Patent Document

FIELD OF THE INVENTION 
     This invention relates to power generating systems such as alternators and power generators, and more particularly to an alternator/inverter having a dual H-bridge for providing 120 volt AC or 240 volt AC power. 
     BACKGROUND OF THE INVENTION 
     Present day portable generators typically make use of a synchronous alternator or a cycloconverter for providing the desired power output, which is typically either 120 volts AC or 240 volts AC. Important considerations for any portable generator are: 
     Voltage regulation; 
     Dual voltage output capability; 
     Idle voltage and frequency; 
     Frequency tolerance; 
     Harmonic distortion: 
     Induction motor operation 
     Charger operation 
     Grounding configuration; 
     4-blade (120-240 volt) twist-lock compatibility; 
     Response to load changes; and 
     Size and weight. 
     With regard to idle voltage and frequency, it is far easier to provide 120 volts and 60 Hz at idle using electronic solutions (i.e., inverter technology) than it is with synchronous alternators. However, sufficient “head room” is still required. To provide 120 volts at 2100 rpm requires a DC bus voltage of 298 volts versus the 225 volts estimated for regulation head room. This higher voltage requires more turns in the alternator coils resulting in an increased coil resistance and reduced system efficiency. 
     Harmonic distortion present in the output waveform of a portable generator is another important consideration that must be addressed. While waveform purity is of little importance to universal motor-powered portable power tools, it is an important consideration when running induction motors and chargers. Induction motors will run on distorted waveforms, but the harmonic content of the input will be converted to heat, not torque. The extra heating from the harmonics must be quantified if a inverter topology which produces a distorted waveform is to be implemented. A sine wave pulse width modulated (PWM) inverter will produce excellent waveforms with only some high frequency noise, but they are likely to require full H-bridges which, traditionally, have not been easily adaptable to the North American grounding convention and the 4-blade twist-lock wiring convention. 
     With regard to grounding configurations, in North America, the standard grounding convention requires that one side (neutral) of each 120 volt circuit is grounded. This means that 240 volt circuits have floating grounds. It is difficult to achieve this standard grounding convention with sine wave PWM inverters that require full H-bridges. It is possible to meet this convention through the use of two half bridges, but such a circuit may be limited to quasi-sine wave outputs which have high harmonic content. 
     Still another important consideration is 4-blade (120-240 volt) twist lock compatibility. This convention requires four wires: ground, neutral, 120 volt line  1  and 120 volt line  2 . Each 120 volt circuit is connected between a 120 volt line and neutral. The 240 volt circuit is connected between the 120 volt line  1  and the 120 volt line  2 . Heretofore, it has been possible to fit the convention with a dual half bridge circuit, but not a full H-bridge circuit that would be required for sine wave PWM inverters. 
     The ability of a generator to respond to load changes is still another important consideration. All inverter topologies will provide a faster response to load changes than a synchronous alternator, due to the large field inductance used by a synchronous alternator. 
     Concerning size and weight, it would also be desirable to make use of inverter topology because virtually any inverter topology will provide size and weight benefits over that of a synchronous alternator. However, trying to produce sine waves from a two half bridge circuit may require large capacitors that would reduce the benefit of volume reduction provided by the inverter topology. 
     In view of the foregoing, it is a principal object of the present invention to provide a generator which meets the grounding convention used in North America through the use of inverter technology. It is still a further object of the present invention to provide a generator using inverter technology which can provide either 120 volt or 240 volt outputs and still meet the grounding convention used in North America. 
     Still further, it is an object of the present invention to provide a generator using inverter topology which meets the 4-blade twist-lock compatibility requirements. 
     SUMMARY OF THE INVENTION 
     The above and other objects are provided by an alternator/inverter system having dual alternator/inverter sections, with each section including a full H-bridge inverter circuit. Each alternator/inverter section incorporates an independent permanent magnet generator winding which is coupled to an independent full wave bridge rectifier circuit. Each rectifier circuit provides a DC voltage to its associated full H-bridge circuit. The first H-bridge circuit includes a first output point and a second output point while the second H-bridge circuit includes a third output point and a fourth output point. The second and third output points are coupled together as a neutral node and connected to ground. A first AC receptacle is coupled across the first output point and neutral. A second AC receptacle is also coupled across neutral and the fourth output point. A third AC receptacle is coupled across the first and fourth output points. Coupled across the third AC receptacle is an electronically controlled switch for selectively shorting the third AC receptacle. The switch is controlled by an electronic controller which also controls operation of each of the H-bridge circuits. A user switch allows a user to select a first mode of operation wherein full power developed by the alternator/inverter system may be drawn from either of the first or second AC receptacles, or a second mode of operation in which the third AC receptacle is operable. In one preferred embodiment, each of the first and second AC receptacles provide 120 volts AC, and the third AC receptacle provides 240 volts AC. In the second mode of operation, only half the total ampere generating capacity of the system is available at either of the first and second 120 AC receptacles as compared to that which would be available if the alternator/inverter system was operating in the first mode of operation. Importantly, the present invention adheres to the grounding convention used in North America in which one leg of each of the first and second AC receptacles is tied to ground. 
