Abstract:
A method and an apparatus optimize performance of a motor drive system. The method according to one embodiment comprises: selecting a property for a rated operation point; selecting inverter system characteristics and motor characteristics such that a motor drive system including an inverter system having the selected inverter system characteristics operatively connected to a motor having the selected motor characteristics will have a rated operation point exhibiting the property; providing an inverter system having the selected inverter system characteristics; operatively connecting a motor having the selected motor characteristics to the inverter system; and minimizing current of the motor drive system including the motor operatively connected to the inverter system in entire operating range of the motor drive system.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present application claims priority under 35 USC §119(e) to U.S. Provisional Application No. 60/631,429 filed Nov. 30, 2004, which is hereby incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to motor drive systems, and more particularly to a technique for design/control of a motor drive system including a DC-AC inverter and a synchronous motor.  
         [0004]     2. Description of the Related Art  
         [0005]     Large vehicles such as aircraft and ships include a multitude of electrical and mechanical systems that perform complex tasks and require large amounts of energy. Synchronous motor drive systems are suitable for large vehicles. Synchronous motor drives are energy conversion systems that can produce large amounts of energy at high power and high speed, as needed for complex equipment onboard large vehicles. Synchronous motor drive systems are especially efficient with the use of permanent magnet synchronous motor (PMSM) systems due to reduced resulting size and weight.  
         [0006]     A typical motor drive system topology for advanced system environments such as “more electric” aircraft with “fly-by-wire” systems is a three phase voltage source DC-AC inverter-driven synchronous motor. The inverter could be installed remotely from the motor, which is integrated into a system such as a cabin air compressor. When the inverter is not in close proximity to the motor, an output AC differential LC filter is used to mitigate high voltage variations occurring on the motor end. Such high voltage variations correspond to high voltage-time derivatives and are caused by long cables connecting the motor to the inverter. The output AC differential LC filter reduces the stress on the motor and helps meet electromagnetic compatibility requirements.  
         [0007]     A motor drive system usually needs to accommodate various input DC bus voltage ranges, which ultimately cause variations in the limits of the inverter output AC voltage range. A motor drive system also needs to accommodate various load conditions. The variability in inverter output AC voltage range and load conditions impacts motor drive system performance.  
         [0008]     Commercial motor drive systems are generally designed for constant torque operation below the rated operation speed, and constant power operation above the rated operation speed. The rated operating point for the motor drive system is located at the rated speed, where maximum power is achieved. Hence, the inverter and the motor are optimized separately based on the rated operation point.  
         [0009]     When high power from a motor drive system is required, the DC bus voltage applied to the motor drive system needs to be increased accordingly, to accommodate the increased back electromotive force resulting from the motor design. The motor current rating is limited by the motor design, which includes the design of motor winding. The winding included in a motor drive system can be large and heavy. Installing, reinstalling, or replacing the winding inside the motor is a difficult task. The DC bus voltage is used to compensate for motor design limitations. For example, medium and high voltage DC buses are designed for megawatts motor drive systems used in commercial applications.  
         [0010]     However, in many applications such as the more electric aircraft, the DC bus voltage that can be used to drive high power motor drive systems is limited. In these applications, separate design optimization of motor and inverter typically results in over-designing of the inverter and the motor to accommodate the peak power/peak current requirements. An additional drawback of separate optimization of motor and inverter design is that the motor drive system design is not optimized for overall system efficiency/power density, especially when an output LC filter is installed between the inverter and the motor. Such a filter is often required in applications such as the more electric aircraft, etc.  
         [0011]     A few publications have studied improved efficiency schemes for motor drive systems. One such technique is described in “Method and System for Improving Efficiency of Rotating, Synchronous, Electrical Machine Interacting with Power Converter”, Roman Bida, US Patent Application 2002/0149336 A1. With the method described in this work, the efficiency of a motor drive is improved by introducing a spectrum of harmonic components in a power converter supplying energy to the motor. The harmonic components control the current of the power converter so that the current becomes identical in shape and phase to the back electromotive force (back EMF) of the motor. This concept, however, does not provide overall optimization of the motor drive system, as it optimizes only the motor section of the system.  
