Performance and Cost Reduction of Permanent Magnet Biased Magnetic Bearings

Abstract

Active magnetic bearings (AMBs) have the well-documented advantage of reduced operational power losses when compared to conventional fluid-film bearings; however, they have yet to be widely adopted in industry due to the high initial costs of manufacturing and supporting power electronics. As AMBs look to become more cost competitive in more widely based applications, permanent magnet biased designs seek to reduce both the operating electrical power losses and the power electronic hardware costs while maintaining normal load and maximum load capacities. In these new designs, permanent magnet components are used to provide the necessary bias magnetic flux in the bearing usually provided by an electrical bias current in traditional all electromagnetic AMB designs. By eliminating electrical bias currents, operating electrical power losses can be significantly reduced while allowing for smaller, cheaper electronic components. This paper provides a comparison of the performance of permanent magnet biased thrust and radial bearing designs with conventional, all electromagnetic bearing designs. The thrust bearings are designed with nominal and maximum load capacities of 1,333 N and 4,000 N, while the radial bearings are designed with nominal and maximum load capacities of 1,000 N and 3,000 N. The shaft diameter is considered to be 70 mm for all bearings. Finite element modeling is used to calculate load capacities and operating electrical power requirements. Power requirements for a number of loads ranging from nominal to maximum capacity are presented for the permanent magnet biased and all electromagnetic bearing designs. A significant reduction in electrical power requirements under maximum load conditions is shown in the permanent magnet biased designs. This reduction is further magnified under nominal load conditions. Additionally, the number of pole wire turns and maximum wire currents are adjusted to realize even greater electrical power losses. The required bias magnetic flux can be generated with reduced wire currents by increasing the number of wire turns. While reducing wire currents also reduces electrical power requirements, the increase in wire turns increases the circuit induction. This increase in induction decreases the bearing slew rate and, in turn, the bandwidth. This study looks at a number of wire turns and current combinations. Tradeoffs between reduced electrical power losses and bearing bandwidth are presented and discussed. The permanent magnet biased AMB designs are shown to significantly reduce electrical power losses having the potential to improve overall machine efficiency. Implications of adopting this technology to both operating and manufacturing costs are discussed. The use of permanent magnets in AMBs is shown to make the costs of these systems more competitive with oil lubricated bearings when compared to conventional AMB designs.