Remarks on Torque Estimation Accuracy in Fractional-Slot Permanent-Magnet Motors

In permanent-magnet (PM) synchronous machines, when using integral-slot windings, the torque computed by using the d-q-axis theory is almost independent of the variation of the flux linkage with the rotor position. This is not verified when fractional-slot PM motors are used. The iron saturation greatly affects the torque behavior, increasing the torque ripple significantly. Particular care is necessary to determine the average torque and the torque ripple when dealing with fractional-slot PM motors. The analysis is carried out by means of a finite-element (FE) model. Since, in synchronous PM machines, the current control imposes constant d- and q-axis currents, the FE solutions are strictly related to the classical d-q-axis theory. The impact of iron saturation is taken into account, comparing a 24-slot 4-pole integral-slot machine and a 24-slot 20-pole fractional-slot-winding machine. Particular attention is given to the interior PM machines.