JP4534887B2 - Method and apparatus for controlling draft device in spinning machine - Google Patents

Description

  The present invention relates to a control method and a control device for a draft device in a spinning machine.

  A spinning machine such as a ring spinning machine or a rough spinning machine includes a large number of spinning spindles along the longitudinal direction of the machine base, and the draft device drives a plurality of bottom rollers on one end side (gear end side) of the machine base. Since these bottom rollers are very long, a twist delay occurs on the out-end side opposite to the gear end at the time of starting. In particular, a bottom apron roller (middle bottom roller) in which an apron is wound around each weight is greatly twisted, and yarn unevenness and thread breakage may occur due to delay in twisting. Further, the yarn unevenness may occur not only due to the delay in twisting of the bottom roller at the start-up but also due to the behavior of the bottom roller twisting back during the stop period.

In recent years, in a high-speed ring spinning machine, the spindle is driven at a high rotational speed of about 25000 revolutions / minute. Then, there has been proposed a spinning machine operation control method for preventing troubles such as yarn breakage from the beginning of winding to 2-3 minutes, and for preventing yarn yarns that are spun at high speed from becoming thinner than desired. (For example, refer to Patent Document 1). In the control method of Patent Document 1, the winding degree of the yarn wound on the bobbin is detected, the rotation speed of the spindle is changed according to the winding degree, and the total draft ratio is set corresponding to the fluctuation of the spindle speed. Change to prevent fluctuations in the spinning yarn count.
JP-A-2-269814

  Patent Document 1 describes that the draft ratio of the draft device is changed to a draft ratio that is not the draft ratio corresponding to the original count during the spinning operation. However, the purpose of the change is to change the total draft ratio corresponding to the speed fluctuation of the spindle, and to prevent the fluctuation of the number in the same pipe yarn. Therefore, no consideration is given to the occurrence of yarn unevenness due to twisting of the bottom roller of the draft device.

  The present invention has been made in view of the above-described conventional problems, and an object thereof is to provide a control method and a control device for a draft device in a spinning machine capable of suppressing the occurrence of yarn unevenness at the time of startup. .

  In order to achieve the above object, the invention according to claim 1 is directed to control of a draft device in a spinning machine including a draft device in which a front bottom roller, a middle bottom roller, and a back bottom roller are independently driven by a motor. Is the method. Then, at least during the machine stop process, the brake draft ratio of the draft device is changed so as to reduce the load torque of the draft device from a predetermined value during steady operation. Here, the “predetermined value during steady operation” means a brake draft ratio that is set without considering the torsion of the bottom roller corresponding to the spinning condition.

  In the present invention, each bottom roller constituting the draft device is independently driven by a motor. Then, at least during the machine stop process, the brake draft ratio of the draft device is changed so as to reduce the load torque of the draft device from a predetermined value during steady operation. That is, since the torque for driving the bottom roller is reduced when spinning is stopped, the amount of twisting return of the bottom roller is reduced when the motor for driving each bottom roller is stopped. As a result, excessive feeding of fibers due to twisting back of the bottom roller while the machine base is stopped is suppressed, and occurrence of yarn unevenness when the machine base is restarted is suppressed.

  The invention according to claim 2 is a method for controlling a draft device in a spinning machine including a draft device in which a front bottom roller, a middle bottom roller, and a back bottom roller are independently driven by a motor. Then, at least during the machine standstill process, the brake draft ratio of the draft device is changed so as to reduce the load torque from a predetermined value during steady operation. Further, the brake draft ratio changed to the machine stand stop process in the machine stand restart process is returned to the original value.

  In this invention, at least during the machine stand stop process, the draft device is controlled in the same manner as in the first aspect of the invention, and the same operations and effects as in the first aspect of the invention are achieved. Also, in the process of restarting the machine base, after the drive is started in a state in which the torque for driving the bottom roller is reduced from the predetermined value during the steady operation, the brake draft ratio is the predetermined value during the steady operation. It is changed to become the value of. Accordingly, the twisting delay of the bottom roller at the time of restarting the machine base is suppressed, and the yarn unevenness is further suppressed.

