Abstract:
Power is supplied to excitation windings in order to generate magnetic fields, preferably for activating superconducting coils. An AC transformer is used, and triggering of the coil is performed via a rectifier having little power loss. Preferably, a two-way rectifier using a freewheeling circuit is utilized in the associate device, thus preventing losses when power is supplied and especially when the power is discharged.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based on and hereby claims priority to German Patent Application No. 10 2005 047 541.8 filed on Sep. 30, 2005 and PCT Application No. PCT/EP2006/066211 filed on Sep. 11, 2006, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a method for supplying energy to and discharging energy from a resistive-inductive load or a pure inductance. 
         [0003]    Applications for the excitation windings can be found in medical appliances or else in synchronous machines, for example. By supplying energy to and discharging energy from the excitation winding, the excitation current is raised or lowered and hence the strength of the magnetic field influenced. 
         [0004]    Often, such excitation windings are also made from HTS (High Temperature Superconductor) materials which need to be kept at a temperature of &lt;80 K in order to maintain superconductivity. To maintain this low temperature, supply of heat from the outside and generation of heat in the cooled area need to be largely avoided. 
         [0005]    The electrical power required for excitation and de-excitation is often very high because the process of excitation and de-excitation needs to take place very quickly in order to achieve a high control quality. To avoid high losses when supplying and discharging electrical power in order to alter the current through the inductive load, it is advantageously possible to use a relatively high voltage and to perform voltage transformation directly before the inductive load is fed. 
         [0006]    To feed the excitation winding directly from the outside, the high current levels in the region of up to a few 100 A mean that it is necessary to use lines having an appropriate cross section which are therefore also good conductors of heat. Instead, it is better for the balance of thermal power loss if relatively high voltages are used for the supply of energy from the hot to the cold area and hence the conductor cross section is reduced. The voltage then likewise needs to be transformed by a transformer in the cold area and rectified in order to feed the excitation winding. 
         [0007]    Specifically in the case of applications for superconductive excitation windings in which the energy is transferred from the ambient temperature (what is known as the “hot area”) to a temperature which allows superconduction (what is known as the “cold area”), this results in reduced losses in the bushings from the hot to the cold area. 
       SUMMARY 
       [0008]    Against this background, it is one possible object to specify a method and circuit device for supplying energy to and discharging energy from an inductance. The inventors propose specifically designed rectifiers and actuating methods for the converter valves contained therein. 
         [0009]    The inventors propose a specific regime of actuation for a specific rectifier circuit which can be used to achieve particularly low-loss operation of the rectifier when feeding resistive-inductive loads by virtue of the current being commutated between different rectifier paths with little loss. This relates both to the phase of the inductance&#39;s excitation and de-excitation in which the magnetic energy stored in the inductance is raised or lowered and to the phase of constant flow of current through the inductance. 
         [0010]    The method can be applied to all rectifier circuits having a plurality of rectifier paths, particularly a multiple full bridge rectifier circuit. 
         [0011]    Optionally, a freewheeling path may be provided which accepts the current for the inductance when it does not need to be altered. 
         [0012]    The method and device can be applied as a whole for all resistive-inductive loads, excitation devices for electric machines and field coils for generating magnetic fields. In this context, it is advantageous that the method is suitable for particularly low-loss conversion on the voltage transformer&#39;s secondary side facing the inductance, the voltage transformer comprising a primary-side inverter, a transformer and a secondary-side rectifier. The voltage transformer can be operated in two quadrants in order to achieve de-excitation and excitation of the inductance while the secondary-side current direction is constant. 
         [0013]    The method can be used particularly advantageously for feeding HTS excitation windings. Alternatively, the method and device can be applied for other windings in electric machines. 
         [0014]    In the case of the circuit arrangement with a specific rectifier, the converter valves comprise power semiconductors. The power semiconductors used are advantageously MOSFETs because they have no pn junction with the associated forward voltage UAK and the resultant losses. Preferably, a converter valve is respectively formed by two reverse-connected series MOSFETs in order to be able to set up a reverse voltage with positive and negative polarity, since an individual MOSFET has no inhibiting action in the reverse direction on account of the intrinsically contained diode. 
