BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a drive circuit for an electric motor having an aerodynamic support of the motor shaft. 
         [0002]    Air bearings are bearings in which two partners moving against one another are separated by a thin air film. They thereby allow an almost friction-free relative movement. 
         [0003]    The air support can in particular be used with electric motors in which the driven motor shaft, i.e. the rotor, is supported by an air gap in the motor housing. A distinction is made between static bearings and dynamic bearings in air bearings. With static bearings, the air gap is generated by introduction of compressed air by external means. Dynamic bearings do not require any compressed air supply since the compressed air required in the air gap is generated by inherent movement. The problem arises here, however, that the two bearing parts which move with respect to one another can contact one another below a characteristic relative speed and are thus subject to wear and increased friction. 
         [0004]    Air-supported motors are used, for example, in air-conditioning units, in particular in air-conditioning units for rail vehicles. The air-supported motors are coupled to a compressor and drive it. Since the air-conditioning system, like other consumers within the rail vehicle, are fed by the supply voltage of the overhead line, this voltage supply can be briefly interrupted at specific transition points. Depending on the interruption period, this can have the result that the speed of the air-supported motors in air-conditioning units is abruptly reduced, down to a standstill, and sufficient compressed air for the air bearing cannot be generated. Harmful friction phenomena thereby arise within the air bearings and the service life of the motor is greatly reduced. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention has therefore set itself the object of showing a solution which prevents the drop of air pressure and of speed within the air-supported motor on a brief or certain interruption of the supply voltage and consequently reduces the wear of the air bearing. 
         [0006]    This object is achieved by the drive circuit in accordance with the features herein. Advantageous embodiments of the drive circuit are the subject of the features herein. 
         [0007]    In accordance with the invention, a drive circuit for an electric motor is proposed having aerodynamic support of the motor shaft or rotor shaft. In accordance with the invention, the drive circuit provides at least one storage means for storing electrical energy. The stored energy of the at least one electrical storage means should now be utilized to maintain an emergency operation of the electric motor on a failure of the actual supply voltage. The electric motor can therefore be fed at least at times via the stored electrical energy of the at least one storage means on a voltage drop in order to be able to maintain a minimum speed of the motor shaft or rotor shaft. The minimum speed is selected so a satisfactory air support of the motor shaft is just provided. 
         [0008]    In an embodiment of the invention, the drive circuit comprises an intermediate DC current circuit for supplying the electric motor with electrical energy. At least one electrical storage means is then a component of the intermediate circuit. On a failure of the intermediate circuit voltage, the at least one electrical storage means takes over the energy supply of the electric motor. 
         [0009]    The electric motor can be configured as an AC current motor and is then fed by means of an inverter from the intermediate circuit voltage. The inverter serves the regulation of the motor speed. 
         [0010]    The at least one storage means can expediently be chargeable by the supply voltage or by the intermediate circuit voltage during regular operation. With a sufficient voltage supply, the at least one storage means is charged up to a specific voltage level. It is thus ensured that on a later voltage failure of the supply voltage or of the intermediate circuit voltage sufficient electrical energy is available in the storage means. 
         [0011]    A control is ideally provided which recognizes a voltage failure of the supply voltage or of the intermediate circuit voltage and switches over to the emergency supply of the electric motor by the at least one storage means in this case. This can be an active switching process of a specific component. There is alternatively the possibility that the at least one storage means is connected to the electric motor such that energy from the at least one storage means can be automatically provided to the electric motor on an interruption of the supply voltage. 
         [0012]    The drive circuit can preferably be designed such that it continues to allow a stabilization of the intermediate circuit voltage during any fluctuations which may occur in the supply voltage or in the intermediate circuit voltage. The control of the drive circuit can preferably be designed such that it recognizes a falling below of a predefined lower limit value of the intermediate circuit voltage. In this case, electrical energy can automatically be provided from the at least one storage means to support the voltage supply of the electric motor. The intermediate circuit voltage can thereby be kept almost constant. 
         [0013]    The at least one air supported electric motor can have at least one coupling means at the output side. It is expedient in this case if the control of the drive circuit has corresponding means for actuating this coupling to switch the motor free of load in the case of an emergency, i.e. on a failure of the supply voltage or intermediate circuit voltage. Ideal motor operating conditions are thereby created to be able to ensure a feeding of the motor from the storage means for as long as possible on a supply interruption. 
