Abstract:
Disclosed is a system for transferring electric energy to a vehicle, in particular to a track bound vehicle such as a light rail vehicle. The system includes an electric conductor arrangement for producing an alternating electromagnetic field and for thereby transferring the energy to the vehicle. The electric conductor arrangement includes at least one alternating current line. Each alternating current line carries one phase of an alternating electric current. The conductor arrangement includes a plurality of consecutive segments. The segments extend along the path of travel of the, vehicle. Each segment includes one section of each of the at least one alternating current line.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a system and a method for transferring electric energy to a vehicle, in particular to a track bound vehicle such as a light rail vehicle (e.g. a tram). 
     The system comprises an electric conductor arrangement for producing an alternating electromagnetic field and for thereby transferring the energy to the vehicle. The conductor arrangement comprises a plurality of consecutive segments, wherein the segments extend along the path of travel of the vehicle. 
     2. Description of Prior Art 
     In particular track bound vehicles, such as conventional rail vehicles, mono-rail vehicles, trolley busses and vehicles which are guided on a track by other means, such as other mechanical means, magnetic means, electronic means and/or optical means, require electric energy for propulsion on the track and for operating auxiliary systems, which do not produce traction of the vehicle. Such auxiliary systems are, for example, lighting systems, heating and/or air condition system, the air ventilation and passenger information systems. However, more particularly speaking, the present invention is related to transferring electric energy to a vehicle which is not necessarily (but preferably) a track bound vehicle. Generally speaking, the vehicle may be, for example, a vehicle having an electrically operated propulsion motor. The vehicle may also be a vehicle having a hybrid propulsion system, e.g. a system which can be operated by electric energy or by other energy, such as electrochemically stored energy or fuel (e.g. natural gas, gasoline or petrol). 
     Track bound vehicles, in particular vehicles for public passenger transport, usually comprise a current collector (alternatively a device) for mechanically and electrically contacting a line conductor along the track, such as an electric rail or an overhead line. At least one propulsion motor on board the vehicle is fed with the electrical power from the external track or line and produces mechanical propulsion force. 
     Trams and other local or regional trains are operated usually via overhead lines within cities. However, especially in historic parts of cities, overhead lines are undesirable. On the other hand, conductor rails in the ground or near the ground cause safety problems. 
     U.S. Pat. No. 5,821,728 describes an apparatus for charging batteries of a moving electric vehicle comprising a series of inductive coils for producing a magnetic field along a centerline of a roadway. The inductive coils are connected to power lines via switches that can be activated by switch activator sensors. When a sensor receives a signal from a transmitter of the vehicle, this will cause the corresponding switch to close and thereby to activate the coil. 
     However, since the coil produces an electromagnetic field the power supply lines are connected to an alternating voltage source. Consequently, the power supply lines permanently produce an electromagnetic field of smaller intensity than the coils, but still strong enough to interfere with any electric or electronic equipment in the surrounding. 
     Inductively transferring energy from the track to the vehicle, i.e. producing electromagnetic fields, is subject to restrictions regarding EMC (electromagnetic compatibility). On one hand, electromagnetic fields may interfere with other technical devices. On the other hand, people and animals should not be subjected to electromagnetic fields permanently. At least, the respective limit values for field intensity must be observed. 
     Alternatively, the power supply may be a DC (direct current) power supply line which may be connected to each coil via an inverter. Typically, inverters comprise a control device for controlling the operation of semiconductor switches in order to produce the alternating voltage on the AC (alternating current) side of the inverter. This control device also requires electric energy for operation. 
     In order to reduce the electromagnetic emission where no vehicle is driving at a time, the conductor arrangement (e.g. the arrangement comprising the coils according to U.S. Pat. No. 5,821,728) which is used to inductively transfer the energy to the vehicle can be divided into consecutive segments extending along the path of travel of the vehicle. These segments may be operated where required only. In particular, the length of each of the segments along the path of travel may be shorter than the length of a vehicle in the travel direction and the segments may be operated only if a vehicle is already occupying the respective region of the path of travel along which the segment extends. In particular, occupied by a rail vehicle means that a rail vehicle is driving on the rails along which the segment extends. Preferably, the segments are operated only if the vehicle is fully occupying the respective region of the path of travel. For example, the rail vehicle is longer (in the direction of travel) than the segment and the vehicle&#39;s front and end are driving beyond the limits of the segment, if viewed from the center of the segment. Therefore it is proposed that the segment is switched on (i.e. the alternating current through the segment is starting to flow) before a receiving device of a vehicle for receiving the transferred energy enters the region of the path of travel along which the segment extends. 