  According to a third aspect of the present invention, in the first or second aspect of the present invention, the predetermined value during the steady operation is obtained by setting the brake draft ratio on the horizontal axis and the torque required for the brake draft on the vertical axis. If the brake curve is greater than the brake draft ratio corresponding to the torque peak value in the curve shown, the brake draft ratio is changed to increase. Further, when the predetermined value during the steady operation is smaller than the brake draft ratio corresponding to the peak value of the torque, the brake draft ratio is changed so as to decrease.

In the present invention, when the brake draft ratio is changed in the machine stand stop process, the torque for driving the bottom roller is surely reduced.
According to a fourth aspect of the present invention, in the invention according to any one of the first to third aspects, the brake draft ratio is changed by changing the speed of the middle bottom roller. As a method of changing the brake draft ratio, there are a method of changing the speed of the middle bottom roller, a method of changing the speed of the back bottom roller, and a method of changing the speeds of the middle bottom roller and the back bottom roller. When the speed of the back bottom roller is changed, the total draft ratio changes unless the speed of the front bottom roller is changed correspondingly. However, in this invention, since the speed of the back bottom roller is the original speed and the brake draft ratio is changed by changing the speed of the middle bottom roller, the total draft ratio is constant even if the brake draft ratio is changed. Control becomes easy.

  According to a fifth aspect of the present invention, there is provided a draft device control device in a spinning machine including a draft device in which a front bottom roller, a middle bottom roller, and a back bottom roller are independently driven by a motor. A brake draft ratio changing means is provided that changes the brake draft ratio of the draft device so as to reduce the load torque of the draft device from a predetermined value at least during the machine stop process. By using the apparatus of the present invention, the control method of the invention described in claim 1 can be carried out.

  According to the present invention, it is possible to suppress the occurrence of yarn unevenness during startup.

(First embodiment)
An embodiment in which the present invention is embodied in a ring spinning machine in which a draft device is provided in a pair of left and right along the longitudinal direction of a machine base will be described with reference to FIG. 1 is a schematic plan view of a draft device in which the top roller is omitted, FIG. 2 is a graph showing the relationship between the brake draft ratio and the torque required for the draft, and FIG. 3 is a graph showing the speed change of the bottom roller during operation of the machine base. It is. In FIG. 1, the upper side is the left side of the machine base and the lower side is the right side of the machine base.

  As shown in FIG. 1, the draft device 11 has a three-wire configuration including a front bottom roller 12, a middle bottom roller 13, and a back bottom roller 14. The front bottom roller 12 is supported at a predetermined position with respect to a roller stand (not shown), and the middle bottom roller 13 and the back bottom roller 14 are fixed to the roller stand so as to be adjustable in the front-rear direction (not shown). ) Is supported through. The middle bottom roller 13 includes an apron (not shown). Further, a front top roller, a middle top roller, and a back top roller (all not shown) are provided in a known configuration corresponding to each of the bottom rollers 12 to 14.

  The left and right front bottom rollers 12 are each formed by a single roller shaft 12a extending over the entire length of the machine base. The left and right middle bottom rollers 13 and the back bottom roller 14 are each divided into two roller shafts 13a, 13b, 14a, 14b, and the two roller shafts 13a, 13b, 14a, 14b are arranged coaxially. Has been. In this embodiment, the middle bottom roller 13 and the back bottom roller 14 are each divided at the center in the longitudinal direction, and the two roller shafts 13a, 13b, 14a, 14b are formed symmetrically.

  Each front bottom roller 12 is driven by a front roller drive motor 15 from one end side of the machine base (the gear end side on the left side in FIG. 1). The rotation of the front roller drive motor 15 is transmitted to each roller shaft 12a via a gear train (not shown).

  Each pair of left and right middle bottom rollers 13 has a pair of roller shafts 13a positioned on the left side in FIG. 1 driven by a first middle roller driving motor 16a from one end side of the machine base, and a pair of left and right middle bottom rollers 13 positioned on the right side in FIG. The roller shaft 13b is driven by the second middle roller driving motor 16b from the other end side (out-end side) of the machine base. The rotation of the first middle roller drive motor 16a is transmitted to each roller shaft 13a via a gear train (not shown), and the rotation of the second middle roller drive motor 16b is transmitted to each roller shaft 13b via a gear train (not shown). It has come to be.

