DESCRIPTION CONTROLLING DEVICE FOR RAILWAY ELECTRIC CAR TECHNICAL FIELD [0001] The present invention generally relates to a controlling device for railway electric cars and specifically relates to a controlling device for railway electric cars that has a slipping/sliding controlling Iriiictlon to :lnhiibit the wKieels from spinninq I.r«Q and a Iidi ng. HACKGROUNn ART IU0U2J Aa a controlling device for railway Biwcij.ic cais/ systems that drive and control an alternate-current motor by using an inverter have already been put into practical use. As is well known, accelerations and decelerations of railway cars are realized by powers that are transmitted between iron rails and iron wheels within the small contact areas therebetween. Thus, a controlling device for railway electric cars needs to control the torque of the electric motor in an appropriate manner so that the wheels do not spin free. In other words, if the torque is too high, the wheels spin free, and a friction coefficient (hereinafter, it may also be referred to as an "adhesion coefficient") between the wheels and the rails decreases, so that the efficiency of transmitting the powers also decreases. As a result, problems arise where the railway electric cars cannot be accelerated in a satisfactory manner and where the wheels and the rails wear down. Conversely, if the torque is too low, although the wheels do not spin free, the railway electric cars cannot be accelerated in a satisfactory manner, and it becomes difficult for the railway electric cars to run on schedule. Also^ the same applies to when a regenerative brake is used. [0003] Conventionally, controlling devices for railway electric cars have a slip controlling system for inhibiting the slipping phenomenon of the wheels as described above. Generally speaking, such a slip controlling system is configured so as to determine a slipping state of the wheels by using rates of change of the wheels' speeds and a speed deviation among a plurality of wheels and to adjust the torque of the electric motor. There may be, however, some substances such as rain, snow, sand, and grease ttetween the rails and the wheels. In addition, the adhesion coefficient greatly changes constantly according to the state of the surfaces of the rails and the wheels, the temperature, and the traveling speed of the railway electric Cars. Thus, physical phenomena of the rails and the wheels are complex, and it is not easy to formulate a control law. For this reason, a large number of methods that can be used by slip controlling systems have been proposed based on theoretical studies from various aspects ii\ui df'itri from teat runs using actual railway electric cars (jieo, for example. Patent Document 1) . 10()0<11 Patent Document 1: Japanese Patent Application T,nid-OF3en No. H06-335106 DISCLOSURE OF INVENTON PROBLEM TO BE SOLVED BY THE INVENTION [0005] The conventional techniques described above, however, have problems as follows: In the regular railway systems such as local trains in Japan, it is easy to recognize the slipping/sliding phenomenon based on the rates of change of the wheels' speeds and the speed deviation among a plurality of wheels, because the rates of change of the wheels' speeds are relatively high, and also, the speed deviation among the wheels is also relatively large when the wheels spin free or slide. However, while a railway electric car is running at a high speed (e.g., approximately 200 kilometers per hour or higher) in a )iigh-speed railway system, the rates of change of the wheels' speeds are low, and the speed deviation among a plurality of wheels is also small when a slipping/sliding is occurring. Thus, a problem remains where it is difficult to recognize a slipping/sliding phenomenon based on the rates of change of the wheels' speeds and the speed deviation among the wheels and it is difficult to distinguish a situation where the railway electric car is in an accelerating state during normal travel, from a situation where a slipping/sliding is occurring. [0006] In view of the problems described above, it is an object of the present invention to provide a controlling device for a railway electric car, the controlling device being able to detect the slipping/sliding phenomenon during, in particular, high-speed travel and to exercise slipping/sliding control in an appropriate manner. MEANS FOR SOLVING PROBLEM [0007] In order to solve the aforementioned problems, a controlling device for a railway electric car according to one aspect of the present invention is constructed in such a manner as to include a plurality of electric motors and a slipping/sliding controlling unit that generates a torque command value so as to inhibit a slipping or sliding based on rotation speeds of the plurality of electric motors, wherein the slipping/sliding controlling unit includes: a reference rotation speed calculator that calculates a first rolGrence rotation speed and a second referonco rotation .'jficMuI by u.slng the rotation speeds of the pluriilily of «'I (tfl t l.c niotors; first adhesion level inclox generating nulls I liat are provided in corresponiJaruMi with I >i«> <^L<.MTl:ric iiU)t(irH reHpectively, and each of th« tit.Ml adliuHion level index generating units receives, as an input, the first reference rotation speed and the rotation speed of a corresponding one of the electric