Why no aluminum inductors?

[cheesehead]

If you use large inductors for audio, you'll be familiar with the insane prices now attached to most inductors - often $15 or more for a single 2MH inductor.

Why are'nt inductors made with aluminum? While a heavier guage of wire would be needed to make up for the higher resistance of aluminum over copper, the fact that aluminum costs a small fraction as much would make them way, way cheaper overall.

 

[PottedMeat]

Hmm good question - never thought of that.
Is aluminum less ductile than copper? Maybe it's more difficult to wind into tight coils.

Is it more difficult to solder to?

Is aluminum harder to coat with enamel?

There are some speakers I believe with aluminum voice coils.

 

[cheesehead]

Originally posted by: PottedMeat
Hmm good question - never thought of that.
Is aluminum less ductile than copper? Maybe it's more difficult to wind into tight coils.

Is it more difficult to solder to?

Is aluminum harder to coat with enamel?

There are some speakers I believe with aluminum voice coils.

Aluminum is more difficult to solder to - strictly speaking, you can't solder to it at all unless you've got some wacky polymer solder.

That said, it's not hard to just crimp ends on to aluminum. Or copper-plate it, which is so easy you can do it with refuse from the art-metal lab at my school. (And I mean things that were literally being thrown out.)

 

[krotchy]

inductor design is much more difficult than you might think. Its not just a wrap it around X times and you get X inductance for Z impedance.

I have actually hand wound inductors for a power inverter in a lab, and the calculations for an inductor are extremely complex, and rarely even work out that well in the end. Typically though the cost of the magnetic core is one of the most expensive parts of an inductor, much more then the copper usually.

Especially when you consider that the magnetic core of the circuit has to be chosen for the frequency of the inductors operation. Remember an inductor that is 2mH at 10KHz might be 1uH at 1Hz or 1MHz depending on the properites of it.

 

[cheesehead]

Originally posted by: krotchy
Especially when you consider that the magnetic core of the circuit has to be chosen for the frequency of the inductors operation. Remember an inductor that is 2mH at 10KHz might be 1uH at 1Hz or 1MHz depending on the properites of it.

Trust me - air-core inductors are pretty linear.

 

[dkozloski]

There is also the factor of "Q" or figure of merit of an inductor which is the inductance compared with intrinsic resistance. An aluminum coil would have greater resistance as aluminum is not as efficienct conductor as copper therefore the figure of merit would be lower. The answer is, of course, to use silver wire.

 

[cheesehead]

Originally posted by: dkozloski
There is also the factor of "Q" or figure of merit of an inductor which is the inductance compared with intrinsic resistance. An aluminum coil would have greater resistance as aluminum is not as efficienct conductor as copper therefore the figure of merit would be lower. The answer is, of course, to use silver wire.

I already said that. The easy answer is to just to use thicker copper.

IIRC, the price of scrap copper is $8.50 a pound, while scrap aluminum is about $1.50 a pound.

 

[f95toli]

Thicker copper won't work. The impedance of a wire depends on the frequency due to the Skin effect whcih can be important (or at least matter) even at audio frequencies.
Also, thicker wire means fewer turns/m which in turn means longer wire and bigger inductors.

Finally, no one would buy them. Audio (hifi) is a business where people are willing to pay large amounts of money for just about everything that they THINK can make a difference, the more exostic the better.
Any manufacturer that tries to reduce cost by "cutting corners" in an "obvious" way by using cheaper materials won't be able sell much.

 

[cheesehead]

Originally posted by: f95toli
Thicker copper won't work. The impedance of a wire depends on the frequency due to the Skin effect whcih can be important (or at least matter) even at audio frequencies.
Also, thicker wire means fewer turns/m which in turn means longer wire and bigger inductors.

Finally, no one would buy them. Audio (hifi) is a business where people are willing to pay large amounts of money for just about everything that they THINK can make a difference, the more exostic the better.
Any manufacturer that tries to reduce cost by "cutting corners" in an "obvious" way by using cheaper materials won't be able sell much.

