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 a thin iron tooth between each couple of permanent magnet. These rotor teeth yield the rotor to be anisotropic, so as its position can be detectable. The inset PM motor considered in this paper is characterized by a fractional-slot stator, that is, 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 10-pole inset PM motor is investigated as far as its self-sensing capability. Both finite element analysis and experimental measurements are presented, showing a satisfactory agreement. It is shown that the prototype of inset PM motor available in laboratory exhibits a good capability to be controlled by sensorless techniques. However, a negative effect is recognized by a non-negligible ripple of torque and flux linkages. They impact on the dynamic performance of the PM motor drive.