Brush less DC Servomotors

In the brush less motor, the construction of the iron cored motor is turned inside out, so that the rotor becomes a permanent magnet and the stator becomes a wound iron core. The permanent magnets, situated on the rotor, require that the flux created by the current carrying conductors in the stator rotate around the inside of the stator in order to achieve motor action. The stator windings are interconnected so that introducing a three-phase excitation voltage to the three stator windings produces a rotating magnetic field. This construction speeds heat dissipation and reduces rotor inertia.

The permanent magnet poles on the rotor are attracted to the rotating poles of the opposite magnetic polarity in the stator creating torque. In the stator the magnetic field rotates at a speed proportional to the frequency of the applied voltage and the number of poles.

In the brush less motor, the flux of the current carrying winding rotates with respect to the stator; but, as the dc motor, the current carrying flux stays in location with respect to the field flux that rotates with the rotor. The chief difference is that the brush less motor maintains position by electrical commutation, instead of mechanical commutation.