Investigation Of Rotor Eddy-Current Loss In High-Speed PM BLDC Motors

Due to small volume, high power density, high-speed motors are extensively used in industry applications such as compressors, centrifuges, vacuum pumps, turbo generators, flywheel applications, tooling machines and so on. Because of high efficiency, large airgap and simple rotor structure, PM BLDC motors are very suitable for high-speed operation. The main problems of high-speed PM BLDC motors are: (1) rotor mechanical design and rotor dynamics, (2) rotor loss and rotor temperature.Rotor loss is a major problem in high-speed PM BLDC motors, as it reduces the power efficiency and may demagnetize the magnets. The rotor eddy current is mainly caused by the space harmonics and time harmonics of armature current, as well as the airgap permeance variation due to slot-openings. This dissertation focuses mainly on the analysis and reduction of rotor eddy-current loss in high-speed PM BLDC motors including optimizing stator structure, slot opening, airgap length and utilizing of a conductive copper shield between the sleeve and magnets.Firstly, the influence of various stator structures (2p-3s non, 2p-6s over and 2p-6s non), slot opening width and airgap length on the rotor loss was studied, with both analytical calculation and FE simulation. Space harmonics of armature mmf are calculated with current-sheet method and Fourier analysis. The rotor loss is then derived with an improved model which takes into account the skin depth and the eddy-current reaction field. The results show that 2p-3s non exhibits high even-order space harmonics of stator mmf, and consequently causes much rotor loss. Bigger airgap length and smaller slot openings are preferred from the point view of reducing the rotor eddy-current loss.Secondly, the effect of segmenting magnets on rotor loss is analyzed. Only when the number of the magnet segments is bigger than 12, the eddy-current loss in magnets can be reduced. The influence of the conductivity of retaining sleeve on the rotor eddy-current loss in high-speed PM BLDC motors is analyzed with FEM. The loss in the Titanium sleeve is much higher than that in the carbon fiber sleeve. However, the eddy current in the Titanium sleeve smoothens the field in the magnets, hence, reduces the loss in the magnets. Furthermore, the influence of the copper shield between the retaining sleeve and magnets is inspected. Although eddy-current loss occurs in the shield, the losses in the other rotor parts are dramatically reduced, resulting in a much lower overall rotor loss. The relationship between the thickness of copper shield and the rotor eddy-current loss is also examined. Thirdly, for a given motor envelope, two high-speed PM BLDC motors with different winding inductances are designed. The research shows that big winding inductance can smooth the winding current and reduce rotor eddy-current loss.Fourthly, influence of parallel and radial magnetization for surface mounted magnets on the performance of a high-speed PM BLDC motor is comparatively studied. It shows that parallel magnetization is better than radial magnetization. Two kinds of mechanical systems with different stator cooling and bearing lubrication are designed. Rotor dynamics of two kinds of bearing supporting systems are comparatively studied and it shows that bearings introduce low-frequency cylindrical and conical vibration modes and the rotor natural frequencies can be improved by changing the bearing supporting from single side to double sides.At the last, a control system based on MC33035 for high-speed PM BLDC is developed to provide a reliable experiment environment for no-load and load operations. The effects of PWM current control and copper shield on the rotor eddy-current loss are investigated. It shows that the rotor eddy-current loss can be dramatically reduced by using copper shield and the PWM current control method will introduce high order time harmonics causing additional rotor eddy-current loss. The experiment results are validated by transient temperature finite element simulation.