Analysis and experimental tests of the sensorless capability of a fractional-slot inset PM motor

The sensorless techniques based on high-frequency signal injection to determine the rotor position is applied to an inset permanent-magnet (PM) motor, whose rotor is characterized by small iron teeth between each couple of PMs. Although these rotor teeth, which are centered along the q-axis, do not significantly contribute to torque production, they yield appropriate rotor saliency to be exploited for the rotor position sensorless detection. The inset PM motor considered in this paper is characterized by a fractional-slot stator, i.e., by a fractional number of slots per pole and per phase. Such a solution is commonly used when a surface-mounted PM machine is considered since it exhibits a higher torque density. A 12-slot 8-pole inset PM motor is investigated as far as its self-sensing capability. Both finite-element analysis and experimental measurements are presented, showing satisfactory agreement. It is shown that the prototype of the inset PM motor available in the laboratory exhibits a good capability to be controlled by sensorless techniques. However, a negative effect is recognized by a nonnegligible ripple of torque and flux linkages. They affect the dynamic performance of the PM motor drive.