Magnetic Bearing Terminology: The ABCs of Magnetic Bearings

Magnetic bearing terminology and nomenclature is changing all the time as the technology evolves, but most of the key terms are fairly well established. Here are some of the most important ones.

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Active Magnetic Bearing (AMB) - A device that uses electromagnetic forces to levitate a shaft in space and maintain its position. The components of an AMB system include magnetic bearing stator, magnetic bearing rotor, position sensors and controller. AMBs can act as either radial or thrust bearings.

Auxiliary Bearings - A bushing or rolling element bearing used to prevent the bearing rotor from contacting the bearing stator in the event of a total loss of power or large transient load. The shaft does not contact the auxiliary bearings during normal machine operation

Control Algorithms - A set of mathematical relationships used to provide feedback control or rotor position within the magnetic bearing controller. For digital controllers, it is implemented in the software.

Electromagnet - A magnet created by winding wire (usually copper) around a ferrous core (usually steel). A magnetic field is created when an electric current flows through the wire windings, which produces a force between the electromagnet and ferrous material in proximity.

Health Monitoring - The ability to monitor, identify and analyze the status of important conditions such as anomalous temperature and vibration in rotating machines. This information can be used to set alarms and schedule maintenance. Much of this information can be obtained through the magnetic bearing user interface without the need of additional health monitoring systems.

Magnetic Bearing Controller - A device consisting of signal processors and power amplifiers. Used to adjust the magnetic field within the bearing to maintain shaft position.

Magnetic Bearing Load Capacity - The maximum force a magnetic bearing can produce in any given direction.

Magnetic Bearing Rotor - The rotating part of the magnetic bearing mounted on the shaft. When electrical current flows in the electromagnets in the stator, the rotor is attracted to the stator, resulting in a magnetic force.

Magnetic Bearing Stator - The stationary part of the magnetic bearing mounted in the machine housing. The stator incorporates the electromagnets and generates the magnetic field.

Passive Magnetic Bearing - A magnetic bearing that uses permanent magnets rather than electromagnets to position and support a shaft. It is impossible to stably support a shaft using only passive magnetic bearings, i.e., active controls are required in at least one direction. Also, the stiffness and damping of passive magnetic bearings are much less than for active magnetic bearings, and therefore passive magnetic bearings are not typically used in machines requiring small clearances between stationary and rotating components.

Position Sensor - Radial and axial sensors that send shaft position signals to the magnetic bearing controller. The controller uses these signals to modify the electromagnetic field in the bearing.

Radial Magnetic Bearing (RMB) - A magnetic bearing that produces magnetic forces in radial (lateral) directions. The radial bearing keeps the shaft centered about the rotational axis of the machine.

Rotor Critical Speeds - When the shaft rotational speed coincides with the natural frequencies of the shaft, it is said to have reached a critical speed. At these critical speeds, the shaft vibration is amplified, which can possibly damage the machine.  Because the stiffness and damping of the magnetic bearing can be electronically optimized, the vibration at critical speeds can be greatly reduced.

Thrust Magnetic Bearing (TMB) - A magnetic bearing that produces magnetic forces in axial (longitudinal) directions. The thrust bearing maintains the shafts axial position in the machine.

Uninterruptible Power Supply (UPS) - A device that contains batteries (or other power source) housed in an enclosure that provide emergency power to the magnetic bearings in the event of a loss of power.