DESCRIPTION MOTOR CONTROLLING APPARATUS TECHNICAL FIELD [0001] The present invention relates to an alternating-current motor for driving an electric vehicle, and more particularly, to a motor controlling apparatus that is suitable for controlling a permanent magnet synchronous motor. BACKGROUND ART [0002] A conventional apparatus for controlling an electrically-driven vehicle generally has a structure that a plurality of induction motors, each attached to each of a plurality of axles on a truck, is driven together in parallel by a single inverter (for example, see Patent Document 1 below). [0003] A technical problem in driving the induction motors together in parallel by a single inverter is in accommodating with different degrees of wear in diameters of a plurality of wheels (hereinafter, referred to as "wheel diameter") that are driven together by a single inverter. [0004] ' It is well known that the rotation speed of an induction motor (=rotor frequency) is a value obtained by adding a slip frequency to an inverter frequency. The slip frequency has a significant meaning when induction motors are driven by a single inverter, because the slip frequency absorbs the difference between the inverter frequencies that are common among the induction motors and the rotor frequencies that are different among the induction motors. [0005] More specific explanation will now be provided, using an example where a plurality of wheels are rotating on rails without slipping thereon. [0006] The rotation speed of a motor becomes ]ower than those of others when a wheel diameter is greater (that is, a circumferential length thereof is longer) than the others. On the contrary, when the wheel diameter is smaller (that is, a circumferential length thereof is shorter), the rotation speed of the motor becomes higher. Because the inverter frequency is common among the motors, a difference in rotation speed is a difference in slip frequency applied to each of the induction motors. At this time, different degrees of torques are generated in the induction motors, correspondingly to the difference in the slip frequencies. However, because a rated slip frequency of an induction motor is generally set in such a manner that an expectable difference in the wheel diameter does not give any influence thereto, the generated torque difference is very limited and practically does not result in a problem. [0007] Therefore, along with other advantages, it is suitable to use induction motors for driving motors together in parallel by a single inverter. In addition, by using the structure to drive induction motors by a single inverter, the number of inverters can be minimized, regardless of the number of induction motors installed on a vehicle. In this manner, the controlling apparatus can further be reduced both in weight and size. [0008] Recently, a permanent magnet synchronous motor driven by an inverter is increasingly applied in fields such as industrial equipment or home appliances. [0009] In comparison to an induction motor, a permanent magnet synchronous motor has advantages of not requiring an excitation current, because magnetic fluxes are established by permanent magnets, and of being highly efficient. because no current flows into the rotor, thus not causing a secondary copper loss. For these reasons, recently, various attempts have been made to apply a permanent magnet synchronous motor as a motor for driving an electric vehicle. [0010] , [Patent Document 1] Japanese Patent Application Laid-open No. 2006-014489 DISCLOSURE OF INVENTION PROBLEM TO BE SOLVED BY THE INVENTION [0011] When applying a permanent magnet synchronous motor as a motor for driving an electric vehicle, minimizing of a structure of an apparatus including a plurality of permanent magnet synchronous motors is a critical matter. [0012] Furthermore, as well known in the art, a permanent magnet synchronous motor operates with the inverter frequency synchronized with the rotor frequency. Therefore, permanent magnet synchronous motors, each differing in the rotation speed, cannot be driven together in parallel by a single inverter. [0013] Therefore, if a permanent magnet synchronous motor is applied to an electric vehicle, a driving inverter will be required for each of the permanent magnet synchronous motors. Because, in an electric vehicle, each wheel is driven by a plurality of motors in a vehicle set, the number of required inverters increases. Therefore, a controller for the increased inverters becomes larger in size and cost. Thus, the controlling apparatus inevitably increases, in size, mass, and cost. [0014] The present invention is made in consideration of the above. An object of the present Invention is to provide a motor controlling apparatus, having a controller for a plurality of inverters provided corresponding to each of a plurality of motors, where each of calculation units, to be arranged accordingly to each of the motors, is arranged effectively within a controller, and operations performed thereby are effectively grouped so as to reduce the size, the mass, and the cost thereof. MEANS FOR SOLVING PROBLEM [0015] In order to solve the afore-mentioned problem and attain the object, a motor controlling apparatus for controlling a plurality of alternating-current motors is constructed in such a manner that it comprioing comprises; a