Quote:
Originally Posted by JohnnyV
For me, doing the control part of it would be relatively easy. However, I have no experience designing circuits for medium/high voltage equipment. If someone has experience with that, I think we could put our heads together and bang out a solution.
Like Shifty said, the biggest concern will be the power MOSFETs and getting them to switch quickly enough, if if you're using multiple ones, in synch with one another.
I would like to see a requirement that the control method use feedback from the motor so you can verify you are running at the desired speed. This also will help with efficiency since you won't have oscillation in the motor speed assuming you set your parameters correctly. The easiest way to implement this is through a PID controller, which, with some effort, can be accomplished via a PIC microcontroller (or any other decent micro).
If going the feedback route, we would want the software for the micro to be easily adjustable for different motors - all motors have inherent parameters that must be taken into account when developing a control system for them.
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Hi,
I think what I would do instead of using the speed as the error signal to the PID controller would be to use the current to the motor as the error signal. A motor is actually a current device, not a voltage device. That is why the PWM works so well in controlling the speed. The output FETs switch on and off at varing dutycycles depending on the desired average current through the motor windings. Think about the way a car works. The accelerator sets the fuel going to the engine not the speed. The driver adjusts the speed depending on driving conditions etc.
As far as output FET switching times...
I have designed Complimentary FET drivers that drive the gates of the power FETs. Made a HUGE difference in the heat dissipated in the power FETs.
The idea here is to turn the power FETs on and off as quickly as possible. Once the FETs are full on or full off the power dissipation is nill. It is the transition switching the kills you. Hence, just design a highspeed driver before the Power FETs
If you parallel the output FETs, you will most likely have to match them. Otherwise, if one FET has a slightly lower ON resistance than the others, it will tend to pass a higher % of the current which could easily excede the individual device rating and *poof* you end up letting the smoke out. Never do you want to let the smoke out; it is the smoke inside the device that makes the part work - let out the smoke and the part stops working
Thinking about how to build a controller without matched FETs makes my head hurt.
That could get REALLY complicated. Although an interesting academic exercise.
Have fun!
Wallace