In the PMSM, by creating a sinusoidal distribution for conductors at the stator surface, the airgap’s flux density is closer to the sinus form . By replacing the field’s winding with a permanent magnet, the brushes, slip-rings, and copper losses could be removed from synchronous motor. The permanent magnet generates motors with permanent excitation. Fig. 1, shows two different ways in which permanent magnets are put into rotor .
The following figure shows the ratio of magnetic torque to reluctance torque in synchronous machines. Regarding the mechanism of torque producing, synchronous machines are divided into three distinct groups. The surface-PMSM (SPMSM), in which arc-shaped permanent magnets are embedded on the surface of cylindrical rotor .
Such machine is a complete PMSM which produces only magnetic torque. Synchronous reluctance machine (synRM) is a pure reluctance machine shown in Fig. 2. f. The inset SPMSM belongs to SPMSM category in terms of magnet arrangement, however it is a combined reluctance-magnetic motor because of its magnetic saliency. Interior PMSM (IPMSM) in which permanent magnets are buried inside the rotor (Fig. 2. c, d, and e) has magnetic saliency, so it is a hybrid reluctance-magnetic motor which can be considered as a PMSM motor regarding its reluctance torque (region II) or a synRM motor regarding its magnetic torque.