We now will follow the same approach as for induction machines, with the surface-mounted permanent-magnet synchronous motor (SMPM-SM), establishing first the electrical equations. This motor has non-salient poles.
This kind of motor is sometimes called brushless by comparison with a DC motor or a universal motor with its brushes and commutator, with which one would have inverted the stator and the rotor; the brushes are done away with by directly supplying the AC current to the three stator phases.
The historical advantage of this kind of motor, used initially with a variable speed for robots and for the table of machine tools, is that one replaces the commutator and the brushes, fragile and also of limited life, with the static commutation of semiconductors such as insulated-gate bipolar transistors (IGBT). The shift of the commutator neutral line of a DC motor was replaced by the phase adjustment of the control voltage vector.
The SMPM-SM does not present an armature reaction because the magnetic permeability of magnets is similar to that of air; it facilitates the control of the air gap flux, starting from measurement of the rotor position by a simple position encoder.
The advantage of this motor type, compared to induction machines, is dual: