  Interior permanent magnet synchronous machine drives are widely employed in
electric traction systems and various industrial processes. However, prolonged
exposure to high temperatures while operating can demagnetize the permanent
magnets to the point of irreversible demagnetization. In addition, direct
measurements with infrared sensors or contact-type sensors with wireless
communication can be expensive and intrusive to the motor drive systems. This
paper thus proposes a nonintrusive thermal monitoring scheme for the permanent
magnets inside the direct-torque-controlled interior permanent magnet
synchronous machines. By applying an external high-frequency rotating flux or
torque signal to the hysteresis torque controller in the motor drive, the
high-frequency currents can be injected into the stator windings. The permanent
magnet temperature can thus be monitored based on the induced high-frequency
resistance. The nonintrusive nature of the method is indicated by the
elimination of the extra sensors and no hardware change to the existing system.
Finally, the effectiveness of the proposed method is validated with
experimental results.
