Patent Publication Number: US-8525492-B2

Title: Electric power generation system with multiple alternators driven by a common prime mover

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 11/809,768 filed on Jun. 1, 2007 now U.S. Pat. No. 7,956,584, which claims the benefit of U.S. Provisional Patent Application No. 60/877,970 filed on 29 Dec. 2006, and is related to U.S. patent application Ser. No. 11/600,937 filed 16 Nov. 2006, U.S. patent application Ser. No. 11/788,942 filed 23 Apr. 2007 and U.S. patent application Ser. No. 11/789,913 filed 26 Apr. 2007, all of which are hereby incorporated by reference each in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to electric power systems, and more particularly, but not exclusively, relates to management of electric power provided by a system including a variable speed generator driven by an engine that has multiple inverters. 
     In many applications of electrical generator systems, steady state load demand is typically low relative to generator power capacity because generator selection is often driven by peak power requirements—resulting in an “oversized” generator most of the time. As an alternative, in certain situations power generation systems could include an electrical energy storage device to supplement generator power during peak usage, which facilitates a reduction in generator size. Alternatively or additionally, a variable speed generator can be used that changes speed based on power demand. A generally fixed AC frequency and voltage output can be provided from a variable speed generator by utilizing appropriate power conversion circuitry. Unfortunately, these systems typically do not offer different voltage outputs simultaneously, such as 120 volts and 240 volts—leading to a need for alternatives. Indeed, there is an ongoing demand for further contributions in this area of technology. 
     SUMMARY 
     One embodiment of the present invention includes a unique technique involving electric power generation. Other embodiments include unique methods, systems, devices, and apparatus involving electric power generation. Further embodiments, forms, features, aspects, benefits, and advantages of the present application shall become apparent from the description and figures provided herewith. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a diagrammatic view of a vehicle carrying an electric power generation system with multiple generator subsystems. 
         FIG. 2  is a schematic view of circuitry associated with one of the generator subsystems shown in  FIG. 1 . 
         FIG. 3  is a control flow diagram for the circuitry of  FIG. 2 . 
         FIG. 4  is a flowchart of one procedure for operating the subsystems of  FIG. 1 . 
         FIG. 5  is a diagram depicting operating logic for controlling the subsystems of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
       FIG. 1  illustrates vehicle  20  in the form of a motor coach  22 . Motor coach  22  includes interior living space  24  and is propelled by coach engine  26 . Coach engine  26  is typically of a reciprocating piston, internal combustion type. To complement living space  24 , coach  22  carries various types of electrical equipment  27  (shown schematically), such as lighting, kitchen appliances, entertainment devices, one or more air conditioners, and/or such different devices as would occur to those skilled in the art. Coach  22  carries mobile electric power generation system  28  to selectively provide electricity to equipment  27 . Correspondingly, equipment  27  electrically loads system  28 . In one form, various components of system  28  are distributed throughout vehicle  20 —being installed in various bays and/or other dedicated spaces (not shown). 
     System  28  includes two primary sources of power: Alternating Current (AC) power from genset  30  and Direct Current (DC) power from electrical energy storage device  70 . Genset  30  includes a dedicated generator engine  32  that is regulated by engine controller  34 , which is sometimes designated an Engine Control Module (ECM). Engine controller  34  is responsive to control signals from power control and conversion circuitry  40  as further described hereinafter. In one arrangement, engine  32  is of a reciprocating piston type. Genset  30  includes a rotary drive member  36  and two generators  38   a  and  38   b  each having a three-phase output. Each generator  38   a  and  38   b  is driven by member  36 . In one form, generators  38   a  and  38   b  are provided by two different, electrically isolated three-phase windings of a stator  35  (e.g., as shown in  FIG. 1 ) of a permanent magnet alternator (PMA) configuration. For this form, generators  38   a  and  38   b  share a common armature that is a form of rotor  39  fixed to a drive shaft form of member  36 , to rotate simultaneously therewith. Alternatively, generators  38   a  and  38   b  can be provided by separate stators/armatures (e.g., as designated in  FIG. 1  by reference numbers  33   a  and  33   b )/rotors or the like and/or may be other than a PMA type. As an alternative to direct shaft mounting, generators  38   a  and/or  38   b  can be mechanically coupled to engine  32  by a mechanical linkage that provides a desired turn ratio, a torque converter, a transmission, a belt, a chain, gears, and/or a different form of rotary linking mechanism as would occur to those skilled in the art. 
     The rotational operating speed of engine  32 , and correspondingly rotational speed of generators  38   a  and  38   b  vary over a selected operating range in response to changes in electrical loading of system  28 . Over this range, genset rotational speed increases to meet larger power demands concomitant with an increasing electrical load on system  28 . Genset  30  has a steady state minimum speed at the lower extreme of this speed range corresponding to low power output and a steady state maximum speed at the upper extreme of this speed range corresponding to high power output. As the speed of genset  30  varies, its three-phase electrical output varies in terms of AC frequency and voltage. 
     Genset  30  is electrically coupled to power control and conversion circuitry  40 . Circuitry  40  includes conversion/inverter circuitry  40   a  coupled to receive variable AC electrical input power from generator  38   a  and conversion/inverter circuitry  40   b  coupled to receive variable AC electrical input power from generator  38   b . Circuitry  40   a  is further designated a primary control/inverter  41   a  and circuitry  40   b  is further designated secondary control/inverter  41   b . Generator  38   a  and circuitry  40   a  are designated collectively power generation subsystem  30   a , and generator  38   b  and circuitry  40   b  are designated collectively power generation subsystem  30   b.    
