Active magnetic bearings (AMB) use electromagnetic actuators in order to control the position of the rotor or levitated object in general. Any degree of freedom can be stabilized using an AMB and in contrast to PMBs, full and stable levitation is possible using only AMBs. The bearings are separate mechanical units with an iron core, bearing windings and attached sensors for determining the rotor position.
AMBs allow to control the stiffness and damping coefficients of the bearing and can, therefore, influence the object’s rotordynamic behaviour during operation.
In an AMB, the flux density difference between two opposing areas of the air gap determines the resulting Maxwell-force on the rotor. These forces depend quadratically on the flux density. Permanent magnets can be used to create bias flux, increasing the absolute flux density values. This shift of the operation point increases the achievable bearing forces. Permanent magnets in AMBs can also be used to compensate static loads, e.g. due to a gravity related shift of the rotor. Both measures reduce the necessary bearing power in the AMB. For further explanations to active magnetic bearings and their components, browse through the video and animation sections at Waukesha Bearings Corporation and SKF/S2M. The vast majority of industrial applications using magnetic levitation rely on active bearings. The chart illustrates the special characteristics of AMBs and their consequential application scope. Even though PMBs also provide many of the listed aspects, the full scope can only be provided by actively controlled bearings.