M. Ganchev, Ch. Kral, T. Wolbank:
"Sensorless Rotor Temperature Estimation of Permanent Magnet Synchronous Motor under Load Conditions";
Talk: IEEE Industrial Electronics Society, Montreal, Canada; 10-25-2012 - 10-28-2012; in: "Proceedings of Conference on IEEE Industrial electronics Society (IECON)", (2012), ISBN: 978-1-4673-2420-5; 1 - 6.

The work proposes a method for estimating the magnet temperature in permanent magnet synchronous
machines (PMSM´s) under load conditions. The method implies an intermittent injection of a voltage pulse in the positive and negative d-axis of the motor. Thus, the difference of the resulting d-current responses depends on the machine operating point which is defined by the d-current, the q-current and the actual magnetization level of the permanent magnets. Since the magnetization of the magnets depends on the temperature, different d-current slopes reflect different temperature levels of the magnets. By applying a voltage pulse in the positive and negative d-axis of the motor, symmetry of the induced voltages can be achieved in a manner that the difference of the current
responses from the positive and negative pulse is to a great extent speed independent.

http://publik.tuwien.ac.at/files/PubDat_210724.pdf