     If a large number of consecutive segments is provided, using a DC power supply line means that an equally large number of inverters is required. Each inverter needs to be provided with electric energy for operation. Therefore, additional equipment for conducting electric energy to the inverter&#39;s control device is required. 
     It is an object of the present invention to provide a system of the kind indicated above, for inductively transferring electric energy to a vehicle and to provide a method of transferring electric energy to a vehicle, which reduce the effort for providing electric energy to devices of the system which control and/or operate the generation of electromagnetic fields in order to transfer the energy to the vehicle. Furthermore, the respective limits for meeting EMC requirements shall be met. 
     SUMMARY OF THE INVENTION 
     The conductor arrangement for producing the electromagnetic field may be located in and/or under the track, for example under the surface of the ground on which the vehicle travels. However, the invention also includes the case that at least a part of the conductor arrangement is located sideways of the track, for example when the track is located in the country side or in a tunnel. 
     The easiest way to transmit the energy, which is needed by the segments of the conductor arrangement to produce the electromagnetic field, is using an additional power supply, wherein the power supply lines of the power supply are carrying an alternating current. Switches at the respective interfaces to the segment can be switched on to commence operating the segment. However, as mentioned above, the power supply lines also produce an electromagnetic field. This field can be shielded from the environment, e.g. by burying the power supply lines in the ground and/or by using metal shields. However, with alternating voltages having high peak voltages, shielding is less effective. In any case, metal shields increase costs and the effort of constructing and realizing the system. On the other hand, high peak voltages increase the efficiency of the power supply. 
     The present invention proposes an alternative way: The power supply carries a mainly constant current (i.e. a direct current) and each segment is connected to the supply line via an inverter. The alternating current which is necessary for producing the electromagnetic field in the respective segment is generated by an inverter. In particular, the inverter produces the alternating current by repeatedly switching on and off the switches, as is principally known in the art of inverters. Preferably, each inverter is placed directly at the end of a segment. In other words: the concept of the present invention is to produce the alternating current locally and preferably where and when necessary. The frequency of the alternating current which flows through the conductor arrangement may be in the range of 5 to 100 kHz, in particular in the range of 10 to 30 kHz, preferably about 20 kHz. 
     In particular, the system comprises a direct current (DC) power supply for conducting electric energy to the segments, wherein the direct current supply comprises a first and a second direct current line, the first direct current line being operated at a different electric potential than the second direct current line. Preferably, the direct current lines extend along the path of a travel of the vehicle. Each segment is connected to the direct current supply via at least one inverter which is adapted to invert a direct current carried by the direct current supply to an alternating current carried by the segment. 
     If a segment is not to be operated, the switches of the inverter are not operated, i.e. are permanently switched off. Since the supply lines mainly carry a direct current, the power supply does not produce an alternating electromagnetic field of significant intensity. Furthermore, only those segments which are switched on (i.e. which are fed by the inverter/inverters) are producing electromagnetic fields. Therefore, the EMC standards can be met easily and the losses of electric energy are reduced to a minimum. The effort for providing and operating switches as parts of inverters is not significantly higher than for providing and operating switches between alternating current supply lines and the segments. The number of switches can even be reduced. 
     Furthermore, using a DC power supply overcomes the disadvantage of an alternating current power supply that alternating current lines comprise an inductance which needs to be compensated, e.g. by capacities at regular intervals along the path of travel. Since the current in the DC supply lines is mainly a direct current, losses due to the inductance are nearly zero. For example, the DC supply line can be fed with electric energy in the same manner as standard DC supply lines (such as live rails) which are to be contacted by a sliding contactor of a vehicle. 
     Inverters for producing a three-phase alternating current (as preferred with the present invention) usually comprise three bridges connecting the DC power supply lines. Each bridge comprises two semiconductor switches (usually IGBTs or GTOs) which are connected in series to each other. The corresponding alternating current line of the bridge is connected to a point in between the two switches. 
     Inverters of this kind comprise drive units for driving the electric currents which are needed to control the switches. In case of IGBTs, these drive units are usually called gate drive units, since they drive the electric currents to and from the gate of the IGBT. To coordinate the operation of the drive units, an inverter control unit is provided, often called drive control unit or DCU. 