         [0015]    Alternatively, the power semiconductors used may also be thyristors, IGBTs, GTOs or IGCTs. 
         [0016]    In the case of the indicated solution to the problem, it is possible to use not only the transformer and the rectifier but also optionally a freewheeling path in parallel with the excitation winding. This freewheeling path can advantageously be used to route the current when its level does not need to be altered. In this case, the current can thus be routed on a short path via a low-impedance converter valve without the need for the current to flow via the transformer&#39;s windings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0017]    These and other objects and advantages of the present invention will become more apparent and more readily appreciated from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings of which: 
           [0018]      FIG. 1  shows a two-path rectifier in a center-tap connection with a freewheeling path, the individual switching elements being formed by MOSFETs which are actuated by a microprocessor, 
           [0019]      FIG. 2  shows the profile of the signals with reference to the primary voltage U P  when the inductance is excited on the basis of the related art, and the associated actuation of the converter valves, 
           [0020]      FIG. 3  shows the profile of the signals with reference to the primary voltage U P  when the inductance is de-excited on the basis of the related art, and the associated actuation of the converter valves, 
           [0021]      FIG. 4  shows the profile of the signals with reference to the primary voltage U P  when the inductance is excited with improved actuation of the converter valves, 
           [0022]      FIG. 5  shows the profile of the signals with reference to the primary voltage U P  when the inductance is de-excited with improved actuation of the converter valves. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0023]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. 
         [0024]    In the following description, particularly in  FIG. 1 , a transformer  50  is assumed in line with a parallel German patent application from the applicant with the same application priority. With regard to the disclosure of the units connected upstream of the transformer, reference is made to this parallel application, inter alia. 
         [0025]      FIG. 1  shows a transformer  3  having a primary side  31  and a secondary side  32 . The primary side of the transformer  3  is actuated by a primary voltage U P  at a suitable frequency, the voltage source and the inverter not being shown in detail. 
         [0026]    The secondary-side rectifier arrangement shown in  FIG. 1  is controlled by a microprocessor  50  on the basis of a sequential program. The sequential program implements a particular actuation regime and is described in detail further below with reference to  FIGS. 2 to 5 . 
         [0027]      FIG. 1  also shows the electrical engineering symbols for the individual elements. In this context:
   T: is a converter transformer with a secondary-side center tap   L: is a resistive-inductive load or inductance to be actuated   V 1 : is a converter valve in the top path   V 2 : is a converter valve in the bottom path   V 3 : is a converter valve in the freewheeling path   V 1   a , V 1   b , V 2   a,  V 2   b,  V 3   a,  V 3   b : are MOSFETs as power semiconductors, including an intrinsically provided body diode   I 1 : is the current in the top path   I 2 : is the current in the bottom path   I L : is the current through the inductance   U P : is the primary-side voltage on the converter transformer   U S : is the secondary-side voltage on the converter transformer per winding—corresponds to ü•U P      U V1 : is the voltage across the top converter valve   U V2 : is the voltage across the bottom converter valve   U L : is the voltage across the inductance/the freewheeling circuit.     
         [0042]    In  FIG. 1 ,  6  and  6 ′ also signify the secondary coils of the converter transformer  5 , and  10  signifies the inductance L to be excited. The converter transformer  5  has two converter valves V 1  and V 2 , each comprising the reverse-connected series MOSFETs V 1   a  and V 1   b  and also V 2   a  and V 2   b,  associated with its secondary side. In addition, a converter valve V 3  comprising the reverse-connected series MOSFETs V 3   a  and V 3   b  is provided for a freewheeling path. 
         [0043]    The individual MOSFETs distinguished by a and b are each reverse-connected in series with one another. Each MOSFET contains an intrinsic body diode, which is likewise shown in  FIG. 1 , by virtue of the principle. Alternatively, extra diodes may be provided outside the MOSFETs in order to relieve the load on the intrinsic body diodes. 
