ENGINE MONITORING DEVICE, ENGINE CONTROL SYSTEM, STEEL ROLLING SYSTEM AND ENGINE MONITORING METHOD

Abstract

Electric motor monitoring device (40, 150, 160, 180) for an electric motor control system (101), wherein the electric motor control system (101) comprises: several electric motors (10-1 to 10-N) configured as three-phase induction electric motors, each comprising a rotating shaft (14); several rotary wheels (16-1 to 16-N) connected to the several rotating shafts (14-1 to 14-N) and connected via a conveying object (12); an inverter (22) connected in parallel with the several electric motors (10-1 to 10-N) and supplying them with an alternating current; a control unit (30) controlling the inverter (22);and a current sensor (41) that detects a current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) flowing through each of the electric motors (10-1 to 10-N), the electric motor monitoring device (40, 150, 160, 180) comprising: a calculation unit (42, 42-1 to 42-N) configured to be connected to the electric motor control system (101) and to calculate a rotational speed (ωrs) of the corresponding electric motor (10-1 to 10-N) according to each of the current values ​​(Iu, Iw, Iu1 to IuN, Iw1 to IwN);and a determination unit (44, 48) configured to detect an overheating condition of any of the electric motors (10-1 to 10-N) and / or an anomaly of a diameter difference between the several rotary wheels (16-1 to 16-N) based on the current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) and the rotational speed (ωrs), wherein the computation unit (42, 42-1 to 42-N) computes several DC current values ​​(Ir1 to IrN') corresponding to the electric motors (10-1 to 10-N) based on the current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN), and the determination unit (44, 48) comprises: a function for calculating a low-speed range current difference (IL) that calculates a difference between a maximum value and a minimum value in the several DC current amounts (Ir1 to IrN') when the rotational speeds (ωrs) of the several electric motors (10-1 to 10-N) are in a low-speed range which is a predetermined speed range;and a function for detecting the overheating state of the electric motor (10-1 to 10-N), corresponding to the minimum DC current amount, based on the low-speed range current difference (IL); and a function for calculating a high-speed range current difference, which is a difference between a maximum value and a minimum value in the several DC current amounts (Ir1 to IrN') when the rotational speeds (ωrs) of the several electric motors (10-1 to 10-N) are in a high-speed range, which is a predetermined speed range; and a function for detecting a rotary wheel diameter difference between the several electric motors (10-1 to 10-N) based on the high-speed range current difference.

Description

Technical field The present invention relates to an electric motor monitoring device, an electric motor control system, a steel rolling system and an electric motor monitoring method. State of the art A technique for monitoring the state of several electric motors while the electric motors are driven in parallel is known. For example, PTL 1 reveals a Electric motor overtemperature protection device applicable to a railway vehicle propulsion system, wherein several electric motors are driven in parallel using one or more inverter devices.The electric motor overtemperature protection device comprises a control device 14, which controls the operation of an inverter device 10, and a protection device 20, which, based on a frequency signal fs with frequency information, detects an overtemperature that may be generated in the electric motors 12a, 12b, when the inverter device 10 performs a control to keep a voltage-frequency ratio constant at the electric motors 12a, 12b, and at least single-phase currents I1, I2 flowing through the electric motors 12a, 12b, generates an overtemperature protection signal Tf, which protects the electric motors 12a, 12b from overtemperature, and outputs the overtemperature protection signal Tf to the control device 14 (see summary). PTL 2 describes an electric vehicle protection system in which the electric motor currents of the first and second induction motor groups are compared to detect a difference in the electric motor currents, thereby detecting a difference between the diameter of a wheel connected to the first induction motor group and the diameter of a wheel connected to the second induction motor group, and protecting an electric vehicle if the difference is greater than or equal to a permissible value (see left box of page 1). Furthermore, PTL 3 discloses a sensorless vector control for parallel-connected induction motors, in which current sensors measure the motor currents.A detection unit determines current differences between the motors and corrects measured current values ​​based on stored reference values ​​to detect wheel slippage or wheel slippage, taking wheel diameter differences into account. Furthermore, PLT 4 describes a method for controlling an induction motor, particularly at low speeds, by superimposing an AC voltage onto the supply voltage. The leakage inductance is determined from the relationship between the superimposed voltage and the resulting current in order to estimate the magnetic flux position and control the motor accordingly. PTL 5 discloses a control method for an induction motor using vector control to achieve high output power. In this method, the internal induced electromotive force is increased proportionally to the speed, and field weakening control is applied as soon as the electromotive force exceeds the value at rated speed. List of counterclaims Patent literature PTL 1: WO 2013 / 035 185 A1PTL 2: JP S61- 210 801 APTL 3: JP 2002 - 281 606 APTL 4: DE 44 13 809 A1PTL 5: EP 1 622 253 A2 Summary of the invention Technical problem However, the PTL 1 technique, which focuses on the relative current difference between multiple electric motors, has a problem in that detection becomes difficult when these electric motors overheat simultaneously. It is difficult to determine whether the temperature increase is caused by a difference in the installation environment of each electric motor or a difference in the diameter of the rotating wheel. Similarly, the PTL 2 technique is difficult to determine whether the current difference between the multiple electric motors is caused by a difference in wheel diameter or other factors. Furthermore, the PTL 2 system requires the installation of sensors, such as a temperature sensor and a speed sensor, for each monitoring target. This presents a problem, as the number of associated devices, including the sensors, increases the overall equipment cost. Another issue is the difficulty of sensor installation when there are dimensional constraints at the electric motor's installation location or under harsh environmental conditions. As the number of sensors increases, ensuring the reliability of the sensor array becomes more challenging, and monitoring accuracy deteriorates. Therefore, there is a need to minimize the number of sensors used. Reducing the number of sensors significantly improves maintenance and reliability. Specifically, it reduces sensor maintenance and inspection work, prevents system shutdowns due to sensor failures, and minimizes the amount of wiring required for the sensor system, thus lowering labor costs. Furthermore, it reduces the risk of wiring problems. The present invention was carried out in view of the above circumstances and an object of the present invention is to provide an electric motor monitoring device, an electric motor control system, a steel rolling system and an electric motor monitoring method that are capable of accurately detecting the condition of an electric motor at low cost. Solution to the problem The invention is described in the attached set of claims. Advantageous embodiments are defined in the dependent claims. Advantageous effects of the invention In the present invention, the state of the electric motor can be accurately detected. Brief description of the drawings [Fig. 1] Fig. 1 is a block diagram representing an electric motor control system according to a first embodiment of the present invention. [Fig. 2] Fig. 2 is a block diagram representing a monitoring device. [Fig. 3] Fig. 3 is a block diagram representing a current calculation unit. [Fig. 4] Fig. 4 is a diagram showing an example of a machine frequency and DC current magnitude during acceleration. [Fig. 5] Fig. 5 is a view showing another example of the DC current magnitude during acceleration. [Fig. 6] Fig. 6 is a view showing yet another example of the DC current magnitude and a proportional signal during acceleration. [Fig. 7] Fig. 7 is a view showing a determination result of an anomaly content based on a current difference between a low-speed range and a high-speed range. [Fig. 8] Fig.Figure 8 is a flowchart representing an anomaly detection routine performed by the monitoring device. [Fig. 9] Figure 9 is a block diagram representing an electric motor control system according to a second embodiment. [Fig. 10] Figure 10 is a block diagram representing an electric motor control system according to a third embodiment. [Fig. 11] Figure 11 is