Because the rotor would stop once it aligned with the magnetic forces in the stator, a commutator is required to keep the rotor moving. The magnetic field is switched along with the DC current as it is continuously switched by the commutator through the windings. In this manner, the rotor can continue to revolve as long as the engine is powered.
The type of commutator used by a Brushless DC Motors and a traditional DC motor differs significantly. Carbon brushes are used for this in a DC motor. These brushes have the drawback of wearing out rapidly. For this reason, Brushless DC Motors require sensors to determine the position of the rotor and a circuit board that serves as a switch, typically Hall sensors. The circuit board processes the sensor input measurements before precisely timing when to commutate while the rotor rotates.
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