Indirect Efficiency Measurement and Levitation Property Determination for High-Speed Bearingless PM Synchronous Machines

The determination of the motor losses and the levitation properties such as the force-current coefficient and the negative magnetic stiffness coefficient from measurements poses a difficulty in high-speed bearingless PM synchronous machines with active rotor position control. The difficulty arises from the high operational speed, which impedes a back-to-back coupling of the bearingless test machine with a load machine due to balancing issues. Also, the use of a force sensor for the determination of the force-current coefficient is difficult, since a stable connection between rotor and sensor at rotor rotation is hard to achieve. Thus, this paper presents indirect methods to determine the motor losses and the levitation properties from measurement. These methods only require the electric power measurement at motor load and motor no-load. Also, we present that the levitation properties are determined from the measured rotor position values and the measured currents of the rotor position control. The measurement methods are exemplified through a prototype machine with the rated data 1 kW / 60000 rpm. We conclude that the measurement results comply with results retrieved from analytical calculations and from 2-dimensional and 3-dimensional finite element method simulations.