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  Tribology and Lubrication CourseNovember 21, 2014,

Arica, Chile

MAGNETIC BEARINGS

Marius Steinfeld,Escuela Universitaria de Ingeniería MecánicaUniversidad de Tarapacá

ABSTRACTThis paper presents a general view of

magnetic bearings and some information aboutcurrent and future challenges. Starting with a

description of the function of Active and Passive

Magnetic Bearings, their properties, including

their advantages/disadvantages and application

with a focus on process technology. 

INTRODUCTIONIn the last decades there were many efforts to

develop magnetic bearing systems for applications

with special requirements. Reasons for that are

economical and technical advantages over

conventional fluid film bearings. The progress in

microelectronic research in the last years madethem favorable for a high spectrum use. Magnetic

 bearings are regarded as the future of bearing

technology.

NOMENCLATUREF: Magnetic force

i: Current intensity

x: Clearance; distance between electromagnet

and rotor

ACTIVE MAGNETIC BEARING (AMB)Active magnetic bearings support the bearing

load with a magnetic force that is created by

electromagnets. They can be arranged as radial or

thrust bearing.

The power amplifier supplies current to the

electromagnets which are embedded in the stator.

Under a steady supply of current they create a

magnetic force field that attracts the rotor (shaft).

The magnet attraction from various sides holds the

rotor in a levitating central position. The magnetic

force that is created can be described by:

This relation between the distance and the force is

not linear. With increasing distance between

electromagnet and rotor, the magnetic force

decreases exponentially, which leads to instability.

A control system is needed, regulating the

magnetic force by adjusting current supply. It

consists of position sensors, a controller and a power amplifier. The sensors measure the current

clearance with electromagnetic induction,

meaning that they use a magnetic field. The sensor

signals are sent to the controller, which compares

them to reference signals. A controller is usually a

microprocessor or a digital signal processor

(DSP). It calculates an input signal with

algorithms that is sent to the power amplifier,

where it leads to an adjustment of current, which

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is supplied to each electromagnet. This control

system ensures that the rotor maintains its

concentric levitating position. Active magnetic

 bearings need auxiliary bearings on the same shaft

to support the load in case of power loss, overload

or shutdown. They are conventional fluid film

 bearings with an extra clearance so that they do

not work until the shaft leaves its center position. 

PASSIVE MAGNETIC BEARINGS (PMB) Passive magnetic bearings use permanent

magnets to keep the rotor levitating. There is

neither a need for a control system nor for a

current supply.

PROPERTIESMagnetic bearings have many advantages

over conventional bearings. They work without

any contact of the surfaces and without lubricant,

which avoids wear and provides zero friction.

Very high rotating velocities are possible,

theoretically without limitation. No coolingsystem is needed. Zero wear and no contamination

of lubricants allow a low maintenance rate.

Because of no friction, machines that use

magnetic bearings have a lower energy

consumption and a longer life expectancy, which

contributes to economic advantages. Another

 property of AMBs is the possibility to receive

measurement data directly. The processor can

calculate directly from the sensor signals data

such as rotational speed, carried load and the

concentricity of the rotor. Automatic

accommodation is the process of compensating

unbalance or vibrations of the shaft automatically

with algorithms. The active creation of vibrations

is also possible. There are some disadvantages as

well. The bearing stiffness is poor and the bearing

itself does not support a high load. Generally it is

a complex system with high acquisition costs.

APPLICATIONMagnetic bearings have many convincing

technological and economical advantages, which

make them favorable for a wide field of

application. Nevertheless, until today they were

only used in special devices because of their costs.

Aerospace, process technology, machine tools and

clean room technology are some examples for

applications wherever high velocity, high

 precision or high purity is required. Other similar

areas are transport systems like the maglev train

TRANSRAPID or tool slides, which use linear

magnetic bearings.

CHALLENGESThe introduction of magnetic bearings to a

wide field of application dominates the current

research and development. Regardless there is a

focus on electric control, like the design of better

algorithms. Another big issue is passive magnetic

 bearings (PMB). They are much simpler thanAMBs and could replace many conventional

 bearings if stability problems were solved. Normal

iron magnets are not sufficient and therefore

momentarily there is an investigation into special

materials like superconductors. Hybrid systems

with active and passive magnetic bearings do

already exist.

REFERENCES[1]  http://ipm.hszg.de/institut/publikationen/publikati

onen-ipm/article/aktive-magnetlager-und-ihre-

anwendungen.html, Publication: 'AktiveMagnetlager und ihre Anwendungen', FrankWorlitz, Zittau/Görlitz University of AppliedSciences

[2] http://www.synchrony.com/knowledge/how-magnetic-bearings-work.php

[3] http://de.wikipedia.org/wiki/Magnetlager  

PICTURESPage 1: Radial AMB profile,

http://www.synchrony.com/knowledge/how-

magnetic-bearings-work.php

Page 2: Thrust bearing with ring magnets,

http://ipm.hszg.de/institut/publikationen/publikationen-ipm/article/aktive-magnetlager-und-ihre-anwendungen.html