SUPERCONDUCTING MAGNETIC LEVITATED TRAIN, TRAIN SYSTEM METHOD OF CONTROLLING THE SAME, AND SUPERCONDUCTING COIL FOR MAGNETIC LEVITATED TRAIN

SUPERCONDUCTING MAGNETIC LEVITATED TRAIN, TRAIN SYSTEM METHOD OF CONTROLLING THE SAME, AND SUPERCONDUCTING COIL FOR MAGNETIC LEVITATED TRAIN The present invention relates to a superconducting magnetically levitated train, a superconducting magnetically levitated train system and a method of controlling the same. More particularly, the present invention relates to a superconducting magnetically levitated train and the method of controlling the same, a superconducting magnetically levitated train system and a superconducting coil used therefor which enables safe lévitation of the train by allowing a large stability margin for the superconducting magnet on a specified car. A superconducting magnetically levitated train employs a system in which a superconducting magnet is located on the train and a normal conductive short-circuit coil is located on the ground. A current is applied to the coil on the ground by the electromagnetic induction caused when the magnetic flux of the superconducting magnet on the train cuts the coil on the ground at the time of starting the train. The repulsive force between the superconducting magnet on the train and the coil on the ground levitates the train. The train is propelled by 2 0 a linear synchronous motor system in which the thrust in the same direction is obtained by inverting the current of a propellant coil provided on the ground separately from the normal conductive short-circuit coil by the interaction between the propellant coil and the superconducting magnet. Since such an induction current is utilized, no levitating force is obtained at the time of stopping, and the lévitation force is insufficient during low-speed travel. As a result, an auxiliary supporting device is required. However, lévitation itself is naturally stable without the need for any control. In a superconducting magnetically levitated train, since a strong magnetic field is obtained from the superconducting magnet on the train, a lévitation height as large as about 100 mm can be realized. In addition, since the superconducting magnet is used in a permanent current mode, no power source for excitation is necessary on the train. On the other hand, a superconducting magnet suffers from the phenomenon of quenching in a normal conducting state, and a light-weight magnet having a high current density magnet such as a superconducting magnet for levitating trains involves the possibility of frequently quenching before the critical value of the superconducting wire is reached due to various disturbances. Since such a quench phenomenon is irreversible, once a quench phenomenon is produced, about 3 0 minutes is required for re-pouring a coolant to the superconducting magnet and exciting the magnet. A conventional superconducting magnetically levitated train will be described in detail hereinbelow. Accordingly, it is an object of the present invention to provide a superconducting magnetically levitated train, a superconducting magnetic levitated train system, a method of controlling the same, and a superconducting coil for a magnetically levitated train which is unlikely to undergo a quenching even during high-speed travel and which has a high reliability. It is another object of the present invention to provide a highly reliable method of controlling a superconducting magnetically levitated train which controls a superconducting coil before it quenches by varying the stability margin of the superconducting magnet and the magnitude of the disturbance so as to prevent the quenching phenomenon even during high-speed lévitation. To achieve this aim, in the present invention, the stability margin of a superconducting magnet on a specific car in which the disturbance energy made to be large on the basis of the fact that the disturbance energy applied to a superconducting magnet is different in cars. According to one aspect of the present invention there is provided a superconducting magnetically levitated train having a plurality of cars connected to each other, a group of superconducting magnets attached to car trucks and a coil provided on the ground so that said superconducting 2037267 magnetically levitated train is levitated by the magnetically induced repulsion between said superconducting magnets and said coil on the ground, characterized in that a superconducting magnet on a specified car has a predetermined stability margin which is larger than the stability margin of a superconducting magnet on the other cars. In accordance with another aspect of the present invention there is provided a method of controlling a superconducting magnetically levitated train having a plurality of cars connected to each other, a group of superconducting magnets attached to car trucks at the respective connecting portions and a coil provided on the ground so that said superconducting magnetically levitated train is levitated at a high speed by the magnetically induced repulsion between said superconducting magnets and said coil on the ground, said method comprising the steps of: detecting the driven state of a superconducting magnet on a specified car; varying the current value in a permanent current mode by an auxiliary power source and a resistor; and varying and controlling the travelling conditions for said train so that the stability margin of superconducting magnets on a specified car is changed in accordance with a command from a train control mean so as to vary the stability of the margin of superconducting magnets on at