Abstract:
A method and apparatus for converting three phase AC voltages on first, second and third input lines to DC voltage across positive and negative DC buses, the apparatus comprising a rectifier including first, second and third rectifier legs, each leg including a switch and a diode wherein the switch is linked between the positive DC bus and an cathode of the diode, an anode of the diode is linked to the negative DC rail and the first, second and third diode cathodes are linked to the first, second and third input lines, respectively, a DC bus voltage sensor linked to the positive DC bus and measuring the DC bus voltage to generate a measured DC bus voltage and a rectifier controller that receives the measured DC bus voltage, a reference voltage value and the three phase AC voltages wherein, when the measured DC bus voltage is at least equal to the reference voltage value, the controller turns on the switches in the first, second and third rectifier legs when the voltages on the first, second and third input lines are positive, respectively.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     This invention relates to DC bus capacitor pre-charging systems and more specifically to a method and apparatus for use in controlling rectification of three phase AC supply voltages to allow pre-charging of a DC bus capacitor prior to full rectifier activity. 
     A typical power converter includes a rectifier stage and an inverter stage wherein the rectifier stage is provided between three phase AC power lines and a DC bus and the inverter stage is provided between the DC bus and a load. The rectifier, as the label implies, rectifies the three phase AC voltages to generate a DC voltage across the DC bus. The DC bus typically includes a DC bus capacitor that charges as the rectifier operates. The inverter receives the DC voltage and is controlled to change that DC voltage into three phase AC voltages that are provided to the load. Typically the inverter can be controlled to control the frequency and amplitude of the three phase voltages supplied to the load. 
     Inverters typically include a plurality of switching devices to convert the DC voltage to three phase AC voltages. As known in the power conversion industry, the inverter switches and or a load linked to an inverter can be damaged if they are exposed to excessive currents. One way to protect inverter switches/loads is to provide one or more fuses in the DC bus which open when excessive current passes there through. 
     When power is initially applied to a converter upon start up, if the DC bus capacitor is uncharged, capacitor appears as a short circuit at the DC bus and therefore, if the three phase AC power is immediately applied to the DC bus, excessive currents can cause the DC bus fuses to blow. For this reason, it is known that, prior to starting full rectification of three phase AC voltages and applying the rectified voltages to the DC bus, the DC bus has to be pre-charged to bring the DC bus potential up to a rated voltage level. 
     The power conversion industry has developed various ways to pre-charge the DC bus prior to full rectification activity. One way to pre-charge the DC bus is to place a parallel resistor and relay in series with the DC bus capacitor and to short out the resistor by closing the relay after the DC bus potential reaches the rated voltage value. The parallel resistor/relay solution works well with small drives where the cost of the relay and resistor is minimal. 
     Another way to pre-charge the DC bus prior to full rectification activity is to construct the rectifier stage using silicon controlled rectifiers (SCRs) and to control the turn on times of the SCRs to charge the DC bus over a period and in a controlled manner. The SCR solution works well in large drives where the cost of a relay required to short out the resistor can often exceed the additional costs associated with the SCR switches and a switch controller. 
     BRIEF SUMMARY OF THE INVENTION 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. This summary is not intended delineate the scope of the invention and the sole purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     It has been recognized that a versatile and inexpensive solution to the DC bus capacitor pre-charging problem is to provide a rectifier bridge in conjunction with a pre-charging circuit where the rectifier bridge includes three controllable switching devices in the top halves of three rectifier legs and three diodes in the bottom halves of the three legs where the controllable switches are open while pre-charging is occurring and are closed during positive half cycles of associated line currents after the DC bus capacitor has reached a target pre-charge level (e.g., the rated voltage of the DC bus). In this way the DC bus capacitor can be charged at a rate that should avoid a large startup current and thereafter normal rectifier activity can commence. 
     In at least some embodiments the three controllable switching devices are silicon controlled rectifiers (SCRs) and the SCRs are controlled to be full on whenever associated phase voltages are positive. In at lease some embodiments a high frequency (e.g., 20 kHz) pulse generator is used to generate firing pulses for the SCRs so that the full on condition can be achieved without requiring components for tracking the phase angles of voltages associated with the SCRs. 
