some say not to add variable speed control to single phase induction
motors

but it is done for consumer products so the key seems to be that you
want to avoid varying the speed to the point where the starter winding
circuit closes and you burn up the starting winding


how to determine the speed of the starter winding

there is usually a click when it opens so the speed could be
measured at that point but are there other ways to calculate
that using the motor specs

fairly certain that the starting winding circuits open around
1500 to 2000 rpm

the cost for a vfd variable frequency drive must be the reason they
are not seen more in consumer products

or are there other reasons

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