THIS MOVES A TON OF AIR. = pick-n-pull salvage - DEMO It squeals a bit a lowest speed because it's used and came off a high mileage wrecked Volvo. 100hz basic freq. Pulse input with 90% duty cycle spins fan at LOW speed. speed. Pulse input with 10% duty cycle spins fan at HIGH speed. Pulse input with 50% duty cycle spins fan at MEDIUM speed. So it's continuous variable from stopped to full speed and any speed in between.