a block diagram representing an electric motor control system according to a fourth embodiment. [Fig. 12] Figure 12 is a block diagram representing a steel rolling system according to a fifth embodiment. [Fig. 13] Figure 13 is a side view representing a railway vehicle according to a sixth embodiment. [Fig. 14] Figure 14 is a bottom view representing a chassis according to a sixth embodiment. [Fig. 15] Figure 15 is a side view representing a vehicle assembly according to a seventh embodiment. Description of embodiments [First embodiment] <Gesamtkonfiguration der ersten Ausführungsform> Fig. 1 is a block diagram representing an electric motor control system 101 according to a first embodiment of the present invention. In Fig. 1, the electric motor control system 101 comprises two electric motors 10-1, 10-2, a drive unit 20, a monitoring device 40 (the electric motor monitoring device), and several current sensors 41. The drive unit 20 includes an inverter 22, a current sensor 24, and a control unit 30. At this point, the electric motors 10-1, 10-2 are a three-phase induction electric motor and are connected in parallel. The rotating shafts 14-1, 14-2 of the electric motors 10-1, 10-2 are connected to gears 16-1, 16-2, with a mechanical component (not shown), such as a gear, inserted between them, or the rotating shafts 14-1, 14-2 are directly connected to the gears 16-1, 16-2. The rotary wheels 16-1, 16-2 move a conveyed object 12 in a tangential direction.Alternatively, a railway track is provided instead of the transport object 12, and the rotary wheels 16-1, 16-2 themselves can move tangentially on the track. Hereinafter, the electric motors 10-1, 10-2 are sometimes referred to collectively as "electric motor 10", the rotating shafts 14-1, 14-2 collectively as "rotating shaft 14", and the rotary wheels 16-1, 16-2 collectively as "rotating wheel 16". The inverter 22 applies a three-phase alternating voltage to the electric motor 10 based on the control of the control unit 30. The control unit 30 comprises hardware typical of a general-purpose computer, such as a CPU (central processing unit), a DSP (digital signal processor), RAM (random access memory), and ROM (read-only memory). The ROM stores a control program executed by the CPU, a microprogram executed by the DSP, various data, and the like. Inside the control unit 30, as shown in Fig. 1, functions implemented by the control program, the microprogram, and the like are represented as blocks. That is, the control unit 30 comprises a command generator 32, a deviation calculation unit 33, a vector control unit 34, a dq / 3Φ converter 36 and a 3Φ / dq converter 38. With these configurations, the control unit 30 performs vector control on the electric motor 10 in order to improve the responsiveness of the electric motor 10. The inverter 22 outputs an alternating current of one U-phase, one V-phase, and one W-phase to the electric motor 10. The current sensor 24 detects a two-phase current. That is, in the example of Fig. 1, the U-phase and W-phase currents are detected, and the detection results are output as current detection values ​​IUS and IWS. Here, a rotational coordinate rotating at a frequency f is assumed. Axes orthogonal to the rotational coordinate are called the d-axis and q-axis, and a current supplied to the electric motor 10 is expressed as a DC current magnitude along the rotational coordinate. The current on the q-axis is a current component that determines the torque of the electric motor 10. Hereinafter, the current on the q-axis is referred to as the torque current. The current on the d-axis is a component that becomes an excitation current of the electric motor 10 and is referred to hereafter as the excitation current. The 3Φ / dq converter 38 outputs an excitation current detection value Id and a torque current detection value Iq based on the current detection values ​​IUS and IWS. The current detection values ​​IUS and IWS typically increase proportionally to the number of electric motors 10. However, the control unit 30 assumes that the number of electric motors 10 is "one," as it is complicated to change the parameters of the control unit 30 according to the number of electric motors 10. For this reason, the 3Φ / dq converter 38 performs normalization by dividing the input current detection values ​​IUS and IWS by the number of electric motors 10 and then calculates the detection values ​​Id and Iq. The command generator 32 receives a torque command value τ* from a host device (not shown) and generates an excitation current command value Id* and a torque current command value Iq* based on the torque command value τ*. The deviation calculation unit 33 outputs deviations Id* - Id, Iq* - Iq based on the current command values ​​Id*, Iq* and the detection values ​​Id, Iq. The vector control unit 34 outputs an excitation voltage command value Vd* and a torque voltage command value Vq* based on the deviations Id* - Id, Iq* - Iq, and so on. The operation of the vector control unit 34 is described in more detail below. The vector control unit 34 performs proportional integration control on the deviations Id* - Id, Iq* - Iq to obtain a frequency command ω1 (not shown), which is a command value for a synchronous speed.As used here, the synchronous speed is the rotational speed of the electric motor 10 when slip is assumed to be "0". The vector control unit 34 receives a phase command θ1 (not shown) by integrating the frequency command ω1. The vector control unit 34 multiplies a vector formed by the current command values ​​Id*, Iq*, with the vector of an impedance of the electric motor 10 and consequently calculates the voltage command values ​​Vd*, Vq*. The dq / 3® converter 36 outputs a PWM signal that controls the inverter 22, based on the voltage command values ​​Vd*, Vq* of the rotational coordinate system. Specifically, the dq / 3Φ converter 36 first converts the voltage command values ​​Vd*, Vq* of the rotational coordinate system into two-phase voltage values ​​of a stationary coordinate system based on the frequency command ω1 and phase command θ1. Furthermore, the dq / 3Φ converter 36 converts the resulting two-phase voltage values ​​into three-phase voltage command values ​​vu*, vv*, vw* (not shown). Furthermore, the dq / 3Φ converter 36 compares the three-phase voltage command values ​​vu*, vv*, vw* with a carrier wave (for example, a triangle wave) to output U-phase, V-phase, and W-phase PWM signals. The inverter 22 performs the switching of the supplied DC voltage (not shown) based on the supplied PWM signal and outputs U-phase, V-phase, and W-phase voltages to the electric motor 10. <Konfiguration der Überwachungseinrichtung 40> Fig. 2 is a block diagram representing the monitoring device 40. The monitoring device 40 comprises hardware similar to a general-purpose computer, such as a CPU, a DSP, RAM, and ROM, similar to the control unit 30. The ROM stores a control program executed by the CPU, a microprogram executed by the DSP, various data, and the like. Inside the monitoring device 40, as shown in Fig. 2, the functions implemented by the control program, the microprogram, and the like are represented as blocks. That is, the monitoring device 40 includes current calculation units 42-1, 42-2 (the calculation unit), a feature size extraction device 44 (determination unit), a memory 46 and an anomaly determination unit 48 (the determination unit, the anomaly determination process). The current calculation units 42-1, 42-2 (hereinafter sometimes collectively referred to as current calculation unit 42) each acquire U-phase current detection values ​​Iu1, Iu2 (equal to the current detection value Iu and the current value) and W-phase current detection values ​​Iw1, Iw2 (equal to the current detection value Iw and the current value) from the corresponding current sensor 41. Based on these detected values, the current calculation unit 42 outputs DC current magnitudes Ir1, Ir2 (equal to the DC current magnitude Ir), proportional signals PLL_P1, PLL_P2 (equal to the proportional signal PLL_P), and machine frequencies ωrs1, ωrs2 (equal to the machine frequency ωrs and the rotational speed). The meaning of the signals output by the current calculation units 42 is described with reference to Fig. 3. Fig. 3 is a block diagram representing the current calculation unit 42. The current calculation unit 42 comprises a 3Φ / αβ converter 52, an arctangent converter 54 (the phase detector), a subtractor 56 (the PLL calculation unit), and a phase calculation unit 60 (the PLL calculation unit, the rotational speed calculation unit, the rotational speed calculation process), a rotational coordinate converter 70, an integrator 72 (the PLL calculation unit), and a multiplier 74. The phase calculation unit 60 comprises multipliers 62 and 64, an integrator 66, and an adder 68. The 3Φ / αβ converter 52 converts the current detection values ​​λi, Iwin into two-phase alternating currents Ii, Ii, which are orthogonal to each other. The arctangent converter 54 calculates an AC phase angle detection value θi* based on the alternating currents Ii, Ii. The subtractor 56 subtracts the AC phase angle detection value θi* from an AC phase angle θi (details will be described later). In the phase calculation