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CHAPTER 1. PECULIARITIES OF MAGLEV AND TECHNICAL REQUIREMENTS
1.1 Introduction. The transportation system of Maglev is designated for transportation of passengers within the same continent with speed 500 km/h and more. It should be safety and not expensive for users and not contaminating the environment. The research made in different countries of the world for the past 40 years drew the outline and structure of future Maglev such as:
MS and PM should ensure a stable motion of the vehicle along an assigned track with a given speed, PS should ensure uninterrupted power supply with three-phased current. Let us consider technical requirements for different parts of the Maglev. A Maglev vehicle should fly with the speed up to 150 m/s along a guideway with a small air gap between them. Its track is designated by guiding surfaces of the guideway, The vehicle speed is determined by a form of the track and distribution of stop stations. The resistance forces of the air to its motion are measured by tons. To overcome these the vehicle should spend megawatts of power. The power will be transferred to the vehicle through magnetic field in the air gap by Lorentz's force exactly as it happens in an ordinary rotating three-phased synchronous motor. In PM the rotor magnets' unwrapped (unrolled) poles are installed on each vehicle, while the stator's three-phased winding is common for all vehicles and extended along the whole track. The winding is powered by three-phased sinusoidal current. Current traveling wave arises in the winding, and interacting with the magnetic field produced by rotor's magnets, propels the vehicle. A part of Maglev guideway between two adjacent stop stations contains acceleration and deceleration sections and may have ascents, descents, and curvatures. A flying vehicle is affected by external forces such as gravity, front air resistance, lateral wind pressure and also considerable centrifugal forces on curvatures. All these forces, except for gravity, may appear unexpectedly and vary considerably. At the same time in each point of its track the vehicle must strictly follow an assigned trajectory and fly with assigned speed. While for aircraft a deviation of hundred meters from its track is admissible, a deviation of magnetically suspended vehicle from its track by 0.0l m leads to an accident. Under these conditions the control of a vehicle speed and its deviations from the guiding surfaces of the guideway becomes the main problem. Let us discuss this problem in detail and outline possible ways of its solution for different parts of Maglev. The working function of MS is to maintain a flying vehicle on its assigned track and counteract external forces and disturbances. This situation is distinguished in principles from the other transportation types. For example, an automobile and a train leans against a solid track, an aircraft -- against air, a rocket -- against incandescent gases. Meanwhile, a Maglev vehicle flies without leaning against any substantial medium. Its MS should permanently produce forces normal to its trajectory, which counteract unforeseen forces of lateral wind pressure, centrifugal farces at vehicle turnings, and gravity. Only inertia and magnetic forces hold it on the assigned track. The vehicle itself carries magnetic field sources. The working function of PM is to ensure a vehicle speed determined at any point of the guideway. For example, the speed should be lowered on curvatures in order to reduce a centrifugal force to a permissible value, then it may be needed to increase again and so on. Simultaneously the propulsion force should vary in accordance with the speed: increase at the vehicle acceleration and guideway ascents and reduce at the vehicle deceleration and guideway descents. PM has to respond to the air resistance which grows proportionally to the square of the vehicle speed. In other words, PM has to work following a precise program. It is different for each guideway part and is the same for each next vehicle passing the same guideway point. It follows from the said above that the correct solution of the main problem is to make Maglev self-regulating. Self-regulation of Maglev means its capability of instant and faultless reaction on any deflection of the vehicle speed from its given value and any deviation from its position on an assigned trajectory by producing certain internal magnetic forces sufficient for eliminating these variations. Self-regulation without involving a servo control system is the most important working function of the system. Only this is capable to ensure safety of passengers, reliability of Maglev, and its competitive ability with the other public transportation systems. Self-regulation is defined as coordinated interaction between mechanical and electro-magnetic processes within the structures of MS and PM, under which any shift of a flying vehicle from its pre-defined position on its track or from pre-defined speed will instantly and faultlessly produce magnetic force sufficient to limit and eliminate such shift. For example, if the vehicle shifts aside from its given trajectory due to a side wind gust pushing it with the force of 5 tons, the shift should produce a force in MS able to limit the shift by 3 mm. This stabilizing force is perpendicular to the trajectory. Delay (or shift) of the car from amplitudes of the wave of current running in the stator winding that occurs due to increased resistance of air to the vehicle movement, should increase propulsion force and keep the car close to the wave. Hence MS and PM should produce significant magnetic forces directed oppositely to any shift and proportional to its magnitude. These forces can be produced by employing resilience of magnetic force pipes. Let us discuss the possibilities of creation of a self-regulating Maglev.