     These and possibly further devices (e.g. detectors for detecting the presence of a vehicle) which are required for operating the inverter and/or the segment, need to be provided with electric energy in order to operate the device or devices. 
     It is a basic idea of the present invention to use the DC power supply at the same time for providing electric energy to these devices. For this purpose, the DC voltage between the two lines of the DC power supply is modulated so that the DC voltage varies as a function of time. However, the peak voltage of the resulting alternating voltage is preferably much smaller than the DC voltage of the DC power supply. For example, the mean value of the voltage between the two power supply lines may be in the region of 500 to 1.000 V. On the other hand, the peak voltage of the overlaid alternating voltage may be in the range of 2 to 10 V, for example. Generally, it is preferred that the peak voltage (the difference between the highest voltage and the mean value) of the overlaid alternating voltage is equal to or less than 5%, most preferred equal to or less than 2% of the direct voltage (i.e. the difference between the mean values of the two DC lines). 
     In particular, the following is proposed: A system for transferring electric energy to a vehicle, in particular to a track bound vehicle such as a light rail vehicle, wherein
         the system comprises an electric conductor arrangement for producing an alternating electromagnetic field and for thereby transferring the energy to the vehicle,   the conductor arrangement comprises a plurality of consecutive segments, wherein the segments extend along the path of travel of the vehicle,   the system comprises a direct current supply for conducting electric energy to the segments, wherein the direct current supply comprises a first and a second direct current line, the first direct current line being operated at a different electric potential than the second direct current line,   each segment is connected to the direct current supply via at least one inverter which is adapted to invert a direct current carried by the direct current supply to an alternating current carried by the segment,   the system comprises a modulation device for modulating the electric potential of at least one of the first and second direct current line using a predetermined modulation frequency, thereby producing a direct voltage having an overlaid voltage alternating with the modulation frequency,   at least some of the inverters comprise a transformer having a primary and a secondary winding, wherein the primary winding is connected to the first or second direct current line so that the direct current flows through the primary winding and wherein the secondary winding is connected to a power supply arrangement adapted to provide electric energy for operating the inverter.       

     Furthermore, a method of transferring electric energy to a vehicle, in particular to a track bound vehicle such as a light rail vehicle, is proposed, wherein
         an electric conductor arrangement is operated to produce an alternating electromagnetic field and for thereby transferring the energy to the vehicle,   the conductor arrangement comprises a plurality of consecutive segments, wherein the segments extend along the path of travel of the vehicle, wherein the segments are operated separately, at least partly and/or at least occasionally,   electric energy for transfer to the vehicle is conducted to the segments via a direct current supply, wherein the direct current supply comprises a first and a second direct current line, the first direct current line being operated at a different electric potential than the second direct current line,   the electric energy is conducted from the direct current supply via at least one inverter to each of the segments, the inverter being operated to invert a direct current carried by the direct current supply to an alternating current carried by the segment,   the electric potential of at least one of the first and second direct current lines is modulated with a predetermined modulation frequency, thereby producing a direct voltage having an overlaid voltage alternating with the modulation frequency,   at least some of the inverters comprise a transformer having a primary and a secondary winding, wherein the primary winding is connected to the first or second direct current line so that the direct current flows through the primary winding and wherein the transformer is used to provide electric energy from the secondary winding for operating a control device of the inverter.       

     The pre-determined modulation frequency is preferably chosen so that there are no interferences with other devices within the system or in the surrounding of the system. For example, the pre-determined modulation frequency may be in the range of 500 to 1000 Hz. 
     As mentioned above, the first direct current line is operated at a different electric potential than the second direct current line. For example, the first direct current line may be at plus potential and the second direct current line may be at minus potential. More generally speaking, the electric potential at the first direct current line is always higher than the electric potential at the second direct current line or vice versa, although the direct voltage is modulated with the pre-determined modulation frequency. The overlaid alternating voltage has a peak value which is smaller than the difference of the mean values between the electric potentials of the first and second direct current line. 
     A device which can be used to produce the overlaid alternating voltage may be a transformer having a primary winding and a secondary winding. The secondary winding may be part of the first or second direct current line so that an alternating current which flows through the primary winding induces the desired modulation of the electric potential at the connections of the secondary winding. 