     Subsystems  30   a  and  30   b  each include three-phase rectifier  42 , a variable voltage DC power bus  44 , a DC-to-AC power inverter  46 , charge and boost circuitry  50 , and controller (CNTL)  100 . Rectifier  42  converts the respective three-phase input to a variable DC voltage on bus  44 . As regulated by controller  100 , electric power on DC bus  44  is converted with inverter  46  to provide a controlled electrical output at a target AC frequency and voltage for each respective subsystem  30   a  and  30   b . Certain aspects of these features of subsystems  30   a  and  30   b  are further described in connection with  FIGS. 2 and 3  hereinafter. 
     Subsystems  30   a  and  30   b  both are coupled to electrical energy storage device  70  to selectively charge it in certain operating modes and supply electrical energy from it (or “boost”) in other operating modes via circuitry  50  as further described hereinafter. Circuitry  40   a  and  40   b  selectively provides DC electric power through respective circuitry  50  to the storage device  70  via the respective DC bus  44 . Various DC loads may be powered from device  70  and/or circuitry  40  (not shown). Device  70  may be provided in the form of one or more rechargeable electrochemical cells or batteries, or of such different type as would occur to those skilled in the art. 
     The two inverters  46  of circuitry  40   a  and  40   b  are coupled together to define a common electrical node “n” corresponding to a neutral line. Inverter  46  of circuitry  40   a  further defines electrical node L 1  corresponding to power line/output of power bus  80   a  and inverter  46  of circuitry  40   a  further defines electrical node L 2  corresponding to another power line/output of power bus  80   b . Buses  80   a  and  80   b  share a common neutral line n due to the connection between inverters  46 . System  28  further includes bus  80  coupled to the outputs of AC power transfer switch  82 . Buses  80   a  and  80   b  of inverters  46  provide one set of inputs to switch  82 ; and external AC electrical power source  90  (shore power) provides another set of inputs to switch  82  via external power interface  92 . It should be appreciated that shore power generally cannot be used when vehicle  20  is in motion, may not be available in some locations; and even if available, shore power is typically limited by a circuit breaker or fuse. When power from source  90  is applied, genset  30  is typically not active. Transfer switch  82  selects between power through inverters  46  (a power source internal to vehicle  20 ) and power form “shore” (external source  90 ) to service various AC electrical loads of equipment  27  on bus  80 . With the supply of external AC power from source  90 , assembly  40  selectively functions as one of these loads, converting the AC shore power to a form suitable to charge storage device  70 . In the following description, AC shore power should be understood to be absent unless expressly indicated to the contrary. 
     Equipment  27  comprises various AC electrical loads that are supplied via AC bus  80  through switch  82 . Included in these loads are two 120 Volt AC (VAC) loads  83   a  and  83   b  each electrically coupled across a different one of lines (nodes) L 1  and L 2 , respectively, to neutral line of (node) n. Also included is 240 VAC load  84  electrically coupled across both lines L 1  and L 2 . It should be appreciated that the AC waveform output phase on lines L 1  and L 2  can be controlled via inverters  46  so that current flow through the neutral line is no greater than the maximum current flow through either line L 1  or L 2  (i.e. lines L 1  and L 2  are 180 degrees out of phase). As a result, any conductors for L 1 , L 2 , and n can be sized the same for such an arrangement. Furthermore, it should be appreciated that this arrangement effectively combines the voltage output by inverters  46  to provide the sum of the voltages across both loads  83   a  and  83   b  to load  84 . In this specific case, 240 VAC for load  84  is nominally about twice that of 120 VAC for loads  83   a  or  83   b.    
     Controller  100  for each of subsystems  30   a  and  30   b  executes operating logic that defines various control, management, and/or regulation functions. This operating logic may be in the form of dedicated hardware, such as a hardwired state machine, programming instructions, and/or a different form as would occur to those skilled in the art. Controller  100  may be provided as a single component, or a collection of operatively coupled components; and may be comprised of digital circuitry, analog circuitry, or a hybrid combination of both of these types. When of a multi-component form, controller  100  may have one or more components remotely located relative to the others. Controller  100  can include multiple processing units arranged to operate independently, in a pipeline processing arrangement, in a parallel processing arrangement, and/or such different arrangement as would occur to those skilled in the art. In one embodiment, controller  100  is a programmable microprocessing device of a solid-state, integrated circuit type that includes one or more processing units and memory. Controller  100  can include one or more signal conditioners, modulators, demodulators, Arithmetic Logic Units (ALUs), Central Processing Units (CPUs), limiters, oscillators, control clocks, amplifiers, signal conditioners, filters, format converters, communication ports, clamps, delay devices, memory devices, and/or different circuitry or functional components as would occur to those skilled in the art to perform the desired communications. Controllers  100  of subsystems  30   a  and  30   b  are both coupled to communication bus  94 . In one form, bus  94  is of a standard Controller Area Network (CAN) type. 
     Referring additionally to the schematic circuit view of  FIG. 2  and the control flow diagram of  FIG. 3 , selected aspects of subsystem  30   a  are further illustrated; where like reference numerals refer to like features previously described. It should be appreciated that subsystem  30   b  is identically configured except that it receives variable AC input power from generator  38   b  instead of generator  38   a , and supplies its AC output on bus  80   b  instead of bus  80   a . Further, while the controller  100  of each subsystem  30   a  and  30   b  are coupled to a common communication bus  94 , they operate in master/slave relationship with respect to certain functional aspects of system  28 , including control of engine  32  via engine controller  34  as described more fully hereinafter. 
