Abstract:
A commutation circuit includes a coil connected to an H bridge, the H bridge including four main switches for reversing polarity and a resulting coil current in the coil. The commutation circuit further includes a voltage source configured to generate a bypass current, and at least one auxiliary switch for controlling the bypass current to thereby decrease a switch current through at least one of the main switches. By generating an appropriate bypass current with help of a voltage source, a switch current through a desired main switch in a leading state can be decreased and eventually brought to zero. Zero current in its turn enables the use of thyristors as main switches as it results in the thyristors to be turned off automatically. Furthermore, decreased switch current at the switching moment reduces switching losses even in different types of switches such as GTOs and IGBTs.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a circuit for consecutively reversing a current direction in a coil. 
       BACKGROUND 
       [0002]    Referring to  FIG. 1 , a conventional commutation circuit  10  comprises an H bridge  20  with four main switches  30 ,  40 ,  50 ,  60 . A coil  70  in which the current direction is to be reversed is connected between a first output terminal  80  and a second output terminal  85  of the H bridge  20 . A first input terminal  90  and a second input terminal  95  of the H bridge  20  are connected to a current source  100  which provides a constant cell current I DC . The four main switches are operated appropriately to make the coil current I L  flow in a desired direction in the coil. For example, according to  FIG. 1  a first main switch  30  and a second main switch  40  are closed, and the coil current I L  flows from left to right in the figure (positive direction  290 ). In order to reverse the coil current I L  to flow from right to left (negative direction  300 ), a third main switch  50  and a fourth main switch  60  are closed, and the first and the second main switches  30 ,  40  are opened. 
         [0003]    The main switches are typically realized as semiconductor devices. Since the commutation circuit topology of  FIG. 1  requires that all the main switches can be actively opened, the respective semiconductor devices need to be of turn-off type i.e. of a type that can be actively turned off. In addition, the semiconductor devices need to have a reverse blocking capability. Examples of such semiconductor devices are symmetrical GTOs and reverse blocking IGBTs. On the other hand, thyristors are not turn-off type semiconductor devices. Therefore, thyristors would not work as main switches in the topology according to  FIG. 1  since a thyristor is only turned off when current through it becomes zero or close to zero. However, thyristors have low losses and they are cheap in comparison with the turn-off type semiconductor devices. It would therefore be desirable to enable the use of thyristors as main switches in an H bridge of a commutation circuit. 
         [0004]    WO2012/062376 discloses a commutation circuit for reversing a coil current in a coil of an electrical machine. The commutation circuit comprises a capacitor arranged to form a resonant circuit with the coil. The main switches in the H bridges disclosed in WO2012/062376 need to be of turn-off type in order for the commutation to work as described. 
       SUMMARY 
       [0005]    One object of the invention is to provide a commutation circuit which enables the use of thyristors as main switches in an H bridge. A further object of the invention is to provide a commutation circuit with low losses irrespective of what kind of switches are used as main switches in the H bridge. 
         [0006]    These objects are achieved by the different features of the present invention. 
         [0007]    The invention is based on the realization that by generating an appropriate bypass current with help of a voltage source, a switch current through a desired main switch in a leading state can be decreased and eventually brought to zero. Zero switch current in its turn enables the use of thyristors as main switches as it results in the thyristors to be turned off automatically. Furthermore, decreased switch current at the switching moment reduces switching losses even in different types of switches such as GTOs and IGBTs. 
         [0008]    According to a first aspect of the invention, there is provided a commutation circuit comprising a coil connected to an H bridge, the H bridge comprising four main switches for reversing polarity and a resulting coil current in the coil. The commutation circuit further comprises a voltage source configured to generate a bypass current, and at least one auxiliary switch for controlling the bypass current to thereby decrease a switch current through at least one of the main switches. The commutation circuit is configured to decrease the switch current through at least one of the main switches without turning on any of the remaining main switches. The ability to controllably decrease switch currents through the main switches reduces switching losses, and provided that the switch currents are reduced sufficiently also enables the use of thyristors as main switches. 
         [0009]    According to one embodiment of the invention, the voltage source is configured to generate a bypass current to thereby bring the switch current to zero. Zero switch current results in thyristors to be turned off automatically in a reliable way. 