         [0012]     Another technique is described in “Back EMF Controlled Permanent Magnet Motor”, D. Fulton and W. Curtiss, U.S. Pat. No. 4,275,343. In this publication, however, only the back EMF control of the motor is optimized. Again, no overall optimization of the motor drive system for design and control of both inverter and motor is performed.  
         [0013]     A disclosed embodiment of the application addresses these and other issues by utilizing a high power density/limited DC link voltage motor drive system, with design and control optimization achieved for the combination of inverter and motor system.  
       SUMMARY OF THE INVENTION  
       [0014]     Embodiments of the present invention are directed to a method and an apparatus for optimizing performance of a motor drive system. According to a first aspect of the present invention, a method of optimizing performance of a motor drive system comprises: selecting a property for a rated operation point; selecting inverter system characteristics and motor characteristics such that a motor drive system including an inverter system having the selected inverter system characteristics operatively connected to a motor having the selected motor characteristics will have a rated operation point exhibiting the property; providing an inverter system having the selected inverter system characteristics; operatively connecting a motor having the selected motor characteristics to the inverter system; and minimizing current of the motor drive system including the motor operatively connected to the inverter system in entire operating range of the motor drive system.  
         [0015]     According to a second aspect of the present invention, a system comprises: a motor having motor characteristics; an inverter system having inverter system characteristics, wherein the motor characteristics and the inverter system characteristics are selected such that a motor drive system including the motor and the inverter system operatively connected to each other has a rated operation point with a predetermined property; and a controller operatively connected to the motor drive system, the controller minimizing current of the motor drive system in entire operating range of the motor drive system. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Further aspects and advantages of the present invention will become apparent upon reading the following detailed description in conjunction with the accompanying drawings, in which:  
         [0017]      FIG. 1  is a block diagram of an electrical/mechanical system containing a motor drive system with design/control optimization according to an embodiment of the present invention;  
         [0018]      FIG. 2  is a block diagram of a motor drive system with design/control optimization according to an embodiment of the present invention;  
         [0019]      FIG. 3  is a flow diagram illustrating operations performed for overall design/control optimization of a motor drive system according to an embodiment of the present invention;  
         [0020]      FIG. 4A  is a block diagram of a motor drive system with design/control optimization including a LC filter in accordance with an embodiment of the present invention;  
         [0021]      FIG. 4B  is a functional block diagram of a motor drive system with design/control optimization including a LC filter in accordance with an embodiment of the present invention;  
         [0022]      FIG. 5  is a vector diagram of the variables in a motor drive system for a mathematical model of the design/control optimization in accordance with an embodiment of the present invention;  
         [0023]      FIG. 6  is a vector diagram of the variables in a motor drive system for a mathematical model of the design/control optimization with zero inverter output filter inductance and capacitance according to an embodiment of the present invention;  
         [0024]      FIGS. 7A-7B  illustrate off-line design optimization of the rated operation point in a motor drive system with design/control optimization in accordance with an embodiment of the present invention;  
         [0025]      FIG. 7C  illustrates an exemplary result of off-line design optimization iteration process of the rated operation point in a motor drive system with design/control optimization in accordance with an embodiment of the present invention;  
         [0026]      FIG. 8A  is a vector diagram for on-line control optimization of the motor driving operation point before the AC output voltage limit has been reached in a motor drive system with design/control optimization according to an embodiment of the present invention;  
         [0027]      FIG. 8B  is a vector diagram for on-line control optimization of the motor driving operation point after the AC output voltage limit has been reached in a motor drive system with design/control optimization according to an embodiment of the present invention;  
         [0028]      FIG. 8C  illustrates the optimized inverter AC voltage vector over the whole operating range with on-line control optimization of a motor drive system with design/control optimization in accordance with an embodiment of the present invention;  
         [0029]      FIG. 8D  illustrates an exemplary result of the on-line control optimization process of the motor driving operation point in a motor drive system with design/control optimization in accordance with an embodiment of the present invention;  
         [0030]      FIG. 9  is a schematic of a complete system for design/control optimization of a motor drive system in accordance with an embodiment of the present invention;  
         [0031]      FIG. 10A  is a functional block diagram of an off-line optimization design unit for design/control optimization of a motor drive system in accordance with an embodiment of the present invention;  
         [0032]      FIG. 10B  is a functional block diagram of an on-line optimization control unit for design/control optimization of a motor drive system in accordance with an embodiment of the present invention; and  
         [0033]      FIG. 11  is a table containing typical values for design and operating parameters of the motor drive system used in the 100 KW/540V aircraft cabin air compressor system. 
