Mill drive system

Abstract

The invention relates to a mill drive system comprising a two-stage planetary gear transmission; a motor; a variable frequency drive, characterized in that the semiconductor bridge of the variable frequency drive comprises a controlled voltage supply, said voltage supply configured to control the voltage level in accordance to a current value to be measured by a current measurement device and compared to a desired current value and a multi-level voltage supply of the controlled voltage supply configured to have at least two voltage levels.

Description

[0001] Mill Drive System

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a mill drive system comprising a two-stage planetary gear transmission; a motor and a variable frequency drive, characterized in that the semiconductor bridge of the variable frequency drive comprises a controlled voltage supply.

[0004] BACKGROUND OF THE INVENTION

[0005] 8 % of the worldwide carbon dioxide emissions are caused by the cement production. A significant part of that (49%) is caused by the deacidification of clinker. The grinding of clinker and cement also contributes a significant part, which is 12 % of the total carbon dioxide emissions in the cement production. The reference of those values is: (Riccardo Maddalena, Jennifer J. Roberts, Andrea Hamilton, 2018). Caused by the financial duty for carbon dioxide emissions, the reduction of carbon dioxide emissions in the grinding process becomes an economical reason to minimize the electrical energy consumption of mill drive systems, by increasing the efficiency.

[0006] Reducing the electrical energy per ton cement will decrease the costs linked to the carbon dioxide emission. A highly efficient mill drive system provides the consumer with a useful or commercial choice.

[0007] A worldwide used drive train of vertical roller mills is described in US 7 905 808 B2. For making the understanding easier, Fig. 1 of US 7 905 808 B2 is given in a more schematic sketch in Fig. 1, without bearings, toothing and in non-scale, but including the motor 1, the connection to the three-phase AC power grid (59), the grinding rollers 11 and the grinded material 12. This drive train has 3 gear transmissions. In the first transmission, the rotor speed of the motor 1 is reduced between the bevel pinion gear 2 and the bevel gear 3. In the second transmission, the speed of the sun-pinion shaft 4 is reduced to the gear ring of part 6 via the planet gears 5. The planet gears 5 are linked via the axil shafts

[0008] 9 to the rotating part 8. In the third gear transmission, the speed of the part 6, which has a second sun gear, is reduced to the speed of the gear ring of part 8 via the planet gears.

[0009] 7. This transmission is in parallel to the link of part 8 to the axels of the planet gears 5 and needs a relation of the gear ratios, which has equivalent transmission ratios of the parallel transmissions between the rotating part 6 and 8, this is explained in detail in US 7 905 808 B2.

[0010] A common method operating a variable frequency drive is to control the voltage by the width of pulses connecting the motor phases via semiconductors to a direct current voltage supply (DC link bus), for example by insulated-gate bipolar transistors (IGBT).

[0011] Those variable frequency drives are controlled by “ Vector control, also called field- oriented control (FOC), is a variable -frequency drive (VFD) control method in which the stator currents of a three-phase AC or brushless DC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor, the other the torque. The control system of the drive calculates the corresponding current component references from the flux and torque references given by the drive's speed control. Typically proportional-integral (PI) controllers are used to keep the measured current components at their reference values. The pulse-width modulation of the variable-frequency drive defines the transistor switching according to the stator voltage references that are the output of the PI current controllers. [1]

[0012] [1] Zambada, Jorge (Nov 8, 2007). Field-oriented control for motors'. MachineDesign.com. Archived from the original on February 16, 2013. ” (Source en.wikipedia.org “Vector control (motors)”)

[0013] Such type of control requires a highly performed processor for the transformations (Clarke-Transformation and Park-Transformation) and data value handling, which must be done during parts of an electrical period (parts of 20 milli seconds at 50 Hz and parts of 8 milli seconds at 125 Hz). The overall concept is to control the voltage by pulsed connection of the motor phases to the voltage supply to achieve the currents, which generate the desired motor torque. For medium voltage and high currents, the switching is challenging, especial for higher motor frequencies. Additional losses occur in the semiconductors by the high frequent switching.

[0014] Very short voltage pulses can cause overvoltage of the threefold up to tenfold voltage of the DC link bus, see page 254 in (Fischer, 2009).

[0015] During maintenance and assembly, the mill drive must be turned with slow speed. Additional axillary drives having a small power motor and an additional gear transmission are culched to the motor shaft of the motor 1 in mill drives in accordance with the prior art are used.

[0016] OBJECT OF THE INVENTION

[0017] It is an object of the present invention to decrease the electrical power consumption of mill drives in the cement production, for the grinding of raw material and cement. From this the electrical energy and the liked carbon dioxide emission, during electrical energy production, will be decreased for cement production by about 50%.

[0018] It is a further object of the present invention to provide an alternative to the prior art, by a simpler VFD, having a bigger entire efficiency of the combination of motor and variable frequency drive, applied in the mill drive arrangement which is illustrated in Fig. 2.

[0019] It is a further object of the present invention to provide an alternative to the prior art, to operate the mill with slow speed during maintenance works with a safety look to prevent the significant higher speed of normal operation, without any additional low speed auxiliary drive.

[0020] It is a further object of the present invention to provide an alternative to the prior art, to prevent overvoltage of voltage pulsing by operating the variable frequency drive with longer voltage pulses and without interrupting the phase current flow, except between the positive and negative phase current amplitudes, and to prevent overvoltage by the prevention of short voltage pulses.

[0021] SUMMARY OF THE INVENTION

[0022] In a first aspect, the invention relates to a mill drive system, said mill system preferably comprises: a two-stage planetary gear transmission; a motor; a variable frequency drive, characterized in that the semiconductor bridge of the variable frequency drive comprises a controlled voltage supply, said voltage supply configured to control the voltage level in accordance to a current value to be measured by a current measurement device and compared to a desired current value and a multi-level voltage supply of the controlled voltage supply configured to have at least two voltage levels.

