BACKGROUND OF THE INVENTION The present invention relates to a power converter unit control apparatus, an electric motor characteristic measurement method and a computer program which can be applied 5 in case where a power converter unit which converts electric power supplied to an AC motor is controlled to measure the electric motor characteristic of the AC motor. Generally, an AC motor which rotates a roller for moving a steel plate to a rolling mill mainly performs vector control to rotate the roller. In order to perform the vector control of the AC motor by using a vector controller, it is necessary to set control constants necessary for 10 the vector control on the basis of motor constants peculiar to the AC motor to be controlled. For this purpose, an operator obtains design values of motor constants beforehand and calculates the motor constants from measured results of various kinds of tests including a resistance measurement test, a no-load test and a constrained test to set the control constants. The motor constants contain, for example, primary resistance (ri) of DC resistance on the primary side, 15 secondary resistance (r2') of DC resistance on the secondary side, combined resistance (ra=ri+r2') on the primary and secondary sides, combined leakage inductance (La=L\+L2') on the primary and secondary sides and the like. Further, it is necessary to set rated torque current and rated excitation current in order to vector-control the AC motor and accordingly in the following description, the motor constants and rated torque current and excitation current 20 characteristic described later are named "electric motor characteristic" generically. Even in the AC motor having the same standards, an error in the motor constants sometimes occurs in each product, although the operation for calculating the motor constants for each motor is troublesome. Accordingly, the technique that the motor constants can be automatically calculated to set the control constants is demanded. For example, JP-A-62- 25 262697 discloses the technique that various measurement conditions are given to a power converter unit control apparatus in the state that an AC motor is connected to the power converter unit control apparatus and the control constants are set on the basis of the motor constants calculated in accordance with an AC voltage command value and a current detection value. 30 Further, JP-A-7-55899 discloses the method of calculating the motor constants by using a power converter unit control apparatus. In the technique disclosed in JP-A-7-55899, a voltage having any frequency is applied to an AC motor while the AC motor is stopped and the motor constants are calculated from the relation of a value of the applied voltage and a current detection value taken out from a PWM inverter to set the control constants. -3- Moreover, JP-A-6-265607 discloses the technique that a primary angular frequency command value and a primary voltage command value are applied to a motor converter unit control apparatus and a mutual inductance of an AC motor which is operated in the steady state is calculated on the basis of a current detection value detected from the AC 5 motor. SUMMARY OF THE INVENTION When an AC motor of which the motor constants are unknown is operated, there is a case where the characteristic of the AC motor can be grasped from only information described in the name plate attached to the body of the AC motor (hereinafter referred to as 10 "name plate information"). The name plate information generally describes only characteristic such as rated frequency, rated voltage and rated current. Accordingly, it is difficult to set the rated torque current and the rated excitation current from the name plate information. Furthermore, the AC motor is sometimes controlled in a weaken magnetic field within an operation range where the AC motor is operated usually. In this case, since the AC 15 motor has the characteristic that the AC motor is magnetically saturated (hereinafter referred to as "saturation characteristic"), excitation current must be changed to weaken the magnetic field in consideration of influence of magnetic saturation. However, it is difficult to set the excitation current for each speed of the AC motor in the control using the weaken magnetic field. It is an object of the present invention to make it possible to calculate motor 20 constants of an AC motor and set rated torque current and rated excitation current which are control constants necessary for vector control of the AC motor and excitation current in case where the AC motor is controlled in a weaken magnetic field from information limited in a simple method. In order to achieve the above object, according to the present invention, a first 25 coordinate transformation unit coordinate-transforms a primary angular frequency command, a d-axis voltage command and a q-axis voltage command into a three-phase AC voltage command and a pulse generation unit converts voltage and frequency of AC power supplied from an AC power supply in response to the three-phase AC voltage command and generates a pulse signal for controlling a power converter unit which drives an AC motor. Next, a second coordinate 30 transformation unit transforms a detection value of AC current outputted by the power converter unit and detected by a current detection unit into d-axis excitation current and iq-axis torque current on the basis of the primary angular frequency command. An electric motor characteristic measurement unit performs control for changing a ratio of voltage and frequency -4- supplied by the power converter unit to the AC motor by supplying the primary angular frequency command, the d-axis voltage command and the q-axis voltage command to the first coordinate transformation unit and performs measurement of primary resistance of the AC motor in a first measurement mode where the electric motor characteristic of the AC motor is 5 measured, measurement of combined resistance and combined leakage inductance of the AC motor in a second measurement mode and calculation of excitation inductance, the rated torque current and the rated excitation current of the AC motor in a third measurement mode on the basis of the d-axis excitation current and the q-axis torque current supplied from the second coordinate transformation unit, so that electric motor characteristic of the AC motor containing at 10 least a rated torque current and a rated excitation current is calculated. According to the present invention, the power converter unit is controlled to change the ratio of the voltage and the frequency supplied to the AC motor. The electric motor characteristic of the AC motor containing at least the rated torque current and the rated excitation current can be calculated in the first to third measurement modes on the basis of the d-axis 15 excitation current and the q-axis torque current supplied from the second coordinate transformation unit. Accordingly, the rated torque current and the rated excitation current can be set automatically from the limited information obtained from the name plate information, for example, to thereby reduce the time required for driving the AC motor greatly and vector-control the AC motor in accordance with the proper confrol constants. 