Control of induction motor is most precisely required in many high performance applications. With the development in power electronic field various control methods for control of induction motor have been developed. Among these Direct torque control (DTC) seems to be particularly interesting, being independent of machine rotor parameters and requiring no speed or position sensors. In addition to the simple structure it also allows a good torque control in transient and steady state conditions. The disadvantage of using DTC is that it results in high torque and flux ripple and variable switching frequency of voltage source inverter, owing to the use of hysteresis controllers for torque and flux loop. In order to overcome these problems, various methods have been proposed by several researchers like variable hysteresis band comparators, space vector modulation, predictive control schemes and intelligent control techniques. However these methods have diminished the main feature of DTC that is simple control structure. This report presents constant switching frequency based torque and flux controllers to replace conventional hysteresis based controllers where almost fixed switching frequency with reduced torque and flux ripple is obtained by comparing the triangular waveforms with the compensated error signals
2. Torque controller
3. Flux controller
Fig.1. Block diagram of conventional DTC method
Fig.2. MATLAB/SIMULINK Model of the DTC Drive
Fig.3. Torque response (a) Conventional DTC scheme (b) Improved DTC Scheme
Fig.4. Speed and Torque response for (a) Conventional DTC scheme (b) Improved DTC scheme
Fig.5. Circular flux locus (a) conventional DTC scheme (b) Improved DTC scheme
Fig.6. 3-phase line-line voltages and currents (a) Conventional DTC scheme (b) Improved DTC scheme
In this paper a detailed comparison between the conventional DTC and improved DTC scheme is made with help of some Matlab simulation results and hence it is shown that a significant reduction in torque and flux ripple can be achieved with the improved DTC scheme also with improved controllers the switching frequency which is constant can be varied by varying the frequency of the triangular carrier waveforms of the torque controller
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