1.2 Self-regulation of MS. Almost two centuries ago Lagrange and Direchlet worked out the theory of stability of so called "conservative" systems [1]. MS utilizing permanent magnets, steel cores and rigid constrains is a conservative system, i.e. the one that preserves its potential magnetic energy. A special peculiarity of this conservative system is that two of its parts - the stator and the levitator - are separated in space, interacting with each other by means of magnetic field. The internal forces in a conservative system are derivatives of potential energy of the magnetic field with respect to coordinates of the shift between its parts. In the absence of external forces, the parts of the conservative system tend to a mutual position in which the system potential energy is reduced. Therefore, if the levitator together with the vehicle is in a position of stable equilibrium (i.e. all internal forces turn into zero) without touching the guideway surface, it can be said that MS is a stable conservative system with its potential energy having a local minimum (not coinciding with the guideway surfaces). In this case, any shift of the vehicle from the equilibrium position under pressure of an external force instantly produces an internal force tending to bring the vehicle back to its equilibrium position, since in the vicinity of the minimum of potential energy its derivatives (i.e., internal forces) along any direction are negative. Hence, in order to make MS self-regulating it is sufficient to ensure the stable equilibrium of a flying vehicle along the whole length of its track. There were numerous attempts to create a transportation mode, utilizing a magnetic field to attain stability suspension for a moving vehicle. Unfortunately, stimulus for serious scientific research was undermined by Earnshaw's theorem, that stated that it was impossible to achieve stable equilibrium of a charged body in an electrostatic field. In the past century Braunbeck [3] published an analogous theorem for magnetized bodies situated in a magnetic field. On the ground of these theorems Maglev developers came to the wrong conclusion that it was impossible to build stable engineering systems made of permanent magnets and iron cores. Research in this direction temporarily ground to a halt that was a big mistake since the unique result of the theory of stability and the most effective way of producing internal magnetic forces were rejected. Analyzing how the Earnshaw's and Braunbeck's theorems were proven, one could draw the conclusion that they are not applicable to engineering systems. In point of fact, these theorems are valid only for magnetized bodies which are made of materials with constant magnetic permeability. In addition the magnetized bodies are supposed not to have rigid constraints. Meanwhile, parts of engineering systems (steel magnetized bodies) are always rigidly connected with each other and steel magnetic permeability is highly dependent on magnetic flux intensity in that steel.
1.3 Self-regulation of PM. Maglev vehicles consume energy from a stationary source powering a PM stator's winding. A high voltage transmission line following in parallel with the guideway track feeds the stator winding divided by separate parts. Each winding part is powered by three-phase sinusoidal current trough a step-down substation. At a vehicle movement just one particular part of the winding passed by the vehicle at the very moment gets power. The energy is transferred to rotor's magnets by Lorentz's force through the air gap. The three-phase sinusoidal current produces a current wave in transverse segments of the winding turns. The current wave travels along the winding with a velocity proportional to the current frequency and the length of the winding turns. Magnetic field of traveling wave coheres with rotor's magnet field and propels the vehicle. Rotation speed in an ordinary synchronous motor's rotor can be regulated by changing the frequency of powering current. In contrast to this, the unfolded winding of a Linear motor allows regulating the traveling wave velocity not only by frequency but also by changing the winding turn length. Consequently, making frequency constant we can make a strict program for regulation of the current traveling wave velocity by means of not uniform distribution of the stator's winding turn length along the assigned track It is crucial to make the current frequency in the system constant. Under this condition energy transformations in Maglev will be simple, reliable, and inexpensive. The propulsion force of PM is proportional to the product of the magnetic flux of the rotor poles and the current flowing in the winding turns. If the rotor is designed in such a way that its magnet pole length are able to change proportionally to the stator winding turns length which the vehicle is passing over at the moment it means that the propulsion force will grow proportionally to a vehicle speed. The forces resisting the vehicle motion grow with the growth of its speed. If their sum exceeds the value of propulsion force then the rotor will fall out synchronism and PM switches off. Therefore it is necessary to make speed distribution along the guideway in such a manner that the value of propulsion force would always exceed the total sum of the resistance forces at any point of the guideway. This is the necessary condition for PM stable work. The forces acting on the vehicle depend on values of its speed and acceleration. Expressing all forces (including the propulsion force) in terms of the winding turns length (i.e., the vehicle speed) and equating them to zero, in accordance with the third Newton's law we obtain a differential equation with respect to the winding turn lengths. Solving this equation we find the distribution of the stator winding turn lengths and cross-sections as well as propulsion force distribution. Then we can find the capacities of all substations corresponding to the propulsion forces. Thus, by varying the rotor's pole length we can ensure the condition of PM stable work still at the stage of its design.
