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T. Wolbank, M. Metwally:
"Spectral Overlap of Saliency Signal Components in Injection Based Sensorless";
Talk: IEEE Energy Congress& Expo, Atlanta, Georgia, USA; 2010-09-12 - 2010-09-16; in: "Proceedings of the IEEE Energy Conversion &Expo", (2010), ISBN: 978-1-4244-5287-3; 3442 - 3447.

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 intermodulation
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.

http://publik.tuwien.ac.at/files/PubDat_189825.pdf