Study On The Mechanical And Electromagnetic Properties Of High Speed Permanent Magnet Machine

The research work of this dissertation is a part of the project-"The distributed high speed generator system driven by micro-turbines and its energy conversion system", which is supported by the National Natural Science Foundation of China. The high speed generator, due to its small volume, high efficiency and great power density, is one of the great concerns in electrical engineering. Two main features: the high rotating speed of the rotor and the high frequency of the winding current and the stator core flux, determine the unique technology of the high speed generator. In this thesis, the mechanical and electromagnetic performances and corresponding key technology of the high speed permanent magnet machine are thoroughly investigated. The main contents are listed as follows:Firstly, the structure design and strength analysis of the PM rotor are carried out. A new type of rotor, which is made of an integral PM enclosed in a nonmagnetic high strength alloy, is brought forward for that the PM could bear little tensile stress but great pressure. To insure the PM rotor's safety, the interference fit between the PM and the enclosure must be carefully chosen for the pre-pressure must be greater than the centrifugal force so that the PM could still bear some pressure when the rotor is running at the rated speed. Based on the elastic thick wall theory and the finite element method, the stress model is built up, the interference value is determined and the stress distribution is calculated. The rotor structure and stress calculation method have been applied to the high speed PM machine prototype design.Secondly, the rotor stiffness analysis and the critical speeds of the rotor system supported by the magnetic bearing are calculated. The supporting isotropy of the magnetic beating is analyzed based on the electromagnetic field analysis and the supporting stiffness of the magnetic bearing is calculated using the static magnetic flux density in the air-gap. Then the dynamic equations of rotor system supported by the magnetic bearings are built up and the critical speeds are calculated by the finite element method. The calculation method has applied to design a high speed PM machine using magnetic bearings and the prototype machine has been running successfully at 60000r/min.Thirdly, the stator design is engaged and a new type of stator ring windings is brought forward. The lower layers of the stator windings are fixed in the inner six slots of the stator core, and the upper layers are fixed in the outer 24 slots of the stator yoke. The ring windings increase not only the cooling areas but also the cooling efficiency for the cooling air could go through the windings' surfaces directly. More important, the axial end of the ring winding is much shorter than that of the traditional 2-pole windings, which reduces the rotor stiffness greatly.Fourthly, the electromagnetic performances of the high speed PM machine are simulated using field and circuit coupled method. The loss and the stationary temperature rise of the high speed PM machine are analyzed. The main advantages of the high speed machine are small volume and great power density. However, the cooling problem is more serious due to the greater loss per volume and the smaller cooling areas. Experiment is done to test the magnetic properties and loss coefficient of the steel lamination for different frequencies and directions. The iron loss, copper loss, air friction loss on the rotor surface and the losses in the rotor are calculated. Then, the stationary temperature rise of the machine is calculated using finite element method.Finally, a high speed PM prototype with rated speed of 60000r/min has been manufactured and tested. The back EMF of the stator winding and the temperature rise of the machine are measured at different speeds. By comparing the test result to the simulation result, the theoretical analysis and design method of the high speed PM machine are partly proved to be correct Based on the study, a new structure of high speed PM machine is proposed.