unit 60, the multiplier 62 multiplies a difference value “θi* - θi” by a predetermined proportional gain KpPLL. The multiplication result of the multiplier 62 becomes the proportional signal PLL_P. The multiplier 64 multiplies the difference value “θi* - θi” by a predetermined integral gain KiPLL, and the integrator 66 integrates the multiplication result. The integration result of the integrator 66 is called the integration signal PLL_I. The adder 68 adds the proportional signal PLL_P and the integration signal PLL_I and outputs the result as a frequency signal ω1s. The integrator 72 integrates the frequency signal ω1s and outputs the AC phase angle θi. The AC phase angle θi is fed to the subtractor 56 and also to the rotary coordinate converter 70. The multiplier 74 multiplies the frequency signal ω1s by "2 / P" (where P is the number of poles of the electric motor 10) and outputs the result of the multiplication as the machine frequency ωrs. At this point, the machine frequency ωrs is a signal corresponding to a synchronous speed of the electric motor 10 (see Fig. 1). The rotary coordinate converter 70 converts the two-phase AC currents Iα, Iβ into the two-axis DC currents Ir, Ii in the rotary coordinate system, which is rotated by the frequency signal ω1s. In this way, the subtractor 56, the phase calculation unit 60 and the integrator 72 function as a PLL calculation unit (phase control loop calculation unit) and output the frequency signal ω1s and the AC phase angle θi, so that the difference value “θi* - θi”, which is output from the subtractor 56, approaches “0”. Referring again to Fig. 2, the feature quantity extraction device 44 extracts several values, called feature quantities, based on the DC current magnitudes Ir1, Ir2, which are the DC current magnitude Irs corresponding to each of the electric motors 10-1, 10-2, the proportional signals PLL_P1, PLL_P2, and the machine frequencies ωrs1, ωrs2. Although details of each of the feature quantities are described later, the “feature quantity” includes a low-speed range current difference IL, a low-speed range proportional signal difference HL1, HL2, a high-speed range current difference IH, and a high-speed range correction current difference IQ. The anomaly detection unit 48 detects the presence or absence of an overheating condition of the electric motors 10-1, 10-2 and a diameter difference between the rotating wheels 16-1, 16-2 (hereinafter referred to as the rotating wheel diameter difference) based on these characteristic values. The anomaly detection unit 48 outputs various alarm signals to the outside based on these characteristic values. The memory 46 stores the contents of the characteristic values ​​and the alarm signals. Any means, such as the illumination of a lamp, the sound emission of an alarm, or the transmission of a radio wave via a wireless communication device capable of notifying the operator, can be used as an alarm signal.If the monitoring device 40 of the first embodiment is installed in a harsh environment, it is preferably enclosed in a monitoring device housing that incorporates dustproof and waterproof measures. If the monitoring device 40 is installed near a device such as the inverter 22 that generates noise, it is preferably equipped with noise reduction measures. Since, as described above, the monitoring device 40 can treat the current flowing through the electric motor 10 as a DC current, a change in the internal state of the electric motor 10 is easily detected by means of a transition phenomenon. Rotational speed information in a variable-speed drive can also be detected by converting the AC current to a DC current, allowing for easy analysis of its correlation with the rotational speed. The AC current can be converted to DC current using a simple algorithm, enabling the edge detection and anomaly identification to be performed directly within the monitoring device. Consequently, the amount of data can be significantly reduced, simplifying analysis and diagnostics. <Zu detektierender anomaler Zustand> (Overheating of the electric motor) In Fig. 1, the resistance values ​​and other parameters of electric motors 10-1 and 10-2 depend on an operating temperature. At this point, a specific temperature (for example, 20 °C) is referred to as the "reference temperature," and a parameter such as a resistance value at the reference temperature is called the "reference value." The resistance value of the electric motor increases as the temperature of electric motor 10 increases. A relationship between an electric motor temperature T and an electric motor resistance value RT is given by the following equation (1). In equation (1), δ is the reciprocal of a resistance temperature coefficient of a wound copper wire, and R20 is an electric motor resistance reference value, namely the electric motor resistance value RT of the electric motor at the reference temperature (20 °C). According to equation (1), for example, if the temperature increases by 40 °C relative to the reference temperature, the electric motor resistance value RT will be approximately 1.16 times the electric motor resistance reference value R20. If the temperature increases by 70 °C relative to the reference temperature, the electric motor resistance value RT will be approximately 1.27 times the electric motor resistance reference value R20. The same formula can be applied to the case where the winding consists of an aluminum wire or the like. If the temperature of the electric motor 10 is distorted due to an individual difference between the several electric motors 10, the specific resistance of the primary and secondary resistors increases in the electric motor 10 at a higher temperature, and the electric motor resistance value RT increases. Consequently, the torque of the electric motor 10 is concentrated in the electric motor 10 at the lower temperature, and a problem arises in that the conveyed object 12 (see Fig. 1) cannot be accelerated uniformly due to a lack of average torque. If this condition persists, the electric motor 10 in which the torque is concentrated sometimes fails due to prolonged overload. (Diameter difference of the rotating wheel 16) In Fig. 1, if a difference in the diameter of the rotating gears 16-1 and 16-2 connected to each electric motor exists, a difference in the rotational speed of each electric motor 10 is generated by this difference in diameter. A difference in the generating torque of each electric motor 10 is generated almost proportionally to the difference in rotational speeds. At this point, the difference in generating torque due to a difference in rotational speed can be prevented if the electric motor 10 is configured such that a large rated slip is achieved. Since the rated slip, on the other hand, significantly affects the efficiency of the electric motor 10, the rated slip is preferably reduced to improve the efficiency of the electric motor 10. If the multiple electric motors 10 are indeed in the configuration of Fig.If the motor is operated with a specific amount of rated slip, the difference in the rotating wheel diameter is ensured. If the diameter of each rotating wheel 16 is strictly controlled, the electric motor 10 can theoretically be designed with low rated slip and high efficiency. However, in this case, the maintenance of the rotating wheel 16 becomes complicated. A method for detecting a large difference in the wheel diameter is discussed below. If a speed sensor is attached to the rotating shaft 14 of each electric motor 10, the increase in the wheel diameter difference can be detected according to the difference in the rotational speed of each electric motor 10. However, if only a small difference in wheel diameter is detected, a highly sensitive sensor is required, which causes a problem by increasing costs and labor for sensor maintenance. For this reason, the electric motor control system 101 of the first embodiment monitors the overheating states of each electric motor 10 and the rotary wheel diameter difference (the diameter difference of each rotary wheel 16) and accurately detects the anomaly when these anomalies are generated. <Prinzip der Anomalitätsdetektion> Fig. 4 is a diagram showing an example of a machine frequency ωrs and DC current magnitudes Ir1, Ir2 during acceleration. In Fig. 4, the machine frequency ωr represents an example of the machine frequency during the acceleration of the electric motors 10⁻¹, 10⁻². Generally, a difference is generated between the machine frequencies ωrs⁹, ωrs⁲ of the electric motors 10⁻¹, 10⁻²; however, the machine frequency ωrs in Fig. 4 is assumed to be one of the machine frequencies ωrs⁹, ωrs⁲. The machine frequency ωrs increases with time, as shown in Fig. 4. Predetermined synchronous speeds fL⁻¹, fL⁻², fH⁻¹, fH⁻² during acceleration exhibit a relationship of "fL⁻¹ < fL⁻² < fH⁻¹ < fH⁻²". Hereinafter, a range of the machine frequency ωrs of "fL⁻¹ < ωrs < fL⁻²" is referred to as the "low-speed range", and a range of the machine frequency ωrs of "fH⁻¹ < ωrs <fH2“ wird als „Hochdrehzahlbereich“ bezeichnet.A period in which the machine frequency ωrs belongs to the low-speed range is called the low-speed range period TL, and a period in which the machine frequency ωrs belongs to the high-speed range is