Gyuláné Vincze, Gergely György Balázs
Budapest University of Technology and Economics Department of Electric Power Engineering
There are several magnetic levitation solutions for basic levitation tasks, such as supporting, side guiding and stabilizing the vehicle. From these tasks supporting the vehicle is the most important, it often determines the traction system.
Developments are aimed at elaborating combined levitation and traction systems that are optimal for the whole vehicle. One element should provide several functions with energy saving operation, if possible.
Magnetic levitation force can be developed with three solutions: with permanent magnets, and with electromagnetic or electrodynamic principles.
Permanent magnet solution (MDS magnetodynamic suspension) is based on repulsive force between two magnets with the same polarity mounted on the vehicle and the rail. Levitation distance is automatic, it cannot be controlled.
Electromagnetic levitation (EMS) system is based on the interaction between controlled excited electromagnets on the vehicle and iron rail or body mounted along the rail. Attractive force between the magnet and the iron provides levitation and side guide of vehicle. As the basic effect is attractive, levitation is similar to contactless suspension. Levitation distance has to be controlled with excitation of electromagnets. If excitation control works then vehicle can be levitated in every speed region (including standing) without any auxiliary mechanical support. Critical disadvantage of electromagnetic levitation is that levitation distance is very small, about 10-15 mm, so very precise and expensive rail system is required.
Electrodynamic levitation (EDS) system is based on the interaction between strong magnetic field generated on the vehicle and the magnetic field appearing in special shape conductive loops (short-circuited coils) generated with motional induction. Magnetic field on the vehicle is generated by permanent magnets, electromagnets or (for example in the Japanese system) by concentrated superconducting magnets. As vehicle moves, the moving magnetic field induces current in conductive loops along the rail. Loops are connected so that their effects are suppressed in normal levitated operation of the vehicle, i.e. magnetic field moving with the vehicle generates minimal resultant current in the loops. If levitation height or side position of the vehicle changes for some reason, this generates counteraction in the short-circuited loops so vehicle recovers its stabile original position, the system is self-controlled. Disadvantage of electrodynamic levitation is that it only works above velocity limit v>v min, where v min=100-150km/h. Below this limit, vehicle has to be lowered to wheels running on the rail. Contrary, advantage of EDS is that levitation distance can be 10…20 cm.
Electromagnetic levitation (EMS Electromagnetic Suspension) system is based on the magnetic attraction between controlled excitation electromagnets placed on the vehicle and iron body placed along the rail. With this solution, both holding of vehicle weight and side guide can be fulfilled. Vertical levitation (or holding) force appears as holding magnet creates attractive force to the laminated or tape shape iron body placed at the bottom of the rail, and pulls the vehicle upwards (Figure 7.2.a). Vehicle acts like a suspended train where rail is realized with iron body. The task of the carrying system is to hold the vehicle at almost constant and stabile distance and prevent contact of vehicle and rail.
The most well-known electromagnetic suspended system with linear synchronous motor drive is Transrapid, its construction scheme is shown in Figure 7.2.b.
As can be seen in the figure, vehicle is realized with arms reaching below the rail on both sides. There are two magnet groups on both arms distributed along the vehicle evenly. One magnet group provides vertical suspension and the other provides side guidance. Magnets acting in vertical direction and mounted on the lower part of the arms have two functions: partly provide suspension, partly act as moving part of the linear synchronous motor. The operation of linear synchronous motor is described in chapter 6.3. “Supporting” magnets has to be realized with alternating polarity, because of their linear synchronous motor functionality, as can be seen in Figure 6.8.
Two side magnet rows, that can be seen in Figure 7.2., control the side (horizontal) position of the vehicle, it is important during curving and cross-wind. Side guide is realized similarly to supporting system so there are magnetically conductive iron rails (tapes) on the side of the main rail, oppotise to the magnets.
Magnetic system for suspension (support) and side guide holds the vehicle in levitating position. Levitation distance has to be controlled with excitation control of the electromagnets. otherwise instability problems may arise. (If levitation distance increases, attractice force decreases, so distance increases more. If distance decreases, attractive force increases more.)
The scheme of the control of levitation distance can be seen in Figure 7.3.
Excitation current of the magnets is controlled. Every magnet has an airgap sensor and separate control. Excitation of every magnet is controlled so that levitation distance is almost constant. (There are 46 suspension or holder magnets in vehicle TR 08.)
