1. A control i^paratus for an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, comprising: an exciting current control imit which performs constant torque control so that the torque is made constant until a rotational speed reaches a predetermined rotational speed set in advance, and performs constant output control so that an output of the induction motor obtained by the torque and the rotational speed is made constant by deaeasing the exciting current at fi-om the predetennined rotational speed to a maximum rotational speed, the exciting current control unit controlling the exciting current in accordance with a magnetic field control pattern in which the exciting current is maintained at a maximum value in the constant torque control and is decreased in accordance with increase in rotational speed in the constant output control; a torque confirming unit which obtains a value of the torque required depending on an operating condition of the induction motor in time series, and determines whether or not the obtained value of the required torque is satisfied by comparing the required torque with a torque control pattern in which the torque is set in advance depending on the rotational speed of the induction motor, a torque control pattern changing unit which changes the torque control pattern so as to increase the maximum value of the exciting current in the constant torque control when the obtained value of the required torque is not satisfied in a speed region lower than the predetermined rotationaJ speed; and a magnetic field control pattern changing unit which changes the magnetic field control pattern in accordance with the changed torque control pattern.

2. The control apparatus for an induction motor according to claim 1, wherein the torque control pattern changing unit changes the torque control pattern so as to reduce the predetermined rotational speed to be lower than the predetermined rotational speed in the normal control when the obtained value of the required torque is not satisfied in the speed region lower than the predetermined rotational speed.

3. The control apparatus for an induaion motor according to claim 1, wherein the torque control pattern changing imit changes the torque control pattern so as to increase the predetermined rotational speed to be higher than the predetermined rotational speed in the normal control when the obtained value of the required torque is not satisfied in a speed region higher than the predetermined rotational speed.

4. The control apparatus for an induaion motor according to claim I, wherein the -29-induction motor is used for rotating a pair of rolls in a mill which rolls a material to be rolled by holding the material between the rolls, and the control apparatus further comprises: a torque current control unit which converts a tension command value of the material to be roiled to a command value of the torque current based on a conversion coeflficient; and a conversion coefficient changing unit which changes the conversion coefficient in accordance with the changed torque control pattern.

5. The control ^paratus for an induction motor according to claim 1, wherein the torque control pattern changing unit calculates a value of a line current determined by the exciting current and the torque current which are supplied to the induction motor after the torque control pattern is changed, determines whether or not a calculation result is within a limit value set in advance, and informs an operator that the control is impossible when the value of the line current exceeds the limit value.

6. A method for controlling an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, including the steps of: performing constant torque control so that the torque is made constant until a rotational speed reaches a predetermined rotational speed set in advance, performing constant output control so that an output of the induction motor obtained by the torque and the rotational speed is made constant by decreasing the exciting current at fix)m the predetermined rotational speed to a maximum rotational speed, and controlling the exciting current in accordance with a magnetic field control pattern in which the exciting current is maintained at a maximum value in the constant torque control and is decreased in accordance with increase in rotational speed in the constant output control; obtaining a value of the torque required depending on an operating condition of the induction motor in time series, and determining whether or not the obtained value of the required torque is satisfied by comparing the required torque with a torque control pattern in which the torque is set in advance depending on the rotational speed of the induction motor; changing the torque control pattern so as to increase the maximum value of the exciting current in the constant torque control when the obtained value of the required torque is not satisfied in a speed region lower than the predetermined rotational speed; and changing the magnetic field control pattern in accordance with the changed torque control pattern.

7. A control apparatus for an induction motor which obtains torque by an exciting -30-cuirent for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, characterized in that constant torque control is performed so that the torque is made constant until a rotational speed reaches a predetermined rotational speed set in advance, and constant output control is performed so that an output of the induction motor obtained by the torque and the rotational speed is made constant at from the predetermined rotational speed to a maximum rotational speed, in normal control, control is performed so that a voltage of an exciting circuit in the induction motor becomes maximum until the rotational speed reaches the predetermined rotational speed, and a terminal voltage applied to the induction motor is increased in accordance with increase in rotational speed and a maximum terminal voltage is applied at the maximum rotabonal speed so as to maintain the maximimi voltage of the exciting circuit in the constant output control, and in high torque control for temporarily obtaining a high torque, control is performed so that the terminal voltage applied to the induction motor until the rotational speed reaches the predetermined rotational speed has a higher value than the terminal voltage at the predetermined rotational speed in the normal control, the voltage of the exciting circuit in the induction motor is adjusted in accordance with the increase in rotational speed so as to prevent the terminal voltage from exceeding a limit value set in advance in the constant output control, and the exciting current becomes higher than that in the normal control by the terminal voltage applied higher than that in the normal control.

8. The control apparatus for an induction motor according to claim 7, wherein, in the high torque control, the control is performed so that the tenninal voltage becomes the maximum voltage until the rotational speed reaches the predetermined rotational speed, and the maximum terminal voltage is maintained in the constant output control.

9. The control apparatus for an induction motor according to claim 7, wherein the predetermined rotational speed in the high torque control is a rotational speed lower than the predetermined rotational speed in the nonnal control.

10. The control apparatus for an induction motor according to claim 7, comprising an exciting current determining unit which determines the exciting current in the high torque control based on the rotational speed of the induction motor and the torque required in accordance with the rotational speed. 11 • The control apparatus for an induction motor according to claim 10, wherein the exciting current determining unit determines the exciting current so that a line current of the -31- induction motor becomes minimum based on the rotational speed of the induction motor and the torque required in accordance with the rotational speed. 12- A method for controlling an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, comprising performing constant torque control so that the torque is made constant until a rotational speed reaches a predetermined rotational-speed set in advance, and performing constant output control so that an output of the induction motor obtained by the torque and the rotational speed is made constant at from the predetermined rotational speed to a maximum rotational speed, in normal control, controlling so that a voltage of an exciting circuit in the induction motor becomes maximum until the rotational speed reaches the predetermined rotational speed, and a terminal voltage applied to the induction motor is increased in accordance with increase in rotational speed and a maximum terminal voltage is applied at the maximum rotational speed so as to maintain the maximum voltage of the exciting circuit in the constant output control, and in high torque control for temporarily obtaining a high torque, controlling so that the terminal voltage applied to the induction motor until the rotational speed reaches the predetermined rotational speed has a higher value than the terminal voltage at the predetermined rotational speed in the normal control, the voltage of the exciting circuit in the induction motor is adjusted in accordance with the increase in rotational speed so as to prevent the terminal voltage from exceeding a limit value set in advance in the constant output control, and the exciting current becomes higher than that in the nonnal control by the terminal voltage applied higher than that in the normal control. 32

13. A control apparatus for an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example.

14. A method for controlling an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example.

