[Kevin Sevcik]

Concur with Matt. All you're doing here is making a very poor imitation of a speed controller. The speed controllers do exactly what you're trying to do, only faster, better, and more reliably. If you're trying to slow the motor down without losing power, you might just try mechanically gearing it down more. With your method, the motor will just sag and be backdriven during the off periods so you'll still be losing "power" just the same as with the speed controllers.