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How Magnetic Bearings Work

A magnetic bearing is a bearing that supports a load using magnetic levitation. Magnetic bearings support moving parts without physical contact.For instance, they are able to levitate a rotating shaft and permit relative motion with very low friction and no mechanical wear.Magnetic bearings support the highest speeds of all kinds of bearing and have no maximum relative speed.

How Magnetic Bearings Work :-


There are three key components in a magnetic bearing system that work together to achieve this new level of performance. First the magnetic bearing itself consists of stationary (stator) components as well as rotating (rotor) components.
  These components create the magnetic field that will support and control the rotor position. The second component is the position sensor. These sensors continuously monitor the position of the bearing rotor relative to the position of the bearing stator.

The third component is the Magnetic Bearing Controller (MBC). This is the ‘brains’ of the system, taking the data from the position sensors and determining how much power it should deliver to each magnetic bearing in order to keep the system stable and under control.

The magnetic bearing stator consists of a stack of steel laminations that are wound with copper wire to form an electromagnet. In operation, a current is supplied to each coil of wire to produce an attractive force that levitates the shaft inside the bearing.

The MBC applies the precise level of current to the coils determined by monitoring signals from the positioning sensors in order to keep the shaft at the desired position throughout the operating range of the machine. Depending on the application, there is typically a 0.5 to 1 mm air gap between the bearing rotor and stator.

 An active magnetic bearing

One if the motst common type of magnnetic bearing is active magnetic bearing (or AMB) is a type of bearing used in high speed rotating machinery that uses electromagnetic forces to levitate a rotating shaft in space, and maintains its position by actively controlling the electromagnets, leaving zero contact between the bearing and the rotating mass.
This magnetic levitation allows a no contact, friction-free operation, elimination of many machine components, and a clean, reliable and efficient machine.

A typical AMB is made up of the following elements:
  • Stator
  • Shaft
  • Sensors
  • Electromagnets
AMBs can be configured as either radial or thrust (axial) bearings.
These elements are shown in the diagram . Power amplifiers supply equal current to two pairs of electromagnets on opposite sides of a rotor.

This constant tug-of-war is mediated by the controller, which offsets the current by equal but opposite amounts of current as the rotor deviates by a small amount from its center position.
 Sensors provide information to the controller for exact rotor position allowing the controller to interpret and control the amount of current provided by the power amplifier.

The sensors are usually inductive in nature and sense in a differential mode. The power amplifiers in a modern commercial application are solid state devices which operate in a pulse width modulation (PWM) configuration. The controller is usually a microprocessor or DSP.

The Application of Magnetic Bearings

For centrifugal air blowers, few technologies can match the energy efficiency and reliability that’s possible with a permanent magnet motor (PMM), active magnetic bearings (AMB) and a variable speed drive (VSD). SKF has designed this advanced combination of components and technologies as a complete package solution that can help manufacturers streamline product design, development and assembly.

Permanent magnet motor
  • Low energy use and cooling requirements
  • High-speed capabilities in a compact design
  • 10%+ more energy efficient than conventional motors at full load and part load
  • Direct drive configuration eliminates gearbox and oil
  • Optimized shaft geometry accommodates large impellers with robust rotor dynamics
Active magnetic bearings
  • Accommodate instant and frequent start-ups and transient surge forces
  • Active control system provides vibration-free performance
  • Capable of speeds in excess of 40 000 rpm
  • Levitate rotating components for friction- and lubricant-free performance
  • Unitized radial and axial bearing modules enable compact packaging and robust performance
Magnetic bearing controller
  • Tracks and registers rotor position up to 15 000 times per second
  • Controls rotor position to within a micron-sized orbit
  • Continuously corrects rotor position by adjusting the power supplied to each electromagnet
  • Instrumentation for integration into the blower control system

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