Add a second pot (wired as a variable resistor, wiper linked to CW end) in series with the feed to the clockwise end of the main speed pot.
That will allow the maximum speed to be set.
An equal value pot (or just a fixed resistor) would give down to 50%, double the existing pot value would give down to one third.
You can add a switch to short that out for low/high speed range control.
That will allow the maximum speed to be set.
An equal value pot (or just a fixed resistor) would give down to 50%, double the existing pot value would give down to one third.
You can add a switch to short that out for low/high speed range control.