PLL motor control

Ive seen many ppl using PLL for DC motor control but why not use a PLL for an ac syncro motor ? Throw a crystal on the input. Use filters to clean up the VCO's output, followed by a nice large BJT output stage to drive the motor. Including the filter and output filter in the PLL's feedback loop wil miminize distortion and phase shifts. You can also throw in a multiplier and programmable divider and you could have very fine adjustments of the oscillator frequency. Just some random ideas of mine. Seems simple enough to me, any reason why this wouldnt work ?
 
The Thorens TD125 used a power wein bridge oscillator to drive a synchronous motor. There was a pot on the top of the turntable chassis for fine frequency adjustment of the oscillator. A neon bulb illuminated a strobe ring on the platter so you could adjust the oscillator frequency correctly. There were two strobe rings, one for 50 Hz and the other for 60 Hz. Very ingenious.
Regards,
Ray
 
Hi.

Why would you use a pll if the crystal alone can be very stable on its own ? If its because of freq. adj., there are other ways to go about that.

Anyway, on such a device, if you want to control 110/220v type motors, the place to burn a few braincells is not in the generation of a stable low-distortion sinewave, but raising it to 220Vrms and still get stability and low distortion.

Of course, and this goes without saying, if you think you have a good idea, go for it ! - If you take it further, I for one would like to know about your progress.

Best Regards,

J.Guilherme