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Strange attractors

Strange attractors

Once limited to niche roles, technological advances and changing user needs are driving the adoption of active magnetic bearings in a wider range of applications, explains Phil Burge of SKF.

Bearing systems in rotating machinery need to achieve a number of design objectives. For example, they must support operating loads, locate shafts accurately, add the minimum of additional friction to the system and run reliably with as little maintenance as possible. And they must do all this at a total cost that suits the end application.

Equipment designers and operators have a wide range of technologies at their disposal to meet these criteria, including the use of plain bushings made of low friction materials, many different types of rolling element systems and hydraulic or air bearing designs. Each of these approaches offers a different balance of performance characteristics, and each has found its own niches in machine design.

There is one lesser-known bearing technology that offers a powerful combination of advantages, however, particularly in the most demanding applications. Active magnetic bearings use high strength electromagnets to 'float' rotating shafts inside a housing. Let's look at few of the benefits that brings.

Because there is no sliding or rolling contact inside the bearing, magnetic beatings are virtually friction-free, which delivers significant energy savings in high power applications. No contact also means no wear, extending component life. For example, SKF S2M magnet bearings installed on a 25MW centrifugal gas compressor at the Wolf Lake Compressor Station in Alberta Canada have been operating continually for more than 25 years.

Nor do magnetic bearings require lubrication. That simplifies equipment design and maintenance procedures. It also offers new design freedoms, by allowing bearings to run in a vacuum or immersed in process gases for example. Take those high power compressors used on natural gas pipelines: conventional compressor designs use hydraulic bearings that require sealing at the shaft between compressor and motor. Inevitably some gas escapes through these seals, and this gas must usually be flared off, wasting valuable product and contributing to process CO2 emissions.

The use of magnetic bearings in this application has allowed the construction of systems in which motor and compressor both run inside the housing, totally eliminating leakage. In a 10MW compressor application this approach can save as much as 220 tonnes of CO2 every year. In addition, the elimination of the hydraulic system saves energy, further reduces maintenance requirements and cuts the risk of fire and leakage associated with high-pressure hydraulic oil.

Magnetic bearings offer important operating benefits too. As their performance doesn't degrade as shaft speeds increase, they are a good solution for very high-speed applications - shaft speeds of up to 100,000rpm have been achieved in some applications. Their strong damping properties come to the fore in situations involving high transient loads and they cope with extreme accelerations or sudden stops. These characteristics help designers to extend the performance envelope of their equipment, allowing operating characteristics that would be hard to achieve with any other technology.

Much of the power of magnetic bearing technology comes from the fact that the magnetic fields supporting the shaft are actively controlled during use, and it is advances in this control technology that provide further important advantages of the approach. The closed loop control used to ensure that the shaft remains centred in the bearing can also automatically compensate for shaft imbalance during operation, reducing vibration and permitting faster, smoother equipment operation. And data collected by the control system as it does this can be used to provide built-in monitoring of machine performance, offering operators early warning of potential problems that manifest themselves as increased vibration.

Naturally, such a comprehensive range of benefits is not without its costs, and the complex and highly sophisticated control systems that magnetic bearings rely upon do mean their initial purchase cost is higher than that of many other solutions. Historically, that has meant that the technology has been most widely adopted in large and highly critical applications. One key market for active magnetic bearings is the oil and gas sector, where they are widely used in compressors, expanders, turbines and high-speed electric motors.

More than 1,000 SKF active magnetic bearings are used in such applications worldwide, for example, thanks to their performance, extremely high reliability and lubricant free operation. In the energy sector, those same benefits have led to the adoption of magnetic bearings in a wide range of rotating machines, from micro turbines to high-speed gen-erators heavy rotating machinery.

The ability of magnetic bearings to operate in deep vacuum conditions, mean-while, has led to their widespread use in semi-conductor manufacturing. More than 120,000 devices are in operation worldwide to date in applications like high performance vacuum pumps. Their ability to offer extremely smooth vibration-free motion, meanwhile, has also lead to magnetic bearings being adopted for a number of scientific applications, including the neutron beam choppers used in particle physics applications.

Today, however, active magnetic bearings are finding applications in many new areas. This change is being driven by two converging trends. First, higher manufacturing volumes and advances in the production of power electronics and control equipment has reduced the unit cost of magnetic bearing systems significantly.

Second, as companies place ever-greater emphasis on efficiency, reliability, availability and long service life in their equipment, they are increasingly recognising the ability of magnetic bearing systems to contribute to these goals. And the bearings' in-built condition monitoring capabilities allow them to be integrated directly into increasingly common condition-based maintenance regimes, saving on the cost of separate shaft vibration monitoring and reporting equipment.

In wastewater treatment, for example, aeration blower systems are widely used to force air into tanks so aerobic bacteria can break down organic waste. A mid-sized sewage treatment facility may have anything from two to five air blowers, which will account for 40 to 80 percent of the site's total energy use. Similarly, energy consumption will make up at least 80 percent of the total lifecycle cost of each blower.

By adopting modern direct drive system using high-speed permanent magnet electric motors and active magnetic bearings in these applications, water treatment plants can achieve energy savings of 10 to 40 percent, reduce their overall cost of ownership and enjoy considerable reliability improvements. Further benefits for the technology in this application include reductions in operating noise, a smaller equipment footprint and reduced risk of water contamination by lubricants.

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