Sensorless control of surface magnet synchronous motors at low speeds using low-frequency signal injection

This paper deals with the application of a low-frequency signal injection method to the sensorless vector control of surface magnet synchronous motors. An ac test signal is superimposed on the stator current of the motor. Due to the mechanical reaction, the response in the stator voltage depends on the orientation of the test signal relative to the permanent magnet flux. This dependency is used to control the orientation error to zero. Derivation of the controller structure is given, and the validity of the method down to zero speed is confirmed by simulations. The method is simple and insensitive to motor parameter errors, and it can be used to improve the sensorless control performance at low speeds.