RE: [TCML] homebrew VFD?

[William Noble <william_b_noble@xxxxxxx>]

square waves will cause significant heating of the motor as well as some really nasty harmonic effects - this is why VFDs go to some significan effort to make sine waves
 
> Date: Mon, 10 May 2010 15:56:38 -0700
> From: evp@xxxxxxxxxxx
> To: tesla@xxxxxxxxxx
> Subject: Re: [TCML] homebrew VFD?
> 
> OK in principle but a practical problem will be in supplying enough 
> current to get the motor started. I've used a similar device to power 
> telescope drive motors but the total power was only 5 watts and they 
> don't draw more on start. You could start the motor off the power line 
> and then switch to the VFD and it would certainly work for frequencies 
> less than power line.
> 
> The motor will be perfectly happy with square waves so no need to 
> filter. In fact, the capacitor on the secondary could screw up the 
> inverter or make it take a lot of excess power. You didn't specify what 
> you planned to use but it does have to put out a PERFECTLY SYMMETRICAL 
> square wave. In my case I used a '2 X' multivibrator oscillator feeding 
> a divide by two circuit to guarantee that.
> 
> Ed
> 
> Scott Bogard wrote:
> 
> > Greetings all,
> > While my intentions with this are completely off topic, the 
> > information could be extremely helpful for those running asynchronous 
> > rotaries, and I know of nobody else to ask this question, thus I fell 
> > no guilt putting it here. I want to know if it is possible to 
> > essentially build a variable frequency drive, this is what I had in 
> > mind. Get a mot, remove the windings then make your own to take 36V 
> > to 120V. Drive the primary with a variable flyback type inverter, 
> > designed for 1-120 Hz. So we have a DC square wave entering the 
> > primary. On the secondary put a capacitor sized to take a square wave 
> > and make it nearly sinusoidal (so perhaps a cap resonate with about 90 
> > Hz?). And then feed your secondary into your induction motor 
> > winding. It seems to make sense in my head but it seems a bit too 
> > simple, perhaps the square wave will not really drive the mot core 
> > correctly because of saturation in which case I don't know what to 
> > do. Any thoughts or should I just abandon this.
> >
> > Scott Bogard.
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> 
> 
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