RSC-20 Variable Speed Control 20 Amp Motor Speed controller - 20 Amps Max Use with mixing drills, or grinders to reduce speed electronically without reducing torque. This handy device will slow down your grinder or your mixing drill. You can now use a regular angle grinder for polishing or edge polishing with our quick change system. The dial lets you select any speed under 4000 Rpm. Will not work on slow start tools.