magnetic bearings are used to in lieu of rolling element or fluid film journal bearings in1 (1)

8/2/2019 Magnetic Bearings Are Used to in Lieu of Rolling Element or Fluid Film Journal Bearings In1 (1)

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CHAPTER 1

INTRODUCTION

A magnetic bearing is a bearing which supports a load using magnetic levitation. Magnetic

bearings support moving machinery without physical contact, for example, they can levitate a

rotating shaft and permit relative motion without friction or wear. They are in service in such

industrial applications as electric power generation, petroleum refining, machine tool operation

and natural gas pipelines. They are also used in the Zippe-type centrifuge used for uranium

enrichment. Magnetic bearings support the highest speeds of any kind of bearing, they have no

known maximum relative speed.

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. Conversely, magnetic bearings in machining

spindles must be extremely stiff and have a very broad bandwidth so that tool position isaccurately controlled. In each case, the dynamic response is a result of the controller used to

stabilize the bearing, rather than a consequence of the bearing's physical design.

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CHAPTER 2

HISTORY

Early magnetic bearing patents were assigned to Jesse Beams at the University of Virginia

during World War II and are concerned with ultracentrifuges for purification of the isotopes of

various elements for the manufacture of the first nuclear bombs, but the technology did not

mature until the advances of solid-state electronics and modern computer-based control

technology with the work of Habermann and Schweitzer. Extensive modern work in magnetic

bearings has continued at the University of Virginia in the Rotating Machinery and Controls

Industrial Research Program. The first international symposium for active magnetic bearing

technology was held in 1988 with the founding of the International Society of Magnetic

Bearings by Prof. Schweitzer, Prof. Allaire (University of Virginia), and Prof. Okada (Ibaraki

University). Since then there have been nine succeeding symposia. Kasarda reviews the history

of AMB in depth. She notes that the first commercial application of AMBs was with turbo

machinery.

The AMB allowed the elimination of oil reservoirs on compressors for the NOVA Gas

Transmission Ltd. (NGTL) gas pipelines in Alberta, Canada. This reduced the fire hazard

allowing a substantial reduction in insurance costs. The success of these magnetic bearing

installations led NGTL to pioneer the research and development of a digital magnetic bearing

control system as a replacement for the analog control systems supplied by the American

company Magnetic Bearings Inc. (MBI). In 1992, NGTL's magnetic bearing research group

formed the company Revolve Technologies Inc. to commercialize the digital magnetic bearing

technology. This firm was later purchased by SKF of Sweden. The French company S2M,

founded in 1976, was the first to commercially market AMBs.

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CHAPTER 3

WORKING PRINCIPLE

An active magnetic bearing (AMB) consists of an electromagnet assembly, a set of poweramplifiers which supply current to the electromagnets, a controller, and gap sensors with

associated electronics to provide the feedback required to control the position of the rotor

within the gap. These elements are shown in the diagram. The power amplifiers supply equal

bias 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 bias current by equal but opposite

perturbations of current as the rotor deviates by a small amount from its center position.

The gap 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

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width modulation (PWM) configuration. The controller is usually a microprocessor orDSP.

Fig4.1 Basic Operation

3.1MAGNETIC LEVITATION TECHNOLOGY

Electromagnetic levitation is based on the attractive force of a controllable electromagnet on a

ferromagnetic body .A control unit adjusts the current in an electromagnet and hence the

magnetic force acting on the ferromagnetic body so that the body is held in suspension. Asensor continuously measures the position of the ferromagnetic body. If the

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ferromagnetic body is above the desired position, the controller reduces the current in the

magnet and with it the magnetic force. If the body is below the desired position, the

current in the magnet is increased.

Fig 3.1 Principle of electromagnetic levitation

CHAPTER 4

MAGNETIC BEARING

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It is difficult to build a magnetic bearing using permanent magnets due to the limitations

imposed by Earnshaw's theorem, and techniques using diamagnetic materials are relatively

undeveloped. As a result, most magnetic bearings require continuous power input and an active

control system to hold the load stable. Because of this complexity, the magnetic bearings also

typically require some kind of back-up bearing in case of power or control system failure.

Two sorts of instabilities are very typically present with magnetic bearings. Firstly attractive

magnets give an unstable static force, decreasing with greater distance, and increasing at close

distances. Secondly since magnetism is a conservative force, in and of itself it gives little if any

damping, and oscillations will cause loss of successful suspension if any driving forces are

present, which they very typically are.