direct-current voltage source; a plurality of inverters that are provided correspondingly to each of the alternating-current motors, and outputs an alternating-current voltage at a predetermined frequency obtained by converting a direct-current voltage supplied from the direct-current voltage source to each of the alternating-current motors; a contactor that opens and closes an output end of each of the inverters; a voltage detector that detects the direct-current voltage supplied to each of the inverters; a current detector that detects a current in each of the alternating-current motors; and a controller that outputs at least a control signal to the inverters based on a control command supplied externally, the voltage detected by the voltage detector, the current detected by the current detector, and a signal indicating conditions of rotations of the alternating-current motors, vjherein the c o n t r o 11 e r includes: a==©©f»B©flr=e«4-e=H'M4rl=©«==in«i*--%l%a% individually caloulatoo and outputo a control oignal -rel-ated to each of-the invortor^ a first common calculation unit having: a sequence processor that generates and outputs a first control signal tliat relates to generation of a torque command, based on a drive command signal input externally, and a protection detector that detects an abnormality in the alternating-current motors and the motor controlling apparatus, and generates a second control signal indicating the abnormality to cause the inverters to stop; a second common calculation unit having a basic torque command generator that generates and outputs a basic torque command that is common to the inverters based on the first control signal received from the first common calculation unit; an individual calculation unit that individually oaloulatoa generates and outputs » third control signals individually related to each of the inverters based on the basic torque command received from the second common calculation unit; and a common logic calculation unit that calculates and outputs a first gate signal for controlling switching of each of the inverters based on the second control signal received from the first common calculation unit and the third control signals received from a plurality of such individual calculation units, and is commonly provided to the individua_ 1 calculation units so as to enable the first gate signals corresponding to each of the inverters tq__be contro 11 ed simultaneously. EFFECT OF THE INVENTION [0016] In a motor controlling apparatus according to the present invention, calculation units in the controller are grouped into: the common calculation unit that calculates and outputs control signals that are common among inverters; the individual calculation unit that individually calculates and outputs control signals related to each of the inverters; and the cojmnon logic calculation unit that outputs gate signals for controlling switching of each of the inverters based on signals received from the common calculation units and the individual calculation units. Thus, operations performed by each of the calculation units are effectively grouped, and each of the calculation units, arranged in accordance with each of the motors, is effectively arranged within a controller. Therefore, the present invention achieves the effects to reduce the size, the mass, and the cost of the motor controlling apparatus. BRIEF DESCRIPTION OF DRAWINGS [0017] [Fig. 1] Fig. 1 is a schematic of a motor controlling apparatus according to an exemplary embodiment of the present invention. [Fig. 2] Fig. 2 is a schematic of a structure of a controller according to the exemplary embodiment. [Fig. 3] Fig. 3 is a schematic of an example of a basic torque command TPO. EXPLANATIONS OF LETTERS OR NUMERALS [0018] 1 power collector 2 rail 3 wheel 4 electric wire 10 controller 20 first common calculation unit 21 sequence processor 22 protection detector 2 3 communication processor 3 0 second common calculation unit 31 basic torque command generator 32 average calculator 40A, 40B individual calculation unit 41A, 4IB torque command processor 42A, 42B slip controller 4 3A, 4 3B INV controller 50 converter controller 51 CNV controller 60 common logic calculation unit 61A, 61B, 62 gate logic 63 speed calculator 6 4 contactor logic 65 high-speed protection detector 6 6 OR circuit 100 controlling apparatus CNV converter CTl first motor current sensor CT2 second motor current sensor ■ CTS input current sensor FC filter capacitor INVl first inverter INV2 second inverter K input-side contactor Ml first permanent magnet synchronous motor M2 second permanent magnet synchronous motor MMKl, MMK2 motor-side contactor PT input voltage detector RZl first rotation sensor RZ2 second rotation sensor SO sequence status TRF transformer BEST MODE(S) FOR CARRYING OUT THE INVENTION [0019] A motor controlling apparatus according to an exemplary embodiment of the present invention will now be explained based on the drawings. Note that the embodiment is not intended to limit the scope of the present invention in any way. [0020] Fig. 1 is a schematic of a motor controlling apparatus according to an exemplary embodiment of the present invention. In this schematic, a controlling apparatus 100 according to the embodiment includes, sequentially from an input-stage side thereof, an input voltage detector PT, an input-side