least one car so that said stability margin is larger than the stability margin of the superconducting magnet on the other cars. In the present invention, it is possible to make the stability margins of the superconducting magnets on the lead car and the rearmost car larger than the stability margins of 3 0 the superconducting magnets on the other cars therebetween. The present invention is more effective when the stability margin of a superconducting magnet mounted on a car is not less than 200 mJ/cc in rated drive. Appropriate control carried out so that the stationary disturbance energy applied to a superconducting magnet on a car is restricted to not more than 100 mJ/cc is effective. In the present invention, use of a superconducting coil composed of a wound composite superconducting wire which is stabilized by aluminum is effective. The composite superconducting wire which is stabilized by aluminum is multifilamentary NbTi wire having a low copper ratio with the outer surface thereof coated with thin high-purity aluminum or multifilamentary NbTi wire having a low copper ratio accommodating high-purity aluminum at the central part of the cross section thereof. According to the present invention, it is possible that the stability margin is variable in correspondence with the disturbance energy which is different depending upon the position of the car. Furthermore, in a superconducting magnet on a specific car, a detector which is capable of detecting the magnitude of a disturbance, an auxiliary current source which is capable of varying the current value in a permanent current mode and a resistor are arranged, so that a centralized train control centre provided on the ground can control the stability margins of the superconducting magnets or the speed of the train. The "stability margin" in the present invention means the difference between a critical characteristic (temperature, magnetic field, current density) of a superconducting magnet and the operating point obtained when the magnet is actually used. The stability margin represents at least one parameter of the temperature, magnetic field and current density of the magnet by an energy per unit volume of the magnet (mJ/cc). In contrast, a factor in the reduction of the travelling stability of a magnetically levitated train is a disturbance 3 0 energy. Causes for a disturbance energy are the proximity of the train to a tunnel, the existence of a high-voltage steel tower and the like, and the disturbance energy produced by such a cause can be obtained by calculation at the time of designing the magnetically levitated train system. As a result of the investigation of the present inventor, it has been found that the stability margin of a superconducting magnet on a specific car which is required to enhance the stability must be so designed as to be constantly larger than the disturbance energy and that the effective difference between the former and the latter is not less than 50 mJ/cc, preferably not less than 100 mJ/cc. If the superconducting magnet, the train and the system are so designed that the stability margin of the superconducting magnet is not less than 200 mJ/cc and the magnitude of the disturbance energy is not more than 100 mJ/cc, especially stable lévitation is possible. Generally, in order that a superconducting coil be stable at a certain operating point, it is necessary that the stability margin which the superconducting coil has is larger than the heat energy produced by the disturbance due to an electromagnetic energy or a mechanical energy. With respect to a superconducting magnetically levitated train, the stability margin and the heat energy produced by a disturbance were quantitatively calculated by experiments. The superconducting coil for a magnetically levitated train does not contain a He cooling channel, in other words, it is what is called a closely wound coil. An epoxy resin is inserted between the turns of the winding for fixture. The winding is accommodated in a liquid He tank which is called an inner tank, and a spacer is intermittently inserted between the winding and the inner tank so that the liquid He exists in a space which corresponds to the thickness of the spacer. It was experimentally made clear that the stability margin in such a coil structure allows a larger value than the heat capacity which is calculated by using an adiabatic stabilization model and which is determined by the temperature margin and the heat capacity of the superconducting wire. In other words, it is necessary to consider a dynamic stabilization model including the sectional area, the electric resistance and the thermal conductivity of a stabilizing material. Therefore, if the following improvements are adopted, the stability margin of a superconducting coil for a magnetically levitated train is increased: (i) The current load factor to the superconducting wire of the superconducting coil is reduced (temperature margin is increased). (ii) A stabilizing material having a large heat capacity at cryogenic temperature is used. (iii) The sectional area of the stabilizing material is increased. (iv) The electric resistance of the stabilizing material at cryogenic temperature is reduced. (v) A stabilizing material having a high thermal conductivity at cryogenic temperature is used. A material which satisfies the above-described conditions, which is light in weight and has a current density which can be enhanced is a composite superconducting wire which is stabilized aluminum. However, aluminum is mechanically so soft that it is impossible to produce multifilamentary NbTi wire having aluminum