape alone will hold it together. I can
still use the coil that way with a regular iron core.

Thanks for all your replies, and discussion.
Regards,
John Crighton
Sydney

 

[Schweinkolben]

Hello All,
the best I can do for a picture is to refer you to this site
http://website.lineone.net/~dave.cushman/transformers.html

Half way down the web page there is a couple of pictures
under Coil Formers or Bobbins.

My coil former looks like the one on the left hand side.
On my former, the side cheeks are made of thin aluminium sheet.
The dark orange centre piece is again made of thin sheet
aluminium (not an extrusion) with folded up 3/4 inch tabs to
hold the cheeks in place. To stop the cheeks falling off the
edge of the centre dark orange section.

Just thinking about this coil. The fact that there was no
iron core or scratch marks on the paint, it probably was
never used or intended to be used for an iron core choke.
The bobbin would be a shorted turn like you guys said.
Not a good idea.
I think someone has made this coil up and then
realised, ooops! metal bobbin, scrap it!

When I find some laminations to fit the 4" by 1" slot
in the bobbin, I will experiment with the aluminium former
and without it just for fun.
All the hard work of winding the heavy conductor has
been done, so tape alone will hold it together. I can
still use the coil that way with a regular iron core.

Thanks for all your replies, and discussion.
Regards,
John Crighton
Sydney

The strange part is the AL wiring on it. Seems like 1960's vintage.
They used AL wiring in houses back then for a while
It could be made to get hot and stay that way, and plastics will flow over
time.
But it should work fine with 50/60 Hz. (if the coils are not shorted).
Some big equipment uses transformers and chokes (weigh 25 to 300 lbs. each)
thick painted gloss black, for vibration tables, AM radio stations, some
medical equipment, etc.

 

[--]

AL losses are higher than copper, and it is the material that conducts the
current.

but in the small amount used in a coil, that difference would be
negligible.

Deep oxide is brittle and causes fractures on AL wire. If you bend it
slightly, it breaks through the surface.

true about the oxide being somewhat brittle - I can't speak to fractures
being caused by oxides on round wire and would imagine that would be
possible, given the outer fiber deflection from the round shape turned in
bending, but i find that unlikely on rectangular pieces, since the outer
fiber stresses have more area to relax into in a rectangle than on a round.
We never had any problem with fractures when anodizing and (generous
radii) bending any of our rectangular architectural or exterior structural
aluminum - mostly 6xxx and 5xxx series ( our sister companies were anodizing
and extruding facilities).

Ditto Copper. However extruding AL is more difficult that Cu, Cu is more
malleable.

Extruding aluminum is fairly easy and a common practice, in my experience.
I have had, and seen, many complex shapes with sharp corners extruded.
I was under the impression from our sister company that copper would not
hold a sharp corner coming out of the dies like aluminum does.

magnetic).

Book is back from the U -
According to Halliday, it is paramagnetic.
also -

http://www.rare-earth-magnets.com/magnet_university/magnetic_levitation.htm

lists it as a paramagnetic material

Weakly anti magnetic. It causes L to decrease as it is put further into the
core. (at high F)

Being repelled by a magnet is referred to as diamagnetic in the physics
text. It is present in all substances. See also the link above.

At low Frequency it is "transparent" to magnetic fields.


nickel is magnetic.

I stand corrected - iron, cobalt, nickel, gadollinium, and dysprosium are
all ferromagnetic elements.

 

[Roy Q.T.]

you know Aluminum:

how long have you had them ?

An Aluminum bobbin may not do much under Normal Current conditions as
you there suspect., but, betcha if you could keep boosting the
ƒrequency to higher levels regardless Vin & control of Aout the
circuit you'd get some notable results ....

aren't those used in signal buzzers?

 

[Roy Q.T.]

Try Your Phyics: aluminum has an explicit conductivity.

you're to lost in electrical eng if you dare state it (Al) has no
conductivity, perhaps you're taking the Clear Gel Subcoating of your
sample for a given in your field.,

[a boost in ƒ and you'll get the coil to sock or arc as customary]

pierce the sample and you'll differ such utterances....

Current: it will seem to stall under superfluos superresistive
conditions but, believe it., it's there; perhaps in {another dimension
or} frequency but it'll always be there if Voltage is applied.

sheesh how can he have said it doesn't conduct ....?


PARENTS: KEEP YOUR KIDS AWAY FROM THESE GROUPS !!!

IF YOU'RE [not] A KID
take care...this is a place where serious harm may result from misguided
handling of products & information.

 

[--]

I am not sure to whose response you are posting

Try Your Phyics: aluminum has an explicit conductivity.



you're to lost in electrical eng if you dare state it (Al) has no
conductivity, perhaps you're taking the Clear Gel Subcoating of your
sample for a given in your field.,

[a boost in f and you'll get the coil to sock or arc as customary]

pierce the sample and you'll differ such utterances....

Current: it will seem to stall under superfluos superresistive
conditions but, believe it., it's there; perhaps in {another dimension
or} frequency but it'll always be there if Voltage is applied.

sheesh how can he have said it doesn't conduct ....?


PARENTS: KEEP YOUR KIDS AWAY FROM THESE GROUPS !!!

IF YOU'RE [not] A KID
take care...this is a place where serious harm may result from misguided
handling of products & information.

 

[--]

Try Your Phyics: aluminum has an explicit conductivity.

electrical or magnetic?

1) it does not conduct magnetic fields, although it is relatively permeable
to magnetic fields

2). Of course aluminum is an electrical condcutor . Aluminum oxide,
however -

you're to lost in electrical eng if you dare state it (Al) has no
conductivity, perhaps you're taking the Clear Gel Subcoating of your
sample for a given in your field.,

to what are you referring?

[a boost in f and you'll get the coil to sock or arc as customary]

pierce the sample and you'll differ such utterances....

Current: it will seem to stall under superfluos superresistive
conditions but, believe it., it's there; perhaps in {another dimension
or} frequency but it'll always be there if Voltage is applied.

sheesh how can he have said it doesn't conduct ....?


PARENTS: KEEP YOUR KIDS AWAY FROM THESE GROUPS !!!

IF YOU'RE [not] A KID
take care...this is a place where serious harm may result from misguided
handling of products & information.

that's for damn sure

 

[Roy Q.T.]

I Exaggerated about the Parental Guidance,

NOTE: Aluminum is not as impervious to Magnetism as suggested, (is
Always a Conductor) in fact it has a low reluctance to magnetism, but,
at High [ƒ] DC Voltages & Currents it has an Electro/Physical
("Magnetic Flux") Effect of it's own ....I believe it's measured in
Gauss as well.
perhaps a bit off the charts.

Ever heard of the term: Ion Wind