Electronic – PWM Motor speed control using PIC; What mosfet to use

In theory, one could use a BLDC motor as a 3-winding (six-phase) stepper motor, controlling rotation speed by controlling the frequency with which the windings are driven. Indeed, for certain situations where would be using a motor at only a fraction of its rated power (e.g. because the cheapest motor one could get was much bigger than necessary) this approach will work fine. A major problem with this approach, however, is that when a motor is driven at a fixed speed, a reduction in mechanical output torque will not cause a corresponding reduction in current draw. Instead, it will cause a massive increase in the amount of heat dissipated by the motor. Although stepper motors are generally able to safely dissipate 100% of rated power as heat, BLDC motors generally aren't.

One might regard the situation as being vaguely analogous to regulating the speed of an automobile purely by using the clutch and brake pedals while the engine was always running at wide open throttle. True, one could for a little while make the car go any desired speed using just two of the three pedals (and if one was going uphill, one could do it with the clutch alone), but fuel economy would be dreadful, and unless one added a truly massive amount of extra cooling to the clutch it would be destroyed in minutes.

1. Put a load on it.

2. From what I can see (given the lack of datasheet), it should generate a PWM w/ the duty cycle proportional to the value of the pot.