15. A control apparatus for an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example.

16. A method for an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example. Dated this 28* day of June 2012 » » ^^U^/^^^ of Anand and Anand, Advocates Agents for the Applicants

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to a control apparatus for an induction motor, and a method for controlling an induction motor, and more particularly to control for temporarily obtainir^ a higher torque than normal. Description of related art In rolling mills, a material to be rolled is rolled by using an induction motor. A general induction motor is controlled by a method of making a torque constant until a base spee4 and by making a motor output constant (making a power constant) from the base speed until a top speed. A rolling mill crushes a material to be rolled between rotating work rolls while applyir^ a tension to the material to be rolled on a mill entry side and a mill exit side, and outputs the piDc^sed material to be rolled therefrom. Rolling is thereby continuously performed. In the rolling mill, a motor is used for applying a tension to the material to. be rolled, and for obt^ning a rotational force required for the processing. As the motor, a motor having a torque-speed characteristic capable of obtaining a torque or an output reqmred for a rolling operation is selected at the time of facility planning, and installed. As examples of a method for changing the torque-speed characteristic of the induction motor, there are a method of increasing a torque in a low speed re^on in return for sacrificing a torque in a high speed region (e.g., see JP-A-2006-42562), and a method of maintaining a high torque in return for reducing an available speed region (e.g., see IP-A-2006-42570). Meanwhile, as a specific process performed when the torque-speed characteristic of the induction motor is changed, a method of using the indurtion motor as a motor having a different characteristic by selectively using a plurality of preliminarily-stored field patterns in motor control (e.g., see JP-A-2000-116199), and a method of obtaining a large motor torque by correcting a field in motor control (e.g., see JP-A-08--706OO) have been proposed. BRIEF SUMMARY OF THE INVENTION A material to be rolled, which becomes a certain product, is assumed in facility -2- planning, and a motor having an output capable of producing the product is seleaed. In this case, if a motor is selected in consideration of a material to be rolled which is not so often produced, the output of the motor will be wasted in most of rolling operations. A material to be rolled wWch requires a torque or an output exceeding the assumption in facility planning may also need to be rolled in some cases. In this case, rolling is performed by, for example, reducing a rolling reduaion and thereby suppressing the torque at present. Since a strip thickness required for each product is determined based on the product specifications, reducing the rolling reduction increases the number of rollings, thereby causing a problem that operation efficiency is deteriorated. To solve the above problems, it is necessary to change a torque characteristic of the motor so as to temporarily obtain a high torque in a BASE speed r^on. The BASE speed generally means a speed corresponding to a highest torque in an available speed range. A torque required in the motor sometimes largely varies depending on operating conditions not only in rolling facilities to which the induction motor is applied, but also in machine tools, railroad cars, and conveyance fecilities. If the required torque is higher than a motor output torque, the operating conditions cannot be satisfied. The problem in such a case can be solved if the torque characteristic of the motor can be changed so as to obtain a high torque. In this case, when the plurality of preliminarily-stored field patterns are selectively used as in JP-A-2000-116199, the motor can be used only in the predetermined field patterns, so that motor poformance cannot be suflficiently exerted. When the large torque is obtained by correcting the field as in JP-A-08-70600, a terminal voltage of the motor increases in a case in which a rotational speed becomes higher When the terminal voltage then reaches a maximum voltage, the field needs to be weakened, so that the output torque is decreased. In the control of the induction motor so as to temporality obtain a high torque as described above, it is necessary to set at least one of a torque current and an exciting current to a higher value than that in normal control. The torque current or the exciting current can be set to the higher value by increasing a voltage applied to the induction motor. However, there is a limit of the voltage which can be accepted by the induction motor. If a voltage exceeding the limit is applied, the induction motor may be broken or deteriorated in durability. It is an object of the present invention to enable control of an induction motor at a higher torque than normal without deteriorating the durability of the induction motor One aspect of the present invention is a control apparatus for an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque according to the magnetic field, the control apparatus performing constant -3-torque control so that the torque is made coastant until a rotational speed reaches a BASE speed, and performing constant output control so that an output of the induction motor is made constant from the BASE speed until a TOP speed, wherein, in normal control, control is performed such that a voltage of an exciting circuit reaches a maximum voltage at the BASE speed, and a terminal voltage is increased in accordance with increase in rotational speed and a maximum terminal voltage is applied at the TOP speed so as to maintain the maximum voltage of the exciting circuit in the constant output control, and in high torque control for temporarily obtaining a high torque, control is performed such that the terminal voltage at the BASE speed has a higher value than the terminal voltage at the BASE speed in the normal control, the voltage of the exciting circuit is adjusted in accordance with the increase in rotational speed so as to prevent the terminal voltage from exceeding a limit value in the constant output control, and the exciting current is made higher than that in the normal control. Another aspect of the present invention is a method for controlling an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, the method including performing constant torque control so that the torque is made constant until a rotational speed reaches a BASE speed, and performing constant output control so that an output of the induction motor is made constant fi"om the BASE speed until a TOP speed, wherein, in normal control, control is performed such that a voltage of an exciting circuit reaches a maximum voltage at the BASE speed, and a terminal voltage is increased in accordance with increase in rotational speed and a maximum terminal voltage is applied at the TOP speed so as to maintain the maximum voltage of the exciting circuit in the constant output control, and in high torque control for temporarily obtaining a high torque, control is performed such that the tenninal voltage at the BASE speed has a higher value than the terminal voltage at the BASE speed in the normal control, the voltage of the exciting circuit is adjusted in accordance with the increase in rotational speed so as to prevent the terminal voltage from exceeding a limit value in the constant output control, and the exciting current is made higher than that in the normal control. Also, one aspect of the present invention is a control apparatus for an induction motor, including a torque characteristic setting apparatus 21 that obtains a value of torque required depending on an operating condition of the induction motor in time series, determines whether or not the value of the required torque is satisfied by comparing the required torque with a torque control pattern in which the torque is set in advance depending on a rotational speed of the induction motor, and changes the torque control pattern so as to increase a maximum value of an exciting current in constant torque control when the value of the required torque is not -4- satisfied in a speed region lower than a predetermined rotational speed, and a magnetic flux command changing device 140 that changes a magnetic field control pattern in accordance with the changed torque control pattern. Another aspect of the present invention is a method for controlling an induction motor, including obtaining a value of torque required depending on an operating condition of the induction motor in time series, determining whether or not the value of the required torque is satisfied by comparing the required torque with a torque control pattern in which the torque is set in advance depending on a rotational speed of the induction motor, changing the torque control pattern so as to increase a maximum value of an exciting current in constant torque control when the value of the required torque is not satisfied in a speed region lower than a predetermined rotational speed, and changing a magnetic field control pattern in accordance with the changed torque control patt«n. By employing the present invention, the induction motor can be controlled at a higher torque than normal without deteriorating the durability of the induction motor. Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING Fig. 1 is a diagram showing the entire configuration of a rolling apparatus according to an embodiment of the present invention, Fig. 2 is a diagram showing the configuration of a motor control apparatus according to the embodiment of the present invention; Fig. 3 is a diagram showing the configuration of a motor control apparatus according to the embodiment of the present invention; Fig. 4 is a diagram showing an equivalent circuit of an induction motor according to the embodiment of the present invention; Fig. 5 is a diagram showing a vector relationship between a current and a voltage of the induction motor according to the embodiment of the present invention; Fig. 6 is a graph showing a torque-speed characteristic in typical induction motor control; Fig, 7 is a diagram showing an expression of an electric torque required for rolling; Figs. 8A-8D are graphs showing examples of a torque-speed characteristic -5- required for the induction motor; Fig. 9 is a graph showing a torque-speed characteristic of the induction motor according to the embodiment of the present invention; Fig. 10 is a graph showing a torque-speed characteristic of the induction motor according to a conventional technique; Fig. 11 is a graph showing a relationship between a torque magnification and a change in a line current; Fig. 12 is a graph showing a torque-speed characteristic of the induction motor according to the embodiment of the present invention; Fig. 13 is a graph showing a torque-speed characteristic of the induction motor according to the embodiment of the present invention; Fig. 14 is a diagram showing the configuration of a control system of the induction motor according to the embodiment of the present invention; Fig. 15 is a diagram showing a state of a rolling operation according to the embodiment of the present invention; Figs. 16A-16E are graphs showing examples of comparing the torque-speed characteristic and a required torque according to the embodiment of the present invention; Figs. 17A-17E are graphs showing examples of comparing the torque-speed characteristic and a required torque according to the embodiment of the present invention; Fig. 18 is a flowchart showing an operation of changing the torque-speed characteristic according to the embodiment of the present invention, Fig. 19 is a flowchart showing an operation of changing the torque-speed characteristic according to the embodiment of the present invention; Fig. 20 is a diagram showing a manner of changing a torque current command according to the embodiment of the present invention; Fig. 21 is a diagram showing the configuration of a motor control ^paratus according to an embodiment of the present invention; Fig. 22 is a diagram showing the configuration of a motor control apparatus according to the embodiment of the present invention; Fig. 23 is a graph showing a torque-speed characteristic in typical induction motor control; Figs. 24A-24C are graphs showing examples of a torque-speed characteristic required for the mduction motor; Fig. 25 is a graph showing a torque-speed characteristic of the induction motor -6-according to the embodiment of the present invention; Fig. 26 is a graph showing a torque-speed charaaeristic of the induction motor; Fig. 27 is a gr^h showing a relationship between a torque magtiification and a change in a line current; and Fig. 28 is a graph showing a torque-speed characteristic of the induction motor according to another embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION In the following, an embodiment of the present invention will be described in detail by reference to the drawings. In the present embodiment, a case in which the present invention is applied to a single-stand rolling mill will be described as an example. Fig. 1 shows the configuration of the single-stand rolling mill. The single-stand rolling mill has an entry-side TR (a tension reel is abbreviated to TR) 2 on an entry side with respect to a rolling direaion of a mill 1, and an exit-side TR 3 on an exit side. Rolling is performed by unwinding a material to be rolled from the entry-side TR 2, rolling the material by the mill 1, and winding the material onto the exit-side TR 3. A roll gap control apparatus 7 that enables to control a strip thickness of the material to be rolled by changing a roll gap, and a mill speed control apparatus 4 that controls the speed of the mill 1 are installed in the mill 1. The entry-side TR 2 and the exit-side TR 3 are driven by a motor An entry-side TR control apparatus 5 and an exit-side TR control apparatus 6 are installed as the motor and a device for driving the motor. A rolling control apparatus 20 outputs a command to the control apparatuses. In the single-stand rolling mill, the entry side and the exit side are reversed depending on the rolling direction since reverse rolling is performed so as to switch the rolling directions. However, as a machine structural definition, a tension reel on the left side of the rolling mill is set to the entry-side tension reel, and a tension reel on the r^ht side of the rolling mill is set to the exit-side tension reel m the present embodiment. At the tune of rolling, a rolling speed setting device 10 outputs a speed command to the mill speed control apparatus 4, and the mill speed control apparatus 4 performs such control as to make the speed of the mill 1 constant. Rolling is stably and effectively performed by applying a tension to the material to be rolled on the entry and exit sides of the mill 1. An entry-side tension setting device 11 and an exit-side tension setting device 12 calculate a tension required for the stable and effective rolling. An entry-side tension-current converting device 15 and an exit-side tension- -7-current converting device 16 respectively obtain current values for obtaining motor torques, which are required to apply the set tension to the material to be rolled, based on entry-side and exit-side tension setting values calculated respectively by the entry-side tension setting device 11 and the exit-side tension setting device 12. The entry-side tension-current converting device 15 and the exit-side tension-current converting device 16 provide the respeaive current values obtained as described above to the entry-side TR control apparatus 5 and the exit-side TR control apparatus 6. The entry-side TR control apparatus 5 and the exit-side TR control apparatus 6 control motor currents so as to obtain the currents provided from the entry-side tension-current converting device 15 and the exit-side tension-curtent converting device 16. A predetermined tension is thereby applied to the material to be rolled by the motor torques applied to the entry-side TR 2 and the exit-side TR 3 based on the motor currents. Although the tension-current converting devices 15 and 16 calculate the current setting values (motor torque current setting values) so as to obtain the tension setting values based on a model of the TR mechanical system and the TR motor control apparatus, the used model has an error. Thus, in entry-side tension control 13 and exit-side tension control 14, the tension setting values are corrected by using actual tensions measured by an entry-side tension meter 8 and an exit-side tension meter 9, and the corrected tension setting values are input into the tension-current converting devices 15 and 16. The entry-side tension meter 8 and the exit-side tension meter 9 are respectively inst^ed on the entry side and the exit side of the mill 1. Accordingly, the current values set in the entry-side TR control apparatus 5 and the exit-side TR control apparatus 6 are corrected. The TR control apparatus here is composed of the motor that drives the TR mechanical system, and the control apparatus. Since the strip thickness of the material to be rolled is important in tenns of product quality, strip thickness control is performed. An exit-side strip thickness control device 18 controls the strip thickness on the exit side of the mill 1 by adjusting the roll gap of the mill 1 by use of the roll gap control apparatus 7 based on an actual strip thickness detected by an exit-side strip thickness meter 17. As described above, in the single-stand rolling mill, constant torque control is employed so as to make a torque generated by the motor constant for the TRs used for winding and unwinding the material, and the tension applied to the material to be rolled is controlled to be constant by correcting a motor current command by use of the actual tensions detected by the tension meters. Since the motor torque to be actually output is determined by a field and a torque current applied to the motor, the current conmiand needs to be changed according to the -8-field so as to make the torque constant. Rolling is performed by unwinding the material to be rolled from the entry-side tension reel 2, rolling the material to be rolled by the mill 1, and winding the rolled material to be rolled onto the exit-side tension reel 3. Fig. 2 shows the speed control configuration of an induction motor as the mill speed control apparatus 4. An induction motor 101 is driven following a speed command Nrcf (a motor rotational speed command) from the rolling speed setting device 10. The speed command Nref is a target value of a rotational speed N (r/min). The induction motor 101 is used in production facilities such as hot rolling facilities, cold rolling facilities, and processing line facilities. A speed controller 104 outputs a torque current command Iqref to a torque current exciting current controller 107 based on a difference (NrerN) between the above speed command Nrcf and the rotational speed N of the induction motor 101 detected by a speed sensor 103. That is, the speed controller 104 functions as a torque current control unit. A magnetic flux commander 105 outputs a magnetic flux command ^irfof a field flux to an exciting current calculator 106 based on the rotational speed N detected by the speed sensor 103. In the magnetic flux commander 105, the magnetic flux command re( is set with respect to the rotational speed N in advance. The magnetic flux command r«f with respect to the rotational speed N is used as a magnetic field control pattern. The magnetic flux commander 105 determines a magnetic flux 0 according to the rotational speed N based on a magnetic flux ^-speed characteristic according to a torque-speed characteristic described below. The magnetic flux O-speed characteristic is the magnetic field control pattern described above. The speed controller 104 outputs Iqcef according to (NFrefN) as described above based on a torque-ourent conversion coefficient according to the torque-speed characteristic described below. The torque-speed characteristic is stored in a magnetic flux command changing device 140. When the torque-speed charaaeristic is changed, the magnetic flux command changing device 140 sets the torque-current conversion coefiRcient and the magnetic flux ^-speed characteristic according to the changed torque-speed characteristic respectively with respect to the speed controller 104 and the magnetic flux commander 105. The exciting current calculator 106 calculates an exciting current of the induction motor 101 based on the magnetic flux command «f from the magnetic flux commander 105, and outputs an exciting current command l^ref to the torque current exciting current controller 107. That is, the magnetic flux commander 105 and the exciting current calculator 106 flinction as an exciting current control unit. A current calculator 109 calculates