On Tue, 16 Sep 1997, Mal Goris wrote:

> I don't think that amplitude control is really the point of using
> magic sinewaves. I didn't even realise that you had any amplitude
> control at all. But you do have accurate phase and frequency control

        You get amplitude control by switching btwn different sequences --
you pack a few hundred into a PIC, and shove them out on command.

> which is important for speed control of AC motors. To implement magic
> sinewaves there are only four drive transistor transitions per cycle,
> so even if you want 500 Hz you have only 2 kHz switching. Although the
> bit sequences that define the magic sinewaves are long it all comes
> down to accurately timing the four transitions. That is, you don't
> necessarily have to shift 1000-bit sequences to generate magic
> sinewaves.

        Not quite -- you have more than 4 transitions, by quite a bit. You
usually have a dozen or so tranistions (which is still a small fraction of
an equivalent PWM scheme) per cycle, and your fundamental soaks up most
of the energy, followed by some very high harmonics (which don't actually
take that much). As you mentioned, phase and frequency control are trivial
with magic sinewaves.

> Magic sinewaves require a dual polarity supply which is less
> convenient when you are operating from a solar panel.

        Turns out that with a delta-wound three-phase motor, you can use
magic sinewaves with a single polarity supply (because you can still get
current to go either way in any winding by simply switching its ends).

        Pierce Nichols