     In  FIG. 3 , blocks formed with heavier line weighting correspond to hardware-implemented functionality, and blocks formed with lighter line weighting correspond to software-implemented functionality provided by programming of controller  100 . Subsystem  30   a  includes Electromagnetic Interference (EMI) filter  38  coupled to three-phase rectifier  42 . In one form, rectifier  42  is implemented with a standard six diode configuration applicable to three-phase AC-to-DC conversion. Rectifier  42  receives the EMI-filtered, three-phase AC electric power output from genset  30  when genset  30  is operational. Filter  38  removes certain time varying characteristics from the genset output that may result in undesirable inference and rectifier  42  converts the filtered three-phase AC electric power from genset  30  to a corresponding DC voltage on bus  44 . 
     At least one capacitor  45  is coupled across DC bus  44  to reduce residual “ripple” and/or other time varying components. The DC voltage on bus  44  is converted to an AC voltage by inverter  46  in response to inverter control logic  104  of controller  100 . In one form, inverter  46  is of a standard H-bridge configuration with four Insulated Gate Bipolar Transistors (IGBTs) that is controlled by Pulse Width Modulated (PWM) signals from controller  100 . In other forms, inverter  46  can be comprised of one or more other switch types such as field effect transistors (FETs), gated thyristors, silicon controlled rectifiers (SCRs), or the like. The PWM control signals from logic  104  selectively and individually drive the gates/switches of inverter  46 . Typically, these control signals are input to intervening power drive circuitry coupled to inverter gates, and the control signals are isolated by opto-isolators, isolation transformers, or the like. Inverter control logic  104  includes a Proportional-Integral (PI) controller to synthesize an approximate sinusoidal AC waveform. Sensing arrangement  45  includes AC voltage sensor  46   a  and AC current sensor  46   b . Control logic  110  receives AC voltage (VAC) from voltage sensor  46   a  and AC current (IAC) from current sensor  46   b  that correspond to the power delivered to bus  80   a  from inverter  46 . The VAC and IAC inputs to logic  104  are utilized as feedback to generate the sinusoidal waveform for the output power with a PI controller. In addition, these inputs are used to calculate power properties required to control sharing functions for the overall system and determine the power factor for the sinusoidal voltage and current outputs to facilitate power factor correction via a PI controller. Control logic  110  receives AC power output information from inverter control logic  104 . This information can be used to determine system power, and is used to compare with the power delivery capacity of genset  30  and device  70  to regulate certain operations described hereinafter. Furthermore, logic  110  uses this AC output information to determine whether a transient power condition exists that warrants consideration. 
     Inductor  47   a  and capacitor  47   b  provide further filtering and conversion of the inverter  46  output to a controlled AC power waveform. EMI filter  48  provides interference filtering of the resulting AC power waveform to provide a regulated single-phase AC power output on bus  80 . In one nonlimiting example, a nominal 120 VAC, 60 Hertz (Hz) output is provided on bus  80 , the genset three-phase output to rectifier  42  varies over a voltage range of 150-250 volts AC (VAC) and a frequency range of 200-400 Hertz (Hz), and the variable voltage on DC bus  44  is between 200 and 300 volts DC (Vdc) 
     In addition to inverter control logic  104 , controller  100  includes genset power request control logic  102  to regulate rotational speed of genset  30  relative to system  28  operations through communication with engine controller  34  via bus  94 . Logic  102  provides input signals to genset  30  that are representative of a requested target electrical load to be powered by genset  30 . Genset governor  103  defined by engine controller  34  responds to logic  102  to adjust engine rotational speed, which in turn adjusts rotational speed of generator  34 . Control by logic  102  is provided in such a manner that results in different rates of engine speed change (acceleration/deceleration) depending on one or more conditions (like transients), as more fully explained in connection with  FIGS. 4 and 5  hereinafter. 
     In one particular form, governor  103  is fully implemented in engine controller  34  that is included with genset  30 . Alternatively or additionally, at least a portion of governor  103  can be included in circuitry  40 . Control logic  102  is responsive to system control logic  110  included in the operating logic of controller  100 , and an engine speed feedback signal provided by engine speed sensor  112  and/or by monitoring AC frequency of the variable output of generator  38   a . Speed adjustment with logic  102  can arise with changes in electrical loading and/or charge or boost operations of device  70 , as further described hereinafter. In turn, logic  102  provides control inputs to charge and power boost control logic  106 . 
     Controllable DC-to-DC converter  60  is electrically coupled to DC bus  44  and electrical energy storage device  70 . In  FIG. 2 , device  70  is more specifically illustrated in the form of electrochemical battery device  75 . Electrical current flow between device  70  and converter  60  is monitored with current sensor  76  and DC voltage of device  70  is monitored at node  78 . In one embodiment, more than one current sensor and/or current sensor type may be used (not shown). For example, in one arrangement, one sensor may be used to monitor current of device  70  for power management purposes (such as a Hall effect sensor type), and another sensor may be used in monitoring various charging states (such as a shunt type). In other embodiments, more or fewer sensors and/or sensor types may be utilized. 