  Each of the pair of left and right back bottom rollers 14 also has a pair of roller shafts 14a located on the left side in FIG. 1 driven by a first back roller drive motor 17a from one end side of the machine base, and a pair of left and right back bottom rollers 14 located on the right side in FIG. The roller shaft 14b is driven by the second back roller drive motor 17b from the other end side of the machine base. The rotation of the first back roller drive motor 17a is transmitted to each roller shaft 14a via a gear train (not shown), and the rotation of the second back roller drive motor 17b is transmitted to each roller shaft 14b via a gear train (not shown). It has come to be.

  Servo motors are used for the drive motors 15, 16a, 16b, 17a, and 17b, respectively. Each drive motor 15, 16a, 16b, 17a, 17b includes a rotary encoder 15r, 16ar, 16br, 17ar, 17br, respectively. The drive motors 15, 16a, 16b, 17a, 17b are connected to servo drivers 18, 19a, 19b, 20a, 20b, respectively, and are controlled by the control device 21 via the servo drivers 18, 19a, 19b, 20a, 20b. .

  The control device 21 includes a CPU (central processing unit) 22, a program memory 23, a work memory 24, an input device 25, an input interface, an output interface, and a motor drive circuit (all not shown). The CPU 22 is connected to the rotary encoders 15r, 16ar, 16br, 17ar, and 17br via an input interface. The CPU 22 is connected to the servo drivers 18, 19a, 19b, 20a, and 20b via an output interface and a motor drive circuit, respectively.

The CPU 22 also controls a motor (not shown) that drives the lifting drive system and the spindle drive system, and the control device 21 also functions as a control device for the spinning machine.
The program memory 23 stores program data and various data necessary for its execution. The above-mentioned various data includes data such as various fiber raw materials, spinning conditions such as spinning yarn number and number of twists, correspondence data of spindle rotation speed during steady operation, rotation speed of draft drive system and lifting drive system motor, etc. is there. Further, the program memory 23 stores a change value of the brake draft ratio in the process of stopping the base corresponding to the spinning condition.

  The work memory 24 temporarily stores the data input by the input device 25, the calculation processing result in the CPU 22, and the like. The input device 25 is used to input spinning condition data such as spinning yarn count, fiber type (raw material), maximum spindle speed during spinning operation, spinning length, lift length, chase length, and used bobbin length. . The work memory 24 includes a backup power source (not shown).

  The CPU 22 drives each drive motor so as to rotate the front bottom roller 12, the middle bottom roller 13, and the back bottom roller 14 so that the brake draft ratio and the total draft ratio are set in advance corresponding to the spinning conditions. 15, 16a, 16b, 17a, 17b are controlled.

  In the stop process of the machine base, the CPU 22 changes the brake draft ratio of the draft device 11 so that the load torque of the draft device 11 is reduced from a predetermined value during steady operation. The relationship between the brake draft ratio of the draft device 11 and the torque required for the draft is not a simple proportional relation, but there is a peak value (maximum value) determined by the spinning conditions as shown in FIG. When the brake draft ratio Bp corresponding to the torque peak value is greater than the brake draft ratio value Bp corresponding to the peak value, the load torque decreases as the brake draft ratio increases, and the torque Is smaller than the brake draft ratio Bp corresponding to the peak value, the load torque decreases as the brake draft ratio decreases. Therefore, the CPU 22 changes the brake draft ratio so as to increase if the brake draft ratio during steady operation is larger than the brake draft ratio Bp corresponding to the peak value, and changes the brake draft ratio to be smaller if the brake draft ratio is smaller. In this embodiment, the brake draft ratio at the time of steady operation is 1.25, which is larger than the brake draft ratio Bp corresponding to the peak value. Therefore, for example, the brake draft ratio is changed to 1.6 so as to increase. The CPU 22 constitutes a brake draft ratio changing means for changing the brake draft ratio of the draft device 11 so as to reduce the load torque of the draft device 11 from a predetermined value at least during the machine stop process.

  The brake draft ratio is changed by changing the speed of the middle bottom roller 13. The brake draft ratio is changed during deceleration after the bottom roller speed change is started to stop the machine base. When the brake draft ratio is changed, the time required to replace all the fibers existing between the back roller and the middle roller with those drafted at the changed brake draft ratio can be secured before the machine stops. Is set.

