Shown here is a simple brushless DC motor control system and a three-phase brushless DC motor driving a rotor which actually has four magnetic poles on it. Now, sometimes there is some confusion as to what is the proper way to drive a brushless DC motor and in fact some people believe it is very similar to the way one drives a stepper motor because of the commutation zones that are associated with the BLDC motor. Now, with the stepper motor what the designer does is just energize the coils in whatever predetermined sequence that one wants the motor to run at and hope that the rotor can keep up with that pattern that is being applied to the stator windings. Well, that can be done with a brushless DC motor but it is probably going to run very rough. Instead, what the designer wants to do first is determine what is the position of the rotor at any given moment in time, and then based upon that information determine which coils to turn on and off to give the best torque response out of the motor. With a brushless DC motor, one needs to at all times have very accurate information as to what angle the rotor is at. Now, the most common or traditional way to do that is with what is shown here in this diagram, is to use Hall effect sensors and the designer can see on the back of this rotor there is a magnetic disc consisting of four magnetic poles and three Hall effect sensors. This basically means that every time one of those North-South boundaries between the magnets on the magnetic disc crosses under one of those Hall effects sensors, the output of the Hall effect sensor will change from a zero to a one or a one to a zero.