     Converter  60  provides for the bidirectional transfer of electrical power between DC bus  44  and device  70 . Converter  60  is used to charge device  70  with power from DC bus  44 , and to supplement (boost) power made available to DC bus  44  to service power demand on bus  80  (see  FIG. 1 ). Converter  60  includes DC bus interface circuitry  54  and storage interface circuitry  64  under the control of charge and power boost control logic  106 . Bus interface circuitry  54  includes a charge inverter  54   a  and power boost rectifier  54   b . Storage interface circuitry  64  includes charge rectifier  64   a  and power boost inverter  64   b . Transformer  58  is coupled between circuitry  54  and circuitry  64 . Charge inverter  54   a  and boost inverter  64   b  can be of an H-bridge type based on IGBTs, FETs (including MOSFET type), gated thyristors, SCRs, or such other suitable gates/switching devices as would occur to those skilled in the art. Further, while rectifiers  54   b  and  64   a  are each represented as being distinct from the corresponding inverter  54   a  or  64   b , in other embodiments one or more of rectifiers  54   b  and  64   a  can be provided in the form of a full wave type comprised of protective “free wheeling” diodes electrically coupled across the outputs of the respective inverter  54   a  or  64   b . For rectifier operation of this arrangement, the corresponding inverter components are held inactive to be rendered nonconductive. 
     Charge Proportional-Integral (PI) control circuit  52  is electrically coupled to charge inverter  54   a  and power boost PI control circuit  62  is electrically coupled to power boost inverter  64   b . Circuits  52  and  62  each receive respective charge and boost current references  106   a  and  106   b  as inputs. Electrical current references  106   a  and  106   b  are calculated by charge and power boost control logic  106  with controller  100 . These references are determined as a function of power demand, system power available, and the presence of any transient power conditions. The total system power is in turn provided as a function of the power provided by inverter  46  to bus  80  (inverter power), the power-generating capacity of genset  30 , and the power output capacity of device  70 . The inverter power corresponds to the AC electrical load “power demand” as indicated by the VAC voltage, IAC current, and corresponding power factor that results from electrical loading of bus  80 . The genset power-generating capacity is determined with reference to genset power/load requested by logic  102 . When the power demand on bus  80  can be supplied by genset  30  with surplus capacity, then this surplus can be used for charging device  70  by regulating converter  60  with PI control circuit  52 ; and when the power demand exceeds genset  30  capacity, supplemental power can be provided to bus  80  from device  70  by regulating converter  60  with PI control circuit  62 . Various aspects of dynamic “power sharing” operations of system  28  are further described in connection with  FIGS. 4 and 5  hereinafter; however, further aspects of converter  60  and its operation are first described as follows. 
     Converter  60  is controlled with system control logic  110  to enable/disable charge and boost operations. Under control of logic  110 , the charge mode of operation and the boost mode of operation are mutually exclusive—that is they are not enabled at the same time. However, it should be appreciated that in system  28 , subsystems  30   a  and  30   b  operate independent of one another in such respects, so that both may be operating in charge mode, in boost mode, or one in each mode. From the perspective of device  70 , it is providing electric power (boosting) or is receiving electric power (charging) based on the quantitative sum of boost or charge being performed by subsystems  30   a  and  30   b . When multiple batteries are used in parallel, subsystems  30   a  and  30   b  can be coupled at different ends of the parallel arrangement to assure a more even current flow throughout the batteries. 
     When charge mode is enabled, the electrochemical battery form of device  70  is charged in accordance with one of several different modes depending on its charging stage. While charging, circuit  52  outputs PWM control signals that drive gates of charge inverter  54   a  in a standard manner. Typically, the PWM control signals are input to standard power drive circuitry (not shown) coupled to each gate input, and may be isolated therefrom by optoisolators, isolation transformers, or the like. In response to the PWM input control signals, inverter  54   a  converts DC power from DC bus  44  to an AC form that is provided to rectifier  64   a  of circuitry  64  via transformer  58 . Rectifier  64   a  converts the AC power from transformer  58  to a suitable DC form to charge battery device  75 . In one form directed to a nominal 12 Vdc output of battery device  75 , transformer  58  steps down the AC voltage output by inverter  54   a  to a lower level suitable for charging storage device  70 . For nonbattery types of devices  70 , recharging/energy storage in the “charge mode” is correspondingly adapted as appropriate. 
     When power boost mode is enabled, boost PI control circuit  62  provides PWM control signals to boost inverter  64   b  to control the power delivered from device  70 . The circuit  62  output is in the form of PWM control signals that drive gates of boost converter  64   b  in a standard manner for a transformer boost configuration. Typically, these control signals are input to power drive circuitry (not shown) with appropriate isolation if required or desired. When supplementing power provided by generator  32 , a current-controlled power boosting technique is implemented with circuit  62 . Circuit  62  provides proportional-integral output adjustments in response the difference between two inputs: (1) boost current reference  106   b  and (2) storage device  70  current detected with current sensor  76 . In response, inverter  64   b  converts DC power from device  70  to an AC form that is provided to rectifier  54   b  of circuitry  54  via transformer  58 . Rectifier  64   b  converts the AC power from transformer  58  to a suitable DC form for DC bus  44 . In one form directed to a nominal 12 Vdc output of device  70 , transformer  58  steps up the AC voltage output from inverter  64   b , that is converted back to DC power for bus  44 . 
       FIG. 4  depicts power management procedure  120  for system  28  that is performed in accordance with operating logic executed by controller  100  for each subsystem  30   a  and  30   b . Also referring to  FIGS. 1-3 , procedure  120  begins with conditional  122  that tests whether shore power from external source  90  is being applied. If the test of conditional  122  is true (yes) then shore power operation  124  is performed. In operation  124 , shore power is applied from bus  80  to charge device  70 , using the “free wheeling” diodes of inverter  46  to rectify from AC to DC power. During operation  124 , shore power is also provided to loads  83  and  84  via switch  82 . 
