Improved dynamic performance of direct torque control of induction machines

Abstract

This thesis proposes simple methods to improve the dynamic performances of a Direct Torque Control (DTC) of induction machines. The principles of direct control of torque and flux based on the selection of appropriate voltage vectors are reviewed. In DTC, the torque is directly controlled by the slip angular frequency which is determined by the irregular motion of stator flux vector. The stator flux is always forced to follow its circular reference by the application of active voltage vectors. During a large torque demand, no zero voltage vector is used. However, the active voltages are switched more often to increase (or decrease) rapidly the slip and the output torque as well. Moreover, this method does not give the fastest dynamic torque response because one of the two possible active voltage vectors switched during torque dynamic is not optimal. The effect of selecting two different voltages on torque dynamic response is investigated and evaluated. Based on the investigation, the most optimized voltage is identified, and it is used to perform a dynamic overmodulation to produce the fastest torque dynamic control. The improved torque dynamic control is verified by simulation and experimental results. Owing to the intrinsic characteristic of DTC switching, it is not possible for stator voltage to perform a six-step mode. The thesis proposes a simple overmodulation strategy for hysteresis-based DTC by transforming the flux locus into a hexagonal shape. This is accomplished by modifying the flux error status before it is being fed to a look-up table. In this way, a smooth transition of stator voltage from Pulse Width Modulation to the six-step mode is achieved as the number of application of zero voltage vectors, i.e. during the motor acceleration, is gradually dropped to zero. With the six-step voltage operation, it allows the DTC to extend a constant torque region and hence results in higher torque capability in a field weakening region. To verify the improvement of the proposed method, simulation and experimentation as well as comparison with the conventional DTC scheme are carried out. It can be shown that the improvement about 20% (in terms of extension of constant torque region) can be achieved through the proposed method. The improvements mentioned above can also be achieved in the DTC with constant frequency torque controller that offers constant switching frequency and reduced torque ripple. The improvements are verified by experimental results.