a better way is to use inverter to drive the electric motor, it needs to be engineered...you can buy a chinese made one for around $125 with spotty reliabiity, or an American made one for $700+ made by Alan Bradley or other US mfgrs,
That is an example.
It is easy to make any project non viable by specifying unrealistic specifications, 0 to 500 rpm is a bit extreme, it can be done of course but not cost effectively.
Regarding the inverter, I over size those so that they run cooler and more reliably. if you are trying to adapt a 3000 rpm motor to 500 rpm you are making things tough on yourself, it will be cheaper to buy an 1150 or 850 rpm motor as a base and then step that down to the speed ranges you want.
If you appreciate my input so far, locate the "rate your expert" link, and rate my work positively... we can go from there.
Lacking that I can spend days with a customer at my own expense,without any compensation. Rating does not increase your charges or close the question and it allows you limited follow up.
Full engineering services of course is another issue, we can discuss later,