Cross-saturation effects on position estimation using BEMF methods in PMAC machines

Under load and in the presence of saturation in particular, PMAC machines exhibit the magnetic axis shift from the rotor position, which in turn causes an error in a control algorithm that uses conventional rotor position estimation. This error is particularly pronounced in the shaft-sensorless position estimation based on the back EMF measurements since most conventional algorithms completely ignore the existence of the cross-saturation i.e. cross-coupling inductance L_qd is assumed to be zero. The described error is a function of operating point and local saturation levels so it is intuative to conclude that it will also be impacted by various machine parameters such as; rotor geometry, winding configuration and so on. In this paper we conduct a comparative study to show the influence of the various PMAC machine topologies on the magnitude of the described error, while utilizing an existing back EMF based position sensorless controller. The impact of the error on the machine performance is also examined. Four major types of PMAC machines are considered; surface and interior permanent magnet machines with concentrated and distributed winding configurations. First, we will use a non-linear mode to analytically characterize this error for an arbitrary PMAC machine and use the FEA to characterize the error for the specific IPM and SPM machines with distributed and concentrated winding topologies. Finally, degradation in the torque-speed capability and efficiency over the entire operating range of the machines due to the error will be also presented.