     The following description and related drawings set forth in detail certain illustrative aspects of the present invention. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
         FIG. 1  is a schematic drawing illustrating a rectifier system according to at lease some aspects of the present invention; 
         FIG. 2  is a schematic drawing illustrating an exemplary SCR gate driver that may be included as a portion of the system of  FIG. 1 ; 
         FIG. 3  is a graph illustrating various signals that may be generated by the system as shown in  FIG. 1 ; and 
         FIG. 4  is a flow chart illustrating an exemplary method according to at least some aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     One or more specific embodiments of the present invention will be described below. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developer&#39;s specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     Referring now to the drawings wherein like reference numerals correspond to similar elements throughout the several views and, more specifically, referring to  FIG. 1 , the present invention will be described in the context of an exemplary rectifier and control system  10  including, among other components, a rectifier controller/processor  12 , a rectifier bridge  16  and a pre-charge circuit  18 . The system  10  receives three phase AC voltages V R , V S  and V T  on three input supply lines  20 ,  22  and  24 , respectively, and converts those AC voltages into a DC voltage/potential across positive and negative DC buses  26  and  28 , respectively. To this end, rectifier bridge  16  includes first, second and third switches or switching devices which, in the illustrated example, include first, second and third silicon controlled rectifiers (SCRs)  30 ,  32  and  34 , respectively, first, second and third diodes  36 ,  38  and  40 , respectively, a snubber circuit including a capacitor  84  and a resistor  86  and a DC bus capacitor  82 . The first SCR  30  and the first diode  36  are linked series between the positive and negative DC buses  26  and  28 , respectively. In this regard, the anode of diode  36  is linked to negative DC bus  28 , the cathode of SCR  30  is linked to positive DC bus  26  and the cathode of diode  36  is linked to the anode of SCR  30  at a node  31 . The R-phase supply line  20  is linked to node  31 . Similarly, the anode of second diode  38  is linked to negative DC bus  28 , the cathode of second SCR  32  is linked to positive DC bus  26 , the cathode of diode  38  is linked to the anode of SCR  32  at a node  33  and the S-phase supply line  22  is linked to node  33 . Continuing, the anode of the third diode  40  is linked to negative DC bus  28 , the cathode of third SCR  34  is linked to positive DC bus  26 , the cathode of diode  40  is linked to the anode of SCR  34  at a node  35  and the T-phase supply line  24  is linked at node  35 . 
     Referring still to  FIG. 1 , capacitor  84  is linked in series with resistor  86  between positive and negative DC buses  26  and  28 . DC bus capacitor  82  is linked between positive and negative DC buses  26  and  28  so as to be in parallel with the series pair including capacitor  84  and resistor  86 . 
     Pre-charge circuit  18  includes first and second pre-charge diodes  60  and  62  and first and second pre-charge resistors  64  and  66 , respectively. First pre-charge diode  60  is linked in series with first pre-charge resistor  64  between supply line  24  and positive DC bus  26  where the cathode of diode  60  is linked to positive DC bus  26 . Similarly, second pre-charge diode  62  and second pre-charge resistor  66  are linked in series between supply line  22  and positive DC bus  26  with the cathode of diode  62  linked to positive DC bus  26 . 
     Referring yet again to  FIG. 1 , rectifier controller/processor  12  includes a high frequency pulse generator  70 , an isolation transformer  72 , an SCR gate driver  74  and a DC voltage comparator module  14 . As the label implies, high frequency generator  70  generates a high frequency (e.g., 20 kHz) pulse signal. To this end, see pulse signal P c  in  FIG. 3 . The high frequency pulse signal is provided to transformer  72  which isolates generator  70  from driver  74 . 
     Comparator module  14  is linked to a voltage sensor  80  that is in turn linked to positive DC bus  26 . Sensor  80  generates a feedback DC voltage value V DC  which is provided to comparator module  14 . In addition to receiving the feedback DC voltage value V DC , module  14  also receives a DC voltage reference value V r  which is input by a system user. The reference value V r  is the rated voltage value for the DC bus. Comparator module  14  compares the feedback DC voltage V DC  to the reference or rated value V r  and generates a control signal α on line  48  which is provided to SCR gate driver  74 . The control signal a generated by module  14  is low when the feedback DC voltage value V DC  is less than the reference voltage value V r . When feedback DC voltage value V DC  is equal or greater than reference voltage value V r , control signal α is high. An exemplary control signal α is shown in  FIG. 3 . 
     In addition to receiving the high frequency pulse signal P c  and the control signals α, gate driver  74  receives the three phase line voltages V R , V S  and V T  on lines  50 ,  52  and  54 , respectively. Gate driver  74  uses all of the received signals P c , α, V r , V S  and V T  to generate first, second and third firing signals R f , S f  and T f  for controlling SCR switching devices  30 ,  32  and  34 , respectively. 
     Referring still to  FIG. 1  and now also to  FIG. 2 , SCR gate driver  74  includes first, second and third A/D converters  110 ,  112  and  114 , respectively, and, first, second and third AND gates  100 ,  102  and  104 , respectively. The R-phase input voltage signals V R  is provided to converter  110  which generates an output signal that is low when the R-phase voltage is negative and that is high when the R-phase voltage is positive. Similarly, converters,  112  and  114  receive the S-phase and T-phase input voltages and generate output signals that are high when the associated input voltages are positive and that are low when the associated input voltages are negative. 
