Active Magnetic Bearing

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A bearing is a machine element, which supports another moving machine element. It permits relative motion between the contact surfaces of the members, while carrying load. Due to relative motion, friction occurs and rubbing surface wears rapidly. To prevent this a lubricant may be used.

A question comes in mind whether it is possible to support a journal without any physical contact so that there is no wear. Is there a force, which can exerted even if there is no contact between two-surfaces. Magnetic force allows such property and thus the idea of magnetic bearing was visualized.

Already in 1842 Earnshaw had demonstrated that passive (permanent) could made but it cannot stable in all spatial direction. For successful operation the unstable direction have to be electrically served. Hence the name ACTIVE MAGNETIC BEARING.

The active magnetic bearing comprises two parts ? the mechanical and electronic. The mechanical parts are similar to electrical motor with a rotor and stator. A core on the stator is wound with the coil through which the electric current that induces the magnetic field. This generates the force that supports the shaft. The electronic part of the active magnetic bearing is the control system. It includes sensors that measure the exact position of the rotor. Even slightest deviation from the desire position will trigger in electronic system to adjust the current flowing through the electromagnets that determines the strength of the magnetic field. The current are adjust so that desired rotor position is maintained even under varying load conditions. The magnetic field is dependant on the current flowing through the coils. The larger the current, the stronger the magnetic field and the load it is able to support. The load an active magnetic bearing is able to support is very high.

1. List of symbols:

  • B???????????????????????????? Flux Density
  • H???????????????????????????? Field Intensity
  • T???????????????????????????? Magneto-motive Force (MMF)
  • Rc??????????????????????????? Resistance of the coil
  • Rext???????????????????????? Resistance of the external circuit
  • R???????????????????????????? Total resistance
  • E????????????????????????????? Electro-motive force (EMF)
  • I?????????????????????????????? Current
  • lav??????????????????????????? Average length of the wire
  • N???????????????????????????? Number of turns
  • Acond?????????????????????? Cross-Sectional Area of the conductor
  • Acoil???????????????????????? Cross-Sectional Area of the coil
  • j?????????????????????????????? Current Density
  • r????????????????????????????? Radius to the centre of the coil
  • ????????????????????????????? Resistivity
  • ?OT???????????????????????? Resistivity at operating temperature
  • ?o??????????????????????????? Permeability of air
  • ROT???????????????????????? Resistance at operating temperature

 

 2. Introduction:
This report is to explain the necessary steps that were taken to achieve the task of theoretically building a Magnetic Bearing Actuator. This specific report entails the design details of a radial 8-pole, hetero-polar magnetic bearing actuator. The design had to be within certain specifications had to adhere to. The bearing had to be optimized in accordance to certain design criteria (such as coil area, resultant force on the journal, minimum core volume etc).
There are two parts to the design a magneto-statics component which was used to obtain the load capacity and a thermal component that determines the temperature operating range of the bearing depending on the insulation class given.
The main aim of the design was to make sure that:

  • The bearing develops the required load capacity (slightly higher) ? result must be confirmed by FE model and relevant calculations.
  • The winding temperature was within the acceptable range for the required insulator class.

 

3. Theory:
Magnetic Bearings:
Magnetic bearings are used to in lieu of rolling element or fluid film journal bearings in some high performance turbo machinery applications. Specific applications include pumps for hazardous/caustic fluids, precision machining spindles, energy storage flywheels, and high reliability pumps and compressors.
Magnetic bearings yield several advantages. Since there is no mechanical contact in magnetic bearings, mechanical friction losses are eliminated. In addition, reliability can be increased because there is no mechanical wear.
Besides the obvious benefits of eliminating friction, magnetic bearings also allow some perhaps less obvious improvements in performance. Magnetic bearings are generally open loop unstable, which means that active electronic feedback is required for the bearings to operate stably. However, the requirement of feedback control actually brings great flexibility into the dynamic response of the bearings. By changing controller gains or strategies, the bearings can be made to have virtually any desired closed-loop characteristics. For example, flywheel bearings are extremely compliant, so that the flywheel can spin about its inertial axis--the bearings serve only to correct large, low frequency displacements.
Typical Bearing Geometry
Conceptually, the typical magnetic bearing is composed of eight of horseshoe-shaped electromagnets. This configuration is shown in Figure 1. The eight magnets are arranged evenly around a circular piece of iron mounted on the shaft that is to be levitated. Each of the electromagnets can only produce a force that attracts the rotor iron to it, so all eight electromagnets must act in concert to produce a force of arbitrary magnitude and direction on the rotor.

 

 

 

Conventional bearings provide support to rotating machinery by allowing relative movement in a plane of rotation.
They allow rotation and provide support in either radial or axial planes of rotation.? The most common types of bearings are rolling element bearings, and oil film or journal bearings.
Rolling element bearings consist of a stationary outer race and a rotating inner race; in between them are the rolling elements ? most common are spherical balls, but cylinders or tapered pins are also used.? During rotation, these 3 items are in contact with each other and the weight being supported is transferred through the rolling elements between the inner and outer races.
Oil film bearings have no rolling elements, but make use of pressurized oil to provide a film of support, and prevent galling between the shaft and the bearing journal.? The oil is circulated so that fresh, cool oil is constantly entering the space between the stationary and rotating pieces.? The shaft rotation shears the oil in this gap, causing it to heat up.? The oil then exits the journal for cooling, filtering, and recirculation.
Magnetic bearings allow contact ? free levitation.? This offers a number of interesting advantages.? Magnetic bearings do not require lubrication, they allow high circumferential speeds at high loads, they do not suffer friction nor wear, the therefore they offer a virtually unlimited lifetime while no maintenance is needed.
Furthermore, the bearing force can be modulated, either for compensating unbalance forces, or for deliberately exciting vibrations.? Because of these advantages, they are used in an increasing number of commercial high-performance applications in the domain of rotating machinery.? These include ultra-high vacuum pumps, canned pipeline compressors and expanders, high-speed milling and grinding spindles, flywheels for energy storage, gyroscopes for space navigation, spinning spindles, and others.

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