1.4 Three possible types of Maglev. There are three possible physical processes utilized for producing internal magnetic forces F in Maglev:
All three processes
to producing internal magnetic forces utilize mechanism of the same nature -
interaction between parallel contours of primary and secondary currents. The
force of interaction is proportional to the product of the current values, and
inversely proportional to an average distance between the contours. The
stiffness of the force (its derivative over shift δ:
It is expedient to note here that the forces of attraction of a rare earth permanent magnet to a steel strip three times bigger than those of an electromagnet and much more times bigger than repulsion forces of magnet from a moving aluminum strip. In the first process the rigid identical spin orientation of all the atoms of a rare-earth magnet (that is equivalent to a current layer flowing without losses over lateral surface of the magnet) makes the primary contour. The value of the primary current is equal to the product of coercive force (H=8.9∙105A/m) and distance between magnet poles. The secondary contour is formed analogously by magnetization currents (of the same direction) simultaneously induced in steel core. Therefore, the magnet attracts and concentrates the magnetization currents in a small domain close to the magnet pole. The average distance between the contours is small. It is comparable to the air gap size. As a result, the stiffness of the interaction force is big. In the second process the order of all values is the same as in the first one. In the third process the primary contour is formed by current flowing in a superconductive coil with its value by an order higher than in the other two processes. The secondary contour is formed by contrarily directed eddy currents (induced by moving superconductive magnets) filling up a volume of aluminum plates. Due to a peculiarity of the design of a superconductive magnet and also because of repulsion of the eddy current from the coil the average distance between the contours is by an order bigger than in the other two processes described above. As a result, the stiffness of the forces is by an order lower. Accordingly there are three types of Magnetic Levitation Transportation Systems (Maglev) have been developed to-date that differ in their ways of producing internal magnetic forces:
EMS and EDS Maglev have been developing for more than 40 years. Detailed descriptions have been published and active models have been built. MDS (Amlev) was invented only seven years ago and at the first time published in 1999 . The scope of this Chapter is a set of problems arising during design and ways of their resolution. At present time development of just two variants of Maglev continues: Transrapid in Germany which utilizes EMS (Electromagnetic Suspension) employing levitator electro-magnets attracted to the stator steel rails , and High Speed Surface Train (HSST) in Japan which utilizes EDS (Electro-dynamic Suspension) employing superconductive magnets repelled from conductive plates (or coils) during motion.
