Abstract:
An AC/DC/AC power converter is constructed without using any electrolytic capacitor, such that it is more compact, durable and reliable. This converter only required a small capacitance for its DC link and this capacitor can be easily obtainable with other types of capacitors such as film or ceramic type. The system further includes means to disconnect both input and output to this DC bus capacitor. A controller capable of fast monitoring the DC bus voltage is also able of quickly disconnecting the capacitor out of either input or output energy path to prevent the capacitor from being charged to over-voltage. The controller also possesses capability of re-connecting the disrupted energy path once the DC bus voltage returns to normal.

Description:
FEDERAL SPONSORED RESEARCH &amp; DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to an alternate current to direct current and then back to alternate current (AC/DC/AC) power conversion system, and more particularly to an improved power structure coupled with a control means of an AC/DC/AC power conversion apparatus designed to convert fixed frequency AC source into variable frequency and variable voltage source for supplying power to an electric motor. 
     2. Background Arts 
     FIGS. 1 and 2 show power structures of conventional AC/DC/AC power converter for driving an AC motor. The converter typically includes a rectifier, a DC link and an inverter. Majority of ac drives do not need to provide regenerative feature and hence a diode rectifying front end is cost effective and suffices as the rectifier. The rectifier directs the AC power from the ac lines to provide power with a DC plus a six times line frequency component as input to the DC link capacitors. The inductor L on either the DC link or the input side is helpful in improving power factor of the power converter. The inverter modulates the DC voltage to generate variable frequency and variable voltage output to a motor. 
     The DC link capacitor section consists of at least two electrolytic capacitors C 1  and C 2  in series to have high enough voltage withstanding capability for the DC bus. Typical voltage rating for an electrolytic capacitor is about 450 VDC, not enough to work under a 650 VDC bus for an power converter. The series stack up of these electrolytic capacitors in turn requires the voltages across each capacitor to be equal and balanced. Such requirement is fulfilled by addition of R 1  and R 2  which may be made up by more than one single resistor to withstand enough watt loss. Resistors R 1  and R 2  supply a current leakage path to balance the capacitor voltages. R 1  and R 2  also serve to discharge C 1  and C 2  when the power converter is not in use. Capacitor C 3  is either a type of film or ceramic capacitor whose capacitance is much smaller than provided by C 1  and C 2 . C 3  is primarily a snubber capacitor for mitigating any high voltage transient caused by high slope switching from within the inverter section. 
     There are drawbacks of conventional power converter using electrolytic capacitors: 
     A large capacitance electrolytic capacitor which must withstand a high level of DC voltage has a relatively limited operating lifetime. The effective operating lifetime of a power converter as whole will in general be determined by this smoothing electrolytic capacitor. 
     Breakdown of such an electrolytic capacitor can cause serious damage to other components of the power converter, since leakage of corrosive electrolyte may occur, or the capacitor may even explode. 
     Because of electrolytic characteristics, other components such as balancing resistors and high frequency film capacitor are required. These additional components increase not only cost but also board space to mount them. 
     The smoothing electrolytic capacitors occupy a relatively large amount of space within the power converter, thereby reducing the freedom available for mechanical design of the converter, and causing the overall size of the power converter to be large. 
     The power converter manufacturers and research community recognize above problem and realize improvement of AC drive product can be made if electrolytic capacitors are to be replaced. U.S. Pat. Nos. 5,729,450, 5,623,399, 6,115,270 and 6,449,181B1 propose to use film capacitor in place of electrolytic ones to provide ripple current capability to the inverter section. U.S. Pat. No. 5,623,399, U.S. Pat. No. 6,115,270 and U.S. Pat. No. 6,449,181B1 are invented for new electric vehicle applications where a battery is readily available to serve as a huge energy storage component in front of the inverter. The other patent U.S. Pat. No. 5,729,450 is in effect teaching for inverting energy from a fuel cell DC source to AC power. The fuel cell has low impedance and again serves as the huge energy storage and imposes smoothing effort on the DC link. Some of above patents, for example U.S. Pat. No. 5,729,450 claims that their approach also applies to conventional AC power converter product with a non-regenerative front end such as diode bridge. We would like to dispute here. If we assume that the energy storage component (in this case battery or fuel cell in stead of electrolytic capacitors) is taken away from the circuit, there is little energy storage capability left by the film capacitor on the DC link. Conventional AC power converter has working condition where transient energy can come back from the motor to the DC link. In this case the DC link is easily over charged, resulting in DC link over-voltage. This over voltage may damage the converter components such as the switching transistors. Thus their solution is not readily applicable to a conventional AC power converter with a diode front end rectifier. 
