Improvements in direct torque control of induction motors

Abstract

Electrical motors have been increasingly employed for more than a century to convert electrical energy into mechanical energy. The wide variety of existing applications requires in many cases a very precise control of the electrical motor by means of a variable speed drive incorporating sophisticated control strategies. Besides the expectations that have to be met concerning the motion control, other aspects such as efficiency, reliability, generation of electromagnetic pollution, etc. have become very important and need to be addressed in the design of variable speed drives.

The work reported in this thesis is devoted to the investigation of speed and torque control methods for electrical motors with particular emphasis been given to a specific control strategy known as Direct Torque Control (OTC). OTC is one of the preferred control techniques for Induction Motor (IM) drives due to the high performance obtained and its structural simplicity. However, it has a number of inherent disadvantages such as: poor performance at low speed operation, unwanted torque and flux ripples, variable switching frequency and a higher harmonic distortion of the stator voltage and current waveforms compared with other methods, such as Field Oriented Control (FOC). It is the aim of this work to improve the control system in order to eliminate or considerably reduce the undesirable features that exist with the Classical OTC method.

The proposed solution investigated consists in the combination of the DTC principle with a multilevel power converter: the three-level Voltage Source Inverter (VSI). This type of VSI can deliver an increased number of voltage vectors when compared to the standard two-level VSI, and this feature enhances the possibilities of the controller when selecting the output voltage vector to be applied to the motor. A new controller has been developed to exploit this potential. The resulting control system is able to achieve a fast and precise control of the stator flux and torque, with a considerable reduction of the ripples. Moreover, the use of the three-level VSI provides reduced voltage derivatives, lower harmonic distortion in the stator voltages and currents, and a reduction of the Common-Mode (CM) voltage and leakage currents. The switching frequency and the voltage supported by the semiconductor devices are also reduced, and their stress and power losses are therefore lower.

Date of Award	2008
Original language	English