On Mon, 15 Sep 1997, Scott Dattalo wrote:

> First a question, do you need DC speed control or an amplitude
> controlled sine wave? I think the former, but just in case...

        Well, the former is what we have now. We want something better,
which is the latter.

> 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 know it's limited! As for dedicating a PIC, they're cheaper than
power MOSFETs anyhow, so that's certainly not an obstacle. As for limited
amplitude control, I think 256+ amplitudes are quite enough :). We can
live with a certain level of low-order harmonics w/o having a cow -- they
don't need to be tiny fractions of a percent to still get a major
efficiency boost (a large part of that is switching from DC to
three-phase).

> I would really like to see how much more efficient the "magic"
> sine waves are compared to the less ethereally PWM generated ones.

        Last I checked, yes. This according to Don Lancaster
(www.tinaja.com).

> 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.
>

        Yes, but you still have a big huge chunk of energy in the carrier
instead of the fundamental. FFTing the magic sinewave indicates that the
harmonics are pushed way up and spread way up, as well as slashing the
total harmonic power.

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

        Ok, thanks.

        Pierce Nichols