         [0010]    According to one embodiment of the invention, the voltage source comprises a capacitor. By means of a capacitor the required voltage can be provided in a simple way. 
         [0011]    According to one embodiment of the invention, the coil current at least partially results from a cell current generated by a current source, and the cell current is furthermore used to pre-charge the capacitor. When the cell current is used to pre-charge the capacitor no additional pre-charge circuit is needed. 
         [0012]    According to one embodiment of the invention, at least one of the main switches comprises a thyristor. A thyristor is a preferred main switch type in a commutation circuit as thyristors are simple, cheap, and have low losses. 
         [0013]    According to one embodiment of the invention, all the main switches are thyristors. 
         [0014]    According to one embodiment of the invention, all the auxiliary switches are thyristors. 
         [0015]    According to one embodiment of the invention, the voltage source is connected in parallel with at least one of the main switches. 
         [0016]    According to one embodiment of the invention, the voltage source is connected in series with the coil. 
         [0017]    According to one embodiment of the invention, an electrical machine comprises a commutation circuit according to any of the embodiments disclosed hereinbefore. 
         [0018]    According to a second aspect of the invention, there is provided a method for reversing a current direction in a coil, the method comprising the steps of: providing a coil connected to an H bridge, the H bridge comprising four main switches; and generating a bypass current and controlling it to thereby decrease a switch current through at least one of the main switches without turning on any of the remaining main switches. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The invention will be explained in greater detail with reference to the accompanying drawings, wherein 
           [0020]      FIG. 1  shows a conventional commutation circuit, 
           [0021]      FIG. 2  shows a commutation circuit according to one embodiment of the invention, 
           [0022]      FIG. 3  shows a commutation circuit according to one embodiment of the invention, 
           [0023]      FIG. 4  shows a commutation circuit according to one embodiment of the invention, 
           [0024]      FIG. 5  shows a commutation circuit according to one embodiment of the invention, 
           [0025]      FIG. 6  shows a commutation circuit according to one embodiment of the invention, and 
           [0026]      FIG. 7  shows four positions to which a voltage source can be connected in relation to an H bridge at different phases of a commutation. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    All embodiments of the invention disclosed herein comprise a sub-circuit corresponding to that shown in  FIG. 1 , the only difference being that at least some of the main switches  30 ,  40 ,  50 ,  60  according to the disclosed embodiments of the invention are realized as thyristors. In the following description the sub-circuit comprising an H bridge  20  with four main switches  30 ,  40 ,  50 ,  60 , a coil  70  connected to output terminals  80 ,  85  of the H bridge  20  and a current source  100  connected to input terminals  90 ,  95  of the H bridge  20  will be termed as a “cell”. 
         [0028]    Referring to  FIG. 2 , in addition to the cell comprising four main switches  30 ,  40 ,  50 ,  60  in the form of thyristors, a commutation circuit  10  according to one embodiment of the invention comprises a voltage source in the form of a capacitor  110  connected in parallel with the H bridge  20  via a first diode  120  and a second diode  130 . The capacitor  110  has a first terminal  115  and a second terminal  116 . The capacitor  110  can furthermore be connected anti-parallel with the H bridge  20  by turning on a first auxiliary switch  140  and a second auxiliary switch  150 . According to the embodiment of  FIG. 2  the first and second auxiliary switches  140 ,  150  are IGBTs each provided with an anti-parallel diode. 
         [0029]    At an initial state of the commutation circuit  10  the cell current I DC  flows through the first main switch  30 , the coil  70  and the second main switch  40 , the coil current I L  in the coil  70  flowing in a positive direction  290  and being equal with the cell current I DC . The capacitor  110  has been charged via the first diode  120  to have a positive polarity on the first terminal  115 . The objective of the commutation is to reverse the coil current I L  to flow through the third and fourth main switches  50 ,  60  and in a negative direction  300  in the coil  70 . At a first phase of the commutation the first and second auxiliary switches  140 ,  150  are turned on, and the capacitor  110  is thereby connected anti-parallel with the H bridge  20 . As a consequence, a capacitor current I C  starts to increase, and the coil current I L  starts to decrease. The capacitor  110  shall be dimensioned large enough to bring the coil current I L  to zero, and when this occurs the first and second main switches  30 ,  40  are turned off automatically as their switch currents (a current through a switch) become zero. 
