Frame alignment stability issues in natural field orientation

Natural field orientation (NFO) is a technique for generating a rotating reference frame position for an induction machine which is aligned with the stator flux. The term "natural" is applied because there is an implicit tendency for the rotating frame to realign with the correct stator flux frame position if there is a perturbation away from this position. However, under regeneration conditions this realignment property may not occur, and the frame position, if perturbed, will move the control reference frame away from stator flux alignment. This paper examines this frame alignment stability problem, and proposes a solution that retains the essential simplicity of the NFO concept.