Patent Publication Number: US-2023163703-A1

Title: Electrical drive system for a work machine having two electric motors that can be regulated independently of one another

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority to German Patent Application No. 10 2021 130 987.5 filed on Nov. 25, 2021. The entire contents of the above-listed application are hereby incorporated by reference for all purposes. 
     TECHNICAL FIELD 
     The disclosure relates to an electrical drive system for an electrical work machine, in particular for a mining truck, having at least two electrical drive motors that can each be regulated via an associated inverter, wherein the drive system has a DC voltage power supply for the energy supply of the at least two drive motors. 
     BACKGROUND AND SUMMARY 
     Electrification and the reduction of polluting emissions are a big topic with work machines. The provision of electrical energy is complex, in particular with large mobile work machines having high power requirements. In mining, for example, electrical overhead lines are already being installed along possible transport roadways between loading and unloading sites to supply large work machines such as mining trucks with the required electrical energy for the traction drive via the overhead line. The overhead lines provide a DC voltage having a constant voltage level. The internal power electronics of the electrical drive system of the work machines is adapted to the specified DC voltage level of the overhead line. 
     As the energy consumption of the vehicles increases and in particular as the distance between the loading and unloading sites increases, the rated voltage of the energy supply system tends to have to be increased in the near future to be able to compensate the increasing electrical transmission losses caused thereby. An increase in the rated voltage of the overhead line has the result, however, that existing work machines are no longer compatible with the overhead line system. The power electronics and the electric motors (or drive systems) of the work machines accordingly have to be redeveloped and replaced, which means substantial effort and costs. 
     An alternative solution is therefore being looked for to also be able to continue to operate the previous drive systems that were dimensioned for a smaller voltage level, also for the operation at an overhead line with a higher voltage level, without a complete replacement of the drive systems being necessary. 
     This object is achieved by an electrical drive system having the features of claim  1 . Advantageous embodiments of the drive system are the subject of the dependent claims. In addition, the disclosure extends to a work machine having such a drive system and to a method of operating an electrical drive system for a work machine. 
     It is proposed in accordance with the disclosure to connect at least one voltage divider in parallel with the DC voltage power supply. The mains voltage provided via the power supply is divided in a defined ratio by means of the voltage divider. At the output side of the voltage divider, two or more partial voltages are provided each having a reduced voltage level in comparison with the mains voltage. The reduced partial voltages are applied to the DC voltage input of the respective inverter. The partial voltages ideally correspond to the respective rated voltage of the inverter. 
     The use of existing inverters or electric motors that are adapted to a smaller rated voltage is also possible at a power supply having a higher voltage level due to the disclosure with an integrated voltage divider. The only condition can be seen as the rated voltage of the respective inverter having to be larger than or equal to the applied partial voltage. 
     In the ideal case, the voltage divider at least theoretically effects a symmetrical voltage division. However, voltage fluctuations occur due to the different load behavior of the at least two drive motors (e.g. when cornering) so that no continuous symmetrical voltage division is present. The electrical drive motors may be asynchronous motors; the inverters connected upstream generate a three phase AC voltage or a three phase AC current from the DC voltage at the input side. Provision can be made that the at least two inverters of the at least two drive motors can be controlled or regulated independently of one another, i.e. the torque and/or the speed of the at least two drive motors can be set or regulated independently of one another and/or there is no mutual influence of the regulations. 
     The electrical drive system can be a traction drive system for a work machine, in particular a mining truck, with the respective drive motors optionally being used to drive independent axles or wheels. The drive system at the deployment site of the work machine can tap a supply voltage from an external DC voltage source via the DC voltage power supply. The external DC voltage source is optionally an overhead line system; the power supply of the drive system is implemented by means of at least one current collector. An alternative connection by means of cable is equally conceivable. 
     Provision can be made in an embodiment that at least one energy store is provided per drive motor or inverter. This at least one energy store is to be understood as an energy store in addition to the intermediate circuit capacitor. The required energy for the operation of the drive motors can be at least temporarily provided to the associated electric motor from the energy store. A parallel supply of the respective drive motor via the power supply and the engaged at least one energy store is also conceivable. In accordance with an embodiment, the energy store also serves the storage of the recovered energy of the drive motor working in generator mode. A charging of the energy store via the power supply is equally conceivable. 
     The at least one energy store can be designed as a battery bank. The use of at least one fuel cell stack having a hydrogen container as the energy store is also conceivable. 
