Improvement of stator flux observer for a direct torque controlled PMSM drive at low speed

Stator flux observation is one of the important parts in direct torque control (DTC) system of permanent magnet synchronous motor (PMSM). The accuracy of the stator flux observation directly influences the performance of the control system. And it depends on stator resistance which is a time-varying parameter and is affected by the motor ambient temperature or voltage frequency in direct torque control system of PMSM. Variation of the stator resistance has a negative effect on the accuracy of stator flux and electromagnet torque observation, especially in the low-speed range, which may cause the control failure of the drive system. An adaptive stator flux observer and an online stator resistance estimator are presented in this paper. The adaptive law of the stator resistance is derived according to Popov hyper-stability theory, which guarantees the stability of the system. The simulation results verify that the proposed adaptive stator flux observer can not only track the actual stator resistance quickly but also ensure the accuracy of stator flux observation even at very low speed.