“Null-E” Magnetic Bearings

Using electromagnetic forces to suspend rotating objects (rotors) without mechanical contact is often an appealing technical solution. However, in real life magnetic suspensions have to satisfy many engineering performance requirements beyond the simple compensation for the rotor weight. These typically include adequate load capacity and stiffness, low rotational loss, low price, high reliability and manufacturability. With recent advances in permanent-magnet materials, the magnitudes of the required forces can often be obtained by simply using the interaction between permanent magnets. While a magnetic bearing based entirely on permanent magnets could be expected to be inexpensive, reliable and easy to manufacture, a fundamental physical principle known as Earnshaw’s theorem maintains that this type of suspension cannot be statically stable. Therefore, some other physical mechanisms must be included. One such mechanism employs the interaction between a conductor and a non-uniform magnetic field in relative motion. Its advantages include simplicity, reliability, wide range of operating temperature and system autonomy (no external wiring and power supplies are required). The disadvantages of the earlier embodiments were high rotational loss, low stiffness and load capacity. It was realized, however, that rotational loss, load capacity and stiffness depend strongly on the topology of the conductors and the magnetic fields. In theory, the rotational loss in the equilibrium position in the absence of external loading can be made zero by designing a system such that no electric field develops in the conductor during the rotor rotation. In this dissertation, we introduce the term ”Null-E” to describe this condition. In the earlier embodiments, the Null-E condition could not be satisfied exactly. Load capacity and stiffness can be also maximized through choosing shapes of the conductors and the fields. From this point of view the field and the conductor shapes used in the so called Null-Flux Bearings were found to be advantageous: the conductors were shaped as planar loops with central openings and the fields were orthogonal to the loop planes and periodic in the direction of the conductor motion. To reduce rotational losses an additional restriction was imposed on the field shape (“Null-Flux” condition): the flux variation within each loop had to be zero in the equilibrium. This condition lowered the average value of the electric field in the conductor, but the topology did not allow making it zero everywhere. The satisfaction of the “Null-Flux” condition was extremely sensitive to manufacturing inaccuracies. This dissertation proposes a novel type of magnetic bearing stabilized by the field-conductor interaction. In contrast to the other bearings based on this principle, the proposed design allows exact satisfaction of the Null-E condition in the equilibrium regardless of the conductor shapes and even in the presence of an axial loading. Because of this we refer to it as the Null-E Bearing. Null-E Bearings also have potential for higher load capacity and stiffness than Null-Flux Bearings. Finally, their performance is highly insensitive to the manufacturing inaccuracies.