6 MARCH/APRIL 2018
higher running speeds, higher efficiencies and longer
machine life than conventional bearings. AMBs can also be
used in harsh environmental conditions, including extremely
low temperatures, zero gravity and corrosive environments.
A Graphical User Interface (GUI) is typically supplied with
commercial AMBs to provide access to many features built-in
to the control firmware, such as calibration, health monitoring,
data logging and troubleshooting. Dynamic properties,
such as stiffness and damping, can be easily measured and
readily changed via interaction between the GUI and AMB
firmware. In contrast, with conventional bearings, changing
the dynamic properties would typically require a complete
redesign, remanufacture, retest and reinstallation.
In addition, the high static stiffness of AMBs provide
more precise control over the nominal shaft center under
load, and AMBs allow for synchronous force rejection
schemes (synchronous cancellation), which virtually eliminate
transmission of rotor unbalance forces to the outside structure.
There are other advantages to using AMBs. Power
consumption of advanced AMBs is typically negligible
compared to the power rating of the machine they are
used in. For example, a homopolar permanent-magnet-biased magnetic bearing system used in a 175 kW
turbocompressor require less than 200 W of power for its
operation. A heteropolar electromagnet-biased magnetic
bearing for the same machine would consume more power
but still less than 500 W.
Challenges and Solutions
There are some inherent losses within AMBs. The radial
magnetic force exerted on a rotor by an AMB
becomes weaker when the rotor spins at a
sufficiently high speed. This is because the
rotor, typically made of conductive soft-magnetic
material, produces induced eddy currents when
spinning in the non-uniform magnetic field
needed to induce a radial force.
In order to reduce the eddy currents in the
rotor, and, subsequently the loss of some radial
force capacity, a portion of the rotor is normally
made of electrically insulated steel laminations.
Thinner laminations have reduced eddy currents
and thus smaller force loss at a given rotational
speed. The loss of radial force also depends
on the frequency of the magnetic field that a
rotor sees when spinning, or for a given spin speed, on a
spatial frequency of the field distribution around the rotor.
For example, a magnetic bearing with a magnetic field
distribution having four cyclic changes around the rotor
(Figure 2) will have lower load capacity at a given speed
than a similar magnetic bearing with a magnetic field
distribution having only one cyclic change.
The optimum solution to these losses is to use homopolar
technology in which the field distribution has only one
cyclic change around the rotor and only when the rotor is
subjected to radial loading, unlike heteropolar designs in
which magnetic field distribution around the rotor has at
least four cyclic changes. A homopolar actuator is shown
in Figure 3. The bias flux in this actuator is generated by
axially polarized permanent magnets arranged around the
circumference of the electromagnet. The bias flux flows
into the shaft through a dead (solid, uncontrolled) pole and
returns through a laminated path into the electromagnet. In
this topology, the bias flux does is distributed in a nominally
uniform manner around the rotor.
Another advantage of the reduced eddy current losses of
the homopolar magnetic bearing technology is significantly
lower heat generation in a spinning rotor. In fact, homopolar
magnetic bearings will have almost no heat generated in the
rotor at speed in an absence of a radial loading, because
the magnetic field will be almost uniformly distributed
around the rotor, thus having no significant eddy currents
generated. In contrast, heteropolar magnetic bearings
generate heat in a spinning rotor even in the absence of
radial loading. Low heat generation in both stationary and
rotating parts of homopolar permanent magnet biased
magnetic bearings make them very energy efficient and well
suited for applications where heat extraction mechanisms
are limited, such as in a vacuum.
Another critical component in an AMB system is the
position sensors, which provide the magnetic bearing
controller with accurate information about the rotor position
unaffected by external factors such as speed, temperature,
dust, working fluids and external magnetic and electric fields.
While conventional magnetic reluctance sensors may work
very well for measuring radial displacements, measurement
Figure 3. In this actuator, the bias flux in this actuator is generated by axially polarized permanent
magnets arranged around the circumference of the electromagnet. The bias flux flows into the
shaft through a dead pole and returns through a laminated path into the electromagnet.
Figure 2. In this depiction
of a complete radial bearing,
there are two control axes (X
& Y), each having a pair of
electromagnets pulling the
rotor in opposite directions.