1.5 Comparison of Maglev different types abilities. Let us show how Transrapid and HSST solve the main problem stated above (in Paragraph 1.1) . Magnetization of steel rails passing by vehicle electromagnets in the Transrapid system produces only destabilizing forces which tend to change the air gap. i e., increase vehicle deviations from the track determined by guideway rails. In order to be capable of performing its working functions the electromagnets of Transrapid suspension must be powered with a current sources regulated by a servo control system monitoring the value of the air gap. As a result, any malfunction in the servo control system may lead to a disaster. Hence Transrapid's EMS is not a self-regulating. Let us consider EDS employed in HSST. A superconductive magnet is not appropriate for high speed public transportation: it represents a health hazard for passengers, bulky, and fragile device incapable of tolerating large mechanical loads. Moreover, it requires permanent cooling by liquid helium. If its temperature within even a small section exceeds its permissible value (about - 250°) the superconductive magnet will explode. Nevertheless, the Maglev creators maintain that only superconductive magnets can ensure self-regulating suspension. They claim that if a concrete guideway channel are lined with an aluminum strip and superconductive magnets are affixed to the vehicle, then repulsion forces will levitate the vehicle and ensure its stability. This statement, which is not supported by calculations, now can be seen as erroneous. Actually, the degree of stability of the MS levitator carrying the vehicle is determined not by the value of the magnetic forces acting on the levitator but by their stiffness, that is, values of their derivatives over coordinates of the vehicle shift in the immediate vicinity of its point of equilibrium. It is true that within the equilibrium position internal forces mutually compensate each other. Their resultant equals zero and, therefore, its value can be expanded in a Maclaurin series with the predominant member proportional to the first derivative of the stabilizing force. The internal forces of a suspension system are forces of interaction between two types of current: primary and secondary. Primary currents produce magnetic field. In superconductive magnets these are currents in superconductive coils. Secondary currents are induced currents. In an aluminum plate these are eddy currents, distributed over the plate volume and consuming considerable energy. The values of the forces are proportional to the product of the primary and secondary current values and are inversely proportional to an average distance between them. Their stiffness is inversely proportional to the square of this distance. A permanent magnet approaching a steel core tip induces molecular currents there flowing without reluctance on the tip surface in the vicinity of the magnet pole. Their values are of the same order, and their direction coincides with the direction of magnetization currents in the magnet. Therefore both currents attract each other, and thus, concentrate the energy of resulting field in a small volume of the air gap between the pole and core tip. This explains why the stabilizing force is so big there. In the case of EDS the secondary eddy currents are less than primary currents. Their directions are opposite to magnetization currents, and both currents repel each other. Therefore, eddy currents are spreading in a significant part of the strip. In this case the energy of the resulting field is distributed in the volume much bigger than in case of steel magnetization. That is the reason that the stabilizing force is small in EDS. At the same time because of thermo insulation covering the superconductive coils, big height of these coils, and the volumetric distribution of the secondary current, the distances between these currents are big. Consequently, even at considerably big values of mmf (magneto-motive force) in the EDS system the values of the internal force stiffness are small nevertheless, and any impulsive action of external force leads to undamped vehicle vibrations with a large amplitude inversely proportional to the value of the stiffness. Thus, although EDS is capable of self-regulation, the internal magnetic forces produced by the system are too small to perform working functions. To obtain internal stabilizing forces possessing sufficient stiffness it is necessary to utilize superconductive magnets with mmf at least an order higher than those currently employed. But utilization of such strong magnets in a transportation system brings virtually not resolvable problems of safety, insufficient magnet durability, and an unacceptably intense magnetic field penetrating the passenger salon. Thus, contemporary EDS produces low stiffness of stabilizing forces with all resultant drawbacks:
The secondary suspension extinguishes rapidly the vibrations of amplitude less than 2mm, but if the amplitude 1cm and more these can lead to fatal outcomes. It appears that preliminary calculations of vibrations in EDS were not performed at the design stage and that these vibrations have been revealed only during tests of a working model. Now we will consider the Linear Synchronous Motors (LSM) in both types of the existing Maglev. In Transrapid the propulsion force is produced by LSM with stator winding turns uniformly laid along the guideway and rotor electromagnets installed under the vehicle. Rotor magnetic flux is constant and the propulsion force is proportional to the stator winding current. LSM speed and propulsion force regulation are provided by electronic converters and inverters of the current frequency located along the entire guideway and controlled by the servo control systems. In HSST linear synchronous motor superconductive rotor magnets are installed along both sides of the vehicle, and the stator winding is uniformly distributed along the wall of a concrete channel. Consequently similar to Transrapid LSM, the propulsion forces as well as speed are regulated by the frequency and value of current. Thus, there is no self-regulation in LSMs of both systems. Let us consider a power system (PS) In Transrapid and HSST the LSM stator is powered according to a scheme employing the following components:
Consequently LSM does not perform the working function (speed regulation of each vehicle along the entire guideway) itself − the last three components of the above scheme are responsible for its working function This leads to tremendous complication of PS and lowering its reliability. Thus, neither Transrapid nor HSST satisfy necessary requirements, nor are they capable of self-regulation. They are not reliable, and a failure of any component would lead to a disaster. The solution of the main problem in the existing versions of Maglev is palliative since it is relayed on servo control systems thus making passengers' lives dependent on the control systems reliability. This has been delaying commercial utilizing Maglev for public transportation. Even if they are built, they will never be able to exist without government subsidies. Despite the fact that the intensive programs of both Germany and Japan have failed to achieve truly successful systems, specialists in the United States have found only these two approaches promising and have recommended EDS for further research. Other approaches were dismissed. This was a major error that forced research into dead-end paths.
