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M. Ganchev, Ch. Kral, T. Wolbank:
"Compensation of Speed Dependency in Sensorless Rotor Temperature Estimation for Permanent Magnet Synchronous Motor";
IEEE Transactions on Industry Applications, 49 (2013), 6; 2487 - 2495.

The work proposes an improved method for estimating the magnet temperature in permanent magnet synchronous machines (PMSM´s) without using any temperature sensors. Originally the method implies an intermittent injection of a voltage pulse in the positive d-axis of the motor while keeping the load current zero. Thus, the resulting d-current response depends on both the initial value of the d-current itself and the actual magnetization level of the permanent magnets. Since the magnetization of the magnets depends on the temperature, different d-current slopes are associated with given temperature levels of the magnets. At higher speeds the current response is additionally affected by induced voltages of various sources which lead to erroneous estimation of the magnet temperature. 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 no longer affected by the induced voltages.

http://publik.tuwien.ac.at/files/PubDat_223963.pdf