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Reply-To: "fred bartoli"
From: "fred bartoli"
References: <email@example.com> <firstname.lastname@example.org>
Subject: Re: How to construct PWM pure sine wave inverter ?
Date: Wed, 16 Oct 2002 13:45:57 +0200
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NNTP-Posting-Date: 16 Oct 2002 13:44:32 MEST
Bill Sloman a écrit dans le message :
> "Booboo" wrote in message
> > >
> > >Suggestion: google for 'magical sine waves'
> > >
> > http://www.tinaja.com/glib/stepsynt.pdf
> > I think he means a ~real~ sine wave.
> A pulse-width modulated digital data stream - just ones and zeros - looks
> exccedingly like a sine wave if low-pass filtered by a good filter.
> The problem with Don Lancaster's magic sine waves - which are just very
> binary sequences - is that the high frequency noise is big and and you
> a multipole filter to get it down to acceptable proportions. Not a problem
> with a fixed frequency source, but for a variable frequency source, the
> technique of driving a D/A converter from a phase accumulator via a
> table is to be preferred.
> For power applications, where you really need PWM, Don Lancaster's magic
> sine waves probably have too many switching edges to be acceptable - each
> transition dumps a lump of energy into the switch and you need to ration
> number of transitions.
> Bill Sloman, Nijmegen
I don't see what's wrong with Don's approch for AC inverters of performences
like the one here.
If I've correctly understood the paper after a breaf reading, there are few
switching edges (from fig 1) 6x4 = 24 pulses over a full sinus cycle. Much
less than a 20-100kHz classic PWM approach. On the contrary, I would think
that magic sin waves is much more favourable in this regard.
Also for the "high freq" noise, the first major harmonics are the 23th and
25th, so it would be easy to build a fixed low pass filter that have it's
corner frequency between 1) the highest fundamental freq (80Hz in this case)
and 2) the 23th harmonic of the lowest freq (30x23 = 690Hz) there's plenty
of room in this window.
In fact, what I find less practical in this approach is the size of the low
pass filter elements, given the low corner frequency and maybe the amplitude
of the obtained fundamental, but I'm not clear on this point having just
thought a few seconds to it.
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