21-01-2013, 12:02 PM
Magnetic Bearings
1Magnetic Bearings.pdf (Size: 771.71 KB / Downloads: 160)
Introduction
The use of bearings is essential to all types of machines, in that they provide the
function of supporting another piece or component in a desired position. Two major
types include radial and axial bearings. Furthermore, bearings are usually implied to
be supporting a rotating object or shaft, which is addressed in this paper, but that is
not always the case. The other situation would be one of translational sliding, which is
merely a linear case of supporting a rotating object.
A further classification can be made into active and passive bearings. Active bearings
are electrically controlled with some type of controller, whereas passive bearings are
not electrically powered and thus have no control mechanism. There are numerous
advantages to using magnetic bearings, the most notable being contact-free, in that the
magnetic force is used to support the object as opposed to contact between two
surfaces. This enables very high rotational speeds to be realized. A magnetic bearing
is free of lubricant, which avoids servicing and also enables use in clean room
environments. Maintenance is also decreased due to the absence of surface wear, so
that as long as the control system functions as intended, there could even be no
maintenance. Lastly, the magnetic properties of the materials used are highly resistive
or immune to changes in temperature, pressure, and the presence of chemicals, further
reasons for use in extreme conditions and highly sensitive applications. One major
disadvantage to using magnetic bearings is their complexity. A very knowledgeable
person in the field is generally required to design and implement a successful system.
Because of the large amount of effort and time required for development and the
increase in the number of components, compared to a traditional bearing, the initial
costs are much higher. However, depending on the application, the return on
investment for these initial costs could be relatively short for a system, for example,
that runs with a much higher efficiency due to the lack of bearing friction resistance.
Magnetism Fundamentals
A magnetic field exists between two poles of opposite polarity, denoted as the north
and south poles. The magnetic field lines are emitted radially from the north pole in
every direction and end up at the south pole, then going through the object back to the
north pole, creating a closed path. Such magnetic field lines can also be created by an
electro magnet, which functions by running current through a wire that is coiled
around a piece of ferromagnetic material. The magnetic field, H, can be calculated
with equation (1), the current through a single loop of wire evaluates to equation (2)
Lorentz Force
The Lorentz force is present when an electrical charge is subjected to an energy field
and/or moves within a magnetic field, provided as equation (5). In the presence of an
induced magnetic field, though, the force due to the electric field is much smaller than
the force due to the magnetic field, simplifying (5) to equation (6).
Force Linearization
For any active magnetic bearing system, the main components consist of the
electromagnet, rotor, sensor, controller, and amplifier. Now the electromagnet-rotor
system will be further examined to see how the governing equations can be obtained
for use in the controller.
The force exerted by a magnet behaves much differently than that of a spring. The
force of a spring increases linearly as displacement increases, whereas the force of a
magnet is inversely proportional to the square of an increase in distance. As the
distance between the magnet and the object subjected to its force decreases, the force
levels off at a point when the material becomes saturated with the magnetic flux. For a
mechatronic system, higher order relationships complicate matters and make it more
difficult to implement a controller. For this reason the magnetic force must be
linearized around the operating point., but to do so, the effects of current and position
must be evaluated independently. With constant current and position defined as
deviation from the operating point, and in the opposite direction as previously defined
in order to achieve a positive correlation, a tangent line to the magnetic force curve at
the operating point is drawn as seen in Figure 3.
Closed Control Loop
For a mechatronic system the controller must be designed to control the system
accurately. For this system, the controller needs to control the system so that the
desired force is applied to the shaft. The input to the controller is the signal for the
position of the rotor, or shaft, and the output from the controller is current that goes to
the electromagnet, which in turn provides the force for the rotor. The input and output
indicate that a controlling equation is needed for providing the appropriate current
while only receiving the position of the rotor.
Electrical Response of System
Like any electrical system, there is a slight delay in signals that are carried through the
wires. In the case of an active magnetic bearing, the overwhelming resistance in the
circuit comes from the inductance of the electromagnets. To get an accurate picture of
system performance, several factors need to be accounted for using Kirchhoff’s Law
for voltage in a closed electrical circuit.
Applications
There are many different areas where active magnetic bearings are used and have
been successful; many more are still being developed. The complexity of the system
makes implementation very difficult in some instances, but the advantages of such a
system can be very enticing.
Some major projects include use in turbomolecular pumps, in long-term energy
storage flywheel systems, and as Maglev trains, to name just a few. The Maglev trains
essentially use a linear version of the basic active magnetic bearing described in this
paper to achieve very high-speed trains that are safe, reliable, and emit less noise
compared to regular trains. Other applications are sure to be developed in the future as
the capabilities of magnetic bearings diversify.