PE$CRlPTION ELECTRIC MOTOR DlAGNOSlS DEVICE TECHNICAL FimLD [0001] The present invention relates to an electric motor diagnosis device which is used in, for example, a control center that is an enclosed switchboard, and which performs diagnosis as to whether or not there is an abnormality in an electric motqr. BACKGROUND ART [0002] Conventionally, a model-based failure detection system has been proposed in which measurement is performed to obtain real-time information about the motor speed and input voltages and currents of an electric motor, a result obtained by performing modeling and the result of the measurement are compared to each other, a comparison result regarding an error generated by performing subtraction on respective signale is evaluated, and the error is analyzed by a diagnostic otserver, thereby determining whether or not a failure has occurred in the electric motor (for example, Patent Document 1). CITATION LIST PATENT DOCUMENT [0003] Patent Document 1: Japanese Translation of PCT International Application Publication Mo. 2000-513097 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0004] In the conventional model-based failure detection , system, the following problems arise: since measurement is performed to obtain real-time information about the motor speed and input voltages and currents of an electric motor, and a result obtained by performing modeling and the result of the measurement are compared to each other, a reference model is required, and furthermore, since whether or not a failure has occurred is determined through comparison with the real-time information obtained by the measurement, it is difficult to apply the system to an electric motor of which the load greatly varies. [0005] The present invention has been made to solve the above problems, and an object of the present invention is to provide an electric motor diagnosis device capable of performing diagnosis as to whether or not there is an abnormality in an electric motor, by performing statistic process on load torques calculated from information about voltages and currents having been sampled and inputted, even if the load of the electric motor varies. SOLUTION TO THE PROBLEMS [0006] An ©lectric motor diagnosis device according to the present invention includes: a load torque calculation unit for calculating load torques of an electric motor with use of voltages and currents having been sampled and inputted from a main circuit to which the electric motor is connected; a histogram calculation unit for calculating an average value and a standard deviation of a probability density function of the load torques which are equal in number to samples and which are calculated by the load torque calculation unit, to obtain a histogram; a normal curve storage unit for obtaining and storing in advance a histogram of load torques in a normal state; and an abnormality determination unit for comparing the histogram in a normal state stored in the normal curve storage unit and the histogram obtained by the histogram calculation unit with each other, to determine whether or not there is an abnormality in the electric motor, EFFECT OF THE INVENTION [Q0Q7] The present invention includes: the load torque calculation unit for calculating load torques of an electric motor with use of voltages and currents having been sampled and inputted from a main circuit to which the electric motor is connected; the histogram calculation unit for calculating an average value and a standard deviation of a probability density function of the load torques which are equal in number to samples and which are calculated by the load torque calculation unit, to obtain a histogram; the normal curve storage unit for obtaining and storing in advance a histogram of load torques in a normal state; and the abnormality determination unit for comparing the histogram in a normal state stored in the normal curve storage unit and the histogram obtained by the histogram calculation unit with each other, to determine whether or not there is an abnormality in the electric motor. Therefore, the present invention exhibits an advantageous effect of obtaining an electric motor diagnosis device capable of performing diagnosis as to whether or not there is an abnormality in the electric motor, by performing statistic process on the load torques which are equal in number to samples and which are calculated from Information about voltages and currents having been sampled and inputted. BRIEF DESCRIPTION OF THE DRAWINGS [0008] [FIG. 1] FIG. 1 is a schematic configuration diagram showing an installation state of an electric motor diagnosis device according to embodiment 1 of the present invention. [FIG, 2] FIG, 2 is a block diagram showing a configuration of a logical calculation unit of the electric motor diagnosis device according to embodiment 1 of the present invention, [FIG, 3] FIG. 3 is a block diagram showing a configuration of a storage unit of the electric motor diagnosis device according to embodiment 1 of the present invention, [FIG. 4] FIG. 4 is a diagram for explaining the relationship between