Direct torque control using cascaded h-bridge multilevel inverter for induction motora

Abstract

This thesis proposes methods to improve the performance of a Direct Torque Control (DTC) of induction motor drives. The basic principle and theoretical aspects of the DTC using a conventional inverter (DTC-Conv) and the DTC using a 5-level Cascaded H-Bridge Multilevel Inverter (DTC-CHMI) are reviewed with emphasis on two major problems: high torque and flux ripple and variable switching frequency. Based on the basic principle of the DTC, torque and flux are directly controlled by selecting appropriate voltage vectors. A DTC-Conv offers eight voltage vectors to increase (or decrease) both torque and flux. Regardless of the torque’s demand, for the DTC-Conv, the application of voltage vector is limited to these eight voltage vectors. This will give a high torque and flux ripple because of the possible voltage vector selected is not optimal for the condition. Based on the investigation, by proposing the DTC-CHMI, a smaller torque and flux ripple can be achieved. Moreover this method offers a good torque response. This is due to the capability of the DTC-CHMI to offer 61 voltage vectors which give more options to choose the most optimum vector for any circumstances. In addition, less switching burden on the switching devices for the DTC-CHMI compared to DTC-Conv, which results in a lower power rating device to be used. It is well known that the implementation of the DTC-Conv consists of a hysteresis-based controller which results in a variable switching frequency in the switching devices. This undesirable condition will affect the inverter design since it is related to the rate of change of the torque which varies with various operating conditions. Therefore, this thesis proposes the proportional-integral controller constant switching frequency together with the DTC-CHMI to replace the DTC-Conv with a hysteresis-based controller. The proposed torque controller consists of three pairs of triangular carrier signals with three pairs of comparators. With this proposed controller, the variation of switching frequency can be narrowed and fixed at the carrier frequency. Furthermore, it minimizes the torque ripples. Design of the proposed controller is thoroughly discussed in this thesis. To verify the enhancement made by the proposed method, simulation and experiment, as well as the comparison with the DTC-Conv were carried out. Results prove that by using the proposed system, torque and flux ripple are reduced by 38.5% and 7.76% respectively. Apart from that, the switching frequency is fixed at 1.667 kHz and a less distorted sinusoidal phase current is obtained.