Abstract:
A converter unit having at least one output. The at least one output is configured to be connected to a coil of an asynchronous machine. The converter unit is configured to provide several voltage levels at the at least one output.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2012/067790, filed on Sep. 12, 2012, which claims the benefit of priority to Serial No. DE 10 2011 084 698.0, filed on Oct. 18, 2011 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to a converter unit for an asynchronous machine and to a battery and a drive unit, which comprise the converter unit according to the disclosure. 
     BACKGROUND 
     It appears that in future battery systems will be used increasingly both in stationary applications and in vehicles such as hybrid and electric vehicles. In order to be able to meet the requirements placed on voltage and available power as set for a respective application, a high number of battery cells are connected in series. Since the current provided by such a battery needs to flow through all of the battery cells and a battery cell can only conduct a limited current, often battery cells are additionally connected in parallel in order to increase the maximum current. This can take place either by virtue of the provision of a plurality of cell coils within a battery cell housing or by externally interconnecting battery cells. However, it is problematic here that, owing to imprecisely identical cell capacitances and voltages, compensation currents can arise between the battery cells connected in parallel. 
     The basic circuit diagram of a conventional electrical drive unit, such as is used, for example, in electric and hybrid vehicles or else in stationary applications such as in the case of the rotor blade adjustment of wind turbines, is illustrated in  FIG. 1 . A battery  10  is connected to a DC voltage intermediate circuit, which is buffered by an intermediate circuit capacitor  11 . A pulse-controlled inverter  12  is connected to the DC voltage intermediate circuit and provides, via in each case two switchable semiconductor valves and two diodes, sinusoidal currents which are phase-shifted with respect to one another at three taps  14 - 1 ,  14 - 2 ,  14 - 3  for the operation of an electric drive motor  13 . The capacitance of the intermediate circuit capacitor  11  needs to be sufficiently high for the voltage in the DC voltage intermediate circuit to be stabilized for a period in which one of the switchable semiconductor valves is on. In a practical application such as an electric vehicle, a high capacitance in the mF range results. 
     If, in the case of the arrangement illustrated in  FIG. 1 , an asynchronous machine is used as the electric drive motor  13 , it is disadvantageous that the achievable power is limited by the eddy current losses in the rotor of the asynchronous machine at high speeds. These losses are caused by the severe harmonics in the current which are caused by the high potential differences of the pulse-controlled inverter  12  and the resultant high values for the change in current over time. In order to reduce these losses, it is nowadays conventional to connect a filter  15  between the pulse-controlled inverter  12  and the asynchronous machine  13 , as is illustrated in  FIG. 2 . By means of this filter  15 , the harmonics in the current are reduced, with the result that the losses are reduced and operation at a high speed is expediently possible for the first time. However, the filter  15  increases the complexity of a required controlled system considerably, takes up installation space and also represents a considerable cost factor. 
     SUMMARY 
     According to the disclosure, a converter unit comprising at least one output is provided. The output of the converter unit is connectable to a coil of an asynchronous machine, and the converter unit is designed to provide a plurality of voltage levels at its output. This makes it possible to operate an asynchronous machine rotating at a high speed without interposing a filter, as is illustrated in  FIG. 2 , without notable eddy current losses arising in the asynchronous machine. Thus, operation of the asynchronous machine at high speeds and high powers becomes possible. Since an asynchronous machine is substantially more favorable in terms of costs in comparison with a synchronous machine and an additional filter can be dispensed with owing to the use of the converter unit according to the disclosure, overall the possibility is provided of providing an inexpensive electric drive unit. 
     It is preferred that the converter unit is designed to provide a substantially sinusoidal voltage signal of a predetermined frequency at its output. Since the voltage at the output of the converter unit can be set in a stepped fashion, a sinusoidal profile can only be achieved at an approximation. In the context of the disclosure, however, it is sufficient to provide a voltage signal which, owing to the use of a sufficiently high number of voltage levels, is brought so close to an ideal sinusoidal profile that the mode of operation of the asynchronous machine is not impaired and the changes in the current in the coil of the asynchronous machine over time are not excessively high. 
