Design considerations of sinusoidally excited permanent magnet machines for low torque ripple applications

Several high performance applications such as electric power steering require, the motor drive to produce smooth torque with very stringent torque ripple requirement. This paper is focused on various machine design considerations that can be used in reducing the torque ripple of a sinusoidally excited permanent magnet (PM) brushless DC (BLDC) motor. The paper quantifies the various sources of torque ripple, which may be minimized by appropriate design considerations. The paper discusses the factors influencing the harmonic content of the induced voltage, effect of slot/pole combination, winding distribution and magnetic saturation. Design optimization is directed to minimize cogging torque and the harmonic contents in the back-emf, thus reducing the overall torque ripple. Comprehensive finite element analysis along with experimental data is provided to validate the theory. The research demonstrates that saturation in the magnetic circuit is another major contributor to the torque ripple and torque nonlinearity as the current increases. A model is developed to study the saturation effect on torque linearity and is verified by FE simulation. Design techniques have been provided to minimize the overall torque ripple and increase the torque linearity.