T. Wolbank, P.W. Nussbaumer:
"Monitoring of Partially Broken Rotor Bars in Induction Machine Drives";
Talk: IECON, Puoenix, AZ, USA; 11-07-2010 - 11-10-2010; in: "Proceedings of the IEEE 36th Industrial Electrical Conference", (2010), ISBN: 978-1-4244-5226-2; 906 - 911.

Detection of rotor bar asymmetries in induction
machines operated by an inverter can be difficult due to
disturbances introduced by the fast switching power devices.
Furthermore interference of the fault indicator with control
dynamics of the load level lower the accuracy of most detection
methods proposed. These disturbances can be avoided if the
detection can be realized at standstill of the drive using the
inverter as a measurement device. In this paper the sensitivity of
a method is analyzed that enables the detection of rotor bar
defects at zero load and around standstill. The method is based
on the excitation of the machine with voltage pulses generated
by the switching of the inverter. Measuring the resulting
response of the machine phase currents, the spatial distribution
of the transient inductance can be identified with high accuracy.
An asymmetry in the rotor bars leads to a distinct change in the
spatial distribution of the transient flux linkage and thus also of
the transient inductance. The accuracy is such that even partial
broken bars can be detected. Measurements on a machine with a
specially manufactured copper rotor cage are presented. The
resistance of a single bar is stepwise increased by changing the
material of the bar from copper to aluminum, brass, and finally
steel. It is shown that even the stepwise increase of a single bar
resistance before the bar is broken can be clearly detected. It is
thus possible to monitor the machine even for a developing rotor
defect.

http://publik.tuwien.ac.at/files/PubDat_189738.pdf