     Typically, the converter unit comprises three outputs, which are connectable to the three coils conventionally used in a stator of the asynchronous machine. 
     In a preferred embodiment of the disclosure, provision is made for the converter unit to comprise at least one battery module string comprising a plurality of battery modules connected in series. Each battery module comprises at least one battery cell, at least one coupling unit, a first connection and a second connection. Each of the battery modules is designed to assume one of at least two switching states depending on an actuation of the coupling unit. In this case, various switching states correspond to different voltage values between the first connection and the second connection of the battery module, i.e. various voltages can be tapped off between the first connection and the second connection. 
     Various embodiments of the coupling unit can advantageously be used. In a first embodiment, the coupling unit is designed to connect the at least one battery cell between the first connection and the second connection in the case of a first control signal and to connect the first connection and the second connection in the case of a second control signal. 
     In a further embodiment, the battery module is designed to optionally assume one of at least three switching states depending on an actuation of the coupling unit. In a first switching state, the first connection and the second connection of the battery module are connected. In a second switching state, the at least one battery cell is connected between the first connection and the second connection with a first (for example positive) polarity. In a third switching state, the at least one battery cell is connected between the first connection and the second connection with a polarity which is opposite the first polarity (in the same example negative). 
     The various embodiments of the battery modules or coupling units can also be mixed within a battery module string. 
     In a further embodiment of the disclosure, the converter unit comprises a multilevel inverter having at least one output. The output of the multilevel inverter forms the output of the converter unit. 
     The two above-described embodiments of the converter unit can also be combined to the extent that the converter unit comprises a battery module string comprising the above-described battery modules and a plurality of center taps are arranged on the battery module string. A potential can be tapped off at a connection between in each case two battery modules at the center taps. The inputs of the multilevel inverter are connected to the center taps. 
     A further aspect of the disclosure relates to a battery, preferably a lithium-ion battery, comprising at least one converter unit according to the disclosure. A further aspect of the disclosure relates to a drive unit comprising at least one asynchronous machine and at least one converter unit according to the disclosure or else a battery comprising the converter unit according to the disclosure. The output of the converter unit is in this case connected to a coil of the asynchronous machine. 
     A further aspect of the disclosure relates to a motor vehicle comprising the drive unit according to the disclosure. 
     Overall, by virtue of the converter unit according to the disclosure, an arrangement is provided in which the difference between a setpoint voltage and a voltage which can actually be tapped off at the output of the converter unit is less than in the case of a converter unit in accordance with the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure will be explained in more detail with reference to the drawings and the description below, wherein identical reference symbols denote identical or functionally identical components. In the drawings: 
         FIGS. 1 and 2  each show an electric drive unit in accordance with the prior art, 
         FIG. 3  shows a coupling unit which can be used in the converter unit according to the disclosure, 
         FIG. 4  shows a first embodiment of the coupling unit, 
         FIG. 5  shows a second embodiment of the coupling unit, 
         FIG. 6  shows the second embodiment of the coupling unit in a simple semiconductor circuit, 
         FIGS. 7 and 8  show two arrangements of the coupling unit in a battery module, 
         FIG. 9  shows the coupling unit illustrated in  FIG. 6  in the arrangement illustrated in  FIG. 7 , 
         FIG. 10  shows an electric drive unit comprising a converter unit in accordance with a first embodiment of the disclosure, 
         FIG. 11  shows an actuation of the converter unit shown in  FIG. 10  by a control device, 
         FIG. 12  shows an embodiment of the coupling unit which makes it possible for a voltage with a selectable polarity to be present between the connections of a battery module, 
         FIG. 13  shows an embodiment of the battery module comprising the coupling unit illustrated in  FIG. 12 , 
         FIG. 14  shows an electric drive unit comprising a converter unit in accordance with a second embodiment of the disclosure, 
         FIG. 15  shows an example of a four-stage multilevel inverter, which can be used in the converter unit in accordance with the second embodiment of the disclosure, and 
         FIG. 16  shows a time profile of a voltage present at one of the outputs of the converter unit according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  shows a coupling unit  30 , which can be used in the converter unit  90  according to the disclosure. The coupling unit  30  has two inputs  31  and  32  and an output  33  and is designed to connect one of the inputs  31  or  32  to the output  33  and to decouple the other input. In specific embodiments of the coupling unit  30 , said coupling unit can also be designed to isolate the two inputs  31 ,  32  from the output  33 . However, no provision is made for both the input  31  and the input  32  to be connected to the output  33 . 