     The other transformer which is mentioned above and which is used to decouple the required electric energy for operating the inverter may be located near the inverter or may be part of a module which comprises the inverter. The alternating voltage which is induced in the secondary winding of this transformer can be used as alternating voltage source. As principally known in the art of providing energy to electric or electronic components, this alternating voltage may be connected to circuitry for conditioning the voltage or current which is needed for operating the inverter and/or other devices. Such circuitry may comprise a rectifier for producing a direct current, capacities for smoothing a direct voltage and/or at least one further transformer for producing a desired alternating voltage level. Alternatively or in addition, the circuitry may comprise other electric or electronic components. 
     Preferably, the electric conductor arrangement comprises three lines, each line carrying a different phase of a three-phase alternating current. However, it is also possible, that there are only two or that there are more than three phases carried by a corresponding number of alternating current lines. For example, in the case of a three-phase system, the phase shift may be 120°, as usual. In particular, each of the segments may comprise sections of each of the lines. The alternating current in each phase may be a sinusoidal or nearly sinusoidal current produced by the inverter or inverters. Alternatively, it may be a nearly rectangular current, i.e. a step function of time. 
     According to a first embodiment, the conductor arrangement is arranged in such a manner that at least two consecutive segments can be operated at the same time, wherein corresponding lines for carrying the same phase of the alternating current in the consecutive sections are connected in series to each other. For example, the interface between the consecutive segments may comprise a switch or an arrangement of switches which may connect or disconnect the corresponding lines. However, it is preferred that the phase lines of consecutive segments are permanently connected in series to each other and that the operation (producing an electromagnetic field) or non-operation (not producing an electromagnetic field) of the segments is controlled by operating (or by not operating) the respective inverters. 
     According to a second embodiment, the lines for carrying the same phase of the alternating current in the consecutive sections are not connected to each other. An advantage of this embodiment is that non-active segments do not produce electromagnetic fields at all, since they are de-coupled from active segments. An example will be described with reference to the figures. 
     The number of consecutive segments which are operated at the same time is not restricted to two. Rather, three or more consecutive segments can be operated at the same time, for example if a long vehicle is travelling on the path, such as a vehicle having receiving devices at different locations. In this case, it is preferred that segments are switched off only when the last receiving device has left the section of the path which corresponds to the segment. 
     The process of starting or stopping the operation of segments may be controlled using at least one of the phase lines of the segments. Preferably, the occupation of a respective section of the track by a vehicle may be detected, in particular by detecting a voltage and/or a current in the lines of the segment which is caused by inductive coupling of the vehicle to the lines and/or which is caused by electromagnetic fields produced by the vehicle. Correspondingly, a measurement device may be connected to at least one of the lines. Preferably, a plurality of or all of the lines of the segment is connected to a measurement device and/or to the same measurement device. The measurement device or devices is/are adapted to detect the occupation of the respective section of the track by a vehicle by detecting a voltage and/or a current in the line or a separate loop which is caused by inductive coupling of the vehicle to the line and/or which is caused by electromagnetic fields produced by the vehicle. Such a measurement device can be combined with one of the inverters and may be provided with electric energy for operation from the power supply arrangement which is connected to the secondary winding of the transformer. 
     Standard inverters known in the art often comprise a capacitor in order to smooth the direct voltage on the DC side of the inverter. It is preferred to use such a capacitor for providing a link for the alternating current component in the power supply, between the two DC supply lines. The capacitor connects the two DC supply lines, but the capacitor is separated from one of the two DC supply lines via the primary side of the transformer. Therefore, the overlaid alternating voltage can be decoupled from the DC supply lines and the voltage at the opposite sides of the bridges of the inverter is nearly constant. The capacitor is functioning as a short cut of the alternating voltage. An example will be described in more detail. Such a link enables the power supply to the secondary side of the transformer, when the inverter is not producing an alternating current, but the control device of the inverter still needs electric energy for operation. Therefore, it is preferred that the inverter comprises a capacitor having opposite connections for connecting the capacitor and wherein the opposite connections are connected to the first and second direct current line. The capacitor may be realized by one capacitor or by any combination of capacitors which are connected in parallel and/or in series to each other. 
     Also, inverters known in the art often comprise a reactor for damping or eliminating frequencies caused by the operation of the inverter. According to an embodiment of the invention, the reactor is used as the primary winding of the transformer. As a result, no additional reactor is needed and the costs of establishing the system are reduced. Furthermore, the electric properties of the inverter are not changed by an additional primary winding. 