motors and generates a first adhesion level index that is an index for an adhesion level between a wheel that is connected to the corresponding electric motor and a surface that is trodden by the wheel, based on an acceleration deviation that is a difference between an acceleration calculated by using the rotation speed of the corresponding electric motor and an acceleration calculated by using the first reference rotation speed and based on a speed deviation that is a difference between the rotation speed of the corresponding electric motor and the first reference rotation speed; a second adhesion level index generating unit that receives, as an input, the second reference rotation speed and generates a second adhesion level index value by multiplying the first adhesion level index value by a gain that has been generated based on an acceleration calculated by using the second reference rotation speed; and a torque command value generating unit that generates the torque command value based on the second adhesion level index value. EFFECT OF THE INVENTION [0008] According to an aspect of the present invention, even in the situation where a slipping or sliding is occurring during high-speed travel and where the acceleration deviation and the speed deviation are small so that it is not effective to exercise slipping or sliding control by adjusting the torque based on the first adhesion level index, it is possible to exercise slipping or sliding control in an appropriate manner by setting the gain generated by the second adhesion level index generating unit to a predetermined value smaller than 1. As a result, an advantageous effect is achieved where, without the need to additionally use new rotation speed information of non-drive shafts or the like, it is possible to recognize a slipping/sliding phenomenon only based on the rotation spoed information of the drive shafts connected to the WIKSOUJ, I () (ietect a slipping/sliding stato btjfore the speed b(K:omes greatly different from an actual value, and to I'XfjciHo .slipping/sliding control in an appropriate manner. riHIKF DlilSCRIPTlON OF DRAWINGS 10009] [Fig. 1] Fig. 1 is a diagram of a controlling device for a railway electric car according to an embodiment of the present invention. [Fig. 2] Fig. 2 is a diagram of a slip controlling unit according to the embodiment. [Fig. 3] Fig. 3 is an operation chart of a speed deviation DFM, an acceleration deviation DFT, and an adhesion level index ADLl, in a situation where a wheel 5A connected to a first shaft spins free. [Fig. 4] Fig. 4 is an operation chart of adhesion level indexes ADLl to ADL4 of mutually different shafts, an adhesion level index ADLO to which a maximum value procesTs has been applied, a torque command TO* corresponding to a steady period, and a torque command T* that has been adjusted by slip control. [Fig. 5] Fig. 5 is an operation chart of a first adhesion level index (galouXatror 7A in a situation where a minor slipping keeps occurring. [Fig. 6] Fig. 6 is an operation chart of levels of FMl to FM4, FMmax, ADLO, SI, S2, ADL, TO* and T* in a situation where a second adhesion level index oaloulara* is functioning. EXPLANATIONS OF LETTERS OR NUMERALS [0010] 1 Slip controlling unit 2 Torque calculator 3 Electric power converter 4A to 4D Electric motor 5A to 5D Wheel 6 Rail 7A to 7D First adhesion level index generating unit 8 Second adhesion level index generating unit 9 Maximum value calculator 10 Minimum value calculator 11, 13, 19 Differentiator i 12 Reference rotation speed calculator 14, 16 Subtracter 15, 17/ 20 Low-pass filter in Judging device 2] Comparator 22, 23 Inverter 2 4 Oft-dolay unit 21) ADL processing unit 28 First-order delay unit 29 Acceleration calculator 30 Acceleration deviation processing unit 31 Differential speed deviation processing unit 32 Acceleration responsiveness lowering unit 33 Slip detecting unit 34 Gain generating unit 35 Time constant setting unit BEST MODE(S) FOR CARRYING OUT THE INVENTION [0011] Exemplary embodiments of a controlling device for a railway electric car according to the present invention, will be explained in detail, with reference to the accompanying drawings. The present invention is not limited to the exemplary embodiments. In addition, although slip control is explained below, the same applies to sliding control. [0012] Exemplary Embodiments Fig. 1 a diagram of a controlling device for a railway electric car according to an embodiment of the present invention. Fig. 2 is a diagram of a slip controlling unit according to the embodiment. [0013] First, a configuration of the controlling duvice for a railway electric car according to the present embodiment will be explained, with reference to Fig. 1. The reference character 1 denotes a controlling unit that performs a torque control so as to resolve a slipping or sliding state. In the following sections, to simplify the description, the controlling unit will be simply referred to as a "slip controlling unit". A torque command value TO* corresponding to a non-slipping state is input to the slip controlling unit 1. After a calculation has been performed on the torque command value TO* while a slipping state is taken into account, the slip cpntrolling unit 1 outputs a torque command value T*, The reference character 2 denotes a torque calculator. The torque calculator 2 receives, as an input, the torque command value T* and outputs a gate control output G. The reference character 3 denotes an electric power converter. The electric power luinvert.er 3 is controlled based on the gate control output G, whirh iM an output of the torque cn.lcnl.n1or ?.. In the oxtiiiiple Hluiwn .in Fig. 1, a plurality of olect.rlc: motora Ah \.o 41) ,11(31 collectively driven. 