       FIG. 4  shows a first embodiment of the coupling unit  30 , which has an inverter  34 , which in principle can only connect one of the two inputs  31 ,  32  to the output  33 , while the respective other input  31 ,  32  is decoupled from the output  33 . The inverter  34  can be embodied particularly simply as an electromechanical switch. 
       FIG. 5  shows a second embodiment of the coupling unit  30 , in which a first and a second switch  35  and  36 , respectively, are provided. Each of the switches  35 ,  36  is connected between one of the inputs  31  or  32  and the output  33 . In contrast to the embodiment shown in  FIG. 4 , this embodiment has the advantage that both inputs  31 ,  32  can also be decoupled from the output  33 , with the result that the output  33  is provided with a high resistance. In addition, the switches  35 ,  36  can be implemented simply as semiconductor switches, such as, for example, metal-oxide semiconductor field-effect transistor (MOSFET) switches or insulated-gate bipolar transistor (IGBT) switches. Semiconductor switches have the advantage of a favorable price and a high switching speed, with the result that the coupling unit  30  can respond to a control signal or a change in the control signal within a short time and high switchover rates can be achieved. 
       FIG. 6  shows the second embodiment of the coupling unit in a simple semiconductor circuit, in which each of the switches  35 ,  36  comprises in each case a semiconductor valve that can be switched on and off and a diode connected back-to-back in parallel with said semiconductor valve. 
       FIGS. 7 and 8  show two arrangements of the coupling unit  30  in a battery module  40 . A plurality of battery cells  41  is connected in series between the inputs of a coupling unit  30 . However, the disclosure is not restricted to such a series circuit comprising battery cells; it is also possible for only a single battery cell to be provided or else for a parallel circuit or mixed series and parallel circuit of battery cells to be provided. In the example shown in  FIG. 7 , the output of the coupling unit  30  is connected to a first connection  42  and the negative pole of the battery cells  41  is connected to a second connection  43 . However, a mirror-image arrangement as shown in  FIG. 8  is possible, in which the positive pole of the battery cells  41  is connected to the first connection  42  and the output of the coupling unit  30  is connected to the second connection  43 . 
       FIG. 9  shows the coupling unit  30  illustrated in  FIG. 6  in the arrangement illustrated in  FIG. 7 . Actuation and diagnosis of the coupling units  30  is performed via a signal line  44 , which is connected to a control device (not illustrated). Overall, it is possible to set either 0 volt or a voltage U mod  between the connections  42  and  43  of the battery module  40 .  FIG. 10  shows an electric drive unit comprising an asynchronous motor or an asynchronous machine  13  and comprising a converter unit  90  in accordance with a first embodiment of the disclosure. 
     Three coils are arranged in a stator of the asynchronous machine  13  in such a way that, on corresponding actuation, a rotating magnetic field is formed. A rotor of the asynchronous machine  13  comprises individual conductors, which run parallel to the axis of rotation and are either short-circuited with one another at their ends or else discharge the current occurring via slip rings. 
     During operation, the rotating magnetic field produced in the coils of the stator induces a voltage in the lines of the rotor which results in a current flow. From the interaction of the current flow with the magnetic field, the torque is formed, with the result that the rotor rotates. 
     In contrast to the case of a synchronous machine, the rotor in an asynchronous machine can in principle not reach the speed of the field, but deviates from this. 
