Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-06-01
2001-10-16
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
Reexamination Certificate
active
06304015
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a high rotation speed, near-frictionless bearing. More specifically, the invention is directed towards a passive magnetic bearing, which provides non-contact suspension of a rotor, rotating above critical speed, without usage of external energy supplies and control systems.
2. Brief Description of the Prior Art
Conventional mechanical bearings provide high load capacity and high stiffness but limited durability especially under high rotation speed. Great friction loss, noise and requirement of lubricants also make use of a mechanical bearing impracticable under high rotation speeds or severe environments such as under low temperature or in vacuum.
Other types of bearing are liquid or gas film bearings, which usually can be classified as self-acting or externally pressed. They all cannot operate in vacuum and are featured by appreciable frictional losses. Besides, the externally pressed bearings require very complicated pneumatic or hydraulic system, including pumps, valves, seals and conduits for their operation. On the other side, self-acting bearings are featured by very small size of a clearance (near 25 &mgr;in), which makes them very sensitive to any contamination.
The most suitable for operation under high speeds and in severe environments are bearings making use of magnetic interaction to achieve non-contact suspension of a rotor. However, when designing such bearings, one needs to consider an important limitation, resulting from physics issue known as Earnshaw's theorem. Applying this theorem to magnetic systems, it can be stated that stable non-contact levitation of a body cannot be achieved by utilizing only interaction between permanent magnets or between permanent magnets and soft-magnetic elements. In particular, for the case of rotation bearings utilizing interaction between permanent magnets or between permanent magnets and soft-magnetic elements to suspend a rotor, Earnshaw's theorem states that if stable suspension is achieved in the axial direction, it will be unstable in the radial direction and vice versa. In conventional active magnetic bearings, stable suspension in all directions is achieved by introducing an external control of magnetic field supporting the rotor. Obviously, for the magnetic field to be controllable it cannot be generated only by permanent magnets but at least partially needs to be generated by electromagnets. This is the cause of such shortages of active magnetic bearings as continuous external energy consumption and requirement of complicated feedback control system. Moreover, active magnetic bearings were found not be able to compensate short high-amplitude force pulses because of the limitations of the current variation speed in the control coils, which are imposed by the coil inductance and maximal voltage, which can be applied by the control unit.
Another type of magnetic bearings exploits the interaction of a superconductor with an external magnetic field. Such systems are absolutely stable and external controls are not needed for their operation. However, the requirement of cooling down superconductors to cryogenic temperatures restricts significantly the area of their applications. Besides, there are some properties of superconducting bearins, which further complicate their application even at low temperatures. First of all it is to be noted that those of practical interest are mainly superconducting magnetic bearings utilizing recently discovered so called high-temperature superconductors, which are capable of operation at the temperature well above the boiling point of liquid nitrogen. However, these materials are extremely brittle, difficult to manufacture and what is more important, they exhibit very complicated electromagnetic properties, which result in a complicated behavior of a bearing making use of them. In particular, most such bearings are featured by a strong force-displacement hysteresis resulting from the remagnetization hysteresis of a superconducting material being exposed to a magnetic field varying upon displacements of the rotor. This force-displacement hysteresis causes unpredictability of the rotor position and may even cause bearing failure under influence of vibrations. The other source of the rotor position ambiguity is so called magnetic flux creep in type II superconductors, which results in changes of the rotor equilibrium position with time even under steady loads. And, finally, there is always a problem of how to set the rotor at the desired equilibrium position at the very beginning when superconductors just turned into superconducting state. In the first approximation, neglecting influence of the remagnetization hysteresis and magnetic flux creep, the rotor in superconducting bearings tends to stay in the position where it was when superconductors turned into superconducting state. If, however, we had kept the rotor in the desired position during transition into superconducting state, then after some load was applied on it after transition ends and then removed, we will find the rotor being significantly displaced from the original position. This will happen because of the flux creep in superconductors, which is much bigger for the first time when we induce a current in a superconductor than for subsequent times. The problems relating to the long-term influences of the superconductor remagnetization hysteresis and magnetic flux creep are successfully solved in superconducting magnetic bearing design described in U.S. Pat. No. 5,789,837. In contrast to other superconducting bearings making use of the interaction of bulk superconductors with magnetic field, this design takes advantage of interaction of currents being induced in shortened superconducting turns mounted on the stator around stator axis with axial magnetic field emanated from a rotor. The currents in the turns are induced automatically whenever the rotor is displaced from an equilibrium position in a radial direction due to the change of the external magnetic fluxes through the turns. The influence of remagnetization hysteresis is reduced in this bearing because only minimal volume of superconducting materials is exposed to varying magnetic field. To reduce long-term influence of magnetic creep in this bearing, it is proposed to rotate the stator together with superconducting turns mounted on it with a low speed about its axis. To provide possibility of such a rotation, stator may be mounted in usual mechanical bearings, which will last for long because the stator rotation speed is low. At the same time rotation speed of the rotor suspended without mechanical contact can be very high. In this case long-term influence of magnetic creep will be essentially eliminated due to periodic exchange of the superconducting turn positions. It is to be noticed, however, that the problem of initial setting the rotor in the desired position is not solved in this bearing either. The method of initial setting proposed in this patent implies that the rotor has a significant displacement from the desired position during transition of superconductors into superconducting state. When the transition ends, it is proposed to rotate stator about its axis not with a low speed, required to compensate long-term influence of magnetic creep, but with a speed high enough to limit rotor displacements under influence of a force oscillating synchronously with the rotor rotation, which will inevitably accompany a constant force pushing the rotor towards the central position during this setting process. When the rotor reaches the central position, there will be no constant force but the oscillating one. The problem is that this oscillating force will be comparable in magnitude with the maximal force, which could be applied on the rotor and it will last for a long time (in fact it will never decay fully). If we remember that the stator was proposed to be installed in mechanical bearings, which cannot work for long time under high speeds (especially in cryogenic environment and under
Filatov Alexei Vladimirovich
Salter Adrian Keith
Dinh Le Dang
Parker and DeStenfano
Ramirez Nestor
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