     Each of the control signal α and the high frequency signal P c  is provided to each of AND gates  100 ,  102  and  104 . In addition, AND gate  100  receives the output of first converter  110 , AND gate  102  receives the output of second converter  112  and the AND gate  104  receives the output of third converter  114 . When all of the inputs to AND gate  100  are high or positive, a firing pulse is generated as signal R f  on line  42 . Similarly, when each of the inputs to AND gate  102  are high or positive, a firing pulse is generated as signal S f  on line  44  and when all of the input to AND gate  104  are positive or high, a firing pulse is generated as signal T f  on line  46 . 
     In operation, referring to  FIGS. 1 ,  2  and  3 , when three phase voltages V R , V S , and V T  are initially applied to lines  20 ,  22  and  24 , DC bus capacitor  82  is initially uncharged and therefore the feedback DC voltage V DC  measured by sensor  80  will have a zero value. When the feedback DC voltage V DC  is zero, the control signal a has a low value and therefore, referring specifically to  FIG. 2 , none of the AND gates  100 ,  102  or  104  generates firing pulses so that, referring to  FIG. 1 , SCRs  30 ,  32  and  34  act as open circuits. In this case, voltage on supply lines  22  and  24  is supplied through resistors  64  and  66  and diodes  60  and  62  to the positive DC bus  26  which starts to pre-charge the DC bus capacitor  82 . As DC bus capacitor  82  charges, sensor  80  begins to generate a non-zero DC voltage value V DC  which, during a pre-charging period, remains below the reference voltage value V r  and, during this pre-charging period, the control signal α remains low. In  FIG. 3 , this pre-charging period corresponds to the period between times τ 0  and τ 1  where control signal α is low and the resulting R-phase firing signal R f  does not include firing pulses. 
     Referring still to  FIGS. 1-3 , eventually DC bus capacitor  82  charges to the point where the DC bus potential as reflected in the feedback DC voltage V DC  exceeds the rated or reference voltage value V r . When the feedback DC voltage V DC  exceeds the rated or reference voltage value V r , module  14  generates a high control signal α (see time τ 1  in  FIG. 3 ). When control signal a goes high, during positive half-cycles of the phase voltages V R , V S  and V T , AND gates  100 ,  102  and  104  generate firing pulses as the firing signals R f , S f  and T f , respectively, for their associated phases. Thus, for instance, referring to  FIG. 3 , for the R-phase, firing signal R f  includes exemplary firing pulses  120  whenever the R-phase input voltage V R  is high and when the high frequency signals P c  is high once control signal a goes high after time τ 1 . As seen in  FIG. 3 , once the R-phase input voltage V R  goes negative at time τ 2 , the R-phase firing pulses stop. Again, at time τ 3  when the R-phase input voltage is positive, the high frequency pulses commence again as signal R f . 
     Referring once again to  FIG. 1 , once SCRs  30 ,  32  and  34  begin to fire, because the SCRs have very little resistance, the pre-charge circuit  18  is effectively opened (i.e., the pre-charge resistors  64  and  66  essentially limit current flow through circuit  18  to a negligible value). 
     Referring now to  FIG. 4 , an exemplary method  130  consistent with at least some aspects of the present invention is shown that may be performed by the system of  FIG. 1 . At block  132  a rectifier bridge is provided that includes SCRs that form the top half of the bridge and diodes that form the bottom half of the bridge. At block  134 , a pre-charge circuit that maybe akin to the circuit  18  shown in  FIG. 1  is provided and is used to commence pre-charging of the DC bus capacitor  82 . At block  136  a voltage sensor (e.g., see  80  in  FIG. 1 ) is used to measure the DC bus voltage and at decision block  138  the feedback DC voltage V DC  is compared to the reference voltage V r . Where the feedback DC voltage V DC  is less than the reference voltage V r , control passes back up to block  134 . Where the feedback DC voltage V DC  is greater or equal to the reference voltage V r , control passes to block  140 . At block  140 , on a phase-by-phase basis, where one of the phase voltages V R , V S  or V T  is negative, control passes back up to block  134 . However, on a phase-by-phase basis, where one of the phase voltages is positive, control passes to block  142  where, on a phase-by-phase basis, high frequency firing pulses R f , S f  and T f  are generated thereby turning on the associated SCRs  30 ,  32  and  34 , respectively (see again  FIG. 1 ). 
     It should be appreciated that a system has been described for relatively inexpensively pre-charging a DC bus capacitor using a hybrid SCR-diode rectifier bridge. It should also be appreciated that the system described does not require any type of phase tracking to determine when during positive half cycles of supply line voltages the SCRs should be turned on. Instead, because of the high frequency of the pulse signal P c , a firing pulse is effectively provided immediately when a phase voltage goes positive so that the SCRs are immediately full on whenever associated line voltages are positive. 
     What has been described above includes examples of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art may recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims for example, while the rectifier is described above as including three SCRs, other embodiments could use other types of switching devices. 
     To apprise the public of the scope of this invention, the following claims are made: