Variable Passive Radial Stiffness in Single-Drive Bearingless Motors for Changing Rotor Natural Frequency

This paper presents variable radial stiffness of single-drive bearingless motors. One-axis actively positioned bearingless motors have been studied for industry applications such as pumps, compressors, and cooling fans. In particular, single-drive bearingless motors can generate torque and active axial force by q- and d-axis currents with only one three-phase inverter. The radial and tilting directions are passively stabilized by repulsive type passive magnetic bearings. In general, the passive radial stiffness is mostly constant around the center position. This paper presents novel characteristics of variable radial stiffness capability in a slotted single-drive bearingless motor. The radial stiffness can be changed with field flux controls, and it is enhanced by flux-weakening control because unbalanced magnetic pull that is generated between the rotor permanent magnets and the stator core is decreased. In 3D-FEM analysis, variable capabilities of the passive radial stiffness are calculated, and the unique phenomenon is experimentally demonstrated in the prototype machine. The variable passive radial stiffness is utilized to change natural frequencies, and it is useful for reducing rotor radial vibrations at critical speeds.