Electrodynamic bearing
Encyclopedia
Electrodynamic bearings (EDBs) are novel, promising systems that can be used to realize contactless electrodynamic suspension
of rotating shafts
.
Relative to active magnetic bearings (AMB) the passive nature of the levitation achieved by EDBs allows a simpler, more reliable and cheaper solution, opening the field of application to medium and large-scale production.
The working principle is based on the induction of eddy currents in a rotating conductor
. When an electrically conducting material is moving in a magnetic field
, a current will be generated in the material that counters the change in the magnetic field (known as Lenz' Law). This generates a current that will result in a magnetic field that is oriented opposite to the one from the magnet
. The electrically conducting material is thus
acting as a magnetic mirror
.
was problematic, but eddy currents and associated power dissipation can be reduced to very low values. The principle of operation is as follows:
A bearing must (1) support a loading force (for example, the weight of a rotor) and (2) provide a force gradient (a restoring force) to hold the rotor in position. Permanent magnets can support weight (in a conventional way, without eddy currents), and without creating destabilizing force gradient
s, but the Earnshaw theorem precludes achieving stability by this means. Eddy currents can provide a stabilizing force gradient without applying a force at the operating position (for example, when a shaft is centered). Creating this force gradient does not require eddy currents (which are induced in proportion to shaft offset). In practice, eddy currents, and hence resistive losses, can be reduced to small values in normal operation. Dynamic bearings of this class, using permanent magnets and ordinary, resistive conductors, can support load and apply restoring force while dissipating little power (and in principle, none).
An improved design approach for bearings of this class was described and analyzed by Dr. Torbjörn Lembke in his PhD thesis at the Royal Institute of Technology
, KTH, in Stockholm, Sweden.
which uses halbach array
s and litz wire loops.
Electrodynamic suspension
Electrodynamic suspension is a form of magnetic levitation in which there are conductors which are exposed to time-varying magnetic fields. This induces currents in the conductors that creates a repulsive magnetic field which holds the two objects apart....
of rotating shafts
Rotordynamics
Rotordynamics is a specialized branch of applied mechanics concerned with the behavior and diagnosis of rotating structures. It is commonly used to analyze the behavior of structures ranging from jet engines and steam turbines to auto engines and computer disk storage...
.
Relative to active magnetic bearings (AMB) the passive nature of the levitation achieved by EDBs allows a simpler, more reliable and cheaper solution, opening the field of application to medium and large-scale production.
The working principle is based on the induction of eddy currents in a rotating conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
. When an electrically conducting material is moving in a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
, a current will be generated in the material that counters the change in the magnetic field (known as Lenz' Law). This generates a current that will result in a magnetic field that is oriented opposite to the one from the magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
. The electrically conducting material is thus
acting as a magnetic mirror
Magnetic mirror
A magnetic mirror is a magnetic field configuration where the field strength changes when moving along a field line. The mirror effect results in a tendency for charged particles to bounce back from the high field region....
.
Avoiding eddy current losses
Before the mid 1990s the eddy currents dampingDamping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
was problematic, but eddy currents and associated power dissipation can be reduced to very low values. The principle of operation is as follows:
A bearing must (1) support a loading force (for example, the weight of a rotor) and (2) provide a force gradient (a restoring force) to hold the rotor in position. Permanent magnets can support weight (in a conventional way, without eddy currents), and without creating destabilizing force gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
s, but the Earnshaw theorem precludes achieving stability by this means. Eddy currents can provide a stabilizing force gradient without applying a force at the operating position (for example, when a shaft is centered). Creating this force gradient does not require eddy currents (which are induced in proportion to shaft offset). In practice, eddy currents, and hence resistive losses, can be reduced to small values in normal operation. Dynamic bearings of this class, using permanent magnets and ordinary, resistive conductors, can support load and apply restoring force while dissipating little power (and in principle, none).
An improved design approach for bearings of this class was described and analyzed by Dr. Torbjörn Lembke in his PhD thesis at the Royal Institute of Technology
Royal Institute of Technology
The Royal Institute of Technology is a university in Stockholm, Sweden. KTH was founded in 1827 as Sweden's first polytechnic and is one of Scandinavia's largest institutions of higher education in technology. KTH accounts for one-third of Sweden’s technical research and engineering education...
, KTH, in Stockholm, Sweden.
Linear magnetic bearing
Linear dynamic magnetic bearings also exist. For example inductrackInductrack
Inductrack is a passive, fail-safe electrodynamic magnetic levitation system, using only unpowered loops of wire in the track and permanent magnets on the vehicle to achieve magnetic levitation. The track can be in one of two configurations, a "ladder track" and a "laminated track"...
which uses halbach array
Halbach array
A Halbach array is a special arrangement of permanent magnets that augments the magnetic field on one side of the array while cancelling the field to near zero on the other side...
s and litz wire loops.