         [0014]    The at least one electrical storage means can be a low voltage store or alternatively a high voltage store. Suitable embodiments of the electrical storage means comprise at least one double layer capacitor and/or at least one rechargeable chemical battery cell, for example an Li ion cell. 
         [0015]    In a preferred embodiment, a measuring means can be provided for detecting the temperature of the at least one storage means. Ideally, the charging voltage of the at least one storage means can then be determined or restricted in dependence on the measured temperature. The service life of the capacitor cells is in particular dependent on the environmental duration and the applied voltage on the use of double layer capacitors. The higher the temperature and the selected charge voltage, the more the service life of the cells is shortened. To decouple the service life of the cells from the environmental temperature, the store temperature is measured and the level of the charge voltage of the at least one storage means is adapted while taking account of the measured temperature. 
         [0016]    The cell voltages are ideally reduced by approximately 0.1 V per 10° C. temperature increase. In this manner, the influence of the temperature on the cell service life is considerably compensated. The invention should, however, not be restricted to the listed example values. 
         [0017]    If a low voltage store is used as the storage means, it can be connected to the supply voltage or to the intermediate circuit via a bidirectional DC/DC converter. The DC/DC converter works in step-down operation for the charging process of the store by the intermediate circuit voltage or the supply voltage. On a failure of the supply voltage, the DC/DC converter can be switched into step-up operation. The air bearing motor can then be supplied from the low voltage store. In this embodiment, the DC/DC converter has to be galvanically separated between the low voltage store and the high voltage store. 
         [0018]    In a simpler embodiment, the at least one storage means is designed as a high voltage store. In this case, a galvanic separation of the bidirectional DC/DC converter is not necessary between the intermediate circuit and the storage means, which produces a substantial simplification of the complexity of the overall system. 
         [0019]    In a further advantageous embodiment of the invention, the at least one storage means is a high voltage store and is connected to the supply voltage or the intermediate circuit via a unidirectional DC/DC converter. The DC/DC converter serves the charging of the at least one storage means with the supply voltage or the intermediate circuit. The DC/DC converter preferably works in step-down operation in this respect. 
         [0020]    The electrical store can furthermore be connected to the air bearing motor via a diode. On a failure of the supply voltage or of the intermediate circuit voltage, the motor is automatically fed with electrical energy from the at least one storage means via the diode. 
         [0021]    There is alternatively the possibility of dispensing with the use of a DC/DC converter. Instead of the unidirectional DC/DC converter, a resistor element is then used via which the at least one store is connected to the supply voltage or to the intermediate circuit. The charging current from the intermediate circuit can be limited via the resistor element. There is the possibility of fixedly defining the resistance value in advance or to use a variable resistor element having an adjustable resistance value. 
         [0022]    The charging of the storage means takes place via the resistor element, whereas the electric motor is fed from the store via a diode on a failure of the supply voltage or of the intermediate circuit voltage. The resistor element is expediently designed as a connected resistor element so that the charging process can be stopped as required, preferably when the storage voltage of the storage means has reached a predefined voltage level. 
         [0023]    The provided control serves the control of the DC/DC converter or the control of the resistor element. The control optionally takes over the switching over of the bidirectional converter between step-up operation and step-down operation. 
         [0024]    It is furthermore expedient if the at least one storage means can be monitored by the control and if its storage voltage can be detected. The control can actuate the DC/DC converter or the switchable resistor element in dependence on the measured storage voltage to control or interrupt the charging process in dependence on the storage voltage. 
         [0025]    The control is furthermore suitable to control or regulate the motor speed via an inverter connected upstream of the electric motor. The control or regulation is consequently connected to a corresponding sensor system for detecting the motor speed. 
         [0026]    In accordance with a further advantageous embodiment of the invention, the inverter can likewise be supplied with energy at least briefly by the at least one storage means on an interruption of the supply voltage/intermediate circuit voltage. The inverter, for example, requires electrical energy for operating the driver circuit or sensor system. An embodiment of the drive circuit is preferred in which a DC/DC converter of low power is connected in parallel with the at least one storage means to provide the supply voltage required for the inverter during the interruption of the supply voltage/intermediate circuit voltage from the at least one storage means and to secure an autonomous of the drive circuit. 