20 BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram schematically illusfrating an inner configuration of a power converter unit confrol apparatus according to a first embodiment of the present invention; Fig. 2 is a block diagram schematically illustrating an inner configuration of an electric motor characteristic measurement unit in the first embodiment of the present invention; 25 Fig. 3 is a flow chart showing an example of first to third measurement modes (modes 1 to 3) changed when the electric motor characteristic measurement unit measures the motor characteristic of an AC motor in the first embodiment of the present invention; Fig. 4 is a graph showing an example of changing a V/f ratio while a rated frequency f is maintained to be fixed in the first embodiment of the present invention; 30 Fig. 5 is a flow chart showing an example of processing of calculating rated torque current IT, excitation current Id and excitation inductance Lm' while the AC motor is operated at rated speed (top speed) in a constant-V/f operation in the first embodiment of the present invention; -5- Fig. 6 is a graph showing the state that the V/f ratio is changed to measure voltage while the AC motor is operated at a base speed in the constant-V/f operation in a second embodiment of the present invention; Fig. 7 shows the relation of voltage, magnetic flux and excitation current at the 5 time that the AC motor is operated in weaken magnetic field in the second embodiment of the present invention; Fig. 8 is a graph showing an example of saturation characteristic of the AC motor in the second embodiment of the present invention; Fig. 9 shows the relation of voltage, excitation inductance and excitation current 10 to be measured when V/f ratio is changed while the AC motor is operated in the constant-V/f operation in the second embodiment of the present invention; Fig. 10 is a flow chart showing an example of processing of deciding excitation inductance LmB' and excitation current IdB at the base speed when the AC motor is operated at the base speed in the constant-V/f operation in the second embodiment of the present invention; 15 Fig. 11 is a block diagram schematically illustrating an inner configuration of an electric motor characteristic measurement unit in a third embodiment of the present invention; Fig. 12 shows the relation of the excitation current Id and the magnetic flux calculation value O measured while the AC motor is operated at base speed in the constant-V/f operation in the third embodiment of the present invention; 20 Fig. 13 is a flow chart showing an example of processing of deciding excitation inductance LmB' and excitation current IdB at the base speed when the AC motor is operated at the base speed in the constant-V/f operation in the third embodiment of the present invention; Fig. 14 shows the relation of magnetic flux O and the excitation current Id obtained from magnetic flux saturation characteristic when the AC motor is operated in weaken 25 magnetic field in the third embodiment of the present invention; Fig. 15 shows the relation of excitation current Id and magnetic flux calculation value O measured while the AC motor is operated at base speed in the constant-V/f operation in a fourth embodiment of the present invention; Fig. 16 shows the relation of the excitation current Id and the magnetic flux 30 calculation value O measured while the AC motor is operated in the constant-V/f operation at the speed between the base speed and the top speed in the fourth embodiment of the present invention; Fig. 17 is a flow chart showing an example of processing of deciding excitation inductance LmB' and excitation current IdB at base speed when the AC motor is operated at the -6- speed between the base speed and the top speed in the constant-V/f operation in the fourth embodiment of the present invention; and Fig. 18 a graph showing an example of the saturation characteristic of the AC motor in the fourth embodiment of the present invention. 5 DESCRIPTION OF THE EMBODIMENTS [FIRST EMBODIMENT] The first embodiment of the present invention is now described with reference to Figs. 1 to 5. In this embodiment, an AC motor control system 10 adapted to calculate motor constants and control constants on the basis of the electric motor characteristic of an AC motor 3 10 driven by AC power is described. The AC motor control system 10 executes a program by a computer to thereby make inner blocks described later cooperate with one another so as to realize an electric motor characteristic measurement method of calculating the motor constants of the AC motor 3 and measuring the electric motor characteristic containing at least a rated torque current and a rated excitation current. 15 Fig. 1 is a block diagram schematically illustrating an inner configuration of the AC motor control system 10. The AC motor control system 10 includes an AC power supply 1 which produces AC power, a power converter unit 2 which converts the AC power supplied from the AC power supply 1 into AC power and voltage having a desired frequency to be outputted, and a power 20 converter unit control apparatus 4 which controls operation of the power converter unit 2. Further, the AC motor control system 10 includes a current detection unit 5 which outputs a detection value of AC current outputted by the power converter unit 2, an input unit 6 through which an operator inputs name plate information, a mode changing unit 18 which changes various modes described later and an AC motor 3. The power converter unit 2 includes 25 switching elements such as transistors and comprises a pulse width modulation (PWM) inverter. The name plate information inputted by the operator from the input unit 6 contains capacity P (watt), rated frequency fi, rated voltage Vi and rated current Ii of the AC motor 3, for example. The power converter unit control apparatus 4 includes an electric motor characteristic measurement unit 15 which measures the electric motor characteristic of the AC 30 motor 3. The electric motor characteristic measurement unit 15 supplies a primary angular frequency command, a d-axis voltage command and a q-axis voltage command to a first coordinate transformation unit 12 to thereby perform control for changing a ratio of voltage and frequency supplied to the AC motor by the power converter unit 2. The electric motor -7- characteristic measurement unit 15 sets rated torque current and rated excitation current for driving the AC motor 3 at rated frequency and rated voltage as motor constants of the AC motor 3. Furthermore, the power converter unit control apparatus 4 includes a vector 5 control unit 11 and a control constant calculation unit 16. The vector control unit 11 calculates various commands supplied to the AC motor 3 so that output torque and speed of the AC motor 3 satisfy the desired electric motor characteristic, so that the AC motor 3 is vector-controlled. The control constant calculation unit 16 supplies to the vector control unit 11 the control constants calculated on the basis of the motor constants supplied from the electric motor 10 characteristic measurement unit 15. Moreover, the power converter unit control apparatus 4 includes a selector unit 17 for changing input from the electric motor characteristic