Active magnetic bearings
The radial magnetic bearing stator comprises electromagnets, usually made of a stack of laminations with copper coils wound around the north and south poles. A current is applied to each coil to produce attractive forces on laminated iron rotor parts so to levitate the shaft inside the bearing. The clearance or magnetic air-gap between the stator and the rotor is 0.5 to 1.0 mm, depending on the application, thus there is no contact friction, no component wear and no lubrication required. The axial bearing design is similar but with solid iron stator cores and rotor parts, rather than laminated.
Other magnetic bearing arrangements are also available by SKF. For instance, a conical magnetic bearing, with just two bearing stators providing both radial and axial forces on the shaft.
SKF proprietary position sensors accurately measure the radial and axial displacements of the shaft. For light rotating machines, a “sensor less” solution is provided, combining the sensor and the electromagnet in a single stator. The power electronics have digital control to supply the currents to the electromagnets and keep the shaft at the pre-defined position. The digital control provides optimized stiffness and damping and with specific functions such as automatic balancing.
The complete active magnetic bearing system consists of:
- the monitoring system that provides real time information on shaft position, vibrations, currents (loads), temperatures, as well as alarms and safety management
- the auxiliary bearings that support the shaft at standstill but are not in contact when the magnetic bearings are energized
- the electric power supply that is secured by batteries or by regeneration from a high speed motor, in order to maintain magnetic levitation in the event of a grid failure
SKF magnetic bearings match a wide diversity of industrial and commercial applications. They cover a large range of load capacities from a few Newtons for the light and high speed vacuum turbomolecular pumps, up to axial bearings providing with more than 30 tons to levitate 8 meter long water turbines shafts. The corresponding power electronics with digital control are available from a few amps @48VDC up to 30 amps@300VDC as used on 30 MW pipeline compressors. The SKF magnetic bearings design focuses on robustness, manufacturability, sustainability and offers key performances:
- high reliability
- high energy efficiency
- no mechanical component wear, thus longer service life
- virtually maintenance-free
- environmentally friendly, oil-free and contamination-free
- very high speeds
- virtually vibration-free
- fit for various process gas or vacuum environments, enabling hermetic machines designs
in built powerful condition monitoring