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Variable Freq Drive

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I'm building a CNC mill and need a way to control the spindle speed from the PC. I've found circuits to control DC motors from the software (TurboCNC). That is OK for my "test vessel" but later I'm gonna need something to control a 2-3HP 3phase AC motor.

I've seen commercial VFD's but no computer controlled ones. So, being a compulsive DIYer I thought of the possibility to built one. The basic idea is shown in the attached pic. I will need a microcontroller to drive the IGBT's (taking commands from the PC) and a rectifier to provide the DC link voltage from 220VAC 1ph (it would be better if it could accept 3ph also).

What do you think? Is it doable or am I in over my head here?


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  • 4 weeks later...

GPG thanks but, being a newbie in electronics I didn't quite understand what this IC does (besides it looks hard to find). I see it can drive the IGBT's using 3 PWM input signals.

Can't I drive the IGBT's directly? I was thinking of trying with a Basic Stamp 2 I have laying around (using the PWMout command).



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Look at your spec sheets. How much current does the IGBT require? Without looking, I doubt that your micro is going to have enough current output to do this.
When you drive one device with another, it is wise to always look at the spec sheet. In some cases, the mismatch can cause a melt down.


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Go with your namesake (PIC). There is an appnote on this very subject at http://ww1.microchip.com/downloads/en/AppNotes/00843a.pdf . You may even be able to do it with one of the 14 bit cores. The PWM will almost certainly be done entirely in software, considering the change in duty cycle.

I'd recommend not using a stamp, not from my own experience but from what I've heard. With the real time interpreter going they can't keep up with the task at hand.

If you go to http://www.tinaja.com/ and put "magic sinewaves" in the search box, you'll be able to find some articles on PWM sinewaves.

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I think its doable.. but tons of work involved in firmware/circuit design.

I have a friend who is building a 3 phase motor controller for an electric car.. He started about the same time I did on my 3 phase inverter, and neither of us are finished yet... (started april 2003) but of course not working on the project constantly...

Most of the work is in firmware design.... the hardware is the easy part.


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