100HI F.ach of the reference charocitai li 'lA l(» S|) donates ri wliuel. The reference character 6 dentitos u lail. The electric motors 4A to 4D are connected to shafts of the wheels 5A to 5D, respectively, and cause the wheels 5A to 5D to rotate, respectively. Due to friction forces generated between the wheels 5A to 5D and the rail 6, the railway electric car obtains a propulsive force via the rotations of the wheels 5A to 5D. Further, the reference characters FMl to FM4 denote speed signals detected by sensors (not shown) that are installed on the electric motors 4A to 4D, respectively. The speed signals FMl to FM4 indicate rotation speeds of the shafts of the electric motors 4A to 4D, respectively. [0015] Next, a configuration of the slip controlling unit 1 will be explained, with reference to Fig. 2. The reference characters 7A to 7D denote first adhesion level index generating units that respectively generate adhesion level indexes ADLl to ADL4, which are indexes of adhesion levels between the wheels 5A to 5D and the rail 6, respectively. The reference character 8 denotes a second adhesion level index generating unit that is provided separately from the first adhesion level index generating units 7A to 7D. The reference character 9 denotes a maximum value calculator that outputs a maximutn valutj FMinax among the rotation speeds FMl to FM4. The reference character 10 denotes a minimum value calculator that outputs a minimum value FMmin among the rotation speeds FMl to FM4. The maximum value calculator 9 and the minimum value calculator 10 are provided within a reference rotation speed calculator 12. The reference rotation speed calculator 12 outputs the FMmin to each of the first adhesion level index generating units 7A to 7D, and also, outputs the FMmax to the second adhesion level indtex generating unit 8. In the present embodiment, the reference rotation speed calculator 12 calculates the minimum value FMmin and the maximum value FMmax, based on the rotation speeds FMl to FM4. However, the present invention is not limited to this example. Any other arrangement is also acceptable as long as two reference' rotation speeds are obtained, based on the rotation speeds FMl to FM'l. |f)ni6| Two signals representing the FMinin and the rotation Hixi'ed FMl are input to the first adhesion level iiulirx T* is satisfied, so that the torque is lowered by "T0*-T*". A time constant used for lowering or recovering the torque is adjusted by using the time constant T used by the first-order delay unit 28. 100261 Kiq. 4 is an operation chart of adhG|jion level liKhixes ADl.l Lo ADL4 of the shafts, the adhuaion lovel lriil<»x ADI.O lo which the maximum value solecLinq process has boon applied, the torque command TO* cor r(mp<»nl I liat. Ihci uljpping is inhibited l>y Ihit Dnf.i ml cHor t;iaed by the second adhesion level index daiMulatrtiii 8. Because a first-order delay is applied to the acceleration SI by the low-pass filter 20, the acceleration S2 exceeds the acceleration detection level SSET between the time t2 and a time t5,'so that the value of ADL becomes "0". As for the torque command T*, the value thereof is lowered from approximately "1" to "0" in the time period from the time t2 to the time t3. The rate of change of the torque command T* is determined by the setting of the time constant x established by the first-order delay unit 28. [0039] As explained above, according to the present embodiment, even in the situation where the acceleration deviation DFT and the speed deviation DFM are both small, and it is difficult to inhibit slippings with the control exercised by the first adhesion level index generating units 7A to 7D, it is possible to inhibit the slippings by using the second adhesion level index generating unit 8. As a result, without the need to additionally use new shaft speed information of non-drive shafts or the like, it is possible to recognize a slipping phenomenon only based on the speed information of the drive shafts, to detect a clipping state before the speed becomes greatly different from the actual value, and to exercise slip control in an appropriate manner. In particular, according to the present embodiment, it is possible to detect and inhibit slippings during, for example, high-speed travel. [0040] The configurations described in the exemplary embodiments above are examples of the contents of the present invention. It is possible to combine the configurations with other publicly-known techniques or the like. Further, needless to say, it is possible to apply modifications to the configurations described above without departing from the gist of the present invention. [0041] Further, the fields to which the slip control according to the present invention may be applied are not limited to controlling devices for railway electric cars. For example, it is possible to apply the slip control according to the present invention to other related fields such as electric automobiles. lN|ilir.'J'l