     U.S. Pat. No. 5,481,451 also proposes film capacitor in place of the electrolytic capacitors however only focuses on how to re-shape the inverter output voltage and current to the motor since the DC bus voltage is no longer only a DC value. As a matter of fact, the patent does not indicate requirement of the kind of front end rectifier and for the same reason as discussed in previous paragraph, we know that the taught technique in U.S. Pat. No. 5,481,451 alone will not work for an AC power converter with diode front end. Additionally this technique limits the output voltage transfer ratio to 0.866 which is not tolerable to most of the AC power converter products. 
     When energy comes back from the AC motor, the DC link voltage can easily be over voltage if the AC drive employs non-regenerative frond end such as a diode bridge. Such over-voltage can be suppressed by a snubber circuit. U.S. Pat. Nos. 6,169,672B1, 5,561,596, 5,157,574, 4,843,533 and 4,646,222 propose this kind of circuits with substantially more components either on DC link, ac input or output to suppress the over voltage spike. There are cases where active switch is employed and further requires active control from a controller. Complexity and part counts are increased and hence also the cost. 
     One particular solution for the DC link over voltage problem is to replace the non-regenerative diode front end with a regenerative-capable rectifier. Kim et al, in a technical paper “ AC/AC Power Conversion Based on Matrix Converter Topology with Unidirectional Switches ” published in IEEE Transaction on Industry Application of January/February 2001, pages 139-145, taught such solution. A regenerative capable rectifier usually consists of a boost type converter which utilizes active switches. Cost again is an issue for those applications that do not require regenerative feature. 
     U.S. Pat. No. 5,825,639 introduces a diode and a resistor to isolate the electrolytic and film capacitors. Such approach intends to let the film capacitor handle majority of the inverter ripple current and to reduce the stress to the electrolytic capacitors and their capacitance. However, a continuous loss through the additional resistor is present in order to render effectiveness from the electrolytic during transients. Besides the system suffers poor efficiency, additional diode and resistor also offset the cost saving from the reduction of electrolytic capacitance. Such arrangement is not able to deliver storage energy from the electrolytic capacitors during ride-through. The electrolytic capacitor, resistor and the diode taught by U.S. Pat. No. 5,825,639 is only a powerful over voltage clamp. 
     Part of above aforementioned inventions teaches inverter structure with reduced capacitance by addition of snubber circuit. The resultant power converter is costly, less efficient as well as bulky. Other part of above aforementioned inventions teaches reduced capacitance inverter to interconnect either to a large storage DC source or a regenerative rectifier. Both configurations results in either regulated or slow moving DC bus voltage. The speed to feedback this bus voltage is furnished by the controller of the power converter, and is not required as fast. When a non-regenerative rectifier is used in place of above large DC source or a regenerative rectifier and feeds input to this reduced capacitance inverter, the bus is neither regulated nor slow moving. DC bus voltage feedback quality by conventional controller deteriorates significantly during fast bus voltage transient. 
     Now it becomes apparent that none of above solutions is suitable or adequate to make a reduced capacitance DC/AC inverter capable of working with a non-regenerative diode rectifier. It is the objective of the present invention to overcome aforementioned drawbacks. 
     SUMMARY OF THE INVENTION 
     It is the objective of the present invention to construct AC/DC/AC power converter systems without using any electrolytic capacitor, such that the converter is more compact, durable and reliable. These converter systems only require a small capacitance easily obtainable with other types of capacitors such as film or ceramic type. The system further includes means to disconnect both input and output to the DC bus capacitor. A controller capable of fast monitoring the DC bus voltage is also part of the system. The controller is also able of quickly disconnecting the capacitor out of either input or output energy path to prevent the capacitor from being charged to over-voltage. The controller further possesses capability of re-connecting the disrupted energy path once the DC bus capacitor voltage returns to normal. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described in connection with certain preferred embodiments with reference to the following illustrative figures so that it may be more fully understood. 
     With specific reference now to the figures in detail, it is stressed that the particular shown are by way of example and for purpose of illustrative discussion of the preferred embodiments of the present invention only. They are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description together with the drawings should make it apparent for those skilled in the art how the two forms of the invention may be embodied in practice. In the drawings: 
     FIG. 1 is a prior art of conventional AC/DC/AC power converter showing details of its DC link structure consisting of electrolytic capacitors C 1  and C 2 , their balancing resistors R 1  and R 2 , a high frequency capacitor C 3  and a DC inductor L. 
     FIG. 2 is another prior art of conventional AC/DC/AC power converter showing details of its DC link structure consisting of electrolytic capacitors C 1  and C 2 , their balancing resistors R 1  and R 2 , a high frequency capacitor C 3 . The inductor is an AC one and now located on the input side. 
     FIG. 3 a  is the first embodiment of a reduced capacitance AC/DC/AC converter, showing an inverter connected to a non-regenerative diode rectifier via a LC filter on the DC bus. The controller has a fast feedback path from the DC bus and controlling paths for engaging the input and output to the link capacitor independently. 
     FIG. 3 b  shows a single phase AC rectifier configuration that can replace the three phase version  10  in FIG. 3 a.    
     FIG. 3 c  shows an alternative precharge comprising of a semiconductor switch  21   c.    
     FIGS. 3 d  to  3   f  shows various arrangements in the DC link. 
     FIG. 4 is a preferred embodiment of the present invention on the controller function how to disconnect and connect the DC bus capacitor in case of over-voltage protection. 
     FIG. 5 a  is the second embodiment of a reduced capacitance AC/DC/AC converter, showing an inverter connected to a non-regenerative diode rectifier via a DC bus capacitor. An AC inductor between the AC input source and the non-regenerative diode rectifier forms the LC filter with the bus capacitor. The controller has a fast feedback path from the DC bus and controlling paths for engaging the input and output to the link capacitor independently. 
     FIG. 5 b  illustrates an arrangement to accept single phase AC source. 
     FIGS. 5 c  and  5   d  shows semiconductor switch in the precharge for three and single phase arrangements respectively. 
     FIG. 5 e  shows the precharge arranged between the AC inductor and rectifier. 
    
    
     DETAIL DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An AC/DC/AC power converter constructed according to the preferred embodiment of the present invention is indicated generally at  1  in FIG. 3 a . A three-phase AC power source  3  supplies energy through wires  81 ,  82  and  83  to converter  1 . Within converter  1 , diodes  11 - 16  forms a unidirectional three-phase rectifier  10  only capable of passing the input energy from AC source  3  to the DC link  30 , not vice versa. Cathodes of diodes  11 ,  13  and  15  are connected together to form a positive terminal  71 . Anodes of diodes  14 ,  16  and  12  are tied similarly to form a negative terminal  72 . Anode of diode  11  and cathode of diode  14  are connected together to wire  81  via node  17 . Similarly anode of diode  13  and cathode of diode  16  joint to node  18  and connect to wire  82 . Anode of diode  15  and cathode of diode  12  tie to node  19  and wire  83 . Rectifier  10  rectifies ac voltage from AC source  3  and presents a voltage consisting of a DC average voltage plus multiples of 6 th  harmonics between positive and negative terminals  71  and  72 . Terminals  71  and  72  serve as the input points to DC link  30  of converter  1 . When converter  1  is fed from a single phase AC source, three phase rectifier  10  in FIG. 3 a  can be replaced by a single phase rectifier  10   b  as illustrated in FIG. 3 b . In FIG. 3 b , single phase AC energy comes from wires  81   b  and  82   b . Wires  81   b  and  82   b  connect to diodes  11  and  13  to form positive bus  71  and to diodes  14  and  16  to form negative bus  72 . 
     DC link  30  consists of a precharge circuit  20 , DC inductors  31  and  32 , as well as a capacitor  40 . A resistor  22  and a relay  21   a  are connected in parallel to form precharge circuit  20 . Alternately the precharge circuit can be formed by substituting relay  21   a  with a semiconductor switch  21   c  like in FIG. 3 c . When converter  1  starts up, capacitor  40  has zero or minimum initial voltage. Relay  21   a  is initially open enabling resistor  22  to be in the precharge path to limit the current inrush from AC source  3  via diode rectifier  10 . Once capacitor  40  is charged, relay  21   a  is closed to provide major current path and hence to bypass resistor  22 . Only after relay  21   a  is closed, converter  1  is allowed to delivery energy to its output, for example an electric motor  2 . The rectifier positive output terminal  71  is connected to one end of precharge circuit  20 . 
