Design and optimization of the bearing system of a high-speed 100 kW electric motor operating in a vacuum

High rotational speeds and operation in a vacuum are particularly demanding operating conditions for certain electric motors and hence demand a holistic design approach of the bearing system. Due to the absence of convection for rotor cooling (Ohmic losses and eddy currents in electric rotor) the heat losses can be rejected only via the bearings. For that reason, thermal properties of the bearings – in particular thermal conductivity - are key properties for bearing system design. In this case, bearing design requires a profound thermal simulation, taking into account the electric motor’s duty cycle, efficiency, cooling system and bearing properties. However, some necessary input parameters can neither be provided by bearing manufacturers, nor be obtained through literature research. Consequently, these properties need to be determined empirically, using testing infrastructure and methods described in this publication. The simulation results regarding system temperatures are compared to actual measurement data of the final motor-generator. The design incorporates high-precision angular contact spindle bearings (25 mm bore) with ceramics balls and grease lubrication, operating at up to 30,000 rpm. The high n*dm values present in this application combined with a slim 7 kg sheet metal rotor also require thorough investigation of rotor dynamics to ensure safe sub-critical operation and avoid unwanted resonances. The insight gained through this partially empirical and partially simulative design approach allowed further improvement of the entire motor. The study is concluded with an in-depth system performance analysis.