<p>Due to the nature of magnetic forces, the forces of a magnetic bearing depend not only on the coil currents but also on the position of the rotor with respect to the stator of the bearing. For conventional radial magnetic bearings with even number of poles, the destabilizing position dependency is modeled as a negative stiffness so that a linear feedback control is applied to stabilize it. Asymmetric bearings having odd number of poles have nonlinear and cross-coupled force-current relationships. In this paper, we use magnetic circuit theory to derive the force-current relationships of asymmetric bearings with odd number of poles. Through a linearization process, a new method of compensating the position dependency is proposed. Analyses show that this compensation scheme com-pletely eliminate the position dependency for 3-pole bearings. For asymmetric bearings with higher number of poles such as 5-pole, 7-pole, and 9-poles, the compensation scheme reduces the position dependency with a varying degree. A test rig consisting of a prototype compressor equipped with 9-pole radial bear-ings is constructed. Test results verify that the compensation is critical to achieve stable levitation. Only with the proposed compensation, the output sensitivities of the bearings satisfy the standard.</p>

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Compensation of Rotor Position Dependency in Magnetic Bearings with Odd Number of Poles

  • Myounggyu D. Noh,
  • Taesan Eom

摘要

Due to the nature of magnetic forces, the forces of a magnetic bearing depend not only on the coil currents but also on the position of the rotor with respect to the stator of the bearing. For conventional radial magnetic bearings with even number of poles, the destabilizing position dependency is modeled as a negative stiffness so that a linear feedback control is applied to stabilize it. Asymmetric bearings having odd number of poles have nonlinear and cross-coupled force-current relationships. In this paper, we use magnetic circuit theory to derive the force-current relationships of asymmetric bearings with odd number of poles. Through a linearization process, a new method of compensating the position dependency is proposed. Analyses show that this compensation scheme com-pletely eliminate the position dependency for 3-pole bearings. For asymmetric bearings with higher number of poles such as 5-pole, 7-pole, and 9-poles, the compensation scheme reduces the position dependency with a varying degree. A test rig consisting of a prototype compressor equipped with 9-pole radial bear-ings is constructed. Test results verify that the compensation is critical to achieve stable levitation. Only with the proposed compensation, the output sensitivities of the bearings satisfy the standard.