Natural field orientation concept: a tutorial

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. This paper presents a tutorial on the different forms of NFO and their various merits and deficiencies relative to each other. In addition it also examines the relationship between NFO and stator flux orientation (SFO). It briefly examines frame alignment stability issues in both NFO and SFO, highlighting the essential differences in the performance. Finally, a rotor flux NFO version is developed and compared to the stator flux oriented NFO. The discussions are backed up by simulation and experimental results where appropriate.