The active magnetic bearing (AMB) is an important element for high-speed systems, such as semiconductor equipment and machine tools. The most popular AMB has eight magnetic poles. We propose a three-pole AMB here. Compared with the eight-pole configuration, it has the advantages of fewer power amplifiers, lower iron loss, and more space for heat dissipation, coil winding, and sensor installation. As a result, the AMB's overall cost can be reduced. Here, we study the three-pole AMB's optimal design, one that minimizes both the number of power amplifiers and the steady-state copper loss. Four design variables are considered: bias current, pole orientation angle, number of coil turns, and pole face. The optimal bias currents minimizing the steady-state copper loss are obtained by the method of Lagrange's multiplier. The coil turns and pole face area are optimal if they occupy equal available bearing space. The optimal three-pole AMB is Y-shaped, i.e., the pole orientation angle is π/6. With this optimal pole orientation, the AMB's two upper poles can share the same bias and control currents, but with opposite winding directions. Thus, only two power amplifiers are required for the optimal three-pole AMB. Finally, it is shown that the three-pole AMB possesses lower steady-state copper loss than the eight-pole AMB.
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