measurement unit 15 or the vector control unit 11 and the first coordinate transformation unit 12. The first coordinate transformation unit 12 coordinate-transforms the d-axis voltage command Vdref and the q-axis 15 voltage command Vqref into a three-phase AC voltage command on the basis of the primary angular frequency command coiref supplied through the selector unit 17. Further, the power converter unit control apparatus 4 includes a pulse generation unit 13 which supplies a pulse signal to the power converter unit 2. The pulse generation unit 13 generates the pulse signal in response to the three-phase AC voltage command supplied from 20 the first coordinate transformation unit 12. The pulse signal can control on and off" of the switching elements constituting the power converter unit 2 to convert voltage and frequency of the AC power supplied from the AC power supply 1, so that the power converter unit 2 for driving the AC motor 3 can be controlled. The pulse signal is calculated so that the value of the AC voltage outputted from the power converter unit 2 is equal to a value of the three-phase AC 25 voltage command outputted from the first coordinate transformation unit 12. Furthermore, the power converter unit control apparatus 4 includes a second coordinate transformation unit 14. The second coordinate transformation unit 14 transforms a detection value of the AC current outputted by the power converter unit 2 and detected by the current detection unit 5 into a d-axis excitation current Id and a q-axis torque current Iq 30 (hereinafter also abbreviated to "Id and Iq") on the basis of the primary angular frequency command coiref supplied through the selector unit 17. The second coordinate transformation unit 14 supplies the transformed Id and Iq to the electric motor characteristic measurement unit 15. The electric motor characteristic measurement unit 15 calculates the electric motor characteristic of the AC motor 3 on the basis of Id and Iq supplied from the second coordinate -8- transformation unit 14. Concrete operation examples of the respective units are now described. The power converter unit control apparatus 4 controls the power converter unit 2 to drive the AC motor 3. At this time, the power converter unit control apparatus 4 calculates 5 the motor constants of the AC motor 3 to obtain the electric motor characteristic and sets the rated torque current and the rated excitation current of the AC motor 3. The power converter unit control apparatus 4 controls the power converter unit 2 so that the value of AC voltage outputted by the power converter unit 2 is equal to the three-phase AC voltage command value outputted by the first coordinate transformation unit 12. The 10 current detection unit 5 supplies the detection value of the AC current detected from the AC power outputted by the power converter unit 2 to the second coordinate transformation unit 14. The electric motor characteristic measurement unit 15 controls the power converter unit 2 so that predetermined AC voltage and AC frequency are applied to the AC motor 3 in first to third measurement modes (expressed as mode 1 to mode 3, respectively) in which the 15 electric motor characteristic of the AC motor 3 is measured. Detailed processing example in the first to third measurement modes is described later. The electric motor characteristic measurement unit 15 measures the motor constants, the rated torque current and characteristic of the rated excitation current (hereinafter referred to as "excitation current characteristic") required for controlling the AC motor 3 at the rated voltage and the rated frequency. 20 Further, the electric motor characteristic measurement unit 15 produces a measurement signal for measuring the electric motor characteristic in the first to third measurement modes. The measurement signal contains the primary angular frequency command coiref, the d-axis voltage command Vdref, the q-axis voltage command Vqref and the like. At this time, the electric motor characteristic measurement unit 15 calculates the motor 25 constants, the rated torque current and the excitation current characteristic on the basis of Id and Iq supplied from the second coordinate transformation unit 14 and the primary angular frequency command coiref supplied to the first coordinate transformation unit 12. The control constant calculation unit 16 calculates the control constants of the vector control unit 11 from the motor constants calculated from the electric motor characteristic 30 such as the motor constants, the rated torque current and the excitation current characteristic calculated in the electric motor characteristic measurement unit 15 in a vector control mode in which the AC motor 3 is vector-controlled. The vector control unit 11 makes the power converter unit 2 vector-control the AC motor 3 in accordance with the primary angular frequency command, the d-axis voltage command and the q-axis voltage command produced on -9- the basis of Id and Iq supplied from the second coordinate transformation unit 14 and the control constants supplied from the control constant calculation unit 16. When the motor constants, the rated torque current and the excitation current characteristic are measured, the power converter unit control apparatus 4 changes the input to the 5 selector unit 17 from the vector control unit 11 to the electric motor characteristic measurement unit 15. The electric motor characteristic measurement unit 15 supplies measurement signals produced by changing measurement conditions variously in the first to third measurement modes to the AC motor 3. At this time, the electric motor characteristic measurement unit 15 supplies the primary angular frequency command coiref, the d-axis voltage command Vdref and the q-axis 10 voltage command Vqref changed on the basis of the measurement conditions to the first coordinate transformation unit 12. On the other hand, when the AC motor 3 is vector-controlled, the power converter unit control apparatus 4 changes the input to the selector unit 17 irom the electric motor characteristic measurement unit 15 to the vector control unit 11 and vector-controls the AC motor 3 in accordance with proper control constants. 15 The selector unit 17 supplies the primary angular frequency command, the d-axis voltage command and the q-axis voltage command inputted from the electric motor characteristic measurement unit 15 in the first to third measurement modes to the first coordinate transformation unit 12. On the other hand, the selector unit 17 changes the input thereto so that the primary angular frequency command, the d-axis voltage command and the q-axis voltage 20 command supplied from the vector control unit 11 in the vector control mode are supplied to the first coordinate transformation unit 12. Fig. 2 is a block diagram schematically illustrating the inner configuration of the electric motor characteristic measurement unit 15. In Fig. 2, configuration of other elements except the electric motor characteristic measurement unit 15 in the AC motor control system 10 25 is simplified and not shown. The electric motor characteristic measurement unit 15 includes a control unit 20 which supplies the primary angular frequency command coiref, the d-axis voltage command Vdref and the q-axis voltage command Vqref to the selector unit 17 when the motor constants, the rated torque current and the rated excitation current characteristic of the AC motor 3 are 30 measured. The control unit 20 is connected to the mode changing unit 18 and the commands to the control constant calculation unit 16 or the selector unit 17 are changed in response to change of the vector control mode or the first to third measurement modes. Further, the control unit 20 is supplied with the name plate information from the input unit 6. In addition, the control unit 20 is supplied with data of Id and Iq from the second coordinate transformation unit 14. -10- Moreover, the electric motor characteristic measurement unit 15 includes an excitation current measurement unit 21 which measures the excitation current from the data of Id and Iq, an excitation inductance measurement part 22 which measures an excitation inductance Lm' and a rated torque current calculation part 23 which calculates the rated torque current. 