Advances in Magnetic Bearings
- Tuesday, 01 June 2010
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 feet to remotecontrol 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.
Historically, the relatively high cost of magnetic bearings has limited the technology’s application. However, through standardization, integration, and manufacturing advances, the cost of magnetic bearings has declined. Also, the engineering effort to integrate the magnetic bearings into a machine is greatly reduced. The net result is that magnetic bearings have become much more economical to use in new and existing rotating machinery.
Examples of Advanced Magnetic Bearing Systems
High Speed Drive Trains. Figure 2 shows a 400-kW, 20,000-rpm drive train with magnetic bearings. The drive train incorporates a high-efficiency permanent magnet (PM) motor/generator. A stub shaft, extending from one end of the drive train, can be used for mounting a pump, compressor, or turbine wheel. As such, the machine may be used either as an electrical motor or a generator. Because of the small size of the magnetic bearing controller, it integrates into the housing of the drive train so that the only required connections to the controller are DC power and an Ethernet network cable. A separate feedthrough for the power leads to the motor/generator is provided. The magnetic bearing controller can also control other aspects of the machine, such as the position of inlet guide vanes. The built-in intelligence of the drive train often eliminates the need for an external controller.
Integrated Magnetic Bearings. Figure 3 illustrates radial and thrust bearings with completely integrated control electronics. The rated load capacity for this radial bearing is 300 pounds, and the rating of the thrust bearing is 1,000 pounds. Despite the integration of the control electronics, the size of these bearings is less than past magnetic bearings that required an additional, large, external controller. Each bearing is powered by 48 VDC and includes a dedicated Ethernet port for high-speed communications and health monitoring. The small size and simplified mechanical and electrical interface make it easy to integrate the bearing into rotating machines such as motors, pumps, fans, and turbines.