The use of an induction-based levitation system present in cutting-edge MAGLEV

technologies, magnetic bearings could do away with complex

Fig4.1 Operation of the magnetic bearing

4.1 INDUSTRIAL MAGNETIC BEARINGS

A magnetic bearing positions and supports a moving shaft using magnetic forces and without

mechanical contact. Because the rotor "floats" in space without contact with the magnets, there

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is no need for lubrication of any kind. Some of the advantages that magnetic bearings offer

over conventional, oil-lubricated bearings include:

Elimination of the oil lubrication system with its associated pumps, valves, coolers,

ducting, sumps, etc.

Higher operating temperatures than would be sustainable by oil lubricants

Higher machine efficiencies due to the elimination of mechanical friction

Improved vibration control

Improved reliability and reduced maintenance lowers downtime for a machine or

process

Extensive health monitoring and protection for the machine

The rotor of the magnetic bearing is mounted on the rotating shaft. Multiple magnets in the

stator surround the rotor, and each one produces a magnetic field that tends to attract the rotor.

Fig 4.2 Industrial Magnetic Bearing

4.2 SUPERCONDUCTING MAGNETIC BEARINGS

The primary factor preventing the application of flywheels to long-term energy storage is loss

in the bearings. Any mechanical bearing with contact between the stationary and rotating parts

will have enough loss to render the system uneconomical .One solution to the problem is to use

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a non-contact active magnetic bearing that employs conventional electromagnets. The

rotational loss of such a bearing is 1-10% that of a mechanical bearing under the same

operating conditions. The problem, however, is that the bearing itself consumes power, which

is dissipated as heat in the copper electromagnets, and the bearing and cooling system power

consumption must be included in the calculation of the overall system efficiency. A reasonable

magnetic bearing consumes a few watts for each kilogram of flywheel weight, depending on

the structure of the bearing and the control system, and this loss is sufficient to make a system

using copper electromagnets uneconomical. Superconducting magnetic bearings, on the other

hand, have demonstrated losses of 10-2 to 10-3 watts per kg for a 2,000 rpm rotor. This translates

to an overall one-day, "round-trip" system efficiency of 84%, which is acceptable.

Fig. 4.3 Superconducting magnetic bearingassembly

4.3 PASSIVE MAGNETIC BEARINGS

Passive magnetic bearings use opposed pairs of permanent magnets on the rotor and stator to

establish the bearings stiffness. Inherently simple, these bearings do not require any shaft

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position sensors or an electronic controller for operation.

Active magnetic bearings offer higher load-carrying capability and can accommodate higher

temperatures. Their electronic controllers tune bearing stiffness and damping properties "on

the fly," allowing for adjustments to system dynamics that affect resonant frequencies and

reduce transmitted vibration. Our proprietary virtual balancing method adjusts the shafts

running position to minimize vibration.

Fig 4.4 Passive Magnetic Bearings

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CHAPTER 5

DESIGN PRINCIPLES

A completely passive and contactless magneto static bearing, stable in all 6 degrees of freedom

(DOF), cannot be realised under normal conditions [7]. In practice, at least one axis has to be

controlled actively by means of electromagnets. Earlier publications on magnetic-bearing

wheels either control one, two or five DOF actively [6, 5, 8]. Table 1 compares these three

options.

Magnetic bearings can be realised by using attractive or repulsive forces. A better mass vs.

stiffness ratio can be achieved by using the attractive force mode [9]. Preference was given tothe 2 DOF option where the wheel is actively controlled along two orthogonal radial directions

where axial movements and all other degrees of rotor freedom are passively controlled by

means of permanent magnets, except for the rotor spin. The two radial axes are independently

controlled by their control loops. This design principle generally results in a flatter geometry,

using less volume and being suitable for panel mounting. Moreover, the 2 DOF actively

controlled bearing allows a high momentum-to-mass ratio of the wheel as parts of the bearing

contribute to the momentum storage capacity. For position detection, four field displacement

type inductive sensors are mounted with 90 degrees angular spacing around the flywheel,

facing the outside rim surface.

In the wheel design both permanent magnets and electromagnetic coils are used. Most of the

DOF are passively controlled - this has the advantages of high reliability and low power

consumption because the amount of electronics is reduced. The permanent magnets produce the

main part of the magnetic flux in the magnetic circuit and the electromagnetic coils modulate

this static bias flux, allowing the control of restoring forces on the wheel to keep it centered.

This modulation is necessary to provide active control in the radial direction in the presence of

imbalance or external forces. Another advantage is the linearised characteristic of force vs.

current through the superposition of permanent magnetic and electromagnetic fluxes. Rare-

earth permanent magnets were chosen because they offer a high energy density and have

advantages in terms of mass and volume.