a torque current Iq and -9- an exciting current Id based on a primary current (a stator current) flowing through the induction motor 101 detected by a current sensor 110. That is, the current calculator 109 converts a line current of the induction motor 101 to a q-axis and d-axis coordinate system rotating in synchronization with a power source frequency of the output of a power converter 112. The current calculator 109 outputs the torque cun-ent I, and the exciting current Id calculated as described above. The torque current exciting current controller 107 outputs to a coordinate converter Ilia torque voltage command Vq for causing the torque current Iq output from the current calculator 109 to follow the torque current command Iqrcf output from the speed controller 104, and an exciting voltage command Vj for causing the exciting current Id output from the current calculator 109 to follow the exciting current command W output from the exciting current calculator 106. Since an output frequency of the current calculator 109 can be set by a known technique, the description thereof is omitted in Figs. 2 and 3. The coordinate converter 111 converts the torque voltage command Vq and the exciting voltage command Vd to a fixed coordinate system, and generates a three-phase voltage command V. The coordinate converter 111 outputs the generated voltage command V to the power converter 112. The powor converter 112 is a PWM (pulse width modulation) inverter, for example. The power converter 112 converts the power of a direct-current power source 113 (for example, PWM conversion) based on the above voltage command V, and supplies a three-phase alternating-current power to the mduction motor 101. By employing the above configuration, the primary current of the induction motor 101 is controlled, so that speed control is performed on the induction motor 101. In the entry-side TR control apparatus 5 and the exit-side TR control apparatus 6, current control is performed so as to make constant the tension applied to the material to be rolled from the TR Fig. 3 shows the current control configuration of an induction motor. Unlike in the case of the speed control, W as the torque current command Iqwf is directly applied in the current control. Thus, the same configuration as the speed control configuration in Fig. 2 is employed except that the speed controller 104 is not provided. Fig. 4 shows a one-phase T-shaped equivalent circuit of the induction motor. The exciting current Id flows through an exciting circuit, and the torque current I and a torque Tq of the induction motor 101 can be calculated by following expressions (1) and (2). <^ = K, • Id = Ki ' ^— = K, ' - —^ C 1 ) CO • LB 6t) T, = K2. <^ . I^ ■ ( 2 ) Here, Ki and K2 are coefficients determined by the characteristic of the induction motor. When an electric constant of the induction motor is obtained, the exciting current I4 can be obtained ifinom the above expression (1). Moreover, when a torque required for rolling is obtained, the torque current I, can be obtained from the expression (2). TTie line voltage Vs for obtaining the exciting current Id and the torque current Iq can be obtained by using following expressions (3) to (5) based on Fig. 5. V, - R, - I, + 6) ■ ( L, + L2 ) • Id + Ed ( 3 ) Vd = CO • ( L, + L2 ) ■ I, - R, • Id ( 4 ) Vs=73"-JV/ + V,2 (5) Next, a control characteristic of the induction motor will be described. Fig. 6 is a graph showing a typical control characteristic of the induaion motor The induction motor is controlled by constant torque control so as to make an output torque constant to a BASE speed as a predetttinined rotational speed set in advance, and controlled by constant output control so as to make a motor output (a power) constant from the BASE speed to a TOP speed. Thus, when -li¬the torque current is constant, the output torque is constant to the BASE speed, and the torque decreases in inverse proportion to the rotational speed at the BASE speed or more. The internal induced electromotive force Ed reaches a maximum value until the rotational speed reaches the BASE speed, and the value is maintained during the constant output control. The magnetic flux commander 105 increases the internal induced electromotive force Ed in proportion to an increase in the rotational speed N (an angular speed ©) of the induction motor 101 according to the expression (1) so as to make a field constant during the constant torque control. After the angular speed o reaches the BASE speed, the internal induced electromotive force E decreases in inverse proportion to the angular speed CD at the BASE speed or more, and the output torque Tq similarly decreases in inverse proportion to the speed. At this time, the line voltage Vs increases along with the increase in the angular speed © based on the expressions (3) to (5). An angular speed co at which the line voltage V, reaches a maximum line voltage VJMAX is set as the TOP speed. The induaion motor cannot rotate at the TOP speed or more. For the motors of the mill and the TR5 in the rolling mill, a speed-torque charactMistic as shown in Fig. 6 is determined based on a maximum speed for performing a rolling operation, a required maximum torque or the like according to the intended purpose of a rolling fecility. A motor having the characteristic correspondii^ thereto is fabricated and installed. Fig. 7 shows one example of an expression of a motor torque required for rolling. Torques Tq^m, T,MILL, TqOTR required for the entry-side TR 2, the mill 1, and the exit-side TR 3 can be respectively obtained based on product specifications and rolling mill specifications. Motors capable of obtaining the above torques at a motor rotational angular speed ffl obtained fi-om a rolling speed required for a rolling operation need to be provided in the rolling facility The motor rotational angular speed © can be obtained from the rolling speed by using the rolling mill specifications. The rolling speed can be determined in consideration of operation efficiency, the product specifications of the material to be rolled, or the like. Figs. 8A to 8C show simple examples of the torque-speed characteristic required for rolling, In Figs. 8A to 8C, a combination of the required torque and the speed is indicated by a star mark. In a case of Fig. 8 A, the torque-speed characteristic required for rolling can be satisfied by selecting a motor having a torque-speed characteristic A. Meanwhile, in a case of Fig. 8B, when the torque-speed characteristic A is employed, the torque-speed characteristic cannot be satisfied in a low speed region. To satisfy -12-the torque-speed characteristic in an entire speed region, a motor having a torque-speed characteristic B is required. Since a motor with a large output is required in this case, the motor and the motor control apparatus need larger capacity, and the amount of capital investment increases. Here, a large torque is required in the low speed region in the example in Fig. 8B. Thus, if one motor is provided with such a torque-speed characteristic as to output a large torque at a low speed, both the demands in Figs. 8 A and 8B can be satisfied by using one motor and one motor control apparatus. Meanwhile, the terminal voltage applicable to the motor is limited, and the maximum output of the motor is also limited. Thus, the BASE speed needs to be lowered so as to obtain a higher torque than normal. That is, the motor is controlled by the motor control apparatus normally as the motor having the torque-speed characteristic A, and as a motor having a torque-speed characteristic C shown in Fig. 8C when the rolling requiring a large torque is performed. Accordingly, a required rolling operation can be performed without increasing the amount of capital investment. To achieve the above state, a motor speed control device 100 according to the present embodiment increases the field $ by increasing Ed in the expression (1). To increase E4, the maximum terminal voltage VJMAX may be obtained at a low speed by increasing the increase rate of the terminal voltage Vs with respect to a speed change as shown in Fig, 9. The internal induced electromotive force Ed as the voltage of the exciting circuit in the induction motor is controlled to be lowered along with the increase in the rotational speed such that the maximimi terminal voltage VMAX is maintained during the constant output control. The power of the termmal voltage V, applied in a larger amount than normal due to the increase in the increase rate is used so as to make the exciting current Id higher than that in the normal control. As a result, the field flux O becomes higher, and the torque T, also becomes higher. In this case, a lower torque than that in the normal control is obtained in a predetermined speed region exceeding the BASE speed as shown in Fig. 9. However, since the object is to obtain a high torque at a low speed, the rolling speed may be limited in a rolling operation. As another method for changing the torque-speed characteristic, there is a method of increasing the torque current command. Fig. 10 shows a torque-speed characteristic in this case. In this case, the torque increases in proportion to an increase in the torque current. When the increase rate of the primary current!