Why Do DC Brushless Motors Need Hall/EMF Sensors?

[VarietyBook]

I understand the basic gist of the whole DC brushless motor, but I don't understand why there needs to be a sensor. Why can't the controller just alternate between phases at a constant rate? What I mean is that if there are a fixed number of ways the coils can be charged, why not just alternate between them and keep going? I hope my question makes sense.

 

[UltrafastPED]

"There has to be a sensor giving feedback to the system indicating when the rotor has reached the desired position. If commutation is done faster than this, the rotor magnets go out of sync with the stator magnetic field and the rotor vibrates instead of rotating. There needs to be a sensing method to determine if the position of the rotor is in sync with the stator, so that the next commutation can be made. There are many types of sensors used in motors: encoders, potentiometers, switches, and others."

From http://www.freescale.com/files/microcontrollers/doc/app_note/AN4058.pdf

Your assumption is that the rotational rates are uniform, and that everything is perfect ... we use feedback-control systems because that ain't so!

 

[MisterX]

It is possible to control a synchronous permanent magnet motor without the use of a sensor*. Instead the position of the rotor is estimated through the V-I relationship (or "back emf"). But there is still some control that somehow involves the rotor position.

*as in, this scheme does not require an additional doodad in the motor itself