Recentadvances in magnetic bearing technology, including controls miniaturization,higher levels of simplicity and integrated product solutions, plus the overallmachinery trend to more directly drive machines without gearboxes, areovercoming past limitations inherent to magnetic bearings. As a result,magnetic bearings are replacing oil-lubricated bearings for many new types ofmachines in a variety of industries.

"Ifyou look at the megatrends in the marketplace, the focus is energy efficiencyand cleaner, more reliable systems," says Troy Jamison, senior vice presidentfor business development at Synchrony Inc. "Magnetic bearingtechnology aligns with both of those trends and provides unique solutions."

Bylimiting frictional losses, systems become more efficient and the need fortoxic lubricants is eliminated. For example, in large oil and gas machines suchas gas compressors, oil-lubricated bearings (which require a high-pressurelubrication system) can be eliminated.

Old Tech Finds NewApplication

Jamisonsays the concept of magnetic bearings has been around 40 years, but recentimprovements have overcome historical issues and changed how these bearings areperceived by engineers. The traditional view has been that magnetic bearingsare very large and used primarily in niche applications. This has historicallymeant that, though they may be suitable for labs, they were not a very goodsolution for real-world applications because of size, cost and complexity ofthe systems.

"Akey factor driving magnetic bearing applications is interest in directly drivenmachines where there is a high-speed motor coupled to the fluid machine," saysJamison. "Instead of needing gearboxes and oil lubrication systems, you can usea more efficient system that incorporates a high speed, permanent magnet motorand magnetic bearings."

Users are adopting the bearings into refrigerationcompressors for chillers and HVAC, for example. Energy recovery applicationsare another growing area using high speed generators that run waste heatthrough an organic Rankine cycle to generate electrical power (The Rankinecycle convertsheat into work. The heat is supplied externally to a closed loop, which usuallyuses water). In addition, magnetic bearings are increasingly used ingeneral industry pump applications to help reduce maintenance costs.

Jamisonsays the typical life of a magnetic bearing is about 10 years. So if a pump isrebuilt every three years, typically the first thing to replace is the bearingswhich can be costly if the application is in environments where it is difficultto remove the bearings, such as nuclear applications, oil and gas or subseapumping.

A Mechatronic Design Solution

Magneticbearings are really the confluence of both mechanical and electrical devices.Traditionally systems have used a large electrical enclosure to house theanalog sensor electronics, A to D converters, digital processor, D to Aconverters and power amplifiers.

Recentadvancements have digitized the whole drive train, and the digital controllershave become much smaller. The control cabinet has been dramatically reduced insize, and magnetic bearings are being applied in applications other than forvery large machines, expensive compressors or turbines.

In2009, Synchrony introduced its Fusion magnetic bearing, where the controller isintegrated directly into the bearing housing. No external controller isrequired, just a 48V input into the bearing itself for providing electricalpower to levitate the shaft.

Withan older system designed for a compressor, the application would use a 3,600rpm motor running at 300 or 400 hp gearing to run the compressor wheel at20,000 rpm. To achieve those speeds, the compressor would also need oil-lubricatedbearings.

"Magneticbearings help remove components, and the train now looks like a direct machinewith a high-speed, permanent magnet motor that uses magnetic bearings," saysJamison. "The application doesn't need the gearbox, the oil lubrication systemand the compressor wheel mount directly on the shaft of the motor."

"Froma complete drive train viewpoint, that is where you really begin to reducecomponents in the system and it becomes a more simplified, more efficient machine,"he says.

Lookingat these developments from a design engineering point of view, the realsolution is in lower costs and the elimination of oil lubrication systems'maintenance cycles. The overall smaller size of the system makes magneticbearings more suitable for a wider range of applications.

Understanding MagneticBearing Operation

Themagnetic bearing controller uses electromagnetic coils around the stator (whichis the fixed component of the magnetic bearing). For a radial bearing configuration,laminations on the shaft itself are attracted by the magnetic field induced bythe electromagnetic coils. Position sensors also monitor the shaft, andclearances are measured in thousandths of an inch.

Energizingthe magnetic bearing levitates the shaft, and it starts to spin driven by themotor and VFD (if one is used in the application). The shaft is levitated andrunning up to speed, but the load is varying.

"Youmay have side loads or other forces acting on the shaft depending on the drivenequipment; regardless the position sensors check to determine if the shaft hasmoved from the center position 15,000 times per second," says Jamison. "Whenthere is movement, the controller readjusts the fields in the electromagnetsusing currents flowing from the power amplifier and into the electromagnets torestore the shaft position."

Anotherreason behind increased interest in magnetic bearings is the availability ofinformation from them. This means that users can now monitor what forces are actingon the shaft. Over time, an application may experience higher degrees ofvibration, high side loads and be able to determine what forces are acting onthe shaft, temperatures and the position of the shaft.

Becausethese products are Ethernet-enabled, engineers can use Ethernet communicationsand direct the data into preventive maintenance models. By monitoring real-timedata based on what is happening at the shaft itself - not a sensor on thecasing of the motor on the machine, users can monitor the forces acting on theshaft. For instance, changes to an impeller due to mechanical failure orbuild-up of process material are the kinds of things that can be viewed by themagnetic bearing using the Ethernet connection.

Jamisonsays this kind of health monitoring is the added benefit of using a magneticbearing over other methods of high-speed bearings.

TheFusion line of magnetic bearings is Synchrony's most integrated product, saysJamison, because of its merging of mechanical and electrical components. "Thisproduct (Fusion) is similar to VFDs that used to be mounted in a control room,but now they are mounted directly on the motor, or integrated with the motor,"he says. "It's the same advantage as moving a controller closer to a machine; inthis case the controller is right on the bearing housing itself."

Thistype of integrated, mechatronic solution is increasingly becoming the realityfor industrial equipment design. After all, any solution that allows OEMmachine builders to do away with the need for feed-throughs or cabling to makeconnections to the controller, because it is now resident on the bearinghousing, is a major design improvement.