«in Fig. 9 and the increase rate of the primary current Ig in Fig. 10 are compared, the increase rate of the primary current Is is found to be smaller in the case of Fig. 9, that is, in the manner employed in the present embodiment while -13-the mcrease rate of the torque Tq is larger in Fig. 9. Generally, the exciting current is about 30% of the torque current. For example, when the torque is to be increased by 10%, there are a method of increasing the exciting current by 10% (the case of Fig. 9), and a method of increasing the torque current by 10% (the case of Fig. 10) as understood from the above expressions (1) and (2). Since the exciting current is about 30% of the torque current as described above, an increase in the line current is smaller when the exciting current is increased by 10%. The line current Is is obtained by a following expression (6). h^J^'JU' + U' (6) Fig. 11 shows a relationship between the torque and the line current. In Fig. 11, a solid line represents a change in the line current according to a torque magnification when the torque current is changed, and a dash line represents a change in the line current according to the torque magnification when the exciting current is changed. As shown in Fig. 11, when the torque is changed by changing the exciting current, the change in the line current is smaller than when the torque current is changed. Accordingly, the torque current is controlled when the torque is decreased, and the exciting current is controlled when the torque is increased. The induction motor torque can be thereby increased while the increase in the line current is being suppressed. When the increase in the line current is suppressed, a heat loss can be also suppressed, and heat generation from the motor and the motor control apparatus can be also prevented. Fig. 8D shows a case in which rolling requires a torque higher than that of the torque-speed characteristic A at a speed exceeding the BASE speed. To obtain a higher torque higher at a speed exceeding the BASE speed, a torque-speed characteristic D may be employed. When the torque-speed characteristic is changed as described above, the BASE speed at which the induced voltage stops being increased according to the speed is raised to a speed at which the terminal voltage reaches the maximum voltage, thereby increasing the constant torque rotational speed region as shown in Fig. 12. The rolling requiring a large torque can be performed by changing the torque-speed characteristic when the rolling is performed at a low speed. In this case, there are of course limiting conditions due to heat generation in the motor, the capacity of the motor control apparatus or the like. Thus, the rolling is performed to a maximum extent imder the limiting conditions. The method can be also employed when rolling requiring a rolling torque not -14-anticipated at the time of planning of the rolling fecility is performed. As described above, the torque-speed characteristic of the motor used for the rolling mill is changed by changing the patten of the field <1> in the motor control, A high torque can be thereby efFeaively obtained within an acceptable applied voltage range for the induaion motor Therefore, when rolling requiring a high torque'is performed or the like, the rolling can be eflfectively performed by providing an optimum torque-speed characteristic without performing such control as to deteriorate the facility durability in present rolling facilities. In Fig. 9, the following case has been described as an example. That is, the BASE speed, i.e., the rotational speed of the induction motor at which the constant torque control is switched to the constant output control is reduced to lower than a default value, that is, a rated value. The terminal voltage Vg is allowed to reach the maximum value V^MAX during the constant torque control until the rotational speed reaches the BASE speed. The power supphed in a larger amount than that in a defeult state at this time is distributed to the exciting current. The internal induced electromotive force Ed is controlled to be gradually lowered in the speed region of the constant output control in which field weakening control is performed. The exciting current h is ther^y decreased. However, the gist of the present embodiment is in that the terminal vohage Vs is allowed to reach VSMAX during the constant torque control until the rotational speed reaches the BASE speed, the power supplied in a largtt" amount than that in the default state is distributed to the exciting current, and the torque T, exerted at the BASE speed is thereby made higher than that in the default state. Thus, it is not always necessary to reduce the BASE speed to lower than that in the default state, that is, the rated value as shown in Fig. 9. The advantage according to the present embodiment may be also obtained even when the above control is performed while the BASE speed is being maintained as shown in Fig. 13. Here, when Figs. 13 and 10 are compared, it is foimd that while there is no change in the BASE speed and the amount of torque increase is also the same in Figs. 13 and 10, Figs. 13 and 10 diflfer in the increase rate of the primary current Is such that the increase rate of the primary current U is smaller in the case of Fig. 13 than in the case of Fig. 10. By employing the control apparatus and the control method for the induction motor according to the present embodiment, the increase in the line current can be made smaller by increasing the exciting current even when the same motor output torque is obtained. On the other hand, the desired torque Tq may not be obtained by the amount of power increase obtained by performing the above control while maintaining the BASE speed. -15- In this case, the desired torque Tq can be obtained at a low speed by reducing the value of the BASE speed as shown in Fig. 9. Please note that the BASE speed is a lowest speed within the range of the speed © for perfonning the constant output control, and the TOP speed is a highest speed within the range. Next, a specific configuration for performing such control as to change the torque-speed characteristic of the induction motor as shown in Figs. 9, 12, and 13 will be described. Fig. 14 shows the configuration of a control system of a case in which the torque-speed characteristic of the induction motor is changed according to the present embodiment. As shown in Fig. 14, the mill speed control apparatus 4 in the control configuration of the single-stand rolling mill is specifically composed of an induction motor 1-101 that drives the work rolls of the mill 1, a speed sensor 1-103 that detects the rotational speed of the induaion motor, and a motor speed control device 141. The entry-side TR control apparatus 5 is composed of an induction motor 2-101 that drives the entry-side TR 2, a speed sensor 2-103 that detects the rotational speed of the induction motor, and a motor current control device 142. Similarly, the exit-side TR control apparatus 6 is composed of an induction motor 3-101 that drives the exit-side TR 3, a speed sensor 3-103 that detects the rotational speed of the induction motor, and a motor current control device 142. The entry-side tension-current converting device 15 ui the rolling control apparatus 20 issues the current command W to the motor current control device 142 of the entry-side TR control apparatus 5. The exit-side tension-current converting device 16 issues the current command Iref to the motor current control device 142 of the exit-side TR control apparatus 6. The rolling speed setting device 10 issues the speed command Nrrf to the motor speed control device 141 of the mill speed control ^paratus 4. The motor speed control device 141 and the motor current control devices 142 respectively control the induction motors 1-101, 2-101, and 3-101 so as to fiilfill the speed command and the current commands. As to the entry-side TR 2 and the exit-side TR 3, the motor torque required for rolling is determined by the tension of the material to be rolled and a reel diameter as shown in Fig. 7. As to the mill 1, the motor torque required for rollmg is determined by the sum of a torque required for processing the material to be rolled and a torque according to the tensions on the entry and exit sides of the mill 1 and a roll diameter driven by the motor. A torque characteristic setting apparatus 21 obtains the torque required for rolling based on the product specifications and the mechanical specifications as shown in Fig. 7. The torque characteristic setting apparatus 21 ensures the motor torque required during rolling by -16- comparing the torque with the motor characteristic. The torque charaaeristic setting apparatus 21 includes a current control torque characteristic setting device 22 that creates a field changing command with respect to the motor current control device 142 driven by the tension reel etc. current command, and a speed control torque characteristic setting device 23 that aeates a field changing command with respect to the motor speed control device 141 driven by the mill etc. speed command. The current control torque characteristic setting device 22 receives a tension coiTunand on each of the entry and exit sides of the mill from the entry-side tension setting device 11 and the exit-side tension setting device 12. The current control torque characteristic setting device 22 determines the torque-speed characteristic of the motor in which the current is controlled such that the set tension can be mamtained during rolling based on a rolling speed pattern, and the reel radius of each of the entry-side TR 2 and the exit-side TR 3. The current control torque characteristic setting device 22 transmits the torque-speed characteristic to the magnetic flux corunand changing device 140 of the motor current control device 142, and thereby changes the torque-speed characteristic. Fig. 15 shows a rolling operation method. Since rolling is performed by winding the material to be rolled fed out of the entry-side TR 2 onto the exit-side TR 3, ^TR is large before the rolling starts, and RDTR is large after the rolling ends. The rolling speed is also gradually raised from 0 from the rolling start, and is gradually lowered to 0 until the rolling end Since the TR radius is changed, the torque requu^ed for the motor always varies during the rolling operation. The entry-side TR requires a largest torque at the rolling start, and the exit-side TR requires a largest torque at the rolling end. In the rolling, speed control is performed based on the speed of the mill. That is, the rolling speed in Fig, 15 is the speed of the mill. Thus, even when the rolling speed is constant, the motor rotational speed is low vAicn the TR radius has a large value, and the motor rotational speed is high when the TR radius has a small value. For example, the torque required by the entry-side TR is as indicated by a soUd line in Fig. 16C. When it is assumed that the torque-speed characteristic of the motor is as indicated by an alternate long and short dash line in Fig. 16D, the torque of the motor with respect to the lapse of time is indicted by an alternate long and short dash line in Fig. 16A, which has a portion lower than the torque required in the entry-side TR indicated by a solid line (a portion with an insufficient motor torque indicated by a dashed ellipse). Since the motor torque is insufficient in the portion, the tension of the material to be rolled carmot be maintained. When the torque-speed characteristic of the motor is changed as shown in Fig, -17- 16E (the BASE speed is raised), the required torque becomes lower than the motor torque throughout the entire rolling as shown in Fig. 16B, so that the tension can be maintained. When the motor torque is insufficient in the low