Benefits of magnetic bearings when used with high-speed motors

Magnetic bearings take engineering into an almost conceptual realm: that of a component with virtually no friction. But these components are very real and have been put to use in demanding applications ranging from highly efficient chillers to oil-drilling machinery.

Magnetic bearings are a non-contact technology. Rather than being separated from other components by a layer of lubricant, they use magnetic levitation to achieve a radial clearance of 0.5-1mm between rotor and stator. These bearings consist of electromagnets that attract the rotor towards the central rotation axis.

The magnetic bearing stator comprises a stack of laminations with copper windings that form a series of north and south poles. A current supplied to each coil produces an attractive force that levitates the shaft inside the bearing. Position sensors keep the shaft in position during operation. The shaft itself is usually driven by another source - such as a turbine, or a motor integrated into the shaft design.

High reliability

As well as ensuring minimal friction, the lack of contact also helps to reduce vibration and wear - leading to higher reliability and longer life. Speeds of up to 100,000rpm can be achieved, while the need for lubrication and maintenance is almost eliminated.

Magnetic bearings sound ideal, but they are not a drop-in solution for everything; they must be applied to the right application. It is crucial to combine them with other devices - such as a high-speed brushless DC motor or permanent-magnet motor - to take full advantage of their benefits.

Despite their futuristic appeal, magnetic bearings have been used for decades. However, for most of that time this has been in very specialised or ultra-high speed applications. Now, improvements in bearing technologies and computing power have reduced the size of the necessary control systems, bringing magnetic bearings within the scope of a far wider range of applications.

Using active monitoring systems - to gather real-time snapshots of key operating parameters such as positions, currents and forces - can help to improve machine reliability and enable better decisions to be made based on what is known about a specific process. Using these types of measurements allows for troubleshooting and enhancement activities.

Staying cool

The York YMC2 range of chillers from Johnson Controls Inc (JCI) is a good example of magnetic bearings - and high-speed motors - being applied in a mainstream application. SKF helped JCI develop the bearings - and three high-speed motors - for this application. A motor-and-bearing combination sits at the heart of an efficient compressor that drives the YMC2 chillers.

These high-speed, direct-drive centrifugal compressors have no gearboxes and use no oil. Together, these features help to make the compressors clean, energy-efficient and low-maintenance.

As previously noted, there is nothing inherently new about either high-speed motors or magnetic bearings. However, this joint development has lowered the cost of these technologies, making the development of the chillers possible.

In particular, the electronics are more compact, while the control system has been redesigned to make it more affordable. Overall, it enabled JCI to incorporate high-end technologies - which would once have been too costly - into a commercial product.

At the same time, the chillers can be manufactured far more efficiently. The three high-speed motors can be used to make hundreds of variants of chiller. With a conventional motor, it would require a multitude of extra mechanical components (such as a different gearbox for each size of chiller). Now, with these permanent magnet motors, the only variant is a change in programming.

Applications in the oil and gas industry

High-speed motors and magnetic bearings have been used in the oil and gas industry for many years, where their superior performance overrides their high price. Now they are finding use in a new generation of machinery, including a sub-sea gas compression system at the Asgard gas field off the Norwegian coast.

Sub-sea gas compression replaces the need for an offshore platform or onshore compression facility, which helps to reduce both capital and operational expenditure. Placing a compressor close to the well can boost production and reduce energy consumption. However, the conditions in such a system are very demanding: it needs to operate at high speed, 300m below sea level, and last for maybe 15 years in the field.

SKF spent five years working with MAN Diesel & Turbo on the design of this system. Using magnetic bearings avoided the need for a gearbox, which helped to shrink the footprint of the system. The ability to run the frictionless bearings at higher speeds also helps to reduce the size of the compressor. At the same time, there was no need for lubricating oil and seals.

The system has been running for nearly two years and is likely to continue operating for many more years: the first pipeline compressor equipped with an SKF magnetic bearing was commissioned in 1985 and is still running today.

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