Sensor-less flux determination in induction motor based upon saturation effects using high-frequency magnetizing current

As real-time computation costs continually decline, both mechanical robustness and economic considerations increasingly favor the replacement of mechanical sensors and transducers by software-based state estimation methods. The elimination of encoders or resolves on induction machine drives is a prime example. For high performance induction motor drives based on 'field-orientation', it is necessary to determine the angle of a magnetic flux vector inside the induction machine (rotor, stator, air-gap) with respect to a stator frame. When this is done without a flux sensor and without a rotor speed/position sensor, the voltage/current, or v/i-model, is the means of determining this flux angle. The v/i-model has drift problems, especially at low flux frequencies. By analyzing the v/i-model in the magnetic field coordinates instead of the fixed stator coordinates, the drift problem becomes a stability problem. Internal feedback methods still leave an operation area where such sensor-less control performance is low: the area around zero flux frequency [1]. In this frequency range the high frequency magnetizing current injection method can provide a way of determining the flux angle, even at zero flux frequency. This method is based upon saturation effects inside the induction machine and its structure is the basis of this proposal.;Prior to this work, existing control methods by injecting high frequency estimation techniques for machines were design dependent [1,2--7]. Furthermore, the limitations of these existing and also newly emerging observer-based estimation techniques were not well understood nor well applied. This work first focuses on evaluating and improving the rotor flux estimation from stator voltage and currents for standard induction machines, without modification to rotor design. In this work, recent developments in the saturated model of induction machines and their use for obtaining the coned location of the rotor magnetic flux for the induction motor are considered. In particular, the motion of stator current vector is transformed to the motion of the rotor current vector through a transformation ratio obtained from the motor magnetic saturation and the motion of the rotor current vector information is extracted from the motor voltage equations [3]. The method is based upon an effect that appears in the region of magnetic saturation. The transfer properties are derived from the equations of current fed (current regulated) induction machines taking saturation into account. Simulation results are presented to show that the algorithm combined with the saturated effect has promising results and would be able to achieve the zero speed control of sensor-less induction motor.