1.6 Amlev is a new variant of Maglev. Now we know that two unsolved problems caused delay in commercial utilization of Maglev. These are how to ensure stable flight of a vehicle, and stable work of a linear motor. Let us now consider a new type of Maglev- Amlev (American version) proposed here It based on rare-earth permanent magnets as and steel cores connected by rigid constrains and utilizes the most intensive way of producing magnetic forces directly performing all working functions. Its Magnetic suspension and and Propulsion motor devices are self-regulating, and its Power system is much simpler, more reliable, and cheaper. A description of Amlev in detail is given below. Here we present just a brief description of its devices, their peculiarities, and interaction within the system. Amlev comprises three parts connected to each other:
All parts of Amlev are essentially distinguished from the existing types of Maglev. The sources of magnetic field are rare-earth permanent magnets Crumax only and steel cores. Amlev stator's winding is powered by sinusoidal current of constant frequency. Next we will consider each part separately. We will start from brief description of the main parts of Amlev’s structure shown in Figs. 1.1- 1.5.
1.7 Magneto-dynamic suspension (MDS). The MDS system of Amlev (Fig. 1.1) comprises six identical interconnected magnetic units, each of them (Fig. 1.2) consists of two parts: movable and stationary. The movable part contains four 0.1 m long permanent magnets of rectangular cross-section assembled in a quadrupole with the help of a steel insert. The stationary part consists of two laminated steel cores of unlimited length of "C"-shaped cross-section. Each core has a long back and two thickened unsaturated tips. The core backs are covered by aluminum screens. The cores are located mirror-symmetrically
to each other and extended along the whole guideway. There is a constant air gap between the thickened tips pertained to the right- and left-hand cores. The quadrupole is inserted into the air gap and can move freely there in all directions. The unit forms two-contour magnetic circuit with each having a source of mmf (permanent magnet) and magnetic reluctances: linear (the distance between a steel insert and a core tip consisting of reluctance of magnet body and air gap between the magnet and the core tip) and non-linear (saturated steel core back). Magnetic fluxes penetrating into core tips produce forces attracting the quadrupole to the cores. A specific force acting on a unitary surface of unsaturated steel (core tip) is oriented perpendicularly to the surface and proportional to the square of magnetic flux density [9 ]. Proceeding from this it will be proven below that:
The peculiarity of the unit is that a quadrupole shift produces not only destabilizing force Fd but also stabilizing force Fs. The values of forces Fd and Fs are proportional to the difference of magnetic fluxes, penetrating correspondingly:
The fluxes in the unit follow Ohm's law for a magnetic circuit. If the quadrupole is displaced to the right making a small lateral shift Δy then the right-hand air gaps and their reluctance are reduced, and the flux increases (at the left side everything is vice versa). In this case parity of the fluxes is broken and a destabilizing force Fd appears, attracting the quadrupole to the right-hand core tip. In this direction the equilibrium of the quadrupole is unstable. At a small vertical shift Δz of the quadrupole in a symmetrical plane with respect to the cores, parity of the fluxes penetrating into the right- and left-hand is retained but their parts entering the upper and bottom halves of the core tip surfaces are redistributed. In this case stabilizing force Fs appears which is perpendicular to Fd and counteracting the vertical shift. In this direction the equilibrium of the quadrupole is stable. Hence, in the immediate vicinity [δ] of the equilibrium internal forces Fd and Fs of the unit can be expanded in Maclaurin's series and expressed by the product of the shift value and their stiffness (i.e., derivatives of the forces with respect of the shift coordinate). At a longitudinal shift Δx of the quadrupole the fluxes will not change and forces are not produced. In this direction the quadrupole is in indifferent equilibrium. The MDS stator consists of a concrete channel of rectangular cross-section (Fig.1.3) having indefinite length and constant width. Six pairs of steel cores are affixed to its floor and walls in such a way that their symmetrical planes are reciprocally perpendicular. Outside the vehicle six quadrupoles are installed along the whole its length: four of them are supporting quadrupoles affixed to the