Comparison Between NdFeB and SmCo Permanent Magnets Regarding Eddy Currents in Bearingless PM Synchronous Machines

PM synchronous machines, used as high-speed drives, typically employ SmCo magnets instead of NdFeB due to the possibly high rotor temperature and the low air gap flux density. SmCo magnets exhibit a lower remanence and a higher electric conductivity than NdFeB material, leading to higher eddy current losses. Additionally, in bearingless PM machines, the asynchronously rotating suspension winding air gap field wave induces eddy currents in the rotor. These lead to a damping and de ection of the air gap field and, thus, to a disturbing rotor force, which may be crucial for the rotor position control. We discuss the occurrence of the eddy current-originated disturbing force, present as an force error angle, for three PM materials: SmCo5, Sm2Co17 and NdFeB. By measurements on two geometrically identical rotors in a prototype machine, employing a Sm2Co17- and a NdFeB-PM in the rotor, we show that the force error angle is considerably bigger for the Sm2Co17-rotor. Still, for the considered small machine dimensions the error angle is small enough to not endanger the rotor position control stability.