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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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