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I have a design based on ucc1625. The rpm needs to vary from 400 to 4000 rpm (closed loop speed control), At low RPMs, the speed control is unstable. I have noticed that the hall effect sensors (120 degree) are not exactly lined up - one is out by 16 electrical degrees. The electronic tack circuit converts this error into a ripply on the tach signal that I believe is caused the motor to run unstable at low rpms--any ideas anyone? I have run loop stability plots with a Venable Analyser and it shows good phase margin and nothing unexpected. I guess there are two questions in here: (1) could the instablilty be caused by tach ripple, and (2) what should I impose a max deviation for hall effect sentor position for the next design?