     
    
     DETAILED DESCRIPTION  
       [0034]     Aspects of the invention are more specifically set forth in the accompanying description with reference to the appended figures.  FIG. 1  is a block diagram of an electrical/mechanical system containing a motor drive system with design/control optimization according to an embodiment of the present invention. The electrical/mechanical system  100  illustrated in  FIG. 1  includes the following components: a power source block  40 ; a motor drive system  45  with design/control optimization; an energy output system  50 ; and individual systems and equipment  55 . Operation of the electrical/mechanical system  100  in  FIG. 1  will become apparent from the following discussion.  
         [0035]     Electrical/mechanical system  100  may be associated with systems with electrical and mechanical components such as a cabin air compressor system, a heating system, a traction system, etc., in an aircraft, a ship, a train, a laboratory facility, etc. Power source block  40  provides electrical power to motor drive system  45 . Power source block  40  handles wattage power that can be hundreds of kW, or MW, and voltages that can be hundreds to thousands of Volts. The output of power source block  40  may be a smoothed DC voltage. Power source block  40  may include transformers, rectifiers, thyristors, filters, and circuit breakers. Motor drive system  45  transforms electrical energy received from power source block  40  into mechanical energy. Motor drive system  45  may include electrical circuits and components, as well as magnetic components such as coils and permanent magnets. Energy output system  50  outputs the energy generated by motor drive system  45  to individual systems and equipment  55 . Energy output system  50  may include shafts, gearboxes, wheels, transmission systems, electrical sensors, and electrical circuits. Individual systems and equipment  55  are systems that enable functioning of services onboard a vehicle, or in a lab. Individual systems and equipment  55  may include a cabin air compressor, an electric generator, a set of wheels, a traction system, a braking system, etc.  
         [0036]      FIG. 2  is a block diagram of a motor drive system  45  with design/control optimization according to an embodiment of the present invention. Motor drive system  45  includes the following components: a DC-AC inverter  65 ; inverter output filters  70 ; and a motor  75 . DC power source block  60  provides electric DC input to DC-AC inverter  65 . DC power source block  60  belongs to power source block  40 . DC-AC inverter  65  converts the electric DC input received from DC power source block  60  into an electric AC voltage. DC-AC inverter  65  is an electrical system that may include semiconductor devices, energy storage components such as capacitors, etc. Inverter output filters  70  eliminate voltage-switching noise caused, for example, by long cables connecting the inverter output filters  70  to motor  75 . Inverter output filters  70  may also eliminate noise in systems where shielding is not preferred, such as “more electric” aircraft systems. Inverter output filters  70  include electrical circuits and components such as inductors and capacitors. The AC voltage output filtered by inverter output filters  70  is input into motor  75 , which outputs mechanical and electric energy. Motor  75  includes electronic and electromagnetic components such as electronic devices, metallic windings, and magnetic cores. Motor  75  may be a permanent magnet synchronous motor.  
         [0037]      FIG. 3  is a flow diagram illustrating operations performed for overall design/control optimization of a motor drive system  45  according to an embodiment of the present invention. The flow diagram in  FIG. 3  implements an optimization technique including both off-line design and on-line control for overall optimization of motor drive system  45  to obtain the maximum efficiency/power density. The optimization technique illustrated in  FIG. 3  takes into account the rated operation point during the off-line design optimization, and the entire operating range during the on-line control optimization of motor drive system  45 . Overall system optimization is achieved when the combination of design/control of inverter  65  and design/control of motor  75  is taken into account during the optimization process. The result is that motor drive system  45  is optimized for inverter unity power factor and motor unity power factor at the rated operating point, and for minimum current for both inverter and motor for the entire specified range of motor drive system operating conditions. According to an embodiment of the present invention, an extensive number of variable factors are considered. The impacts of the variable factors on overall capability of motor drive system  45  to reach optimized performance are assessed. An analytical solution that achieves the optimization goal is then established.  
