Performance of FFT-rotor slot harmonic speed detector for sensorless induction motor drives

The paper presents new results relating to real-time speed measurements obtained from the spectral estimation of rotor slot harmonic frequencies present in the stator currents of a cage induction motor. Derivations of the measurement accuracy and a method for attaining the maximum accuracy are presented. The paper derives expressions for predicting the performance under low motor loads and derives the region of signal indeterminacy in the torquespeed plane arising from interference by PWM harmonics. Tracking algorithms to reduce this indeterminacy are presented. A real time experimental implementation allows investigation of dynamic behaviour under speed transients. Use of the detector in speed controllers and in particular sensorless vector controllers is discussed. The research forms part of a larger programme aimed at achieving field-oriented control performance from induction motor drives without a mechanical sensor.