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RE: [TCML] homebrew VFD?
I would encourage you to look around - the "larger" units are not that expensive, used, partuclarly if you get higher voltages like 440 - something as small as 5 to 10 hp in 440 can be had for under $100 if you are patient, and you can spend the energy tricking the electronics into accepting single phase input - you will end up with a nicer system with bells and whistles. small (5 hp) 440V 3 phase motors are also really cheap. I got rid of a 2 hp 220/440V motor a couple of months ago for scrap (it was in fine condition), sold it by weight
magazines like EDN have had articles on how to synthesize sine waves - basically it's like a giant D/A and you step up/down the voltage in the desired amounts -even a 2 bit d/a will be better than nothing, and 8 bits should be pretty good - for 220 in, an 8 bit converter will give you about a volt per step if you carry the sign separately, 2V/step if the sign is one of the 8 bits.
> Date: Tue, 11 May 2010 13:40:39 -0400
> From: sdbogard@xxxxxxxxx
> To: tesla@xxxxxxxxxx
> Subject: Re: [TCML] homebrew VFD?
>
> 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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> >>> http://www.pupman.com/mailman/listinfo/tesla
> >>>
> >>>
> >>>
> >>
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> >
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