contactor K, an input current sensor CTS, a converter CNV, a filter capacitor FC, a first and a second inverters INVl and INV2, a first and a second motor current sensors CTl and CT2, and a first and a second motor-side contactors MMKl and MMK2. [0021] Furthermore, as shown in Fig. 1, the primary-side end of a transformer TRF is connected to an electric wire 4 via a power collector 1, and the other end is connected to a rai.l 2 that is at a ground potential via a wheel 3. In other words, electric power supplied from an electric power substation (not shown) is received via the electric wire 4, the power collector 1, the wheel 3, and the i'ai,l 2. [0022] An arrangement, connections, functions, and operations of each of these units m tlie con L rol 1 i lu] apparatus 100 will now be explained. [0023] (Input Voltage Detector PT) In Fig. 1, the secondary-side of the transformer TRF is connected to the controlling apparatus 100, and the voltage output from the TRF is input to the input-side contactor K that functions to isolate the controlling apparatus 100 from the transformer TRF. An input voltage VS that is the voltage at the secondary-side of the transformer TRF is input to a controller 10 via the input voltage detector PT. Because the voltage at the secondary side of the transformer TRF is usually high (approximately 1500 volts), a low-voltage winding may be provided in the transformer TRF, and the input voltage VS may be obtained therefrom. [0024] (Input-Side Contactor K) The input-side contactor K is a contactor hciving a capability to open and close a current of several-hundred amperes, and is set to OFF when the controlling apparatus 100 is to be stopped or some abnormality occurs, and set to ON during usual operations. The controller 10 outputs a control signal KC to the inpuL-side contactor K to turn ON or OFF an internal closing coil provided therein, so as to control the opening and the closing of a main contact. A status of the main contact in the input-side contactor K is returned to the controller 10 as a contact status signal KF through, for example, an auxiliary contact and alike mechanically cooperating therewith. [0025] (Input Current Sensor CTS) At the next stage to the input-side contactor K, the 10 input current sensor CTS is provided to detect an input current IS. The input current IS detected by the input current sensor CTS is input to the controller 10. [0026] (Converter CNV) At the next stage to the input current sensor CTS, the converter CNV is provided to convert the input alternating-current voltage to a direct-current (DC) voltage VD and to output the DC voltage VD to the filter capacitor I'C. The converter CNV includes a bridge circuit having switching devices such as insulated gate bipolar transistors (IGBTs), and generally is structured to be a so-called voltage-type pulse width modulation (PWM) converter that causes each of the switching devices to perform PWM operation,. The converter CNV receives a gate signal CG from the controller 10 for each of the switching devices, and reversely outputs an operation status signal CGF of each of the switching devices to the controller 10. Because the structure of and the operation performed by the voltage-type PWM converter are well known in the art, a detailed explanation thereof is omitted herein. [0027] (Filter Capacitor FC) The filter capacitor FC is connected to the output side of the converter CNV. The first inverter INVl and the second inverter INV2 are connected to positive and negative terminals of the filter capacitor FC in parallel, and are each supplied with DC voltage VD that is the voltage output from the converter CNV. [0028] (First and Second Inverters INVl and INV2) The first inverter INVl includes a bridge circuit having switching devices such as IGBTs, and generally is structured to be a so-called voltage-type pulse widtli modulation (PWM) inverter that causes each of the switching devices to perform PWM operation. The first inverter INVl 11 receives a gate signal IGl from the controller 10 for each of the switching devices, and reversely outputs an. operation status signal IGFl of each of the switching devices to the controller 10. Because the structure of and the operation performed by the voltage-type PWM inverter are well known in the art, a detailed explanation thereof is omitted herein. Moreover, because the structure of and the operation performed by the second inverter INV2 are the same as those of the first inverter INVl, explanations of the structure and the operation performed thereby are omitted herein. [0029] (First and Second Motor Current Sensors CTl and CT2) At the output side of the first inverter INVl, the first motor current sensor CTl is provided to detect the output current of the first inverter INVl (that is, a motor current). A first motor current II detected by the motor current sensor CTl is input to the controller 10. Moreover, at the output end of the second inverter INV2, the second motor current sensor CT2 is provided, and the output current detected by the second motor current sensor CT2 is input to the controller 10. [0030] (First and Second Motor-Side Contactors MMKl and MMK2) At the niExt stage to the motor current sensor CTl, the first motor-side contactor MMKl is provided. The first motor-side contactor MMKl is a contactor having a capability to open and close