         [0044]    Instead of the two-pulse rectifier circuit with a center tap shown in  FIG. 1 , it is possible to use any other rectifier circuit with a plurality of rectifier paths, particularly multiple full bridge rectifier circuits. 
         [0045]      FIGS. 2 to 5  respectively show the time on the abscissa and alternately plot the following variables on the associated ordinate:
   U P : primary-side voltage on the converter transformer   U V1 : voltage across the top converter valve   U V2 : voltage across the bottom converter valve   U L : voltage across the inductance L/the freewheeling circuit   I 1 : current in the top converter valve   I 2 : current in the bottom converter valve     
         [0052]      FIG. 2  produces the graphs  21  to  26 ,  FIG. 3  produces the graphs  31  to  36 ,  FIG. 4  produces the graphs  41  to  42  and  FIG. 5  produces the graphs  51  to  55 . 
         [0053]    In this context,  FIGS. 2 and 3  first of all show the operation of the two-pulse rectifier based on the related art and  FIGS. 4 and 5  show the improved operation of the two-pulse rectifier with reduced power loss as a result of improved commutation of the current. 
         [0054]      FIGS. 2 and 3  show significant times t x  for the commutation based on the related art as follows:
   t 1 , t 4 : polarity change for the primary-side transformer voltage   t 2 : power semiconductors V 1   a  and V 1   b  switched on and V 2   a  and V 2   b  switched off   t 3 , t 6 : end of the current commutation   t 5 : power semiconductors V 2   a  and V 2   b  switched on and V 1   a  and V 1   b  switched off.     
         [0059]    In the case of the described design and actuation of the rectifier shown in  FIG. 1 , it is fundamental that the energy losses during commutation of the current between the converter valves are reduced. 
         [0060]    A freewheeling path in parallel with the excitation winding can carry or short the current when its level does not need to be altered. This means that the current does not need to be routed via the inevitably higher nonreactive winding resistances of the transformer but rather can be routed on a short path via the low-impedance converter valve V 3 . In addition, the hysteresis loss in the transformer disappears because the voltage can be switched off during the freewheeling phase. 
         [0061]    In the steady state, in which the excitation current is neither raised nor lowered, the excitation device therefore needs to be activated only occasionally to compensate for residual losses when superconducting inductances are actuated, the low-loss freewheeling path being active for most of the time. 
         [0062]    A fundamental cause of the losses in the rectifier is the commutation operation from one converter path or converter valve to the other converter path or converter valve. By way of example, in the case of the rectifier shown in  FIG. 1 , the current needs to be commutated from the converter valve V 1  to the converter valve V 2 —and accordingly also back. 
         [0063]    When the valves based on the related art are actuated, this is done by switching on the two MOSFETs V 2   a /V 2   b  together and at the same time or immediately afterwards switching off the two MOSFETs V 1   a /V 1   b  together, times t 2  and t 5  in  FIGS. 2 and 3 . The current which is driven further by the excitation winding L cannot commutate from one converter valve of the rectifier to the other at infinite speed, however, because the magnetic energy first needs to be lowered or raised in the leakage inductances of the transformer&#39;s current-carrying windings. In the case of the stated type of valve control based on the related art, this is done by virtue of the converter valve in the path from which commutation is to take place (i.e. the path which the current leaves) raising a reverse voltage, which is essentially corresponds to the drain-source breakdown voltage U (BR)DSS  of the MOSFET V 1   a  (V 2   a ) used in the converter valve, from time t 2  or t 5  onward. This voltage brings about a fall in the current in the path from which commutation is to take place and a rise in the current in the accepting path, the sum of both currents always corresponding to the current I L  through the inductance L. For the time of the commutation up to the time t 3  or t 6 , this is associated with high losses in the converter valve of the path from which commutation is to take place, particularly the MOSFETs V 1   a  and V 2   a,  as can be seen from the increased voltage across the converter valves. 
         [0064]    This means that the energy stored in the leakage inductances is converted to heat upon every commutation operation in the converter valves in the known method of actuation. Besides the unwanted introduction of heat, the valve is therefore used at a critical operating point, which has an adverse effect on reliability. 
