Conference Session B5
Paper 6036
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THE ROLE OF SUPERCONDUCTIVE ELECTROMAGNETS IN ELECTRODYNAMICALLY
SUSPENDED COMMERCIAL TRAINS
Zachary Yoder, zdy1@pitt.edu, (Tu Th 2-4, Vidic), Abby Fenn, amf159@pitt.edu, (Tu Th 10-12, Sanchez)
Abstract—Commercial magnetically levitating trains are a
sustainable and more efficient alternative to traditional trains.
These trains use two distinct mechanisms to magnetically
levitate. Some trains use electromagnetic suspension where
attractive electromagnets are used to suspend the train.
Conversely, electrodynamic suspension style trains are
levitated using repulsive magnetic forces. There are two ways
in which the magnetic field is created in electrodynamic
suspension, one uses room temperature, permanent magnets,
called inductrack, while the other uses cooled,
superconducting magnets. This paper investigates
superconducting magnets in maglev trains and analyzes the
current development and future outlook of this type of
technology. From a functional standpoint, electrodynamically
suspended trains travel much faster than traditional rail
trains, electromagnetically suspended trains, and inductrackstyle electrodynamically suspended trains. The development of
the superconducting magnets is especially important as it has
significant potential in many different fields. In order to pursue
this topic, we explore the development and implementation of
magnetically levitating trains in general. We also focus more
specifically on the technical details behind the
superconducting electromagnets used in electrodynamically
suspended trains as well as some of the current issues this
technology faces and reasons why it is not currently widely
implemented.
Key Words—Maglev, superconductors, electromagnets,
electrodynamic suspension, trains, transportation
AN INTRODUCTION TO MAGLEV
TECHNOLOGIES
Overview
Although magnetically levitating trains have been around
for many years, recent developments in maglev technologies
and superconductors are beginning to pave the way for full
integration into our world [1]. These trains hover using either
repulsive or attractive magnetic forces, and can travel much
faster than traditional trains. There are two major forms of
suspension that maglev trains utilize to levitate:
University of Pittsburgh Swanson School of Engineering 1
Submission Date 16-03-04
electrodynamic suspension and electromagnetic suspension
[1]. Electrodynamic suspension uses the repulsive forces
between a permanent magnet mounted on the train and a
superconducting magnets on the railway, while
electromagnetic suspension uses the attractive forces between
electromagnets mounted on the underside of the railway and
the train wrapped around to the underside of the railway [2].
Both types allow trains to travel while lifted off the railway.
Maglev technologies used in trains have many benefits
over traditional trains. The longevity of the trains and railways
is drastically increased due to lack of contact with the railway;
the only friction on the train is that of the air. Much less repair
work is required for both the train and the railway because of
this [3]. These trains are also nearly impossible to derail due to
mounted guiding magnets, and factors including rain, snow,
and ice have very little impact on operation [4]. Places with
extreme weather conditions could find this as a beneficial
transportation method when other means of transportation are
too unsafe to be used. Magnetically levitated trains are also
much more environmentally friendly. These trains can make
turns that are about one fifth the radius of traditional trains,
which means that the railway will have less environmental
disturbance [5]. A smaller turning radius means that the trains
would be able to better navigate through certain areas and
avoid any serious environmental disturbances. Since these
trains do not run on wheels for the majority of their operation,
there is a great elimination of noise pollution compared to that
of traditional trains- about a twenty decibel drop [5]. Not only
does this cause less disturbance in the natural world, but also
allows trains to move through cities during any time of day
without any negative effects on those living in the surrounding
area.
Beginning in the 1970’s, Germany and Japan have been
leaders in this field and have researched and developed the use
of magnetic levitation in trains [3]. The maglev trains in
Germany mainly implement electromagnetic suspension.
Their design, called TransRapid, has exceeded speeds of 260
miles per hour [3]. The Japanese have been researching and
devoting more time specifically to electrodynamic suspension
using superconducting electromagnets. This specific focus is
due to Japan’s susceptibility to earthquakes. Electrodynamic
suspension creates a greater gap between the train and railway
which makes them much safer in these events [3]. The
Abby Fenn
Zach Yoder
magnetic forces which make these trains extremely hard to
derail also makes this technology safer in the event of an
earthquake. These leading countries are setting an example by
advancing technologies for future use. With more development
and time, these technologies could become more widely used.