called the high-speed range period TH. The DC current values ​​Ir101 and Ir201 are an example of the DC current values ​​Ir1 and Ir2 (see Fig. 2) during the acceleration of the machine frequency ωrs. In this example, it is assumed that the temperature of electric motor 10-2 (see Fig. 1) is higher than the temperature of electric motor 10-1 and that the impeller diameter difference between electric motors 10-1 and 10-2 is zero. Since the temperature of electric motor 10-2 is higher than that of electric motor 10-1, the DC current value Ir201 for electric motor 10-2 is smaller than the DC current value Ir101 for electric motor 10-1. Hereinafter, "Ir1 - Ir2" in the low-speed period TL is referred to as the "low-speed current difference IL". The difference between "Ir1-Ir2" in the high-speed period TH is referred to as the "high-speed current difference IH". For the small difference in wheel diameter, the low-speed current difference IL and the high-speed current difference IH have essentially the same value.The DC current values ​​Ir102 and Ir202 are another example of the DC current values ​​Ir1 and Ir2 during the acceleration of the machine frequency ωrs. In this example, it is assumed that the temperatures of the electric motors 10-1 and 10-2 (see Fig. 1) are equal and that a difference in the rotor diameter is generated. That is, the diameter of rotor 10-1 is a predetermined reference value, and the diameter of rotor 10-2 is smaller than the reference value. Since rotor 10-2 has the smaller diameter, the DC current value Ir202 is slightly smaller than the DC current value Ir102, even during the low-speed period TL. However, the low-speed current difference IL, which is the difference between the DC current values ​​Ir202 and Ir202, remains at a small value.In the high-speed range period T, on the other hand, the influence of the rotary wheel diameter difference appears strong and the high-speed range current difference IH becomes a relatively large value. As can be seen in Fig. 4, the influence of the electric motor resistance value RT, namely the temperature, is dominant in the low-speed current difference IL, and the influence of the impeller diameter difference is dominant in the high-speed current difference IH. Consequently, the heat generated by the electric motor 10 can be detected based on the measurement result of the low-speed current difference IL to provide a warning. Similarly, the impeller diameter difference can be detected based on the measurement result of the high-speed current difference IH to provide a warning. However, in the actual machine, the influence of the heat generated by the electric motor 10 and the influence of the impeller diameter difference are generated simultaneously.For this reason, the influence of the heat generated by the electric motor 10 and the influence of the difference in the rotating wheel diameter are preferably separated from each other in order to accurately evaluate the influence of the heat generated by the electric motor 10 and the influence of the difference in the rotating wheel diameter. Fig. 5 shows another example of the DC current values ​​Ir1, Ir2 during acceleration. Since the machine frequency ωrs changes with time in the same way as in Fig. 4, a description is omitted. In Fig. 5, the DC current values ​​Ir103 and Ir203 are shown, as is another example of the DC current values ​​Ir1 and Ir2 during the acceleration of the machine frequency ωrs (see Fig. 4). In this example, the temperature of electric motor 10-1 is assumed to be the reference temperature, and the temperature of electric motor 10-2 is assumed to be the reference temperature + 70 °C. The diameter of the rotating wheel 16-1 (see Fig. 1) is assumed to be a predetermined reference value, and the diameter of the rotating wheel 16-2 is assumed to be smaller than this reference value. Since the temperature of electric motor 10-2 is higher than the temperature of electric motor 10-1, the low-speed current difference IL is sufficiently large. Because the diameter of impeller 16-2 is smaller than the diameter of impeller 16-1, the difference between the DC current values ​​Ir103 and Ir203 increases over time and becomes a large value in the high-speed current difference IH. The high-speed current difference IH is a value influenced by both the temperature difference and the impeller diameter difference between electric motors 10-1 and 10-2. In this case, the corrective high-speed current difference IQ (not shown) can be obtained from "IQ = |IH| - |IL|". The corrective high-speed current difference IQ has a value where the influence of the impeller diameter difference appears, while the influence of the temperature difference is prevented. In Fig. 5, DC current values ​​Ir104, Ir204 are shown, along with another example of DC current values ​​Ir1, Ir2 during the acceleration of the machine frequency ωrs (see Fig. 4). In this example, the temperature of electric motor 10-1 is assumed to be the reference temperature, and the temperature of electric motor 10-2 is assumed to be the reference temperature + 70 °C. The diameter of the rotating wheel 16-1 (see Fig. 1) is assumed to be smaller than a predetermined reference value, and the diameter of the rotating wheel 16-2 is assumed to be the reference value. Since the temperature of electric motor 10-2 is higher than the temperature of electric motor 10-1, the absolute value |IL| of the low-speed range current difference is sufficiently large. Since the diameter of the rotating wheel 16-2 is smaller than the diameter of the rotating wheel 16-1, the difference between the DC current values ​​Ir104, Ir204 decreases over time and vice versa.In this case, the correction high-speed range current difference IQ (not shown) can be obtained from "IQ = |IH| + |IL|". The correction high-speed range current difference IQ also has a value where the influence of the impeller diameter difference appears, while the influence of the temperature difference is prevented. In the example shown in Fig. 5, it is assumed that one of the temperatures of the electric motors 10-1, 10-2 is a reference temperature, while the other is higher than the reference temperature. However, if the cooling functions of the electric motors 10-1, 10-2 deteriorate simultaneously, the difference between the DC current values ​​Ir1, Ir2 sometimes appears insignificant. In such cases, it is difficult to detect the overheating states of the electric motors 10-1, 10-2 based solely on the difference between the DC current values ​​Ir1, Ir2. An example is described with reference to Fig. 6. Fig. 6 is a view that shows yet another example of the DC current values ​​Ir1, Ir2 and the proportional signal PLL_P1 during acceleration. In Fig. 6, the DC current values ​​Ir105 and Ir205 are examples of the DC current values ​​Ir1 and Ir2 when both electric motors 10-1 and 10-2 are at the reference temperature, while the difference in wheel diameter is 0. The DC current values ​​Ir106 and Ir206 in Fig. 6 are examples of the DC current values ​​Ir1 and Ir2 when both electric motors 10-1 and 10-2 are at the reference temperature +70 °C, while the difference in wheel diameter is 0. Since in the example a significant difference between the difference value “Ir105- Ir205” and the difference value “Ir106- Ir206” does not appear, it is difficult to detect the overheating states of the electric motors 10-1, 10-2 on the basis of the difference between the difference value “Ir105- Ir205” and the difference value “Ir106- Ir206”.Referring again to Fig. 3, the difference value “θi* - θi” is multiplied by a proportional gain KpPLL and an integral gain KiPLL, respectively, in the multipliers 62 and 64 of the phase operating unit 60. The gains KpPLL and KiPLL are set such that the difference value “θi* - θi” converges as soon as possible when the corresponding electric motor 10 is at the reference temperature. However, if the actual temperature of the electric motor 10 deviates from the reference temperature, the proportional gain KpPLL and the integral gain KiPLL move away from their optimal values ​​at this temperature, so that the fluctuation range of the proportional signal PLL_P becomes larger compared to the case where the proportional signal PLL_P is at the reference temperature. For this reason, by monitoring the fluctuation amplitude of the proportional signal PLL_P, it can be detected whether the corresponding electric motor 10 is in an overheating state. The proportional signal PLL_P105 in Fig. 6 shows an example of a waveform of the proportional signal PLL_P1 (that is, the proportional signal PLL_P for the electric motor 10-1) in the low-speed period TL (see Fig. 4). At this point, the temperature of the electric motor 10-1 is at the reference temperature. The maximum value of the proportional signal PLL_P1 in the low-speed period TL is called PLL_P1max, the minimum value is called PLL_P1min, and the difference between the maximum and minimum values ​​is called the low-speed proportional signal amplitude HL. The low-speed proportional signal amplitude HL for the proportional signal PLL_P105 is specifically called the low-speed proportional signal amplitude HL105. The proportional signal PLL_P106 in Fig. 6 provides another example of the waveform of the