By Mark Hinckley, Director, Mechatronics Platform, SKF USA Inc., Lansdale, PA
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Magnetic bearings help achieve performance levels not possible with traditional rolling element bearing or oil film bearing technology.
The concept of magnetic bearings has been around for at least 100 years. For many of those years, though, it was the stuff of university experiments and studies. However, as computing power has improved so has the viability of using magnetic bearings in commercial applications.
The progression in magnetic bearing technology has followed the evolution of computer technology. The improvement in electronics and software has enabled better monitoring and control of motion. And magnetic bearing technology has been both the beneficiary of this technology improvement as well as the cause of certain applications becoming more likely to benefit from magnetic bearings. Especially for industrial machinery, increasingly environmental impact and the cost of energy are becoming the top priorities. In these applications, magnetic bearings can provide higher speed operation with greater reliability and efficiency.
However, magnetic bearings are not simply drop-in replacements for traditional rolling element or fluid film bearing designs. They are a little more complicated than that, but for the right application, well worth the investment. In order to apply magnetic bearing technology to an application, it does take a significant amount of work for the machine designer working closely with the magnetic bearing supplier.
To take advantage of the performance advantages of magnetic bearings, you can combine them with a high-speed brushless DC motor or permanent magnet motor. The combination of the two technologies creates a synergistic effect, allowing higher speed operation.
Perhaps the greatest benefit of magnetic bearings is their ability to rotate at extremely high speeds. The bearings themselves are limited by the strength of the material to a peripheral speed limit of 180 m/s. For special applications, it is possible to increase this limit to 200 m/s with more exotic materials. Another factor is the nearly frictionless performance of the bearings which is the direct result of the rotor floating on a contact-free magnetic field. The elimination of moving mechanical components also means that there is no lubrication required for the bearing system, which makes it a very clean system to operate. All of these factors make magnetic bearings highly reliable, which drastically reduces maintenance requirements. Magnetic bearing systems also have low vibration levels even during high-speed operation, and low audible noise levels.
A cross-sectional view shows a typical magnetic bearing system with both axial and radial bearings together with the position sensors.
For these reasons, magnetic bearings can help you achieve performance levels that were not attainable before. If you have reached the performance limits in any one of these areas with traditional solutions, then you should consider magnetic bearing technology.
How they work
Magnetic bearings are basically a system of bearings which provide non-contact operation, virtually eliminating friction from rotating mechanical systems. Magnetic bearing systems have several components. The mechanical components consist of the electromagnets, position sensors and the rotor. The electronics consist of a set of power amplifiers that supply current to electromagnets. A controller works with the position sensors which provide feedback to control the position of the rotor within the gap.
The position sensor registers a change in position of the shaft (rotor). This change in position is communicated back to the processor where the signal is processed and the controller decides what the necessary response should be, then initiates a response to the amplifier. This response should then increase the magnetic force in the corresponding electromagnet in order to bring the shaft back to center. In a typical system, the radial clearance can range from 0.5 to 1 mm.
This process repeats itself over and over again. For most applications, the sample rate is 10,000 times per second, or 10 kHz. The sample rate is high because the loop is inherently unstable. As the rotor gets closer to the magnet, the force increases. The system needs to continuously adjust the magnetic strength coming from the electromagnets in order to hold the rotor in the desired position.
Magnetic bearing systems can also be actively monitored and therefore dramatically reduce maintenance costs.
The general magnetic principle is illustrated here. A change in shaft position is registered by the position sensor which is sent to the processor.
The controller then sends the necessary response to the amplifier, which increases the magnetic force in the corresponding electromagnet to bring the shaft back to center.
A monitoring system can offer real-time snapshots of positions, currents, and forces in both time and frequency domains in addition to real-time representations of position, current or force orbits. Alarm logs can also capture all system variables on either side of an unusual event while trending tools can record variables for short or long-term trending.
Such active monitoring systems take information that is automatically available in the magnetic bearings and present it in a useful interface. This helps you not only improve machine reliability but ultimately get the most out of your systems. You can make better decisions based on what you know and document about your process. What you are able to measure, you are able to improve. And here, you can capture measurements on your process to help trouble shoot and improve your setup.
Magnetic bearings are an especially good fit in the oil and gas industry primarily because of their high reliability. In fact, one of the earliest applications of magnetic bearing technology was in the natural gas industry, where there are still machines operating in the field for decades without any reliability issues. The high-speed capability of magnetic bearings also helps to improve the efficiency of the machines in the field as more of the energy in the pipeline can be delivered to the end-use site.
In an application involving a turbocompressor, instead of independent radial and axial bearings, this design combines the axial and radial bearings into one cartridge unit.
The symmetrical bearings are located on each end of the PM motor.
More recently, an application involving cooling turbocompressors posed the challenge of providing quiet and efficient compression of refrigerants for industrial chillers. A traditional solution involved using rolling element bearings and a single speed induction motor. However, the magnetic bearing design offered the advantage of an oil-free semi-hermetic unit that accommodated the large operating range, with speeds ranging from 12,000 to 40,000 rpm, including surge conditions. The oil-free centrifugal compressor with variable speed drive increased the part-load and the full-load energy efficiency by at least 10% over geared/hydrodynamic designs. The magnetic bearing design also lowered noise levels and provided vibration-free operation, in addition to reducing maintenance by providing built-in monitoring.
SKF USA Inc.
Filed Under: Factory automation, Bearings, Motion control • motor controls, Mechatronics