Rotor Time Constant Identification on Sensorless Induction Motor Drives by low Frequency Signal Injection

Speed control performance on most sensorless field-oriented control induction motor drives is sensitive to parameters variation. In fact, the speed feedback signal is normally obtained from the rotor flux angular frequency through the machine mathematical model and its precision is strongly affected by errors in estimating the rotor time constant. This paper describes a new method to on-line estimate the rotor time constant, which is accomplished by injecting a low frequency current signal and by tracking to a minimum the produced slip angular frequency ripple. The proposed technique does not require modifications of the controller structure and does not require additional sensors, while featuring a good accuracy and fast convergence to the correct value of the rotor time constant. Although it can be implemented on any kind of sensorless induction motor drive, a straightforward practical implementation can be however accomplished on MRAS based sensorless induction motor drives. Simulation and experimental tests confirm the effectiveness of the proposed approach