Pumps & Systems, February 2009
In this age of environmental consciousness, OEMs around the world are competing to build better, safer and greener machines. In striving toward such lofty goals, many industries are rediscovering a fundamental principle-magnetics. Magnetic bearings improve reliability, reduce friction, minimize vibration and offer advanced health monitoring and diagnostics-all without the disadvantages of lubricants. New advances in magnetic bearings, including miniaturization, simplicity and integration, have accelerated their use in rotating machinery in industrial, aerospace and defense sectors.
How a Magnetic Bearing Operates
Figure 1 shows the basic layout of a five-axis magnetic bearing system. Typically, two radial magnetic bearings support and position the shaft in the lateral (radial) directions, and one supports and positions the shaft along the longitudinal (axial) direction. The magnetic bearing offers little frictional resistance to rotation.
Figure 1. Basic operation of a magnetic bearing.
Each magnetic bearing consists of a rotor that rotates with the shaft and a stator that contains electromagnets and position sensors. An active magnetic bearing maintains clearance between the rotor and stator using a closed-loop feedback system that controls the shaft's position. Sensors detect the local displacements, a digital processor interprets these signals, and power amplifiers provide currents to the electromagnets. The cycle of position sensing, processing and amplification repeats an impressive 15,000 times per second.
Like other bearings, a magnetic bearing provides stiffness and damping. However, unlike other bearings, the performance may be optimized by simply changing control parameters. Advanced control algorithms minimize machine vibration even under high levels of unbalance.
Reducing the Magnetic Bearing Size
In magnetic bearings, the bearing pressure is less than oil-lubricated bearings. Therefore, for the same load capacity, magnetic bearings are larger. Historically, this has made integrating magnetic bearings into rotating machines difficult, limiting the range of applications.
Through recent design innovations, the size of radial magnetic bearings has been reduced by more than 30 percent. Increasing the amount of steel at the bore of the stator, while reducing the amount of steel elsewhere, has improved the bearing pressure for radial bearings. The length of the radial magnetic bearing has also been reduced by developing position sensors that can be integrated into the electromagnets.
Reducing the Controller Size
The controller consists of sensor conditioning electronics, analog-to-digital (A/D) converters, digital processors, digital-to-analog (D/A) converters, power amplifiers and a communications interface. Design innovations have systematically miniaturized or eliminated each of these components. For instance, high speed A/D converters are eliminated by using frequency-modulated (FM) position signals that are directly converted to digital values. An integrated processor architecture combines processing, power amp switching and communications, while eliminating D/A converters. Finally, new control algorithms make it possible to reduce the size of the power amplifiers.
These innovations have dramatically reduced the controller size, once as bulky as a household refrigerator, to little more than the size of a DVD player. Smaller controllers can more easily be mounted on the rotating machine, thereby eliminating the need for a separate enclosure. In fact, it is now possible to buy magnetic bearings with the controller completely integrated into the bearing itself, which eliminates the need for a separate controller.
In the past, engineers encountering a magnetic bearing system for the first time were often shocked by the quantity and complexity of the cables and connectors. Electromagnets require dozens of wires carrying high currents and high switching frequencies. Sensors also require more than a dozen wires carrying high frequency signals. These wires were routed up to 300-ft to remote control rooms where the magnetic bearing controller was located. Long cables caused the emission of electromagnetic interference (EMI) and electrical noise pickup from the sensor wires.
The new, compact controllers significantly simplify the system. A compact controller integrates into the casing of the machine by external mounting or integration into the bearing. Remote power (typically 48 - 300 VDC) is supplied to the controller. Short magnetic bearing cables between the controller and the machine simplify connections, reduce EMI and eliminate special sensor tuning.
Improving Health Monitoring
In the past, health monitoring of a rotating machine required a dedicated vibration monitoring system. These large, expensive systems consist of proximity probes, conditioning electronics, high speed data acquisition systems, digital processors and alarming hardware.
However, a machine already equipped with a magnetic bearing system can also perform health monitoring without additional hardware investment. Inherent in the magnetic bearings are high resolution position sensors, digital processing and communications. When networked to an external computer, visualization of orbits, advanced diagnostics, trending, archiving and alarming are all possible. Health monitoring is achieved by simply extending the functionality with additional computer software.