     The alternator/inverter of the present invention further provides excellent control over the harmonic distortion of the output waveform. The use of inverters allows a faster response to load changes than what would be obtainable with a synchronous alternator. 
     In alternative preferred embodiments, both analog and digital voltage regulation circuits are incorporated into the alternator/inverter system of the present invention. The voltage regulation circuits are employed together with active rectifiers to control the DC bus voltage across each inverter between predetermined upper and lower limits. In this manner, losses associated with electrical cabling coupled to the outputs of the two inverters, as well as losses associated with the inverters themselves, can be compensated for. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a simplified schematic diagram of an inverter/alternator with dual H-bridges for providing 120 volt or 240 volt operation, in accordance with a preferred embodiment of the present invention; and 
     FIG. 2 is a simplified schematic drawing of an alternative preferred embodiment of the present invention incorporating a digital DC voltage regulation system for compensating for the DC bus voltage drop caused by a power cabling coupled to the alternator/inverter; 
     FIG. 3 is a simplified schematic drawing of another alternative preferred form of the voltage regulation circuit, which involves the use of an analog comparator; and 
     FIG. 4 is still another alternative preferred embodiment of the voltage regulator circuit used with the alternator/inverter system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
     Referring to FIG. 1, there is shown an alternator/inverter system  10  in accordance with a preferred embodiment of the present invention. The system  10  is ideally suited for use in a portable electric power generator, however, it will be appreciated that the invention is not so limited and may find utility in a variety of related power generating applications. 
     The system  10  includes two identical alternator/inverter sections or subsystems  12  and  14 . Alternator/inverter section  12  includes a three phase permanent magnet generator (PMG)  16  for providing a three phase AC output signal to a full wave bridge rectifier circuit  18 . Rectifier circuit  18  is coupled across DC bus lines  20  and  22  which form a DC bus. Coupled across the DC bus is a first, full H-bridge circuit  24  comprised of four identical power switching devices  26   a - 26   b . An inductor  28  and a capacitor  30  are coupled across points  32  and  34  and form an LC filter for attenuating harmonic distortion in the output waveforms generated by the H-bridge  24 . Point  36  forms a first output and point  34  forms a second output. Each of the power switching devices  26   a - 26   d  may comprise a variety of suitable power switching components, but in one preferred form comprise insulated gate bi-polar transistors (IGBTs). A DC bus capacitor  38  is also coupled across the DC bus. 
     The second alternator/inverter section  14  is identical in construction to the first alternator/inverter section  12  and includes a three phase permanent magnet generator  40  providing an AC output to a full wave bridge rectifier circuit  42 . Bridge rectifier circuit  42  is coupled across DC bus lines  44  and  46  and across DC bus capacitor  48 . The DC output from the rectifier  42  drives a second, full H-bridge circuit  50  having four power switching devices, which in this example are illustrated as IGBTs  52   a - 52   d . Coupled between points  54  and  56  are a capacitor  58  and an inductor  60  which form an LC filter for attenuating harmonic distortion in the output waveforms produced by the H-bridge  50 . Point  54  forms a third output point and point  62  forms a fourth output point. 
     A first AC power receptacle, in this example a 120 volt AC receptacle  64 , is coupled across first output point  36  and the second output point  34  by the connection to ground. A second AC power receptacle, illustrated as a 120 volt AC receptacle  66 , is similarly coupled between the fourth output point  62  and the third output point  54 , via the connection to ground. Coupled across output points  36  and  62  is a third AC receptacle, which in this example is illustrated as a 240 volt AC receptacle  68 . 240 volt AC receptacle  68  also has coupled in parallel with it a power relay  70  which is controlled by a controller  72 . The controller  72  operates to switch the contacts of the power relay  70  between an open condition, wherein the 240 volt AC receptacle  68  receives the output across points  36  and  62 , and a closed position in which the receptacle  68  is shorted by the power relay  70 . A user switch  74  allows a user to provide a signal to the controller  72  to select whether the 240 volt AC receptacle  68  is switched “ON” for use or not. The controller  72  also provides pulse width modulated (PWM) control signals to each of the H-bridges  24  and  50  to control switching of the IGBTs  26  and  52  to produce the desired AC output waveforms across points  34 ,  36  and  54 ,  62 . 
     In operation, a DC bus voltage of preferably around 200-220 volts is provided across the DC bus lines  20 ,  22  and  44 ,  46 . The controller  72  controls the first H-bridge  24  such that IGBTs  26   a  and  26   b  are switched on while IGBTs  26   c  and  26   d  are off. IGBTs  26   a  and  26   b  are then turned off while IGBTs  26   c  and  26   d  are turned on. The second H-bridge  50  is controlled in the same fashion by first turning on IGBTs  52   a  and  52   b  while IGBTs  52   c  and  52   d  are turned off, and then turning on IGBTs  52   c ,  52   d  while IGBTs  52   a  and  52   b  are turned off. The controller  72  switches the H-bridges  24  and  50  on and off using a well known sine wave PWM pattern that produces a constant frequency sine wave output. In the present embodiment, this provides 120 volts AC across capacitor  30  and 120 volts AC across capacitor  58 . 
