Getting Technical
CHARLES NICOLSON
Charles Nicolson has a physics and chemistry degree from Natal University which he subsequently put to
good use by applying speciality chemicals in mining and industrial processes where water is a major factor.
This created an enduring interest in water technology, a passion that expanded to the HVAC industry
in 1984 when he joined BHT Water Treatment. Since then, water technology in HVAC water circuits has
continued to be an abiding interest.
INTRODUCTION TO
MAGNETIC BEARINGS
By Charles Nicolson
General understanding of magnetic bearings is that they operate without
needing any contact between a static section of a machine or device
and the rotating section which is usually a shaft of some kind.
A
lthough there are magnetic units which provide only
partial support working in conjunction with conventional
mechanical bearings, the majority of magnetic bearings do, in
fact, run without any additional medium in either vapour or liquid
forms between the fixed and moving surfaces of the bearings. In
other words, magnetic bearings require no lubrication and can run
in normal ambient atmospheric conditions or even in a vacuum.
By supporting spinning shafts or other moving parts without
physical contact, magnetic bearings allow the highest rotational
speeds of any kind of bearing up to (theoretically) an unlimited
maximum.
Historically, the relatively high cost of magnetic bearings
has limited the technology’s application. However, through
standardisation, integration, and manufacturing advances, the
cost of magnetic bearings has continued to decline. Also, the
engineering involvement in designing and installing magnetic
bearings in machines has become far more standardised. The
overall result is that magnetic bearings have become more
economical to use in new and existing rotating machinery
although costs of standardised magnetic bearings are now
increasing in line with general rising costs of engineered machines
and similar technical products.
There are basically two types of magnetic bearings, termed
‘passive’ or ‘active’ bearings.
Passive magnetic bearings use permanent magnets and,
therefore, do not require any input power. However, passive
bearings tend to be bulky and have limited load bearing ability
according to the strength and complexity of their multiple
permanent magnetic components.
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RACA Journal I January 2020
Most magnetic bearings are active magnetic bearings,
using electromagnets which require continuous power input and
an active control system to maintain loading stability. There are
also combined designs in which permanent magnets are used to
carry the static load and active electromagnetic sections are used
to maintain dynamic stability. Active or partially active magnetic
bearings typically require some sort of mechanical back-up
bearing in case of any power or control system failure.
The basic operating principle of an active magnetic bearing is
that ferrous objects such as cylindrical steel shafts are attracted
to magnets which can be, as stated above, either permanent
magnets or electromagnets or combinations of the two types.
It is important to note here that the force between any magnet,
either a permanent magnet or an electromagnet, and any
ferrous object is always attractive – it cannot be repulsive.
Radial
Bearings
Position
Sensors
Stator
Rotor
Thrust Bearing
Electromagnets
Figure 1: Five-axis active magnetic bearing assembly available
commercially in standard sizes.
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