[0023] The mill drive system according to the present invention reduces the losses by 50%. The mill drive system according to the present invention is a combination of a motor, connected to the grid by the simplified variable frequency drive (Fig. 3) and a toque transformer (Fig. 2), which is a two-stage planetary gear transmission. The gear ratio is adapted to a motor speed, for which the motor losses are the smallest. The efficiency of the milling process is also depending on the rotation speed of the mill. Depending on the variating raw material composition and the humidity, the optimal mill speed varies also. This can be handled by the variable frequency drive, which is proposed in the following. The frequency drive (Fig. 3) according to the present invention decreases the losses of the semiconductors by pulsing each phase-connecting semiconductor by a single pulse per electrical period. Higher electrical frequencies can be operated by this, for which the motor efficiency is higher. The entire mill drive system, with its additional losses has smaller losses than those, which are prior art. Overvoltage, caused by short voltage pulses are avoided by the inventive concept. By this medium voltage, winding insulations will have a longer lifespan and choke coils can be avoided. The avoidance of choke coils has the result of the avoidance of losses, caused by those and the cooling of those.

[0024] The frequency drive (Fig. 23), having a low voltage level and a safety look, which prevent the supply of the semiconductor bridge by the higher voltage supplies, can be used for operating the mill in low maintenance speed with the existing motor and without any auxiliary drive.

[0025] In the mill drive system according to the present invention, the variable frequency drive is preferably configured to control the output voltage by comparing the current value to additional current values, at least additional to and the voltage supply configured to have at least the three voltage levels, which are connected to the semiconductor bridge by the controlled voltage supply, depending on the measured current value.

[0026] The controlled voltage supply may be configured to be disconnected from the multilevel voltage supply by a safety look, and the semiconductor bridge is configured to connected durable to the lower voltage level by the safety look via the bypass for low-speed maintenance operation, wherein contains a diode, and where the value of the voltage level is smaller than the pole wheel voltage at the speed, which shall not be exceeded for safety reasons.

[0027] In another preferred embodiment of the present invention, the maintenance speed is preferably configured to be controlled by the phase shift angle between the phase angle of the pole wheel voltage and the phase angle of the voltage, which is the output of the semiconductor bridge. The maintenance speed may be configured to be controlled by variating the desired current value in accordance with the needed torque, and wherein the desired current value is configured to be achieved by switching between the voltage levels and in the configuration, wherein the value of the voltage level is nil for positive currents, and the higher voltage levels are disconnected by a safety look switch.

[0028] The motor preferably comprises a magnet arrangement (Fig. 4), having low eddy current losses in comparison to the surface mounted magnet arrangement, by arranging the magnets apart from the airgap and its alternating magnetic fields, between the rotor poles, in a flux concentration arrangement. The variation of the magnetic flux in a permanent magnet in a surface mounted arrangement is illustrated in the white circles in Fig. 28, by the different black coloured area for different rotor positions, during the rated operation. The magnetic flux of the preferred arrangement (Fig. 4) does not change significantly, because the permanent magnets are not directly exposed to the altering magnetic field of the stator windings at the airgap, see dashed black circles in Fig. 29 for different rotor positions. Caused by the smaller flux variation, the maximal current density in the permanent magnet of the preferred arrangement is smaller (0.31 A / mm2, see Fig. 31) than for the surface mounted magnets (1.1 A / mm2, see Fig. 30). The maximal density of losses in the magnets of the surface mounted magnets (Fig. 32) are by factor 41 bigger than in the magnets of the preferred magnet arrangement (Fig. 33). The motors of the compared example have equivalent speed and motor power. The motor speed is 750 rpm and the electrical supply frequency is 125 Hz for both examples. At 50 Hz and 300 rpm, the motor with the surface mounted magnets has an efficiency of 97.7 %*), which is typical for such type of permanent magnet motor. At 750 rpm and at an electrical frequency of 125 Hz, the efficiency of this motor decreases to 95.8 %*). The significant contribution to the high losses at the higher frequency are the high eddy current losses in the permanent magnets of 3.6 % related to the output power. The relative eddy current losses in the magnets of the preferred arrangement of the preferred (Fig. 4) are 0.01 % and the motor efficiency is 99.2 %*

[0029] *) The total motor efficiencies for all examples are smaller, because bearing friction and air friction are not considered in the results of both arrangements.

[0030] The preferred magnet arrangement in (Fig. 4) additional is a flux concentration arrangement, so called because the surface of the poles at the magnetic airgap is smaller than the cross section of the magnets orthogonal to the magnetic flux. Those magnets cannot be demagnetized by short circuit currents, because in such case the dynamoelectric core sheets of the poles at the airgap will go in saturation and the magnetic field at the magnets cannot achieve values for demagnetizing of the magnets. This flux concentration is an additional reason why the magnetic flux is very stable in different rotor positions, which prevents eddy currents.

[0031] In a preferred embodiment of the present invention, the two-stage gear transmission is without the losses of a bevel gear transmission and of its bearings.

[0032] The motor may further comprise a horizontal shaft axis, which comprises a magnet arrangement illustrated in Fig. 4 having low eddy current losses is supplied by the variable frequency drive which is delineated in the present document.

[0033] The mill drive system may have a horizontal motor axis and a horizontal axis of the two- stage planetary gear transmission supplied by the variable frequency drive.

[0034] The semiconductors are preferably formed by MOSFET or IGBT, and the semiconductors are arranged in a multilevel arrangement for medium voltage.

[0035] In another embodiment of the present invention, the semiconductor bridge is preferably configured not to be connected to the voltage level during the motor start before the motor speed exceeds a certain value, where the pole wheel voltage is far below the voltage level, to avoid short pulses of the switching of the voltage level and by this to avoid overvoltage as a consequence of short and high voltage pulses.

[0036] In a second aspect, the invention relates to a mill comprising the mill drive system according to any of the preceding embodiments.

[0037] In a third aspect, the invention relates to the use of the mill drive system, according to any of the preceding embodiments, in a mill.

[0038] The first, second and third aspects of the present invention may be combined.

[0039] In the present context, a number of terms are used in a manner being ordinary to the skilled person. Some of these terms are detailed below:

[0040] IGBT - is preferably used to mean an insulated-gate bipolar transistor

[0041] VFD - is preferably used to mean a variable-frequency drive

[0042] DOL - is preferably used to mean a Direct on line MOSFET - is preferably used to mean a Metal-oxide-semiconductor field-effect transistor

[0043] BLDC motor - is preferably used to mean a Brushless direct current motor

[0044] DC - is preferably used to mean Direct current

[0045] AC - is preferably used to mean an Alternating current

[0046] PWM - is preferably used to mean a pulse width modulating

[0047] Pl-controller - is preferably used to mean a proportional-integral controller

[0048] BRIEF DESCRIPTION OF THE FIGURES

[0049] The figures show one way of implementing the present invention and are not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

[0050] Fig. 1 : Schematically illustrates the prior state of a mill drive system.

[0051] Fig. 2: Schematically illustrates the drive train of the mill drive system of the present invention without the inventive power electronics.

[0052] Fig. 3: Modules of the Inventive VFD 34, connected to the motor 13.

[0053] Fig. 4: One pole pair of an example of a highly efficient motor, having 10 pole pairs, and a magnet arrangement apart from the magnetic airgap and having a flux concentration.

[0054] Fig. 5: Efficiency and shaft power depending on the phase shift of the pole wheel voltage in relation to the phase angle of the sinusoidal current.

[0055] Fig. 6: Multilevel voltage supply.

[0056] Fig. 7: Phase shift of phase branch current and phase element voltage.

[0057] Fig. 8: Pulses subjecting the semiconductors which are connecting the motor phase to the positive voltage and to the negative pole.

[0058] Fig. 9: Phase element voltage U (thin curve) and phase current of one of the 5 branches between motor terminal U and star-point.

[0059] Fig. 10: Comparison of currents from sinusoidal current sources to those currents caused by the VFD 31.

[0060] Fig. 11 : Desired current value 37 - dashed line, current value 48 measured by 35 wide solid curve and voltage at the output of 33 -thin solid curve.

[0061] Fig. 12: Semiconductor - bridge 31, illustrated by one-level IGBTs.

[0062] Fig. 13: Algorithm for the switching signal of semiconductors, illustrated for one of six of a three-phase motor. Fig. 14: Controlled voltage supply 33, illustrating the semiconductors by IGBTs.

[0063] Fig. 15: Comparison of shaft power ripples for sinusoidal motor currents (dashed curve) and VFD motor currents (solid curve).

[0064] Fig. 16: VFD configuration having an additional lower voltage level 45 for the motor start.

[0065] Fig. 17: Diagrams of a motor start without the higher voltage level 47.

[0066] Fig. 18: Phase currents of one branch.

[0067] Fig. 19: Additional switching of the voltage level 47 (short-dashed curve), after exceeding 375 rpm (50% of rated speed).

[0068] Fig. 20: Phase currents of one branch at the transition at the limit of the low-speed value 50 (375 rpm).

[0069] Fig. 21 : Switching at rated speed (750 rpm).

[0070] Fig. 22: Phase currents, phase element voltage (phase U) and torque at rated speed (750 rpm).

[0071] Fig. 23: VFD configuration 56 with safety look 57 and the bypass 58 of 33.2.

[0072] Fig. 24: Uncontrolled rise of the rotation speed at the voltage level 45 (115 V) of the real moment of inertia of the drive train (500 kg m2per 2 pole pairs, 2500 kg m2of the entire drive train related to the motor shaft) with the switching angle of maximal torque (40°), considering friction causing 1 kNm load torque.

[0073] Fig. 25: Rise of torque by increasing the phase shift angle of advanced voltage phase angle in relation to the phase angle of the pole wheel voltage, which is given by the rotor position angle.

[0074] Fig. 26: Decrease of torque by decreasing the phase shift angle of advanced voltage phase angle in relation to the phase angle of the pole wheel voltage, which is given by the rotor position angle.

[0075] Fig. 27: VFD configuration 61 having the lower voltage level 45, which has the value nil for positive supply currents.

[0076] Fig. 28: Variation of the magnetic flux in the magnets of the surface mounted magnet arrangement, between two rotor positions during rated operation.

[0077] Fig. 29: Magnetic flux in the magnets of the preferred magnet arrangement for two rotor positions during rated operation. Fig. 30: Current density in the magnets of the surface mounted magnet arrangement, caused by the variation of the magnetic flux, having a local maximal value of 1.14 A / mm2.

[0078] Fig. 31 : Current density in the magnets of the preferred magnet arrangement, caused by the variation of the magnetic flux, having a local maximal value of 0.31 A / mm2.

[0079] Fig. 32: Density of ohmic-loss in the magnets of the surface mounted magnets, having a local maximal value of 7.80 mW / mm3. (For the efficiency calculation, the losses are integrated over the entire cross section of the magnets and are averaged along one electrical period.)

[0080] Fig. 33: Density of ohmic-loss in the magnets of the preferred magnet arrangement (Fig. 4), having a local maximal value of 0.19 mW / mm3. (For the efficiency calculation, the losses are integrated over the entire cross section of the magnets and are averaged along one electrical period.)

[0081] DETAILED DESCRIPTION OF THE INVENTION

[0082] For the detailed description, the mill drive system is compared to an existing state of the art mill drive system, which is typical for the prior state of the configuration in Fig. 1. The prior state motor in this example (1 in Fig. 1) is an asynchronous motor, generating a shaft power of 4400 kW at 895 rpm at 60 Hz supply frequency. This motor operates directly connected to the grid 59 and does not need any VFD for fixed speed. The efficiency of the prior state motor is 96.5 %. The rated voltage of the three-phase asynchronous motor is 4160 V in star-connection.

[0083] In a preferred embodiment, the mill drive system according to the present invention comprises a two-stage planetary gear transmission 30; a motor 13; a variable frequency drive 34, characterized in that the semiconductor bridge 31 of the variable frequency drive 34 comprises a controlled voltage supply 33, said voltage supply 33 configured to control the voltage level in accordance to a current value 48 to be measured by a current measurement device 35 and compared to a desired current value 37 and a multi-level voltage supply 32 of the controlled voltage supply 33 configured to have at least two voltage levels 46 and 47. The frequency drive 51 is configured to control the output voltage of 33.2 by comparing the current value 48 to additional current values, at least additional to 49 and the voltage supply 32.2 configured to have at least the three voltage levels 45, 46 and 47, which are connected to the semiconductor bridge 31 by the controlled voltage supply 33.2, depending on the measured current value 48. The controlled voltage supply 33.2 is configured to be disconnected from the multilevel voltage supply 32.2 by a safety look 57 and the semiconductor bridge 31 is configured to connected durable to the lower voltage level 45 by the safety look via the bypass 58 for low-speed maintenance operation, wherein 58 contains a diode 60, and where the value of the voltage level 45 is smaller than the pole wheel voltage of 13 at the speed, which shall not be exceeded for safety reasons.

