Investigation of Various Rotor Retaining Sleeve Structures in High-Speed PM Brushless Motors

A rotor retaining sleeve is normally a must in high-speed permanent magnet brushless motors to firmly retain the magnets at their positions and ensure the rotor integrity. The rotor retaining sleeve can be of different materials and shapes as long as it can withstand the high centrifugal forces exerted on the magnets. On the other hand, it is practically very difficult to remove the heat from the rotor in these motors. As the inevitable rotor temperature rise results in severe performance deterioration and even irreversible demagnetization, the overall rotor loss, especially the eddy current loss, is of paramount importance for these high-speed permanent magnet brushless motors. In this paper, the influences of retaining sleeve configurations, including laminated sleeve, circumferentially grooved sleeve, copper shielded sleeve, and irregular-shaped laminated sleeve, on the rotor mechanical stress and eddy current loss in a proposed high-speed permanent magnet brushless motor have been comprehensively investigated. The conductivity of the retaining sleeve and the permanent magnet has a huge impact on the rotor eddy current loss and the loss versus equivalent conductivity curve approximates to a Gaussian distribution. The study reveals the proposed laminated sleeve configuration can not only reduce the rotor eddy current loss effectively but also provide a robust rigid magnet rotor construction.