An encoderless induction motor drive with improved current control at low speed

In a speed-sensorless induction machine drive, the accuracy of the motor flux estimation deteriorates at low frequency due to the relatively large errors introduced by thermal variation of the stator resistance and the flux processing circuitry. This in turn introduces errors in the estimation of the motor speed and the orientation angle of synchronous reference frame. Undesirable phenomena such as current waveform distortion, motor torque pulsation and instability in speed control can be observed. A simple control method for improving the motor current and speed control at low frequency is reported. A variable control structure approach is incorporated to ensure the stability at both low speed and high speed. Experimental results verify the control approach