METHOD FOR ACTUATING A MOTOR FOR STARTING A MILL

Abstract

In a method for actuating a motor for starting up a mill containing material to be ground, the motor for triggering sedimentation of the material to be ground in the mill is initially fed by a first inverter. The motor is subsequently disconnected from the first inverter, and in a further step the motor is connected to a first supply which has a first supply voltage. The maximum output voltage of the first inverter is lower than the first supply voltage.

Claims

1.-15. (canceled)

16. A method for controlling a motor for starting a mill containing material to be ground, said method comprising: feeding the motor by a first converter to remove any sedimentation of the material to be ground in the mill; disconnecting the motor from the first converter; and connecting the motor to a first supply having a first supply voltage, wherein a maximum output voltage of the first converter is lower than the first supply voltage.

17. The method of claim 16, wherein the maximum output voltage of the first converter is defined as max. 50% of the first supply voltage of the first supply.

18. The method of claim 16, further comprising operating the motor at a constant rated frequency while connected to the first supply, with the rated frequency being greater than a maximum output frequency of the first converter.

19. The method of claim 18, wherein the maximum output frequency of the first converter is defined as max. 30% of the rated frequency of the first supply.

20. The method of claim 16, further comprising operating the first converter from a second supply providing a second supply voltage.

21. The method of claim 16, further comprising connecting the motor to a second converter, after disconnecting the motor from the first converter and prior to connecting the motor to the first supply, with the second converter having a maximum output voltage which corresponds to the first supply voltage.

22. The method of claim 21, further comprising synchronizing the motor with the first supply by the second converter.

23. The method of claim 16, further comprising: operably connecting at least one further motor to a further mill; and connecting the motor and the further motor in a time-offset manner at least to the first converter.

24. The method of claim 16, wherein the first converter is a low-voltage converter.

25. The method of claim 24, further comprising operating the low-voltage converter from a low-voltage supply.

26. The method of claim 16, wherein the first supply is a medium-voltage supply.

27. The method of claim 21, wherein the second converter is a medium-voltage converter.

28. A control device for controlling a motor for starting a mill containing material to be ground, said control device being configured to: feed the motor by a first converter to remove any sedimentation of the material to be ground in the mill; disconnect the motor from the first converter; and connect the motor to a first supply having a first supply voltage, wherein a maximum output voltage of the first converter is lower than the first supply voltage.

29. A drive unit, comprising a control device as claimed in claim 28.

30. A mill arrangement, comprising: a mill containing material to be ground; a first converter having a maximum output voltage; a first supply having a first supply voltage; and a drive unit operably connected to the mill, said drive unit including a motor configured to start the mill and a control device configured to feed the motor by the first converter to remove any sedimentation of material to be ground, to disconnect the motor from the first converter, and to connect the motor to the first supply, wherein the maximum output voltage of the first converter is lower than the first supply voltage.

31. The mill arrangement of claim 30, wherein the maximum output voltage of the first converter is defined as max. 50% of the first supply voltage of the first supply.

32. The mill arrangement of claim 30, further comprising a second supply providing a second supply voltage for operating the first converter.

33. The mill arrangement of claim 30, wherein the first converter is a low-voltage converter.

34. The mill arrangement of claim 30, wherein the first supply is a medium-voltage supply.

35. The mill arrangement of claim 30, further comprising a second converter, said motor being connected to the second converter, after disconnecting the motor from the first converter and prior to connecting the motor to the first supply, with the second converter having a maximum output voltage which corresponds to the first supply voltage.

Description

[0027] The invention will now be described and explained in greater detail with reference to the exemplary embodiments illustrated in the accompanying drawings in which:

[0028] FIG. 1 shows a three-dimensional representation of a vertical mill arrangement,

[0029] FIG. 2 shows a schematic layout of a first embodiment of a mill arrangement,

[0030] FIG. 3 shows a schematic layout of a second embodiment of a mill arrangement and

[0031] FIG. 4 shows a schematic layout of a third embodiment of a mill arrangement.

[0032] Identical reference characters have the same meaning in the different figures.

[0033] FIG. 1 shows a three-dimensional representation of a vertical mill arrangement 2 comprising a drive unit 4 and a mill 6. The drive unit 4 is mechanically connected to the mill 6 via a gearing 8. The mill 6 comprises a grinding cylinder 10 containing a grinding tool 12. The grinding tool 12 can be implemented as a stirrer, spindle, screw or in some other way and has a shaft 14 which is connected to the gearing 8. However, the shaft 14 of the grinding tool 12 can also be connected directly to a shaft of the drive unit 4, e.g. via a flanged joint. The shaft of drive unit 4, which is not shown in FIG. 1, is driven by a motor 16 which is preferably fed by a converter 18. The converter 18 can also be disposed at another location, e.g. in a separate room. The motor 16 is preferably designed as an asynchronous squirrel-cage motor that can be operated at a power of at least 100 kilowatts.