for the whole matrix in the present wire working technique. It has been found that it is possible to greatly increase the stability margin by using a superconducting wire having Cu and aluminum for a composite matrix, wherein a slight amount of aluminum which enables plastic working of such a composite matrix wire suffices. In a superconducting magnetically levitated train which travels at a super high speed and which has a plurality of connected cars, the kind and the magnitude of the disturbance applied to a superconducting magnet on the train will be inferred. As a disturbance is applied to a general superconducting magnet, there is a movement of the 3 0 superconducting wire, a cracking of the epoxy resin between the turns of the windings and a deformation of the superconducting coil as a whole. These are caused by the electromagnetic force produced by the excitation of the superconducting coil and generate heat resulting in the production of quench at not more than a rated value. However, these disturbances can be detected by testing before mounting the coil on the train. Disturbances caused by the fact that w v> 2037267 the superconducting magnet is located on a train must also be taken into consideration. These, in situ disturbances are the generation of an AC loss produced on the superconducting coil by the fluctuating magnetic field from the coil on the ground, the increase in heat ingress due to vibration and the generation of heat due to the impact load, vibration and wind pressure produced when the train enters a tunnel or the train passes another train. It is difficult to quantify the magnitudes of these disturbance energies in the present state of the art. However, it is certain that the disturbance energy applied to a superconducting magnet is different depending upon the position of the car in a superconducting magnetically levitated train having a plurality of connected cars. This fact is one of the basis of the present invention. That is, a superconducting magnet on the lead car receives the largest disturbance energy, and it is the lead car and the rearmost car which are susceptible to the influence of a lowered lévitation force at the time of quench. In the present invention, the car which is liable to receive a disturbance is specified and the stability margin of the superconducting magnet on that specified car is made larger than the stability margin of the superconducting magnet on other cars. In the present invention, a car which is susceptible to the influence of a lowered lévitation force at the time of quench is discriminated and the stability margin of a superconducting magnet on that specified car is made larger than the stability margin of a superconducting magnet on other cars. The actual stability margin was calculated by experiment. 3 0 Four kinds of superconducting wires having stabilizing materials having different sectional areas and different materials were respectively wound so as to have a cross section similar that of a superconducting coil for a magnetically levitated train and an epoxy resin was inserted between the turns of the winding, thereby producing superconducting coils. In order to simulate a disturbance energy, a pulsating current was applied to a heater wire which M' had been previously embedded in the winding, thereby generating a thermal disturbance. In this way, the minimum energy necessary for quench was obtained by experiment. As a result, the stability margin of the coil of a conventional multifilamentary NbTi wire having a copper ratio of 1.0 was 100 mJ/cc at the rated value of the coil and it was found that the stationary disturbance energy (except a disturbance due to a non-stationary disaster such as an earthquake, a fire and an explosion) applied to a superconducting magnet must be not more than 100 mJ/cc according to the results of the current test operations of a superconducting magnetically levitated train. From the results of the experiments on the stability margins of the coils of wires having different copper ratios, it was made clear that a stability margins of not less than 200 mJ/cc is necessary in order to secure a stable margin of a superconducting magnet on a train with a high reliability. As is clear from the above explanation, the disturbance energy on a superconducting magnetically levitated train varies with the speed of the train, the position of the car and the environmental conditions of the travelling train. Another feature of the present invention is that the stability margin of each superconducting magnet is made variable in correspondence with temporally and spatially different disturbance energies. With respect to a spatially different disturbance energy, superconducting magnets having different stability margins are mounted at predetermined positions in advance. With respect to temporally different disturbance energies, it is necessary to detect the magnitude of the disturbance during high-speed travel and to control the stability margins of the superconducting magnets on the train or the speed of the train. The control of the speed of the train is easily carried out by varying the frequency of the current which is applied to the propellant coil on the ground. For example, a method of reducing the disturbance energy 3 5 applied to the superconducting magnets by reducing the speed of the train immediately before the train enters a tunnel or the train passes another train is effective. Furthermore, it is possible to control the coil current on the train in a permanent current mode by a centralized train control centre provided on the ground. By virtue of the features