a current of several-hundred amperes, and is set to OFF when the controlling apparatus 100 is to be stopped or some abnormality occurs, and set to ON during usual operations. The controller 10 outputs a control signal MKCl to the first motor-side contactor MMKl to turn ON or OFF an internal closing coil provided therein 12 so as to control opening and closing of the main conta.ct. The status of the main contact in the first motor-side contactor MMKl is returned to the controller 10 as a contact status signal MKFl through, for example, an auxiliary contact and alike mechanically cooperating therewith. Furthermore, at the next stage to tlie motor current sensor CT2, the second motor-side contactor MMK2 is provided. Because a function and an operation performed thereby are the same as those of the first motor-side contactor MMKl, explanations of the function and the operation performed thereby are omitted herein. [0031] (First Permanent Magnet Synchronous Motor Ml) At the next stage to the first motor-side contactor MMKl, the first permanent magnet synchronous motor Ml is connected. The first permanent magnet synchronous motor Ml is mechanically connected to the wheel 3, and is structured to drive the wheel 3. In addition, a first rotation sensor RZl is connected to the first permanent magnet synchronous motor Ml, and a detected value Rl is input to the controller 10. [0032] (Second Permanent Magnet Synchronous Motor M2) At the next stage to the second motor-side contactor MMK2, the second permanent magnet synchronous motor M2, mechanically connected to another wheel 3 that is not Llie wheel 3 connected to the first permanent magnet synchronous motor Ml, is connected. Moreover, to the second permanent magnet synchronous motor M2, a second rotation sensor RZ2 is connected, and a detected value R2 is input to the controller 10. [0033] (First and Second rotation Sensors RZl and RZ2) r3oth of the first rotation sensor RZl and the second rotation, sensor RZ2 are so-called encoders or resolvers, and the detected values Rl and R2 detected by the rotation 13 sensors are signals indicating an absolute position of a rotor in each of the motors. A so-called sensorless controlling scheme is also commercialized, whicli obvial:es a robation sensor that obtains the absolute position oi Lhe rotor of a motor by performing a calculation based on the voltage and current of the motor. If the soMisorless controlling scheme is to be used, the first rotation sensor RZl and the second rotation sensor RZ2 are not required. [0034] (Controller 10) The controller 10 includes a microcomputer (MC) or a logical circuit, and a control source that supplies a power thereto. The controller 10 outputs a control signal (KC, CG, IGl, IG2, MKCl, and MKC2) to each of these units following predetermined procedures to control each of the units based on a drive command signal CMD input from a cab (not shown) and alike of the electric vehicle and a status signal received from each of the units described above (at least the input voltage VS, the contact status signal KF at the input-side contactor K, the input current IS, the operation status signal CGF at the switching devices of tlie converter, the DC voltage VD, the operation stalus signaJ IGFl at the switching devices in the first inverter, the operation status signal IGF2 at the switching device in the second inverter, the first motor current II, the second motor current 12, the contact status signal MKFl at the first motor-side contactor MMKl, the contact status signal MKF2 at the second motor-side contactor MMK2, the detected value Rl at the first rotation sensor RZl, and the detected value R2 at the second rotation sensor RZ2). If the received status signal indicates an abnormal value, the controller 10 performs a control operation such as stopping each of these units by way of the control signal to be provided thereto. 14 [0035] In addition to these control signals, the controller 10 outputs a status notifying signal STD, and receives the drive command signal CMD. The status notifying signal STD is a signal for indicating an operation status or an abnormal status of each of tlie units in the controlling apparatus 100, and is output in a form of, for example, a data communication or a contact signal to an external cab or an equipment status mojiitoring apparatus (neither of which is shown) and alike. The drive command signal CMD at least includes signals corresponding to a move-forward/backward command, a power runninq command and strength thereof, a brake command and strength thereof. 