5 The excitation inductance measurement unit 22 measures the excitation inductance Lm' from the relation of current and voltage applied to the AC motor 3. The rated torque current calculation unit 23 uses a calculation expression described later to calculate the rated torque current from Id and Iq supplied from the second coordinate transformation unit 14. Further, the electric motor characteristic measurement unit 15 includes a rated 10 voltage calculation unit 24. The rated voltage calculation unit 24 calculates a rated voltage calculation value V of the AC motor 3 by changing a measurement voltage while a V/f ratio of voltage and frequency applied to the AC motor 3 is maintained to be constant. In the following description, the ratio of voltage and frequency of the AC power supply 1 to be supplied to the AC motor 3 is named "V/f ratio". Operation that the AC motor 3 is operated while the V/f ratio is 15 maintained to be constant is named "constant-V/f operation". Furthermore, the electric motor characteristic measurement unit 15 includes a measurement condition judgment unit 25. The measurement condition judgment unit 25 judges measurement conditions of the AC motor 3 at the time that the rated voltage calculation value V is equal to rated voltage Vi obtained from the name plate information. The rated voltage 20 calculation unit 24 calculates the rated voltage calculation value V on the basis of the rated voltage Vi read from the name plate information and inputted through the input unit 6 by the operator and Id and Iq supplied from the rated torque current calculation unit 23. The measurement condition judgment unit 25 decides the measurement conditions of the AC motor 3 when the rated voltage Vi is equal to the rated voltage calculation value V and calculates the 25 rated excitation current and the rated torque current to realize the rated operation of the AC motor 3. The measurement condition judgment unit 25 notifies the rated excitation current and the rated torque current in the decided measurement conditions to the control unit 20. The control unit 20 supplies the notified rated excitation current and rated torque current to the 30 control constant calculation unit 16 when the control unit 20 controls operation of the AC motor 3. Thereafter, the control constants calculated by the control constant calculation unit 16 are supplied to the vector control unit 11, so that the vector control unit 11 vector-controls the AC motor 3. Fig. 3 is a flow chart showing an example of first to third measurement modes -11- (modes 1 to 3) changed when the electric motor characteristic measurement unit 15 measures the electric motor characteristic of an AC motor 3 and in which the electric motor characteristic of the AC motor 3 is measured. First, the operator operates the input unit 6 to input the name plate information of 5 the AC motor 3 to the control unit 20 of the electric motor characteristic measurement unit 15 beforehand (step SI) and the name plate information is used in the first to third measurement mode. The name plate information is used when a setting range of plural measurement voltages (Vmode 1(1), Vmode 1(2), ...) applied to the AC motor 3, the standard of the AC current detection value, the calculation standard of the rated torque current IT, the judgment standard of 10 the measurement conditions and the like are decided. Next, the selector unit 17 changes the mode to the first mode, in which the AC motor 3 is subjected to single-phase DC excitation to measure a value of primary resistance ri of the AC motor 3 (step S2). In the first measurement mode, the control unit 20 sets the primary angular frequency command (Oiref=0, the q-axis voltage command Vqref=0 and plural 15 measurement voltages (Vmode 1(1), Vmode 1 (2),...) for the d-axis voltage command Vdref as measurement conditions to be supplied to the AC motor 3. The control unit 20 supplies the d-axis voltage command Vdref to the first coordinate transformation unit 12. Thereafter, the power converter unit 2 applies the measurement voltage obtained by converting the d-axis voltage command Vdref to the AC motor 3. 20 At this time, the second coordinate transformation unit 14 transforms the detection value of AC current detected by the current detection unit 5 into Id and Iq to be supplied to the electric motor characteristic measurement unit 15. The electric motor characteristic measurement unit 15 calculates the value of primary resistance ri of the AC motor 3 from the relation of the measurement voltage applied to the AC motor 3 and Id and Iq obtained 25 by transforming the AC current outputted by the power converter unit 2 in a corresponding manner to the measurement voltage. Next, the selector unit 17 changes the mode to the second measurement mode, in which the AC motor 3 is subjected to single-phase AC excitation. In the second measurement mode, the control unit 20 supplies, as the measurement conditions, the primary angular 30 frequency command coiref=0, the q-axIs voltage command Vqref=0 and a sinusoidal measurement voltage (Vmode2xsin (comode2xt)) for the d-axis voltage command Vdref for performing the single-phase AC excitation to the AC motor 3. The control unit 20 supplies the d-axis voltage command Vdref set as the sinusoidal measurement voltage to the first coordinate transformation unit 12. Thereafter, the power converter unit 2 applies the measurement voltage -12- which is a transformed value of the d-axis voltage command Vdref to the AC motor 3. The electric motor characteristic measurement unit 15 calculates values of a combined resistance ro (=ri+r2') and a combined leakage inductance La (=Li+L2') of the AC motor 3 from the relation of the voltage applied to the power converter unit 2 and Id and Iq 5 which are transformed values of AC current outputted by the power converter unit 2. Next, the selector unit 17 changes the mode to the third measurement mode, in which the AC motor 3 is operated while the top speed of the AC motor 3 and the V/f ratio are maintained to be constant (step S4). In the third measurement mode, the control unit 20 sets the d-axis voltage command Vdref=0, the primary angular frequency command (J0iref=coimode3 and 10 the q-axis voltage command Vqref=Vmode3 as the measurement