     If the test of conditional  122  is false (no), procedure  120  continues with conditional  126 . Conditional  126  tests whether system  28  is operating in a quite mode. If the test of conditional  126  is true (yes), then the storage/battery only operation  128  is performed. Quite mode is typically utilized when the noise level resulting from the operation of genset  30  is not permitted or otherwise not desired and when shore power is not available or otherwise provided. Correspondingly, in operation  128  genset  30  is inactive, and power is provided only from storage device  70 . For operation in this quiet mode, power delivered by storage device  70  is voltage-controlled rather than current-controlled, supplying a generally constant voltage to DC bus  44  to facilitate delivery of an approximately constant AC voltage on bus  80 . Conditionals/operators  122 - 128  are performed at the same time by both subsystems  30   a  and  30   b.    
     Operator Input/Output (I/O) device  115  is operatively connected to controllers  100  to provide various operator inputs to system  28  and output status information. In one nonlimiting form, device  115  is mounted in a cabin of coach  22  and is in communication with controllers  100  of circuitry  40  via bus  94 . 
     If the test of conditional  126  is false (no), then conditional  130  is encountered. Conditional  130  tests whether power share mode is active. In response to changes in electrical loading of system  28 , the power share mode dynamically adjusts the speed of genset  30  under direction of controller  100  of subsystem  30   a  based in part on communications with controller  100  of subsystem  30   b . Boost/charge is independently controlled by the subsystems  30   a  and  30   b . It should be appreciated that this regulation is based on total power for each subsystem  30   a  and  30   b , which accounts for: (a) AC power output from inverter  46  as measured by inverter voltage and current, (b) the DC power as measured at the storage device, and (c) the power loss intrinsic to circuiting  40   a  and  40   b . The loss calculation facilitates determination of a target genset speed and boost rate for steady state operation, as further discussed in connection with operation  138 . 
     If the test of conditional  130  is true (yes), then conditional  132  is executed. Conditional  132  tests whether a power level change or transient has been detected. If the test of conditional  132  is true (yes), then transient handling operation  150  is performed as further described in connection with  FIG. 5 . Different criteria or “tests” may be used to detect different kinds of transients that are subject to conditional  132 , as further described hereinafter. If the test of conditional  132  is false (no), then the power is at steady state in the power share mode. Steady state power delivery occurs in one of two ways contingent on the steady state electrical load magnitude, as implemented by conditioned  134 , which tests whether the electrical load is below a selected threshold related to available genset  30  power (steady state genset rating). This test involves adding the DC and AC power levels, accounting for losses, and comparing the total power to the genset power rating to determine if simultaneous charging of device  70  can be performed. If so, the test of conditional  134  is true (yes) and operation  136  is performed. 
     In operation  136 , a “genset plus charge” power share mode is supported that uses excess genset capacity for charging device  70 , as needed (charge enabled/boost disabled). The genset plus charge power share mode of operation  136  typically reaches steady state from a transient condition as further described in connection with operation  150 . The total genset power in the genset plus charge mode is determined as the measured AC power output plus the measured DC charging power less estimated charger losses. In one form, the charger loss is estimated by reference to one or more tables containing the loss of the charger circuitry as a function of battery voltage and charge current. The target genset speed is then determined based on the normalized load. The genset speed is set to support the DC and AC loads. When the genset reaches the rated charge level, its speed may be reduced. As the AC power requirement approaches the genset rating, the charge rate may be reduced in order to maintain load support with genset  30 . 
     If the test of conditional  134  is false (no), then operation  138  results. In operation  138 , genset  30  and device  70  are both utilized to provide power to the electrical load at steady state in a “genset plus boost” power share mode. The desired boost rate is calculated based on total AC and DC power requirements less loss. This rate controls boost current to reach the desired power share between the genset and the storage device. The boost rate is calculated by determining the desired storage power contribution to the system load and referencing one or more tables that represent the loss of boost circuitry as a function of battery voltage and current. Conditional  134  and operations  136  and  138  are also performed independently by each subsystem  30   a  and  30   b.    
     Typically, for this steady state condition, genset  30  is operating at an upper speed limit with additional power being provided from device  70  in the boost enabled mode. It should be understood that this genset plus boost power share operation also typically reaches steady state from a transient condition as further described in connection with operation  150  as follows. In one form, the load calculations are normalized to a percent system rating, a percent boost capability and a percent genset load to facilitate system scaling for different genset and boost sizes. The implementation of operation  150  by subsystems  30   a  and  30   b  is further described in connection with  FIG. 5  hereinafter. 
     Continuing with  FIG. 4  first, operations  124 ,  128 ,  136 , and  138  proceed to conditional  140 . Conditional  140  tests whether to continue operation of process  120 . If conditional  140  is true (yes), process  120  returns to conditional  122  to re-execute the remaining logic. If conditional  140  is false (no), process  120  halts. 
     In operation  150 , there may be multiple types of transient conditions identified. For transients of a smaller degree, initially they may be handled by subsystems  30   a  and  30   b  independent of one another. In contrast, for certain large transient conditions imposed on one of the subsystems  30   a  or  30   b , its existence may dictate the response irrespective of the transient status of the other of the subsystems  30   a  or  30   b . For example, a large increase in electrical load on one of subsystems  30   a  or  30   b  can result in the desire to increase the speed of genset  30  with maximum acceleration despite electrical loading of the other subsystem  30   a  or  30   b.    
