Comparative analysis of limited field weakening capability of surface mounted permanent magnet machines

This investigation focuses on the comparison of five different surface mounted magnet rotor designs for electric vehicles with limited field weakening range. Using finite element calculations both base speed and maximum speed under field weakening conditions are taken into consideration. An algorithm determines the minimum current needed to suit specific torque and maximum voltage requirements. Furthermore, the algorithm reacts to the presence of saliency by slightly shifting the q-axis current into the d-axis, thus reducing the overall current. The results show that filling the pole gap with rotor iron and thereby introducing reluctance gives the best results in terms of high torque/current ratio both at base speed and maximum speed under field weakening conditions.