A stable operation of induction type bearingless motor under loaded conditions

A control method of induction type bearingless motors, which realizes stable operations under loaded conditions, is proposed. Without a proposed control method, it is shown that both the amplitude and the direction of radial force vary in accordance with loads. These variations result in interference of radial force commands in two perpendicular axes. The interference becomes a serious problem as a load increases. A shaft finally touches down without proper compensation. It is found that the decoupling of the radial force commands can be realized with compensation of both amplitude and phase angle of 4-pole motor current. A decoupling controller is proposed with a principle of field oriented controller of conventional induction motors. The results of experiments show the validity of compensations. It is shown that induction type bearingless motors can be operated stably under loaded conditions, as well as transient conditions