     The three coils of the asynchronous machine  13  shown in  FIG. 10  are connected to ends of three battery module strings  50 - 1 ,  50 - 2 ,  50 - 3 , which together form the converter unit  90  in accordance with the first embodiment of the disclosure. Each of the three battery module strings  50 - 1 ,  50 - 2 ,  50 - 3  comprises a plurality of battery modules  40 - 1 , . . . ,  40 -n which are connected in series, which each comprise a coupling unit  30  and are constructed as illustrated in  FIG. 7 or 8 . When battery modules  40 - 1 , . . . ,  40 -n are combined to form one of the battery module strings  50 - 1 ,  50 - 2 ,  50 - 3 , in each case the first connection  42  of a battery module  40 - 1 , . . . ,  40 -n is connected to the second connection  43  of an adjacent battery module  40 - 1 , . . . ,  40 -n. In this way, a stepped output voltage can be produced in each of the three battery module strings  50 - 1 ,  50 - 2 ,  50 - 3 . 
     A control device  60  shown in  FIG. 11  is designed to output a first control signal to a variable number of battery modules  40 - 1 , . . . ,  40 -n in the three battery module strings  50 - 1 ,  50 - 2 ,  50 - 3  via a data bus  61 , by means of which first control signal the coupling units  30  of the thus actuated battery modules  40 - 1 , . . . ,  40 -n connect the battery cell (or the battery cells)  41  between the first connection  42  and the second connection  43  of the respective battery module  40 - 1 , . . . ,  40 -n. At the same time, the control device  60  outputs a second control signal to the remaining battery modules  40 - 1 , . . . ,  40 -n, by means of which second control signal the coupling units  30  of these remaining battery modules  40 - 1 , . . . ,  40 -n connect the first connection  42  and the second connection  43  of the respective battery module  40 - 1 , . . . ,  40 -n, as a result of which the battery cells  41  thereof are bypassed. 
     By suitable actuation of the plurality of battery modules  40 - 1 , . . . ,  40 -n in the three battery module strings  50 - 1 ,  50 - 2 ,  50 - 3 , three sinusoidal output voltages can thus be generated, between which there is a phase shift of 120°. As a result, sinusoidal currents with a phase shift of 120° flow in the coils of the stator of the asynchronous machine  13 . 
     Provision is made in a further embodiment for the battery modules  40 - 1 , . . . ,  40 -n used in the three battery module strings  50 - 1 ,  50 - 2 ,  50 - 3  to be designed to switch their battery cells  41  between the first connection  42  and the second connection  43  in such a way that a polarity of the voltage present between the first connection  42  and the second connection  43  is selectable depending on an actuation of the coupling unit. 
       FIG. 12  shows an embodiment of a coupling unit  70  which makes this possible and in which a first, a second, a third and a fourth switch  75 ,  76 ,  77  and  78  are provided. The first switch  75  is connected between a first input  71  and a first output  73 , the second switch  76  is connected between a second input  72  and a second output  74 , the third switch  77  is connected between the first input  71  and the second output  74 , and the fourth switch  78  is connected between the second input  72  and the first output  73 . 
       FIG. 13  shows an embodiment of the battery module  40  with the coupling unit illustrated in  FIG. 12 . The first output of the coupling unit  70  is connected to the first connection  42  and the second output of the coupling unit  70  is connected to the second connection  43  of the battery module  40 . The battery module  40  constructed in this way has the advantage that the battery cells  41  can be connected to the connections  42 ,  43  via the coupling unit with a selectable polarity, with the result that an output voltage with a different mathematical sign can be produced. It may also be possible, for example by closing of the switches  76  and  78  and simultaneous opening of the switches  75  and  77  (or else by opening of the switches  76  and  78  and closing of the switches  75  and  77 ), to connect the connections  42  and  43  conductively to one another and to generate an output voltage of 0 V. Overall, it is therefore possible to set either 0 volt, the voltage U mod  or the voltage −U mod  between the connections  42  and  43  of the battery module  40 . 
       FIG. 14  shows an electric drive unit comprising an asynchronous motor or an asynchronous machine  13  and comprising a converter unit  90  in accordance with a second embodiment of the disclosure, which comprises a multilevel inverter  80 . 
