Advanced control strategies for stability improvement of natural field orientation

Natural Field Orientation (NFO) is a patented control strategy for induction machines that was first proposed in 1980's. In recent work by R.E.Betz and G.Mirzaeva it was found that the NFO control algorithm has a stability problem in regeneration mode. The authors proposed an augmentation to the basic algorithm in the form of an auxiliary feedback in the angular velocity estimation. This augmentation resulted in improvement of the NFO performance in regeneration, however, limited to relatively small torque magnitudes and slow dynamics in regeneration. It was expected that flexible control of the auxiliary feedback gain and / or application of a PI block in the auxiliary feedback may be beneficial for further performance improvement of the NFO algorithm. This paper develops a full dynamic model of the NFO controlled system and presents results of its linearised stability analysis. Based on the above, it proposes some more advanced versions of the control over the parameters of the auxiliary feedback. The paper further discusses advantages and limitations of the proposed modifications of the NFO algorithm. To support the discussion, it includes results of simulations and experiments obtained for a test induction machine.