[0084] Subsequently, the maintenance speed is configured to be controlled by the phase shift angle between the phase angle of the pole wheel voltage and the phase angle of the voltage, which is the output of the semiconductor bridge 31.

[0085] The maintenance speed is configured to be controlled by variating the desired current value 48 in accordance with the needed torque, and wherein the desired current value 48 is configured to be achieved by switching between the voltage levels 46 and 47 in the configuration 61, wherein the value of the voltage level 46 is nil for positive currents, and the higher voltage levels are disconnected by a safety look switch.

[0086] In yet a preferred embodiment, the motor 13 comprises a magnet arrangement, having low eddy current losses.

[0087] The motor 13 comprises a cooling system.

[0088] The motor 13 further comprises horizontal shaft axis, which comprises a magnet arrangement, having low eddy current losses, supplied by the variable frequency drive 51. The semiconductors of 51 are formed by MOSFET or IGBT, and the semiconductors are arranged in a multilevel arrangement for medium voltage.

[0089] The semiconductor bridge 31 is configured not to be connected to the voltage level 47 during the motor start before the motor speed exceeds a certain value, where the pole wheel voltage is far below the voltage level 47, to avoid short pulses of the switching of the voltage level 47 and by this to avoid overvoltage as a consequence of short and high voltage pulses. The example for the mill drive (Fig. 2) according to the present invention is dimensioned for the substitution in the exiting example in figure 1. Content of this drive is a motor (13 in Fig. 2) having a permanent magnet arrangement in accordance with US 946 7014 B2. The geometry of one pole pair for the present example is content of Fig. 4. The intermediate pole 28 in Fig. 4 is fixed by a form fit between the rotor core poles 29. The rotor has a diameter of 1600 mm. The axial length is 536 mm. The shaft power of 4400 kW of the entire motor having 10 pole pairs at 750 rpm and at a frequency of 125 Hz is the same as of the compared motor, which is prior state (1 in Fig. 1). Two coils of each three phases are in serial connection to the star-point, and 5 of those branches are in parallel connection to a star point. The motor 13 is connected to the grid via the inventive VFD 34 (Fig. 3).

[0090] The major saving of losses, of 3 % in this example, is achieved by eliminating the first gear transmission between the bevel pinion gear 2 and the bevel gear 3 (Fig. 1).

[0091] The second major saving of losses is achieved by the motor 13, having a vertical motor shaft 14, is arranged below the two-stage planetary gear transmission, see Fig. 2. The motor shaft 14 drives the sun wheel 4a (Fig. 2). The rest of the drive train is principal the same as in Fig. 1, but it is adapted to the motor speed where the motor efficiency is the highest. One reason of the significant smaller motor losses of about 1.5 % of 13 (Fig. 1) in comparison to 3.5 % of the asynchronous motor 1 (Fig. 2) is no need of any rotor current for the excitation of the rotor. Additionally, eddy current losses occur in the permanent magnets, but those are significantly smaller in the arrangement, which is shown in Fig. 4, then in permanent magnet motors, which have surface mounted magnets, which are subjected directly to the alternating magnetic field of the stator windings at the airgap. Those alternating magnetic fields cause significant eddy currents in surface mounted permanent magnets.

[0092] The losses of the bevel gear transmission are about 3 % and the entire losses in the losses of the two bevel gear transmissions are about 2 %. The losses in the (prior state) example for the 4400 kW asynchronous motor having 4 pole pairs are 3.5 % (related to the shaft power) at a rotation speed of 895 rpm at a supply frequency of 60 Hz. Such a motor can be operated directly connected to the grid 59, and no losses of any frequency converter occur. The sum of the entire losses of this drive train is about 8.5 %. The summation of losses of the mill drive according to the present invention is 4.5%, given by 2 % losses of the planetary gear transmission 30, 1.5 % of the motor and below 1.0 % of the inventive VFD 34. The solution according to the present invention decreasing the losses of the drive train by about 50 % (4 % Losses / 8.5 % Losses prior).

[0093] An additional saving of energy can be achieved by optimizing the mill table speed via the motor speed, which is controlled by the VFD, which is not possible by the prior mill drive with fixed speed.

[0094] Additional savings off the electric power system are the avoidance of those high startcurrents, which DOL-motors need, and the idle power DOL motors, if the DC-link buses of the VFD are directly connected by diode bridges to the grid (59).

[0095] The following results are achieved by a finite element simulation of one branch with two pole pairs in serial connection between motor terminal and star point. The ohmic resistance of the two litz wire coils in serial connection is 23.42 mQ, at 120 °C. The length of each coil, having 24 winding turns, is 36.77 m including winding overhang. The copper cross section of the 8.69 mm by 13.65 mm litz wire is 78.05 mm2, considering a fill factor of 65.8%. With reference to the real existing mill drive motor the power of the motor must achiever - With the limitation of the winding temperature and under consideration of the efficient cooling in accordance with PA 2022 00972, the current in the copper cross section is limited below 3 Amper per square millimetre. This current density multiplied by the copper cross section of 78 mm2has the result of 234 A, which is the limit of the effective current in the coil. The serial connection of two coils and 5 of those branches in parallel connection between the motor terminals and the starpoint give the result of the maximal motor phase current of 1170 A. In the following, simulations are made for two pole pairs with the serial connection of two coils. The result of the voltage is the same as for the entire motor. The simulation results of the phase currents and of the torque must be multiplied by 5, which is the result of the pole pair number divided by the number of serials connected coils.