         [0038]     Minimizing the inverter  65  current and motor  75  current in the entire specified operating range of motor drive system  45  is the main criteria for the design and control optimization of the inverter/motor system design for high efficiency/power density. A combination of off-line design optimization (S 124 ) and on-line control optimization (S 134 ) is implemented to accomplish overall design/control optimization of motor drive system  45 .  
         [0039]     In the off-line design optimization step S 124 , unity power factor is achieved for both inverter and motor at the rated operating point which is selected as the maximum efficiency point. To optimize the maximum efficiency point, an extensive number of motor drive system  45  parameters are selected (S 125 ). The selected parameters are input into an optimization block where off-line optimization design constraints are applied (S 126 ). The off-line optimization design step S 126  uses an optimization iteration step S 142  to find the optimized values of the selected parameters that minimize inverter  65  current (S 127 ), minimize motor  75  current (S 128 ), and bring the power factors of motor  75  and inverter  65  to unity (S 129 ). For this purpose, combinations of motor drive system  45  parameters that satisfy the off-line design optimization constraints in steps S 127 , S 128  and S 129  are found.  
         [0040]     Motor back electromotive force (EMF) and inductance value selection for motor  75 , as well as filter parameters for inverter output filters  70  are found in optimization step S 143 . Minimum inverter current is achieved when DC voltage utilization for inverter  65  is maximized (S 130 ). Motor back EMF and inductance value selection for motor  75  are optimized to ensure minimum motor current under the constraint imposed by the DC bus voltage limitation of inverter  65  (S 131 ). Filter parameters for inverter output filters  70  are optimized so that minimum inverter and motor currents are obtained at the same time when both inverter  65  and motor  75  operate at unity power factor (S 132 ). Step S 124  for off-line design optimization of the rated operation point outputs optimized filter  70  parameters, and optimized motor back EMF and inductance for motor  75  (S 133 ). The output filter and motor parameters help achieve minimum motor current, minimum inverter current and minimum overall current when power factors for both inverter and motor are at unity value, and maximum efficiency/power density is achieved for motor drive system  45  (S 133 ).  
         [0041]     The second step of design/control optimization of motor drive system  45  is on-line control optimization of the entire motor driving operation range (S 134 ). In this step, the minimal current operation point for the optimized efficiency at all operating points is selected. As motor drive system  45  starts operation, DC bus voltage is turned on and connected to the inverter (S 135 ). The inverter AC output voltage has an upper limit that is defined by the DC bus voltage. While the inverter AC output voltage is within the range defined by the AC output voltage upper limit (S 136 ), inverter current, or motor current, or both, are minimized (S 137 ), by applying a control strategy defined by I md =0 for the motor drive system  45  (S 138 ). This strategy aligns the motor current to the motor back EMF. When the AC output voltage hits the upper AC output voltage limit (S 136 ), the constraint I md =0 is not appropriate anymore, as the DC bus voltage cannot continue to produce the expected inverter AC voltage. In this case, maximum voltage control, which maintains the maximum DC bus utilization, becomes the primary goal in the on-line control optimization of motor drive system  45  (S 139 ). The inverter current and motor current are next minimized at the controlled operating point (S 140 ). Hence, with DC bus utilization maintained at the maximum value, the inverter  65  current, or the motor  75  current, or both, are at the minimum.  
         [0042]      FIG. 4A  is a block diagram of a motor drive system  45 A with design/control optimization including a LC filter  70 A in accordance with an embodiment of the present invention. Motor drive system  45 A with design/control optimization includes three main components: a three phase DC-AC inverter  65 A; an inverter output filter  70 A; and a synchronous motor  75 A. Inverter output filter  70 A is an AC differential LC filter that includes an inverter output filter inductor  90  with inductance L and an inverter output filter capacitor  95  with capacitance C. Synchronous motor  75 A includes a motor stator  105  and a rotor  110 . The winding of motor stator  105  acts as an inductor with inductance L m . The synchronous motor  75 A optimized winding inductance L m  is in a specific per unit value range suited for applications in which motor drive system  45 A may be used. Rotor  110  generates a motor back electromotive force (motor back EMF) E. Three phase DC-AC inverter  65 A converts a DC current input I dc  and voltage input V dc  into a three phase AC waveform with inverter bridge output current I ac  and inverter bridge output voltage V ac . Current I c  passes through inverter output filter capacitor  95 . Motor current I m  passes through synchronous motor  75 A, and creates motor terminal voltage V m . Inverter output filter capacitor  95  is in parallel with synchronous motor  75 A. Therefore motor terminal voltage V m  is equal to inverter output filter capacitor voltage V c .  