         [0030]    At a second phase of the commutation the first and second auxiliary switches  140 ,  150  are turned off, and the first input terminal  90  is thereby again brought into contact with the first terminal  115  via the diode  120 , while the output terminal  95  is brought into contact with the second terminal  116  via the diode  130 . The capacitor  110  starts to recharge positive polarity on the first terminal  115 , and the third and fourth main switches  50 ,  60  are turned on. The turning on of the third and fourth main switches  50 ,  60  can be delayed in order to control the amount of energy on the capacitor  110  at the end of the commutation. The recharging of the capacitor  110  continues after the turning on of the third and fourth main switches  50 ,  60 , and eventually the energy stored on the capacitor  110  becomes sufficient for the next commutation. The objective of the commutation is now reached, and the commutation circuit  10  is at a state that is identical with the initial state except the fact that the coil current I L  flows in the negative direction  300 . A next commutation can now be carried out in a corresponding manner as the commutation just described. 
         [0031]    Referring to  FIG. 3 , according to one embodiment of the invention the voltage source in the form of a capacitor  110  is connected either between the first output terminal  80  and the second input terminal  95  or between the second output terminal  85  and the second input terminal  95 , depending on the states of a third auxiliary switch  160 , a fourth auxiliary switch  170 , a fifth auxiliary switch  180  and a sixth auxiliary switch  190 . All the main switches  30 ,  40 ,  50 ,  60  and the auxiliary switches  160 ,  170 ,  180 ,  190  are thyristors. 
         [0032]    At an initial state of the commutation circuit  10  the coil current I L  flows in a positive direction  290  and is equal with the cell current I DC . The capacitor  110  is pre-charged by means of a pre-charge circuit  200  to have a positive polarity on the first terminal  115 . The objective of the commutation is to reverse the coil current I L  to flow in a negative direction  300 . At a first phase of the commutation the third main switch  50  and the third auxiliary switch  160  are turned on. As a consequence, the first main switch  30  is automatically turned off as a negative voltage is applied over it and the switch current is brought to zero, and the cell is short circuited through the third and second main switches  50 ,  40 . 
         [0033]    The capacitor  110  now forms a resonance circuit together with the coil  70 , and due to the pre-charge of the capacitor  110  the coil current I L  slightly increases before decreasing to zero as the voltage over the capacitor  110  changes polarity and eventually stores all the energy of the resonance circuit. The fourth auxiliary switch  170  is turned on to allow the resonance to continue, and the coil current I L  starts to increase in the negative direction  300  and eventually becomes equal with the cell current I DC . As this occurs the second main switch  40  is automatically turned off, and as soon as the voltage over the capacitor  110  changes polarity the fourth main switch  60  is turned on. 
         [0034]    The objective of the commutation is now reached, and the capacitor  110  needs to be pre-charged with the same polarity as initially before a next commutation can be carried out. The commutation just described was carried out by connecting the capacitor  110  between the first output terminal  80  and the second input terminal  95 . A next commutation is carried out in a corresponding manner, the only difference being that the capacitor  110  is now connected between the second output terminal  85  and the second input terminal  95  by appropriately operating the fifth and sixth auxiliary switches  180 ,  190 . 
         [0035]    Referring to  FIG. 4 , according to one embodiment of the invention the commutation circuit  10  comprises a voltage source in the form of a combination of a first capacitor  110  and a second capacitor  111 . The first capacitor  110  is pre-charged using the cell current I DC  (for example by turning on a tenth auxiliary switch  240  and the second main switch  40 ) to have a positive polarity on a first terminal  115  and a negative one on a second terminal  116 . The second capacitor  111  comprises a third terminal  117  and a fourth terminal  118 , and it is not pre-charged. The first capacitor  110  needs to be dimensioned to contain sufficient amount of energy to turn off a single thyristor while the second capacitor  111  needs to be dimensioned to store somewhat more energy than the coil  70  at cell current I DC . The first capacitor  110  is therefore typically much smaller than the second capacitor  111 . The commutation circuit  10  of  FIG. 4  further comprises a seventh auxiliary switch  210 , an eighth auxiliary switch  220  and a ninth auxiliary switch  230 . All the main switches  30 ,  40 ,  50 ,  60  and the auxiliary switches  210 ,  220 ,  230 ,  240  are thyristors. 