     At least one or the at least two energy stores is/are sensibly each connected to the voltage divider and/or to the inverter of the associated drive motor via a bidirectional DC voltage converter (DC-to-DC converter). The bidirectional DC-to-DC converter or converters optionally serves/serve the provision of the required energy for the traction drives from the at least two energy stores. This in particular applies to deployment areas, for example driven distances without an external power supply, for example routes without overhead lines. The DC-to-DC converters here optionally work as boost converters. 
     The bidirectional DC-to-DC converters optionally also serve the charging of the respective energy stores from the DC voltage power supply and/or via the drive motors working as generators. The DC-to-DC converters here optionally work as buck converters. 
     The DC-to-DC converters used can also be used with a suitable control to balance the generated partial voltages at the output of the voltage divider when the work machine or the drive system is supplied with energy via the DC voltage power supply. A suitable regulation of the energy flow from or into the energy store compensates deviations of the partial voltages so that symmetrical voltage ratios can be supplied to the input of the inverters as the end result. 
     It is likewise possible for the at least two energy stores to be interchangeably connected to one another via a charge balancing means. Such a charge balancing means can optionally be implemented by a further bidirectional DC-to-DC converter whose connectors are connected to at least two energy stores. The charge balancing means allows a charge balance between the at least two energy stores. The charge balancing means can in particular be used on an exclusive supply of the drive motors from the energy stores, i.e. during operating times without an external supply via the DC voltage power supply. The charge balancing means can achieve a balancing of the partial voltages on the voltage divider by an additional charge balance between the two energy stores for this case. The aforesaid bidirectional DC-to-DC converters associated with the energy stores are controlled to set the corresponding voltages U 1  and U 2 . The resulting energy flow (current) in the drive trains to the individual drive motors results from the load of the individual drive trains. A charge balancing means, i.e. an additional bidirectional DC-to-DC converter therefore has to be used for the balancing of the charges of the energy stores, said additional DC-to-DC converter being able to be designed as smaller in power than the two bidirectional DC-to-DC converters associated with the energy stores. 
     The voltage divider itself can comprise a series connection of at least two impedances Z 1  and Z 2  connected in parallel with the power supply, with each impedance optionally comprising a parallel connection of at least one capacitor and at least one resistor. The DC voltage provided per drive motor is then tapped above a portion of the impedances. 
     As has already been indicated above, the energy stores can provide energy in support of the power supply by means of the DC-to-DC converters. A balancing or another desired adaptation of the partial voltages of the voltage divider at the output side, i.e. the input voltage at the inverter, can thereby also take place by a suitable control of the DC-to-DC converters associated with the energy stores. Against this background, a separate balancing device of the voltage divider can be dispensed with if the drive system is supplied with energy via the DC voltage power supply. However, there is nothing to contradict providing at least one additional voltage balancing device instead of or in addition to the energy stores and/or associated DC-to-DC converters and/or charge balancing means to balance the partial voltages of the voltage divider provided at the output side. The additional voltage balancing device can be active or passive. 
     Each drive motor can additionally be connectable in parallel to a braking resistor to convert the recovered power into thermal energy. Such a brake resistor can optionally be temporarily engaged so that a conversion into thermal energy is only carried out under certain circumstances. Since the recovered energy primarily serves the charging of the energy stores or is fed into the external network, the respective brake resistor can be dimensioned as considerably smaller since only a small residual portion, if any, of the recovered energy has to be eliminated via the resistor. 
     The drive system is optionally equipped with a control unit to control the individual components of the drive system in dependence on certain actual and desired values. The inverters and/or bidirectional DC-to-DC converters and the charge balancing means and/or an optional active balancing device of the voltage divider can be controlled, for example. The voltage level of the power supply and/or the levels of the generated partial voltages of the voltage divider and/or one or more values of the drive motors characteristic for the drive such as the actual speed of the drive motors, and/or the power consumption of the drive motors and/or temperature values of the drive motors, and/or the charge state of the energy stores are, for example, supplied to the control unit as possible actual values. The control unit generates suitable control variables for the inverters and/or DC-to-DC converters and/or the charge balancing means in dependence on certain desired specifications, for example the required desired motor power, in particular the desired torque and the desired speed, and/or a desired braking demand. A control variable can equally be generated for activating or deactivating the braking resistor and a corresponding desired value can be generated for the control of the balancing device arranged at the input for the balancing of the partial voltages. 
