[ASK] Pro & Con of rectangular shaped enameled copper wire ?

[BravoV]

Inductors using a rectangular shaped enameled copper wire, what are the pros & cons ?

Example, random image from google.

[KMoffett]

I'm thinking these were for induction furnaces for melting metal.  The rectangular cross section allows a very low resistance in a a very small space for passing a very high current.

Ken

[CatalinaWOW]

Moffett is right about that application.

Pros: Lower resistive loss within a given coil volume allowance if wrapped properly.  Cheaper than silver as solution to resistive loss problems.

Cons: More expensive conductor, more difficult to wind.  Can have "interesting" skin currents/arcing potential, particularly at high frequency.  With the exception of the inductive heating application, most uses I have seen are low frequency. Under 100 kHz.

[TimFox]

I used rectangular wire in a DC electromagnet to improve the fraction of the "coil" volume that was taken up with copper, and to improve the heat conduction out of the coil.  This was grad school, and I was lucky to find a surplus roll of insulated rectangular wire that was originally intended for a transformer.

[T3sl4co1l]

Not so much -- I never seen 'em used for induction heating.

Why?  No cooling!  The poor thing will melt, from the inside out, in under a minute (and that's at low power levels of several kW).

Square and rectangular tubing is used a lot, though: but not because of losses.  It's easier to fabricate.  You can chop a piece into segments by making miter cuts (sealed with silver solder).  It bends easily without kinking, even for fairly tight inside corners.

Indeed, the losses are higher.  Much higher!  For a coil with dimensions as pictured above, there would be extremely high eddy currents induced in the coil itself (the eddy currents flow across and around the conductor, while the load current flows lengthwise).  This forces the load current towards the inside corners (path of least impedance).  Indeed, the effect is so strong that the outside corners will carry opposing currents (going backwards!), if not as intense as the inside corners' current.

For any application where low losses are required (RF tank coils, most induction heating apps), round tubing is best, spaced with a pitch twice the tubing OD.  This reduces the eddy currents, caused by the turns being so close together, without reducing the overall inductance too much (because it is stretched, relative to having pitch as close to diameter as possible).

To put concrete numbers on it: the pictured coils might have a Q in the 20-50 range, depending on shape and frequency.  A loosely spaced coil of round wire/tubing will have Q ~ 100-200, all else being equal.  That's a big deal when you're resonating over 100kVAR!

They do work nicely for cored inductors, and lower power levels.  All the big name manufacturers have lines of inductors made this way: for example, Vishay IHLP series, lots of Bourns and Cooper (now Eaton) high current parts, Coilcraft high-current parts, etc.

And of them, the powdered iron types are actually molded around the winding, so the core material completely fills the gaps and crevices between turns.  You can't beat the amount of inductance available this way -- these things have dense ratings -- lots of inductance and amps in a small package.

The Q factor improves about proportional to permeability, so they don't get terribly hot in the process, either.  Add a permeability of 10, and Q shoots up from 20 to 200.  In practice, it's worse than that, because of geometry factors and core losses.  Typical Q for this type is 50-200: quite reasonable for a power conversion application that only needs to circulate 100s of VARs.

And what is square/rectangular wire most ideal for?  The applications that don't need to worry about skin and proximity effect: mains frequency transformers!  The high winding factor (most copper area in the winding space; packing factor) allows a good ~20% improvement in current capacity, over round wire, for a transformer of identical construction (same wire width, same number of turns).

Tim

[Richard Crowley]

JBL (and perhaps others?) used to boast about their edge-wound voice coils.

An edge-wound voice coil has an improved "space factor" compared to a multilayer round wire coil. Bell Labs claimed the advantage to be about 27% in their late 1920s literature. Their original 555W compression driver used rectangular aluminum wire of about .002" by .014" wound on edge to make the voice coil. An edge-wound coil is also stiffer mechanically and less prone to warping out of shape.

Maximizing the space factor would seem to be an important goal. Achieving high gap flux density is expensive, for one thing. Increasing the gap width a bit to accomodate a less space efficient coil would require quite a bit more magnet to restore the same flux density, as the reluctance of the air gap is a square of the width of the gap.

Having said all that, it happens that most of my all time favorite drivers, i.e. the Lansing and RCA prewar field coil theatre units, all have round wire voice coils; go figure. Rudy Bozak was a pioneer in fitting edge wound aluminum coils to Cinaudagraph cone drivers in the late 1930s. Edge wound coils didn't come into common use in cone drivers until after the war, when Western Electric's new line featured them and Jim Lansing used them in the new Altec and JBL designs.

Aluminum wire is used mostly in high frequency driver voice coils, due to its low weight. As I recall, aluminum wire of a given size has 70% of the conductivity of copper but only 1/3 the mass. Often the aluminum is clad with copper, about ten percent by cross section, to permit easier and more reliable connections to the voice coil leads.
http://www.audioheritage.org/vbulletin/showthread.php?22328-Edge-wound-market-hype

[CatalinaWOW]

JBL (and perhaps others?) used to boast about their edge-wound voice coils.

An edge-wound voice coil has an improved "space factor" compared to a multilayer round wire coil. Bell Labs claimed the advantage to be about 27% in their late 1920s literature. Their original 555W compression driver used rectangular aluminum wire of about .002" by .014" wound on edge to make the voice coil. An edge-wound coil is also stiffer mechanically and less prone to warping out of shape.

Maximizing the space factor would seem to be an important goal. Achieving high gap flux density is expensive, for one thing. Increasing the gap width a bit to accomodate a less space efficient coil would require quite a bit more magnet to restore the same flux density, as the reluctance of the air gap is a square of the width of the gap.

Having said all that, it happens that most of my all time favorite drivers, i.e. the Lansing and RCA prewar field coil theatre units, all have round wire voice coils; go figure. Rudy Bozak was a pioneer in fitting edge wound aluminum coils to Cinaudagraph cone drivers in the late 1930s. Edge wound coils didn't come into common use in cone drivers until after the war, when Western Electric's new line featured them and Jim Lansing used them in the new Altec and JBL designs.

Aluminum wire is used mostly in high frequency driver voice coils, due to its low weight. As I recall, aluminum wire of a given size has 70% of the conductivity of copper but only 1/3 the mass. Often the aluminum is clad with copper, about ten percent by cross section, to permit easier and more reliable connections to the voice coil leads.
http://www.audioheritage.org/vbulletin/showthread.php?22328-Edge-wound-market-hype

Excellent reminder that there are always a lot of factors that go into the "best" solution for any problem.

[T3sl4co1l]

Excellent reminder that there are always a lot of factors that go into the "best" solution for any problem.

Indeed -- note that my complaints of poor electrical performance, are easily overruled by the other requirements of a voice coil: light weight for one, and for that matter, poor efficiency is actually a desirable trait (since coil resistance leads to acoustic dampening).

Voice coils are also a low frequency application (as the wire is concerned), which puts them in the same regime as power transformers: better fill factor means more power in the same space!

Tim