Load Capacity and Power Consumption Comparison between 8-Pole Biased and 9-Pole Unbiased Magnetic Bearings

Typical radial magnetic bearings are structurally symmetrical with eight poles. When horseshoe windings are used, bias currents are employed to use a linear controller and to ensure dynamic performances. However, bias currents increase ohmic losses and the size of the drive. Unsymmetrical nine-pole bearings have been proposed to eliminate the need for bias currents and to provide similar dynamic characteristics. In this paper, we compare the nine-pole bearings with equal-capacity eight-pole bearings in terms of load capacity and the power losses under static loads. It is found that 9-pole bearing can carry 9.7% higher load than 8-pole bearing if the bearing sizes are the same. Also, the power loss of 9-pole bearing is indeed less than that of 8-pole bearing if the bias current is half of the maximum current as is normally the case and if the bearing load is close to the capacity. The results are confirmed by experimental observations.