Average and oscillating torque determination in fractional-slot PM motors

This paper deals with the analysis of the torque behavior by means of finite element (FE) model. Since in synchronous PM machines, the current control imposes constant d- and q-axis currents, the FE solutions are strictly associated with the classical d - q axis theory. When using integral-slot windings, the torque computed by using the d - q axis theory is almost independent of variation of the flux linkage with the rotor position. This is not verified when fractional-slot PM motors are used. The iron saturation affects greatly the torque behaviour. In particular, the torque ripple increases significantly. A particular care to determine the average torque and the torque ripple is necessary when dealing with fractional-slot PM motors. An analysis of the iron saturation effect applied to different fractional-slot windings is provided, with a particular attention to the interior permanent magnet machines.