An inner-rotor flux-modulated permanent-magnet machine for low-speed high-torque applications
Applications for low-speed, high-torque electric machines are widespread, including marine propulsion and wind power generation. These applications often have stringent space and weight limitations. Hence, due to the size of the machine being proportional to the required torque, these applications call for machine designs that put a premium on reducing total mass while still achieving the requirements of the application. Possible approaches to reducing mass and volume include the use of mechanical gears that increase the speed and lower the torque requirements of the machine. This, however, results in increased noise, increased maintenance, and decreased reliability. An alternative to mechanical gears that does not have these limitations is magnetic gearing. Magnetic gears replace the meshing teeth of a mechanical gear with meshing permanent magnets through an array of steel pieces . Integrating magnetic gears into the machine structure avoids the need for a separate gearbox. There are a number of approaches to integrating the gear into the machine. The machine structure proposed in this work consists of a combination of a magnetic gear internal to a permanent magnet synchronous machine (PMSM) to create an inner-rotor, flux-modulated, permanent-magnet synchronous machine (IRFMM). Unlike previously proposed arrangements, the proposed machine utilizes a more desirable inner-rotor configuration and avoids multiple magnet arrays. The inner rotor is the only moving part in this machine structure. In this work, a finite element analysis (FEA) design model is set forth. The FEA model consists of a structured mesh that is able to adapt to changing machine geometry and materials. This adaptability is required in order for the model to be used in the design environment utilized in this work. The IRFMM design is formulated as an optimization problem that utilizes a genetic algorithm to explore a large search space. In addition to the novel machine structure and FEA model proposed, this work sets forth two design studies. The first is a single-objective optimization based design with mass as the objective. This study demonstrates a marked improvement in mass for the IRFMM over a conventional PMSM and a 10 percent improvement over an alternatively geared machine structure for a given low-speed, high-torque application. The second study is based upon a marine propulsion application and is a multi-objective optimization based design method of the proposed machine structure. This study presents a tradeoff between the mass of the machine and its loss. The losses include resistive, skin and proximity effects, core losses, and the semiconductor losses associated with the power electronics that are inherently paired with the machine. This tradeoff enables a clear understanding of the advantages of combining the magnetic gear within the machine. The multi-objective study demonstrates a marked improvement in mass and loss for the IRFMM over a conventional PMSM for the given marine propulsion application.
Sudhoff, Purdue University.
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