sampling speed, and forgetting coefficient and scale factor, in the electric motor diagnosis device according to embodiment 1 of the present invention, [FIG. 5] FIG. 5 is a flowchart for explaining an operation of the electric motor diagnosis device according to embodiment 1 of the present invention. [FIG. 6] FIG. 6 is a diagram for explaining a histogram of load torques, in the eisctric motor diagnosis ■ device according to embodiment 1 of the present invention. [FIG. 7] FIG, 7 is a diagram for explaining a case where a peak part of a histogram in an abnormal state is shifted from that of a histogram in a normal state, in the electric motor diagnosis device according to embodiment 1 of the present invention. [FIG. 3] FIG. 8 is a diagram for explaining a Mahalanobis distance of the histogram in the electric motor diagnosis device according to embodiment 1 of the present invention. [FIG, 9] FIG. 9 is a diagram for explaining threshold-value-based determination performed with use of the Mahalanobis distance, in the electric motor diagnosis device according to embodiment 1 of the present invention. [FIG, 10] FIG. 10 is a diagram for explaining a case where there are two peak parts of a histogram in an abnormal state unlike a histogram in a normal state, in the electric motor diagnosis device according to embodiment 1 of the present invention. [FIG. 11] FIG. 11 is a diagram for explaining a peak ratio of the two peak parts of the histogram in an abnormal state, in the electric motor diagnosis device according to embodiment 1 of the present invention. [FIG, 12] FIG, 12 is a diagram for explaining threshold-value-based determination performed with use of the peak ratio, in the electric motor diagnosis device according to embodiment 1 of the present invention. DESCRIPTION OF EMBODIMENTS [0009] Hereinafter, an embodiment of the present invention will be described. In the drawings, the same or the corresponding components are denoted by the same reference characters. Embodiment 1 FIG. 1 is a schematic configuration diagram showing sin installation state of an electric motor diagnosis device according to embodiment 1 of the present invention, FIG. 2 is a block diagram showing a configuration of a logical calculation unit of the electric motor diagnosis device according to embodiment 1 of the present invention, FIG. 3 is a block diagram showing a configuration of a storage unit of the electric motor diagnosis device according to embodiment 1 of the present invention, FIG. 4 is a diagram for explaining the relationship between sampling speed, and forgetting coefficient and scale factor, in the electric motor diagnosis device according to embodiment 1 of the present invention. FIG. 5 is a flowchart for explaining an operation of the electric motor diagnosis device according to embodiment 1 of the present invention. FIG, 6 is a diagram for explaining a histogram of load torques, in the electric motor diagnosis device according to embodiment 1 of the present invention, FIG, 7 is a diagram for explaining a case where a peak part of a histogram in an abnormal state is shifted from that of a histogram in a normal state, in the electric motor diagnosis device according to embodiment 1 of the present invention. FIG. 8 is a diagram for explaining a Mahalanobis distance of the histogram in the electric motor diagnosis device according to embodiment 1 of the present invention. FIG, 9 is a diagram for explaining threshold-value-based determination performed with use of the Mahalanobis distance, In the electric motor diagnosis device according to embodiment 1 of the present invention. FIG. 10 is a diagram for explaining a case where there are two peak parts of a histogram in an abnormal state unlike a histogram in a normal state, in the electric motor diagnosis device according to embodiment 1 of the present invention. FIG. 11 is a diagram for explaining a peak ratio of the two peak parts of the histogram in an abnormal state, in the electric motor diagnosis device according to embodiment 1 of the present invention, FIG. 12 is a diagram for explaining threshold-value-based determination performed with use of the peak ratio, in the electric motor diagnosis device according to embodiment 1 of the present invention. [0010] In FIG. I, a main circuit 2 which is led in from a power system 1 is provided with; a wiring circuit breaker 3; an electromagnetic contactor 4; a voltage detector 5 such as an instrument voltage transformer for detecting, for two inter-phase portions, inter-phas© voltages of the three-phase main circuit 2; and a current detector 6 such as an instrument current transformer for detecting, for two phases, load currents of the main circuit 2, An electric motor 7 such as a three-phase induction motor which is a load is connected to the main circuit 2, and mechanical equipment 8 is driven so as to be operated by the electric motor 7, An electric motor diagnosis device 9 is used mainly in a control center that is an enclosed switchboard, and the electric motor diagnosis device 9 is composed of an input unit 10, a logical calculation unit 20, a storage