The basic idea behind a brushless dc motor is that it regulates its own
speed.  Trying to control the speed externally without messing w/ the
electronics inside is not going to give you a very good/nice range.

The motor already has feedback, usually in the form of a hall effect
chip which senses how fast the rotor is going, and when it is in a
certian position.  The rest of the circuitry then sends appropiate
currents through the coils to make it spin.

Some fans (notably processor fans such as the Pentium II and III) have a
third wire which either 1) sends out a pulse every revolution (generated
by the sensor) or 2) is in a certian state (high or low) when the fan is
operating correctly, and switches state when there is a fault which may
cause the fan to stop moving.

-Adam

Paul Brown wrote:
>
> Has anyone tried to use a PIC to control a brushless DC motor?  I have
> worked with controlling brush type DC motors using encoders, pwm, and a
> driver chip.  However, a brushless motor seems to be quite a bit more
> complex.  Has anyone tried this?  Can you point me to any code or
> references?
>
> Is there a way to implement a feedback control loop on these without using
> encoders?
>
> Paul