       FIG. 5  depicts control logic  120  for regulating operation of primary inverter  41   a  of subsystem  30   a  and secondary inverter  41   b  of subsystem  30   b  with respect to transient conditions, including the regulation of load imbalances therebetween; where like reference numerals refer to like features. Logic  120  is depicted as primary operational logic  100   a  for primary inverter  41   a  and secondary operational logic  100   b  for secondary inverter  41   b . Logic  100   a  and  100   b  are each implemented with the corresponding controller  100  of each inverter  41   a  and  41   b , respectively. For the depicted arrangement, inverter  41   b  operates relative to inverter  41   b  in a slave/master relationship, and communicate over bus  94 . Likewise, connector  44   a  communicates with engine controller  34  and bus  94 . 
     The function of logic  100   a  and  100   b  is next described in conjunction with the corresponding logical elements shown in  FIG. 6 . Logic  100   a  includes transient response (TR) logic  102   a  that determines the primary inverter target capacity designated as P and logic  100   b  includes TR logic  102   b  that determines the secondary inverter target capacity designated as S. Target capacity S is sent to logic  100   a  of inverter  41   a  that sums target capacity S and P together with adder  142   a . Adder  142   a  outputs the balanced target power unit capacity designated as BT (where BT=S+P). It has been discovered that when the loads on inverters  41   a  and  41   b  become unbalanced, it is sometimes desirable to have extra power capacity to take into account the effect unbalanced loading can have. This added capacity permits generation of the DC bus voltage at a level sufficient to keep distortion of the AC 120 V voltage output for each inverter  41   a  and  41   b  at a minimum. Accordingly, a variable unbalanced target power unit capacity designated by UT is added to the balanced target power unit capacity BT by adder  142   b  of logic  100   a . The unbalanced target power unit capacity UT is determined from an unbalanced load Look Up Table (LUT)  144  based on the inputs of primary target capacity P and secondary target capacity S. 
     The specific output of LUT  144  is empirically (experimentally) determined for a given arrangement/application. In a typical application, the unbalanced target capacity UT output by LUT  144  is greatest when one of the two inverters  41  and  41   b  has a zero or a negligible load size while the other inverter has a load size that corresponds to half or more of the maximum engine speed (i.e. 50% target load capacity or more), and may be negligible (zero) when the smaller of the two inverter load sizes is below a certain threshold (in one application this threshold is about 22% of the maximum target power unit capacity). It may also be negligible if the two inverter load sizes are about or near the same sizes (i.e. balanced loading). It should be understood that the UT value can be provided in other ways such as from multiple LUTs, one or more mathematical expressions, schedules, a combination of these, or such different manner as would be known to those skilled in the art. 
     The output of adder  142   b  is the total target power unit capacity T (where T=BT+UT). Total target capacity T is sent to engine controller  34 . In response, engine controller  34  adjusts rotational engine speed of engine  32  appropriately as designated by engine speed logic (ECM SP)  34   a . Engine controller  34  also determines the power unit capacity of engine  32 , designated as C, and sends power unit C to primary inverter  41   a . Within logic  100   a , power unit capacity C is reduced by the amount of unbalanced target capacity UT through the operation of negative multiplier  150  that outputs the negative form of UT and adder  142   c  that sums both C and negative UT (−UT) together. In other words, the output of adder  142   c  is the difference between C and UT (C−UT). 
     Each inverter  41   a  and  41   b  initially addresses transient responses with its corresponding TR logic  102   a  and  102   b . As previously described, this logic is based on the individual power unit capacity of the corresponding inverter  46 . To provide appropriate primary and secondary inverter power unit capacities (PC and SC) for TR logic  102   a  and  102   b , respectively; logic  100   a  takes into account the ratio of each of the primary and secondary inverter target power unit capacities (S and P) to the total balanced target power unit capacity (BT) and otherwise provides an offset for UT. More specifically, logic  100   a  determines the ratio BT/P with divider  146   a  that is output as a primary power capacity ratio PR. The output of divider  146   a  (BT/P) is provided as the denominator and C-UT is provided as the numerator to divider  148   a  to provide primary inverter power unit capacity PC in ratio terms with any nonzero value of UT removed. Logic  100   a  also determines the ratio BT/S that is output by divider  146   b  as secondary power capacity ratio SR. SR is input as a denominator and C-UT is input as a numerator to divider  148   b  to provide secondary inverter power unit capacity SC in ratio terms with any nonzero value of UT removed. Secondary inverter power unit capacity SC is provided to TR logic  102   b  of secondary inverter  41   b  to perform transient response handling. It should be understood that if P is equal to zero then PC is set to zero without performing division with dividers  146   a  and  148   a . Similarly, if S is equal to zero then SC is set to zero without performing division with dividers  146   b  and  148   b.    