unit 30, and an output unit 40, [0011] The input unit 10 is provided with: a voltage input unit 11 to which voltages detected by the voltage detector 5 are inputted at a preset sampling speed (hereinafter, referred to as "sampled and inputted"); a current input unit 12 to which currents detected by the current detector 6 are sampled and inputted; a sampling speed setting unit 13 for setting a sampling speed,* and a rating information input unit 14 to which rating information about the electric motor 7 and the like are inputted. [0012] The sampling speed setting unit 13 performs setting such that sampling is performed for 10 seconds at a sampling speed of 100 samples per second, for example. Accordingly, 1000 sampled data are to be obtained. The set sampling speed and the like are stored in a sampling speed storage unit 31 shown in FIG. 3, of the storage unit 30, [0013] In the rating information input unit 14, a power supply frequency, the rated output, the rated voltage, the rated current, the rated rotation rate, etc., of the electric motor 7, and the number of magnetic poles, a winding resistance value, etc., used for calculation of load torque, are inputted in advance, and the inputted rating information and the like are stored in a rating information storage unit 32 shown in FIG. 3, of the storage unit 30. The rating information and the like are information that can be easily acquired through reference to a catalog of the manufacturer of the electric motor 7 or a rating plate attached to the electric motor 7. it is noted that, if there are a plurality of the electric motors 7 to be diagnosed, rating information about all the electric motors 7 to be diagnosed needs to be inputted in advance. [0014] The configuration of the logical calculation unit 20 will be described with reference to FIG. 2, The logical calculation unit 20 is provided with: a dq conversion calculation unit 21 for converting voltage inputted from the voltage input unit 11 and current inputted from the current input unit 12, into voltage and current in the d-axis direction and the q~axis direction (hereinafter, referred to as dq-axis directions) suitable for calculation of load torque; a load torque calculation unit 22 for calculating load torque with use of the voltage and the current that are obtained as a result of the conversion by the dq conversion calculation unit 21; a histogram calculation unit 23 for obtaining a histogram with use of the load torques which are equal in number to samples and which are calculated by the load torque calculation unit 22; and an abnormality determination unit 24 for determining whether or not there are abnormalities in the electric motor 7 and the mechanical equipment 8, through comparison between the histogram obtain by the histogram calculation unit 23 and a histogram in a normal state stored in advance in a normal curve storage unit 33 shown in FIG, 3, of the storage unit 30, [0015] In addition, a winding resistance value calculation unit 22a for obtaining a winding resistance value Rs of a stator winding of the electric motor 7, and a forgetting coefficient calculation unit 22b and a scale factor calculation unit 22c for calculating a forgetting coefficient kf and a scale factor «, respectively, from a sampling speed, are provided as the units for obtaining and setting, in advance, data to be used in the load torque calculation unit 22. Generally, when the temperature of the stator winding of the electric motor 7 rises, the winding resistance value Rs increases, and thus, it is possible that, for example, a correlation between the temperature and the winding resistance value Rs obtained through experiments conducted by the manufacturer of electric motor 7 or the like is set in the rating information storage unit 32 in advance, and. the temperature of the stator winding at the time of sampling of voltages and currents is detected and inputted, whereby the winding resistance value Rs is accurately Obtained from the set correlation by the winding resistance value calculation unit 22a. Alternatively, the winding resistance value Rs may also be obtained by being calculated from detected voltage and current. Thus, the method for calculation by the winding resistance value calculation unit 22a is not limited to the above-described method. [0016] There is a correlation as shown in FIG. 4 between sampling speed, and forgetting coefficient kf and scale factor ex. Therefore, for example, a correlation in FIG. 4 obtained as a result of actual measurement performed in advance by the manufacturer of the diagnosis device is set as a data table in the storage unit 30, and, when a sampling speed and the like are set by the sampling speed setting unit 13, a forgetting coefficient kf and a scale factor a are obtained by the forgetting coefficient calculation unit 22b and the scale factor calculation unit 22c, respectively, with use of the stored data table, and are set and stored in the sampling speed storage unit 31 shown in FIG. 3. Once the sampling speed is first set, the sampling speed under the game condition is applied to all the