Pierce Nichols wrote:

>         Because there is a concept, called a magic sinewave, that allows
> us to get very low distortion power sinewaves (3-phase, too), but it is
> new enough not be in any commercial products. We have a DAC PWM DC speed
> control now. We think we can build a very good magic sinewave inverter
> with these magic sinewaves and get better efficiency and lower motor
> weight.

First a question, do you need DC speed control or an amplitude
controlled sine wave? I think the former, but just in case...
Count on dedicating one PIC for the magic sinewave synthesizer.
Of all the possible bit streams for a given sequence of bits, the
number of unique amplitude controlling low order harmonic distorting
ones is limited. Your alternatives are to 1) live with limited
amplitude control 2) allow more lower order harmonics 3) Increase
the length of the bit stream 4) maybe someone else has a fourth
reason (how about get a Scenix chip then you can have magic software
running on magic hardware).


I would really like to see how much more efficient the "magic"
sine waves are compared to the less ethereally PWM generated ones.
Has anyone done the arithmetic, or perhaps less demanding, has
anyone else even thought about it (or even less demanding does any
one else give a damn)? The number of transitions over a fixed time
interval is lower for the magic sine waves. However, the intermediate
harmonics are higher. Let me quantify: with magic you can exactly cancel
all of the even, and the third, fifth, seventh, and perhaps ninth
harmonics. However, the 11th, 13th, and etc. are relatively large
(please excuse the vagueness). Now with PWM for the same frequency
sine wave there might be a residual 2nd, 3rd harmonic, etc. but the
amplitude doesn't increase until you approach the carrier frequency.

BTW, here's a fairly decent tutorial on PWM motor control:
http://www.argonet.co.uk/education/4qd/ccts/pwm.html


Scott