         [0036]    At an initial state of the commutation circuit  10  the coil current I L  flows in a positive direction  290  and is equal with the cell current I DC . The objective of a first commutation is to reverse the coil current I L  to flow in a negative direction  300 . At a first phase of the first commutation the seventh auxiliary switch  210  is turned on. As a consequence, the positively pre-charged first terminal  115  is brought into contact with the second input terminal  95 , and the second main switch  40  is automatically turned off as a negative voltage is applied over it and the switch current is brought to zero. The discharge of the first capacitor  110  continues until it changes polarity, at which instant the eighth auxiliary switch  220  is turned on. The cell current I DC  continues to charge the first and the second capacitors  110 ,  111  in parallel. The duration of this charging can be controlled such that an appropriate amount of energy is available in the first capacitor  110  for completing the commutation. 
         [0037]    At a second phase of the first commutation the fourth main switch  60  is turned on, and resonance between the coil  70  and the first and second capacitors  110 ,  111  begins. The coil current I L  decreases to zero as the first and second capacitors  110 ,  111  eventually store all the energy of the resonance circuit, and the seventh and eighth auxiliary switches  210 ,  220  turn off automatically. The fourth terminal  118  and the second terminal  116  both retain a positive polarity at the end of this process, which polarity of the second terminal  116  will be utilized during a subsequent second commutation. 
         [0038]    At a third phase of the first commutation the ninth auxiliary switch  230  is turned on, and the coil current I L  starts to increase in the negative direction  300  and eventually becomes equal with the cell current I DC . As this occurs the first main switch  30  is automatically turned off, and the cell current I DC  continues to charge the second capacitor  111  with a positive polarity on the third terminal  117 . As soon as the voltage over the second capacitor  111  changes polarity the third main switch  50  is turned on, which results in the ninth auxiliary switch  230  being automatically turned off. The objective of the first commutation is now reached, and the second commutation can be carried out when desired. The voltage stored in the first capacitor  110  has a positive polarity on the second terminal  116 , while the voltage stored in the second capacitor  111  is close to zero. The objective of the second commutation is to reverse the coil current I L  again to flow in the positive direction  290 . 
         [0039]    At a first phase of the second commutation the tenth auxiliary switch  240  is turned on. The positive polarity on the second terminal  116  applies a negative voltage over the third main switch  50  which automatically turns off as the switch current is brought to zero. The discharge of the first capacitor  110  continues until it changes polarity, at which instant the ninth auxiliary switch  230  is turned on causing the cell current I DC  to charge the first and second capacitors  110 ,  111  in parallel. The duration of this charging can be controlled such that an appropriate amount of energy is available in the first capacitor  110  for completing the commutation. 
         [0040]    At a second phase of the second commutation the first main switch  30  is turned on, and the first and second capacitors  110 ,  111  now form a resonance circuit together with the coil  70 . The ninth and tenth auxiliary switches  230 ,  240  turn off automatically as the coil current I L  reaches zero. The first capacitor  110  is thereby left with a positive polarity on the first terminal  115 , and the second capacitor  111  with a positive polarity on the third terminal  117 . The cell at this phase is short circuited through the first and fourth main switches  30 ,  60 , and the coil current I L  is zero. 
         [0041]    At a third phase of the second commutation the eighth auxiliary switch  220  is turned on. The coil current I L  starts to increase in the positive direction  290  and eventually becomes equal with the cell current I DC . As this occurs the fourth main switch  60  is automatically turned off, and as soon as the voltage over the second capacitor  111  changes polarity the second main switch  40  is turned on. This causes the eighth auxiliary switch  220  to be automatically turned off. The objective of the second commutation is now reached, and a next commutation can be carried out when desired as the first capacitor  110  is already pre-charged with the same polarity as initially. 