bottom, the other two guiding quadrupoles affixed to the vehicle's walls - one on each side. When the vehicle moves along the channel the quadrupoles get inserted in the air gaps between the core tips of the corresponding pair of cores. When the units are assembled in such a way the stabilizing forces of the supporting units compensate for the destabilizing forces of the guiding units, and, vise versa, the stabilizing forces of the guiding units compensate for the destabilizing forces of the supporting units. It will be proven that if the stiffness of the stabilizing force exceeds the stiffness of the destabilizing force (the condition of stability) in every single unit then MDS is stable and self-regulating. This means that any small shift of the vehicle as well as its small turns results in instantly arising internal force or torque stabilizing the vehicle.
It follows from the stability condition that units in MDS must be designed in such manner to produce the least destabilizing force at lateral shift. Considering magnetic circuit, this means a slight dependence of the fluxes penetrating into the right- and left-hand core tip on the change of a unit air gap reluctance at the lateral shift of the quadrupole. It is known that flux in a contour is determined by its total reluctance. Therefore, non-linear magnetic reluctances (increasing with the growth of the flux) are placed in the contour and connected in series with air gap reluctances. This stabilizes the total contour reluctance and reduces working flux dependence on the lateral shift (Fig. 1.2). Steel core back inserted in the magnetic circuit is a non-linear magnetic reluctance which increases rapidly with the growth of its magnetic flux (Fig.2.7). However there are no natural insulators for magnetic flux. When the steel core back becomes saturated its magnetic reluctance rf grows, forcing the magnetic flux to flow out through its lateral surfaces. As a result the leakage flux reduces the steel saturation level and its magnetic reluctance. This peculiarity of saturated steel hampers its utilization for magnetic circuits. Nevertheless, exploiting the electromagnetic induction phenomenon we can overcome this difficulties. Cutting the long levitator magnets into ν equal parts, and then turning each its even part by 180°, alternating magnetic flux (including leakage flux) appears in the stator cores at the vehicle motion. Moreover, if the lateral core surfaces are covered with an aluminum layer, then the leakage flux induces eddy currents inhere with the magnetic field oriented contrarily to the leakage flux. It means that an electromagnetic barrier is raised which almost completely suppresses the leakage flux and maintains core saturation at the required level (this phenomenon will be considered in detail below). In this case the stability condition is fulfilled
1.8 Self-regulating motor based on permanent magnets with extending poles. Permanent Magnet Linear Motor - PMLM (Figs.1.3-1.6.) comprises an extended stator winding which is common for all vehicles and. a permanent magnet rotor installed on each vehicle. The winding is divided into different parts with each powered by sinusoidal current from a separate step-down transformer. Current frequency in winding is constant however the length and cross section of its turns vary from one part to another [7]. In addition PMLM is supplied with synchronizing devices changing its magnet pole length that makes it possible to regulate its propulsion force. Each winding phase comprises four parallel bus-bars placed in eight slots of a tooth holder interchanging with bus bars pertaining to another phases as schematically showed in Fig.1.6. In the bus-bars placed in the even slots of each phase current runs from the beginning of the winding part (i.e., from the feeding transformer) to its end. In the bus-bars that are bent back and placed in the odd slots the current runs in the opposite direction (i.e., toward the transformer). The transverse turn segments form a U- shaped propulsion traction channel (Fig.1.3). The longitudinal segments (equaled to the turn lengths) are gathered in two facing bars disposed on the external walls of the channel. It will be shown below that all currents in each phase in transverse turn segments have the same direction, but in the longitudinal segments current directions are opposite. Therefore, in the bus-bars disposed on the bottom and on the walls of the channel currents running forward and back produce a current wave traveling with the velocity V proportional to the turn length while the wave produced by the longitudinal currents in the left- and right-side bars travels with the same velocity but displaced by a quarter of a period.