     It is possible here that the DC-to-DC converters associated with the energy stores be controlled in dependence on the measured partial voltages of the voltage divider to be able to compensate possible deviations between the partial voltages by a suitable control of the DC-to-DC converters from the energy stores. 
     In addition to the drive system in accordance with the disclosure, the disclosure equally relates to a work machine, in particular a mining truck, having a drive system in accordance with the disclosure so that the same advantages and properties result for the work machine as were already shown above with reference to the drive system in accordance with the disclosure. 
     The at least two drive motors of the drive system here optionally serve the drive of separate wheels of the work machine. The work machine moreover comprises at least one current collector for electrically contacting an external DC voltage source, in particular an overhead line. Provision can be made here that the work machine can selectively be supplied with the required energy for the traction drive via the overhead line or alternatively via the internal energy stores. In the ideal case, the total work machine is emission free, i.e. it does not comprise any internal combustion engine for the implementation of the traction drive or any other drives of the work machine. 
     Finally, the disclosure relates to a method of operating an electrical drive system for a work machine, in particular an electrical drive system in accordance with the present disclosure. It is proposed in accordance with the disclosure to split the mains voltage applied to the power supply of the drive system to partial voltages by a use of a voltage divider so that independent electrical drive trains of the drive system can be supplied with a reduced voltage level. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Further advantages and properties of the disclosure will be explained in more detail in the following with reference to the embodiments shown in the Figures. There are shown: 
         FIG.  1   : a circuit diagram of the electrical drive system in accordance with the disclosure for a mining truck having a voltage divider comprising two identical impedances Z 1  and Z 2 , a conventional balancing device, and two separate storage systems; 
         FIG.  2   : a modification of the drive system of  FIG.  1   , but without an additional balancing device in the region of the power supply; 
         FIG.  3   : a further development of the drive system of  FIG.  2    with an additional DC-to-DC converter between the two energy stores; 
         FIG.  4   : an alternative embodiment of the system in accordance with  FIG.  2    with fuel cells as the internal energy supply system; and 
         FIG.  5   : an exemplary implementation of the voltage divider comprising two impedances. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    now shows the schematic design of the drive system in accordance with the disclosure for a mining truck. The traction drive of such a mining truck is by electric motor, i.e. at least the two rear wheels of such a mining truck can be driven by separate electric motors, here the motors M 1  and M 2 . The two motors M 1  and M 2  are typically asynchronous motors that are fed via a three phase DC voltage. 
     The drive motors M 1 , M 2  are components of two independent electrical drive trains that each comprise an inverter  10  that is connected upstream of the drive motor M 1 , M 2  and that converts the DC voltage applied at the input side into the required three phase AC voltage for the motors M 1 , M 2 . In addition to this, a battery bank  50  is provided per drive train that is connected to the DC voltage intermediate circuit via an associated DC-to-DC converter  60 . 
     The deployment site of such mining trucks is often in large landmines to transport large amounts of the removed soil from a loading site to an unloading site. Overhead line systems  30  that provide a DC voltage having a constant potential U DC  along the course of the lines are installed along the transport roadway for the required energy supply. The mining truck can tap the DC voltage potential U DC  by means of the current collectors installed at the mining truck and can use it for the energy supply of the drive system. 
     Due to the increasing transport roadways and the increasing power consumption of the machines, it is desirable to raise the voltage potential due to impending power losses. 
     Since the components such as the converters  10  and the drive motors M 1 , M 2  of the drive trains are adapted for a certain rated potential, a voltage divider  40  is integrated in accordance with the disclosure, whereby the use of the drive system is also possible at an overhead line system  30  whose DC voltage U DC  exceeds the rated voltages of the inverters  10 . Specifically, the DC voltage U DC  of the overhead line system  30  tapped via the current collectors  20  is divided by means of the voltage divider  40  arranged at the input side to two partial voltages U 1 , U 2  whose voltage levels are each below the potential U DC , ideally both partial voltages each correspond to U DC /2. The voltage divider  40  comprises a series connection of the two impedances Z 1 , Z 2 , with the partial voltage U 1  being applied above the impedance Z 1  and the partial voltage U 2  above the impedance Z 2 . Only the reduced partial voltages U 1 , U 2  are now applied at the inverters  10 , with the partial voltage U 1  being applied at the input of the first inverter of the motor M 1  and the partial voltage U 2  at the second inverter  10  of the motor M 2 . 