proportional signal PLL_P1 in the low-speed period TLan (see Fig. 4). At this point, the temperature of the electric motor 10-1 is at the "reference temperature +70 °C". The low-speed proportional signal amplitude HL for the proportional signal PLL_P106 is specifically referred to as the low-speed proportional signal amplitude HL106. When the proportional signals PLL_P105 and PLL_P106 are compared, a significant difference between the low-speed proportional signal amplitudes HL105 and HL106 is observed. Consequently, the overheating condition of the electric motor 10 can be detected by monitoring the low-speed proportional signal amplitude HL106. Sometimes the low-speed proportional signal amplitudes HL of electric motors 10-1, 10-2 are referred to as “HL1”, “HL2”. Fig. 7 is a view that illustrates an example of state determination based on the low-speed and high-speed current differences IL, IH. That is, Fig. 7 is a comprehensive table of the contents described with reference to Fig. 4, Fig. 5 to Fig. 6. In Fig. 7, it is determined that the impeller diameter difference between impellers 16-1, 16-2 is "normal" when both the low-speed range and high-speed range correction current differences IL, IQ are small. On the other hand, the temperature rise of the electric motors 10-1, 10-2 is determined according to the low-speed range proportional signal amplitude HL. That is, it is determined that the signal for the small low-speed range proportional signal amplitude HL is normal (see, for example, HL105 in Fig. 6). Conversely, it is determined that the signal for the large low-speed range proportional signal amplitude HL is anomalous (in the overheating state) (see, for example, HL106 in Fig. 6).It can be determined that one of the electric motors 10 is in an overheating (anomalous) state, while the impeller diameter difference between the two electric motors 10 is normal when the low-speed range current difference IL is large, while the high-speed range correction current difference IQ is small. It can be determined that the anomaly in the impeller diameter difference between the two electric motors is generated when the low-speed range current difference IL is small, while the high-speed range correction current difference IQ is large. On the other hand, the temperature rise of the electric motors 10-1, 10-2 is determined according to the low-speed range proportional signal amplitude HL.It can be determined that the anomaly is generated both in the temperature rise of one of the electric motors 10 and in the rotary wheel diameter difference between the two electric motors 10 when both the low-speed range and correction high-speed range current differences IL, IQ are large. <Funktionsweise der ersten Ausführungsform> Fig. 8 is a flowchart illustrating an anomaly detection routine performed by the monitoring device 40. The anomaly detection routine is performed at each predetermined sampling period when the electric motors 10-1, 10-2 are currently accelerated. In Fig. 8, when the processing continues to step S2, a current measurement processing is performed. That is, in the monitoring device 40 (see Fig. 2) the current calculation units 42-1, 42-2 detect the current detection values ​​Iu1, Iw1, 1u2, and Iw2 from the current sensor 41 (see Fig. 1). When processing continues to step S4, the current calculation units 42-1 and 42-2 calculate the states of the electric motors 10-1 and 10-2. Specifically, the current calculation units 42-1 and 42-2 output the DC current values ​​Ir1 and Ir2, the proportional signals PLL_P1 and PLL_P2, and the machine frequencies ωrs1 and ωrs2. When processing continues to step S5, the monitoring device 40 determines the speed range of the machine frequencies ωrs1 and ωrs2, and subsequent processing branches according to this determination result. If both machine frequencies ωrs1 and ωrs2 are in the low-speed range, specifically if both "fL1 < ωrs1 < fL2" and "fL1' < ωrs2 < fL2" are satisfied, the processing steps of S6 are executed. If both machine frequencies ωrs1 and ωrs2 are in the high-speed range, specifically if both "fH1 < ωrs1 < fH2" and "fH1 < ωrs2 < fH2" are satisfied, the processing steps of S20 are executed. If neither of these conditions is met, the anomaly detection routine processing terminates. (Processing in the low-speed range) In step S6, the feature quantity extraction unit 44 (see Fig. 2) performs low-speed feature quantity extraction processing. That is, the low-speed current difference IL = Ir1 - Ir2 and the low-speed proportional signal differences HL1, HL2 (HL in Fig. 6) of the electric motors 10-1, 10-2 are calculated. When the processing proceeds to step S8, the anomaly detection unit 48 determines whether the absolute value |IL| of the low-speed current difference IL exceeds a predetermined threshold value ILT. If the result is determined to be "Yes", processing proceeds to step S10 to determine whether "Ir1 > Ir2" is satisfied. If the result in step S10 is "Yes", processing proceeds to step S12, and the anomaly detection unit 48 outputs an electric motor overheating alarm signal to the outside, indicating that the electric motor 10-2 is in an overheating state. Conversely, if the result in step S10 is determined to be "No", processing proceeds to step S14, and the anomaly detection unit 48 outputs the electric motor overheating alarm signal to the outside, indicating that the electric motor 10-1 is in an overheating state. If the absolute value |IL| is less than or equal to the threshold value ILT, it is determined as "No" in step S8, and processing proceeds to step S16. In step S16, the anomaly detection unit 48 determines whether "HL1 > GL and HL2 > GL" are satisfied. As used here, GL is a predetermined threshold constant that determines whether the amplitude of the proportional signal PLL_P is too large. If it is determined as "Yes" in step S16, processing proceeds to step S18, and the anomaly detection unit 48 outputs the electric motor overheat alarm signal to the outside, indicating that both electric motors 10-1 and 10-2 are in an overheating state. If it is determined as "No" in step S16, the processing of the anomaly detection routine ends. In step S16 described above, it is determined that both electric motors 10-1 and 10-2 are in an overheating state if both "HL1 > GL" and "HL2 > GL" are satisfied. At this point, if only one of "HL1 > GL" and "HL2 > GL" is satisfied, the determination that the corresponding electric motor is in an overheating state is considered. However, since the overheating state of one of the electric motors is also determined using the low-speed range current difference IL (step S8), if the determination is based on the low-speed range proportional signal amplitude HL, two determinations are made for the same event (the overheating state of one of the electric motors).For this reason, in the first embodiment, the overheating state of one of the electric motors is detected using the low-speed range current difference IL (step S8) and the overheating states of both electric motors 10-1, 10-2 are detected on the basis of the low-speed range proportional signal amplitude HL (step S16). (Processing at high speeds) When both machine frequencies ωrs1, ωrs2 are in the high-speed range, processing continues through step S5 described above to step S20, and the high-speed range feature size extraction process is performed by the feature size extraction device 44 (see Fig. 2). That is, the high-speed range current difference IH = Ir1 - Ir2 is calculated. When processing continues to step S22, the feature size extraction device 44 performs a current value correction process to calculate the correction high-speed range current difference IQ. At this point, the high-speed range current difference correction IQ is set to "IH - IL" for "IH > 0", and the high-speed range current difference correction IQ becomes "IL - IH" for "IH < 0". As described above, the anomaly detection routine is performed during the acceleration of the electric motors 10⁻¹, 10⁻². Therefore, when the high-speed processing of step S20 is performed, the low-speed processing in step S6 is performed before the high-speed processing of step S20, and the low-speed range current difference IL is calculated. The low-speed range current difference IL used in step S22 is the low-speed range current difference IL measured in the low-speed range, which appears immediately before the high-speed current range. When processing proceeds to step S24, the anomaly detection unit 48 determines whether "IQ > IQH" is satisfied. As used here, IQH is a predetermined threshold constant that determines whether the correction high-speed range current difference IQ is excessive. If it is determined as "No," the anomaly processing routine terminates. If, on the other hand, it is determined as "Yes," processing proceeds to step S26, and the anomaly detection unit 48 outputs the rotary wheel diameter differential alarm, indicating that the rotary wheel diameter difference between rotary wheels 16-1 and 16-2 is anomalous, and the anomaly detection routine terminates. The detailed contents of the rotary wheel diameter differential alarm signal are determined according to the values ​​of the low-speed range and high-speed range current differences