conditions and supplies a voltage command for performing the constant-V/f operation to the first coordinate transformation unit 12. The control unit 20 supplies the measurement voltage to the first coordinate transformation unit 12. Thereafter, the power converter unit 2 supplies the measurement voltage which is a converted value of the voltage command for performing the 15 constant-V/f operation to the AC motor 3. The electric motor characteristic measurement unit 15 calculates the voltage calculation value V at the top speed on the basis of the relation of the voltage applied to the power converter unit 2 in order to calculate the motor constants and Id and Iq supplied from the second coordinate transformation unit 14. Control for approaching the voltage calculation 20 value V to the rated voltage Vi used as the judgment standard is repeated. When the voltage calculation value V at the top speed is equal to the rated voltage Vi, the excitation inductance Lm' at the top speed is decided and the d-axis excitation current Id is set as the rated excitation current to thereby set the rated torque current IT calculated from the rated excitation current Id. The excitation inductance Lm', the rated torque current and the rated excitation current of the 25 AC motor 3 are calculated as the electric motor characteristic of the AC motor 3 at the top speed and the excitation current characteristic considering influence of the magnetic flux saturation is calculated. Finally, the control constant calculation unit 16 calculates the control constants using the calculated motor constants, rated torque current and excitation current characteristic 30 (step S5) and writes the control constants in a memory not shown. After the above processing has been completed, the electric motor characteristic measurement unit 15 ends the first to third measurement modes. The selector unit 17 changes the measurement mode to the vector control mode and the vector control unit 11 drives the AC motor 3 at a variable speed by vector control. Next, a processing example of measuring the rated torque current and the -13- excitation current characteristic in the third measurement mode performed in step S4 of Fig. 3 is described. In order to vector-control the AC motor 3, it is necessary to decide the rated torque current (q-axis current Iq) and the rated excitation current (d-axis current Id) at the d- and 5 q-axes intersecting with each other at right angles. However, when design values of the AC motor 3 cannot be obtained beforehand, information of the AC motor 3 is limited to the name plate information. In order to vector-control such an AC motor 3, the motor constants are calculated in accordance with the processing shown in the flow chart of Fig. 3 described above and the control constants used in the vector control are set. However, in the prior art, it is 10 difficult to set the rated torque current IT and the excitation current Id properly. In the embodiment, as shown in step S4 of Fig. 3, the AC motor 3 is operated in the constant-V/f operation to measure the excitation inductance Lm'. In this measurement, the rated torque current IT and the rated excitation current IdT are set as follows. The rated frequency fi at the top speed obtained from the name plate information is used to operate the AC 15 motor 3 in the constant-V/f operation, so that the excitation inductance LmT' and d-axis excitation current IdT at the top speed are measured. In the following description, the excitation inductance Lm' and the d-axis excitation current Id measured at the top speed of the AC motor 3 (also named "maximum rotation speed") are expressed as "LmT"' and "IdT", respectively. Further, the excitation inductance Lm' and the d-axis excitation current Id 20 measured at the base speed of the AC motor 3 (also named "basal rotation speed") are expressed as "LmB"' and "LdB", respectively. As described above, the excitation inductance Lm' has been already measured from the relation of current and voltage applied to the AC motor by the excitation inductance measurement unit 22. The rated torque current calculation unit 23 calculates the rated torque 25 current IT by the following expression (I) using the rated current Ii read from the name plate information and the measured excitation current Id. IT = ^If - IdT' ...(1) In order to calculate the proper rated torque current IT and rated excitation current IdT at the top speed of the AC motor 3, it is necessary to decide the measurement conditions required when the constant-V/f operation is performed. In the embodiment, the rated voltage 30 Vi read from the name plate information is used as the judgment standard for deciding the measurement conditions. -14- Accordingly, the rated voltage calculation unit 24 uses the primary resistance ri measured in the first measurement mode, the combined leakage inductance La measured in the second measurement mode and the excitation inductance LmT', the rated torque current IT and the rated excitation current IdT at the top speed measured in the third measurement mode to 5 calculate the rated voltage calculation value V in accordance with the voltage equations shown in the following equations (2), (3) and (4). The rated voltage calculation value V is calculated by calculating Vd and Vq for d- and q-axes, respectively, and adding squares of Vd and Vq. Vd=ri xldT-co, xLcjxIT ... (2) Vq=ri xIT+coi xLaxIdT+coi xLmT' xJdT = ri xIT+tOi x(Lo+LmT')xIdT ... (3) V = 7vd'+Vq' ...(4) where coi=(Dr+(os. Slip frequency cos is calculated from primary resistance rl measured in the first measurement mode, combined resistance ra measured in the second measurement mode and 10 excitation inductance LmT' measured in the third measurement mode in accordance with the following expression (5). ro - r, IT 008 = ^x •••(5) LmT' IdT Next, an example where the V/f ratio is changed while the rated frequency fi is maintained to be constant is described. Fig. 4 is a graph showing an example of changing the V/f ratio by changing the 15 voltage V while the rated frequency fi at the top speed is maintained to be constant. The V/f ratio is changed by changing the measurement voltage while the rated frequency fi ((Oiref=a)imode3=2x7rxfi) of electric power supplied to the AC motor 3 by the power converter unit 2 is maintained to be constant, so that the rated voltage calculation value V is calculated. In the embodiment, the q-axis voltage command Vqref=Vmode3 which is the 20 measurement condition in the third measurement mode is changed to select the measurement value on the measurement condition that the rated voltage calculation value V is equal to the rated voltage Vi. Next, an example of the operation processing performed in the third measurement - 15- mode changed in step S4 of Fig. 3 is described in detail. Fig. 5 is a flow chart showing an example of processing of calculating the rated torque current IT, the excitation current Id and the excitation inductance Lm' while the AC motor is operated in the constant-V/f operation at the rated speed (top speed). 