I would like to design a variable frequency drive (VFD) to control a small wind tunnnel exhibit for a science museum. The reason I think this needs to be a (semi)custom design, is because of some of the requirements in the user interface.
variable speed can be adjusted with a dial/pot. 2. specific speeds can be "remembered", i.e. setpoints with buttons (3). The motor is likely to be in the neighborhood of 1HP and 115VAC (1 or
3 phase), so relatively small. I know very little about the "art" of VFD designs. I need help with the actual synthesis and driver electronics (i.e. dealing back EMF,etc). I can handle the control part of it, if I know what the parameters are.
Are there any good books or application notes about VFDs ?
Of course, if there are reasonably priced VFD that will do what I need or is hackeable, that might be good too.
If you have good hacking skills, then hacking an off-the-shelf VFD might be the way to go. If it has an ordinary pot for its speed control, just run its wires out to your front panel, and for presets, just switch in fixed resistors in place of the pot.
I have done some work with this using Microchip PICs. I actually rewound a
3/4 HP 120 VAC single phase 6 pole motor (1200 RPM at 60 Hz) so it would be essentially an 8 VAC three phase 12 pole motor (600 RPM at 60 Hz). I built a very simple 3 phase VFD drive using MOSFETs, and was able to run the motor at various speeds from a 12 VDC battery, until one of the MOSFETs burned up. I didn't have good overvoltage and overcurrent protection, and this is where the "art" of the design comes in.
I also ran it from a GE/Fuji AF-300 E11 VFD drive that I bought on eBay for about $75. I used 120 VAC single phase for input, and used 120V/12V stepdown transformers for the output. I was able to run it at very low speed, and also at its 60 Hz speed of 600 RPM, and then even at 1800 RPM with 180 Hz output. The controller would have allowed me to take it to 400 Hz, for 4000 RPM, but I didn't want it to explode! Theoretically, it should be possible for this motor to put out over 2 HP, but I have not yet built the dynamometer to test it.
Anyway, the
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website has many application notes on PWM and VF drives, and they also have a demo package for design of drives, but it's a bit pricey at about $400. I would definitely use a three phase motor for your application. They are ideal for variable speed control, and are very reliable, quiet, efficient, and very cheap on the surplus market. The VF drives are also inexpensive, but you have to know what you are getting. Many are designed for 3 phase input, but most can be used on single phase at reduced power. The drive I have includes an RS485 communication link that allows you to change speed settings through a serial port from a PLC or computer. The unit I bought was new in the original box, and several were available at that time (about 6-8 months ago). The list price is probably about $300.
Good luck on your projects. If you need any specific help after reading some of the literature, let me know. It would probably be a good learning experience for both of us.
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Paul E. Schoen, President
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