speed region as shown in Fig. I7A, a torque characteristic B shown in Fig. 17D is changed to a torque characteristic C shown in Fig. 17E (the BASE speed is lowered, and the field current is increased in the constant torque control region to the BASE speed). The tension can be thereby mamtained. Fig. 18 shows the operation of the current control torque charaaeristic setting device 22. Although a case in which the torque-speed characteristic is changed for the entry-side TR. 2 will be described here, the torque-speed characteristic is similarly changed for the exit-side TR 3. As shown in Fig. 18, when acquiring a tension setting from the entry-side tension setting device 11, the current control torque characteristic setting device 22 calculates a required torque according to the reel radius by using the expression described in Fig. 7 based on the acquired tension setting (S1801). The current control torque characteristic setting device 22 obtains the value of a torque required according to various operating conditions of the induction motor m time series, and determines whether a present torque-speed setting is higher than the required torque as described using Figs. 16A and 17A (S1802). That is, in S1801 and S1802, the current control torque characteristic setting device 22 fiinctions as a torque confirming unit. When the present torque-speed setting is not lower than the required torque in any portion as a result of the determination in Si 802 (S1802/NO), the process is terminated since the tension can be mamtained. Meanwhile, when the present torque-speed setting is lower than the required torque in any portion (S1802A^S), a process of changing the torque-speed setting is started since the tension cannot be maintained. In this case, the process diflfers depending on whether or not a reel speed in the portion of the present torque-speed setting lower than the required torque is equal to or lower than the BASE speed. When the reel speed in the portion lower than the required torque is equal to or lower than the BASE speed (S1803A^S), the field current is increased and the BASE speed is lowered so as to increase the torque as shown in Figs. 8C and 9 (S1804). A new torque-speed characteristic is thereby created (S1806). That is, in S1806, the current control torque characteristic setting device 22 fiinctions as a torque control pattern changing unit. As described above, the field current may be also increased while the BASE speed is being maintained. For example, when the reel speed in the portion lower than the required torque is a speed just before reaching the BASE speed, the field current is increased while the BASE speed is being maintained so as to satisfy the required torque. Meanwhile, when the reel speed is higher than the BASE speed, the BASE speed -18- is raised by changing a speed timing at which the exciting current starts to be reduced so as to change the torque-speed characteristic as shown in Figs. 8D and 12. A new torque-speed characteristic is thereby created (SI806). The new torque-speed characteristic created in SI806 needs to be achievable by the entiy-side taision reel motor 2-101 and the motor current control device 142 as a control target. To this end, the line voltage needs to be set to a limit value or less. The current control torque characteristic setting device 22 thus peifonns a predictive calculation of the line voltage based on the exciting current and the torque current. When the line voltage exceeds the limit value (S1807/NO), the current control torque characteristic setting device 22 does not change the torque-speed characteristic, and outputs an alarm to inform an operator that the tension cannot be maintained when the rolling is continued (SI808). In this case, the operator selects to increase the torque current or reduce the tension, and continues the rolling. The operator may also select to stop the rolling, of course. When the line voltage is within the limit value (SISOTAHES), the present torque-speed characteristic is changed to the "new torque-speed characteristic" (Si809). The current control torque charaaeristic setting device 22 outputs the new torque-speed characteristic to the magnetic flux command changing device 140 in the motor current control device. The magnetic flux command changing device 140 obtains a magnetic flux -speed characteristic so as to obtain the given torque-speed characteristic, and sets the magnetic flux 4>-speed characteristic in the magnetic flux commander 105. That is, the magnetic flux command changing device 140 fonctions as a magnetic field control pattern changing unit. Here, the magnetic flux-speed characteristic can be obtained from the torque-speed characteristic by a known technique based on the electric characteristic of the induction motor, When the torque-speed characteristic of the motor is changed, the torque-current conversion coefficient is also changed. The entry-side tension-current converting device 15 converts the torque obtained by the entry-side setting tension and the entry-side reel radius to a current by using the torque-current conversion coefficient, and outputs the current to the motor current control device 142. When the magnetic flux $-speed characteristic of the magnetic flux commander 105 is changed according to the change in the torque-speed characteristic, the magnetic flux coomiand changing device 140 correspondingly changes the torque-current conversion coefficient of the speed controller 104 (S1810). That is, the magnetic flux command changing device 140 also functions as a conversion coefficient changing unit that changes the conversion coefficient according to the change in the torque-speed characteristic. The case in which the torque is calculated from the entry-side tension setting -19-value set by the entry-side tension setting device 11 has been described here. When the entry-side tension control 13 is performed using a tension detection value of the entry-side tension meter 8 as shown in Fig, 1, the entry-side tension setting value plus the output of the entry-side tension control 13 is employed as the entry-side taision command, so that the torque may be calculated by using the command. The speed of the motor 1-101 of the mill 1 is controlled to follow the command by the motor speed control device 141. Since the material to be rolled is processed by rolling in the mill 1, the torque required therefor also needs to be provided from the motor. It is thus difficult to calculate the required torque as in the entry-side TR and the exit-side TR. Although it is possible to calculate the torque TqMnx from the product specifications and the mechanical specifications as shown in Fig. 7, an actually required mill torque T,(MILL is often different from the calculated value due to a calculation error of a torque arm coefficient and a rolling load expression. Thus, the speed control torque characteristic setting device 23 calculates the torque required for the motor from the torque current command Iqref, which is the output of the speed controller 104 actually controlled by the motor speed control device 141, and changes the torque-speed characteristic by using the result. Fig. 19 shows the general operation of the speed control torque characteristic setting device 23. As shown in Fig. 19, the speed control torque characteristic setting device 23 receives a speed controller torque current command as the output of the speed controller 104 from the motor speed control device 141, and calculates a required torque by using the torque-current conversion coefficient (S1901). The follovving steps are the same as those of the current control torque charaaeristic setting device shown in Fig. 18. In the case of the mill, an eTdt-side strip thickness setting of the material to be rolled may be changed in the mill (re-set to a larger thickness), or the tension may be changed (re-set to a larger tension) as an option selected when rolling cannot be performed. The torque-current conversion coefficient changed hi S1910 is used in SI901 in a next process. The torque-speed characteristic is given to the magnetic flux command changing device 140 in the motor speed control device 141 to create the magnetic flux-speed characteristic. At the same time, it is necessary to change the torque current command output from the speed controller 104 so as to make the output torque of the motor constant since the magnetic flux is changed. Fig. 20 schematically shows a process. In the speed controller 104, the torque current command I,rrf is obtained by proportional-integral control of a speed error AN. In this case, the torque current command Iqref is the sum of a proportional term IqPref and an integral term Iqiref as shown in a following expression (7). -20- ^^ref - IB at a field characteristic switch point. In this case, when a torque current command m the field (A) is represented as I decreases in inverse proportion to the angular speed o at the BASE speed or more, and an output torque Tq similarly decreases in inverse proportion to the speed. At this time, a line voltage V, increases along with the increase in the angular speed CD based on the expressions (3) to (5). An angular speed © at which the line voltage V, reaches a maximum line voltage VSMAX is set as the TOP speed. The induction motor cannot rotate at the TOP speed or more. For the motors of the mill and the TRs in the rolling mill, a speed-torque characteristic as shown in Fig. 23 is determined based on a maximum speed for performing a rolling operation, a required maximum torque or the like according to the intended purpose of a rolling facility. A motor having the characteristic corresponding thereto is fabricated and installed. One example of an expression of a motor torque required for rolling is as shown in Fig. 7. Torques TQETR, T<,MILU TqoxR required for the entry-side TR 2, the mill I, and the exit-side TR 3 can be respectively obtained based on product specifications and rolling mill specifications. Motors capable of obtaining the above torques at a motor rotational angular speed 0) obtained from a rolling speed required for a rolling operation need to be provided in the rolling facility. The motor rotational angular speed CD can be obtained from the rolling speed by using the rolling mill specifications. The rolling speed can be determined in consideration of -24- operation eflficiency, the product specifications of the material to be rolled, or the like. Figs. 24Ato 24C show simple examples of the torque-speed charaaeristic required for rolling. In Figs. 