Magnetic Bearings Information
Magnetic bearings use an electromagnet to provide non-contacting, friction-free motion in rotary applications. They are constructed of a rotating and a stationary part (rotor and stator, respectively) separated by an air gap. Magnetic bearings operate by applying an electric current to ferromagnetic materials used in both the rotor and stator, creating a magnetic flux path that includes the rotor, stator, and air gap. Magnetic bearings do not require lubrication and are virtually maintenance-free. They are available in two different functional types: active (AMB) and passive (PMB).
Active magnetic bearings (AMB) require specialized control and software. They are comprised of three distinct parts: the bearing itself, the electronic control system, and the auxiliary bearings.
- The purpose of the electronic control system is to control the position of the rotor by varying the current in the electromagnets. The electronic control system can be adjusted and adapted to the disturbance frequency of the machine itself. This information can be used to adjust and optimize performance due to process changes.
- Active magnetic bearings have higher stiffness and damping characteristics than similar size passive bearings. The control current for active magnetic bearings can be calculated by taking the square of the rotor-stator gap.
- The auxiliary bearings are used to support the rotor when the machine is stationary with the electronic control system switched off, or, in the event of a failure, in the magnetic suspension system to allow the rotor to run down without damage to the rotor itself or the stator of the magnetic bearings.
Passive magnetic bearings (PMB) do not require control hardware or software for operation. They have lower stiffness and damping characteristics than similar size active bearings. Passive bearings can be constructed with:
- permanent magnets
- diamagnetic materials
- electrodynamic effects
Magnetic bearings are available in three common configurations: radial, axial, and conical; although custom and specialized versions can be manufactured according to need.
Radial magnetic bearings are used for rotary motion where the primary external loads are in the radial direction, or perpendicular to the axis of rotation. They function via four electromagnets, each driven by an amplifier, which are arranged around the rotor to form the bearing. In horizontal applications, the magnet centerlines are orientated at 45° to the perpendicular so that gravity can act upon the upper two adjoining magnets. This increases the stability of the system and adds to the stability of the load.
Axial, or thrust, magnetic bearings are used for rotary motion where the primary external loads are parallel to the axis of rotation. They use a flat, solid ferromagnetic disc, secured to the rotor as the collar, for the axial thrust bearing. Solid disc electromagnets are situated on either side of the collar and operate in a similar manner to the radial bearing above it but in one dimension only.
Conical magnetic bearings are used when the bearing may be subjected to both radial and axial loads. These bearings are designed with a conical shaped stator and rotor.