Analysis of Axial Flux Machine with Sinusoidal Permanent Magnet Shape and Consequent Pole for Low Torque Ripple

Abstract

This dissertation presents the axial flux permanent magnet (AFPM) machine with sinusoidal shaped poles for high speed and low torque applications. The idea of using the axial flux topology for high speed application is to utilize its attractive features as compared to the radial flux machines. The axial flux machines have proved to have better performance in terms of higher torque to volume density than their radial flux counterparts. Not only this, axial flux machines do not reduce the effective air gap of the machine due to the retaining sleeve unlike in the radial flux high speed machines. Furthermore, the inertia of an axial flux machine is easier to adjust than the radial flux machine by simply increasing the rotor core thickness or the axial length. Due to these reasons, the AFPM proves to be a good choice to be considered for high speed applications. The proposed AFPM machine consists of slotless stator and dual-rotor structure with two poles on each rotor The slotless stator have simple airgap winding forming a simple structure in order to eliminate the stator slot harmonics. Therefore, cogging torque of the machine can be completely eliminated. The rotor poles are designed in the form of a sinusoidal shape using analytical modeling. The sinusoidal permanent magnet (PM) poles induce an ideal sinusoidal back-electromotive-force (back EMF) without any harmonics. The sinusoidal back EMF produces the negligible torque ripples in the machine when applied with a sinusoidal current source. The torque ripple factor is a crucial parameter in the design of high speed machines which is mainly responsible for the smooth operation with minimum noise and vibrations. A consequent pole (C-Pole) rotor model is also designed and analyzed considering the sinusoidal pole shape which uses one PM pole and the other as iron pole. The sinusoidal consequent pole inherits the properties of both the production of sinusoidal back EMF as well the higher torque to PM volume density. The C-Pole model reduces the amount of highly expensive SmCo PMs without affecting the performance of the machine. In this model, the pole thicknesses of the two rotors are kept of unequal to generate a net axial force. The net axial force primarily balances the weight of the rotor to reduce the bearing stress. This feature is the inherent property of the dual rotor AFPM machines. The feasibility and the performance of the proposed sinusoidal C-Pole is verified using the FEM simulations and the results shows improved performance in torque to PM volume density while maintaining the low torque ripples. The overload current torque capability analysis is also performed to verify the operation of the iron pole at the overload current. The sinusoidal PM rotor poles are caged into a special designed sleeve to form a robust rotor structure as well as increase the mechanical strength of the overall rotor structure. The sleeve is made up of aluminum material due to its almost zero permeability and high mechanical strength. The mechanical stress analysis is performed using finite element method (FEM) to validate the effectiveness of the proposed sleeve. A prototype of the proposed high speed machine is manufactured and the back EMF is measured at the rated speed of 32,000 rpm. The experimental results verify the FEM simulation results within marginal error. A drive system is built to run the proposed machine at the rated speed. The prototype runs well at the and the experimental results are included in the dissertation.