Spectral overlap of saliency signal components in injection based sensorless controlled induction machines

Sensorless control techniques of ac machines based on fundamental wave methods have a critical point of operation at zero frequency. Using signal injection methods zero frequency operation is possible, however another critical point of operation may exist at overlap frequencies which, so far has not been addressed by research. The resulting signals from injection based sensorless control methods are composed of components caused by saturation, slotting or anisotropy saliencies, as well as inter-modulation effects. The point of operation where the frequency of the slotting saliency equals the frequency of the saturation saliency is critical in terms of control performance as then a reliable separation is only possible feed forward. A mathematical description of the relation between the mechanical speed and load torque at the overlap of the slotting and saturation frequencies is given. Measurement results at specific points of operation are shown to point out that the critical point of operation is no longer zero speed or zero frequency but may be one of the operating states where an overlap in frequencies exists.