24A to 24C, a combination of the required torque and the speed is indicated by a star mark. In a case of Fig. 24A, the torque-speed characteristic required for rolling can be satisfied by selecting a motor having a torque-speed charaaeristic A. Meanwhile, in a case of Fig. 24B, when the torque-speed characteristic A is employed, the torque-speed characteristic cannot be satisfied in a low speed region. To satisfy the torque-speed characteristic in an entire speed region, a motor having a torque-speed charaaeristic B is required. Since a motor with a large output is required in this case, the motor and the motor control apparatus need larger capacity, and the amount of capital investment increases. Here, a large torque is required in the low speed region in the example in Fig, 24B. Thus, if one motor is provided with such a torque-speed characteristic as to output a laige torque at a low speed, both the demands in Figs. 24A and 24B can be satisfied by using one motor and one motor control apparatus. Meanwhile, the terminal voltage applicable to the motor is limited, and the maximum output of the motor is also limited. Thus, the BASE speed needs to be lowered so as to obtain a higher torque than normal. That is, the motor is controlled by the motor control apparatus normally as the jnotor having the torque-speed characteristic A and as a motor having a torque-speed characteristic C shown in Fig. 24C when the rolling requiring a large torque is performed. Accordingly, a required rolling operation can be performed without increasing the amount of coital investment. To achieve the above state, a motor speed control device 100 according to the present embodiment increases the field O by increasing Ea in the expression (1). To increase Ed, the maximum terminal voltage VSMAX may be obtained at a low speed by increasing the increase rate of the terminal voltage V, with respect to a speed change as shown in Fig. 25. The internal induced electromotive force Ea as the voltage of an excituig circuit in the induction motor is controlled to be lowered along with the increase in the rotational speed such that the maximum terminal voltage VRMAX is maintained during the constant output control. The power of the terminal voltage V, applied in a larger amount than normal due to the increase in the increase rate is used so as to make the exciting current Id higher than that in the normal control. As a result, the field flux becomes higher, and the torque Tq also becomes higher. In this case, a lower torque than that in the normal control is obtained in a predetermined speed region exceeding the BASE speed as shown in Fig. 25. However, since the objea is to obtain a high torque at a low speed, the rolling speed may be limited in a rolling -25- operation. As another method for changing the torque-speed characteristic, there is a method of increasing the torque cuirent command. Fig. 26 shows a torque-speed charaaeristic in this case. In this case, the torque increases in proportion to an increase in the torque current. When the increase rate of the primary current I* in Fig. 25 and the increase rate of the primary current Ig in Fig. 26 are compared, the increase rate of the primary current I^ is found to be smaller in the case of Fig. 25, that is, in the manner employed in the present embodiment wiiile the increase rate of the torque T, is larger in Fig. 25. Generally, the exciting current is about 30% of the torque current. For example, when the torque is to be increased by 10%, there are a method of increasing the exciting current by 10% (the case of Fig. 25), and a method of increasing the torque current by 10% (the case of Fig. 26) as understood from the above e3q)ressions (1) and (2). Since the exciting current is about 30% of the torque current as described above, an increase in the line current is smaller when the exciting current is increased by 10%. The line current Is is obtained by a following expression (10). Fig. 27 shows a relationship between the torque and the line current. In Fig. 27, a solid line represents a change in the line current according to a torque magnification when the torque current is changed, and a dash line represents a change in the line current according to the torque magnification when the exciting current is changed. As shown in Fig. 27, when the torque is changed by changing the exciting current, the change in the line current is smaller than when the torque current is changed. Accordingly, the torque current is adjusted when the torque is decreased, and the exciting current is adjusted when the torque is increased. The induaion motor torque can be thereby increased while the increase in the line current is being suppressed. When the increase in the line current is suppressed, a heat loss can be also suppressed, and heat generation from the motor and the motor control apparatus can be also prevented. In a rolling operation, a rolling speed is important for operation efficiency As the rolling speed becomes higher, the operation efiiciency is improved. Thus, it is preferable to output the torque required for the motor to a highest possible rolling speed. However, the increase in the line current can be made smaller by increasing the exciting current even when the same motor output torque is obtained. Thus, by employing a filed pattern in which the -26- maximum fine voltage is obtained at a maximum rotational speed of the motor required for the rolling operation, the line current can be minimized. A rolling control calculator that controls the rolling facility exists at a higher-order position of the motor control apparatus. Since the rolling control calculator recognizes what kind of rolling operation is to be performed, the rolling control calculator can appropriately calculate a torque-speed curve based on the maximum torque required for the motor and the maximum rotational speed. Accordingly, the torque-speed characteristic may be provided from the higher-ordM- position, and the motor control apparatus may create a speed-field pattern corresponding thereto. The information of the torque-speed characteristic calculated by the higher-order rolling control calculator is stored in the magnetic flux commander 105 shown in Figs. 21 and 22. The magnetic flux commander 105 determines the magnetic flux command ^nsf according to the rotational speed input from a speed calculator 115 based on the information of the stored torque-speed characteristic. The exciting current calculator 106 determines and outputs the exciting current command I^cf according to the magnetic flux command Orer input from the magnetic flux commander 105. That is, the magnetic flux commander 105 and the exciting current calculator 106 function as an exciting current determining unit. Rolling requiring a large torque can be performed by changing the torque-speed characteristic when the rolling is performed at a low speed. In this case, there are of course limiting conditions due to heat generation in the motor, the capacity of the motor control apparatus or the like. Thus, the rolling is performed to a maximum extent under the limiting conditions. The method can be also employed when rolling requiring a rolling torque not anticipated at the time of planning of the rolling facility is performed. As described above, the torque-speed characteristic of the motor used for the rolling mill is changed by changing the patten of the field in the motor control. A high torque can be thereby effectively obtained within an acceptable applied voltage range for the induction motor. Therefore, when rolling requiring a high torque is performed or the like, the rolling can be effectively performed by providing an optimum torque-speed characteristic without performing such control as to deteriorate the facility durability in present rolling facilities. In Fig. 25, the following case has been described as an example. That is, the BASE speed, i.e., the rotational speed of the induction motor at which the constant torque control is switched to the constant output control is reduced to lower than a defeult value, that is, a rated value. The terminal voltage Vj is allowed to reach the maximum value VJMAX during the -27- constant torque control until the rotational speed reaches the BASE speed. The power supplied in a larger amount than that in a default state at this time is distributed to the exciting current. The internal induced electromotive force Ed is controlled to be gradually lowered in the speed region of the constant output control in which field weakening control is performed. The exciting current Id is thereby decreased. However, the gist of the present embodiment is m that the terminal voltage V^ is allowed to reach VSMAX during the constant torque control until the rotational speed reaches the BASE speed, the power supplied in a larger amount than that in the default state is distributed to the exciting current, and the torque Tq exerted at the BASE speed is thereby made higher than that in the default state. Thus, it is not always necessary to reduce the BASE speed to lower than that in the default state, that is, the rated value as shown in Fig. 25. The advantage according to the present embodiment may be also obtained even when the above control is performed while the BASE speed is being maintained as shown in Fig. 28. Here, when Figs. 28 and 26 are compared, it is found that while there is no change in the BASE speed and the amount of torque increase is also the same in Figs. 28 and 26, Figs. 28 and 26 diflfer in the increase rate of the primary current I, such that the increase rate of the primary current I., is smaller in the case of Fig. 28 than in the case of Fig. 26. By employing the control apparatus and the control method for the induction motor according to the present embodiment, the increase in the line current can be made smaller by increasing the exciting current even when the same motor output torque is obtained. On the other hand, the desired torque Tq may not be obtained by the amount of power increase obtained by performing the above control while maintaining the BASE speed. In this case, the desired torque Tq can be obtained at a low speed by reducing the value of the BASE speed as shown in Fig. 25. Please note that the BASE speed is a lowest speed within the range of the speed co for performing the constant output control, and the TOP speed is a highest speed within the range. Although the single-stand rolUng mill has been described as an example as shown in Fig. 1 in the above embodiment, the present invention