This paper proposes a direct field-oriented controller forinduction machine drives based on a new sensorless speed measurementscheme. Current harmonics which arise from rotor slotting andeccentricity are analyzed using digital signal processing techniques. Anon-line initialization algorithm determines slot harmonic parameterswhich depend on machine structural characteristics. A digital spectralestimation algorithm then analyzes multiple current harmonics to produceaccurate results at any nonzero speed, independent of time-varying motorparameters. This robust speed information is used to tune the parametersof a mechanical model of the motor and the load. A speed observer basedon the mechanical model provides a speed signal to the field-orientedcontrol algorithm. A controller utilizing this tachometer-less observerhas the advantage over other sensorless schemes of being able to obtainhigh accuracy speed information down to very low speeds. This is due tothe fact that this scheme does not rely on flux (voltage) measurement,which is significantly degraded at low speed. The algorithm applies to abroad class of induction motors and requires no special machinemodifications
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