Re: [TCML] homebrew VFD?

[Steve Ward <steve.ward@xxxxxxxxx>]

Keep in mind that to get a constant torque out of the motor, you want
constant current, not constant voltage.  This means at low RPM, if applying
a constant voltage, the back EMF is low and this allows a lot of current
through the windings of the motor, which would likely be extremely stressful
and damage it.  For this reason, real VFDs have a current control loop at
the heart of operation (the output voltage will be lower at low RPM).

Your kludge sounds like it offers no current regulation, and that alone
would be bad.  If stressing the motor was OK, stressing the MOT for the same
reason is probably not so good, im thinking it will saturate at anything
slightly less than 60hz at its designed volts/turn.

Some things are just not easily thrown together from junk, unless your junk
happens to be old VFDs.

Steve

On Mon, May 10, 2010 at 4:27 PM, Yurtle Turtle <yurtle_t@xxxxxxxxx> wrote:

> I assume you're trying to make a single phase VFD. I ran a single phase
> motor from a 3 phase vfd by disabling it's phase imbalance monitoring. It
> worked OK up to a point. If I ran it too slow, the motor's internal
> centrifugal switch started disengaging. I considered adjusting the weights,
> so it would engage at a lower speed, but ended up buying a 3 phase motor. My
> smaller sync motor doesn't have this switch, so I'm sure it would have
> worked just fine.
>
> Adam
>
> --- On Mon, 5/10/10, Scott Bogard <sdbogard@xxxxxxxxx> wrote:
>
> > From: Scott Bogard <sdbogard@xxxxxxxxx>
> > Subject: [TCML] homebrew VFD?
> > To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> > Date: Monday, May 10, 2010, 1:34 PM
> > 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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