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454Casull 17th January 2005 12:59 AM

How much flux do you lose if...
you cut a 0.1mm horizontal gap in the pole piece of a conventional motor /w undercut pole piece? Let's say it produces 0.7T in the coil gap - what will it be after you cut that thin slice?

EDIT: I'm assuming the effect will be the same no matter the height you make the cut, unless it's somewhere near the T-flange at the top (let's assume it isn't).

454Casull 17th January 2005 02:18 AM

Modeled it myself in FEMM. Preliminary results suggest a negligible (~1%) loss in flux density in the coil gap.

Filling the gap with Kapton MTB or a similar high-performance insulating film shouldn't change flux much, and so it looks like this is a viable method to reduce eddy current levels.

SmarmyDog 17th January 2005 03:08 AM

Interesting thought. Using a thin gap to break eddy currents is the theory behind using insulated laminations in transformers. I have always wondered how a speaker driver that used laminations instead of bulk iron/steel would perform. Keep us posted if you do any further experimentation…

-Casey Walsh

454Casull 17th January 2005 03:33 AM

A laminated iron pole piece would definitely be interesting.

AndrewT 17th January 2005 04:23 PM

i'm trying to get my head round your thoughts, unsuccessfully.
Could you explain further? The need and the desired result.
regards Andrew T.

DanWiggins 17th January 2005 04:38 PM

Laminations work really well...

We use them on lots of our drivers. Properly designed and built, there's really no appreciable loss of flux at all.

For the vertical cut (which you'd need for eddy currents; a cut parallel to the top plates won't reduce eddy curents in the pole) your loss in flux is roughly equivalent to the loss of surface area of the pole.

Dan Wiggins
Adire Audio

454Casull 17th January 2005 05:46 PM

Wow, that motor sure is real purty! :P What driver is that for, anyway?

DanWiggins 17th January 2005 05:56 PM

Just a sample for an OEM, showing some one of the cosmetic options we have. We can nickel plate (black or shiny), copper plate (up to 8mm thick if needed - great for poles!), or use a standard gold chromate.

In this case the copper plate is around 0.015mm, with a thin clear lacquer over that, just for cosmetic purposes.

Dan Wiggins
Adire Audio

SmarmyDog 17th January 2005 06:19 PM

Hi Dan:

It was great meeting you, Chris and David at CES!

This post just confirms that you folks at Adire Audio have waaaay too much fun! :D

-Casey Walsh

Joules 17th January 2005 06:26 PM

Allthough I have never read any technical discutions on this, as applied to speaker motors (meaning, I could be wrong), this is how I view the so called eddy current problems in pole pieces based on what I know about magnetic fields in conductors.
I don't think currents circulating around the circumfrence of the pole piece is the primary problem, although it may be one of them. The main problem as I see it, is where a line of magnetic flux exits the surface of the pole piece. When this line is pushed on by feilds from the voice coil it tries to move in the pole piece, there by reducing the pushing force to the cone and increasing distortion. Currents will circulate around a field line moving in a conductor. These currents "eddy currents" will try and resist the movement of this line. The higher the conductivity of the pole piece the greater these eddy curents and the greater the resistance to movement and the lower the condutivity of the pole piece the easyer to push around the flux lines. Steel is much more resistive than copper. So copper cladding the pole pieces would reduce "flux modulation" by increasing the eddy curents around each field line as they exit the pole piece.

Transformers use laminated cores because its easer to reverse magnetic field direction in a smaller piece of magnetic material than a larger one. I don't know what effect this would have on a pole piece that is supose to have a very solid and still field structure.

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