Optimal levitation distance is 10 mm for type TR 08, deviation allowed is ±2mm. If the train is laying on the rail (because levitation magnets are off) then distance is 160 mm. Side guide magnets has the same distance control.
Vehicle is able to move only if it is levitating. Effects happening when levitation force disappears:
If holding levitation magnet is switched off totally then levitation force disappears. The arms holding the magnets (Figure 7.2.a) are formed so that the train slips with a “landing skid” on the narrow metal stripes mounted on the surface of the rail. Under normal operation, this contact happens only when vehicle stands.
If some holding magnets malfunction during travel then holding force can be balanced with increasing the excitation of the remaining magnets. This is done automatically with separate distance (airgap) control if magnetic field of the holder magnets can be increased with excitation control.
Critical characteristic of electromagnetic levitation is that it works with very small, approx. 10-15 mm, levitation distance so it requires very precise and expensive rail construction. Power required for levitation magnets is similar to the power required for air conditioning inside the train. Vehicle has linear motor drive and can provide motion force along the full length of the vehicle. Force is not limited by sliding risk.
Auxiliary power supply of levitated vehicle type TR 08 is realized with linear generator and moving transformer, as it is described in chapter 3.4. Linear generator consumes a part of motive power so increases the tractive resistance of the vehicle. This can be seen on the tractive resistance characteristic of Transrapid type Tr 08 (see Figure 7.4.). In the figure, the biggest component is windage depending on the head surface, and the side resistance (due to side effects) and the force resulting from the linear generator are added. There is no rolling resistance. Brake force of the linear generator against movement depends on the load current of the linear generator. The figure is for maximal load.
As can be seen in the figure, linear generator can be loaded with maximal current till about 140 km/h, while its voltage increases with velocity. At about 140 km/h, generator reaches its maximal power, after that its load has to be limited hyperbolically.
Voltage of linear generator is proportional to the velocity of the vehicle. It cannot be used at low speed, around stations. Moving transformer for auxiliary supply, described in section 3.4., was developed to solve this.
Electrodynamic levitation (EDS) system is based on the inductive magnetic interaction between strong magnetic field generated on the vehicle and the magnetic field appearing in special shape conductive loops placed along the rail. Magnetic field can be generated with electromagnets or superconducting magnets (in case of Japanese vehicles series ML), or permanent magnets (for example in Inductrack system). Winding along the rail can be realized with simple short-circuited conductive loops, 8-shape, or figural 8-shape short-circuited coils.
The biggest advantage of EDS system is that levitation is inherently stable, no feedbacked position control is required. A small deviation of the levitation distance returns the vehicle to its original position, because of the counteraction developed in the short-circuited loops.
An important disadvantage of EDS system is that at low speed (v<100…150km/h) current induced in the short-circuited loops is not high enough to create the required lifting force that can counteract the weight of the vehicle. Vehicle must be lowered to wheels in this case, until it reaches speed where levitation force is enough to hold the vehicle. As the vehicle must be able to stop everywhere, the whole rail must be constructed so that it must be capable of operating at both low and high speeds.
An example for an EDS electrodynamic levitation system based on superconducting magnets is the vehicle series type ML (magnetic levitation) developed in Japan. Development of technical solutions in the vehicles can be investigated from 1974. Important stages in the development are shown in Figure 7.5.
During first attempts, levitation, side guide and traction functions were separated (Figure 7.5.a). Separate superconducting magnets were used for levitation and side guide. In novel solutions, the number of superconducting magnets was reduced, and they use combined winding system as shown in Figure 7.5.b and c. Development in the placement of superconducting magnets can also be seen in Figure 7.6.
The most novel solution, combined placement of superconducting magnets at the ends of the waggons in bogies can be seen in Figure 7.6.c. This arrangement helps placing the superconducting magnets far away from the passengers.
There are two main streams in the development of superconducting magnets, LTS (low temperature below 4,2K) and HTS (high temperature above 20K) superconductors. The structure of an LTS superconducting magnet unit can be seen in Figure 7.7.
Superconducting magnets can create about 700kAturn excitation. Unit shown above consists of four magnets placed at about 1-2m distance and with alternating polarity. Superconducting magnet can hold its few tesla flux density with a small decreasing, about 0.44% per day.
Shape and distribution of winding elements along the rail is a result of an optimization design process, just as the selection of superconducting magnet system, which is a long development work.
