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Registered Member #1806
Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136
Oh my, I sorry. At no point was I unhappy with any of your contributions. If I conveyed any sort of unhappiness, it was totally unintentional and I am deeply sorry. On the contrary, I greatly appreciate all of your contributions. Thank you. :)
The reason for my original question was to see if this process would be as straight forward as I was thinking it might be. If it were, I would be able to build this relatively fast. It does not appear that it will be as simple as I had hoped, and therefore, the development process will likely take longer than I have available time right now.
It is my understanding that any motor powered directly from single phase AC will have torque ripple. That configuration accounts for a large per-cent of the line powered motors out there. Whereas only with a 3-phase motor is torque ripple attenuated to insignificance. If that is the case, then torque ripple on the output should not be an issue for me; at least from a usability standpoint, assuming my understanding is correct.
From what I've read about SMPS design, I get the impression that without a very good understanding of the situation as a whole, it is a very time and effort intensive process to make something that works properly. This is why I wanted to avoid the converter, but also for cost and complexity reasons. I guess it's possible that even without considering the power factor as an issue, chopping the mains into the motor is enough of a converter to give me all of the fun converter issues like needing resistor/diode/capacitor snubbers to keep the motor from blowing the mosfets, along with possibly significant RFI coming off of the motor, and so forth.
I would agree that having a voltage supply behind the inverter would be easiest, but it seemed reasonable enough to monitor rotor position with a few hall sensors and watch the current to adjust the pwm into the motor. In this case, one would control which winding gets the current based on rotor position feedback, limit the current, and not bother with caring about the voltage. Maybe it would strongly resemble a bipolar chopper stepper motor driver. In the end, it could end up like so many other things where is seems reasonably straight forward until you get into the design, and only then is the true complexity seen. Mountains seem small when you're far off and only get bigger the closer you get.
I found some apparently suitable 200v mosfets along with a 200v 3-phase (3 half bridge) gate driver chip. I would then just use one of the AVRs I have setting around to send pwm out to the bridge driver chip, watch the current, and read the hall sensors. ...if I proceed with this project.
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brushless dc motor drive from the mains
