Scott Dattalo wrote:
>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.

<snip>

>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.

The nice thing about magic sinewaves is that the distortion comes in mainly at
 the higher harmonics.  If you are using them to feed a motor (with it's nice
 inductive windings), you are essentially pushing your power right into a very
 large low-pass filter.  As such, the high-order harmonics aren't much of a
 problem.

Take a look at Don Lancaster's pages at http://www.tinaja.com for a lot more
 info on magic sinewaves.

 - Rick "PIC'ing up the thread" Dickinson

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