Advances in Magnetic Bearings Overcome Past Restraints

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

"If you look at the megatrends in the marketplace, the focus is energy efficiency and cleaner, more reliable systems," says Troy Jamison, senior vice president for business development at Synchrony Inc. "Magnetic bearing technology aligns with both of those trends and provides unique solutions."

By limiting frictional losses, systems become more efficient and the need for toxic lubricants is eliminated. For example, in large oil and gas machines such as gas compressors, oil-lubricated bearings (which require a high-pressure lubrication system) can be eliminated.

Old Tech Finds New Application

Jamison says the concept of magnetic bearings has been around 40 years, but recent improvements have overcome historical issues and changed how these bearings are perceived by engineers. The traditional view has been that magnetic bearings are very large and used primarily in niche applications. This has historically meant that, though they may be suitable for labs, they were not a very good solution for real-world applications because of size, cost and complexity of the systems.

"A key factor driving magnetic bearing applications is interest in directly driven machines where there is a high-speed motor coupled to the fluid machine," says Jamison. "Instead of needing gearboxes and oil lubrication systems, you can use a more efficient system that incorporates a high speed, permanent magnet motor and magnetic bearings."

Users are adopting the bearings into refrigeration compressors for chillers and HVAC, for example. Energy recovery applications are another growing area using high speed generators that run waste heat through an organic Rankine cycle to generate electrical power (The Rankine cycle converts heat into work. The heat is supplied externally to a closed loop, which usually uses water). In addition, magnetic bearings are increasingly used in general industry pump applications to help reduce maintenance costs.

Jamison says the typical life of a magnetic bearing is about 10 years. So if a pump is rebuilt every three years, typically the first thing to replace is the bearings which can be costly if the application is in environments where it is difficult to remove the bearings, such as nuclear applications, oil and gas or subsea pumping.

A Mechatronic Design Solution

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

Recent advancements have digitized the whole drive train, and the digital controllers have become much smaller. The control cabinet has been dramatically reduced in size, and magnetic bearings are being applied in applications other than for very large machines, expensive compressors or turbines.

In 2009, Synchrony introduced its Fusion magnetic bearing, where the controller is integrated directly into the bearing housing. No external controller is required, just a 48V input into the bearing itself for providing electrical power to levitate the shaft.

With an older system designed for a compressor, the application would use a 3,600 rpm motor running at 300 or 400 hp gearing to run the compressor wheel at 20,000 rpm. To achieve those speeds, the compressor would also need oil-lubricated bearings.

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

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

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

Understanding Magnetic Bearing Operation

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

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

"You may have side loads or other forces acting on the shaft depending on the driven equipment; regardless the position sensors check to determine if the shaft has moved from the center position 15,000 times per second," says Jamison. "When there is movement, the controller readjusts the fields in the electromagnets using currents flowing from the power amplifier and into the electromagnets to restore the shaft position."

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

Because these products are Ethernet-enabled, engineers can use Ethernet communications and direct the data into preventive maintenance models. By monitoring real-time data based on what is happening at the shaft itself - not a sensor on the casing of the motor on the machine, users can monitor the forces acting on the shaft. For instance, changes to an impeller due to mechanical failure or build-up of process material are the kinds of things that can be viewed by the magnetic bearing using the Ethernet connection.

Jamison says this kind of health monitoring is the added benefit of using a magnetic bearing over other methods of high-speed bearings.

The Fusion line of magnetic bearings is Synchrony's most integrated product, says Jamison, because of its merging of mechanical and electrical components. "This product (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; in this case the controller is right on the bearing housing itself."

This type of integrated, mechatronic solution is increasingly becoming the reality for industrial equipment design. After all, any solution that allows OEM machine builders to do away with the need for feed-throughs or cabling to make connections to the controller, because it is now resident on the bearing housing, is a major design improvement.