Design Guidelines and Scaling Effects for Bearingless PM Synchronous Machines Accounting for Eddy Current Reaction Field

Apart from mechanical constraints, the maximum size and output power of high-speed bearingless PM synchronous machines is mainly limited by two effects with disturbing influence on the bearing force vector. One is the interference between the stator drive field and the suspension air gap field. The other one is the air gap field deformation by rotor eddy currents due to the asynchronously rotating stator field wave, which generates a nonmoving bearing force vector. The eddy current reaction field lowers the bearing force amplitude and leads to a spatial error angle. Apart from material properties, these two effects are mainly influenced by the split ratio and the permanent magnet height. It is discussed how the machine's main dimensions and pole count shall be chosen in order to keep the influences of these effects small. It is also shown how the motor efficiency suffers from these effects and how the corresponding limitations vary with machine size and electromagnetic utilization.