Analysis of frame alignment issues in natural field orientation including non-linear and leakage inductance effects

The natural field orientation (NFO) algorithm is a form of stator flux orientation (SFO) that has the desirable property that it does not use integration to determine the position of the stator flux reference frame. It is also supposed to implicitly correct reference frame errors. However, under certain modes of operation this does not occur. The purpose of this paper is to develop non-linear analytical expressions that accurately model this problem and then verify them against simulation and experimental results. This objective is achieved by using two non-linear modelling approaches - a simplified one which assumes that the machine has no leakage, and one where leakage is considered. The accuracy of two approaches are compared with each other. Furthermore, a solution to the frame instability is also proposed and analysed. The analytical results are verified by simulation and experiment.