Bearingless Axial-Force/Torque Motor with Reduced Number of Power Switches

Abstract—In this work, full six degree-of-freedom bearingless operation of a special motor with only four power switches (or two half bridges) is presented. The utilized motor was introduced as bearingless axial-force/torque motor (BLAFTM), controlling two degrees of freedom (axial deflection and motor torque) actively, while all other degrees of freedom are passively stabilized by two permanent-magnetic ring bearings. This extremely economic power electronics setup was achieved introducing a split-capacitor (SC) leg, where both motor phases are connected to. However, balancing the SC voltage remains problematic, as it is affected by active axial force generation. This issue was solved implmenting a nested control scheme with an outer SC voltage controller regulating the axial position properly, as a deflected rotor leads to force currents that balance the voltage. This concept was implemented, commissioned and tested. Measurements proof full bearingless motor functionality.