     The conductor arrangement for producing the electromagnetic field may comprise three lines (each line for one phase of the alternating-current) extending in the direction of travel. However, each line preferably comprises sections which extend transversely to the direction of travel. As a result, each line may follow a serpentine-like path. 
     Preferably, each line follows the same serpentine-like path along the path of travel, wherein the lines are shifted in the direction of travel by one third of the distance between consecutive sections of the same line extending transversely to the direction of travel. Serpentine-like means that sections of the line which extend in the direction of travel are followed in each case by a section which extends transversely to the travel direction which in turn is followed again by a section which extends in the direction of travel. 
     Such lines with transversely extending the line sections produce—at each point in time while the alternating electric current is flowing through the line—a row of successive magnetic poles of an electromagnetic field, wherein the successive magnetic poles have alternating magnetic polarities. At a given point in time the alternating current produces a magnetic field having a magnetic field vector which is oriented in a first direction in a first region of the line, followed (in the direction of travel) by a second region of the line where the field vector of the magnetic field is oriented in the opposite direction of the first direction, followed by another region of the line where the magnetic field vector is oriented again in the first direction and so on. As a result, the field intensity sideways of the path of travel is very small, since the fields produced by the transversely extending line sections compensate each other. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments and examples of the present invention will be described with reference to the attached figures. The figures show: 
       DETAILED DESCRIPTION OF THE INVENTION 
         FIG. 1  an arrangement having segments for transferring electric energy to a vehicle, wherein the segments are connected to a DC power supply via inverters at interfaces between the segments, 
         FIG. 2  schematically a rail vehicle travelling along a track with a plurality of consecutive segments, wherein an inverter is arranged at each interface between two consecutive segments, 
         FIG. 3  Schematically an arrangement comprising consecutive segments, wherein the lines of the segments for carrying the phases of the alternating current are not connected in series to each other, 
         FIG. 4  a constant current source for producing a constant alternating current in one of the consecutive segments and 
         FIG. 5  a circuit diagram showing a three-phase inverter which is connected to the DC power supply. 
     
    
    
       FIG. 1  shows a circuit diagram. A row of consecutive segments  137 ,  138 ,  139  of an electric conductor arrangement for producing an electromagnetic field is partially shown in the figure. Only one segment is fully shown, namely segment  138 . The segments  137 ,  138 ,  139  each comprise three phase lines  135   a ,  135   b ,  135   c . Each phase line  135  of each segment  137 ,  138 ,  139  comprises at one end of the phase line  135  a capacity  140  for compensating the inductance of the phase line  135 . As a result, the impedance is zero. Each phase line  135  is symbolized by a chain of semi-circles in order to indicate the inductance. However, the phase line may be a straight line, a serpentine-like line, a coil or any other kind of lines. 
     At the interfaces between the consecutive segments  137 ,  138 ,  139 , each phase line  135  is connected to plus and minus potential of a DC power supply comprising DC lines  141   a ,  141   b  via in each case one switch  147 ,  148 . The six switches at each interface are parts of an inverter. For example, phase line  135   a  is connected via connection  144   a  to plus potential (line  141   a ) and minus potential (line  141   b ). Within connection  144   a , the switch between phase line  135   a  and plus potential is denoted by reference numeral  147  and the switch between the phase line  135   a  and minus potential is denoted by  148 . The connections  144   b ,  144   c  of the phase lines  135   b ,  135   c  to plus and minus potential (lines  141   a ,  141   b ) are constructed in the same manner. This description applies to interface  142  between segment  137  and segment  138 . At the interface between segment  138  and segment  139 , the connections between the phase lines  135  and the DC supply line  141  are denoted by reference numerals  145   a ,  145   b ,  145   c . The switches between the phase lines  135  and the plus potential of line  141   a  are denoted by  149  and the switches to the minus potential are denoted by  150 . 
     Consequently, each interface  142 ,  143  can be connected and disconnected to/from the supply line  141  by operating switches  147 ,  148  or  149 ,  150 . The switches  147 ,  148  constitute a first inverter, together with a control of the switches  147 ,  148  which is not shown in  FIG. 1 . In the same manner, switches  149 ,  150  and a corresponding control for controlling the switching operations of these switches constitute a second inverter at interface  143 . During operation of the inverters, the switches of the inverter are repeatedly switched on and off to produce a desired alternating current at the interface  142 ,  143 , i.e. at the end of one of the segments  137 ,  138 ,  139 . For example, the connection  144   a  for connecting the DC supply line  141  to phase line  135   a  therefore comprises a series connection of switch  147  and switch  148  wherein a connection is made between phase line  135   a  and a contact point between the switches  147 ,  148 . 
