Magnetic Bearings Come Of Age

More-powerful controllers and better software lead to broader applications for magnetic bearings. Magnetic radial bearings have a stator and rotor. The surface area of the magnetic poles, number of coil windings, air-gap dimensions, and applied voltage and current all affect bearing ratings. Applying electric current to a bearing's ferromagnetic material creates an electromagnetic force that suspends the shaft. Digital electronics and sophisticated control algorithms permit simultaneous control of five separate axes — two axes for each radial bearing and a fifth axis running lengthwise along the shaft. Magnetic bearings exhibit an excellent power-to-speed relationship, compared with other bearing types. Magnetic-bearings, which support shafts with magnetic levitation rather than mechanical contact, have been in industrial use for decades. Magnetic bearings offer a host of advantages to users, including high-speed capabilities and the ability to operate lubrication-free and in vacuum environments. They generate no friction, experience minimal wear, and operate contamination free with extremely low vibration. And the bearings can precisely control shaft position, measure external forces acting on the shaft, and even monitor a machine's operating condition. Recent technological developments, especially in digital processing and control, have made magnetic bearings a more-robust and cost-effective design solution than ever. Today's bearings are suitable for a wide range of applications, from semiconductorfabrication equipment to microturbines, from refrigeration compressors to vacuum pumps. Magnetic-bearing systems electromagnetically suspend shafts by applying electric current to a bearing's ferromagnetic materials. The systems have three main elements: bearing actuators, position sensors, and controller and control algorithms. Typical units consist of two magnetic radial bearings and one magnetic thrust bearing. They control the shaft along five axes: two axes for each radial bearing and a fifth axis along the shaft. Magnetic bearings have stationary and rotating components — the stator and rotor, respectively. The radial magnetic bearing stator resembles an electricmotor