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I've been thinking about designing a controller as well (or at least the control portion) and i've been thinking about AC. The gist of it is this - frequency of the wave determines rotation speed of the motor. rpm = 60* pole pairs * Hz. Amplitude determines current which relates to torque. I believe, however that there is also a relationship between rotation speed and the voltage required to get a particular amount of current - ie, at a stop, 60VAC will use more current than 60VAC at high speed.
From what I gather, the control portion is NOT hard, just the power portion is hard. Generating 3 AC waves from 3 PWM ports on a processor is easy. Relating RPM and throttle position to frequency and amplitude is "not hard".
I have been meaning to find a small fan to try to power with an audio amplifier (excellent DC to AC converter with variable freq and amplitude, optimised for under 85Hz)
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