     As shown in  FIG. 2 , a plurality of segments  147   a  to  147   f  may be arranged as consecutive segments of a conductor arrangement  160  along the path of travel of a vehicle  162 . The vehicle  162  (e.g. a tram) may comprise at least one receiving device  161  for receiving the electromagnetic field produced by one or more than one of the segments  157 . In the situation shown in  FIG. 2 , the receiving device  161  is located above segment  157   c  and at least this segment  157   c  is operated to produce an electromagnetic field and to provide energy to the vehicle. Furthermore, the vehicle may comprise energy storages  163   a ,  163   b  which may be used to operate the vehicle if not sufficient energy is received from the segments  157 . 
     At each interface between two consecutive segments  157 , an inverter  152   a  to  152   e  is provided. For example, the inverters  152  may be realized according to the circuit diagram of  FIG. 1 . The DC supply lines  141   a ,  141   b  are also shown in  FIG. 2 . They are connected to an energy source  151 , such as a power station for producing a direct current. 
       FIG. 3  shows a rail vehicle  214  at the top of the figure. The vehicle  214 , for example a tram, comprises two receiving devices  218   a  and  218   b  for receiving the electromagnetic energy from segments  211  of the track. The figure shows at least parts of in total seven consecutive segments  211   a - 211   g , although—in practice—there are usually much more consecutive segments. At the time which is depicted in  FIG. 3 , three of the segments, namely segments  211   b ,  211   c  and  211   d  are active, i.e. the lines of theses segments carry an alternating current thereby producing the electromagnetic field for transferring electromagnetic energy to the receiving devices  218  of the vehicle  214 . Each of the segments comprises three lines and each of the line carries one of three phases of the alternating current while active. 
     Each segment  211  is connected to a DC-supply line  213   a ,  213   b  via a constant current source arrangement A, B, C which is shown in  FIG. 4 . Arrangement A, B, C, can be housed in a single box, for example. As shown in  FIG. 3 , the DC-supply line  213  is connected to a supply station  215  which delivers the required DC-current to the DC-supply line  213 . Optionally, supply station  215  may also comprise a device for modulating the DC voltage between supply lines  213 . An example of a modulation device will be described with reference to  FIG. 5 . 
     The constant current source A, B, C comprises an inverter C having connections for connecting the DC-supply lines  213   a ,  213   b . The connections are combined with an inductivity  219   a ,  219   b . One side of the inductivity  219  is connected to the respective DC-line  213  and the other side of the inductivity  219  is connected to the three current paths  221   a ,  221   b ,  221   c  for producing the three phases of the alternating current. The inductivities serve to eliminate or at least to damp frequencies which are caused by the operation of the switches in paths  221 . 
     Each current path  221  comprises two semiconductor power switches which are connected in series to each other and the alternating current lines are connected to a point in between the switches in the respective current path. Regarding the semiconductor switches and the current path the inverter may be a conventional three phase inverter and, therefore, further details are known to a skilled person. 
     However, there is a further connection between the secondary sides of the inductivities  219  which comprises two capacities  223   a ,  223   b  which are connected in series to each other. A point in between the capacities  223  is connected to each of the alternating current lines via in each case at least one first inductivity  225   a ,  225   b ,  225   c . Furthermore, the point in between the two capacities  223  is connected to another point in each of the alternating current lines  216   a ,  216   b ,  216   c  via a second capacity  227   a ,  227   b ,  227   c . These connection points are located an opposite side of an inductivity  226   a ,  226   b ,  226   c  in the alternating current line  216   a ,  216   b ,  216   c  with respect to the connection where the first capacity  225   a ,  225   b ,  225   c  is connected to the alternating current line  216   a ,  216   b ,  216   c . The connections where the first capacities  225  are connected to the alternating current line  216  are shown in  FIG. 4  within block B and the connections where the second capacities  227  are connected to the alternating current lines  116  are shown within block A in  FIG. 4 . Block A also comprises the inductivities  226 . 
     Blocks A, B constitute a so called six-pole filter, including compensation capacitors which compensate for the inductivities of the lines in the segments which produce the primary side electromagnetic field. 