     The logic  102   a  and logic  102   b  each output a corresponding binary large transient signal PLT and SLT indicative of whether a large transient status applies to the respective inverter  41   a  or  41   b . These signals are provided as inputs to a binary OR operator  140   a  that outputs the result as binary large transient signal LT. This logic insures that when at least one of the two inverter load sizes sets the large transient condition, the signal LT is set to true. Signal LT is input to engine controller  34  to provide a maximum speed increase in response to the state of the LT signal that indicates a large transient status of either inverter  41   a  and/or  41   b . In effect, an active (true) large transient signal LT by passes any determination of engine speed based on the target power unit capacity T determined from both subsystems  30   a  and  30   b . In contrast, when large transient LT is inactive (false), the target power unit capacity T governs, which may not involve maximum acceleration of engine  32 . Instead, acceleration or deceleration of engine  32  may be constrained to a fixed rate of change less than the maximum rate of change available, as further described in U.S. patent application Ser. No. 11/600,937 filed 16 Nov. 2006 (previously incorporated by reference). In one implementation, this rate is selected to reduce human perception of engine noise indicative of a speed change. Alternatively or additionally, adjustment in boost or charge by TR logic  102   a  and/or  102   b  can influence the corresponding target power capacity signals P and S from the primary inverter  41   a  and secondly inverter  41   b -influencing the variable T output to engine  32 . 
     Logic  102   a  and logic  102   b  also each include logic operators  104   a  and  104   b  that output binary vent and coolant pump commands to regulate thermal aspects of operation. Specifically, operator  104   a  selectively outputs primary vent fan command PVF and primary coolant pump command PCP, and operator  104   b  selectively outputs secondary vent fan command SVF and secondary coolant pump command SCP. Commands PVF and SVF are provided as inputs to binary OR operator  140   b , which outputs the dual inverter vent fan command VF that is sent to engine controller  34  as the logical “or” function of PVF and SVF. Commands PCP and SCP are provided as inputs to binary OR operator  140   c , which outputs the dual inverter coolant pump command CP that is sent to engine controller  34  as the logical “or” function of PCP and SCP. Commands CP and VF are provided to ECM Pump Control (ECM PC) module  34   c  that adjusts pump operation as appropriate. 
     To maintain synchronization between inverters  41   a  and  41   b  the primary inverter  41   a  counts every three AC cycles it generates as delineated by a zero crossings. With every three cycles, the primary inverter  41   a  communicates a synchronization signal to the secondary inverter  41   b  to facilitate synchronization of its output waveform with the primary inverter  41   a . In one form, this synchronization signal can be treated like an interrupt signaling the start of the generation of three AC cycles. The secondary inverter  41   b  also sends a synchronization signal at the end of its three AC cycles to indicate that it is performing the inverting function. 
     In a further embodiment, it is desirable that start-up of the inverters  41   a  and  41   b  be synchronized such that an initial full 240 V output is provided minimal distortion. For this embodiment, an initial synchronization sequence can be executed to insure that the two inverters  41   a  and  41   b  begin inverter operation within a negligible delay of one another. In one form, this sequence includes: (a) the primary inverter  41   a  sending a synchronization signal to the secondary inverter  41   b  when it is ready to start inverting (i.e. generating the 120 V AC voltage signal) and then waits to receive a synchronization signal (as an acknowledgement) from the secondary inverter  41   b ; (b) the secondary inverter  41   b  waits to receive a synchronization signal from the primary inverter  41   a  when it is ready to start inverting; (c) when the secondary inverter  41   b  receives the first synchronization signal from the primary inverter  41   a , the secondary inverter  41   b   1  sends a synchronization signal to the primary inverter  41   a  (as an acknowledgement) and starts inverting in its next interrupt cycle; and (d) the primary inverter  41   a  waiting for the acknowledgement synchronization signal from the secondary inverter  41   b , starts inverting after receiving the acknowledgement synchronization signal from the secondary inverter  41   b . This initial synchronization is only performed at the start of inverting for both inverters  41   a  and  41   b . If the primary inverter  41   a  does not get acknowledged after it has sent its first synchronization signal to the secondary inverter  41   b  for a pre-determined amount of time (in one implementation 3 sec), the primary inverter  41   a  shuts down, which in turn causes the secondary inverter  41   b  to shut down. Once synchronization is initially successful, synchronization is maintained by sending a periodic signal from the primary inverter  41   a  to the secondary inverter  41   b  (in one form, every three AC cycles as described above). After a successful initial synchronization is performed, a lack of reception of a predetermined number of consecutive synchronization signals causes the inverter  41   a  or  41   b  not receiving the synchronization signal to shutdown, which in turn causes the other inverter  41   a  or  41   b  to shutdown. In one instance, the predetermined number is 6. In the case of a shutdown of one of inverters  41   a  or  41   b , the other is also shutdown, regardless of cause, which can be implemented through the use of an external (fault) shutdown signal that is sent from one inverter  41   a  or  41   b  to the other inverter  41   a  or  41   b . In another variation there are two generators driven by a common engine with two inventors as shown for system  28 , but only one has a boost/charge capability. In this case, the boost can include two isolated outputs that could be routed to either inverter as desired. 
     Many other embodiments of the present application exist. For example, one or more fuel cell devices, capacitive-based storage devices, and/or a different form of rechargeable electrical energy storage apparatus could be used as an alternative or addition to an electrochemical cell or battery type of storage device  70 . Furthermore, one or more fuel cells (including but not limited to a hydrogen/oxygen reactant type) could be used to provide some or all of the power from genset  30  and/or energy storage device  70 . Engine  32  can be gasoline, diesel, gaseous, or hybrid fueled; or fueled in a different manner as would occur to those skilled in the art. Further, it should be appreciated that engine  32  can be different than a reciprocating piston, intermittent combustion type, and/or coach engine  26  can be used in lieu of engine  32  to provide mechanical power to generator  34  or to supplement mechanical power provided by engine  32 . In still another embodiment, the vehicle carrying system  28  is a marine vessel. In one variation of this embodiment, rotational mechanical power for generator  34  is provided from a propulsion shaft (such as a propeller shaft) with or without engine  32 . Alternatively or additionally, generator  34  can be of a different type with adaptation of circuitry/control to accommodate such different generator type, as desired. In yet other embodiments, more or fewer controllers are utilized and may or may not be arranged in a master/slave relationship. 