     The multilevel inverter  80  has (n+1) inputs  81 - 1 , . . . ,  81 -(n+1) and three outputs  82 - 1 ,  82 - 2 ,  82 - 3  and is designed to output one of the potentials at each of its outputs  82 - 1 ,  82 - 2 ,  82 - 3 , which potential in each case is present at one of its inputs  81 - 1 , . . . ,  81 -(n+1). The outputs  82 - 1 ,  82 - 2 ,  82 - 3  of the multilevel inverter  80  are connected to the coils of the asynchronous machine  13 . Since most of the available electric motors are designed for operation with three phase signals, the multilevel inverter  80  preferably has precisely three outputs  82 - 1 ,  82 - 2 ,  82 - 3 . The inputs  81 - 1 , . . . ,  81 -(n+1) of the multilevel inverter  80  are connected both to (n−1) center taps  73 - 1 , . . . ,  73 -(n−1) and to the poles  71 ,  72  of a battery module string  50 , which, similar to as in the first exemplary embodiment, comprises n battery modules  40 - 1 , . . . ,  40 -n with coupling units. Owing to the fact that the potential at each of the outputs  82 - 1 ,  82 - 2 ,  82 - 3  of the multilevel inverter  80  is variable and is dependent on the potential values at its inputs  81 - 1 , . . . ,  81 -(n+1) and the potential values present at these inputs  81 - 1 , . . . ,  81 -(n+1) can in turn be set by suitable actuation of the n battery modules  40 - 1 , . . . ,  40 -n, there are a plurality of possible combinations for the actuation of the battery module string and the multilevel inverter  80 , which generate an identical phase signal at the outputs  82 - 1 ,  82 - 2 ,  82 - 3  of the multilevel inverter  80 , preferably an approximately sinusoidal AC voltage. 
     The phase signals at the outputs  82 - 1 ,  82 - 2 ,  82 - 3  of the multilevel inverter  80  can thus be set in stages. By setting a stepped profile of the potential at the outputs  82 - 1 ,  82 - 2 ,  82 - 3  of the multilevel inverter  80 , the losses in the asynchronous machine  13  can be reduced since the conventional change between the positive and negative intermediate circuit potential is absent in the arrangement according to the disclosure. In this way, an improvement to the electromagnetic compatibility of the drive of the asynchronous machine  13  is achieved since the changes in the potential at the inputs of said drive are less pronounced. Likewise, an improvement to the efficiency of the power electronics in the arrangement according to the disclosure is achieved by virtue of the fact that switches comprising metal-oxide semiconductor field-effect transistors (MOSFETs) instead of insulated-gate bipolar transistors (IGBTs) can be used in the multilevel inverter  80 . 
     Owing to the fact that a plurality of possible combinations for the actuation of the battery module string and the multilevel inverter  80  are provided for generating a predetermined phase signal at the outputs  82 - 1 ,  82 - 2 ,  82 - 3  of the multilevel inverter  80  and therefore also in the coils of the asynchronous machine  13 , the actuation can be optimized to the extent that the battery modules  40  can be discharged uniformly and therefore, for example, no undesired reduction in the range of an electric vehicle results, which is caused by non-uniform utilization of the battery cells  41 . This has the advantage that the advantages known from the prior art in respect of a multilevel inverter, in particular its high efficiency, can be used in electric drives without different discharge of the individual battery modules  40  needing to take place in a manner which is dependent on the load. 
       FIG. 15  shows an example of a four-stage multilevel inverter, which can be used in the converter unit  90  according to the disclosure and comprises five inputs  81 - 1 , . . . ,  81 - 5  and three outputs  82 - 1 ,  82 - 2 ,  82 - 3 , wherein the latter are connected to the inputs of the asynchronous machine  13 . The phase signals at the three outputs  82 - 1 ,  82 - 2 ,  82 - 3  are each controllable by switching elements, which are arranged in one of three strings  85 - 1 ,  85 - 2 ,  85 - 3  for each of the three outputs  82 - 1 ,  82 - 2 ,  82 - 3 . The mode of operation of the multilevel inverter  80  illustrated in  FIG. 15  is described by way of example below with reference to the string  85 - 3 , which determines the phase signal at the output  82 - 3 . 
