--- In feet_forward@yahoogroups.com, "Arthur Middleton" <artmidd@...> wrote:

snip

> It's application specific whether brushed or brushless is superior. I forget
> the details.
>
> Arthur.
>

I was not aware that DC motors could be truly brushless.

AC motors are usually brushless (induction motors).

VSD's used in industry all start with AC (1 or 3 phase) rectify to produce a DC
bus (often several hundred volts) then build a 3 phase output using power
electronics to generate the 3phase output at a varaible voltage and frequency.

The way that brushless DC motors in model aircraft work is as follows:
First the motor is turned inside out. The wound 'rotor' is stationary (hence
becomes the stator). The 'stator' - the outer casing including bonded rare earth
magnets, rotates.

This approach is ok in a model, but there is no housing stopping contact with
the moving parts.

The second innovation in the new 'brushless DC' motors is that they aren't
really DC.

They work similarly to the above description of as industrial VSD except instead
of producing a 3 phase AC waveform (3 sine waves 120 degress out of phase to
induce a rotating magntic field) they simulate 3 sine waves by chopping the DC.

The motor has 3 leads - 3 phases. The waveform per phase switches between full
battery voltage and 0 volts. The pulse width is modulated from minimum to
maximum in a sinusoidal manner. The three pseudo sine waves are 120 degrees out
of phase

Since permanent magnets are used, the motors are syncronous, rather than relying
on slip to cut the lines of flux.


The above suggest to me the following reasons this technology may not be
scalable:

1) The use of permanent magnets - heavy and expensive. Typically larger motors
use windings rather that magnets.
2) Industral VSDs generate sine wave AC rather than chopping DC (I beleive).
This suggests that short cuts are taken for models to keep down the size, weight
and cost on the speed controllers - at the expense of flying time.

The speed controllers in models run very hot. Mine is the size of a postage
stamp and about 8 mm think including the heat sink. It peaks out at in excess of
300W from a 11.1 V 3 cell lipo

As Royce says - you need a good electrical engineer if you want to get an entry
to the start line.

No point with a good Agni motor if your speed controller is inefficient.

Peter