A torque ripples compensating technique based on disturbance observer with wavelet transform for sensorless induction motor drives

In a sensorless vector control system of the induction motor, it is difficult to estimate rotor speed accurately in the lower speed region because the dead times of the switching devices in a inverter distort the waveforms of the voltages and currents, and these distorted currents affect on the estimated speed as a fluctuation. The motor torque thus has ripples whose frequency are 1 and 6 times those of the primary frequency. In this paper, the authors propose an accurate speed controller in the lower speed region for a speed sensorless vector control system. In the system, the load torque is estimated by a disturbance observer and in order to extract the ripple of torque, they adopt the wavelet transform technique to the estimated torque. From experimental results obtained using this system, sensorless vector controlled induction motors can be driven accurately in the lower speed region.