Asghar Taheri

PhD Thesis Defense Session

Abstract:
Recently, multiphase machines have been received great deal of attention. Some of the advantages of these machines can be listed as: higher redundancy and reliability under fault conditions, lower torque pulsations, reduced rotor harmonic currents, lower DC-link voltage requirement, lower power per phase, and a reduction in the stator copper loss. The six-phase induction machine (6PIM) is known as an interesting and the most discussed types of multi-phase machines. Until recent years, the literature of 6PIM drives has covered different issues.  Some papers addressed the multi-phase drive efficiency; improving efficiency of six-phase induction machine is an important issue that addressed less in the articles. Six-phase induction motor has 64 space voltage vectors which causes more complexity in selecting of inverters switching. Unsuitable selection of switching table leads to large harmonics especially in low speed and it reduces drive efficiency. The six-phase induction machine has large zero sequence harmonic currents of order . These harmonic currents are varied in various techniques. Decreasing this loss is so essential in six-phase induction machine. Improving of ST-DTC of Six-phase induction machine to reduce voltage and current harmonics and torque pulsation is addressed newly in this thesis. Selecting a suitable method by minimizing these harmonics is very important in DTC of six-phase induction motor. Motors are designed to have maximum efficiency at their nominal point. If the motor load or velocity is under this point, flux should be reduced to have appropriate efficiency. There are a number of approaches for choosing a suitable control method including Loss Model based Control (LMC), Search Control (SC). A flux search controller is proposed to improve the efficiency of direct torque control and field oriented control of six-phase induction machine. The proposed flux search controller is based on adaptive gradient descent of motor flux value with fast response and easy implementation. A proper loss model of six phase induction motor in conjunction with the proposed method has been presented. Stator frequency variation with respect to flux reduction is considered as a new aspect of fast flux search control. Reduction of the flux in the search control technique leads an increase in stator frequency and an error in core loss calculation is occurred if the frequency variation is ignored. The proposed search controller greatly improves efficiency by reducing core loss as well as harmonics loss. The approach not only is easy to implement and adaptive about parameter variations but also requires no additional hardware for practical implementation. The simulation and experimental results presented in this paper verify the effectiveness of the proposed method in improving efficiency of direct torque control and field oriented control of six-phase induction motor drive. Adaptive gradient variation of flux technique decreases the convergence time, noise impact, and increases accuracy of algorithm.  Although using the frequency variation affects the core loss calculation, the SC technique causes an increase in accuracy of the core loss calculation. The proposed loss modeling of DTC of 6PIM can describe dynamic variation of losses in the SC technique not only in 6PIM but also in IM. Flux variation in the SC technique results in variation of core loss; a matter previously worked in literature. However, the flux variation leads to a variation in frequency as well as core loss coefficients. In this thesis, the accuracy of loss calculation and efficiency in the SC technique is improved considering the frequency.

:  Date of Defense
Saturday 3 sep. 2011, 15 pm
Electrical Engineering Department, class NO. 305