Design and electrodynamic analysis of active magnetic bearing actuators
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For more than a century, engineers have imagined bearing systems that use magnetic fields to levitate rotors in rotating machines, eliminating contact between bearing surfaces. In the past twenty-five years, magnetic bearing systems have moved from laboratory novelty to an accepted industrial product, and are now being used in an impressive variety of applications. This dissertation deals with the development and verification of design codes for permanent magnet bias, homopolar magnetic bearing actuators. A design code using magnetic circuit analysis is developed that can provide quick evaluation of candidate bearing actuators. Non-linear material properties are represented, and force versus current bearing characteristics can be calculated as a function of operating speed. Details of code development and description of a user interface created with a commercially available spreadsheet program are presented. To verify magnetic circuit design code predictions, three-dimensional finite element analysis is performed for a magnetic bearing system of interest. For experimental verification, an inside-out topology test bearing actuator and testing fixture were designed and fabricated in which bearing parameters were directly measured. Test results are presented and compared to theoretical predictions of the circuit analysis code and finite element program. In high-speed rotating machines, rotating losses are a prime concern because heat transfer mechanisms to remove rotor heat are limited. Losses inherent in permanent magnet bias homopolar magnetic bearings are discussed and the dependence of losses on bearing geometry is explored. Studies to reduce rotor losses by optimizing stator winding slot geometry are presented. Finally, a thrust-bearing concept designed to further reduce bearing losses is evaluated. In this concept, the static rotor weight of a vertical-axis machine can be supported by bearing actuator bias fields, minimizing required control effort. This concept holds the promise of reducing actuator power input and bearing losses, thus increasing bearing system efficiency.