Flux observer model for sensorless control of PM BLDC motor with a damper cage

Flux observer is a common and practical way to estimate the rotor position and speed for sensorless control of the permanent magnet (PM) AC motors. Currents in the damper cage influent the accuracy of the observed rotor position and speed severely. However, it is really difficult to measure the currents in the damper cage. According to the generation principle of the currents in the damper cage, the currents in the damper cage are higher harmonics. That's to say, the fundamental component of the stator currents become DC components under the rotor rotating coordination system, while the currents in the damper cage are still AC components. Therefore, if only the fundamental components of the stator currents, flux linkages, terminal voltage, etc. are considered, the currents in the damper cage can be ignored. Because the rotor position is only related to the fundamental component of the rotor PM flux linkage, an improved flux observer mathematical model is derived for the BLDC motor with a damper cage in this paper. The accuracy of the proposed flux observer is verified by both of numerical analysis results and experimental results.