[0096] Motor Efficiency for Sinusoidal Currents depending on the Phase Angle

[0097] The angle of the abscissa in the diagram in Fig. 5 is the phase angle of the pole wheel voltage related to the phase angle of the phase current, which is nil. A high torque value and motor power is achieved if the phase angle of sinusoidal motor currents is equivalent to the phase angle of the pole wheel voltage (0° in Fig. 5), which is not the highest, see page 345 (“Bild 6.73”) in (Fischer, 2009) in Rolf Fischer “Elektrishe Maschinen” ISBN 978-3-446-41754-0 (2009). Shaft power and efficiency in the diagram in Fig. 5 are achieved with 10 pole pairs of the illustrated pole pair in Fig. 4. The effective value of the sinusoidal currents in the coils is 227.7 A, which is below the limit of 234 A given by the maximal current density of 3 A per square millimetre copper cross section of the motor coils. Two coils are in serial connection between the motor terminals and the star-point. Five of those branches are in parallel connection. The entire effective phase current is the fivefold value, 1139 A. To obtain the shaft power of 4400 kW with this phase current the phase shift between pole wheel voltage and phase current must be about 1.5 degree (4414 kW at 1.5° in the diagram in Fig. 5). The efficiency in this operation is below 99%, see dashed line in Fig. 5.

[0098] The efficiency increases above 99% in operations without phase shift and for negative angles of the phase shift. But the shaft power decreases significantly for negative phase shift angles. For a negative phase shift angle of -3°, the efficiency increases a little to 99.21%, but the shaft power is 23% below the maximal achievable value. Beside this, the operation with negative angles can occur unstable operation, see page 314 in (Fischer, 2009).

[0099] In the present example, the motor must achieve 4400 kW. Therefor a 0.7 % bigger motor phase current of 1147 A is needed, which is still below the limit of 1170 A, given by the maximal current density of the coils. The operation with this current and without phase shift between pole wheel voltage and phase current causes an efficiency of 99.06 % and a shaft power of 4405 kW, and the power factor is 0.76 (5820 kVA apparent power). The power factor of the prior asynchronous motor is 0.88.

[0100] Practically no sinusoidal current sources exist. The challenge is to configurate a VFD which controls voltage supplies in the way near to the operation above, with approximately sinusoidal currents with the phase current angle which has no shift related to the pole wheel voltage.

[0101] Inventive Variable-Frequency Drive (VFD) in Operation in the Rated Speed Range

[0102] The speed of the mill operation is nearly constant. The variation for the optimal mill speed is small in relation to the rated speed. The present chapter describes the operation of the inventive VFD for this operation at nearly constant speed. The start of the mill from still stand is described in a separate chapter. The VFD according to present invention connects the motor phases to a voltage source during the entire duration of a positive or negative current amplitude. No additional losses occur by any additional switching operations of the semiconductors of the semiconductor bridge 31 (Fig. 3). Each switching of a semiconductor causes losses during changing the resistance between the states of fully conducting and nonconducting. The saving of switching causes the saving of losses and increases the efficiency of the inventive VFD. Additional savings of losses are caused by more continued current curves of the motor, operated at the inventive VFD. In prior art, overvoltage is prevented by chock coils, which cause additional losses in those coils of 2 kW in the example of the present application plus the power of the necessary power for the cooling aggregate.

[0103] In the VFD according to the present invention, additional losses occurring during the switching operation of the semiconductors of the controlled voltage supply 33 (Fig. 3), but the number of those semiconductors multiplied by the small switching frequency is smaller than the number of the semiconductors of a three-phase semiconductor bridge multiplied by the number of switching of a VFD in accordance with the prior art. Additionally, VFD with a semiconductor bridge in the voltage supply as the input of the DC-link bus in accordance with the prior art have additional losses in such a bridge during the switching of those semiconductors.

[0104] The current, and by this the motor torque, is controlled by the input voltage of the semiconductor-bridge 31. An inventive component is the controlled voltage supply 33. The input of 33 are the voltages of a multi-level voltage supply 32. The current value 48 measurement by the current measurement 35 is the basis of the controlling.

[0105] Such variable frequency drives without the controlled voltage supply 33 but having an inductivity in the DC link bus are known as current-source inverter to the prior art. Those current-source inverter can have shorter switching blocks to reduce the torque and by this the motor speed. The losses in drive systems with current-source inverter to the prior art can increase to 25%, see Page 248 in (Fischer, 2009).

[0106] In accordance with the prior art, the voltage supply of the semiconductor bridge are diode bridges or semiconductor bridges, having a single voltage level. Such supply has a condenser for pulse width modulating (PWM) variable frequency drives or a serial inductivity for current source inverters. Such supplies in accordance with the prior art do not have any multilevel voltage supply 32 and no controlled voltage supply 33, which are components of the inventive VFD 34. An example of a multilevel voltage supply 32 is illustrated in Fig. 6.

[0107] The switches of the VFD are illustrated by insulated-gate bipolar transistor (IGBT) as an example. The semiconductors, respectively switches can be alternative types, for example metal-oxide-semiconductor field-effect transistors (MOSFET). For medium voltage, those switches are configurated in a multi-level structure. For a better understanding, the simple structure of semiconductor switches are represented by single IGBTs.

[0108] The diagram in Fig. 7 illustrates the phase shift between phase current and phase element voltage of one branch between the motor terminal U and the star-point, for sinusoidal currents which are in phase with the pole wheel voltage. The voltage curve crosses the abscissa 40° (degrees of the electrical period) earlier than the current curve. For this reason, the voltage curve must be controlled in the way to subject the motor 40° earlier than the phase angle of pole wheel voltage. This phase shift value 38 is one input of the semiconductor - bridge control 36, see Fig. 3. The simulation model is built to have the pole wheel voltage of the phase U in phase of the rotor angle multiplied by the number of pole pairs. The shift of the voltage switching 40° earlier than the pole wheel voltage of phase U is illustrated in Fig. 8 by the solid curve in the first diagram for the semiconductor, which is connecting the motor phase to the positive voltage.

[0109] The diagrams in Fig. 8 illustrate the semiconductor switching of the semiconductor bridge 31 (Fig. 12) during one electrical period, which has the duration of 8 milli seconds at 125 Hz for 10 pole pairs. In the present example, the pole wheel voltage is in phase with the scale of the abscissa in Fig. 8. After the IGBT1 (Fig. 12) is switched of and a waiting time, to prevent a short circuit in 31, the semiconductor (IGBT2) is connecting the motor phase U to the minus pole. The same is done for the phases V (diagrams 3 and 4 in Fig. 12) and W (diagrams 5 and 6 in Fig. 12), using the IGBTs 3 and 4 for the phase V, and 5 and 6 for the phase W. The switching of the semiconductors of the phase V is 120° later and of phase W is 240° later than the switching of the phase U.