Magnetic levitation in trains has shown many benefits for the
future.
supply that will prevent the train from crashing in the event of
a power outage [6]. Although these trains are easier to
implement than electrodynamically suspended trains, they are
not as stable and therefore not as safe [4].
Electrodynamic Suspension
Electrodynamic suspension is a relatively new form of
maglev technology that utilizes repulsive magnetic forces
rather than attractive ones [4]. The magnets are placed on the
underside of the train and on the topside of the railway. This
method also has permanently mounted magnets used for
guidance. Figure 2 below shows the positioning of the train
cabin on the railway using repulsive forces to levitate.
Electromagnetic Suspension
Electromagnetic suspension was the first form of maglev
technology developed, but it is opposite of the conventional
understanding of maglev train systems. Electromagnetic
suspension uses powered electromagnets to produce magnetic
fields [6]. In electromagnetic suspension systems, the cabin is
suspended by attractive electromagnets located on the cabin
wrapped beneath the railway and the underside of the railway,
with additional magnets on the sides of the track for guidance
[1]. The guidance magnets ensure that the train does not move
too far off the center of the track [4]. This increases the safety
by making sure the train will not shift side to side and possibly
drag against the railway.
Figure 2 [7]
Train and railway cross-section for electrodynamic
suspension
There are two methods of electrodynamic suspension. One
method, called Inductrack, uses permanent magnets, while the
other uses superconducting magnets [2]. Both of these
methods still use repulsive forces to levitate the trains. The
Inductrack method is simpler and does not require an electric
power supply, but rather uses room temperature permanent
magnets to create the magnetic field [6]. Permanent magnets
used in the Inductrack method are essentially the same as
magnets for refrigerators, but are implemented at a much larger
scale. However, this method can only be implemented in small
systems because the permanent magnets used are not powerful
enough to levitate large and heavy trains [5]. There are no
current commercially used Inductrack rails or full scale
prototypes [8].
The method using superconducting magnets is much more
powerful, but requires that the superconducting magnets be
kept at extremely low temperatures during operation.
Additional cooling is required because of the heat generated
by the induced currents [5]. Liquid helium is the most common
coolant used in these systems [5]. Superconducting magnet
Figure 1 [7]
Train and railway cross-section for electromagnetic
suspension
Figure 1 shows how the train is positioned above the
railway in electromagnetic suspension systems with attractive
forces between the levitation magnets. This method has a few
benefits over electrodynamic suspension. These trains are able
to remain levitated at rest, while electrodynamically suspended
trains require the use of wheels until they reach a critical speed.
However, a drawback to this method of suspension is that
it requires a precise, very small air gap between the magnets
that can become very difficult to control at high speeds.
Because of this, electromagnetically suspended trains cannot
travel as fast as electrodynamically suspended trains can. Since
this method relies on onboard, powered electromagnets, the
train must be equipped with an emergency battery power
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trains are the fastest of all forms of maglev trains. In 2015, a
maglev system in Japan beat its own 2003 world record speed
of 590 kilometers per hour, setting the new world record at
603 kilometers per hour [9]. In commercial use, this would
provide for extremely fast travel and commutes .
Electrodynamic suspension has many benefits over
electromagnetic suspension. Many of these benefits lie in the
fact that this style uses superconducting magnets rather than
traditional magnets. Superconductors allow these magnets to
be extremely strong and can therefore levitate the train higher
off the track than electromagnetic suspension [2]. The air gap
between the train and rail does not have to be controlled as in
electromagnetic suspension because of the very high magnetic
stability [5]. They are also much more reliable, and can even
continue to conduct in the event of a power failure because of
such strong magnetic forces, so there is no need for an
emergency battery power supply [2].
A significant drawback to electrodynamic suspension is
that the trains must roll on rubber tires until they reach off a
liftoff speed of about 100 kilometers per hour, which is when
they are able to levitate under their own power [6].
Conversely, the wheels can actually be an advantage. In the
case of a power failure or other emergency, when the train
must decelerate quickly, the wheels will be there to roll on the
guideway to safely come to a stop. With electromagnetic
suspension, there are no wheels, meaning that if the emergency
battery fails, the train would crash into the railway.
Electrodynamic suspension proves to be a faster and safer
method.
