Magnetic Bearings – The mechanical engineer’s guide

  • There are two main types of magnetic bearings, active and passive, although active magnetic bearings are the preferred option
  • Magnetic levitation eliminates friction and excessive heat generation as well as mechanical wear and tear
  • Active and passive magnetic bearings have in many circumstances been replaced by new pioneering electrodynamic bearings which make better use of rotational motion
  • When using magnetic bearings there is no limit to the rotating speed as there is no friction.

As the name suggests magnetic bearings use magnetic levitation to support the rotating shafts, without directly contacting them, thus there is no friction. As we all know, less friction means less energy wasted and greater overall efficiency. The very first patent relating to the magnetic suspension of rotating elements was submitted in the 1940s. However, the first widespread applications did not occur until the seventies. The delay was a result of practical limitations which caused problems with the design and realisation of reliable magnetic bearings until then.

How Magnetic Bearings Work

When using permanent magnets they can be both a force of attraction and a force of repulsion – how you use this is the key.

 

The force of interaction between a ferromagnet and a source of a magnetic field is always a force of attraction. The conductive non-ferromagnetic material and the alternating magnetic field source are always repelled. Two conductors with electric current can both mutually attract and also repel each other.

Types of Magnetic Bearings

Magnetic bearings can be grouped into eight types in accordance with the principle of operation:-

  1. Permanent magnets
  2. Active magnets
  3. LC resonant
  4. Induction
  5. Conduction
  6. Diamagnetic
  7. Superconducting
  8. Magnetohydrodynamic

The most common are two different types of magnetic bearings, namely:

  • Passive – with permanent magnets
  • Active – with electromagnets and a control device

Passive Magnetic Bearings

The first type of magnetic bearings are the passive ones that use permanent magnets. Due to the inability of magnetic force regulation, these bearings must be designed and manufactured with great precision. However, there is no such thing as a perfect magnetic bearing and it is impossible to reach an absolute electrostatic field stability.

The main disadvantage of permanent magnet bearings is that it is impossible to create a magnetic levitation of a rotor using only permanent magnets. This restriction is a consequence of the:-

  • Earnshaw’s theorem (the equilibrium position of the active element in a static force field is unstable if force and distance are related by the inverse square law)
  • Brownback’s statements (stable levitation of a physical object in the constant magnetic field is possible if the magnetic permeability of its material is less than a magnetic permeability environment).

These laws and statements lead to the fact that in a constant magnetic field (under normal conditions) only physical objects  made of diamagnetic or superconductors can steadily levitate. Ferromagnetic objects can steadily levitate only in an environment with a higher magnetic permeability (for example, in a ferromagnetic fluid).

For this reason, passive magnetic bearings are usually utilised in sets. Each set is devoted to the role of stabilising different directions in the degrees of freedom of the rotating shaft such as:-

  • Axial
  • Radial
  • Tilting

Passive magnetic bearings can perform very well when combined with temperature range limiters and extra dampening elements. These help reduce the vibrations and disturbances that can cause instability and imbalancing of the rotating shaft. However, permanent magnet bearings are not preferred over active magnet bearings and are generally considered to be less effective.

Active Magnetic Bearings

An active magnetic bearing is a controlled electromechanical device. The stable position of the rotor is a consequence of the action of magnetic attraction forces. The principle of the active magnetic bearing ensures that:-

  • The rotor is held in a certain position in the radial or axial directions by means of electromagnetic fields
  • Electromagnetic fields, in turn, are generated by controlled electromagnets
  • These electromagnets are located on the stator of the electromagnetic bearing

The currents that are applied to the windings of these electromagnets vary depending on the position of the rotor in space, in accordance with a given control law. Thus, in addition to electromagnets, the control system also includes position sensors (as well as rotational speed sensors), which produce the error signals. The error signals through the amplifiers control the currents in the electromagnets to return the rotor to the nominal (center) position. The control laws that are programmed in the control system determine the characteristics of the active magnetic bearing.

Active magnetic bearings are the most popular type of magnetic bearings. This is because electromagnets are easier to control and adjust, so the operation is more reliable and predictive. To aid the overall operation, active magnetic bearings also feature gap and rotation sensors. So the distance from the rotating shaft to the inner edge of the bearing is detected. This ensures that intervening corrective action such as the increase or decrease of electric power on a particular magnetic element (electromagnet) can be taken automatically through the main controller — which is typically a microprocessor nowadays.

Applications and Advantages of Magnetic Bearings

The obvious advantage of magnetic bearings is that they allow the rotating shaft to rotate almost completely free of friction. Therefore there is no mechanical wear, no excessive heat generation, no rotating speed limit and it is more efficient. Moreover, and in the case of the active types, the rotating shaft can be very well balanced to automatically rotate around its center of mass thanks to the controller and sensors. As a consequence there is no need to adjust or configure anything as long as it is within an acceptable value deviation range.

In practice this means that imbalanced axles can rotate almost with no vibrations as long as they pass through an active magnetic bearing. The magnetic bearings themselves are much more resilient than other traditional types of bearings. They are not affected by:-

  • Temperature
  • Pressure
  • Load change
  • Rotating speed changes
  • Gravity
  • Chemicals
  • Friction – therefore no lubrication is required

All this makes them ideal for use in applications where low noise, instantaneous start/stop actions, high transient loads, frequent shaft unbalancing (and need for immediate re-stabilisation) and energy efficiency are required.

Combining such benefits makes magnetic bearings very popular in a wide range of application fields. These include industrial machines such as:-

  • Electric motors
  • Pumps
  • Compressors
  • High-speed turbines

As well as specialised applications such as maglev trains and equipment that has to operate in vacuum conditions – where the lubrication of traditional bearings is practically impossible or not feasible.

The disadvantages of magnetic bearings are mainly their very high cost, quite larger size and weight. In the case of active magnetic bearings, the need for a secondary traditional bearing as back-up incase the power is out at any point adds to the expense.

Today and Future

Right now, active magnetic bearings are the established choice for those who need this type of bearing. The reputable SKF has been manufacturing and developing active magnetic bearings for the last four decades. The likes of NSK, Waukesha, AST, FAG, and RBC are leading developments in the field with their R&D innovations.

However, the future seems to be heading back to the passive type of bearing. New advancements in the field of homopolar electrodynamic bearings have managed to successfully bypass the issues analysed in Earnshaw’s theorem – while still not using any kind of a controller to operate. This new type of dynamic bearing is capable of stabilising the rotating shaft by taking advantage of the rotational motion to generate stabilising and position restoring forces. This completely avoids eddy current losses and makes the operation much more energy-efficient than anything currently available. For more details you can read the actual thesis that describes this particular type of pioneering electrodynamic bearing.

About: Bill Toulas

Passionate engineer and new technologies advocate, writing about the ways they shape our world and amplify our very existence. Believes that engineering is the art of changing this world forever, every day, little by little, and sometimes all at once.

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