[0110] The currents, caused by this voltage switching are the wide curves in the diagrams in Fig. 10 (solid curve - phase U, dashed curve -phase V and dot-dashed curve -phase W). Those curves are near the sinusoidal currents, which are illustrated in the diagrams in Fig. 10 by the thin curves. The rotor angle 39 of the motor 13 is the second input of the semiconductor - bridge control 36 (Fig. 3). In 36 the rotor angle is multiplied in the operator 41 by the number of pole pairs to achieve the electrical rotor angle, see Fig. 13. If this angle is not equivalent to the phase angle of the pole wheel voltage, the shift is eliminated by a constant shift angle. The phase shift value 38 is added in the summation operator 42 to this angle. The achieved angle is the control angle, for which the output must cause the switch on of the semiconductor IGBT1 in 31 (Fig. 12) during 0° and 180°, for example by exclusively the positive values of a sinus function 42 by the threshold operation 43. For the semiconductor IGBT2, which is connected to the negative voltage, a second threshold operator generates a switching signal for negative values of the sinus function. This algorithm is not inventive and can be done in a different way, having the same result, for example generating the switching signal directly from the value of the angle, without sinus function with lower calculation effort. The input angle has a shift to the angles of the abscissas of the diagrams in Fig. 7 to Fig. 11.

[0111] For the switching control of the semiconductors of the other phases, the shift of 120° is subtracted from the input value of 42 for the phase V and 240° is subtracted for the phase W.

[0112] The values of the currents are controlled by the controlled voltage supply 33 (Fig. 3). The input 48 of 33 is measured by the current measurement 35 (Fig. 3) of the entire current of the semiconductor - bridge 31 (Fig. 3). The second input is the desired current value 37 (Fig. 3). Those inputs and the voltage output are illustrated in the diagram in Fig. 11.

[0113] The controlled voltage supply is connected to a multi-level voltage supply 32 (Fig. 3). If the measured current 35 exceeds the desired current value 37, the semiconductor IGBT7 of 33 (Fig. 14) is switched off from the higher voltage input level P2 and the semiconductor IGBT8 is switched on to connect the lower voltage level input P3 to the output P5. If the measured current is below or equivalent to the desired current value 37, the semiconductor IGBT7 is switched on and the semiconductor IGBT8 is switched off.

[0114] In the present example, the synchronous frequency of the motor is 125 Hz, and the number of pulses of the two semiconductors of the controlled voltage supply are the sixfold of the electrical periods per time, respectively 750 pulses per second. Each semiconductor of the semiconductor - bridge 31 has only one single pulse per electrical period (125 Hz). Each switching of a semiconductor causes losses inside the semiconductor during the time where the resistance of the semiconductor changes from nil to non-conducting and during the change from non-conducting to nil resistance. The inventive VFD has the lowest possible switching frequency of the six semiconductors of the semiconductor - bridge 31 and a low switching frequency of the additional two semiconductors of the controlled voltage supply 33. The additional advantage of the multi-level voltage supply is never to interrupt the current during positive or negative amplitudes of the motor current completely. The phase element voltage is not pulsed to nil, see the curve of the phase element voltage U in Fig. 9. The only nil value occurs between the plus and minus amplitudes. The real motor currents are near the sinusoidal curves (Fig. 10), where the motor losses are minimal.

[0115] Typically, the torque ripple of VFD driven motors increase compared to motor with sinusoidal currents because it is difficult to operate a motor with sinusoidal currents at a VFD. The dashed curve in the diagram in Fig. 15 illustrates the typically ripples of power, caused by the torque ripples of the permanent magnet motor, having single coils, at sinusoidal currents. The ripples, peak to peak in this example are 8.7%. The same motor has torque ripples of 23.0% (minimal to maximal value) in the present example at the VFD, causing currents which are not exact sinusoidal (Fig. 10).

[0116] The efficiency of the motor during those operations decreases not significantly, from 99.06 % for sinusoidal to 99.03 % for the operation at the VFD. The efficiency of the VFD increases for the inventive concept proportional to the decrease of the semiconductor pulse frequency. Friction losses are not included in the efficiency above.

[0117] A huge advantage is the simplicity of the inventive VFD. It switches the semiconductors simply in accordance with the input of the rotor angle 39 and limits the current by a single current measurement compared to the desired current value, which is proportional to the desired torque. There is no need for a highly performed processor for any transformations (Clarke-Transformation and Park-Transformation) and data value handling, which must be done during parts of an electrical period (parts of 20 milli seconds at 50 Hz and parts of 8 milli seconds at 125 Hz).

[0118] Motor Start

[0119] At still stand (0 rpm), no pole wheel voltage occurs, and the inductive resistance of the motor coils are nil, because the electrical synchronous frequency is nil. Even the motor rotor starts turning the voltages generated by the motor, opposite the supply voltages are minor. In VFD in accordance with the prior art and a single voltage level of the DC link bus, the limits of the phase currents are achieved by short voltage pulses. Voltage wave reflections can be caused by short high frequent pulses. In accordance with the theory of wave propagation, the voltage amplitude can be the twofold value of the value of the pulsed voltage, in case of one reflection. Because there is more than one reflection, the overvoltage can increase to the ten-fold value of the source voltage, compare page 254 in (Fischer, 2009).

[0120] A voltage level above the highest voltage level 47 is avoided in the VFD configuration 51 (Fig. 16) by the avoidance of switching the voltage level 47 during low-speed operation and during the motor start. Before exceeding a specified low-speed limit 50. In the present example, this value is 50% of the rated speed. The voltage level of the middle voltage level 46 is 50% of 47.

[0121] The diagrams in Fig. 17 illustrate the values during the motor start until a rotor speed of 92 rpm. The moment of inertia of the rotor part of two pole pairs is 50 kgm2in this simulation, which 10% of the real value to make the acceleration faster for giving a better overview of the pulse patterns during the acceleration phase.