[0034] The grinding cylinder 10 contains material 20 to be ground, also termed the charge 20, to which a grinding motion is imparted by the grinding tool 12 by rotation of the shaft 14. If the operation of a mill 6 at least partially charged with material 20 to be ground is interrupted e.g. for maintenance purposes and the mill stands idle for a lengthy period time, the material 20 to be ground may solidify in the mill 6 during this downtime. Such sedimentation of the material 20 to be ground necessitates a very high starting torque which has to be produced by the motor 16.

[0035] For example, for mills 6 in which sedimentation of this kind is likely to occur, special asynchronous motors with squirrel-cage rotors and high starting torque or wound-rotor motors with increased starting torque are used. Although wound-rotor, motors require relatively small starting currents, these wound-rotor motors are very high-maintenance.

[0036] The converter 18 must generate a high current for such a high starting torque. At the same time, a high voltage is required from the converter 18 for the rated speed required during operation. In the case of a converter 18, the costs are essentially determined by the power. A converter 18 that is suitable both for the high current for generating a high torque for removing the sedimented charge 20 and for the high voltage for operating the mill 6 at rated speed is therefore very expensive.

[0037] FIG. 2 schematically illustrates a first embodiment of a mill arrangement 2. As shown in FIG. 1, a motor 16 of a drive unit 4 is mechanically connected to a mill 6, wherein a gearing 8 can be provided between motor 16 and mill 6. The motor 16 can be connected to a first supply 21. The first supply 21 is designed as a medium-voltage supply 22 in order to be able to provided sufficient power for the mill arrangement 2. The connection to the medium-voltage supply 22 is via a main power switch 23. The medium-voltage supply 22 has a supply voltage in the range up to 15,000 volts. During grinding operation, the motor 16 is operated directly from the medium-voltage supply 22 at a constant rated frequency 24.

[0038] If the material 20 to be ground has solidified in the mill 6, an increased torque compared to normal milling operation, and therefore a higher current, is required to remove such a sedimentation of the material 20. The motor 16 of the mill 6 is therefore fed by a first converter 25 which is implemented as a low-voltage converter 26. However, the maximum output voltage of the low-voltage converter 26 is no more than 50% of the supply voltage of the medium voltage supply 22. In particular, the maximum output voltage of the low-voltage converter 26 is up to 1000 volts.

[0039] The maximum output frequency 28 of the low-voltage converter 26 is max. 30% of the rated frequency 24. The first converter 25 implemented as a low-voltage converter 26 is operated from a second supply 30 which is implemented as a low-voltage supply 31 having a voltage up to 1000 volts. The connection to the low-voltage supply 31 is established using a main switch 23.

[0040] After removal of the sedimentation, the motor 16 is disconnected from the first converter 25 via a first switch 33 implemented e.g. as an MV disconnecter, and, in a further step, connected to the first supply 21. The drive unit 4 comprises a control device 35 which controls the switching operations and the first converter 25.

[0041] The control device 35 also allows the motor 16 not to be connected to the first supply 22 but left connected to the converter 27 for occasional low-speed operation.

[0042] FIG. 3 schematically illustrates a second embodiment of a mill arrangement 2 which, in comparison with the mill arrangement 2 in FIG. 2, additionally has a second converter 37 which is implemented as a medium-voltage converter 38. The medium-voltage converter 38 can be connected to the medium-voltage supply 22 via a main switch 23. The medium-voltage converter 38 can be connected to the motor 16 via a second switch 40. The maximum output voltage of the medium-voltage converter 38 essentially corresponds to the supply voltage of the medium-voltage supply 22.

[0043] As shown in FIG. 2, the low-voltage converter 26, which can be fed from the low-voltage supply 31, is used to dislodge a sedimented charge in the mill 6. After removal of the sedimentation, the motor 16 in FIG. 3 is connected to the medium-voltage converter 38 which increases its output voltage, preferably continuously, and synchronizes the motor 16 with the medium-voltage supply 22. In particular, the phase and/or frequency of the output voltage is matched to the medium-voltage supply 22 by the medium-voltage converter 38. The control device 35 controls the switching operations and the two converters 25, 37. The mill arrangement 2 is otherwise as shown in FIG. 2.

[0044] The controller 35 also allows the motor 16 not to be synchronized with the first supply 22 but to remain connected to the second converter 37 for operation with the variable-speed option.

[0045] The control device 35 also allows the motor 16 to be occasionally connected to the first converter 25 for low-speed operation.

[0046] FIG. 4 schematically illustrates a third embodiment of a mill arrangement 2 having, by way of example, three motors 16 each assigned to a mill 6 and driving same. The three motors 16 are controlled by the converters 25, 37 in a time-offset manner via switches 42, 43, 44. Each motor 16 is first connected to the first converter 25 and then to the second converter 37 in order, as described above, to remove any sedimentation and synchronize the respective motor 16 to the medium-voltage supply 22 with the option of leaving a motor 16 connected to the second converter 37.