5 First, the control unit 20 of the electric motor characteristic measurement unit 15 takes in the rated frequency fi, the rated voltage Vi, the rated current Ii, the primary resistance ri measured in the first measurement mode and the combined resistance ra and the combined leakage inductance La measured in the second measurement mode (step SI 1). Next, the control unit 20 operates the AC motor 3 in the constant-V/f operation in 10 the third measurement mode (step SI2). At this time, the control unit 20 sets the d-axis voltage command Vdref=0 and the primary angular frequency command Q)iref=coimode3 (=2x7cxf|) as the measurement conditions and sets the q-axis voltage command Vqref as an initial value Vini. The rated voltage Vi obtained from the name plate information, for example, is set as the initial value Vini. The control unit 20 supplies the voltage command for performing the constant-V/f 15 operation to the power converter unit 2. Consequently, the power converter unit 2 impresses the measurement voltage to the AC motor 3 so that the AC motor 3 performs the constant-V/f operation. Next, the current detection unit 5 detects AC current in the third measurement mode outputted by the power converter unit 2 and the second coordinate transformation unit 14 20 outputs data of Id and Iq. The excitation current measurement unit 21 takes in data of Id and Iq from the second coordinate transformation unit 14 (step SI3). Next, the excitation inductance measurement unit 22 substitutes parameters in a function F for calculating the excitation inductance Lm' to calculate a value of the excitation inductance Lm' of the AC motor 3 (step SI 4). This parameters contain the primary resistance 25 rl, the combined leakage inductance La, the primary angular frequency commandcoiref and the q-axis voltage command Vqref taken in step SIl and Id and Iq taken in step SI 3. Next, the rated torque current calculation unit 23 calculates the rated torque current IT from the data Id taken in step S13 and'the rated current Ii taken in step Sll in accordance with the expression (1). Further, the rated torque current calculation unit 23 30 calculates the slip frequency cos from the primary resistance ri and the combined resistance ra taken in step SI 1, the excitation inductance Lm' calculated in step SI4, the excitation current Id and the rated torque current IT in accordance with the expression (5) described above. Further, the electric motor characteristic measurement unit 15 calculates the rated frequency coi=2x7txfi+a)s (step S15). - 16- Next, the rated voltage calculation unit 24 calculates the rated voltage calculation value V in accordance with the expressions (2), (3) and (4) (step SI6). Then, the measurement condition judgment unit 25 compares the rated voltage calculation value V with the rated voltage Vi (V1/V3 in case of line voltage) of the name plate information taken in step SI 1 (step S17). 5 When the rated voltage calculation value V is larger than the rated voltage Vi, a reduction voltage Vstep (for example 5V) is subtracted from the q-axis voltage command Vqref (for example 385V) set in step S12 (step S18) and the processing is returned to step S12. The processing of steps S12 to SIS is repeated until the rated voltage calculation value V is equal to the rated voltage V]. 10 On the other hand, when the rated voltage calculation value V is equal to the rated voltage Vi, the measurement condition judgment unit 25 ends the measurement of the electric motor characteristic. At this time, the measurement condition judgment unit 25 sets the excitation inductance Lm' and the excitation current Id on the measurement condition that the rated voltage calculation value V is equal to the rated voltage Vi as the rated excitation 15 inductance LmT' and the rated excitation current IdT, respectively. Further, the torque current calculated from the rated excitation current IdT is decided as the rated torque current IT (step SI9) and the decided value is written in a memory not shown. This value is read out from the control constant calculation unit 16 in the vector control mode and is used for calculation of the control constants. 20 According to the electric motor characteristic measurement unit 15 in the first embodiment described above, in the first to third measurement modes, the AC motor 3 is operated in a rated manner using the rated voltage Vi and the rated current Ii obtained from the name plate information and accordingly the motor constants, the rated excitation current IdT and the rated torque current IT are selected automatically. Consequently, when the control constant 25 calculation unit 16 sets the control constants from the calculated electric motor characteristic in the vector control mode and changes the operation to vector control to operate the AC motor 3, the AC motor 3 can be operated in the rated manner as described in the name plate information. Further, the electric motor characteristic measurement unit 15 repeats the control for increasing the voltage applied to the AC motor 3 from the value lower than the rated voltage 30 Vi or reducing the applied voltage from the rated voltage V) until the voltage calculation value V at the top speed is equal to the rated voltage Vi, so that the voltage calculation value V nearer to the rated voltage Vi can be obtained. In Fig. 5, judgment of the rated voltage calculation value V is started from the voltage higher than the rated voltage Vi and the measurement voltage is reduced until the rated -17- voltage calculation value V is equal to the rated voltage Vi. To the contrary, even if setting of the ratio V/f is started from the voltage equal to a half of the rated voltage Vi, for example, and the measurement voltage is increased until the rated voltage calculation value V is equal to the rated voltage Vi, the measurement conditions can be judged similarly. 5 Moreover, the value of the primary resistance used in calculation of the expressions (2), (3) and (5) is set in consideration of increased temperature in the ordinary operation as compared with the numerical value measured at normal temperature. Accordingly, the value of the primary resistance changed by increased temperature of the AC motor 3 is considered in calculation of the voltage calculation value V at the top speed. Consequently, the 10 motor constants at the top speed, the rated excitation current IdT and the rated torque current IT can be selected so that the rated voltage calculation value V and the current value calculated when the rated operation is performed at normal operation are equal to the rated voltage V| and the rated current Ii obtained from the name plate information, respectively. [SECOND EMBODIMENT] 15 The second embodiment of the present invention is now described with reference to Figs. 6 to 10. Fig. 6 is a graph showing the state that the V/f ratio is changed to measure a voltage while the AC motor 3 is operated in the constant-V/f operation at the base speed. The processing of performing the constant-V/f operation at the base speed is modification of the 20 processing shown in step S4 of Fig. 3. When the AC motor 3 is driven in the weaken magnetic field, the excitation current Id is changed between the base speed and the top speed to obtain the excitation current IdB at the base speed on the basis of the value of the excitation current IdT at the top speed decided in Fig. 5. 