[0122] The voltage level 47 is not switched on, see value “0” in the first diagram in Fig. 17. There are short pulses of the voltage level 46, see second diagram in Fig. 17. The major parts of the voltage are long pulses (dot-dashed curve in diagram 3 in Fig. 17) of the voltage level 45, which is 115 V, which is 2.5% of the higher voltage level 47.

[0123] Below 63 rpm (doted curve in Fig. 17) even this small voltage level let exceed the current limit (wide solid line in Fig. 17), which is not critical during a short time. Otherwise, the voltage level 45 must be decreased, because the concept of the inventive VFD is not to interrupt the current flow, except between the change of the polarisation of the phase voltage and the avoidance of overvoltage, caused by this.

[0124] The switching of the semiconductors connecting the motor phases to the plus pole of 31.2 are illustrated by the long-dashed curves in Fig. 17. Each switching of the semiconductor bridge causes a current drop and caused by the current control a switching on of the voltage level 46 (solid curve in diagram 2 in Fig. 17). From the motor start at 0 rpm on the phase currents are symmetrical, see solid curve (phase U), dashed curve (phase V) and dot-dashed curve (phase W) in Fig. 18. The phase element voltage of phase U is illustrated by the thin solid curve in Fig. 18.

[0125] The additional switching of the higher voltage level after exceeding the limit of the low- speed limit 50% at 375 rpm is illustrated by the short-dashed curve in the first diagram in Fig. 19. Those short pulses become longer with increasing rotation speed and increasing pole wheel voltage. The impact to the phase currents by this transition is illustrated in Fig. 20. The higher peaks of the phase element voltage are illustrated by the thin solid voltage curve, and the increase of acceleration by the bigger current amplitudes is visible in the long-dashed curve of the rotor speed in the same diagram.

[0126] At the rated speed of 750 rpm, the switching of the higher voltage level 47 is longer than at 50% rated speed (see first diagram in Fig. 21) and the number of switching of the lower voltage level 45 become less (see third diagram in Fig. 21). The phase currents at rated speed have a significant better approximation to sinus curves than at low speed (Fig. 22). Sinusoidal currents cause the lowest torque ripples. The torque is illustrated by the doted curve in Fig. 22, and the voltage of the phase element U is illustrated by the thin solid curve.

[0127] After achieving the desired speed, the desired current value 37 and the 49 must be decreased to stop the acceleration by a control in accordance with the desired speed, and it must be increased by this control during the load torque occurs.

[0128] Maintenance Operation

[0129] During maintenance and assembly of the mill, the mill drive must be turned with slow speed. Beside the achievement of a slow turning speed of the mill table 10 for welding it, after it is worn by the mill process, for safety reasons the operational speed must be prevented, because working people are in the mill during the those works.

[0130] The slow speed and the prevention of rated operation speed can be achieved by the inventive VFD and the motor 13, without additional auxiliary drive, which would be the prior state of the art. For maintenance operation, the VFD 56 (Fig. 23) contains the safety look 57, which disconnects the controlled voltage supply 33.2 from the multilevel voltage supply 32.2 and connects the semiconductor bridge 31 directly durable to the voltage level 45 via the bypass 58. The diode 60 in the bypass 58 prevents negative currents in the supply circuit, during the pole wheel voltage of the motor exceeds the voltage level 45 of the supply at higher speed.

[0131] The pole wheel voltage, which increases proportionally with the motor speed, prevents higher speed, because it is opposite to the torque-generating current. Even if the control 36 of the semiconductor bridge 31 fails, the motor speed cannot exceed the value, where the pole wheel voltage is bigger than the voltage level 45.

[0132] The degressive acceleration above 50 rpm is illustrated by the dashed curve in Fig. 24. The wide solid curve in diagram 3 in Fig. 24 is the phase element voltage. The amplitude of it is a stepped voltage curve, caused by the voltage level 45 and the switching of 31. The gear transmission ratio in the present example is 40 and causes a mill table speed of 3 rpm. The voltage level 45 must be dimensioned in accordance with the speed limit for safety reasons. Additional safety equipment is necessary for a fast stop of rotation, by a hardware switch for connecting the phases in short circuit via load resistors for a fast dissipation of the kinetic energy of the drive train. During this stopping operation, the motor works as a generator, transforming the kinetic energy of the moment of inertia into electrical energy which disappears as heat energy at the load resistors.

[0133] The operation at a maintenance speed of 38 rpm (0.95 rpm mill table speed) is illustrated in the diagrams in Fig. 25 and Fig. 26. In both diagrams, the torque of two pole pairs is 4.36 kNm during the phase shift angle of the advanced voltage phase angle is electrically 20° (wide solid curve) in relation to the phase angle, which is given by the rotor position angle. During the entire operation, the semiconductor bridge 31 is durable connected to the voltage level 45, which has a value of 115 V.

[0134] For increasing the average torque from 4.36 to 6.47 kNm, the shift angle is increased from electrical 20° to 30°. The torque is illustrated by the long-dashed curves. The phase currents of the two pole pairs are illustrated by the thin curves, which are solid for phase U, dashed for phase V and dot-dashed for phase W. The amplitudes of the currents increase with the increase of the phase shift angle.

[0135] For decreasing the average torque from 4.36 to 2.71 kNm, the shift angle is decreased from 20° to 10° (wide solid curve in Fig. 26).

[0136] The rotor speed measured as 52 can be controlled by the variation of the torque. The torque can be variated by the variation of the phase shift angle, as demonstrated in the diagrams in Fig. 25 and Fig. 26. The approximate sinusoidal phase currents in the diagrams are achieved for a maintenance speed which is depending on the voltage level 45 and the pole wheel flux, because the pole wheel voltage and the supply voltage must be in a comparable range.

[0137] The complete speed range, up to the limit of the safety speed limit (see in diagram in Fig. 24) can be controlled by the multilevel voltage method, which is explained for the inventive frequency drive 34, but where the lower voltage level 45 has the value nil, see VFD configuration 61 in Fig. 27. For the speed control, the torque is controlled by the variation of modifying the desired current value 37.

[0138] For both methods, the variation of the value of 38 or the variation of the value of 37 must be done by classical controllers, like Pl-controller, based on the rotation speed.