  Next, the operation of the apparatus configured as described above will be described. Prior to the operation of the spinning machine, spinning conditions such as fiber raw material, spinning yarn count, and number of twists are input to the control device 21 by the input device 25. When the operation of the spinning machine is started, each drive motor 15, 16a, 16b, 17a, 17b is spun via the servo drivers 18, 19a, 19b, 20a, 20b on the basis of a command from the control device 21. Control is performed so as to rotate at a rotation speed at which a total draft ratio and a brake draft ratio are set in advance corresponding to the output conditions. Further, the spindle drive system and lifting system drive motors (not shown) are also controlled to have a predetermined rotational speed.

  When the spinning machine is operated, the roving yarn passes through the back roller, the middle roller, and the front roller of the draft device 11 and is drafted, and then wound around a bobbin that rotates together with the spindle at a winding unit (not shown). Taken.

  When the machine base is stopped due to doffing or stopping halfway, the brake draft ratio of the draft device 11 is a predetermined value (for example, 1.25) at the time of steady operation, and the load torque is higher than when driving at that brake draft ratio. Is changed to a value (eg, 1.6). The CPU 22 decreases the deceleration of the middle bottom roller 13 so that the brake draft ratio becomes 1.6, that is, the rotation speed of the middle bottom roller 13 becomes higher than the original rotation speed. After the brake draft ratio is changed to 1.6, the middle bottom roller 13 is decelerated so as to maintain the brake draft ratio. That is, as shown in FIG. 3, the middle bottom roller 13 is decelerated at a constant deceleration that maintains the original brake draft ratio (1.25) during deceleration during the machine stop process, and then in the middle of deceleration. Deceleration is reduced. Then, after the brake draft ratio is changed to 1.6, the brake draft ratio is decelerated to a stop at a constant deceleration higher than that when the brake draft ratio is 1.25. In addition, the part shown with a dashed-two dotted line in FIG. 3 shows the original deceleration state. Further, the front bottom roller 12 and the back bottom roller 14 are decelerated at a constant deceleration from the start of deceleration to the time of stop, and the total draft ratio does not change.

  In the process of restarting the machine base, after the middle bottom roller 13 and the back bottom roller 14 are driven in a state where the brake draft ratio (1.6) at the time of the machine stop is reached, the original brake draft ratio (1 .25), the acceleration state of the middle bottom roller 13 is changed. That is, the middle bottom roller 13 is changed to low acceleration after starting acceleration in an acceleration state higher than the original acceleration state indicated by a two-dot chain line. After reaching a predetermined brake draft ratio (1.25), the vehicle is accelerated to a predetermined rotational speed so as to maintain the brake draft ratio.

  Since the torque for driving the middle bottom roller 13 is reduced at the time of deceleration for stopping the machine base, the middle and the second middle roller driving motors 16a and 16b for driving the middle bottom roller 13 are stopped. The amount of twist return of the bottom roller 13, that is, the roller shafts 13a and 13b is reduced. As a result, excessive feeding of fibers due to twisting back of the bottom roller while the machine base is stopped is suppressed, and occurrence of yarn unevenness when the machine base is restarted is suppressed.

  In the process of restarting the machine base, the middle bottom roller 13 and the back bottom roller 14 are first driven with the brake draft ratio (1.6) changed in the process of stopping the machine base. That is, after the driving is started in a state where the torque for driving the bottom roller is reduced, the brake draft ratio is changed so as to become a predetermined value (1.25) during the steady operation. Accordingly, the twisting delay of the bottom roller at the time of restarting the machine base is suppressed, and the yarn unevenness is further suppressed.

This embodiment has the following effects.
(1) In the draft device 11, the front bottom roller 12, the middle bottom roller 13, and the back bottom roller 14 are independently driven by the drive motors 15, 16a, 16b, 17a, and 17b, and at least in the process of stopping the machine, The brake draft ratio of the device 11 is changed so as to reduce the load torque of the draft device 11 from a predetermined value during steady operation. Therefore, in the state where the motor driving each bottom roller is stopped, the amount of twist return of the bottom roller is reduced. As a result, excessive feeding of fibers due to twisting back of the bottom roller while the machine base is stopped is suppressed, and occurrence of yarn unevenness when the machine base is restarted is suppressed.

