On Wednesday, October 19, 2016 at 8:06:10 AM UTC-7, Electric Comet wrote:
some say not to add variable speed control to single phase induction
motors

That's because running slower than design speed for most AC motor
types causes excess heat. They burn up.
Some very small motors (for fans) are intended to tolerate this.

want a grinder that can go 200 rpm or so but also do 3500

prety sure that to get that low and maintain some torque will
require a vfd

Not necessarily; stepped pulleys, or a cone drive, or a gearbox can
do the job. Or, you could use a universal motor, or a pneumatic one.