of the present invention, it is possible to safely drive the superconducting magnetically levitated train at a super high speed with the greatest efficiency by specifying a car which is susceptible to a disturbance and increasing the stability margin of the superconducting magnet mounted on the specified car. The present invention will be described in detail hereinbelow with the aid of the accompanying drawings, in which: Fig. 1 shows a relationship between the stability margin and the disturbance energy exemplifying the advantage of the present invention; Fig. 2 is a cross sectional view of a superconducting magnetically levitated train; Fig. 3 is a vertical sectional view of a superconducting magnetically levitated train; Figs. 4 and 5 schematically show the structure of a superconducting magnetically levitated train according to the present invention; and Fig. 6 is a circuit diagram showing a method of controlling a superconducting magnet according to the present invention. Prior to discussing the drawings in detail it should be noted that like elements have been given like reference numerals throughout. In a conventional superconducting magnetically levitated 3 0 train such as shown in, for example, Figs. 2 and 3, two superconducting magnets 2 are accommodated in one coolant container 3 and four coolant containers (eight superconducting magnets) are attached to a car truck 10 provided at a lower part of a car body 1, in each car. An auxiliary supporting device 7 supports the train at the time of stopping and during low-speed travel, while during high-speed travel, the train is levitated by the repulsive force between a propellant guiding ground coil 4B, a supporting ground coil 4A and the superconducting magnets 2 on the train. Additionally, it is proposed that in the near future, the car trucks 10 carrying the superconducting magnets 2 be located between the cars in order to reduce the magnetic field leaking from the superconducting magnets 2 to a passenger car. In such a superconducting magnetically levitated train, when a specific superconducting magnet is quenched during travel, the symmetrically positioned superconducting magnet be demagnetized so as to prevent a loss in the balance of the train as a whole. The train is therefore caused to safely travel and land on an emergency landing device 8. A connecting system for preventing the loss of the balance on the left-hand and right-hand sides by modifying the arrangement of the group of superconducting magnets so that the S-pole, N-pole and S-pole are alternately disposed in the longitudinal direction of the train and the same poles are connected in series to each other. For a superconducting wire in a conventional magnetically levitated train, multifilamentary NbTi wire having a low copper ratio is used in order to reduce the weight of the superconducting magnet and to increase the current density of the superconducting coil. This is described on pages 36 to of "The Journal of the Japan Society of Mechanical Engineering", vol. 91, Nc. 835, June (1988). A Cu/NbTi wire containing 1.0 Cu based on NbTi in cross sectional ratio (hereinunder referred to as "copper ratio ") is used for recent ELTU 002. Although a wire using aluminum in place of Cu or multifilamentary NbTi wire using aluminum in place of some of the Cu is proposed in order to reduce the weight, such a wire has a large aluminum cross sectional area and it is not used for superconducting magnetically levitated trains. In any of the above-described prior art, the fact that when a specific superconducting magnet on the train quenches, a superconducting magnetically levitated train, during high-speed travel, must be suddenly stopped and that a long time is required for restoration of the superconducting magnet has not been taken into-consideration. Therefore, such a conventional superconducting magnetically levitated train lacks reliability for use as a train handling traffic. In addition, a superconducting coil composed of a wound multifilamentary NbTi wire having a low copper ratio, especial a wire having a copper ratio of 1.0 is poor in electromagnetic stability. A magnetically levitated train with such a superconducting coil mounted thereon travels at a high speed, the superconducting coil frequently quenches due to an increase in disturbance energy. In a wire using aluminum in place of Cu, or a wire using aluminum in place of some Cu, a coil having a high current density is not considered because the lévitation force is inconveniently small. Referring now to Fig. 4, according to this figure, a superconducting magnetically levitated train has, for example, eight cars, namely, a lead car 11, 6 intermediate cars 21 and a rearmost car 31. The total length of the train is about 200 m, the weight is about 180 t and the maximum speed of the train is 500 km/h. The size of the lead car 11 is 28.0 m in length, 2.8 m in width and 2.65 m in height. The size of the intermediate cars 21 is 21.6 m in length, 2.8 m in width and 2.65 m in height. The size of the rearmost car 31 is the same as that of the lead car 11. Each car includes 4 superconducting magnets arranged in 2 poles x 2 lines are disposed on the car truck at the connecting portion of each car. The superconducting magnets other than the superconducting magnets on the lead car 11 are of the same racetrack type, each having a length of 2.3 m and a width of 0.5 m, a pole-pitch of 2.7 m and a magnetomotive force of 700 kA. In the structure of the superconducting magnetically levitated train, the superconducting magnet on the lead car is a coil of a composite superconducting wire consisting