The rotor comprises mirror-symmetrical (front and rear with respect to the plane YOZ) halves of steel yoke inserted into a longitudinal slit on the vehicle bottom, which are capable to move apart and together along a slit operated by a synchronizing mechanism Fig.1.4. Each half have cells containing permanent magnets capable to move upward and downward within the cells each operated by a synchronizing device. All magnets are of rectangular form and have pole shoes. The magnets pertained to the front and rear parts of the steel yoke have opposite polarities. In its turn the cross-section of each yoke half is made of two mirror- symmetrical C- shaped cores with core shoes. The core backs loosely embrace the stator winding bars. The upper (long) core shoes perform functions of slide rods for moving magnets up and down. The lower (short) core shoes together with pole shoes of the dropped down magnets form a working gap involving the walls of the propulsion channel. Lifting the magnets or dropping them down in the channel (Figs.1.5 b, c) the synchronizing devises create the front and rear rotor poles of the needed length. Only two central magnets are fixed in the yoke in their lower position (Fig.1.5a). The steel yoke increases considerably the magnetic flux in the working gap. As distinguishes from the LSM of Transrapid it neither produce destabilizing force, nor increase the stator winding inductance. The installation of the bus bars in the certain order mentioned above simultaneously performs three functions:
PMLM design, its peculiarities and mode of operation in detail are presented in Chapter III. Now we will give just its brief description. When vehicle launches from a stop station it draws-out special start side wheels supporting it during its motion along the start section until its velocity gets value Vo= 25 m/s that is sufficient to achieve stability. There are two ways to accelerate a vehicle to speed 25m/s/. The simplest and the cheapest one is to connect the pull-out wheels with induction motors powered by three bus-bars situated on the guideway wall.. It is expedient to use the above motors in emergency cases (such as returning a stuck vehicle to a nearest stop station).
As was said above propulsion force is proportional to the product of current in the winding and magnetic flux in the working gap. The PMLM design allows to change the rotor propulsion force as its speed changing. To attain this the width of the bars of the winding parts and also power of the feeding transformers are made proportional to the winding turn length. In addition the length of the rotor magnets and pole pitches varies synchronously in accordance with the turn length of the stator winding part which the rotor is passing over at the moment. This takes place during the vehicle motion with the help of two synchronizing devices: one moves apart or together the rotor yoke halves thus continuously changing its pole pitch length in accordance with the winding turn length while another gradually increases or decreases the rotor pole length. The purpose of their joint work is to ensure permanent coincidence of the rotor magnet poles with central pieces of the traveling wave halves where linear current density (and, consequently, the propulsion force) is the biggest. The forces resisting the vehicle movement increase as its speed grows. Knowing dependencies of all forces acting on the vehicle on its speed and applying the third Newton's law, we can find such distribution of the winding turn length along the whole track (taking into account curvatures, slopes, acceleration sections etc.) that ensures propulsion force exceeding the resistance to the vehicle motion at any point of the track. Such winding together with signal indicators (switching the synchronizing devices and distributed along the the winding) is itself a strict program of PMLM self-regulation eliminating its rotor falling out synchronism and thus ensuring its stable work. A stator winding fragment of the length equaled to travel wave length λ = 6m is shown in Fig.1.6 (see description in detail in Chapter 3).
1.9 Power system (PS). As was said above, power system of Amlev is a three-phased system of constant frequency (f=25Hz). A high voltage line extending along the Amlev track supplies with power step-down transformers feeding the parts of the stator propulsion winding. A vehicle flying over a part of the track switches on or off the next winding part. Electronic inverters and converters of small capacity (consuming about 3% of the transformer power) are installed together with step-down transformers They are designated both for launching a vehicle from a stop station and in case of emergency. Proposed PS is simple, reliable, and not expensive. |
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) is
inversely proportional to the square of this distance. Hence, the less is the
distance, the greater is the stiffness. These considerations allow us to
compare the order of forces produced by these various processes.