         [0027]    In addition to the drive circuit, the invention relates to an air conditioning system having an air supported motor for driving a compressor and a drive circuit in accordance with the present invention or an advantageous embodiment of the drive circuit. The air conditioning system is consequently characterized by the same advantages and properties as the drive circuit in accordance with the invention. A repeat description is therefore dispensed with. 
         [0028]    The electric motor is preferably connected to the compressor via a coupling means which switches the electric motor load-free on an interruption of the supply voltage. 
         [0029]    The air conditioning system in particular serves the use in rail vehicles, wherein the supply voltage is preferably provided by an overhead line. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    Further advantages and properties of the invention will be explained in the following with reference to a plurality of embodiments shown in the drawings. There are shown: 
           [0031]      FIG. 1 : a circuit diagram of a drive circuit for controlling an air supported motor in accordance with a first embodiment in accordance with the invention; 
           [0032]      FIG. 2 : the drive circuit in accordance with the invention in accordance with a second embodiment; 
           [0033]      FIG. 3 : the drive circuit in accordance with the invention in accordance with a third embodiment; 
           [0034]      FIG. 4 : the drive circuit in accordance with the invention in accordance with a fourth embodiment; and 
           [0035]      FIG. 5 : the drive circuit in accordance with the invention in accordance with  FIG. 4  with an extension to supply the inverter. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]    All five circuit diagrams of  FIGS. 1 to 5  show a drive circuit having an intermediate circuit  10  for the energy supply of the electric motor  20 . The motor  20  is an electric motor having an aerodynamic support of the motor shaft which serves the drive of an air conditioning compressor and is connected thereto via a coupling, not shown. 
         [0037]    The electric motor  20  is an AC current motor. The inverter  30 , which converts the DC voltage U ZK  of the intermediate circuit  10  into the required AC voltage for feeding the motor, is therefore connected upstream of the motor  20 . In addition, the frequency of the generated AC voltage for regulating the speed of the motor  20  can be adjusted. The desired control of the inverter  30  takes place by the control  40  of the drive circuit. The rotational speed ω of the motor serves as the regulation value which is measured at the motor shaft and which is transferred to the control  40 . The control  40  then sets the required frequency of the inverter  30  in dependence on the desired speed. 
         [0038]    The circuit design shown having an air supported motor  20  for driving a compressor is frequently used in air conditioning systems for rail vehicles. The air conditioning system like other consumers, too, are fed inside the rail vehicle from the overhead line. The voltage supply can therefore be briefly interrupted at the transition points and can fail. Since the motor  20  has an aerodynamic support, the effect of the air bearing depends on the speed of the motor  20 . If the speed falls below a critical limit value, there is a contact between the bearing counter-points and unwanted friction phenomena arise within the air bearing, whereby the service life is substantially reduced. 
         [0039]    For this reason, in accordance with the invention, an electrical store  50 ,  51  is integrated into the intermediate DC voltage circuit  10  so that the required energy can be provided during a voltage failure of the intermediate circuit voltage U ZK  for the disturbance-free operation of the air-supported motor  20 . The electrical energy of the storage means  50 ,  51  ensures a minimum speed of the motor  20  to be able just to maintain the air pressure within the air bearing. 
         [0040]    During the normal state, the air-supported motor is fed by the voltage supply system and the store  50 ,  51  is simultaneously charged to the intermediate circuit voltage U ZK . If the intermediate circuit voltage U ZK  is interrupted for a brief period, for example at transition points of the overhead line, the control  40  recognizes the voltage failure and switches the energy supply of the motor  20  to the store  50 ,  51 . The air bearing motor  20  is simultaneously decoupled from the load. This takes place by the output-side coupling, not shown, between the motor  20  and the compressor. Ideal operating conditions for the air bearing motor  20  are hereby provided to ensure a maximum supply duration of the motor  20  from the storage means and consequently a maintenance of the minimal speed for as long as possible. 
         [0041]    The electrical store  50 ,  51  can comprise a plurality of double layer capacitors or of rechargeable battery cells such as Li ion cells. 
         [0042]    Various embodiment variants are suitable for the integration of the store  50 ,  51  into the intermediate circuit which will be explained in more detail in the following with reference to  FIGS. 1 to 4 . However, a general definition for the dimensioning of the store  50 ,  51  should be given beforehand. It is assumed that the minimal energy consumption of the motor  20  during the interruption time T is equal to E where the formula 
         [0000]    
       