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CHAPTER 6

ADVANTAGES OF MAGNETIC BEARINGS

Magnetic bearings have specific properties that differentiate them from mechanical bearings.

The principle advantages relate to the absence of physical contact and electronic control of the

rotor position.

No lubrication

No abrasion

No generation of particles

Easy to clean and sterilize

Ideal for clean-room operation

Can operate under difficult environmental conditions like heat, cold, steam, vacuum,

aggressive chemicals

Excellent thermal insulation of rotor and stator

Hermetic sealing (canning) possible

Low vibration and noise

Electronic adjustment of damping and stiffness

Electronic unbalance compensation

Electronic fine positioning of the rotor within the air gap

Permanent monitoring of bearing load, rotor deflection and unbalance without

additional equipment

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CHAPTER 7

DISADVANTAGES

Require strong applied magnetic field.

Initial cost and maintenance cost is high.

Many of the suspension techniques have a fairly narrow region of stability.

Magnetic fields have no built-in damping. This can permit vibration modes to exist that

can cause the item to leave the stable region. Eddy currents can be stabilizing if a

suitably shaped conductor is present in the field, and other mechanical damping

techniques have been used in some cases.

Power requirements can be large.

Superconductors require very low temperatures to operate.

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CHAPTER 8

APPLICATIONS

Magnetic bearing advantages include very low and predictable friction, ability to run withoutlubrication and in a vacuum. Magnetic bearings are increasingly used in industrial machines

such as compressors, turbines, pumps, motors and generators. Magnetic bearings are commonly

used in watt-hour metersby electric utilities to measure home power consumption. Magnetic

bearings are also used in high-precision instruments and to support equipment in a vacuum, for

example in flywheel energy storage systems. A flywheel in a vacuum has very low windage

losses, but conventional bearings usually fail quickly in a vacuum due to poor lubrication.

Magnetic bearings are also used to support maglev trains in order to get low noise and smooth

ride by eliminating physical contact surfaces. Disadvantages include high cost, and relatively

large size.

A very interesting new application of magnetic bearings is their use in artificial hearts. The use

of magnetic suspension in ventricular assist devices was pioneered by Prof. Paul Allaire and

Prof. Houston Wood at the University of Virginia culminating in the first magnetically

suspended ventricular assist centrifugal pump (VAD) in 1999

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CHAPTER 9

CONCLUSION

Calculations based on the finite element method give a deeper insight into the phenomena in

magnetic bearings. The performed analysis plays an important role in the design procedures of

magnetic bearing and helps in the verification of the construction assumptions. It is essential to

use appropriate materials for the shaft and stator. The pole shoes arrangement, their sizes and

coil parameters strongly influence the electromagnetic force value produced by the

electromagnet. The design procedure of magnetic bearings is a complex task consisting of a

few elements: analysis of the magnetic bearings operating mode parameters, calculation of

electromagnetic force, selection of materials and calculation of magnetic field properties to

obtain the desired force value, and the choice of controller architecture. Thus the magnetic field

analysis plays an important role in the magnetic bearings development process. Current

research is focused on 2D and 3D modeling and analysis using the electromagnetic module

with Simulink interactions to examine the static and dynamic magnetic bearings behavior in the

real operation environment.

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REFERENCE

1) Sperber, F. 1996, International Communication and Experimental Satellite in a

High-Elliptical Orbit. International Symposium on Small Satellites.

2) Studer, A, January 1972, Magnetic Bearings for Instruments in the Space

Environment.

3) Studer, A. 1978, Magnetic Bearings for Spacecraft. NASA Technical Memorandum

78046, Goddard Space Flight Center, Greenbelt.

4) Robinson, A.A. May 1981, Magnetic Bearings - the Ultimate Means of Support for

Moving parts in Space. ESA Bulletin 26.

5) Robinson, A.A.: 1982, A Leightweight, Low-Cost, Magnetic-Bearing Reaction Wheel

for Satellite Attitude Control Applications.

6) Anstett, Souliac, M, Rouyer, C, Gauthier, M. 1982,SPOT - The Very First Satellite

to Use Magnetic Bearing Wheels.

7) Earnshaw, S, 1842. On the nature of molecular forces which regulate the constitution

of the limiferous ether Issue no: 7.

8) Bichler, U., Eckart,T. 1993, A Gimballed Low Noise Momentum Wheel. 27th

Aerospace Mechanisms Symposium, NASA Ames Research Center.

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magnetic bearings are used to in lieu of rolling element or fluid film journal bearings in1 (1)

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