subsequent diagnoses for that electric motor. [0017] The histogram calculation unit 23 is provided with: an average value calculation unit 23a for calculating the average value of the load torques which are equal in number to samples and which are obtained by the load torque calculation unit 22; and a standard deviation calculation unit 23b for calculating a standard deviation. The abnormality determination unit 24 is provided with a Mahalanobls distance determination unit 24a and a number-of-peaks determination unit 24b for determining whether or not there ia an abnormality in accordance with the state of abnormality, of which operations will be described in details later. [0018] The configuration of the storage unit 30 will be described with reference to FIG. 3. The storage unit 30 is provided with: the sampling speed storage unit 31 for storing the sampling speed set by the sampling speed setting unit 13, and the like; the rating information storage unit 32 for storing rating information about the electric motor 7 and the like inputted from the rating information input unit 14; the normal curve storage unit 33 for storing the histogram in a normal state serving as a reference for determination as to abnormality; and a histogram saving unit 34 for saving, in time series, the histograms obtained by the histogram calculation unit 23, in addition, the normal curve storage unit 33 is provided with an average value storage unit 33a and a standard deviation storage unit 33b for respectively storing an average value and a standard deviation which are the base data for the histogram in a normal state. The histogram saving unit 34 is provided with an average value saving unit 34* and a standard deviation saving unit 34b for respectively saving average values and standard deviations which are the base data for the histograms. [0019] The output unit 40 is provided with: a statistical result output unit 41 for displaying and outputting, for example, a comparison result obtained by the abnormality determination unit 24 comparing histograms in a normal state and in an abnormal state with each other, a result obtained by the Mahalanobis distance determination unit 24a performing trend analysis on Mahalanobis distances, and a result obtained by the number-of-peaks determination unit 24b performing trend analysis on peak ratios; and an alarm output unit 42 for performing alarm output by means of, for example, emission of an alarm sound or lighting of an abnormality lamp when the abnormality determination unit 24 has determined that there is an abnormality. [0020] Next, operations will be described with reference to a flowchart shown in FIG. 5. The electric motor diagnosis device 9 is activated at predetermined time intervals, e.g., every 10 minutes, and performs diagnosis as to abnormality in the electric motor 7 and the like. In step 101, voltages vuv and vvw for two inter¬phase portions are inputted from the voltage input unit 11 at a sampling speed that is set by the sampling speed setting unit 13 and that is stored in the sampling speed storage unit 31, and currents iu and iv for two phases are sampled and inputted from the current input unit 12. [0021] in step 102, the dq conversion calculation unit 21 converts the inputted voltages and currents into voltages and currents in the dq-axis directions. Calculation by the dq conversion calculation unit 21 will be described. First, since, in dq conversion of three-phase currents, only current information for two phases ig sufficient if three phases u, v, and w are balanced, a current id in the d~axis direction and a current iq in the q-axis direction are calculated with a conversion formula shown in expression (1), using the currents iu and iv for two phases inputted from the current input unit 12, [0022] [Mathematical l] [0023] similarly, also in the case of voltage, only voltage information for two phases is sufficient. However, ordinarily, voltage is often measured as line-to-line voltage. Therefore, using voltages vuv and ww for two inter-phase portions (the subscript uv denotes ""between u phase and v phase", and the subscript vw denotes "between y phase and w phase") inputted from the voltage input unit 11, a voltage vd in the d-axls direction and a voltage vq in the q-axis direction are calculated with a conversion formula shown in expression (2) , [Q024] [Mathematical 2] [0025] In step 103, the load torque calculation unit 22 calculates load torques with use of the voltages and the currents in the dq-axis directions obtained as a result of conversion by the dq conversion calculation unit 21. The calculation by the load torque calculation unit 22 will be described. The load torque calculation unit 22 calculates a load torque Te by means of expression (3) with use of; the currents id and iq in the dq-axis directions obtained by being calculated by the dq conversion calculation unit 21; the number Pp of magnetic poles of the electric motor 7, which is inputted from the rating information input unit 14 and stored in the rating information storage unit 32 in advance; and interlinkage magnetic fluxes