[0139] If the control by the desired current value 37 is used, for safety during maintenance works, the higher voltage levels must be disconnected by a safety look from the higher voltage levels.

[0140] A mill may comprise the mill drive system according to any of the preceding embodiments.

[0141] The mill drive system according to any of the preceding embodiments may be used in a mill.

[0142] Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. It should also be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.

[0143] References:

[0144] (1) Prior state motor of a mill drive

[0145] (2) Bevel gear pinion

[0146] (3) Bevel gear

[0147] (4) Sun pinion first planetary gear stage (4a sun-wheel)

[0148] (5) Planet gears of the first planetary gear stage

[0149] (6) Ring gear of the first planetary gear stage and sun gear of the second planetary gear stage (7) Planet gears of the second planetary gear stage

[0150] (8) Ring gear of the second planetary gear stage

[0151] (9) Planet axel

[0152] (10) Rotating mill table

[0153] (11) Grinding roller

[0154] (12) Grinding material

[0155] (13) Highly efficient motor

[0156] (14) Motor shaft

[0157] (15) Rotor core

[0158] (16) Cooling ducts of the rotor

[0159] (17) Permanent magnets

[0160] (18) Stator core

[0161] (19) Stator coils

[0162] (20) Colling ducts of the stator

[0163] (21) Motor casing

[0164] (22) Stator core

[0165] (23) Cooling ducts of the stator

[0166] (24) Stator coils

[0167] (25) Rotor core

[0168] (26) Cooling ducts of the rotor

[0169] (27) Permanent magnets

[0170] (28) Intermediate pole

[0171] (29) Rotor core pole

[0172] (30) Two stage planetary gear transmission

[0173] (31) Semiconductor bridge

[0174] (32) Multi-level voltage supply / (32.2) Multi voltage supply, having 3 voltage levels.

[0175] (33) Controlled voltage supply / (33.2) Controlled voltage supply, having 3 voltage levels.

[0176] (34) Inventive variable-frequency drive

[0177] (35) Current measurement

[0178] (36) Semiconductor - bridge control

[0179] (37) Desired current value (38) Phase shift angle

[0180] (39) Measured value of the rotor angle

[0181] (40) Signals for the switching of the semiconductors / (40.2) Delayed signals for switching of the semiconductors

[0182] (41) Multiplication operator

[0183] (42) Summation operator

[0184] (43) Transformation into a sinus function

[0185] (44) Threshold operation

[0186] (45) Lower voltage level

[0187] (46) Middle voltage level

[0188] (47) Higher voltage level

[0189] (48) Current value

[0190] (49) Lower current limit value

[0191] (50) Low speed limit

[0192] (51) Variable-frequency drive, having 3 voltage levels

[0193] (52) Measured value of the rotor speed

[0194] (53) Motor phase U

[0195] (54) Motor phase V

[0196] (55) Motor phase W

[0197] (56) Variable-frequency drive with safety look for maintenance

[0198] (57) Safety look

[0199] (58) Bypass

[0200] (59) Three-phase AC power grid

[0201] (60) Diode

[0202] (61) Variable-frequency drive having the value nil as lower voltage level 45

Claims

Claims:

1. A mill drive system comprising: a two-stage planetary gear transmission (30); a motor (13); a variable frequency drive (34), characterized in that the semiconductor bridge (31) of the variable frequency drive (34) comprises a controlled voltage supply (33), said voltage supply (33) configured to control the voltage level in accordance to a current value (48) to be measured by a current measurement device (35) and compared to a desired current value (37) and a multi-level voltage supply (32) of the controlled voltage supply (33) configured to have at least two voltage levels (46) and (47).

2. A mill drive system according to claim 1, wherein said variable frequency drive (51) is configured to control the output voltage of (33.2) by comparing the current value (48) to additional current values, at least additional to (49) and the voltage supply (32.2) configured to have at least the three voltage levels (45), (46) and (47), which are connected to the semiconductor bridge (31) by the controlled voltage supply (33.2), depending of the measured current value (48).

3. A mill drive system according to claim 2, wherein the controlled voltage supply (33.2) is configured to be disconnected from the multilevel voltage supply (32.2) by a safety look (57) and the semiconductor bridge (31) is configured to connected durable to the lower voltage level (45) by the safety look via the bypass (58) for low-speed maintenance operation, wherein (58) contains a diode (60), and where the value of the voltage level (45) is smaller than the pole wheel voltage of (13) at the speed.

4. A mill drive system according to claim 3, wherein the maintenance speed is configured to be controlled by the phase shift angle between the phase angle of the pole wheel voltage and the phase angle of the voltage, which is the output of the semiconductor bridge 31.

5. A mill drive system according to claims 3 or 4, wherein the maintenance speed is configured to be controlled by variating the desired current value (48) in accordance with the needed torque, and wherein the desired current value (48) is configured to be achieved by switching between the voltage levels (46) and (47)in the configuration 61, wherein the value of the voltage level (46) is nil for positive currents, and the higher voltage levels are disconnected by a safety look switch.A mill drive system according to any of the preceding claims, in which the motor (13) comprises a magnet arrangement, having low eddy current losses, by arranging said magnets (27) apart from the airgap and its alternating magnetic fields, between the rotor poles (28) and (29), in a flux concentration arrangement.

6. A mill drive system according to any of the preceding claims, comprising only a two-stage gear transmission without a third bevel gear transmission, and without the losses of the third gear transmission.

7. A mill drive system with horizontal drive axis, comprising the mill drive according to claim 7, further comprising a horizontal motor axis and a horizontal axis of the two-stage planetary gear transmission supplied by the variable frequency drive (51).

8. A mill drive system according to claim 2, wherein the semiconductors of (51) are formed by MOSFET or IGBT, and the semiconductors are arranged in a multilevel arrangement for medium voltage.

9. A mill drive system according to claim 2, wherein the semiconductor bridge (31) is configured not to be connected to the voltage level (47) during the motor start before the motor speed exceeds a certain value, where the pole wheel voltage is far below the voltage level (47), to avoid short pulses of the switching of the voltage level (47) and by this to avoid overvoltage as a consequence of short and high voltage pulses.

10. A mill comprising the mill drive system according to any of the preceding claims.

11. Use of the mill drive system according to any of the preceding claims in a mill.

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