     The partial voltages cannot be equal due to the deviations in the impedances Z 1  and Z 2 . To be able to balance the partial voltages, a balancing device  90  is connected to the voltage divider  40 . The outgoing partial voltages U 1 , U 2  are balanced by a suitable control of the balancing device  90  by means of the control unit  100 . 
     There is the possibility in ongoing operation to supply the drive motors M 1 , M 2  selectively with energy from the overhead line system  30 , the energy store  50 , or from both sources  30 ,  50 . The vehicle can thus also bridge certain travel distances outside the overhead line system by energy from the internal energy store  50  and can thus ideally be operated completely free of emissions. Recovered braking energy of the drive motors M 1 , M 2  can be fed back via the bidirectional DC-to-DC converter  60  into the batteries  50  or alternatively via the current collectors  20  into the overhead line system  30 . Excess energy can additionally be converted into thermal energy via separate, temporarily engageable braking resistors  70  of the drive trains. The braking resistors  70  can accordingly be dimensioned as small since the braking energy is primarily fed back for the charging of the stores  50  or into the network  30 . 
     The central control device  100  of the drive system of  FIG.  1    receives the voltage level U DC  of the overhead line system  30 , the charge states of the battery banks  50 , and the generated partial voltages U 1 , U 2  of the voltage divider  40  as input values. The control unit  100  furthermore receives actual characteristic values of the drive motors M 1 , M 2 , inter alia the current motor temperature and the motor speed of the motors M 1 , M 2 . The control unit  100  receives the required desired toque M of the drives M 1 , M 2 , the desired speed V of the traction drive, and the required desired acceleration a via desired valuators for the control of the drives M 1 , M 2 . A current braking demand is furthermore communicated to the control unit  100  via a further desired valuator. 
     The control unit generates the desired control variables on the basis of the aforesaid input values and controls the two inverters  10  accordingly to generate the desired torque by the motors M 1  and M 2 . 
       FIG.  2    shows a modification of the drive system of  FIG.  1   ; the same components are marked by identical reference numerals in both Figures. In this embodiment, the DC-to-DC converters  60  can take over the work of the balancing device  90  in accordance with  FIG.  1    when the mining truck is connected to overhead lines. A suitable regulation ensures an energy flow of the individual drive trains that sets symmetrical voltage ratios (U 1 ≈U 2 ). The voltage levels in the two drive trains, i.e. the voltage level applied at the inverter, can be regulated independently of one another by the two bidirectional DC-to-DC converters  60  controllable independently of one another. It is thereby possible, for example, to balance asymmetries of the partial voltages U 2 , U 2  in driving operation. The additional balancing device  90  in accordance with  FIG.  1    for the balancing of the partial voltages U 2 , U 2  can thus be dispensed with by the suitable control of the DC-to-DC converter  60 . 
     The DC-to-DC converters  60  have to be controlled to charge the stores  50  during the braking process. Excess braking energy is either fed back into the overhead line system  30  or is alternatively removed via the respective activated braking resistor  70 . 
       FIG.  3    shows a first further development of the drive system of  FIG.  2   ; the same components are marked by identical reference numerals in both Figures. An additional bidirectional DC-to-DC converter  80  is then installed as a charge balancing means between the two energy stores  50 . A charge balance is thereby implemented between the two storage systems  50  when the vehicle is separated from the overhead lines  30  and is only supplied by the batteries  50 . In this case, the DC-to-DC converters  60  will set the corresponding voltages U 1  and U 2 ; the energy flow (current) results from the load of the individual drive trains. An additional bidirectional DC-to-DC converter  80  therefore has to be used for the balancing of the charges of the battery banks  50  at the left and right, said additional DC-to-DC converter  80  being able to be designed as smaller in power than the two DC-to-DC converters  60 . The bidirectional DC-to-DC converter  80  is also suitably controlled by the control unit  100 . 
     In the modification of  FIG.  4   , a fuel cell system  110  has been installed instead of the battery banks  50  with respect to  FIG.  2   . The fuel cell system can likewise be controlled by the control unit. 
       FIG.  5    shows a possible embodiment of the voltage divider  40  in accordance with  FIGS.  1  to  4    that is implemented by two impedances Z 1  and Z 2 . The impedances Z 1  and Z 2  each may comprise an additional parallel circuit of capacitor C 1 , C 2  and resistor R 1 , R 2 . 
     The individual modifications of  FIGS.  1  to  4    are also combinable with one another as desired, i.e. the embodiments of  FIGS.  1 ,  2 , and  3    can also be used with fuel cells  110  in accordance with  FIG.  4   .