IH. For "IL > 0 and IH > 0", the content of the rotary diameter differential alarm signal indicates that "the diameter of rotary wheel 16-2 is excessively small". For "IL > 0 and IH < 0", the content of the rotary diameter differential alarm signal indicates that "the diameter of rotary wheel 16-2 is excessively large". For "IL < 0 and IH < 0", the content of the rotary diameter differential alarm signal indicates that "the diameter of rotary wheel 16-1 is excessively small". For "IL < 0 and IH > 0", the content of the rotary diameter differential alarm signal indicates that "the diameter of rotary wheel 16-1 is excessively large". <Effekte der ersten Ausführungsform> As described above, in the first embodiment the electric motor monitoring device (40) comprises the calculation unit (42), which calculates the rotational speed (ωrs) of the corresponding electric motor (10) according to each current value (Iu, Iw), and the determination unit (44, 48), which detects the overheating state of any of the electric motors (10) and / or the anomaly of the diameter difference between the multiple rotating wheels (16) on the basis of the current values ​​(Iu, Iw) and the rotational speed (ωrs). Consequently, in the first embodiment, the overheating condition of the electric motor (10) or the anomaly in the diameter difference between the several rotating wheels (16) can be detected based on the current values ​​(Iu, Iw). That is, the temperature sensor, the speed sensor, and the like can be omitted, thus preventing problems in advance and saving maintenance time. The determination units (44, 48) further include the function of calculating the low-speed range current difference (IL), which is the difference between the maximum value and the minimum value in the several DC current amounts (Ir) when the rotational speeds (ωrs) of the several electric motors (10) are in the low-speed range, which is a predetermined speed range, and the function of detecting the overheating state of the electric motor (10), which corresponds to the minimum DC current amount (Ir), on the basis of the low-speed range current difference (IL). Consequently, the overheating state can be determined based on the low-speed range current difference (IL). The processing unit (42) comprises the phase detector (54), which obtains the AC phase angle detection value (θi*) based on the current value (Iu, Iw) for any one of the electric motors (10), and the PLL processing unit (56, 60, 72), which calculates the proportional integration on the difference value (θi* - θi) between the input AC phase angle (θi) and the AC phase angle detection value (θi*), and outputs the AC phase angle (θi) such that the difference value (θi* - θi) becomes smaller. The processing unit (42) outputs the proportional signal (PLL_P) that is proportional to the difference value (θi* - θi). The determination units (44, 48) have the function of detecting the overheating state in one of the electric motors (10) on the basis of the proportional signal (PLL_P). Consequently, the overheating condition that is generated simultaneously in the multiple electric motors (10) can be detected. The determination units (44, 48) further include the function of calculating the high-speed range current difference (IH), which is the difference between the maximum value and the minimum value in the several DC current amounts (Ir) when the rotational speeds (ωrs) of the several electric motors (10) are in the high-speed range, which is a predetermined rotational speed range, and the function of detecting the rotational wheel diameter difference of the several electric motors (10) on the basis of the high-speed range current difference (IH). Consequently, the difference in the diameter of the rotating wheel between the multiple electric motors (10) can be detected. The detection unit (44, 48) emits the alarm signal when the overheating condition of any of the electric motors (10) and / or the anomaly of the diameter difference between the several rotating wheels (16) is detected. Consequently, a manager can be notified of various anomalies. [Second embodiment] Fig. 9 is a block diagram illustrating an electric motor control system 102 according to a second embodiment of the present invention. In the following description, components corresponding to the components of the first embodiment are designated with the same reference numeral, and the description is sometimes omitted. In Fig. 9, the electric motor control system 102N (N is a natural number of 3 or more) comprises electric motors 10-1 to 10-N and N rotary wheels 16-1 to 16-N connected to the N electric motors 10-1 to 10-N, with rotary shafts 14-1 to 14-N inserted between them. Two current sensors 41 (2N total) are attached to the U-phase and W-phase of each of the electric motors 10-1 to 10-N, and current detection values ​​Iu1 to IuN, Iw1 to IwN are supplied to a monitoring device 150 (electric motor monitoring device). Although a configuration of the monitoring device 150 is not shown, N current calculation units are provided instead of the two current calculation units 42-1, 42-2 in the monitoring device 40 of the first embodiment (see Fig. 2). The low-speed and high-speed current difference IL, IH (see Fig. 4), calculated by the feature quantity extraction device 44, is equal to a result in which the minimum value is subtracted from the maximum value of the N-system DC current magnitudes Ir1 to IrN in the low-speed or high-speed range. The other configurations and operations of the second embodiment are essentially the same as those of the first embodiment. [Third embodiment] Fig. 10 is a block diagram illustrating an electric motor control system 103 according to a third embodiment of the present invention. In the following description, components corresponding to the components of the first and second embodiments are designated with the same reference numeral, and sometimes the description is omitted. In Fig. 10, the electric motor control system 103 includes a monitoring device 160 (electric motor monitoring device) instead of the monitoring device 40 (see Fig. 2) of the first embodiment. The monitoring device 160 is essentially the same as the monitoring device 40. However, the anomaly detection unit 48 outputs the electric motor overheating alarm signal and the rotary wheel diameter differential alarm signal (see Fig. 2) and issues a control instruction to the command generator 32 of the drive device 20, as required. As used here, for example, the control instruction is an instruction to stop or slow down the electric motors 10-1, 10-2, thereby enabling the electric motors 10-1, 10-2 to be operated more safely and efficiently. As described above, in the third embodiment the determination unit (44, 48) issues the control instruction to change the control state to the control unit (30) when the overheating state of any of the electric motors (10) and / or the anomaly of the diameter difference between the multiple rotary wheels (16) is detected. Consequently, the control state in the control unit (30) can be changed to the appropriate state. (Fourth embodiment) Fig. 11 is a block diagram illustrating an electric motor control system 104 according to a fourth embodiment of the present invention. In the following description, components corresponding to the components of the first to third embodiments are designated with the same reference numeral, and sometimes the description is omitted. In Fig. 11, the electric motor control system 104 comprises a drive and a monitoring device 170, the electric motors 10-1, 10-2 and rotary wheels 16-1, 16-2 which are connected to the electric motors 10-1, 10-2, with the rotary shafts 14-1, 14-2 inserted between them. The drive and monitoring device 170 comprises the control unit 30, the inverter 22, an anomaly detector 180 (electric motor monitoring device) and an averaging unit 182. The averaging unit 182 calculates an average of the current detection values ​​Iu1, Iu2 and an average of the current detection values ​​Iw1, Iw2, and outputs the averages as current detection values ​​Ius, Iwsaus. The configurations of the control unit 30 and the inverter 22 are the same as those of the first embodiment (see Fig. 1), and the configuration of the anomaly detector 180 is the same as that of the monitoring device 160 (see Fig. 10) of the third embodiment. Consequently, the drive and monitoring device 170 of the fourth embodiment have a function in which the functions of the drive device 20 and the monitoring device 160 of the third embodiment are combined. The fourth embodiment can also be configured by adding the anomaly detector 180 and the averaging unit 182 to the existing drive device 20 (see Fig. 10). [Fifth embodiment] Fig. 12 is a block diagram illustrating a steel rolling system 105 according to a fifth embodiment of the present invention. In the following description, components corresponding to the components of the first to fourth embodiments are designated with the same reference numeral, and sometimes the description is omitted. In Fig. 12, the steel rolling system 105 comprises the N (N is a natural number of 3 or more) electric motors 10-1 to 10-N, the N rotary shafts 14-1 to 14-N and the N rotary wheels 16-1 to 16-N, the drive device 