Active magnetic bearings prove attractive for demanding application

Paul Boughton

Active magnetic bearings have been available for decades, yet demand from design engineers for higher performance and reduced energy consumption is giving rise to heightened interest in these systems. Jon Severn reports on some of the latest developments and applications.

For the vast majority of applications, engineers can choose the most appropriate rolling-element or plain bearings. Air bearings are suitable for only a limited range of applications, but there is now increasing interest in magnetic bearings or, more precisely, active magnetic bearings.

From early childhood, would-be engineers understand that opposite poles of permanent magnets attract each other and like poles repel. By using electromagnets instead of permanent magnets, and by having a means to control the electromagnets, it is simple to conceive of a rotating shaft that is supported and guided in a non-contact manner by virtue of electromagnetic forces - and gravity, of course, depending on the orientation of the shaft.

Clearly the main advantage of the magnetic bearing is that it is non-contact and, therefore, wear-free and virtually friction-free. This has benefits in terms of high reliability, reduced maintenance requirements, long and predictable life, and lower energy consumption - though the electromagnets will consume an amount of energy, typically in the order of tens of Watts, depending on the shaft size and radial loading. In some applications, such as turbomachinery, it is advantageous to eliminate the lubrication system that would otherwise be required for a conventional bearing arrangement. The lack of contact and lubrication also enables magnetic bearings to operate in hostile environments including those where temperature extremes (typically -256 to 220°C), vacuum, high pressure, steam or corrosive chemicals are encountered. Being non-contact components, magnetic bearings do not restrict the speed of rotation, though manufacturer SKF suggests an upper limit for the surface speed of 250m/s or 4.5 million DN, where DN is the diameter of the rotor in mm multiplied by the speed in revolutions per minute (Fig. 1). To put this in context, conventional bearings with sophisticated lubrication systems would only be able to achieve speeds around one-quarter of this.

Bearing arrangement

A typical active magnetic bearing (AMB) system consists of a set of stationary electromagnets located around the ferromagnetic rotor (shaft), corresponding gap sensors, a control unit and power amplifiers for feeding power to opposite pairs of electromagnets.

The controller uses data from the gap sensors to adjust the power to the electromagnets in order that the shaft's position is maintained to within microns, regardless of external influences such as radial loads.

Whereas early controllers were analogue, the current generation of controllers use digital signal processing and can cycle through the control loop thousands of times per second, enabling shaft speeds to be maintained in excess of 100,000 revolutions per minute.

Radial bearing assemblies

Most AMB systems have a pair of radial bearing assemblies, one at each end of the shaft, and a thrust bearing to maintain the shaft's axial position. Because each radial bearing maintains the shaft's position in two axes, and the thrust bearing controls the shaft's position along its axis, the overall AMB arrangement is often referred to as a five-axis system.

We have already mentioned turbomachinery as one application for which AMB systems are suitable; others include centrifuges for purifying nuclear isotopes, natural gas compressors, equipment for use in manufacturing semiconductors, and high-throughput blowers (see panel).

In addition, the high-speed capability of AMBs makes them suitable for machine tool spindles. SKF, which manufactures AMB systems, quotes a force density of 40-60 N/cm2 and says that AMBs can be scaled to carry any load - though the company also points out that the force density is lower than for other bearing types, so AMBs are likely to be bigger than conventional bearing arrangements with a similar load-carrying capacity.

Unlike other types of bearing, AMB systems have to include a back-up power source and auxiliary bearings (also called emergency bearings or touch down bearings) to prevent the equipment from suffering damage in the event of a failure in the sensing, control or power amplifier components. Remember, however, that the electrical and electronic components have a far longer design life than conventional mechanical bearings, so the likelihood of a failure in these components is low.

Given the exceptional longevity of AMBs, the life cycle cost can be lower than that of conventional bearings, even though the initial purchase prices is usually higher due to the increased complexity.

Remote monitoring

A recent development from Waukesha Magnetic Bearings, which is a division of Waukesha Bearings Corporation and part of the Dover group, raises the possibility of further reductions in lifetime costs. This is through integrated remote connectivity that enables the machine builder or Waukesha's engineers to monitor vibration, bearing load and rotor stability remotely. The bearing controller is equipped with a TCP/IP connection, so access is possible from any suitable device connected to the internet.

As well as aiding troubleshooting during operation, the remote connectivity also offers benefits during commissioning and later in the bearing's life, when retuning may be necessary. The remote diagnostics also enable the static clearances to be checked either automatically or on demand, with this information providing an indication of the condition of the auxiliary bearings.

One of the drawbacks with conventional AMBs is that the controller has to be housed separately from the bearing. However, USA-based Synchrony has recently introduced the Fusion family of active magnetic bearings that integrate the control system within the bearing housing (Fig. 2). This simplifies system integration considerably, provided the increased bulk of the bearing can be accommodated and the operating environment is not too severe, as the maximum operating temperature at the stator outside diameter is 60° C.

Each Fusion unit is equipped with an Ethernet port to aid commissioning, condition monitoring and troubleshooting, and MTBF (mean time between failures) is quoted as greater than 80,000 hours.

To give an indication of the performance capabilities of the integrated bearings, the model FR 35-10 has a stator outside diameter of 178mm (7 inches), can accommodate a shaft diameter of 56mm (2.2 inches), has an overall length of 97mm (3.8 inches), a load capacity of 1.9kN (424 lb) and a maximum operating speed of 34,000 revolutions per minute.

In contrast, the largest model in the range, the FR 90-20, has a stator outside diameter of 381mm (15 inches), can accommodate a shaft diameter of 147mm (5.8 inches), has an overall length of 279mm (11.7 inches), a load capacity of 27kN (6053 lb) and a maximum operating speed of 13,000 revolutions per minute. Fully integrated thrust bearings are also available.

Reduced energy consumption

Active magnetic bearings are not a new concept, but the demand from design engineers for higher performance and reduced energy consumption has led to a resurgence in interest in these bearings.

As can be seen from the above, manufacturers have responded, with the result that new developments are creating exciting and innovative opportunities for these ultra-low-friction bearings.

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