of multifilamentary NbTi wire having a copper ratio of 1.0 with the outer surface thereof coated with high-purity aluminum having a thickness of 0.2 mm, and the superconducting magnet w " 12 2037267 on the other cars is a coil o-4 multifilamentary NbTi wire having a copper ratio of 1.0, as in the prior art. The superconducting magnet on the lead car is of a racetrack type and the magnetomotive force thereof is 700 kA. It has been made clear that this structure produces a stability margin of 600 mJ/cc on the superconducting magnet on the lead car at the rated value, which is six times as large as the stability margin of the superconducting magnet on the other cars. It has been proved that the superconducting magnetically levitated train of this set does not quench even if it is levitated at the maximum speed of 500 km/h, and it can withstand a disturbance caused when entering a tunnel or passing another train. In this embodiment, although the weight of the lead car slightly increases and the sectional area of the coil increases so that the lévitation force caused by the magnetically induced repulsion is slightly lowered, since a large lifting power is produced on the lead car when the train travels at a speed as high as 500 km/h, it is possible to obtain a lévitation force which is balanced as a whole. Therefore, a highly reliable superconducting magnetically levitated train is realized without the need for changing the magnetomotive force of the superconducting magnet depending upon the position of the car. In another embodiment of a superconducting magnetically levitated train in accordance with the present invention, the superconducting magnet on each car is of a racetrack type, each having a length of 2.3 m, a width of 0.5 m, a pole pitch of 2.7 m and a magnetomotive force of 700 kA. In the structure of the superconducting magnetically levitated train, the superconducting magnet on the lead car and the rearmost car is a coil of a composite superconducting wire consisting of multifilamentary NbTi wire having a copper ratio of 0.8 which accommodates high-purity aluminum 0.2 in aluminum ratio at the central portion of the cross section thereof, and the superconducting magnet on the other cars is a coil of multifilamentary NbTi wire having a copper ratio of w ,3 2037267 1.0, as in the prior art. This structure produces a stability margin of 200 mJ/cc on the superconducting magnet on the lead car and the rearmost car at the rated value, which is twice as large as the stability margin of the superconducting magnet on the other cars. As a result, it has been proved that the superconducting magnetically levitated train of this set does not quench even if it is levitated at the maximum speed of 500 km/h, and it can withstand a disturbance caused when entering a tunnel or passing another train. In this embodiment, since it is possible to slightly reduce the weight of the lead car and the rearmost car to that in the prior art and the cross sectional area of the coil is the same as in the prior art, a large lévitation force is produced on the lead car and the rearmost car. It is therefore possible to reduce the magnetomotive force of the superconducting magnet on the lead car and the rearmost car in order to balance the superconducting magnetically levitated train as a whole which has a plurality of connected cars. Thus, this embodiment is advantageous in that it is possible to further increase the stability margin by reducing the magnetomotive force. In accordance with yet another embodiment of a superconducting magnetically levitated train constructed in accordance with the present invention, the superconducting magnets other than the superconducting magnet on the lead car are of the same racetrack type, each having a length of 2.3 m and a width of 0.5 m, a pole pitch of 2.7 m and a magnetomotive force of 700 kA. In the structure of this superconducting magnetically levitated train, the superconducting magnet on the lead car is multifilamentary NbTi wire having a copper ratio of 2.0, and the superconducting magnet on the other cars is a coil of multifilamentary NbTi wire having a copper ratio of 1.0, as in the prior art. The superconducting magnet on the lead car is of a racetrack type and the magnetomotive force thereof is 700 kA. This structure produces a stability margin of 2 00 mJ/cc on the superconducting magnet on the lead car at the rated 2037Z67 value, which is twice as large as the stability margin of the superconducting magnet on the other cars. As a result, it has been proved that the superconducting magnetically levitated train of this set does not quench even if it is levitated at the maximum speed of 500 km/h, and it can withstand a disturbance caused when entering a tunnel or passing another train. In this embodiment, although the weight of the lead car slightly increases and the sectional area of the coil increases so that the lévitation force caused by the magnetically induced repulsion is slightly lowered, since a large lifting power is produced on the lead car when the train travels at a speed as high as 500 km/h, it is possible to obtain a lévitation force which is balanced as a whole. Therefore, a highly reliable superconducting magnetically levitated train is realized without the need for changing the magnetomotive force of the superconducting magnet depending upon the position of the car. In accordance with a further embodiment of a 2 0 superconducting magnetically levitated train constructed in accordance with the present invention, the superconducting magnet on each car is of a racetrack type, each having a length of 2.3 m, a width