         [0065]    One improved option is to switch on the two converter valves V 1  and V 2  together for a certain time and hence to set up a short circuit. If this is done at a time when the primary-side and hence secondary-side voltage of the converter transformer is oriented such that the current I 1  is lowered and the current I 2  is raised then the current I 1  will be reduced and the current I 2  will be increased, this being brought about solely by the secondary-side voltage. The valve V 1  now needs to be switched off exactly at the zero crossing in the current I 1 . 
         [0066]    If the valve is switched off too early then a magnetic residual energy remains stored in the unavoidable leakage inductances of the transformer and in turn needs to be lowered by the drain-source breakdown voltage U (BR)DSS →see above. 
         [0067]    If the valve is switched off too late, the current I 1  will become negative after its zero crossing and the current I 2  will rise beyond the current I L . The negative I 2  is again switched off by the source breakdown voltage U (BR)DSS . 
         [0068]    In both cases, sudden switching off results in high voltage spikes and consequently high power losses. 
         [0069]    Since the commutation time is dependent on the level of the current through the inductance which is to be commutated, the time difference between switching on V 2  and switching off V 1  cannot be kept constant, which means that pure time control is eliminated. By contrast, detecting the current&#39;s zero crossing is complex and susceptible to error. 
         [0070]    It is now fundamental that the two MOSFETs in the converter valve are not actuated together but rather separately such that first of all only the MOSFET V 1   b  is switched off. The intrinsic MOSFET diode or else an additionally provided, parallel connected diode means that the converter valve V 1  acts as a freewheeling valve across the MOSFET V 1   a  which is still switched on and the diode in V 1   b . At the same time, the two MOSFETs V 2   a /V 2   b  are switched on together. If this is done at a time at which the voltage on the secondary windings of the transformer is oriented such that the current I 1  is lowered and the current I 2  is raised (see above) then the commutation is forced by the voltage provided by the transformer. 
         [0071]    Following the lowering of the current I 1  in the commutation phase, the current I 1  in the converter valve V 1  is interrupted by the diode of V 1   b  automatically at the zero crossing, so that the voltage provided by the transformer cannot drive a reverse current. Losses are therefore caused only by the forward voltage U AK  of the diode. When the current I 1  has subsided to zero, the second power semiconductor V 1   a  is also switched off. This switching off of V 1   a  is not coupled directly to the zero crossing of  11 , but rather can take place at a certain interval thereafter. This interval is proportioned such that the maximum excitation current can be commutated. However, V 1   a  must be switched off before the voltage on the secondary windings of the transformer changes its polarity. 
         [0072]    The signal profiles for the improved method are shown in  FIG. 4  for the excitation and in  FIG. 5  for the de-excitation. These Figs show significant times t x  as follows:
   t 1 , t 4 : polarity change for the primary-side transformer voltage   t 2 : power semiconductors V 1   a  and V 1   b  switched on an V 2   b  switched off   t 3 , t 6 : end of current commutation   t 2 ′: V 2   a  switched off   t 5 : power semiconductors V 2   a  and V 2   b  switched on and V 1   b  switched off   t 5 ′: V 1   a  switched off   
 
         [0079]    Whether the circuit is operated in excitation mode or de-excitation mode is decided by the position of the times t 2 , t 2 ′, t 3 , t 5 , t 5 ′ and t 6 , identifying the current commutation, relative to the times of the polarity change t 1  and t 4  for the primary-side transformer voltage. 
         [0080]    It becomes clear that exactly switching off at the zero crossing at the diode in the valve V 1   b  avoids a loss of energy in the commutation operation. 
         [0081]    This current commutation from one of the converter valves V 1  or V 2  in the freewheeling path, embodied by the converter valve V 3 , takes place in similar fashion. 
         [0082]    The invention has been described in detail with particular reference to preferred embodiments thereof and examples, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention covered by the claims which may include the phrase “at least one of A, B and C” as an alternative expression that means one or more of A, B and C may be used, contrary to the holding in  Superguide v. DIRECTV , 69 USPQ2d 1865 (Fed. Cir. 2004).