Magnetic Bearing Terminology

Magnetic Bearing: A type of bearing, particularly for rotating shafts, that uses electromagnetic forces to support the shaft with no mechanical contact.

 

Active Magnetic Bearing (AMB): A type of magnetic bearing in which non-contact support of a shaft is achieved by using closed-loop control. Such bearings use attractive magnetic forces where at least part of the magnetic force is produced by actively controlled electromagnets. The AMB includes a magnetic actuator, position sensor and magnetic bearing controller (MBC) with a control device, power amplifiers and power supply. Most industrial and commercial magnetic bearings are of this type.

 

Passive Magnetic Bearing: A type of magnetic bearing that does not require active control. While there are a wide variety of passive magnetic bearings, they have not found significant commercial applications due to low stiffness and low force density per unit surface area.

 

Electromagnetic (EM) Biased Magnetic Bearing: A type of AMB in which the magnetic field has two components – a constant bias field and a variable control field, with the bias magnetic field being generated by an electrical current in a coil. The primary purpose of the bias is to linearize the applied force vs. coil current relationship of the actuator. Almost all commercial magnetic bearings are either EM Biased AMBs or Permanent Magnet (PM) Biased AMBs (see below).

 

Permanent Magnet (PM) Biased Magnetic Bearing: A type of AMB in which the magnetic field has two components – a constant bias field and a variable control field, with the bias magnetic field being generated by a permanent magnet (PM). As with the EM Bias AMB (above), the primary purpose of the bias is to linearize the applied force vs. coil current relationship of the actuator. All commercial AMBs are either EM Biased AMBs or PM Biased AMBs.

 

Radial Magnetic Bearing: Magnetic Bearing in which magnetic forces produce radial support for the shaft in two orthogonal axes.

 

Axial Magnetic Bearing (or, Thrust Bearing): Magnetic Bearing in which magnetic forces produce axial support for the shaft.

 

Combination Radial and Axial Magnetic Bearing (or, Combo bearing): Magnetic Bearing in which magnetic forces provide both axial and radial supports for the shaft.

 

Magnetic Bearing Controller (MBC): Electronic hardware with all of the components needed to operate an AMB system. The MBC includes the electrical power supplies, power amplifiers, and analog and/or digital control processing components. All modern MBCs now use digital control as this provides the most flexibility for use with a wide variety of machines, but it also enables a wide variety of commissioning, diagnostic and monitoring capabilities.

 

Magnetic Bearing Actuator (or simply, “Actuator”): A device capable of producing magnetic forces in a magnetic bearing.

 

Magnetic Bearing Position Sensor (or simply, “Position Sensor”):A device capable of sensing shaft displacement (position) with a wide bandwidth. In AMBs, information from the position sensors is used by the MBC to control the actuator coil currents, and thus the magnetic force on the shaft. Most often the position sensor will include a sensor head (transducer) in the machine adjacent to the magnetic actuator and drive/demodulation electronics in the MBC.

 

Magnetic Bearing Speed Sensor (or simply, "Speed Sensor"): A speed sensor is a device capable of measuring rotational speed of the shaft about its spin axis. Information about the shaft speed is used by the MBC to ignore the synchronous component of the shaft position variations, thus allowing the shaft to spin about its center of mass. This feature minimizes transmission of the shaft vibrations due to unbalance to the housing of the machine and reduces the load on the magnetic bearings.

 

Magnetic Bearing System: Actuators, position sensors, speed sensors and magnetic bearing controller, together form the complete control system – called Magnetic Bearing System or AMB System – to control the position of the shaft in an air-gap. In general, a magnetic bearing system has five control axes: four radial axes to control translation and rotation in two orthogonal planes and one axial axis to control translation in the axial or thrust direction.

 

Back-up Bearing (also Auxiliary Bearing or Touchdown Bearing): Mechanical bearings, such as ball bearings or retainer rings, used to support the shaft and protect components from damage in the event that there is power interruption, there is a temporary overload of the magnetic bearing, or there is a failure of some component of the AMB system.

 

Levitation: Supporting a body (rotor) on a magnetic field. When an AMB is activated and controls the rotor to the desired position (the set point) via magnetic forces, the rotor is said to be levitated.

 

Magnetic Center of an AMB: Location in an AMB, typically near the geometric center, where the attractive magnetic forces are balanced in all directions and there is no net force on the shaft centered at this position.

 

Magnetic Bearing Negative Stiffness (or simply "Negative Stiffness"): The actuator position stiffness of the actuator due to bias flux at the nominal rotor position without an external load in an EM Biased or PM Biased AMB,. In a conventional spring/mass system, displacing the mass from the equilibrium position results in a spring force that pushes the mass back toward equilibrium – this is positive stiffness. In a biased AMB with no control, displacing the mass (shaft) from the quasi equilibrium position at magnetic center results in a force that pulls the shaft further away from the center – this a negative stiffness. Because of the negative stiffness, AMBs are unstable without closed-loop control.