On Tue, 27 Sep 2005 09:00:14 GMT, xray
wrote:


I have a lathe that runs faster than what I would like on the slowest
speed. Two answers seem to be available. Replace the motor with 3-phase
motor and a VFD controller, or replace the motor with a DC motor and PWM
controller.

The DC version, I think I understand electronically, although maybe not
effective powerly (I've heard lower DC power motors effectively look
like higher power AC motors -- ?).

The 3-phase VFD (Variable Frequency Drive) seems simple enough in
principle, but I'm wondering what's in the best cost effective versions.

The one I want will take single phase 230V AC and convert it to 3-phase
motor drive output that is variable in freq.

First question: what happens to the input? If it is single phase, I
assume it goes through a full wave rectifier to get it as smooth as
possible. Then what? Caps wouldn't help much at this kind of power I'd
guess, and commercial units are small. Just ignore the bumps?

I think the drive to the three phases is a form of PWM, probably
microprocessor generated, and done by IGBTs.

So, It's obvious I don't know a lot about this except my first-level
assumptions. Can anyone provide a basic description of what is happening
in these VFDs and what might make better or worser implementations?

Thanks. Hope it generates some interesting observations. I know this can
get deep, but at a first level I'd like to hear the the basic theory
about how the input power might be adapted and controlled.


My father has been at me for years to make him a single phase to 3
phase VFD for his lathe. I have finally got around to it, just
tweaking power supplies ATM (never use a voltage mode smps for
industrial control) and writing the code.

The unit I have designed is based on a microchip design. Single phase
rectified and filtered. Microcontroller using sine look-up tables to
generate PWM. An IR IGBT module that takes care of all the high side
switching stuff. My estimate is that it will cost under AU$100 for a
hobbiest to make (no certifictions) providing that an aussie company
will stock the IR modules. So far it is working quite well, cept the
damn smps to drive the uP and other stuff.

I wish someone would design the switcher for me (hint hint), i just
cant seem to get a smps to work reliably for me. It would be nice to
have it working with a UC3842 with a non isoloated output of +5 and
+15dc )))) (hint hint again) )) I will get there...