Re: [TCML] homebrew VFD?

[Scott Bogard <sdbogard@xxxxxxxxx>]

For this particular application I need a much larger motor (5~hp) than 
is typically used for an ASRSG, which is why I'm so keen on making my 
own to run off 36VDC, the cost on those bigger units is prohibitive, 
while the induction motors themselves are cheap.  If I could get a 
single phase prototype to work I could make a three phase one in theory, 
though efficiency might be less than ideal.  Plus serious bragging 
rights come with building your own VFD!  Another thought I am having is 
putting a cap in series with the primary, now current will be limited 
over time, and a freewheeling diode can be installed to drain the cap 
every cycle.  I am concerned about core saturation associated with 
square wave, is there any easy way to take a square wave and make it a 
sine wave?  Just some thoughts.

Scott Bogard.

On 5/10/2010 7:51 PM, William Noble wrote:
> 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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