     When the power relay  70  is in the closed position, the first AC receptacle  64  and the second AC receptacle  66  are coupled in parallel. Thus, each AC receptacle  64  and  66  is able to receive the full ampere output from the system  10 . By that it is meant that the full ampere generating capacity of the system  10  is available to either AC receptacle  64  or  66 . If both AC receptacles  64  and  66  are used, then the full current generating capacity of the system  10  will be split between the AC receptacles  64  and  66  according to the loads imposed by the devices coupled to the AC receptacles  64  and  66 . The 240 volt AC receptacle  68  is shorted and inoperable when the power relay  70  is closed. 
     When a 240 volt AC load is to be driven by the system  10 , the user selects switch  74 , which in turn sends a signal to the controller  72  to open the switch contacts of the power relay  70 . In this condition (shown in FIG.  1 ), the 240 volt AC receptacle  68  is then effectively placed across output points  36  and  62 . The controller  72  also controls the second. H-bridge  50  such that the 120 volt AC output across capacitor  58  is 180° out of phase with the 120 volt AC output across capacitor  30 . Thus, a 240 volt potential difference exists between output points  36  and  62 . It will be appreciated, however, that the first H-bridge  24  could also be controlled by the controller  72  such that its output is changed in phase by 180° instead of the output of the second H-bridge  50 . When the 240 volt AC receptacle  68  is operable, only one half of the total ampere generating capacity of the system  10  will be available to each of the first AC receptacle  64  and the second AC receptacle  66 . 
     Importantly, the system  10  adheres to the wiring convention used in North America which provides for one leg of each 120 VAC receptacle  64  and  66  to be tied to ground. The system  10  also provides 4-blade (120-240 volt) twist-lock compatibility. The use of inverters provides a faster response to load changes than would otherwise be possible with a conventional synchronous alternator with its typically large field inductance. The use of inverter technology also allows the system  10  to be made smaller and lighter than what would be possible with a conventional synchronous alternator. 
     Referring now to FIG. 2, an alternator/inverter  100  in accordance with an alternative preferred embodiment of the present invention is shown. The alternator/inverter  100  is identical in construction to the alternator/inverter  10  with the exception of a pair of voltage regulation circuits  180  and  182 . For convenience, the components of system  100  identical to those of system  10  have been labeled with reference numerals increased by  100  over those used in connection with system  10 . The overall operation of the two alternator/inverter circuits  112  and  114  is identical to that provided in connection with the description of operation of system  10 , and will therefore not be repeated. Furthermore, since the components of each of the voltage regulation circuits  180  and  182  are identical in construction and operation, only the construction and operation of circuit  180  will be described. 
     It will be appreciated that good voltage regulation is an important attribute of any electric power generation system. Since the user will generally be using power at the end of an extension cord, it is desirable to compensate for the voltage drop in the electrical power cable. This can be done by monitoring the AC output voltage and current in the inverters  124  and  150 , but measuring DC currents and voltages is easier and can be done faster. Thus, the voltage regulation circuits  180  and  182  operate to control the DC bus voltage of each alternator/inverter section  112  and  114  independently and compensate for not only the voltage drop of the extension cord, but the drops caused by the inverters  124  and  150  as well. 
     Referring further to FIG. 2, a plurality of three silicon controlled rectifiers (SCRs)  118   a  are substituted for three of the conventional diodes used with rectifier  18  of system  10 . Each of the SCRs  118   a  has its gate  118   b  coupled to an output of a gate driver circuit  186 . The gate driver circuit  186  receives an output from a microcomputer  188 , which in turn receives a signal from a conventional current sensing circuit (i.e., shunt)  190  and a differential DC voltage signal representing the potential difference between the two DC bus lines  120  and  122 . The microcomputer  188  preferably comprises an 8-bit microcontroller such as the MC68HC08MR4 available from Motorola, but it will be appreciated that a variety of other suitable controllers could be implemented as well. 
     In operation, the current sensing circuit  190  senses a change in the DC current flowing in DC bus line  122  and provides an output indicative of same to the microcomputer  178 . Simultaneously, the microcomputer  188  measures a differential voltage between bus lines  120  and  122  via circuit lines  192  and  194 . The microcomputer  188  includes an internal look-up table for providing a “V ref ” value needed to adjust the DC output voltage of the system  100 . The V ref  vs. DC current look-up table is constructed using an assumed value of internal resistance (H-bridge and AC filter) and an assumed value of extension cord resistance. An exemplary table, as shown below, increases the V ref  (and, therefore the DC bus voltage) such that the output voltage of the system  100  increases linearly with increased current until the output voltage reaches 126 volt (a limit set by regulatory agencies). At this point, the slope of the DC bus voltage vs. current curve changes so as to maintain the 126 volts at the output terminals of the H-bridge  124 . The voltage at the end of the cable will equal the output voltage minus the IR drop of the particular cable used. 
     
