An improved natural field orientation control of a current fed induction machine

Natural field orientation (NFO) is a simplified version of stator flux oriented (SFO) control for an induction machine. The essential difference from traditional SFO is that NFO does not estimate the stator flux magnitude but assumes that it's equal to the reference value. The NFO algorithm only requires knowledge of the stator resistance and is extremely simple to implement. However, it has been found that NFO has stability problems under regeneration. Also when implemented on a current fed induction machine, the control frame has an undesirable tendency to misalign with respect to the stator flux vector. This paper examines both the above issues in detail. It proposes a solution that mitigates this undesirable behaviour and, at the same time, retains the inherent simplicity of the NFO control scheme. The findings of the paper are substantiated by simulation and experimental results.