of 0.5 m, a pole pitch of 2.7 m and a magnetomotive force of 700 kA. In the structure of this superconducting magnetically levitated train, the reference numerals 2' and 2" in Fig. are superconducting magnets, respectively. The superconducting magnets 2' and 2" are provided with different stability depending on the magnitude of the disturbance applied to each superconducting magnet during high-speed travel of the superconducting magnetically levitated train and the taking into consideration how the superconducting magnet quenches due to a disturbance. The superconducting magnet 2' is a coil of a composite superconducting wire consisting of raultifilamentary NbTi wire having a copper ratio of 0.8 which accommodates high-purity aluminum 0.2 in aluminum ratio at the central portion of the cross section thereof, and the superconducting magnet 2" is a coil of multifilamentary NbTi wire having a copper ratio of 1.0, as in the prior art. This structure can make the stability margin of the superconducting magnet 21 twice as large as the stability margin of the superconducting magnet 2". As a result, it has been proven that the superconducting magnetically levitated train of this set does not guench even if it is levitated at the maximum speed of 500 km/h, and it can withstand a disturbance caused when entering a tunnel or passing another train. In this embodiment, since it is possible to slightly reduce the weights of the cars carrying the superconducting magnets 21 with respect to the weights the other cars and the cross sectional area of the coil is the same as in the prior art, a large lévitation force is produced on cars carrying the superconducting magnets 21. It is therefore possible to reduce the magnetomotive force of the superconducting magnet 21 in order to balance the superconducting magnetically levitated train as a whole which has a plurality of connected cars. Thus, this embodiment is advantageous in that it is possible to further increase the stability margin by reducing the magnetomotive force. Fig. 6 shows the circuit structure of the superconducting magnet 2 which enables control over the stability margin of a superconducting magnet on a superconducting magnetically levitated train during high-speed travel. The superconducting magnet 2 assumes a permanent current mode between the two superconducting coils 4 0 and a permanent current switch 42, thereby generating a constant magnetic field during stationary travel. Each superconducting coil is provided with a detector 3 0 41 for detecting the magnitude of a disturbance. When the magnitude of the disturbance exceeds the limitation, the power source 45 for a heater 43 of the permanent current switch 4 2 is activated accordance with the command from a centralized train control centre 49, whereby a gate wire 44 of the 3 5 permanent current switch 42 assumes a normal conductive state and the current applied to the coils is consumed by a resistor 46 on the train. When the coil current is lowered to a predetermined value, the power source 4 5 for the heater 4 3 is shut off, whereby the permanent current mode is formed between the superconducting coils 40 and the permanent current switch 42. When the value of the current applied to the superconducting coils is increased, the coil current is increased in the state in which the permanent current state is cancelled by an auxiliary power source 47 provided on the train. When the coil current rises to a predetermined value, the power source 45 for the heater 43 is shut off, whereby the permanent current mode is again formed between the superconducting coils 40 and the permanent current switch 42. The upper limit and the lower limit of the coil current are set at values which are determined by the value of a magnetic field sensor 48 for detecting the magnetic flux density which is generated from each superconducting coil 40. In this embodiment, it is possible to vary the value of the coil current of the superconducting coil on the train which is driven in the permanent current mode in correspondence with the magnitude of a disturbance. Because the value of the coil current is variable, the stability margin of the superconducting coil is variable. In this embodiment, it is possible to detect the magnitude of a disturbance and vary the speed of the train by varying the frequency of the propellant coil on the ground in accordance with a command from the centralized train control centre 49 which is provided on the ground. Varying the train speed is equivalent to varying the magnitude of the disturbance applied to the superconducting coil. In any case, since it is possible to detect the magnitude of a disturbance and control the stability margin of the superconducting coil or the magnitude of the disturbance applied to the superconducting coil before the superconducting coil quenches, it is possible to continue lévitation at a high speed without suddenly stopping the train due to a quench. In this embodiment, it is also possible to estimate the magnitude of a disturbance applied to the superconducting coil in accordance with the travelling pattern of the train and 2037Z67 control the stability margin of the superconducting coil on the superconducting magnetically levitated train during high-speed travel or the magnitude of a disturbance applied to the superconducting coil by program control. Superconducting coils having a sectional area similar to that of a superconducting coil for a superconducting magnetically levitated train were produced by varying the structure of a composite superconducting wire such as the copper ratio and the aluminum ratio. Fig. 1 