# [amsat-bb] Torque Coil turns vanish?

Thomas Doyle tomdoyle1948 at gmail.com
Tue Oct 28 13:52:44 UTC 2014

```Bob,

Often the simplest things turn out to be the most interesting. Your post
reminded me of something a bit similar that happened while I worked for
Motorola. There was 450MHz mobile radio in the product line that had 3.5
Watts of audio power for the speaker. The brain trust in the marketing
re-design the audio section. The design had been optimized for maximum
efficiency in the small space available for the speaker but none the less

The speaker had to be re-designed - the wire used for the voice coil was
too small to handle the 5 watts. When the wire size was increased to handle
the increased current there was room for fewer turns of wire in the voice
coil. The end result was the radio now had 5 watts of audio power but
actually generated several dB lower sound levels than the previous
efficient design with 3.5 watts of audio power. Marketing was happy - we

W9KE tom...

On Mon, Oct 27, 2014 at 3:35 PM, Robert Bruninga <bruninga at usna.edu> wrote:

> I keep coming up with the number of turns of a torque coil vanishing with
> respect to torque (for a given wire size).
>
>
>
> Given that Torque = A x B x I x n where A is area, B is field, I is current
> and n is number of turns.
>
> But I is inversely proportional to the length of wire (resistance). I = V/R
> but R is proportional to n.
>
> Hence the number of turns.  So I is proportional to  V/n.
>
>
>
> But V is fixed in the cubesat, (5v) area is fixed (where we can put the
> coil) and so plugging in,
>
> T = A x B x V/n x n and the n’s cancel.
>
>
>
> So I can get the same torque with –any- number of turns for a given size
> wire.
>
>
>
> What changes with turns, of course, is the current.  So to save spacecraft
> power, the more turns, the less current, and thus, the least power for the
> same torque.
>
> Interesting result.
>
>
>
> We ended up with 600 turns of #34 wire to keep the current at 25 mA so we
> can drive it directly from a CPU pin.
>
>
>
> I wonder where’s the limit?  Infinitie number of turns… Zero current?  I
> guess you run out of space.
>
>
>
> AH HA!  Yes, that was it.  We started with #30 wire but it took so many
> turns to get to below the 25 mA, that there wasn’t room for the coil.  So
> we went to smaller wire and ended up with the #34 to give us a ¼” thick
> coil with 600 turns to give 25 mA.
>
>
>
> But I just always found it interesting that once you have chosen the wire
> size, the number of turns is only determined by the current you want.  The
> Torque is constant.
>
>
>
> Bob, WB4APR
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--

Sent from my computer.

tom ...
```