         [0042]    A great advantage of the embodiment according to  FIG. 4  in comparison with that of  FIG. 3  is that a separate pre-charge circuit  200  is not needed. Instead, the commutation circuit  10  of  FIG. 4  utilizes the cell current I DC  for pre-charging the first and second capacitors  110 ,  111 . 
         [0043]    Referring to  FIG. 5 , according to one embodiment of the invention the commutation circuit  10  comprises a voltage source in the form of a capacitor  110  connected either between the second output terminal  85  and the second input terminal  95  or between the second output terminal  85  and the first input terminal  90 , depending on the states of an eleventh auxiliary switch  250  and a twelfth auxiliary switch  260 . All the main switches  30 ,  40 ,  50 ,  60  and both auxiliary switches  250 ,  260  are thyristors. 
         [0044]    At an initial state of the commutation circuit  10  the coil current I L  flows in a positive direction  290  and is equal with the cell current I DC . The capacitor  110  is pre-charged by means of a pre-charge circuit  200  to have a positive polarity on the first terminal  115 . The pre-charge energy needs to be just enough to turn off a single thyristor. Because according to the embodiment of  FIG. 5  the capacitor  110  needs to be dimensioned to store somewhat more energy than the coil  70  at cell current I DC , the pre-charged energy is only a fraction of the capacitor&#39;s  110  capacitance. The objective of the commutation is to reverse the coil current I L  to flow in a negative direction  300 . 
         [0045]    At a first phase of the commutation the eleventh auxiliary switch  250  is turned on. As a consequence, the second main switch  40  is automatically turned off as a negative voltage is applied over it and the switch current is brought to zero, and the coil current I L  starts charging the capacitor  110  with a positive polarity on the second terminal  116 . This charging of the capacitor  110  is allowed to continue to store a desired amount of energy in the capacitor  110 . At a second phase of the commutation the fourth main switch  60  is turned on, and the cell is thus short circuited through the first and fourth main switches  30 ,  60 . The capacitor  110  now forms a resonance circuit together with the coil  70 , and the coil current I L  decreases to zero as the capacitor  110  eventually stores all the energy of the resonance circuit. As this occurs the eleventh auxiliary switch  250  is automatically turned off. At this moment the capacitor  110  should contain enough energy to carry through the following phases of the commutation i.e. somewhat more than the energy of the coil  70  at cell current I DC . 
         [0046]    At a third phase of the commutation the twelfth auxiliary switch  260  is turned on, and a resonance circuit between the capacitor  110  and the coil  70  is again formed. The coil current I L  starts to increase in the negative direction  300  and eventually becomes equal with the cell current I DC . As this occurs the first main switch  30  is automatically turned off, and as soon as the voltage over the capacitor  110  changes polarity the third main switch  50  is turned on and the twelfth auxiliary switch  260  is automatically turned off. The objective of the commutation is now reached, and a next commutation can be carried out in a corresponding manner as the commutation just described. The capacitor  110  needs to be pre-charged with an opposite polarity than initially before the next commutation can be carried out, and the next commutation is initiated by turning on the twelfth auxiliary switch  260 . 
         [0047]    Referring to  FIG. 6 , according to one embodiment of the invention the commutation circuit  10  comprises a voltage source in the form of a capacitor  110  connected in series with the coil  70 . A thirteenth auxiliary switch  270  and a fourteenth auxiliary switch  280  are connected in parallel with the capacitor  110 , the two auxiliary switches  270 ,  280  being anti-parallel in relation to each other. The first and third main switches  30 ,  50  and both auxiliary switches  270 ,  280  are GTOs, and the second and fourth main switches  40 ,  60  are thyristors. 
         [0048]    At an initial state of the commutation circuit  10  the coil current I L  flows in a positive direction  290  and is equal with the cell current I DC . The thirteenth auxiliary switch  270  is turned on and the capacitor  110  is thus bypassed and has a zero voltage. The objective of the commutation is to reverse the coil current I L  to flow in a negative direction  300 . 
         [0049]    At a first phase of the commutation the thirteenth auxiliary switch  270  is turned off, and the fourth main switch  60  is simultaneously turned on. As a consequence, the cell is short circuited through the first and fourth main switches  30 ,  60 . The capacitor  110  now forms a resonance circuit together with the coil  70 , and the coil current I L  decreases to zero as the capacitor  110  eventually stores all the energy of the resonance circuit. As this occurs the second main switch  40  is automatically turned off. 