     Precharge circuit  20  is in series with inductor  31 . Inductor  31  is arranged to be in the positive DC bus path. Inductor  31  is further connected to one side of the capacitor  40 , forming a positive terminal  73  to an inverter section  50 . The other side of the capacitor  40  is connected to another DC inductor  32 , forming a negative node  74 . Nodes  73  and  74  are the output terminals from DC link  30  to inverter  50 . The other side of the DC inductor  32  is connected to negative terminal  72  of rectifier  10 . It is understandable that inductors  31  and  32  can be constructed either in separate magnetic cores or in one core. In addition to possess differential inductances, inductors  31  and  32  may also be constructed to possess some common mode inductances impeding to any leakage current running through ground. Construction of such common mode impedance from the same core material from inductors  31  and  32  is well known in the art. It is also understandable that precharge circuit  20  is not limited to locate at the positive terminal  71  of rectifier  10  and before inductor  31 . Its three other locations within DC link  30  are between inductor  31  and capacitor  40  (FIG. 3 d ), capacitor  40  and inductor  32  (FIG. 3 f ), as well as between inductor  32  and negative terminal  72  of rectifier  10  (FIG. 3 e ). 
     In order to avoid disadvantages from electrolytic capacitors and their associated circuit, a capacitor of other kind, such as film or ceramic, is used for capacitor  40 . To have a voltage rise limit ΔV C     —     limit  based upon maximum amount of ripple current I ripple  required from inverter  50 , capacitor  40  is selected with following formula:              C   =         I   ripple          T   s         Δ                   V     C_lim                 it                   (   1   )                                
     where T s  is the switching cycle time within inverter  50 . As an example, a 0.75 Hp, 460V electric motor requires 1A I ripple , the capacitance is calculated to be 2.5 μF if inverter  50  is to operate at 4 kHz and to allow a 100V DC voltage rise. A typical conventional AC/DC/AC drive incorporates 250 μF capacitance, a total of 100 times higher value. A 2.5 μF capacitor is easily achievable by material other than electrolytic. 
     Inverter section  50  takes the DC power from DC link circuit  30  via positive bus terminal  73  and negative bus terminal  74 . Inverter  50  consists of six diodes  91 - 96  and six semiconductor switches  51 - 56 , such as insulated gate bipolar transistors (IGBT). Switch  51 , when gated on, can only conduct unidirectional current from top to bottom. Switch  51  blocks current from either direction when gated off. Diode  91  is connected to switch  51  in parallel such that diode  91  conducts free-wheeling current from bottom to top. Diode  91  and switch  51  form a diode-switch block  97 . In similar manner the other switches  52 - 56  and diodes  92 - 96  are arranged for controlled and free-wheeling current paths. Two diode-switch blocks are connected in series to form a single leg, with their middle point serving as one of the inverter output. Since we have 6 diode-switch blocks, we can form three phase legs and have three output terminals  84 - 86 . Via wires  84 - 86  output nodes  87 - 89  give variable frequency and magnitude power source to load of converter  1 . The load of converter  1  can be, for example, an electric motor  2 . 
     In order to complete an AC/DC/AC power converter system with reduced DC link capacitance, a controller  60  is further incorporated. Controller  60  is responsible for observing necessary system variables, such as output currents, DC link voltage and temperature etc. Based upon observed variation of these variables, controller  60  adjusts gating outputs to the controllable components in converter  1 , i.e. switches  51 - 56  in inverter  50  and relay  21  in the precharge circuit  20 . Normal control for converter  1  to deliver variable output voltage source is known in the art. 
     Since capacitance value of capacitor  40  within DC link  30  is reduced hundreds of times, there is little energy storage capability within converter  1 . There are cases where energy flow through converter  1  can be disrupted and the DC bus voltage across capacitor  40  is charged up rapidly to its over-voltage range. This over-voltage on capacitor  40  also appears on inverter bridge  50  and may exceed voltage rating of individual semiconductor device, resulting in damage and hence malfunction of converter  1 . One scenario where DC link voltage may rise to its over-voltage range occurs after line loss. Since AC power energy at input of converter  1  is not guaranteed to be available all time, there is occasion when AC power source  3  losses its energy for a portion of time, usually comparable to its frequency cycle (16 ms for 60 Hz utility). During this line loss interval, converter  1  continues to supply power to its load  2 , depleting energy out from capacitor  40  quickly. Consequently DC bus voltage across capacitor  40  drops very fast. Conventional converter stores enough energy with much bigger electrolytic capacitors in DC link  30 , allowing a much slower depletion, hence slower DC bus voltage drop. Conventional controller is designed to work with such slow moving DC bus voltage and hence is slow reacting to such fast transient if used for reduced capacitance power structure. When the AC power source  3  recovers, relay  21  may be still closed with such conventional controller. At this moment AC power source  3  is to charge the capacitor  40  from a low initial voltage, resulting in high inrush current and fast voltage swing to over-voltage range. 
