Reduced Torque Ripple And Switching Frequency Using Optimal DTC Switching Strategy For Open-End Winding Induction Machines

Direct Torque Control (DTC) of induction machine has received wide acceptance in many Variable Speed Drive (VSD) applications due to its simple control structure and excellent torque dynamic control performances. However, the DTC which employs a two-level inverter and hysteresis controllers produces two major drawbacks, namely, larger torque ripple and variable switching frequency, which might produce a very high switching frequency (or power loss), particularly at a very low speed operation. The root causes of the problems can be identified as follows; 1) delay actions in controlling the torque (which is commonly resulted in digital implementation of hysteresis controller) causes the torque cannot be exactly restricted within the hysteresis band, and hence produces a larger torque ripple 2) inappropriate selection of voltage vector (among a limited number of voltage vectors available in a two-level inverter) cannot restrict the increase of switching frequency in the hysteresis controller, as the torque slopes regulated in hysteresis bandwidth vary during operating conditions. This thesis proposes an optimal DTC switching strategy to reduce torque ripple and switching frequency for open-end winding induction machines. The open-end winding induction machine is supplied by a dual-inverter which can offer a greater number of voltage vectors and hence, gives more options to select the most optimal voltage vectors to minimize the problems. The most optimal voltage vectors for every speed range are identified as the vectors that can produce the minimum torque slopes. By minimizing the torque slopes, the torque ripple and switching frequency can be reduced. The identification is made by investigating the torque slope behaviours and torque control capabilities for every speed range. The selection of the most optimal voltage vectors is accomplished by using a modification of torque error status and a look-up table. To obtain a constant switching frequency, a Constant Switching Frequency Torque Controller (CSFTC) is proposed without the use of a PI controller and a knowledge of machine parameters. Some improvements obtained in the proposed strategy were verified via simulations and experimentations, as well as comparison with the conventional DTC. The improvements obtained are as follows; 1) reduction of torque ripple and switching frequency with the proposed optimal DTC switching strategy, 2) a constant switching frequency with the proposed CSFTC. The main benefit of the proposed strategy is its simplicity, where the DTC improvements can be obtained without the common approach, i.e. the use of Space Vector Modulation (SVM) which involves complex control algorithms. It also shown that the average improvement about 39% and 43% can be achieved toward reduction of torque ripple and switching frequency

New torque and flux controllers for direct torque control of induction machines

The new torque and flux controllers with constant switching frequency and low torque and flux ripples for direct torque control induction machine drives are presented. The core of these proposed controllers is based on the comparison between the compensated error signals with high frequency triangular waveforms, thus does not require complex calculation to generate the inverter switching signals. The controllers are therefore can be implemented using analog and/or digital circuits. Modeling and simulation of the new controllers are presented and the results show that the torque and flux ripples are reduced significan

Engineering design project : series 1

This book is a compilation of engineering design projects which illustrates a detail explanation on the design process including all tools and methods involved in each design stage. The presented projects have been selected to prepare engineering students with the adequate knowledge and skills required in organizing product based project, formulating problems, generating and evaluating alternatives, preparing technical documentation and considering the sustainable and economic constraints for the product development and realization stage using the integration of various disciplines in mechanical and manufacturing engineering. This book is generally suitable as a reference to project-oriented problem-based learning (POPBL) activities at engineering institutions

Predictive current control of asynchronous machines by optimizing the switching moments

In this paper a model-based predictive control (MBPC) scheme for the current control of induction machines is presented. The controller directly selects the optimal switch state of the inverter. The proposed scheme uses a longer prediction horizon and a limited amount of optimal switching instants to reduce the average switching frequency. The next iteration of the MBPC-scheme is performed at the established optimal switching instant, as such suppressing the receding horizon property for short time spans. The proposed method is compared to a more conventional MBPC-scheme with a very short prediction horizon. Both simulations and experiments clearly show a significant reduction in average switching frequency. However, with a reduction in switching frequency the torque ripple is increased. To correctly asses the properties of the different schemes, a key performance indicator is proposed that offers a fair and unbiased comparison in terms of switching frequency and torque ripple

Direct torque control of brushless DC drives with reduced torque ripple

The application of direct torque control (DTC) to brushless ac drives has been investigated extensively. This paper describes its application to brushless dc drives, and highlights the essential differences in its implementation, as regards torque estimation and the representation of the inverter voltage space vectors. Simulated and experimental results are presented, and it is shown that, compared with conventional current control, DTC results in reduced torque ripple and a faster dynamic response

An improved two-vector model predictive torque control based on RMS duty ratio optimization for pmsm

This paper proposes an improved two-vector model-predictive torque control (MPTC) strategy to reduce the average torque ripple and improve the flux tracking performance. When determining the duty ratio of vector combination, this method aims at restricting the root mean square (RMS) error of both torque and flux during the whole control period. Every vector combination and corresponding time duration are evaluated in the cost function, which leads to global restriction of torque ripple and flux ripple. In order to avoid increasing switching frequency and computational burden, a restriction is added on the second vector. The three candidates of the second vector are the two adjacent vectors of the first one and zero vector. Simulation results are provided to show the effectiveness of the proposed strategy

Multiphase induction motor drives - a technology status review

The area of multiphase variable-speed motor drives in general and multiphase induction motor drives in particular has experienced a substantial growth since the beginning of this century. Research has been conducted worldwide and numerous interesting developments have been reported in the literature. An attempt is made to provide a detailed overview of the current state-of-the-art in this area. The elaborated aspects include advantages of multiphase induction machines, modelling of multiphase induction machines, basic vector control and direct torque control schemes and PWM control of multiphase voltage source inverters. The authors also provide a detailed survey of the control strategies for five-phase and asymmetrical six-phase induction motor drives, as well as an overview of the approaches to the design of fault tolerant strategies for post-fault drive operation, and a discussion of multiphase multi-motor drives with single inverter supply. Experimental results, collected from various multiphase induction motor drive laboratory rigs, are also included to facilitate the understanding of the drive operatio