     A further embodiment of the present application includes: an internal combustion engine, a first variable speed generator driven by the engine, a second variable speed generator driven by the engine, a first inverter to convert variable electric power from the first variable speed generator to provide a first controlled AC voltage at a first electrical output node, and second inverter to convert variable electric power from the second variable speed generator to provide a second controlled AC voltage and a second electrical output node. The first inverter and the second inverter are electrically connected together with a common electrical node to provide a third controlled AC voltage across the first output node and the second output node greater than each of the first controlled AC voltage and the second controlled AC voltage. 
     In another embodiment, an apparatus comprises: a motor coach, a trailer, a camper, a truck, a bus, or a watercraft including an internal combustion engine, a first variable speed generator driven by the engine, a second variable speed generator driven by the engine, a first inverter to convert variable electric power from the first variable speed generator to provide a first controlled AC voltage, and a second inverter to convert variable electric power from the second variable speed generator to provide a second controlled AC voltage. 
     Yet another embodiment includes: a first variable speed generator, a second variable speed generator, a first rectifier connected to the first variable speed generator, a first DC bus connected to the first rectifier, a first inverter connected to the first DC bus, a second rectifier connected to the second variable speed generator, a second DC bus connected to the second rectifier, a second inverter connected to the second DC bus, and at least one energy storage device electrically coupled to at least one of the first DC bus and the second DC bus. 
     Still another embodiment includes: with a vehicle, carrying a first variable speed electric generator, a second variable speed electric generator, a first inverter coupled to the first generator, and a second inverter coupled to the second generator; driving the first variable speed electric generator and the second variable speed electric generator, and controlling the first inverter and the second inverter to provide a first electric power output with a first controlled AC voltage across one of the first inverter and the second inverter, and a second electric power output with a second controlled AC voltage across both the first inverter and the second inverter, the second AC voltage being greater than first AC voltage. 
     A further embodiment comprises: carrying an electrical power generation system with a vehicle, the system including an internal combustion engine with a drive shaft, a first variable speed electric power generator, a second variable speed electric power generator; turning the first generator and the second generator with the drive shaft to provide a first power output from the system with a first controlled AC voltage and second power output from the system at a second controlled AC voltage greater than the first controlled AC voltage; and propelling the vehicle with another engine. 
     Another embodiment of the present application comprises: operating a vehicular electrical power generation system including a variable speed internal combustion engine, a first variable speed electric power generator, a second variable speed electric power generator, a first inverter coupled to the first generator, and a second inverter coupled to the second generator; driving the first generator and the second generator with the internal combustion engine operating at a first speed to supply electric power through the first inverter and the second inverter; determining a degree of electrical load imbalance between the first inverter and the second inverter; and controlling operating speed of the internal combustion engine as a function of the degree of the electrical load imbalance. 
     Yet another embodiment of the present invention includes a vehicular electrical power generation system with the variable speed internal combustion engine, a first variable speed electric power generator, a second variable speed electrical power generator, a first inverter coupled to first generator, and a second inverter coupled to the second generator. Also included are means for driving the first generator and the second generator with an internal combustion engine operating a first speed to supply electric power through the first inverter and second inverter, means for determining a degree of electrical load imbalance between the first inverter and the second inverter, and means for controlling operating speed of the internal combustion engine as a function of the degree of the electrical load imbalance. 
     Still a further embodiment includes: operating a vehicular electric power generation system including a variable speed internal combustion engine, a first variable speed electric power generator, a second variable speed electric power generator, a first inverter coupled to the first generator, and second inverter coupled to the second generator; driving the first generator and the second generator with the internal combustion engine operating at a first speed to supply electricity through the first inverter and the second inverter; determining a change in electrical loading of the first inverter and the second inverter; and in response to the change in electrical loading, adjusting the internal combustion engine to operate at a second speed different than the first speed. 
     Another embodiment of the present application includes a vehicular electric power generation system with a variable speed internal combustion engine, a first variable speed electric power generator, a second variable speed electric power generator, a first inverter coupled to the first generator, and a second inverter coupled to the second generator. Also included are means for driving the first generator and the second generator with the internal combustion engine operating at a first speed to supply electricity through the first inverter and the second inverter, means for determining a change in electrical loading of the first inverter and the second inverter, and means for adjusting the internal combustion engine to operate at a second speed different than the first speed in response to the change in electrical loading. 
     Any theory, mechanism of operation, proof, or finding stated herein is meant to further enhance understanding of the present invention and is not intended to make the present invention in any way dependent upon such theory, mechanism of operation, proof, or finding. It should be understood that while the use of the word preferable, preferably or preferred in the description above indicates that the feature so described may be more desirable, it nonetheless may not be necessary and embodiments lacking the same may be contemplated as within the scope of the invention, that scope being defined by the claims that follow. In reading the claims it is intended that when words such as “a,” “an,” “at least one,” “at least a portion” are used there is no intention to limit the claim to only one item unless specifically stated to the contrary in the claim. Further, when the language “at least a portion” and/or “a portion” is used the item may include a portion and/or the entire item unless specifically stated to the contrary. While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the selected embodiments have been shown and described and that all changes, modifications and equivalents that come within the spirit of the invention as defined herein or by any of the following claims are desired to be protected.