One word: Gainclone.

Look it up.

 

[f95toli]

I know what a Gainclone is (I started building one years ago but, as usual, never got round to finishing it).

I am not sure why Gainclone(s) would be relevant to the discussion. I assume you are refering to the fact that they are quite popular while still being cheap to build. However, AFAK the original Gainclone was a clone of Gaincard which was a commercial product and very expensive.
Moreoever, if you look at various DIY sites you will find plenty of people who have spent a small fortune on Black Gate capacitors, ALPS pots, silve-in-telfon wires and other very expensive components when they built their gainclones.

 

[ScottMac]

I'm not sure how well it would work ... aluminum is "reluctant" to electromacnetic fields.

In seventh grade electronics class, we had a demonstration with an electromagnet which had an iron bar standing on it. With the electromagnet energized, an aluminum ring was placed on the bar, encircling it with just enough clearance for the bar to fit inside the opening of the ring.

The aluminum ring "floats" on the bar ... if you force it to the bottom, it heats up ... it heats up alot. If you push the ring to the bottom and let it go, it flys off the bar (ours hit the ceiling) It was explained that the aluminum is more reluctant (to the electromagnetic field) than the air is (ala path of least resistance).

I believe this is also the foundational operation of the "Star Wars" program's rail gun. It uses electromagnetic fields to drive an aluminum puck to some pretty fantastic speeds.

So, while I have no idea as to efficiency of using aluminum for the coils of an inductor, I believe that putting aluminum in captive proximity to another fairly strong electromagnetic field would generate a dangerous amount of heat, and probably wouldn't work very well, if at all.

Conducting electricity is one thing, accepting a magnetic field is something else.

FWIW

Scott

 

[gsellis]

Also, Al has oxzidation issues beside being +3 vs +1 and hating to let those electrons go.

 

[highwire]

For many year aluminum has been used in preference to copper for high voltage transmission lines because Al is more conductive per pound than Cu as well as lower cost per pound. There is, I believe, a recent widening in price per pound, also. So, that will lead to even more use of aluminum where normal copper conductor had been used.

However, in electromagnetic applications ie, inductors, transformers, motors, solenoids, increased conductor size is a very unfavorable factor for aluminum, since it will cause the whole magnetic structure to increase in size and weight. So, here, aluminum is not going to replace copper as easily for this fundamental reason.

But, don't bet aluminum wound audio inductors won't happen as copper chases silver in the metals market.

 

[BrownTown]

just a quick look in the internets shows that people do in fact manufacture aluminum inductors, but clearly they are not the dominate part.

 

[Howard]

AFAIK, the inductance of an inductor is due only to its geometry (number of turns and such).

Originally posted by: f95toli
Thicker copper won't work. The impedance of a wire depends on the frequency due to the Skin effect whcih can be important (or at least matter) even at audio frequencies.
Also, thicker wire means fewer turns/m which in turn means longer wire and bigger inductors.

Finally, no one would buy them. Audio (hifi) is a business where people are willing to pay large amounts of money for just about everything that they THINK can make a difference, the more exostic the better.

Not all hifi, just audiophilia.

Skin effect is negligible up until the very high treble.

 

[f95toli]

Originally posted by: Howard

Not all hifi, just audiophilia.

Skin effect is negligible up until the very high treble.

If someone is building a speaker and is even considering using an air-core inductor I would say that are probably at least trying a build something that would be considered hifi. In a mid- or low-fi speaker you will quite often find iron-core (or some other material o increase mu) inductors.

 

[Howard]

Originally posted by: f95toli

Originally posted by: Howard

Not all hifi, just audiophilia.

Skin effect is negligible up until the very high treble.

If someone is building a speaker and is even considering using an air-core inductor I would say that are probably at least trying a build something that would be considered hifi. In a mid- or low-fi speaker you will quite often find iron-core (or some other material o increase mu) inductors.

Sure, using air core inductors is sensible, but that has nothing to do with people spending ridiculous amounts of money on things that will make no practical, audible difference.