Some brushless motors use hall effect sensors to sense the magnet position and provide feedback. However, most of the brushless motors used for RC are sensorless. When energizing the 3 coils, at any given time one wire is driven positive, one is driven negative and the third is undriven. The magnets on the spinning rotor induce a voltage in the undriven coil which the sensorless controller detects and uses to determine when to switch to the next step in the sequence.
One problem with the sensorless motors is that voltage induced in a coil is proportional to the speed of the magnet moving past it. At very low speed there is not enough voltage to detect so at startup, the controller has to drive the motor without feedback until it is spinning fast enough to induce a detectable voltage.
4-POLE BRUSHLESS DC MOTOR ANIMATION
