Indeed, if the rotor isn’t moving close to the synchronous speed, then the LSM will not move the stator at all!
If the rotor is close enough to the synchronous speed, then the LSM will be able to accelerate the rotor up to the synchronous speed.
Note: the synchronous speed of a LSM can be altered in two main ways:
1. by varying the frequency of the three phase currents
2. by varying the number of wires per unit length in the stator and rotor.
Discussion
LSM’s, like LIM’s, have the ability to move the rotor relative to the stator without any physical contact. This drastically reduces mechanical wear.
LSM’s do NOT have the ability to accelerate the rotor from rest up to the speed of a quickly moving magnetic field. This means that Maglev trains that use LSM must either make the synchronous speed start very slowly and increase slowly or use a secondary propulsion system for acceleration.
A general-looking setup for a LSM.
from Linear Motion Electromagnetic Systems, page 26.
The Final Analysis
The Advantages
Energy Efficiency
The basic physics of magnetic lift and electrical propulsion are the essentials behind Maglev’s energy efficiency. For example, Maglev consumes per trip about one seventh of the energy used by a Boeing 737-300 for a 125-620 mile trip. In addition, Maglev operation is not dependent upon petroleum for its energy; the electrical power can be derived from other sources. The energy efficiency is due to the mechanical efficiency resulting from drastically reduced friction and less enegry lost as heat in the operation of the vehicle.
Speed
High speed is an inherent characteristic of maglev. Because the operation of the vehicle occurs without physical contact, high speeds are within technological limits. Speeds up to 500 miles per hour are possible, with top speeds usually limited not by physical constraints, but rather by economic considerations. Commercial rail today usually travels at only 200 miles per hour, while Maglev promises at minimum a 300 mile per hour top speed. For short trips, Maglev is competitive with short airplane flights of up to 500 miles!
Freight Transport
Maglev could be used to effect a fully automated transport system, with goods arriving within seconds of their scheduled time. Combining the reliability and speed advantages, this looks to be a promising possibility.
Low Wear and Maintenance
Because lift and guidance forces are distributed over a large area, contact stresses are at a minima. The Linear Motor allows noncontact propulsion and braking, in contrast to conventional rail where severe stresses occur from wheel/rail contact and by power transfer. A low cost maintenance program is a certain advantage associated with this technology.
Important Issues
Drag Forces: Magnetic and Aerodynamic
When a conductor moves through a magnetic field, the changing flux induces electric currents as reviewed in the induction page of the Basic Physics section.
These eddy currents then react with the magnetic field in such a way as to brake motion through the field. Due to of this phenomenon, a part of the propulsion energy is spent counteracting the drag force. As the table from Linear Motion Magnetic Systems shows, the drag force increases as the speed increases, for the most part. As a maglev gains speed, it requires more and more energy just to remain at cruising speed.
In addition to the magnetic drag force, conventional aerodynamic drag is present. Although both forms of drag are undesirable in many ways, there are some ways of utlizing them to our advantage. The drag forces can help brake a maglev train quite efficiently. In the introduction, there is a photograph of a Maglev train with aerodynamic brakes extended.
The Disadvantages
Technical Problems – A Study in the Feasibility of the SCM
The major technical barrier to the mass adoption of Maglev as a new transportation system lies in problems with the superconducting magnets (hereafter SCM) used to levitate and power the trains.
To date most Maglev trains have utilized an SCM made of NbTi. The SCM develops extremely high temperatures during operation, during which it must be cooled down to four degrees Kelvin to maintain its properties. Liquid helium is usually used for this purpose. The heat side effects are unavoidable, and thus engineers have focused on the efficacy of the cooling systems rather than designing a “cooler” SCM.
Severe difficulties lie in the storage of the helium vapor, and the reliquification of the vapor once it has absorbed the tremendous heat of the SCM. Currently, this has been the slowest front for progress in Maglev technology.
The current thought is that using cryorefrigeration techniques which constantly cool the magnet without flashing the helium prove to be the most promising. By winding the coolant through tubes surrounding the magnet, an even, constant cooling process will be effected.
This maglev diagram known as the LM-500-01, also from Linear Motion Magnetic Systems, page 338, shows many of the onboard systems relating to refrigerating the superconducting magnets.
These techniques, available today, is limited only by its tremendous expense, which points toward a bottom line for Maglev technology.
The Bottom Line: Economics
Economical considerations have historically been a huge hurdle to otherwise very promising technological advances. This certainly rings true with the SCM and Maglev. The superconducting magnets themselves cost millions, and the cooling system technologies associated with the SCM’s cost millions more. While in the lab the technologies have been very interesting, the conventional systems have so far won out, merely because of costs.
Essentially, we must look at the opportunity costs involved to fully come to a conclusion with regards to the efficacy of this technology. Adopting the Maglev system worldwide would have severe costs, but with a tangible payoff over the next 20 years. We would see a definite reduction in operating costs and a great leap in efficiency, but only after the initial investment in the new technology. SCM’s and cooling system R&D have already cost us millions even billions of dollars, yet we are not as yet ready to commit to the Maglev system. Feasibility studies conducted by the US Department of Transportation have shown a great need for the technology, yet no group ready to invest because of the shear number of R&D dollars still needed with no real examples of Maglev success in this country.
Judging from the progress of other countries, it is our recommendation that the United States take steps toward a greater use of Maglev to reduce its long term public transportation problems and take advantage of the low cost operation, reliability, and energy efficiency associated with Maglev.
References
We’ve stood on the shoulders of giants, and now it’s time to enumerate them . . .
Bibliography
References
We’ve stood on the shoulders of giants, and now it’s time to enumerate them . . .
Books
1. D. Halliday, R. Resnik & K. Krane, Physics, vol. 2, 4th ed. John Wiley & Sons Inc., 1992.
2. Liang Chi Shen and Jin Au Kong, Applied Electromagnetism, 3rd ed. PWS Publishing Company, 1995.
3. I. Boldea and S.A. Nasar, Linear Motion and Electromagnetic Systems. John Wiley & Sons Inc., 1995.
Web Sites
· The High Speed / Automated Transportation Home Page
· The Argonne National Laboratories Home Page
· The Railway Technical Research Institute
· Magnetbahn: The Unofficial Transrapid Homepage