Novel Dual Stator Axial Flux Brushless Doubly Fed Reluctance machine for Improving Torque Density

Abstract

This thesis focuses on the development of new axial flux brushless doubly fed reluctance machine having the higher torque to volume density compared to its radial flux counterpart. In particular, the rotor pole design which is very important for such machines has been focused and analyzed to increase the effectiveness and advantages of the proposed AF-BDFRM. Furthermore, sample prototype of 1.0 HP was selected for testing considering the manufacturing cost and available testing facilities to verify the proper working of the novel AF-BDFRM. The proposed machine is most suitable for applications that require speed control over a limited range like in wind turbines and compressors. As it utilizes only slip power which reduces the rating of the converter to be used with control winding. Therefore, the overall cost of the system goes down, and it becomes a viable option for such variable speed applications. Firstly, the design concept including the basic design criterion in single and dual air gap machines is explained. Then, the axial flux design consideration was taken into account to design the proposed AF-BDFRM. Then the design procedure including the design, analysis and optimization method for the rotor pole of proposed machine are discussed in detail. The proposed machine model has two stators and one salient pole reluctance rotor. The salient pole rotor is made of only iron and winding free. The absence of permanent magnets, ability to control power factor and stable mechanical structure of the machine make it overall cheaper and more rugged. The absence of copper losses in the rotor also increases the overall efficiency of the proposed machine compared to doubly fed induction machine. Proposed machine can operate in sub-synchronous, synchronous or super-synchronous mode depending on whether a negative sequence frequency, DC or positive sequence frequency is applied to the control winding, respectively. The unaligned placement of two stators suppresses the cogging torque and torque ripple in the proposed machine. Torque to volume ratio of the AF-BDFRM is higher than that of a radial flux BDFRM because torque is directly proportional to D^3 and not to D^2L. However, its air gap length is double than RF-BDFRM which will cause more leakage flux in the proposed machine but the surface current density also can be increased in the proposed design as it has two stators whereas RF-BDFRM has only one stator, and its surface current density has to be limited to K = K_1 + K_2. A transient 3D FEM analysis was done to analyze the performance of the proposed AF-BDFRM and the performance was compared with the already developed RF-BDFRM. The rotor pole shape was modified to a diamond shape to reduce cogging torque and torque ripple. Transient FEA results show that the proposed machine exhibits 91% higher torque density when compared with the already developed RF-BDFRM whereas the weight of the proposed machine is 22.5% lighter. However, it was observed that the rotor pole shape could be further improved and optimized for higher torque density and lower torque ripple. For the optimization of the rotor pole in the proposed machine, the objective function and design variables were selected. Inner overhang, outer overhang, and rotor span were selected as the design variables. Rotor pole span was chosen as a design variable to increase the lower reluctance path for the flux. Then, the Latin Hypercube (LHC) sampling method was used to design the experiments. After that, the Kriging method was used to approximate the objective function and a genetic algorithm (GA) was utilized to find optimal values for the selected design variables. A basic sinusoidal shape instead of the previous diamond shape was selected as the starting point for optimization of the rotor pole. The back EMF increased from 378 V for the former diamond shape to 453 V for optimized sinusoidal shape. The output torque performance was also greater for the optimized rotor pole corresponding to an increased torque density of the machine. Furthermore, the torque ripple was observed 16% for the optimized sinusoidal shape which was 26.6% for the diamond shape. Torque output, which was previously 323 Nm, increased to 371 Nm. To verify the FEA and the proper working of the novel AF-BDFRM, a 1.0 HP prototype was fabricated for experimentation. Because this prototype was needed only to verify the proper working of the proposed AF-BDFRM, therefore, considering the manufacturing cost and the available laboratory facilities, a 1.0 HP prototype is selected to perform experiment. A basic sinusoidal shaped rotor pole with no overhang and a 30o mechanical span was selected. This 1.0 HP prototype was built and tested in the laboratory. The rotor and stator poles were chosen to fulfill the speed requirement and to reduce the unbalanced magnetic pull (UMP) which is necessary to consider for the mechanical stability of axial flux machines. Stacking factor and the coil area of the constructed prototype was found to be lowered during manufacturing which reduced the flux carrying capability of the iron core and linkage capacity of winding coils. Due to this reason, a difference of 10% was observed between the simulated and experimentally induced back EMF. However, Experimental results verified the proper working of the novel AF-BDFRM and its smooth functioning over a range of variable speed.