Figure 3 [11]
Graph indicating critical temperatures of nonsuperconductors and superconductors
The
beginning
of
today’s
understanding
of
superconductivity lies in the work of two brothers- F. and H.
London, who laid the foundation for the modern day BCS
theory of superconductivity [10]. They were able to expose
relationships between the electric field, magnetic flux density,
and the current density, that were nearly opposite of what was
normally observed in a conductor, as shown in figure 3. For
example, current density is proportional to the applied electric
field in normal metals. However, the findings of the London
brothers showed that there was a possibility of having a current
flow through a metal without electric field, the definition of
perfect conductivity [10]. Although they did not discover or
build a superconductor, they laid the foundation to today’s
understanding of superconductivity.
First, an understanding of subatomic particles is necessary.
Subatomic particles fall into two different distinctions:
Fermions and bosons [10]. Fermions have a spin of either ½ or
-½, and increase, starting from 1, by a factor of two [12].
Fermions cannot go into the exact same state or have the exact
same energy. When looking at an atom, no more than two
electrons can occupy any given orbital, as they behave like
fermions. On the other hand, bosons are particles that have a
spin of integer n. These particles have no net spin, so there is
no limit to the number of bosons can be in the same energy
state [12].
In the normal atomic structure of metal, electrons acting as
fermions fly free around the nucleus, and their negative charge
attracts other protons from other nuclei, forming the rigid
lattice of the substance [13]. Occasionally, electrons will move
closer to the nucleus of the atom and cause the surrounding
lattice to become charged more positively. This increased
positive charge can sometimes cause electrons to come close
together and, due to an arbitrarily small attractive force
between them, pair up. These are called cooper pairs [10].
Since one of those electrons will have spin ½ and the other will
have spin -½ their net spin will be zero [12]. These cooper pairs
then will behave as boson particles, meaning they can occupy
the same energy level and will condense into their lowest
energy level- their ground state [12].
However, these cooper pairs are relatively uncommon
because their tiny bond energy can easily be overwhelmed by
thermal energy. At room temperature it is very unlikely for this
phenomena to occur. However, the Fermi level introduces a
new set of rules. The Fermi level is a term that is used to
describe electron energy levels at zero Kelvin, or absolute zero
[10]. The thermal energy is nearly zero, and now the bond
strength between electrons is actually stronger than the thermal
energy surrounding them. Because of the Pauli exclusion
principle, no two electrons can be in the exact same energy
state at a given time [12]. These electrons still can behave like
boson particles and have no magnetic spin, allowing them to
exist at energy levels that are virtually the same. So, at absolute
SUPERCONDUCTING MAGNETS
Superconducting Magnets In Detail
Superconductivity is a phenomenon where, at
extremely low temperatures, a metal or material can have a
resistance of zero. This allows for perfect conductivity, which
means an unlimited amount of current can flow through the
superconductor. [10] However, once the material’s
temperature rises above a certain critical point, they exhibit the
same resistance as normal conductors. This can be seen in
Figure 3 below.
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Zach Yoder
zero, all these electrons will go into the lowest energy levels
possible and ‘stack up’ into a ‘Fermi sea’ [10].
In order to fully understand superconductors one has to
understand something called the bandgap, or the energy gap.
This can be visualized as bands- the valence band and the
conduction band. Electrons are either locked into place in the
valence band and cannot move, or they are able to move
around the material in the conduction band [10]. If all the
electrons are locked in place and cannot move, the material is
an insulator. But, if many electrons are able to move from atom
to atom then the material is a conductor [12]. In between those
states is an energy gap.
Now, the valence band has essentially turned into a
conduction band as all the electrons are in the fermi level.
Figure 5 shows the states that are now empty, allowing for
unobstructed and free flow of electrons through the material,
which is now a superconductor [10].
Superconductor Materials
All superconductors are made up of some type of
metal, but some metals conduct better than others. All
elements fall into three categories when discussing
superconductivity: type I superconductors, type II
superconductors, and non-superconductors. The difference
between type I and II lies in the critical magnetic field. Just as
these materials have critical temperatures, the highest
temperature at which they can still superconduct, is the point
where they also have a critical magnetic field, which is the
strongest magnetic field that still allows for superconductivity.
In fact, there is a strong correlation between the critical
temperature and the critical magnetic field.
