Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why Do DC Brushless Motors Need Hall/EMF Sensors?

  1. Oct 31, 2013 #1
    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.
  2. jcsd
  3. Oct 31, 2013 #2


    User Avatar
    Science Advisor
    Gold Member

    "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!
  4. Oct 31, 2013 #3
    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
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook