Slow speed performance of an NFO-controlled induction machine

Natural field orientation (NFO) control was first developed in 1980s. It can be defined as a simplified version of the stator flux oriented (SFO) control performed in the reference frame which is aligned with the stator flux vector. A number of interesting properties result from this simplification, including a natural self-alignment of the control frame with the stator flux position and a very low sensitivity to the machine parameter error. High tolerance of the NFO control algorithm to the stator resistance error in particular, suggests that the algorithm may have a superior performance at slow speed to other "voltage model" based control techniques. This paper investigates NFO operation under slow speed conditions. It explains the principles of NFO operation and the physical reasons of its good tolerance to the machine parameter errors. It then analyses the performance limitations associated with the low speed values and suggests possible ways to overcome these limitations. It presents simulation and experimental results that show good initial agreement with theoretical conclusions.