Evaluation of various slip estimation techniques for an induction machine operating under field oriented control conditions

Sensorless speed control of an induction machine can be achieved by synthesizing a slip estimate solely from stator voltages and currents. Stator or rotor flux linkages can likewise be estimated, thereby allowing alignment of such flux linkages with a rotating coordinate axis to produce rapid torque and speed responses. Six published methods of rotor slip estimation are evaluated with simulated data. Only two of these show signs of being usable. These are both based on the steady-state machine model and produce fair slip accuracy for a machine running under perfect field alignment conditions. It is not ascertained whether the other techniques yielded poor results because they required a closed-loop speed controller, because they were presented incorrectly in the literature, or because they have fundamental deficiencies. Based on the experience gained from these evaluations, a new technique has been developed, and both measured and predicted results are presented to show that it works well. Moreover, this new technique has many features that make it theoretically and practically more attractive than most of the other existing techniques.<>