25 Fig. 7 shows the relation of the top speed, the base speed, voltage, magnetic flux and excitation current at the time that the AC motor 3 is operated in the weaken magnetic field. Fig. 7A shows an example of voltage Vemf (=(orex4)) called speed electromotive force, Fig. 7B shows an example of magnetic flux O in weaken magnetic field and Fig. 7C shows an example of excitation current IdB at base speed and excitation current IdT at top speed. 30 As shown in Fig. 7B, the magnetic flux O is set to be strong fixed state (forced magnetic field) until the base speed is reached and accordingly the voltage Vemf is increased in proportion to the speed (Fig. 7A). Further, as shown in Fig. 7B, the magnetic flux is reduced (weaken magnetic field) in inverse proportion to the speed from the base speed to the top speed. When the weaken magnetic field is set in this manner, the voltage Vemf is fixed and the AC - 18- motor 3 has the fixed output characteristic between the base speed and the top speed. At this time, the magnetic flux O is calculated as "0=Lm'xId". Accordingly, when the excitation inductance Lm' is fixed, the excitation current Id is changed in inverse proportion to the speed in accordance with change of the magnetic flux O as shown by solid line 5 of Fig. 7C. The excitation current characteristic from the base speed to the top speed shown by solid line of Fig. 7C can be decided on the basis of the rated excitation current IdT at the top speed decided in the first embodiment described above. Fig. 8 is a graph showing an example of a saturation characteristic of the AC motor 3. 10 When the magnetic flux O of the AC motor 3 has the saturation characteristic, the magnetic flux O in the weaken magnetic field (non-saturation point) is equal to the magnetic flux OT=LmT'xIdT at the top speed and the magnetic flux O in the forced magnetic field (saturation point) is equal to the magnetic flux OB=LmB'xldB at the base speed. Here, the excitation inductance is LmB' measured while the AC motor 3 is operated at the base speed in the constant-V/f operation. Fig. 12A shows an example in which the measurement voltage is 30 increased in the state that the V/f ratio is high while the base speed is maintained to be constant and Fig. 12B shows an example of the characteristic of the magnetic flux measured while the AC motor 3 is operated in the constant-V/f operation at a base speed. Fig. I5A shows an example in case where the voltage V is increased in the high V/f ratio while the base speed is maintained to be constant and Fig. 15B shows an example of characteristic of the magnetic flux O changed in accordance with the excitation 25 current Id. There is a case where there is no condition that the magnetic flux O is equal to the judgment standard magnetic flux OB as shown in Fig. I2B because of the saturation characteristic of the magnetic flux O of the AC motor 3 and excessive current flows into the AC motor 3. At this time, the excitation current Id is excessive before the magnetic flux O reaches 30 the judgment standard magnetic flux OB as shown in Fig. 15B and the excitation current IdB at the base speed cannot be specified. Here, the excitation current limit value Idlim is provided as the protection function for suppressing the excitation current Id to a fixed value so that the measured excitation current Id is not increased to an excessive value and the electric motor characteristic measurement unit 30 can measure the excitation current characteristic within the -27- range of permissible current of the power converter unit 2. The excitation current limit value Idlim is calculated in accordance with the following expression (8) in consideration of the permissible current linv of the power converter unit 2 muhiplied by a safety factor a and an overload factor OL multiplied by the rated torque current IT. Idlim = V(Iinvxa)'-(ITxOL)' ...(8) 5 However, when the measurement of the excitation current characteristic is limited by the excitation current limit value Idlim, it is impossible to produce the magnetic flux O necessary in the base speed due to the saturation characteristic of the AC motor 3. This means that the relation of the voltage Vemf, the magnetic flux O and the excitation current Id in the weaken magnetic field operation as shown in Fig. 7 is not realized and a fixed-output 10 characteristic is not attained between the base speed and the top speed. Accordingly, it is necessary to set the optimum base speed according to the saturation characteristic of the AC motor 3 and set the excitation current IdB at the base speed. Fig. 16 shows an example of the relation of the excitation current Id and the magnetic flux calcuJation value O measured while the AC motor 3 is operated in the cinstant-V/f 15 operation at the speed between the base speed and the top speed. Fig. 16A shows an example where the speed of the AC motor 3 is changed between the base speed and the top speed and Fig. 16B shows an example where the excitation current IdB at the base speed is decided on the basis of the judgment standard magnetic fluxOB. As shown in Fig. 16A, when the measurement of the excitation current 20 characteristic is limited by the excitation current limit value Idlim, the electric motor characteristic measurement unit 30 of the fourth embodiment changes the speed of the AC motor 3 to a first intermediate speed (corelc) which is an intermediate speed between the base speed and the top speed set at the beginning. An initial value of the measurement voltage is set to the point in which the V/f ratio is equal to that on the measurement conditions decided at the top 25 speed. Thereafter, the electric motor characteristic measurement unit 30 increases the measurement voltage gradually to make measurement and judges whether the magnetic flux is equal to the judgment standard magnetic flux Olc obtained from the speed ratio calculated using the expression (7) or the measurement is limited by the excitation current limit value Idlim. When the measurement of the excitation current characteristic is limited by the 30 excitation current limit value Idlim, the measurement is repeated at a second intermediate speed which is an intermediate speed between the flrst intermediate speed (corelc) shown in Fig. 16A -28- and the top speed. To the contrary, when the measured magnetic flux O is equal to the judgment standard magnetic flux Olc, the measurement is performed at a third intermediate speed which is an intermediate speed between the base speed set at the beginning and the first intermediate speed again. In this manner, the processing of changing the speed is repeated, so 5 that the optimum base speed and excitation current IdB can be set in accordance with the saturation characteristic of the magnetic flux exhibited from data obtained in this process. Next, the calculation processing performed in the third measurement mode shown in step S4 of Fig. 3 is described with reference to the flow chart of Fig. 17. Fig. 17 is a flow chart showing an example of processing of deciding the 10 excitation inductance LmB' and the excitation current IdB at the base speed while the AC motor 3 is operated at the speed between the base speed and the top speed in the constant-V/f operation. The electric motor characteristic measurement unit 30 maintains the frequency at which the base speed of the AC motor 30 is fixed in the third measurement mode. The electric motor characteristic measurement unit 30 performs the control for approaching the magnetic flux 15 calculation value O calculated while the ratio of voltage and frequency applied to the AC motor 3 is changed to drive the AC motor 3 in weaken magnetic field to the judgment standard magnetic flux OB calculated at the ratio of the base speed and the top speed from the magnetic flux at the top speed. In this process, when the d-axis excitation current inputted from the second coordinate transformation unit 14 exceeds the excitation current limit value, the speed of 20 the AC motor 3 is changed between the base speed and the top speed. When the magnetic flux calculation value 0 is equal to the judgment standard magnetic flux calculated from the magnetic flux at the top speed at the ratio of the changed speed and the top speed, the changed speed is set to the base speed and the excitation inductance and the excitation current are obtained as the electric motor characteristic of the AC motor 3 at the base speed. 