     When operated, i.e. while the semiconductor switches of inverter C are repeatedly switched on and off, the constant current source produces an alternating current having a constant effective mean value independent of the power which is thereby produced within the segment  211  and is transferred to the receiving devices  218  of the vehicle  214 . 
       FIG. 5  shows a module  35  which comprises a three-phase inverter  301  and some other circuit components. A first direct current line  41   a  of the module  35  is connected to a first direct current supply line  141   a  of a DC power supply. A second direct current line  41   b  of module  35  is connected to the second direct current supply line  141   b  of the direct current supply. 
     As schematically shown in the top left region of  FIG. 5 , the first direct current supply line  141   a  is connected to the secondary winding  37   a  of a first transformer  37  which is used to modulate the direct voltage between the direct current supply lines  141  at a pre-determined modulation frequency. In order to produce the corresponding overlaid alternating voltage between the direct current supply lines  141   a ,  141   b , the first transformer  37  comprises a primary winding  37   b  which may be connected to an alternating current source which is producing an alternating current at the pre-determined modulation frequency. Such an alternating current source may be a frequency generator, the frequency of which can be adapted. The voltage at the secondary winding  37   a  is determined by the voltage across the primary winding  37   b  and the relation of the windings of the transformer. Therefore, the voltage of the alternating current source may be adaptable as well. 
     The first direct current line  41   a  of module  35  comprises the primary winding  39   a  of a second transformer  39  which is part of the module  35 . This second transformer  39  is used to decouple the electric energy from the direct current power supply which is needed to operate the inverter. Other than shown in  FIG. 5 , the first transformer and the second transformer do not have to be connected to each other directly. Rather, one of the transformers may be connected to the first direct current supply line and the other transformer may be connected to the second direct current supply line. 
     The first and second direct current lines  41   a ,  41   b  of the module  35  are connected to each other via a resistor  44  and via capacitors  45   a ,  45   b ,  45   c , forming a capacitance between the first and second direct current lines  41   a ,  41   b . The resistor  44  is provided for safety reasons. The high-Ohmic resistance of the resistor  44  allows for a slow equalization of the different electric potentials of direct current lines  41  if module  35  is disconnected from the DC power supply, e.g. by opening switch  40  in line  41   a.    
     Capacitor  45  is used to smooth fluctuations between the electric potential across resistor  44 . Such fluctuations are undesired, since the DC voltage across the series connections of in each case two switches  147 ,  148  should be constant. 
       FIG. 5  shows a specific embodiment having a preferred feature according to the present invention which may be realized in other specific embodiments as well. According to this feature, the capacity is connected to the direct current power supply via the primary winding of the second transformer which is used to decouple the energy from the direct current power supply. This means that the overlaid alternating voltage which exists in the DC power supply (between lines  141   a ,  141   b  in the example of  FIG. 5 ) is at least partly eliminated by the decoupling of the electric energy performed by the second transformer ( 39  in  FIG. 5 ). In other words, the capacitor  45  produces a short circuit for the alternating voltage. 
     The function and operation of the semiconductor switches  147 ,  148  may be similar or identical to the arrangement shown in  FIG. 1  and, therefore, will not be described again here. The alternating current lines  135   a ,  135   b ,  135   c  are connected to a point in between the series connection of the semiconductor switches  147   a ,  148   a ;  147   b ,  148   b ;  147   c ,  148   c , respectively. A free-wheeling diode  47 ,  48  is connected anti-parallel wise to each of the semiconductor switches  147 ,  148 . 
     However, further details of the inverter are shown in  FIG. 5 . Also representing the corresponding control devices of the other semiconductor switches, a gate drive unit  55  is shown which is connected to the gate of semiconductor switch  147   b  for driving the currents which are necessary to change the gate charge in order to switch the semiconductor switch  147   d . As indicated by a dotted line, gate drive unit  45  is connected to a drive control unit  53  which controls the operation of all gate drive units of semiconductor switches  147 ,  148 . The energy which is needed to operate the drive control unit  53  and optionally as well the energy which is needed to operate the gate drive units is provided by circuitry  51 . This circuitry  51  is connected to the secondary winding  39   b  of the second transformer  39 . Circuitry  51  comprises the electric and/or electronic components to adapt the alternating voltage at the secondary winding  39   b  to the needs of drive control unit  53  and/or drive units  54 . 
       FIG. 5  also shows a measurement unit  53  for measuring, for example, current and voltage. The measurement values may be used by the drive control unit  53  for controlling the gate drive units  54 .