is not limited to the single-stand rolling mill, and may be also q)phed to a tandem rolling mill in which a plurality of mills are arranged in series. It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims. CLAIMS: 1. A control i^paratus for an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, comprising: an exciting current control imit which performs constant torque control so that the torque is made constant until a rotational speed reaches a predetermined rotational speed set in advance, and performs constant output control so that an output of the induction motor obtained by the torque and the rotational speed is made constant by deaeasing the exciting current at fi-om the predetennined rotational speed to a maximum rotational speed, the exciting current control unit controlling the exciting current in accordance with a magnetic field control pattern in which the exciting current is maintained at a maximum value in the constant torque control and is decreased in accordance with increase in rotational speed in the constant output control; a torque confirming unit which obtains a value of the torque required depending on an operating condition of the induction motor in time series, and determines whether or not the obtained value of the required torque is satisfied by comparing the required torque with a torque control pattern in which the torque is set in advance depending on the rotational speed of the induction motor, a torque control pattern changing unit which changes the torque control pattern so as to increase the maximum value of the exciting current in the constant torque control when the obtained value of the required torque is not satisfied in a speed region lower than the predetermined rotationaJ speed; and a magnetic field control pattern changing unit which changes the magnetic field control pattern in accordance with the changed torque control pattern. 2. The control apparatus for an induction motor according to claim 1, wherein the torque control pattern changing unit changes the torque control pattern so as to reduce the predetermined rotational speed to be lower than the predetermined rotational speed in the normal control when the obtained value of the required torque is not satisfied in the speed region lower than the predetermined rotational speed. 3. The control apparatus for an induaion motor according to claim 1, wherein the torque control pattern changing imit changes the torque control pattern so as to increase the predetermined rotational speed to be higher than the predetermined rotational speed in the normal control when the obtained value of the required torque is not satisfied in a speed region higher than the predetermined rotational speed. 4. The control apparatus for an induaion motor according to claim I, wherein the -29-induction motor is used for rotating a pair of rolls in a mill which rolls a material to be rolled by holding the material between the rolls, and the control apparatus further comprises: a torque current control unit which converts a tension command value of the material to be roiled to a command value of the torque current based on a conversion coeflficient; and a conversion coefficient changing unit which changes the conversion coefficient in accordance with the changed torque control pattern. 5. The control ^paratus for an induction motor according to claim 1, wherein the torque control pattern changing unit calculates a value of a line current determined by the exciting current and the torque current which are supplied to the induction motor after the torque control pattern is changed, determines whether or not a calculation result is within a limit value set in advance, and informs an operator that the control is impossible when the value of the line current exceeds the limit value. 6. A method for controlling an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, including the steps of: performing constant torque control so that the torque is made constant until a rotational speed reaches a predetermined rotational speed set in advance, performing constant output control so that an output of the induction motor obtained by the torque and the rotational speed is made constant by decreasing the exciting current at fix)m the predetermined rotational speed to a maximum rotational speed, and controlling the exciting current in accordance with a magnetic field control pattern in which the exciting current is maintained at a maximum value in the constant torque control and is decreased in accordance with increase in rotational speed in the constant output control; obtaining a value of the torque required depending on an operating condition of the induction motor in time series, and determining whether or not the obtained value of the required torque is satisfied by comparing the required torque with a torque control pattern in which the torque is set in advance depending on the rotational speed of the induction motor; changing the torque control pattern so as to increase the maximum value of the exciting current in the constant torque control when the obtained value of the required torque is not satisfied in a speed region lower than the predetermined rotational speed; and changing the magnetic field control pattern in accordance with the changed torque control pattern. 7. A control apparatus for an induction motor which obtains torque by an exciting -30-cuirent for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, characterized in that constant torque control is performed so that the torque is made constant until a rotational speed reaches a predetermined rotational speed set in advance, and constant output control is performed so that an output of the induction motor obtained by the torque and the rotational speed is made constant at from the predetermined rotational speed to a maximum rotational speed, in normal control, control is performed so that a voltage of an exciting circuit in the induction motor becomes maximum until the rotational speed reaches the predetermined rotational speed, and a terminal voltage applied to the induction motor is increased in accordance with increase in rotational speed and a maximum terminal voltage is applied at the maximum rotabonal speed so as to maintain the maximimi voltage of the exciting circuit in the constant output control, and in high torque control for temporarily obtaining a high torque, control is performed so that the terminal voltage applied to the induction motor until the rotational speed reaches the predetermined rotational speed has a higher value than the terminal voltage at the predetermined rotational speed in the normal control, the voltage of the exciting circuit in the induction motor is adjusted in accordance with the increase in rotational speed so as to prevent the terminal voltage from exceeding a limit value set in advance in the constant output control, and the exciting current becomes higher than that in the normal control by the terminal voltage applied higher than that in the normal control. 8. The control apparatus for an induction motor according to claim 7, wherein, in the high torque control, the control is performed so that the tenninal voltage becomes the maximum voltage until the rotational speed reaches the predetermined rotational speed, and the maximum terminal voltage is maintained in the constant output control. 9. The control apparatus for an induction motor according to claim 7, wherein the predetermined rotational speed in the high torque control is a rotational speed lower than the predetermined rotational speed in the nonnal control. 10. The control apparatus for an induction motor according to claim 7, comprising an exciting current determining unit which determines the exciting current in the high torque control based on the rotational speed of the induction motor and the torque required in accordance with the rotational speed. 11 • The control apparatus for an induction motor according to claim 10, wherein the exciting current determining unit determines the exciting current so that a line current of the -31- induction motor becomes minimum based on the rotational speed of the induction motor and the torque required in accordance with the rotational speed. 12- A method for controlling an induction motor which obtains torque by an exciting current for generating a magnetic field, and a torque current for generating torque in accordance with the magnetic field, comprising performing constant torque control so that the torque is made constant until a rotational speed reaches a predetermined rotational-speed set in advance, and performing constant output control so that an output of the induction motor obtained by the torque and the rotational speed is made constant at from the predetermined rotational speed to a maximum rotational speed, in normal control, controlling so that a voltage of an exciting circuit in the induction motor becomes maximum until the rotational speed reaches the predetermined rotational speed, and a terminal voltage applied to the induction motor is increased in accordance with increase in rotational speed and a maximum terminal voltage is applied at the maximum rotational speed so as to maintain the maximum voltage of the exciting circuit in the constant output control, and in high torque control for temporarily obtaining a high torque, controlling so that the terminal voltage applied to the induction motor until the rotational speed reaches the predetermined rotational speed has a higher value than the terminal voltage at the predetermined rotational speed in the normal control, the voltage of the exciting circuit in the induction motor is adjusted in accordance with the increase in rotational speed so as to prevent the terminal voltage from exceeding a limit value set in advance in the constant output control, and the exciting current becomes higher than that in the nonnal control by the terminal voltage applied higher than that in the normal control. 32 13. A control apparatus for an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example. 14. A method for controlling an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example. 15. A control apparatus for an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example. 16. A method for an induction motor which obtains torque by an exciting current for generating a magnetic field, substantially as herein described with reference to accompanying drawings and example. Dated this 28* day of June 2012 » » ^^U^/^^^ of Anand and Anand, Advocates Agents for the Applicants