Here's a document from TI about modulation techniques. If this link doesn't work, try post# 69 on DIYEV:
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Toyota IPM Motor Controller Design Details - Page 7 - DIY Electric Car Forums
The problem we're going to encounter (especially with internal permanent magnet BLDC motors) is that we may have to change modulation techniques as we approach the top speeds (around 10,500 RPM )
Quote:
Originally posted by Coulomb at DIYEV:
I think it just means using square voltage waves, or trapezoid voltage waves, to drive the motors.
If you apply say 110% modulation (where 100% modulation uses all your available DC bus voltage, i.e. you are using everything from 0% to 100% PWM ratio), then the peaks of your generated sine waves will be clipped. In other words, your PWM ratio will go to 100% and 0% for a significant part of the sine wave; you'd like to go to 105% and -5%, but of course you can't.
As you increase the modulation past 100%. you "go further into the over modulation region", and the clipping gets worse and worse. Your sine waves go from correctly rounded to a little clipped to trapezoidal to (eventually) practically square waves.
Of course, IPM motors are typically designed to be driven like this. I think I read somewhere that the Prius switches from PWM modulation to actual direct square waves (in the latter, you don't bother chopping up the signal at a PWM frequency, you just switch hard on and hard off at the electrical frequency of the motor). This switch is based on motor speed; PWM below that speed, square waves above that speed.
I don't know what the shape is below that speed (it would presumably be sine waves or trapezoid waves, or maybe something that better matches the back-EMF shape.)
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In general, there's the problem of the BEMF approaching the DC Bus voltage. This can be mitigated by advancing the current angle so that the reluctance torque increases as a percentage of the total torque. This is why I was dancing around the room when I saw you were doing a FOC 3-phase controller.
It will require pretty good resolution of both physical rotor angle and current angle (probably within 5 or 10 degrees) There's a graph of this effect on page 62 of the ORNL 2010 Prius Evaluation.
Ok, the other problem is the switching speed, combined with the amount of poles and the motor's rotating speed. At some point the controller has a hard time keeping up. At play here is the PWM resolution, microcontroller clock frequency, number of calculations necessary, and even the IGBTs' switching speed.
It seems to me that changing switching methods could result in some problems; at the very least, inefficiencies and power loss at high speeds because it's no longer optimized.
So, I'm wondering how possible it would be to design the switching method to be the same, not go to some big compromise like big on-off blocks above a paticular speed. This may take some compromises in PWM resolution and calculation resolution, and some real cleverness with efficient (mathmatically) calculations.
Thoughts? or am I making a mountain out of a molehill?