20 and a monitoring device 150. The steel rolling system 105 also includes a table, called the hot runner table 200, which is arranged horizontally. Each of the rotary wheels 16-1 to 16-N includes a bearing 210 and a roller 220. The steel rolling system 105 of the fifth embodiment is provided in a single stage downstream of a finishing mill (not shown), and a high-temperature steel sheet (not shown) is conveyed from the finishing mill. The conveyed steel sheet is placed between the roller 220 and the hot runner table 200, and the roller 220 is rotated by the electric motors 10-1 to 10-N, thereby rolling the steel sheet. The rollers 220 of the rotary wheels 16 are all driven under the same conditions. Consequently, the electric motors with the same specifications are used for the electric motors 10-1 to 10-N, which are the drive sources for the rollers 220. The other configurations of the fifth embodiment are the same as those of the second embodiment (see Fig. 9). The steel sheet conveyed on the hot runner table 200 is at a considerably high temperature, and the bearings 210, couplings (not shown), and electric motors 10 are subjected to strict operating conditions. Therefore, to prevent situations such as line stoppages, sensors such as a temperature sensor and a speed sensor are generally installed at each individual diagnostic target, such as the bearing 210 and the electric motor 10, and the detection signal is analyzed to diagnose the anomaly. However, in the fifth embodiment, the need to install the temperature sensor, speed sensor, and the like at each electric motor 10 is eliminated, thus preventing situations such as a steel rolling line stoppage in advance and at low cost. [Sixth embodiment] Fig. 13 is a schematic view representing a railway vehicle 310 according to a sixth embodiment of the present invention. In the following description, components corresponding to the components of the first to fifth embodiments are designated with the same reference numeral, and sometimes the description is omitted. In Fig. 13, a railway vehicle 310 comprises a vehicle body 312, chassis 314, 315, wheels 316, 317, 318, 319, and a drive and monitoring device 320. The chassis 314, 315 support the vehicle body 312. The wheels 316, 317 are mounted on the chassis 314, and the wheels 318, 319 are mounted on the chassis 315. Fig. 14 is a bottom view of the chassis 315. Two electric motors 10-1 and 10-2 are mounted on chassis 315. These motors are connected to axles 334 and 344, with reduction gears 332 and 342 inserted between them. A pair of left and right wheels 318 is mounted on axle 334, and a pair of left and right wheels 319 is mounted on axle 344. Although the details of chassis 314 are not shown, chassis 314 is configured similarly to chassis 315. That is, two electric motors 10-3 and 10-4 (not shown) are mounted on chassis 314, and the railway vehicle 310 comprises a total of four electric motors 10-1 to 10-4. Returning to Fig. 13, the drive and monitoring device 320 is configured similarly to the drive and monitoring device 170 of the fourth embodiment (see Fig. 11), and the drive and monitoring device 320 drives the four electric motors 10 contained in the railway vehicle 310 and monitors the condition of the electric motors 10, etc. That is, if one or more electric motors 10 overheat, the drive and monitoring device 320 detects the overheating of the electric motor 10 and outputs the electric motor overheating alarm signal. In the example of Fig. 13, the wheels 318, 319 have the turning wheel diameters d1, d2 and exhibit a relationship of “d1 < d2”.When the difference in the diameter of the rotating wheel between the wheels 318, 319 becomes large, the drive and the monitoring device 320 detect, as in the first to fifth embodiments, that the difference in the diameter of the rotating wheel is large and outputs the difference in the diameter of the rotating wheel alarm signal. When the temperature of the electric motor 10 rises, the specific resistance of its primary and secondary resistors increases, and the electric motor resistance value RT increases. Consequently, the torque of the multiple electric motors 10, driven in parallel, is concentrated in the electric motor with the lowest resistance value RT, creating a phenomenon where the railway vehicle 310 cannot accelerate uniformly due to the lack of average torque. If this phenomenon persists, there is a possibility that the standard electric motor 10 will also fail due to long-term overload. In the sixth embodiment, this phenomenon can be detected to optimize the electric motor, thus preventing a major problem in advance and achieving more efficient power savings for the electric motor 10. The method for managing the wheel diameter of a railway vehicle depends on the railway company; however, maintenance is generally carried out in such a way that the wheel diameter difference falls within a specified value for the vehicle. While strictly managing the wheel diameter difference of each wheel can prevent output imbalances in the multiple electric motors 10, this makes maintenance cumbersome and impractical. In the sixth embodiment, the need to install sensors, such as temperature and speed sensors, for each electric motor 10 is eliminated, and the overheating condition of the electric motor and the anomaly in the wheel diameter difference can be detected solely from the alternating current flowing through the electric motor.Since the drive pattern for the railway vehicle is essentially constant, the overheating of the electric motor 10 can be accurately detected by a temporal change in the evaluation information in the same running state or by statistical processing of the vehicle occupancy data. [Seventh embodiment] Fig. 15 is a side view showing a configuration of a vehicle structure according to a seventh embodiment of the present invention. In the following description, components corresponding to the components of the first to sixth embodiments are designated with the same reference numeral, and sometimes the description is omitted. In Fig. 15, a vehicle assembly 300 comprises railway vehicles 310, 360 and a connecting unit 350 between wagons, which connects the railway vehicles 310, 360. The railway vehicle 310 is the same as that of the sixth embodiment (see Fig. 13). The railway vehicle 360 ​​is configured similarly to the railway vehicle 310. This means that the railway vehicle 360 ​​comprises chassis 364, 365, a vehicle body 362, wheels 366, 367, 368, 369, and a drive and monitoring device 370. The chassis 364, 365 support the vehicle body 362. The wheels 366, 367 are mounted on the chassis 364, and the wheels 368, 369 are mounted on the chassis 365. In the seventh embodiment, the drives and monitoring devices 320, 370 exchange information with each other and detect anomalies in the overheating state of the electric motor and the difference in wheel diameter of the wheels 316 to 319, 366 to 369 based on information about the vehicle itself and information about the other vehicle. Assuming that each of the railway vehicles 310, 360 is a "group", the vehicle assembly 300 of the seventh embodiment is a "multi-group drive system" and determines the anomaly of the electric motor or the difference in wheel diameter based on a comparison with the other group. [Modifications] The present invention is not limited to the embodiments described above, but various modifications are possible. The embodiments described above are examples for the purpose of facilitating understanding of the present invention, but do not necessarily include all described configurations. Part of the configuration of one embodiment can be replaced by a configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Another configuration can be deleted from or added to, and replaced by, a part of the configuration of each embodiment.The control and information lines shown in the drawings indicate those considered necessary for the description, but do not necessarily represent all control and information lines required for the product. In fact, it can be assumed that almost all components are interconnected. Possible modifications of the above embodiments are, for example, as follows. (1) Since the hardware of the control unit 30, the monitoring devices 40, 150, 160, and the anomaly detector 180 in the above embodiments can be constructed by a general-purpose computer, the algorithm in Fig. 2 and Fig. 3 and the program in Fig. 8 can be stored in a storage medium or distributed via a transmission path. (2) The algorithm in Fig. 2 and Fig. 3 or the program in Fig. 8 is described as software processing using the program in each of the above embodiments. Alternatively, part or all of it can be replaced by hardware processing using an ASIC (application-specific integrated circuit), an FPGA (user-programmable logic gate), or the like. (3) In the configuration of Fig. 1 and the like, only one inverter 22 is provided. Alternatively, several inverters can be provided.If the inverter is provided for each electric motor 10, the electric motor 10 can be a synchronous electric motor.