10036] In Fig. 1, an example of an AC-fed electric vehicle is shown as an exemplary embodiment of tlic) motor controlling apparatus. The motor controlling apparatus niay also be applied to a DC-fed electric vehicle that is widely used in subways and suburban railways. If the motor controlling apparatus is to be applied to a DC-input electric vehicle, the transformer TRF and the converter CNV are not required, and the DC voltage (generally approximately DC600 volts to 3000 volts) supp].i,ed from the electric wire 4 is directly applied to the filter capacitor PC as the DC voltage VD. [0037] (Detailed Structure of Controller 10) Detailed structure of the controller 10 will now be explained. Fig. 2 is a schematic of a structure of the controller 10 according to the exemplary embodiment. As shown in Fig. 2, the controller 10 includes a first common calculation unit 20, a second common calculation unit 30, individual calculation units 40A and 40B, a converter controller 50, and a common logic calculation unit 60. [0038] (Structure of First Common Calculation unit 20) The first common calculation unit 20 includes a 15 sequence processor 21, a protection detector 22, and a communication processor 23. [0039] (First Common Calculation unit 20 - Sequence Processor 21) The sequence processor 21 receives the d.rive command signal CMD provided externally and the status signals (VS, KF, IS, CGF, VD, IGFl, 1GF2, II, 12, MKFl, MKF2, FMl, and FM2) from each of the units in the controlling appciratus 100. Based on the drive command signal CMD, the sequence processor 21 outputs, with a prescribed sequence logic, a control signal CS including a code of a torque command corresponding to a drive forward/backward command, a power running command, a brake command, a torque cut instruction and alike to a basic torque command generator 31 to be described later. At the same time, the sequence processor. 21 outputs KC that is a close command for the input-side contactor K, MKCl that is a close command for the first motor-side contactor MMKl, and MKC2 that is a close command for the second motor-side contactor MMK2 to the common logic calculation unit 60 to be described later. [0040] (First Common Calculation unit 20 - ProlecLjon Detector 22) The protection detector 22 generates a control signa]. SWH based on the status signals, and outputs ,Lhe coiiLro] signal SWH to the common logic calculation mij.t 60. In addition, when a voltage, a current, and a.like of each of the units in the controlling apparatus exceed a predetermined value, the protection detector 22 determines the situation as abnormal and outputs a signal upon determination of the abnormality as abnormality detected status signal PF to the communication processor 23. [0041] (First Common Calculation unit 20 - Communication Processor 23) The communication processor 23 receives the status signals (VvS, KF, IS, CGF, VD, IGFl, IGF2, II, 12, MKFl, MKF2, FMl, and FM2) from each of the units in tlie controlling apparatus, the abnormality detected status signal PF from the protection detector 22, and a sequence status SO from the sequence processor 21. The communication processor 23 outputs the status notifyijig signal STD to the cab of the electric vehicle, the equipment status monitoring apparatus (neither of whicli is shown), and alike, in a form of, for example, a data communication or a contact signal. [0042] (Structure of Second Common Calculation unit 30) The second common calculation unit 30 includes the basic torque command generator 31 and an average calculator 32. [004 3] (Second Common Calculation unit 30 - Basic Torque Command Generator 31) The control signal CS from the sequence processor 21 is input to the basic torque command generator 31. The basic torque command generator 31 generates a basic torque command TPO using the power running command, the brake command, and the commands of respective strengths thereof. The basic torque command TPO is a value determined at least based on the power running command, the brake command, blie commands of respective strength thereof, and the speed of the electric vehicle. [0044] (Second Common Calculation unit 30 - Average Calculator 32) The speed of the electric vehicle, used for generating the basic torque command TPO, is generated at the average calculator 32. The average calculator 32 performs an averaging operation to a speed FMl of the first permanent magnet synchronous motor generated from the detected value 17 R.1 obLa;ined a I:, tlie first rotati.on sensor Ji7A , mid a s[joed FM2 of tlie second permanent magnet synchronous moLoi: generabed from the detected value R2 obtained a I; the second rotation sensor RZ2, and outputs the output of I:lie averaging operation to the basic torque command generator 31 as an average motor speed FMA. [0045] Fig, 3 is a schematic of an example of the basic torque command TPO, The horizontal axis thereof represents the average motor speed FMA generated by the average calculator 32, and the vertical axis thereof represents the basic torque command TPO generated by the basic torcfue command generator 31. As shown in Fig. 3, the basic torque command TPO has several profiles that are dependent on the average motor speed, and these profiles are switched by the control signal CS output from the sequence processor 21. [0046] (Structures of Individual Calculation units 40A and 40B) The basic torque command TPO is input to the individual calculation units 4 0A and 4OB. The individual calculation unit 4 0A corresponds to controls of the first permanent magnet synchronous motor Ml, and tl:ie J.nd.Lvidua] calculation unit 4OB corresponds to controls of the second permanent magnet synchronous motor M2. Although not especially shown in the schematic, the number of motors controlled by the controller 10 is not limited to two. When control a third and a fourth permanent magnet synchronous motors is required, individual calculation units, each corresponding thereto, may be added. A structure of each of the individual calculation units is as shown in Fig. 2. Although the reference letters and numerals are different, the structure, the arrangement, the function, and ali]