shows the results of a measurement of the stability margin of each of these superconducting coils to which a thermal disturbance was applied by a heater provided on the coil. The curve 50 shows the stability margin of a superconducting coil for a superconducting magnetically levitated train at the rated value which is composed of a coil of multifilamentary NbTi wire using only Cu as a stabilizing material. The curve 51 shows the stability margin of a superconducting coil for a superconducting magnetically levitated train at the rated value which is composed of a coil of multifilamentary NbTi wire using a stabilizing material containing Cu and high-purity aluminum. From these results, it is obvious both in the curves 50 and 51 that when the copper ratio or the (copper + aluminum) ratio increases, the coil weight also increases and the current density of the coil as a whole is lowered, thereby lowering the lévitation power. On the other hand, the disturbance energy applied to a superconducting coil for a superconducting magnetically levitated train can be inferred to be represented by the curves 60 and 61 in Fig. 1 from operating tests carried out on an experimental train line. The curve 61 shows the maximum disturbance energy applied to the superconducting coil when the travelling speed of the train is 500 km/h and the curve 60 shows the maximum disturbance energy applied to the superconducting coil when the travelling speed of the train is 350 km/h. 3 5 To state the above results in more detail, in order that the superconducting coil be operated stably while the train is levitated at a speed of 500 km/h, a copper ratio of not less than 1.6 is necessary in a conventional superconducting coil having only Cu as a stabilizing material and a (copper + aluminum) ratio of not less than 0.9 is necessary in a superconducting coil which uses a composite of Cu and high-purity aluminum as a stabilizing material. It is naturally possible to vary the stability margin of the superconducting coil by varying the constitution of Cu and high-purity aluminum even if the (copper + aluminum) ratio is constant. However, multifilamentary NbTi wire having a low copper ratio which contains a higher ratio of high-purity aluminum factor cannot be produced by present plastic working techniques. The ratio of high-purity aluminum is therefore limited. In a superconducting magnetically levitated train having a plurality of connected cars, since the magnitude of a disturbance is different depending upon the position of the car, the superconducting wire used for a superconducting magnet which is mounted on a car suffering from a small disturbance may adopt a conventional multifilamentary NbTi wire having a copper ratio of 1.0 without producing any particular problem. As described above, according to the present invention, a highly reliable superconducting magnetically levitated train is provided by having a superconducting magnet which does not quench even if a superconducting magnetically levitated train is levitated at a high speed. According to the embodiments of the present invention, if the stability margin of the superconducting magnet on the train is set at not less than 2 00 mJ/cc during the rated 3 0 operation, or the stationary disturbance energy applied to the superconducting magnet on the train is set at not more than 100 mJ/cc, it is possible to safely drive a superconducting magnetically levitated train at a super high speed with sufficient tolerance. 3 5 According to the embodiments of the present invention, since the stability margin of the superconducting magnet on the superconducting magnetically levitated train during high-speed travel, or the magnitude of a disturbance applied to the superconducting magnet can be controlled by a centralized train control centre provided on the ground, it is possible to safely drive the superconducting magnetically levitated train at a super high speed with efficiency. WHAT IS CLAIMED IS : CUJ/LO/ 1. A superconducting magnetically levitated train having a plurality of cars connected to each other, a group of superconducting magnets attached to car trucks and a coil provided on the ground so that said superconducting magnetically levitated train is levitated by the magnetically induced repulsion between said superconducting magnets and said coil on the ground, characterized in that a superconducting magnet on a specified car has a predetermined stability margin which is larger than the stability margin of a superconducting magnet on the other cars. 2. A superconducting magnetically levitated train having a plurality of cars connected to each other, a group of superconducting magnets attached to car trucks at respective connecting portions, and a coil provided on the ground so that said superconducting magnetically levitated train is levitated by the magnetically induced repulsion between said superconducting magnets and said coil on the ground, characterized in that a superconducting magnet on the lead car and the rearmost car has a stability margin which is larger than the stability margin of a superconducting magnet on the other intermediate cars. 3. In a superconducting magnetically levitated train system having a plurality of cars connected to each other, a group of superconducting magnets attached to car trucks at respective connecting portions and a coil provided on the ground so that said superconducting magnetically levitated train is levitated at a high speed by the magnetically induced repulsion between said superconducting magnets and said coil on the ground, a superconducting magnetically levitated train characterized in