(4) In step S16 (see Fig. 8) of each of the above embodiments, if both “HL1> GL” and “HL2> GL” are satisfied, both electric motors 10-1, 10-2 are determined to be in an overheating state. Alternatively, if only one of “HL1> GL” and “HL2> GL” is satisfied, the corresponding electric motor can be determined to be in an overheating state. Reference symbol list 10, 10-1 to 10-N Electric motor 14 Rotating shaft 16 Rotary wheel 22 Inverter 30 Control unit 40 Monitoring device (electric motor monitoring device) 41 Current sensor 42, 42-1 to 42-N Current calculation unit (calculation unit) 44 Feature quantity extraction unit (determination unit) 48 Anomaly determination unit (determination unit, anomaly determination process) 54 Arc tangent converter (phase detector) 56 Subtractor (PLL calculation unit) 60 Phase calculation unit (PLL calculation unit, rotational speed calculation unit, rotational speed calculation process) 72 Integrator (PLL calculation unit) 101 to 104 Electric motor control system 105 Steel rolling system 150,160 Monitoring device (electric motor monitoring device) 180 Anomaly detector (electric motor monitoring device) 220 Roller θi* AC phase angle detection value θi*-θi Difference value θi AC phase angle τ* Torque command ωrs Machine frequency (rotational speed) IH High-speed range current difference IL Low-speed range current difference IQ Correction high-speed range current difference (high-speed range current difference) Iq* Torque current command value Iq Torque current detection value Iu, Iw, Iu1 to IuN, Iw1 to IwN Current detection value (current value) PLL_P Proportional signal,

Claims

Electric motor monitoring device (40, 150, 160, 180) for an electric motor control system (101), wherein the electric motor control system (101) comprises: several electric motors (10-1 to 10-N) configured as three-phase induction electric motors, each comprising a rotating shaft (14); several rotary wheels (16-1 to 16-N) connected to the several rotating shafts (14-1 to 14-N) and connected via a conveying object (12); an inverter (22) connected in parallel with the several electric motors (10-1 to 10-N) and supplying them with an alternating current; a control unit (30) controlling the inverter (22);and a current sensor (41) that detects a current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) flowing through each of the electric motors (10-1 to 10-N), the electric motor monitoring device (40, 150, 160, 180) comprising: a calculation unit (42, 42-1 to 42-N) configured to be connected to the electric motor control system (101) and to calculate a rotational speed (ωrs) of the corresponding electric motor (10-1 to 10-N) according to each of the current values ​​(Iu, Iw, Iu1 to IuN, Iw1 to IwN);and a determination unit (44, 48) configured to detect an overheating condition of any of the electric motors (10-1 to 10-N) and / or an anomaly of a diameter difference between the several rotary wheels (16-1 to 16-N) based on the current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) and the rotational speed (ωrs), wherein the computation unit (42, 42-1 to 42-N) computes several DC current values ​​(Ir1 to IrN') corresponding to the electric motors (10-1 to 10-N) based on the current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN), and the determination unit (44, 48) comprises: a function for calculating a low-speed range current difference (IL) that calculates a difference between a maximum value and a minimum value in the several DC current amounts (Ir1 to IrN') is when the rotational speeds (ωrs) of the several electric motors (10-1 to 10-N) are in a low-speed range which is a predetermined speed range;and a function for detecting the overheating state of the electric motor (10-1 to 10-N), corresponding to the minimum DC current amount, based on the low-speed range current difference (IL); and a function for calculating a high-speed range current difference, which is a difference between a maximum value and a minimum value in the several DC current amounts (Ir1 to IrN') when the rotational speeds (ωrs) of the several electric motors (10-1 to 10-N) are in a high-speed range, which is a predetermined speed range; and a function for detecting a rotary wheel diameter difference between the several electric motors (10-1 to 10-N) based on the high-speed range current difference. Electric motor monitoring device (40, 150, 160, 180) according to claim 1, wherein the calculation unit (42, 42-1 to 42-N) comprises: a phase detector (54) which obtains an AC phase angle detection value (θi*) based on the current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) for any of the electric motors (10-1 to 10-N); and a PLL calculation unit (56, 60, 72) that performs proportional integration on a difference value between the input AC phase angle and the AC phase angle detection value (θi*) and outputs the AC phase angle (θi) such that the difference value becomes smaller, the calculation unit (42, 42-1 to 42-N) outputs a proportional signal (PLL_P) that is proportional to the difference value, and the determination unit (44, 48) has a function for detecting the overheating state of one electric motor (10-1 to 10-N) on the basis of the proportional signal (PLL_P). Electric motor monitoring device (40, 150, 160, 180) according to claim 1, wherein the detection unit (44, 48) outputs an alarm signal when the overheating condition of any of the electric motors (10-1 to 10-N) and / or the anomaly of the diameter difference between the multiple rotary wheels (16-1 to 16-N) is detected. Electric motor monitoring device (40, 150, 160, 180) according to claim 1, wherein the determining unit (44, 48) issues a control instruction to change the control state to the control unit (30) when the overheating state of any of the electric motors (10-1 to 10-N) and / or the anomaly of the diameter difference between the multiple rotary wheels (16-1 to 16-N) is detected. Electric motor control system (101) comprising: several electric motors (10-1 to 10-N) configured as three-phase induction electric motors, each comprising a rotating shaft (14); several rotary wheels (16-1 to 16-N) connected to the several rotating shafts (14-1 to 14-N) and connected via a conveying object (12); an inverter (22) connected in parallel with the several electric motors (10-1 to 10-N) and supplying them with an alternating current; a control unit (30) controlling the inverter (22); a current sensor (41) detecting a current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) flowing through each of the electric motors (10-1 to 10-N); and an electric motor monitoring device (40, 150, 160, 180) according to one of claims 1 to 4. Steel rolling system (105) comprising: an electric motor control system (101) according to claim 5, wherein each of the rotary wheels comprises a roller that presses a steel sheet. An electric motor monitoring method carried out by an electric motor monitoring device (40, 150, 160, 180) in an electric motor control system (101) comprising: several electric motors (10-1 to 10-N) configured as three-phase induction electric motors, each comprising a rotating shaft (14); several rotating wheels (16-1 to 16-N) connected to the several rotating shafts (14-1 to 14-N) and connected via a conveying object (12); an inverter (22) connected in parallel with the several electric motors (10-1 to 10-N) and supplying them with an alternating current;a current sensor (41) that detects a current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) flowing through each of the electric motors (10-1 to 10-N), and the electric motor monitoring device (40, 150, 160, 180), wherein the electric motor monitoring method comprises: a rotational speed calculation process for calculating a rotational speed (ωrs) of the corresponding electric motor (10-1 to 10-N) according to each of the current values ​​(Iu, Iw, Iu1 to IuN, Iw1 to IwN); a process for calculating several DC current values ​​(Ir1 to IrN') corresponding to the electric motors (10-1 to 10-N) based on the current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN);and an anomaly detection process for detecting an overheating condition of any of the electric motors (10-1 to 10-N) and / or an anomaly of a diameter difference between the multiple rotary wheels (16-1 to 16-N) based on the current value (Iu, Iw, Iu1 to IuN, Iw1 to IwN) and the rotational speed (ωrs), wherein the anomaly detection process comprises: calculating a low-speed range current difference (IL) which is a difference between a maximum value and a minimum value in the multiple DC current values ​​(Ir1 to IrN') when the rotational speeds (ωrs) of the multiple electric motors (10-1 to 10-N) are in a low-speed range which is a predetermined speed range;Detecting the overheating state of the electric motor (10-1 to 10-N), corresponding to the minimum DC current amount, based on the low-speed range current difference (IL); calculating a high-speed range current difference, which is a difference between a maximum value and a minimum value in the multiple DC current amounts (Ir1 to IrN') when the rotational speeds (ωrs) of the multiple electric motors (10-1 to 10-N) are in a high-speed range, which is a predetermined speed range; and detecting a rotary wheel diameter difference between the multiple electric motors (10-1 to 10-N) based on the high-speed range current difference.

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