     In order to prevent above fast rising over-voltage, energy flow path to capacitor  40  has to be disconnected by fast controller action. For example, controller  60  is to open relay  21  as soon as the bus under-voltage is detected. Bus under-voltage is an indication of line loss condition. This requires a fast and accurate link voltage feedback path  61 , preferably in tens of micro-seconds respond time. Typical conventional bus feedback takes in ten of milliseconds feedback time. Controller  60  will turn off switches  51 - 56  if it judges that the bus over-voltage is due to energy back flow from load  2  via inverter  50 . 
     FIG. 4 illustrates the preferred embodiment of the present invention for features of controller  60  for fast converter protection by sensing DC link bus voltage across capacitor  40 . The sampling frequency of this voltage is a fast loop within the controller, preferably within tens of micro-seconds. When sample time comes, voltage across capacitor  40  is sampled. Due to the requirement of fast sampling, feedback path from capacitor  40  should not present much delay (for example, preferably less than 1 micro-second). The sampled voltage V cap  is compared to a line loss threshold. If V cap  is less than the threshold, it indicates that AC power source  3  experiences a line loss. Controller  60  reacts to turn off relay  21  of FIG.  3 . If V cap  is not less than the line loss threshold, it continues to compare to a over-voltage threshold. If the comparison result indicates a bus over-voltage condition, controller  60  inhibits gating signals to switches within inverter  60  via line  62 . Sequence of comparing V cap  to line loss and bus over-voltage thresholds can be exchanged. After stopping energy coming from its source, controller  60  starts to monitor the bus voltage V cap . When V cap  recovers back to its normal range, normal operation of converter  1  is re-issued by controller  60 . 
     A second AC/DC/AC embodiment of the present invention is illustrated in FIG. 5 a . Converter  5  accepts three-phase AC power from source  3 . The converter  5  consists of an AC input section  300 . AC input section  300  further includes three-phase AC contactor and a three-phase inductor  304 . AC source  3  connects to a three-phase AC precharge contactor  200 . AC three-phase contactor  200  has two single phase contactors  201  and  202 . Two resistors  203  and  204  are connected across these two contactors  201  and  202 . When the AC power source  3  is first turned on, contactors  201  and  202  are open. Resistors  203  and  204  limit the inrush current to converter  5 . Once the DC link bus voltage across capacitor  40  is charged up, contactor  201  and  202  are commanded to close from controller  60  via control line  63 . It is understandable that the same precharge feature is preserved if contactors  201  and  202  and their associated resistors  203  and  204  are arranged to any two phases of three phase input lines  305 - 307 . Normally a three-phase contactor with three single phase contacts is more readily available than one with two single phase contacts. In this case the third phase contact is inserted between input line  306  and inductor  302 . For the same precharge function, another alternative to AC contactor  304  is to use a DC relay, as  20  from FIG. 3, between the diode rectifier  10  and capacitor  40 . It is also possible to configure converter  5  to accept single phase AC power by replacing the three-phase input blocks  300  and  10  with  300   b  and  10   b  of FIG. 5 b . Semiconductor switches from FIG. 5 c  and  5   d  can be used in place of  201  and  202 . 
     Three-phase contactor  200  connects to a three-phase inductor  304  consisting of three single phase inductors  301 - 303 . Three-phase inductor  304  is in the circuit for input current power factor improvement and blocking high switching frequency noise from entering to AC power source  3 . Inductors  301 - 303  can be wound on a single magnetic core or common mode inductance can be provided through three independent cores. The precharge circuit  200  and three-phase inductor  304  can be also arranged as shown in FIG. 5 e  with inductor  304   e  in front of precharge circuit  200   e . A three-phase diode bridge  10  is connected to this three-phase inductor  304 . Construction of rectifier  10  from diodes  11 - 16  is the same as from FIG.  3 . The single DC link capacitor  40  is connected to rectifier  10  directly. Capacitance selection of  40  still follows equation (1). In this embodiment capacitor  40  and three-phase AC inductor  304  form a filter effectively preventing high switching noise from going back to input lines  305 - 307 . 
     Terminals from capacitor  40  are DC link bus terminals  73  and  74 . Inverter  50  takes DC power from these two terminals  73  and  74 . Controller  60  incorporates features from FIG. 4 for fast system protection and sends out commanding signals via  62  to inverter  50  for switch actions among switches  51 - 56 . Regulated variable magnitude and frequency voltage is delivered to load  2  via output wires  84 - 86 . 
     It will be evident to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments and that the present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive. The scope of the invention will be indicated by the appended claims rather than by the foregoing description. And all changes, which come within the meaning and range of equivalency of the claims, are therefore intended to be embraced therein.