Dynamic Instability Induced by Iron Loss Unbalance in Rotor-Active Magnetic Bearing System

There are many papers that discuss instability caused by inadequate controller design for rotor-active magnetic bearing (AMB) systems, but it is not well known that the iron losses affect their control stability. Since the unbalance force in rotational coordinate stays in a constant phase angle at fixed speed, the magnetic force that compensates the unbalance force also stays in a constant phase angle. This causes a stationary distribution of iron losses on the rotor and that makes a thermal bow. This paper presents the thermally induced unstable vibration in a rotor-AMB system. Focusing the interaction between an unbalance control and a thermal bow through iron losses reveals the mechanism of the instability. A model including the thermal dynamics is proposed and the instability condition is derived. Lastly, how to avoid the thermal instability is discussed.