         [0050]    At a second phase of the commutation the third main switch  50  is turned on, and a resonance circuit between the capacitor  110  and the coil  70  is again formed. The coil current I L  starts to increase in the negative direction  300  and eventually becomes close to equal with the cell current I DC . In a lossless circuit the coil current I L  in the negative direction  300  could indeed become equal with the cell current I DC , and in a circuit with low losses the coil current I L  in the negative direction  300  could become high enough to automatically turn off the first main switch  30  should it be a thyristor. However, according to the embodiment of  FIG. 6  the first main switch  30  is a GTO, and it is actively turned off as soon as the voltage over the capacitor  110  changes polarity and the switch current through the first main switch  30  becomes close to zero. 
         [0051]    At a third phase of the commutation the fourteenth auxiliary switch  280  is turned on simultaneously with the turning off of the first main switch  30 . This will bypass the capacitor  110  now that the coil current I L  flows in the negative direction  300 . The objective of the commutation is now reached, and the commutation circuit  10  is at a state that is identical with the initial state except the fact that the coil current I L  flows in the negative direction  300 . A next commutation can now be carried out in a corresponding manner as the commutation just described. 
         [0052]    Further referring to  FIG. 6 , an alternative mode of operating the commutation circuit  10  is suggested. Instead of simultaneously turning off the thirteenth auxiliary switch  270  and turning on the fourth main switch  60  at the first phase of the commutation, the thirteenth auxiliary switch  270  is turned off first. The cell current I DC  will now charge the capacitor  110  with extra energy which can be utilized later for compensating losses within the circuit. After an appropriate delay the fourth main switch  60  is turned on and the following commutation phases occur as described hereinbefore. The advantage of charging the capacitor  110  with extra energy is that the capacitor  110  now contains enough energy to increase the coil current I L  to be equal with the cell current I DC . Consequently, the first main switch  30  in the form of a GTO can be replaced with a thyristor and the commutation will still succeed. The same applies to the third main switch  50  when a corresponding delay is introduced when turning on the second main switch  40  during commutation in an opposite direction. 
         [0053]    The commutation circuits  10  described herein can be utilized in controlling coil currents I L  in coils of an electrical machine, such as a transformer, an electrical motor or a generator. In such applications the voltage generated by an electromotive force (emf) between the first and second output terminals  80 ,  85  of the H bridge  20 , as well as losses (electrical energy transformed into heat or mechanical energy), shall be taken into consideration when dimensioning the components and when operating the respective commutation circuit  10 . 
         [0054]    The invention is not limited to the embodiments shown above, but the person skilled in the art may modify them in a plurality of ways within the scope of the invention as defined by the claims. Thus, numerous other topologies than those disclosed above can be utilized to achieve the technical effect of the invention i.e. to decrease a switch current through a desired main switch  30 ,  40 ,  50 ,  60  in a leading state.  FIG. 7  shows four positions to which the voltage source (here in the form of a capacitor  110 ) can be connected in relation to the H bridge  20  at different phases of the commutation. Furthermore, the current source  100  may be replaced with any suitable source that generates current to the coil  70 , such as a voltage source in series with an inductance. Such source shall in the context of the present invention be considered as a current source  100 . 
         [0055]    In order for the commutation to work as described when using the disclosed topologies it needs to be assumed that the capacitors, the switches, eventual pre-charge circuits and other components are appropriately dimensioned and that the switches are appropriately controlled. Instead of using capacitors as voltage sources according to the embodiments shown above any other suitable voltage sources can be used. Furthermore, any current generated by a voltage source shall be considered as a bypass current if it results in a decrease of a switch current through at least one of the main switches  30 ,  40 ,  50 ,  60 , even if the respective current not necessarily bypasses the respective switch. Even if all the embodiments shown above contain at least one thyristor as a main switch  30 ,  40 ,  50 ,  60 , the invention can also be applied on commutation circuits  10  where none of the main switches  30 ,  40 ,  50 ,  60  is a thyristor.