      
       E=P·T  
      
     
         [0000]    applies and P represents the minimally required power so that the air-supported motor is in a secure operating state with a secure speed. The required storage capacity C is then calculated as follows: 
         [0000]        E=P—T= 1/2 C ( U   SP   2   −U   min   2 ) 
         [0043]    It follows from this: 
         [0000]    
       
         
           
             C 
             = 
             
               
                 2 
                  
                 
                   P 
                   · 
                   T 
                 
               
               
                 
                   U 
                   SP 
                   2 
                 
                 - 
                 
                   U 
                   min 
                   2 
                 
               
             
           
         
       
     
         [0044]    Where U SP  is the voltage of the store and U min  is the minimally required intermediate circuit voltage for a secure motor operation. 
         [0045]      FIG. 1  now shows the block diagram of the drive system which is capable of emergency operation and in which the storage means is designed as a low voltage store  51 . The integration of the storage means  51  in the intermediate circuit  10  takes place via the bidirectional DC/DC converter  60  which moreover provides a galvanic separation between the intermediate circuit voltage U ZK  and the low voltage store  51 . 
         [0046]    If the voltage supply system is in the normal state, this is recognized by the control  40  and the motor is fed by the voltage supply, i.e. the intermediate circuit voltage U ZK , via the inverter  30 . At the same time, the low voltage store  51  is charged by the DC/DC converter  60 , with this working as a step-down converter for this purpose and transforming the intermediate circuit voltage U ZK  with a higher volt figure into a lower DC voltage for the low voltage store  51 . 
         [0047]    On a failure of the voltage supply or of the intermediate circuit voltage U ZK , the control  40  switches the DC/DC converter  60  into the step-up mode, whereby the low volt voltage of the store  51  is stepped up and feeds the motor  20  via the inverter  30  with electrical energy for maintaining the minimum speed. The required intermediate circuit voltage is provided by the store  51  here. 
         [0048]      FIG. 2  shows an embodiment which is modified with respect to  FIG. 1  and in which a high voltage store  50  is now used instead of a low voltage store  51 . The switching design, in particular the integration of the store  50  into the intermediate circuit  10 , is simplified by the use of the high voltage store  50 . Due to the lower voltage difference, the bidirectional DC/DC converter  61  does not have to ensure any galvanic separation between the intermediate circuit voltage U ZK  and the voltage level of the high voltage store  50 . 
         [0049]    In the embodiment of  FIG. 3 , a high voltage store  50  is likewise used as a storage means analog to  FIG. 2 . The DC/DC converter  62  now works only unidirectionally, and indeed in the step-down mode. The intermediate circuit voltage U ZK  is thereby set to the required voltage of the high voltage store  50 . In the embodiment shown, the store  50  is consequently charged by the intermediate circuit voltage U ZK  during the normal operation until a predefined voltage level is reached. Once this voltage level is reached, the control  40  switches off the DC/DC converter  62 . 
         [0050]    On the failure of the intermediate circuit voltage U ZK , the charging process is stopped and the air bearing motor  20  is automatically fed by the store  50  via the diode  70  connected to the intermediate circuit  10  in the direction of flow. 
         [0051]    In the embodiment of  FIG. 4 , the use of a DC/DC converter is now completely dispensed with. Instead, the high voltage store  50  is charged in normal operation via a resistor  80  by the intermediate circuit voltage U ZK . A switch  90  is connected upstream of the resistor  80  to be able to interrupt the charging process on reaching a predefined storage voltage of the high voltage store  50 . The control  40  monitors the storage voltage U Sp  and opens the switch  90  where necessary as soon as the desired voltage is reached. The charging process is then interrupted. 
         [0052]    On the failure of the intermediate circuit voltage U ZK , the high voltage store  50  automatically supplies the motor  20  with electrical energy via the diode  70  connected to the intermediate circuit  10  in the direction of flow. 
         [0053]    The resistor  80  limits the charging current during the charging phase, in particular when the store is completely discharged. The resistance value R of the resistor  80  is calculated as follows: 
         [0000]    
       
         
           
             
               R 
               = 
               
                 
                   U 
                   ZK 
                 
                 
                   I 
                   
                     SP 
                      
                     
                         
                     
                      
                     _ 
                      
                     
                         
                     
                      
                     Max 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where U ZK  is the intermediate circuit voltage and I Sp     —     Max  is the maximum permitted charging current of the store  50 . 
         [0054]    The embodiment In accordance with  FIG. 5  substantially corresponds to that in accordance with  FIG. 4 . In addition, the DC/DC converter  95  of small power is connected in parallel to the high voltage store  50  to provide the supply voltage required for the inverter  30  for the operation of the internal driver circuit as well as the sensor system during an interruption of the intermediate circuit voltage and to secure an autonomous operation of the drive circuit.
The invention relates to a drive circuit for an electric motor having an aerodynamic support of the motor shaft, wherein the drive circuit comprises at least one storage means for storing electrical energy by which the electric motor can be fed with electrical energy on a failure of the supply voltage or intermediate circuit voltage to obtain a minimum speed of the motor shaft required for the air support at least at times.