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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