magnetic bearings
A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic bearings support moving machinery without physical contact; for example, they can levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear. Magnetic bearings are in service in such industrial applications as electric power generation, petroleum refining, machine tool operation, and natural gas pipelines. They are also used in the Zippe-type centrifuge[1] used for uranium enrichment. Magnetic bearings are used in turbomolecular pumps, where oil-lubricated bearings would be a source of contamination. Magnetic bearings support the highest speeds of any kind of bearing; they have no known maximum relative speed.
It is difficult to build a magnetic bearing using permanent magnets due to the limitations described by Earnshaw's theorem, and techniques using diamagnetic materials are relatively undeveloped. As a result, most magnetic bearings require continuous power input and an active control system to hold the load stable. Many bearings can use permanent magnets to carry the static load, and then only use power when the levitated object deviates from its optimum position. Magnetic bearings also typically require some kind of back-up bearing in case of power or control system failure and during initial start-up conditions.
Two sorts of instabilities are very typically present with magnetic bearings. Firstly, attractive magnets give an unstable static force that decreases with greater distance and increases at close distances. Secondly since magnetism is a conservative force, in and of itself it gives little if any damping, and oscillations may cause loss of successful suspension if any driving forces are present, which they very typically are.
With the use of an induction-based levitation system present in maglev technologies such as the Inductrack system, magnetic bearings could do away with complex control systems by using Halbach Arrays and simple closed loop coils. These systems gain in simplicity, but are less advantageous when it comes to eddy current losses. For rotating systems it is possible to use homopolar magnet designs instead of multipole halbach structures, which reduces losses considerably. An example of this - that has solved the Earnshaws theorem - is the homopolar electrodynamic bearings invented by Dr Torbjörn Lembke
An active magnetic bearing (AMB) works on the principle of electromagnetic suspension and consists of an electromagnet assembly, a set of power amplifiers which supply current to the electromagnets, a controller, and gap sensors with associated electronics to provide the feedback required to control the position of the rotor within the gap. These elements are shown in the diagram. The power amplifiers supply equal bias current to two pairs of electromagnets on opposite sides of a rotor. This constant tug-of-war is mediated by the controller which offsets the bias current by equal but opposite perturbations of current as the rotor deviates by a small amount from its center position.
The gap sensors are usually inductive in nature and sense in a differential mode. The power amplifiers in a modern commercial application are solid state devices which operate in a pulse width modulation (PWM) configuration. The controller is usually a microprocessor or DSP.
Active bearings have several advantages, they do not suffer from wear, they have low friction, and they can often accommodate irregularities in the mass distribution automatically, allowing it to spin around its centre of mass with very low vibration.