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 Vref vs. DC Current Look-up Table 
               
             
          
           
               
                   
                 I load   
                 V bus (DC)   
                 V out (RMS)   
                 V cable end (RMS)   
               
               
                   
                   
               
             
          
           
               
                   
                 0 
                 169.7 
                 120.0 
                 120.0 
               
               
                   
                 2 
                 173.4 
                 120.6 
                 120.0 
               
               
                   
                 4 
                 177.1 
                 121.3 
                 120.0 
               
               
                   
                 6 
                 180.9 
                 121.9 
                 120.0 
               
               
                   
                 8 
                 184.6 
                 122.5 
                 120.0 
               
               
                   
                 10 
                 188.3 
                 123.2 
                 120.0 
               
               
                   
                 12 
                 192.0 
                 123.8 
                 120.0 
               
               
                   
                 14 
                 195.8 
                 124.4 
                 120.0 
               
               
                   
                 16 
                 199.5 
                 125.1 
                 120.0 
               
               
                   
                 18 
                 203.2 
                 125.7 
                 120.0 
               
               
                   
                 20 
                 206.4 
                 126.0 
                 119.7 
               
               
                   
                 22 
                 209.3 
                 126.0 
                 119.0 
               
               
                   
                 24 
                 212.1 
                 126.0 
                 118.4 
               
               
                   
                 26 
                 214.9 
                 126.0 
                 117.7 
               
               
                   
                 28 
                 217.8 
                 126.0 
                 117.1 
               
               
                   
                 30 
                 220.6 
                 126.0 
                 116.5 
               
               
                   
                 32 
                 223.4 
                 126.0 
                 115.8 
               
               
                   
                 34 
                 226.2 
                 126.0 
                 115.2 
               
               
                   
                 36 
                 229.1 
                 126.0 
                 114.6 
               
               
                   
                 38 
                 231.9 
                 126.0 
                 113.9 
               
               
                   
                 40 
                 234.7 
                 126.0 
                 113.3 
               
               
                   
                   
               
             
          
         
       
     
     The output of the current sensing circuit  190  “I” is a measure of the “IR” drop due to the resistance of the cables coupled to the outlets  164 ,  166  orf  168 , and the voltage drop due to the losses associated with the inverter  112  and the output filter formed by inductor  128  and capacitor  130 . The microcomputer  188  uses the measured DC current “I” to obtain the current value for V ref  from its internal look-up table. The microcomputer also measures the DC bus voltage, “V bus ” between the two DC bus lines  120  and  122 . When the microcomputer  188  detects that the DC bus voltage, “V bus ”, is lower than the current value of V ref , then it signals the gate driver circuit  186  to turn on the SCRs  1118   a , thus charging the DC bus capacitor  138 . The rectifier  118  functions as a normal six diode bridge when the SCRs are on. When the microcomputer  188  detects that the DC bus voltage exceeds the present value for V ref , then it signals the gate driver circuit  186  to turn off the SCRs  118   a . In this manner, the microcomputer  188  continuously monitors and adjusts the DC bus voltage to compensate for the above-described losses. The PWM duty cycle of the signal used to control H-bridge  124  is not changed during the process of adjusting the SCRs  118   a  to compensate for changes in the DC bus voltage. 
     Referring to FIG. 3, another alternative preferred embodiment  200  of the present invention is shown. Embodiment  200  is also identical in construction and operation to the system  100  of FIG. 2 with the exception of the use of a pair of analog voltage regulation systems  280 . Again, the components in common with the system  10  are designated by reference numerals increased by 200 over those used in connection with FIG.  1 . 
     The voltage regulation system  280  comprises a current shunt  282 , a voltage divider network  284 , a gate driver circuit  285 , and a “V+IR” compensation circuit  286 . The current shunt  282  is inserted into the lower DC bus rail  222  to measure DC current (“I”). The voltage across the lower resistor of the divider network  284  is a fraction of the DC bus voltage. The center node of the divider network is connected to the inverting input of a comparator  288  of the compensation circuit  286 . The current signal from the left side of the shunt  282  will be negative with respect to the signal ground when the bus capacitor  238  is supplying power to the H-bridge  224 . Therefore, the current signal is inverted and amplified via an inverting amplifier  290  of the compensation circuit  286 , with a gain of “R”. The “IR” signal is added to a fixed voltage reference “V ref ”. The output of an adder  292  of the compensation circuit  286  (V ref +IR) is fed to the non-inverting input of the comparator  288 . When the DC bus voltage (Vbus) across the DC bus capacitor  238  exceeds the value of “V ref  +IR”, the comparator  288  sends a low signal to the gate driver circuit  285  which turns off all the SCRs  218   a . When the DC bus voltage is lower than the value of “V ref +IR”, then the comparator  288  sends a high signal to the gate driver circuit  285 &#39;which turns on all of the SCRs  218   a . The 3-phase bridge rectifier circuit  218  then recharges the DC bus capacitor  238 . 
     Referring to FIG. 4, the system  200  is shown with simplified voltage regulation circuits  300  and  302  incorporated. Since the circuits  300  and  302  are identical in construction and operation, only circuit  300  will be described. Circuit  300  represents an even less complicated means for implementing the “V+IR”control described above. Circuit  300  includes a current shunt  304  which is inserted into the bottom rail  222  of the DC bus to measure current (“I”). A resistor divider network  306  is again coupled across the DC bus lines  220  and  222 , but now it is located to the left of the current shunt  304 . The signal ground on the bottom DC bus rail  222  is still to the right of the current shunt  304 . The center node of the divider  306  is still connected to an inverting input of a comparator  308 . 
     The “I” signal from the left side of the shunt  304  still will be negative with respect to the signal ground when the DC bus capacitor  238  is supplying power to the H-Bridge  224 . The “IR” drop of the current shunt  304  will be negative with respect to the DC voltage (Vbus&#39;) at the center node of the divider  306 . Therefore, the signal to the inverting input of the comparator  308  will be “Vbus&#39;−IR”. The non-inverting input of the comparator  308  is connected to the reference voltage (“V ref ”). When “Vbus&#39;−IR” is greater than V ref , the comparator  308  sends a low signal to a gate driver circuit  310  which turns off all the SCRs  218   a . When “Vbus&#39;−IR” is less than V ref , the comparator 308 sends a high signal to the gate driver circuit  310  which turns on all the SCRs  218   a . However, “Vbus &#39;−IR”&gt;than V ref  is equivalent to Vbus&#39;&gt;V ref  +IR, and Vbus&#39;−IR &lt;V ref  is equivalent to Vbus&#39;&lt;V ref +IR. Thus, the same function is achieved with fewer parts. 
     The voltage regulation circuits  180 ,  280  and  300  thus provide a means for controlling the DC bus voltage of each of the inverters of the present invention to thereby compensate for losses associated with electrical cabling coupled to the AC receptacles, as well as internal losses of each of the inverters. 
     The various preferred embodiments of the present invention also provide for an alternator/inverter system which meets the grounding convention used in North America, as well as providing compatibility with the 4-blade twist lock wiring convention. The inverters of the present invention provide excellent control over total harmonic distortion of the output waveforms produced, and are able to respond faster to load changes than conventional synchronous alternators. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.

Technology Category: 5