P.W. Nussbaumer, T. Wolbank:
"Saliency Tracking based Sensorless Control of AC Machines Exploiting Inverter Switching Transients";
Talk: IEEE International Symposium on Sensorless Control for Electrical Drives, Padova, Italy; 07-09-2010 - 07-10-2010; in: "Proceedings of IEEE International Symposium on Sensorless Control for Electrical Drives", (2010), ISBN: 978-1-4244-7034-1; 237 - 242.

Control of ac machines at zero frequency
usually requires the presence of a rotor speed sensor. Sensorless
control is so far possible around zero frequency when
exploiting inherent saliencies of the machine. Current
techniques use injected high frequency signals or the machine´s
step response to extract and identify magnitude and position of
inherent saliencies. In this paper, a new method is presented to
determine the inherent asymmetries using only the information
provided during the switching transients of the machine
current. While known techniques need an injected high
frequency voltage, specific switching patterns, or modifications
in the PWM scheme to exploit the current reaction, the
proposed method is able to extract the necessary information
from the transient current signal that directly follows the steep
voltage change of the switching transition. Thus, the standard
PWM scheme can be applied without modifications.
Measurement results are presented to show the influence of the
saliencies on the switching transition current ripple.

http://publik.tuwien.ac.at/files/PubDat_189833.pdf