Zero- flux levitation structure is shown in Figure 7.8 with the usual coordinate system. x axis is in the direction of the vehicle speed, z axis is the direction of the vertical levitation and y axis is in side guide direction.
Winding is realized with connected 8-shape loops placed next to each other running along both sides of the rail, as can be seen in Figure 7.5.b. Zero-flux levitation is based on the fact that magnetic field moving with the vehicles induces voltage with the same direction in the upper and lower loops which partly suppress each other because of the 8-shape connection. Current from the resultant voltage create magnetic fields with opposite direction in the upper and lower loops. The system is self-controlling, i.e. minimal remaining current is flowing (zero-flux). Remaining current is set by the force required to hold the weight of the vehicle, and how much decreasing between the superconducting magnet and the cross of the loop is required. Derivative function of levitation force F z by levitation distance is dF z /dz, the coefficient of rigidity, which describes with how much dynamics the system goes back to equilibrium, if it deviates for any reason. Vertical levitation height, i.e. position of the vehicle, is determined by the geometrical position and side height of the loops.
To side guide the vehicle, loops on opposite sides of the rail, shown in Figure 7.8.a, are cross-connected as shown in Figure 7.8.b. Superconducting magnets on the opposite sides of the vehicle has opposite polarity.
Levitation operation can be indicated with N-S (north-south) polarity of the magnetic fields of the loops (Figure 7.9):
For vertical levitation, the polarity of magnetic fields of lower loops is the same as the polarity of the superconducting magnets (placed on the vehicle) so repulsive (lifting) force is developed. The polarity of the upper loops is opposite to the polarity of the superconducting magnets so attractive force is developed which also lifts the vehicle. If the vehicle deviates from its lateral central position then the currents flowing in the left and right loops become different. Cross-connection eliminates this difference so it creates force which helps to pull back the vehicle to the centre.
Winding system inside the rail is shown in Figure 7.10.
Levitation coils cross-connected to 8-shape pairs, according to Figure 7.8, are indicated with red colour. Winding for traction linear motor is indicated with blue. In the figure, two-layer three-phase winding is shown but one-layer solution is also possible. In case of one-layer winding, it can be integrated with four levitation coils.
Driving system of EDS levitated Japanese vehicle series signed ML is similar to linear synchronous motor drive described in chapter 6.3. Difference is required because of the superconducting magnets, comparing to Transrapid, as an example there. Superconducting magnets used in EDS levitation create high flux density and have larger geometric dimensions so only some of them is used in one waggon, as can be seen in Figure 7.6.c. Magnetic poles are opposite next to each other, and have a distance of τ p polar pitch, similar to Figure 6.8. Instead, superconducting magnet groups are placed to a distance integer multiple of 2τ p. Tractive force is local, concentrated at places where superconducting magnet groups are, similarly to trains with bogies. Opposite to this, tractive force is distributed along the whole body of Transrapid. Because of the geometry of superconducting magnets, pole pitch of the magnets is in the range of about τ p≈2m, opposite to pole pitch τ p≈25,8cm at Transrapid. Because of larger pole pitch, frequency of the fundamental harmonic of the three-phase current feeding the linear motor can be much (ten times) less than for Transrapid. As f=v/λ, wavelength is λ=2τ p, at the same vehicle speeds v=500km/h (~140m/s), maximal frequency f≈35Hz is enough for feeding.
Normal operation brake in EDS levitated vehicle MLU002 is solved with regenerative brake, linear synchronous motor recuperates energy into the supply network through the inverter and the DC link circuit. For the case if supply fails, an alternative brake system must be used, which can be resistance, frictional or aerodynamic brake. Resistance brake transforms kinetic energy to heat on a brake resistance connected to the DC link circuit of the inverter. Resistance brake is effective over a certain velocity. At lower speed, below 350km/h, frictional brake can be used in addition to resistance brake. However, if speed is higher than 350km/h, mechanical frictional brake cannot be used, even if resistance brake fails. At higher speeds, aerodynamic running resistance (windage) can be used to brake, securely and effectively, even if both electric and mechanic brake fails.
To increase windage, by hydraulically opening brake plates on the front sides of the vehicle and each waggon, frontal area of the vehicle can be increased. These brake can be operated during levitation or running on wheels, too. Instability and oscillation does not appear, and increasing the surface affects in about one second. Tests proved that braking is secure even if one of the plates (e.g. side plate) does not open or plates do not open at the same time, for example on the front and the rear wagons.
(References used in this section are: …)