     The string  85 - 3  comprises eight switching elements  83 - 1 , . . . ,  83 - 8 , which each consist of a semiconductor valve which can be switched on and off and a diode which is connected in parallel therewith. The switching elements  83 - 1 , . . . ,  83 - 8  are divided into complementary pairs ( 83 - 1 ,  83 - 5 ), ( 83 - 2 ,  83 - 6 ), ( 83 - 3 ,  83 - 7 ), ( 83 - 4 ,  83 - 8 ). The actuation of each of the complementary pairs ( 83 - 1 ,  83 - 5 ), ( 83 - 2 ,  83 - 6 ), ( 83 - 3 ,  83 - 7 ), ( 83 - 4 ,  83 - 8 ) is performed in such a way that, when one of the switching elements is closed, the complementary switching element is opened. If the open state is illustrated by  0  and the closed state is illustrated by  1 , a potential is thus output, as follows, at the output  82 - 3  of the multilevel inverter by a combination of the states of the switching elements  83 - 1 , . . . ,  83 - 8 , which potential is equivalent to the potential at one of the inputs  81 - 1 , . . . ,  81 - 5  of the multilevel inverter  80 : 
     potential at output  82 - 3 =potential at input  81 - 1 : 11110000; 
     potential at output  82 - 3 =potential at input  81 - 2 : 01111000; 
     potential at output  82 - 3 =potential at input  81 - 3 : 00111100; 
     potential at output  82 - 3 =potential at input  81 - 4 : 00011110; 
     potential at output  82 - 3 =potential at input  81 - 5 : 00001111. 
     If, for example, the switching combination 00011110 is selected, with the result that the potential at the output  82 - 3  is equivalent to the potential at the input  81 - 4 , for the case where the inputs  81 - 1 , . . . ,  81 - 5  are connected to taps of the battery module string  50  between which in each case only one battery module  40  is arranged, a voltage can thus be generated, depending on the actuation of the battery modules  40 , which voltage corresponds to a value between 0 V and the sum of three module voltages, wherein this voltage can be set in stages. 
     The multilevel inverter illustrated in  FIG. 15  functions with voltage limitation via a network of diodes  84 . These diodes are used for feeding the taps  71 ,  73 - 1 , . . . ,  73 -(n−1),  72  of the battery module string  50  to the switching elements  83 - 1 , . . . ,  83 - 8  whilst at the same time preventing a short circuit of battery modules  40 , which could take place in the event of a direct connection without diodes  84 . The diodes  84  need to be dimensioned differently in respect of their blocking ability. The highest reverse voltage in the region of the switching elements  83 - 5 , . . . ,  83 - 8  needs to be assumed, for example, by that diode  84  which is connected between the input  81 - 2  and the switching element  83 - 5 . There is a corresponding mirror-image response in the case of the diodes  84  in the region of the switching elements  83 - 1 , . . . ,  83 - 4 . 
     One or more battery modules  40  each having a coupling unit  30  or  70  can be arranged between adjacent taps  71 ,  72 ,  73  of the battery module string  50  and therefore can each generate two or three potential values. 
     All of the embodiments of the converter unit  90  according to the disclosure have the common feature that a substantially sinusoidal voltage signal of a predetermined frequency is made available at the three outputs of the converter unit  90 . 
       FIG. 16  shows a time profile of a voltage U 1 (t) present at one of the three outputs of the converter unit  90  according to the disclosure. The voltage U 1 (t) assumes in each case constant values for specific time intervals and in this case approximately follows the sinusoidal profile of the setpoint voltage U 2 (t). The sinusoidal setpoint voltage profile U 2 (t) can have, for example, an amplitude U 0  of 20 V and a frequency of 50 Hz. 
     Those times are marked on the time axis in  FIG. 16  at which a stepped increase or reduction in the voltage U 1 (t) takes place, for example as a result of battery modules being connected or bypassed in one of the battery module strings  50 - 1 ,  50 - 2 ,  50 - 3  in the first embodiment of the converter unit  90  according to the disclosure. If a higher number of battery modules is provided in one of the battery module strings  50 - 1 ,  50 - 2 ,  50 - 3 , the profile U 1 (t) comes closer to the setpoint voltage profile of U 2 (t), and eddy current losses in the rotor of the asynchronous machine  13 , which can be attributed to severe changes over time in the voltage profile U 1 (t), are reduced.