Figure 4 [14]
Bandgaps metal, semiconductors, and insulators
If the band gap is too large, and the electrons are unable to
cross over it, then the material is an insulator. If the gap is small
enough, some electrons can cross over into the conduction
band, making it a semi conductor. But, if the two bands
overlap, then the electrons can easily move through the
conduction band and the material is a conductor [10]. Figure 4
illustrates this. In a superconductor, the cooper pairs of
electrons have descended into the fermi sea, leaving a gap
above them.
Figure 6 [16]
Critical temperature vs. critical magnetic field of
various elements
As the critical temperature of the material increases, so
does the critical magnetic field. Figure 6 shows the strong
linear correlation between these two characteristics. Type I
superconductor, also known as “soft” superconductors, are
typically raw elements and have a much lower critical
magnetic field than type II materials. Type II superconductors
are known as “hard” superconductors, and are usually
compounds of multiple different elements. Not only are they
physically harder than type I materials, but they also have
significantly higher critical magnetic fields. In the presence of
higher powered magnetic fields, they actually exhibit behavior
of being in a mixed state between superconducting and regular
conducting. This is demonstrated in figure 7.
Figure 5 [15]
Fermi level empty states
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Zach Yoder
Maglev Cooling Systems
Superconducting magnets are integrated in an
electrodynamic suspension system through a complex array of
refrigeration and shielding systems. Currently, liquid helium
and liquid nitrogen systems are used. An example is presented
in Figure 9 below.
Figure 7 [16]
Type I vs. Type II superconducting behavior
Achieving Necessary Temperature
The biggest challenge in using superconducting
magnets is achieving the near absolute zero temperature that is
required for perfect conductivity. Many elements have been
used as coolants, such as helium, nitrogen, hydrogen and neon,
but there are very few that remain a liquid at temperatures near
0 Kelvin [17].
Liquid helium is used for many reasons. Helium stays
a liquid at temperatures below 4 Kelvin, making it perfect for
a coolant. Helium has a very small atomic radius leading to
minimal dispersion forces [17]. Consequently, helium induces
very few temporary dipoles, resulting in a very low freezing
point [18].
Figure 9
The refrigeration cooling system used in
electrodynamic suspension systems [14].
Helium turns to a liquid at 4 Kelvin, making it the perfect
substance for supercooling superconductors. The liquid helium
is supplied to the refrigeration unit by a common compressor,
and is circulated through the refrigerator until it liquefies at 4
Kelvin [19]. This is then used to cool the magnet. The liquid
nitrogen is supplied also by a compressor to an 80 K
refrigerator, and is used primarily to cool the radiation
shielding systems that are vital for safe and effective operation
[19].
In order to cool superconducting magnets themselves,
cryogenic cooling systems are implemented. These are
methods that use liquid helium or liquid nitrogen to supercool
superconductors [20]. There are two main forms of cryogenic
refrigerators. The first is called an open cycle cooler and the
second is a closed cycle cooler [20]. In the open cycle, the
liquid coolant comes into direct contact with the
superconductor, while in the closed cycle it does not come into
direct contact [20]. Open cycle cryogenic refrigerators are
usually used for greater heat loads, and therefore are more
commonly implemented for cooling superconductors [20]. In
order to cool the electromagnets that are mounted on the train
itself, the refrigerators are mounted onboard the vehicle [21].
They must be lighter than stationary cooling systems in order
to be more efficient. Each vehicle has six units called “bogies”
mounted on them. A bogie is the unit mounted on the vehicle
which holds the magnets that levitate and propel the train [22].
On either side of each bogie, there are two cryogenic cooling
systems, each of which contain eight magnets [21]. The liquid
helium or liquid nitrogen is circulated from a source on the
vehicle to each bogie so that the flow divides and cools all of
the magnets [21]. This cooling method is highly effective, but
also very complex and expensive. The continued use of helium
as a coolant could bring about difficulties as well.
Figure 8 [14]
Phase diagram of Helium 4
Figure 8 shows a phase diagram of Helium 4, the most
common isotope of Helium. It is possible for helium to become
a solid, but that requires near absolute zero and extreme
pressure. Two to four Kelvin is the most common temperature
of liquid helium used for cooling superconductors, as helium
is in the normal liquid state at that temperature [17]. Below two
Kelvin, at normal pressures, helium condenses into a
superfluid, where it has zero viscosity [17]. Liquid helium is
the most viable coolant for cooling superconductors to the
needed temperatures; however, it is an irreplaceable resource,
meaning that once it is used up it is gone permanently.
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Zach Yoder
and require lots of energy to produce such temperatures.
Ninety-two Kelvin is -181.15 degrees Celsius, or -294.07
degrees Fahrenheit [24].
A recent breakthrough in high temperature
superconductors occurred in 2015 at the Max Planck Institute
for Chemistry in Mainz, Germany [25]. They were able to
achieve superconductivity at -94 degrees Fahrenheit [25].
Unfortunately, this only occurred in an environment where the
pressure was about 200 gigapascals, or two-thirds of the
pressure of the center of earth [25]. So, although the high
temperature was a significant discovery, the pressure is so
extreme that we are no closer to room temperature
superconductors than before.
Room temperature superconductors would also solve
the helium problem. Expensive coolants and cooling systems
would no longer be required, which would greatly simplify the
implementation of superconductors. We would no longer rely
on a finite supply of coolant. As a result, the cost to make and
use superconductors would plummet.
The
applications
of
room
temperature
superconductors could bring huge breakthroughs in maglev
technologies. For example, immense quantities of energy are
lost in power lines [12]. If these power lines could exhibit
perfect conductivity, no power would be lost whatsoever.
Transportation would be revolutionized with energy efficient
and environmentally friendly public transportation.
Potential Difficulties
There are a few drawbacks to this system which need to be
noted to future developments and uses. Thorough shielding
systems are required to protect passengers from the extreme
magnetic field required to for levitation. If there were no
shielding systems, passengers with electronic health devices,
such as pacemakers, would be at significant risk [6]. Any
electronic devices could be ruined, and credit cards and ID’s
could be wiped [6].
Another problem with this technology is the extreme
temperatures required to keep the superconducting magnets in
operation. Because of the heat produced in the induced
magnetic currents, the superconductors need to be kept at four
Kelvin otherwise they cannot operate [6]. Liquid helium
refrigeration systems are used, but liquid helium is
complicated and expensive to produce, and countries that are
unable to create their own liquid helium must import it, which
is even more expensive. [17]
Superconducting magnets also waste a lot of helium
through their operation. If the helium is not kept as a liquid it
will immediately evaporate into the air and be lost forever.
Additionally, magnets sometimes have to be “quenched,”
where they boil away a large amount of liquid helium in order
to continue to superconduct.
Currently, liquid helium is, by far, the most effective
coolant used to supercool materials to such low temperatures.
However, helium is a finite resource. Currently, the Federal
Helium Reserve in Amarillo provides 30% of the world’s
helium, but was nearly shut down in 2014 [17]. If these
reserves were shut down helium would run dry, and
superconductors everywhere would be in big trouble.
Another difficulty with implementing maglev trains
is the cost. The price of implementing new maglev trains,
along with the rails, is much more than the cost of traditional
trains. A maglev train in Japan is proposed to run a distance of
290 kilometers, costing about $47.7 billion to construct [23].
Many countries do not have the ability to invest this much
money into their railway systems just to build faster trains,
which hinders the widespread implementation of this
technology.
IMPLEMENTATION
Future Outlook
Maglev trains are not currently widely used due to
one simple reason: it is simply too expensive to build and
operate them. The future of maglev trains largely depends on
the ability to significantly decrease their cost. There are
multiple ways in which this technology could be developed
and advanced in an effort to make this happen.
Room temperature superconductors would solve
virtually all the problems facing maglev development. If
superconductors were able to operate at room temperature, the
cost to operate these trains would decrease. There would also
be no need for liquid helium, which is another major downside
of implementing superconductors since liquid helium is a
resource which could potentially run out. However, it appears
that we will have to wait a while before that could become a
reality. But, that does not mean that maglevs cannot be
implemented until that discovery is made. There are multiple
other ways to drastically decrease maglev costs while still
using low temperature superconductors.
A large cost associated with superconductors is the
cost of buying, producing and keeping cool the liquid helium
used as coolant. The current refrigeration process is long,
complicated and relatively wasteful [17]. A promising new
discovery could change that. A pulse tube refrigerator is a
cooling system that is able to cool material to very low
temperatures using less power a much smaller platform [19].
The Future of Superconductors: High Temperature
Superconductivity
The future of superconductors lies in room
temperature superconductors. The extremely low temperature
required currently makes superconductors too expensive and
impractical for many major applications. One of the first hightemperature superconductors was Lanthanum-Barium-Copper
Oxide which could superconduct at temperatures up to 30
Kelvin. Under standard pressure, the material with the highest
critical temperature is Yttrium-Barium-Copper Oxide which
exhibits superconductivity at 92 Kelvin. Although these may
seem high compared to 4 Kelvin, they are still extremely low
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Zach Yoder
Currently, it’s best performance has been cooling liquid
nitrogen to 80 K with a cooling power of 150 watts [19].
Through continued development, the pulse tube refrigerator is
a likely candidate to replace the current refrigeration process.
The maglev train is a more environmentally friendly
option of mass transportation than current methods of
transportation, including trains and buses. As fossil fuels
become scarcer and their cost of extraction increases, maglev
trains may one day become a cheaper alternative to fossil fuels.
Additionally, as more and more politicians begin to recognize
the huge problem of climate change, which is widely accepted
within the scientific community, there will be a stronger push
for environmentally friendly transportation methods. Maglev
trains will be a perfect candidate.
https://nationalmaglab.org/education/magnetacademy/history-of-electricity-magnetism/museum/maglevtrains-1
[2] “Maglev Suspension Systems.” Maglev Trains.
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[3] S. Brown. (May 2010). “Revolutionary RAIL.”
Scientific
American.
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=JkF1dGhUeXBlPWlwLHVpZCZzY29wZT1zaXRl#AN=50
575288&db=aph
[4] C. Wilson. “Maglev: Magnetic Levitating Trains.”
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[5] H. Lee, K. Kim, J. Lee. (July 2006). “Review of Maglev
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[8] G. Mempin. (03 April 2012). “Inductrack.” U.S.
Department of Energy Energy Innovation Portal. (Web.)
http://techportal.eere.energy.gov/technology.do/techID=758
[9] J. McCurry. (21 April 2015). “Japan’s maglev train
breaks world speed record with 600km/h test run.” The
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[12] Nero, David. (25 February 2016). "Superconductors."
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http://www.thephysicsmill.com/blog/wp-
CONCLUSION
Although magnetic levitation in trains has been
researched for years, high speed trains using these
technologies have yet to become widely used. Many factors
contribute to this, with the main being how expensive they are.
The fastest of these maglev trains are the kind that utilizes
electrodynamic suspension, where the cabin levitates above
incredibly powerful superconducting electromagnets. These
magnets get their extreme force only when kept at very low
temperatures. The low temperatures allow the magnets to
exhibit the phenomenon of superconductivity, allowing
unlimited amount of current to flow through them. The
repulsive forces between like ends of the magnets provide the
upward force that keeps the cabin levitating. This levitation
allows for the train to speed atop the rail without actually
touching it. The benefit of this is that there is less maintenance
cost.
Recent advances in various technologies point
towards a future with maglev integration not only in trains, but
in all kinds of public transportation and even space flight.
Developments that increase the critical temperature of
superconductivity and allow for materials to superconduct
without the need for extreme cold would help curb the
dependence of maglev trains on liquid helium. The pulse tube
refrigerators are one example of a technology that could reduce
the requirements needed for maglevs to run, which makes their
implementation simpler and cheaper. Growth in
superconductors themselves have hugely consequential results
and could transform our world. Additionally, the
environmental benefits of maglev, including less habitat
disturbance and less reliance on fossil fuels, could provide a
huge piece of the puzzle in solving the pressing problem of
climate change. As this growth continues, the developments
and change that will be made will be astounding.
content/uploads/half_full_band1.png
REFERENCES
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ACKNOWLEDGEMENTS
We would like to thank Dr. Nero for taking time to
help explain superconductivity to us. We would also like to
thank our roommates for giving us quiet spaces to work.
Finally, we would like to thank our parents for encouraging us
and helping to keep us focused.
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economical way to enhance the performance of present
HTSMaglev systems by utilizing the anisotropy property of
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E51F1D693928E6.c6.iopscience.cld.iop.org
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