Abstract
Many high speed applications employ a surface permanent magnet (PM) machine topology with a retaining sleeve due to its robustness and ability to achieve high overall peripheral speeds as well as efficiencies. One often overlooked feature in the mechanical design of such machines, which has not achieved sufficient attention to date is the anisotropic thermal expansion of rare earth magnets, the degree of which varies for different magnet technologies. This paper investigates the effects of the aforementioned on the mechanical design of a high speed PM spindle machine with NdFeB magnets. The maximum allowable interference is found to be limited by the working temperature of the magnets while the minimum required interference is increased due to their anisotropic thermal expansion. Based on this, appropriate conditions are formulated to integrate a Neodymium Iron Boron (NdFeB) PM in high speed rotors. These modifications considering the shaft together with the magnet anisotropic thermal expansion are included in a proposed rotor design and validated using simulations in ANSYS mechanical environment.
Highlights
High speed electrical machines have increased their market uptake in applications like microturbines, turbochargers, turbomolecular pumps and gas compressors [1]
Surface mount permanent magnet synchronous machine (PMSM) with a retaining sleeve is selected to realize the wide ranging requirements imposed on the motor like ultra high speed, compactness and high efficiency
The permanent magnet (PM) is in tension for type I, II and III PM unlike the result predicted by considering isotropic thermal expansion
Summary
High speed electrical machines have increased their market uptake in applications like microturbines, turbochargers, turbomolecular pumps and gas compressors [1]. The temperature considered here is the maximum estimated operating point resulting from rotor losses [11] and not the brief value attained by PM upon interacting with the heated retaining sleeve during fabrication. In existing literature axisymmetric shrink fit analysis is performed considering isotropic thermal expansion of PM if a metallic retaining sleeve is used [10,12,17]. Both NdFeB and SmCo PMs exhibit anisotropic thermal expansion [18,19].
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