25 First, the control unit 20 of the electric motor characteristic measurement unit 30 takes in the primary resistance ri measured in the first measurement mode and the combined resistance ra and the combined leakage inductance La measured in the second measurement mode 2 for use in calculation (step S41). Next, the control unit 20 sets the frequency fB at the base speed to an initial value 30 fini of the frequency command as the measurement condition in the third measurement mode and sets the primary angular frequency command (iOiref=a)imode3 (=2x7cxfini). The control unit 20 uses the magnetic flux OT and the top frequency roreT at the top speed and the set primary angular frequency command coiref to calculate the judgment standard magnetic flux Or in accordance with the expression (7) (step S42). -29- The control unit 20 sets d-axis voltage command Vdref=0 and q-axis voltage command Vqref as initial values Vini. As the initial value, the voltage having the V/f ratio on the measurement condition decided in measurement at the top speed may be set. Then, the control unit 20 outputs the d-axis voltage command Vdref and the q-axis voltage command 5 Vqref for performing the constant-V/f operation and applies the measurement voltage to the AC motor 3 (step S43). Next, when the current detection unit 5 detects AC current, the excitation current measurement unit 21 takes in data of Id and Iq from the second coordinate transformation unit 14 (step S44). The measurement condition judgment unit 25 compares the excitation current Id 10 taken in step S44 with the excitation current limit value Idlim calculated in accordance with the expression (8) (step S45). When the excitation current Id is larger than the excitation current limit value Idlim, the electric motor characteristic measurement unit 30 changes the frequency command that is the measurement condition and increases the speed of the AC motor 3 (step S46). At this 15 time, for example, after an intermediate value between the top speed and the value set last time is calculated, the processing is returned to step S42 and the processing from steps S42 to S45 is repeated. In the following description, speed set last time after the processing from steps S42 to S45 has been performed is named "last set value" and speed set this time after the processing fi"om steps S42 to S45 has been performed is named "latest set value". Accordingly, when the 20 processing in step S46 is repeated, the intermediate value approaches the top speed gradually. Next, when the excitation current Id taken in step S44 is smaller than or equal to the excitation current limit value Idlim, the primary resistance rl and the combined leakage inductance La taken in step S41, the primary angular frequency commandcoiref and the q-axis voltage command Vqref set as the measurement conditions and data of Id and Iq flowing in a 25 corresponding manner to voltages applied for measurement are used to calculate the value of excitation inductance Lm' of the AC motor 3 (step S47). Next, the electric motor characteristic measurement unit 30 calculates the magnetic flux calculation value 0 from the data Id taken in step S44 and the excitation inductance Lm' calculated in step S47 in accordance with the expression (6) (step S48). The 30 magnetic flux calculation value O is compared with the judgment standard magnetic flux Or calculated in step S42 (step S49). When the magnetic flux calculation value (D is smaller than the judgment standard magnetic flux Or, an increment Vstep is added to the q-axis voltage command Vqref set in step S43 (step S50). Thereafter, the processing is returned to step S43 and the processing from steps -30- S42 to S49 is repeated until the magnetic flux calculation value O is equal to the judgment standard magnetic flux Or. On the other hand, when it is judged that the magnetic flux calculation value O is equal to the judgment standard magnetic flux Or, the excitation inductance Lm' and the 5 excitation current Id are decided as excitation inductance Lmc' and excitation current Idc at the set measurement frequency (speed), respectively (step S51). Next, a saturation degree judgment standard value AO/AId showing a change amount of the excitation currents Id to a change amount of the magnetic flux calculation values O in the last measurement and the latest measurement is used as a saturation degree judgment 10 standard value and the saturation degree judgment standard value is compared with the standard value do (step S52). The saturation degree judgment standard value AO/AId is a value indicating change in magnetic flux to change in excitation current at the base speed from a value indicating change in magnetic flux to change in excitation current at the top speed. The electric motor characteristic measurement unit 30 decides the speed of the AC rhotor 3 in case where the 15 value of change in magnetic flux to change in the d-axis excitation current measured by changing the speed of the AC motor 3 is equal to the saturation degree judgment standard value as the base speed and also decides the excitation current as the excitation current at the base speed. It is necessary to widen the weaken magnetic field range to prevent the excitation current from flowing uselessly. The relation of the excitation current Id and the magnetic flux 20 O in the saturation characteristic of the AC motor 3 is described with reference to Fig. 18. Fig. 18 is a graph showing an example of the saturation characteristic of the AC motor 3. It is supposed that the excitation current IdB smaller than the excitation current limit value Idlim is an optimum point. In the weaken magnetic field area of the non-saturation 25 point, the saturation degree judgment standard value is AO/AId ^ LmT' and accordingly the magnetic flux OT at the top speed can be calculated by LmT'xIdT. On the other hand, as the magnetic flux O is saturated, the weaken magnetic field range is widened and the excitation current Id is made smaller to prevent useless excitation current from flowing, so that the excitation current Id is limited within the range that the 30 saturation degree judgment standard value AO/AId is smaller than the standard value dO (saturation degree judgment standard value AO/AId < standard value dO) (0