Getting Technical
Ferrous objects (such as steel shafts) are commonly thought of
as being ‘magnetic’ which can lead to confusion as magnetic
substances are not magnets themselves and therefore do not
have any magnetic polarity.
The most commonly occurring magnetic bearing consists
of a steel shaft rotor surrounded by electromagnets and gap
sensors. Active magnetic bearings control clearance between the
rotating shaft and the electromagnets using closed-loop feedback
systems to control the shaft’s position. Inductive sensors detect
the gaps, digital processors interpret the signals and power
amplifiers provide currents to the electromagnets.
Figure 1 details a more complex layout typical of a commercial
five-axis magnetic bearing system available in standard sizes. Two
radial magnetic bearings support and position the shaft in the
lateral (radial) direction, and one supports and positions the shaft
along the longitudinal (axial) direction.
Similar to the single radial bearing, electrical control panels
for these bearings are supplied separately for convenience of
installation either close to or remote from the bearing assembly.
The electrical and electronic components in the control panels
operate at sensing, processing and amplification cycles of up to
18 000 per second.
Passive magnetic bearings have been in use since the early
1900s mainly on rotating shafts which run at specific revolutions
and constant radial loadings. Practical development of active
bearing types has been generally limited to fewer specific
installations mainly where machines such as turbines and
centrifugal pumps run at constant revolutions but require more
shaft support than passive magnetic assemblies can provide.
In common with many other technological advances which
were facilitated during World War II under the almost unlimited
financial aspects of the Manhattan Project, new active magnetic
bearings were developed early in 1943 for ultracentrifuges needed
to enrich uranium sufficiently to weapons grade level for the first
atomic bombs.
However, after the end of World War II, further development
work on active magnetic bearings reverted mainly to research and
pilot plant runs for almost fifty years until the early 1990s.
One of the notable users of active magnetic bearings at this
time was the NOVA Gas Transmission Ltd. (NGTL) operating
gas pipelines in Alberta, Canada. NGTL found that using active
magnetic bearings in gas compressors had an additional
benefit of allowing for the elimination of oil lubricant reservoirs
which reduced the fire hazard resulting in a substantial
reduction in insurance costs. The success of these magnetic
bearing installations led NGTL to pioneer further research and
development of a digital magnetic bearing control system as
a replacement for the analogue control systems currently in
use. In 1992, NGTL's magnetic bearing research group formed a
company, Revolve Technologies Inc. for commercialising digital
www.hvacronline.co.za
magnetic bearing technology. The company was successful and
was subsequently purchased by SKF of Sweden in 2007.
Ongoing advances in magnetic bearings, including
miniaturisation, simplification, and integration have expanded
their use in rotating machinery in commercial and industrial
sectors. Like other bearings, magnetic bearings provide stiffness
and damping. However, unlike other bearings, their performance
may be optimised by simply changing control parameters. For
example, advanced control algorithms enable minimising of
machine vibration even under relatively high levels of imbalance.
REDUCING MAGNETIC BEARING SIZE
In magnetic bearings, bearing pressures are less than those
in oil and other liquid-lubricated bearings. Therefore, for the
same load capacity, magnetic bearings are larger. Historically,
this has made integrating magnetic bearings into some types
of rotating machines difficult and expensive thereby limiting
the range of applications.
Through ongoing design innovations, the size of radial
magnetic bearings has been reduced by more than 30%.
Increasing the amount of steel at the bore of the stator while
reducing the amount of steel elsewhere has improved bearing
pressures in radial bearings. Lengths of radial magnetic bearings
have also been reduced by developing position sensors that can
be integrated directly into the electromagnets.
“Historically, the relatively
high cost of magnetic bearings
has limited the technology’s
application.”
REDUCING CONTROLLER SIZES
Controllers consist of sensor conditioning electronics, analogue
to digital converters, digital processors, digital to analogue
converters, power amplifiers, and a communications interface.
Design innovations have systematically miniaturised or even
eliminated some of these components.
For instance, high-speed analogue converters are
eliminated by using frequency modulated position signals
that are directly converted to digital values. An integrated
processor architecture combines processing, power switching
and communications while eliminating digital to analogue
converters. Finally, new control algorithms make it possible to
reduce the size of power amplifiers.
These innovations have dramatically reduced the
controller size, once as bulky as a household refrigerator
RACA Journal I January 2020
65