  (2) In the draft device 11, the front bottom roller 12, the middle bottom roller 13 and the back bottom roller 14 are each independently driven by a drive motor, and the brake draft ratio of the draft device 11 is steadily operated at least during the machine stop process. It changes so that load torque may be reduced from the predetermined value of time. Then, the brake draft ratio changed to the machine stand stop process in the machine stand restart process is returned to the original value. Therefore, after the drive is started in a state where the torque for driving the bottom roller is reduced compared with the case of driving at the original brake draft ratio in the initial stage when the machine is restarted, the brake draft ratio is in a steady operation. The time is changed to a predetermined value. As a result, the twisting delay of the bottom roller at the time of restarting the machine base is suppressed, and the yarn unevenness is further suppressed.

  (3) When the brake draft ratio during steady operation is larger than the brake draft ratio Bp corresponding to the peak value of the torque in the curve with the brake draft ratio on the horizontal axis and the load torque required for the brake draft on the vertical axis Is changed so that the brake draft ratio increases in the process of stopping the machine base. Further, when the brake draft ratio during the steady operation is smaller than the brake draft ratio Bp corresponding to the peak value of the torque, the brake draft ratio is changed so as to decrease during the stoppage of the machine base. Therefore, when the brake draft ratio is changed during the machine standstill process, the torque for driving the bottom roller is surely reduced.

  (4) The brake draft ratio is changed by changing the speed of the middle bottom roller 13. As a method of changing the brake draft ratio, in addition to a method of changing the speed of the middle bottom roller 13, a method of changing the speed of the back bottom roller 14, a method of changing the speeds of the middle bottom roller 13 and the back bottom roller 14 There is. When the speed of the back bottom roller 14 is changed, the total draft ratio changes unless the speed of the front bottom roller 12 is changed correspondingly. However, in this embodiment, the speed of the back bottom roller 14 is assumed to be the original speed, and the speed of the middle bottom roller 13 is changed to change the brake draft ratio. Therefore, even if the brake draft ratio is changed, the total draft ratio is constant, so that control becomes easy.

The embodiment is not limited to the above, and may be embodied as follows, for example.
○ At least in the machine standstill process, the brake draft ratio of the draft device 11 may be changed so as to reduce the load torque of the draft device 11 from a predetermined value during steady operation. It is not necessary to start driving the draft device 11 from the brake draft ratio changed in the stand stop process. For example, the driving of the draft device 11 is started so that the brake draft ratio becomes a predetermined value at the time of steady operation, or the brake draft ratio changed during the stand stop process and the predetermined value at the time of steady operation. Alternatively, the driving of the draft device 11 may be started so as to be an intermediate value.

  In order to change the brake draft ratio, the speed of the back bottom roller 14 may be changed instead of changing the speed of the middle bottom roller 13. When changing the speed of the back bottom roller 14, the speed of the back bottom roller 14 is reduced when the draft ratio is increased, and the speed of the back bottom roller 14 is increased when the draft ratio is decreased. In this case, in order to keep the total draft ratio constant, it is necessary to change the speed of the front bottom roller 12 corresponding to the speed change of the back bottom roller 14. FIG. 4 shows the speed change of each bottom roller when the back bottom roller 14 is shifted to change the brake draft ratio. In FIG. 4, a portion indicated by a two-dot chain line indicates a change in rotational speed when the original acceleration or deceleration is performed.

In order to change the brake draft ratio, the speeds of both the middle bottom roller 13 and the back bottom roller 14 may be changed.
The brake draft ratio during steady operation is not limited to a value greater than the brake draft ratio Bp corresponding to the peak value of the torque in a curve with the brake draft ratio on the horizontal axis and the torque required for the brake draft on the vertical axis. It may be set to a small value. In that case, the change of the brake draft ratio is changed so that the brake draft ratio decreases in the process of stopping the base.

  ○ The time to change the brake draft ratio in the machine stop process is not necessarily during the deceleration of each bottom roller. For example, change of the brake draft ratio is started at the same time as the deceleration of each bottom roller, or the change of the brake draft ratio is completed before the deceleration of each bottom roller is started, and the vehicle is decelerated at the constant brake draft ratio after the change at the time of deceleration. You may do it.

  ○ The value of the brake draft ratio after the change in the machine stand stop process is input with the input device 25 when inputting the spinning condition of the machine stand instead of using the value stored in the program memory 23 in advance. You may make it do.

  The drive motors 15, 16 a, 16 b, 17 a, and 17 b that drive the front bottom roller 12, middle bottom roller 13, and back bottom roller 14 are not limited to servo motors and may be any motor that can control the rotational speed. For example, an induction motor controlled by an inverter may be used.

  The present invention may be applied not only to the 3-wire draft device 11 but also to a 4-wire or higher draft device. In the 4-wire draft device, the second bottom roller and the third bottom roller around which the apron is wound are middle bottom rollers. In the configuration in which the apron is wound only on the second bottom roller in the four-wire draft device, the second bottom roller is the middle bottom roller, and the third bottom roller is the back bottom roller.

  ○ Not only the configuration in which the middle bottom roller 13 and the back bottom roller 14 are driven by the drive motors 16a, 16b, 17a, and 17b from both sides of the machine base, but some of the bottom rollers (for example, the middle bottom roller 13) are mounted on the machine base. The back bottom roller 14 may be driven from one side of the machine base. Further, depending on the number of weights of the machine base, the front bottom roller 12, the middle bottom roller 13 and the back bottom roller 14 may all be driven only from one side of the machine base.

  ○ Not only ring spinning machines that spin from roving yarn, but also ring spinning machines that draw sliver directly without spinning and spinning spinning yarns directly, spinning machines such as binding spinning machines or roving machines, You may apply to the other spinning machine provided with the draft apparatus of the structure where the roller shaft of the bottom roller was long and the apron was wound around the middle bottom roller.

The technical idea (invention) that can be grasped from the respective embodiments will be described below.
(1) In the invention according to any one of claims 1 to 5, the spinning machine is a ring spinning machine.

  (2) In the invention according to claim 5, the brake draft ratio changing means includes a memory in which the brake draft ratio in the machine stop process and the corresponding speeds of the middle bottom roller and the back bottom roller are stored. The motor that drives the middle bottom roller and the back bottom roller is controlled so as to achieve the speed.

The schematic plan view of the draft apparatus which abbreviate | omitted the top roller. The graph which shows the relationship between a brake draft ratio and the torque which a draft requires. The graph which shows the speed change of a bottom roller at the time of a machine stand operation. The graph which shows the speed change of the bottom roller at the time of the driving | operation of the machine base in another embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Draft apparatus, 12 ... Front bottom roller, 13 ... Middle bottom roller, 14 ... Back bottom roller, 15 ... Front roller drive motor as a motor, 16a ... Similarly 1st middle roller drive motor, 16b ... Similarly 2nd Middle roller drive motor, 17a ... same first back roller drive motor, 17b ... same second back roller drive motor, 21 ... control device, 22 ... CPU constituting brake draft ratio changing means.

Claims (5)

A method for controlling a draft device in a spinning machine including a draft device in which a front bottom roller, a middle bottom roller, and a back bottom roller are independently driven by a motor,
A method for controlling a draft device in a spinning machine, wherein the draft draft ratio of the draft device is changed so as to reduce the load torque of the draft device from a predetermined value during steady operation, at least in a machine standstill process. A method for controlling a draft device in a spinning machine including a draft device in which a front bottom roller, a middle bottom roller, and a back bottom roller are independently driven by a motor,
At least in the machine stop process, the brake draft ratio of the draft device is changed so as to reduce the load torque of the draft device from a predetermined value during steady operation, and the machine stop process in the machine restart process A control method of a draft device in a spinning machine for returning a changed brake draft ratio to an original value.   If the predetermined value during the steady operation is larger than the brake draft ratio corresponding to the peak value of the torque in a curve with the brake draft ratio on the horizontal axis and the torque required for the brake draft on the vertical axis, The ratio of the draft device in the spinning machine according to claim 1 or 2, wherein the ratio is changed so as to increase, and if the brake draft ratio is smaller than the brake draft ratio corresponding to the peak value of the torque, the brake draft ratio is changed to decrease. Control method.   The method for controlling a draft device in a spinning machine according to any one of claims 1 to 3, wherein the brake draft ratio is changed by changing a speed of a middle bottom roller. A control device for a draft device in a spinning machine including a draft device in which a front bottom roller, a middle bottom roller, and a back bottom roller are independently driven by a motor,
A control device for a draft device in a spinning machine comprising brake draft ratio changing means for changing a brake draft ratio of the draft device so as to reduce a load torque of the draft device from a predetermined value at least in a machine standstill process.

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