         [0043]      FIG. 4B  is a functional block diagram of a motor drive system  45 A with design/control optimization including a LC filter in accordance with an embodiment of the present invention.  FIG. 4B  illustrates all electrical components of the electrical circuit for motor drive system  45 A in  FIG. 4A . Inverter bridge output current I ac  gives rise to inverter bridge output voltage V ac  and inverter output filter inductor voltage V L . Inverter bridge output current I ac  is split into current I c  that creates voltage V c  on capacitor  95 , and motor current I m  that gives rise to voltage V Lm  on winding inductance L m  of motor stator  105 . The following mathematical relationships describe the electrical circuit of motor drive system  45 A, where “j” is the complex unity, and “ω” is the angular frequency of the three phase AC signal output of inverter  65 A: 
   Z   L   =jωL   (1)    Z   Lm   =jωL   m   (2)                Z   C     =     1     j   ⁢           ⁢   ω   ⁢           ⁢   C               (   3   )             
  {right arrow over (I)}   ac   ={right arrow over (I)}   m   +{right arrow over (I)}   C   ( 4 ) 
   {right arrow over (V)}   Lm   =Z   Lm   {right arrow over (I)}   m   =jωL   m   {right arrow over (I)}   m   (5)    {right arrow over (V)}   L   =Z   L   {right arrow over (I)}   ac   =jωL{right arrow over (I)}   ac   (6)                  V   →     C     =         Z   C     ⁢       I   →     C       =         I   →     C       j   ⁢           ⁢   ω   ⁢           ⁢   C                 (   7   )             
  {right arrow over (V)}   m   ={right arrow over (E)}+{right arrow over (V)}   Lm   ( 8 ) 
 {right arrow over (V)} C ={right arrow over (V)} Lm   (9)    {right arrow over (V)}   ac   ={right arrow over (V)}   L   +{right arrow over (V)}   m   (10)  
         [0044]      FIG. 5  is a vector diagram of the variables in a motor drive system  45 A for a mathematical model of the design/control optimization in accordance with an embodiment of the present invention. Motor drive system  45 A, which is a three phase machine, can be described as a two phase machine through a transformation from a three phase coordinate system to a two phase coordinate system. The two-phase coordinate system has unit vectors {right arrow over (d)} ( 152 ) and {right arrow over (q)} ( 150 ) which are perpendicular to each other. In the dq motor axis coordinate system, motor current {right arrow over (I)} m  has a d-axis component {right arrow over (I)} md  ( 158 ) and a q-axis component I mq  ( 156 ). The complex unit “j” is along the {right arrow over (q)} motor axis. Using the mathematical relationships (1)-(10) listed above, together with vectors {right arrow over (E)} ( 154 ), {right arrow over (I)} m  ( 1160 ), {right arrow over (V)} Lm =jωL m  {right arrow over (I)} m  ( 162 ), {right arrow over (V)} m  ( 164 ), {right arrow over (I)} C =jωC{right arrow over (V)} C =jωC{right arrow over (V)} m  ( 166 ), {right arrow over (I)} ac  ( 168 ), {right arrow over (V)} L =jωL {right arrow over (I)} ac  ( 170 ), and {right arrow over (V)} ac  ( 172 ) in  FIG. 5 , and aligning the {right arrow over (q)} axis to the motor back EMF vector {right arrow over (E)}, the mathematical model for the design/control optimization in the motor drive system  45 A is described by the equations below: 
   {right arrow over (E)}= 0+ jE   (11)    {right arrow over (I)}   m   =I   md   +jI   mq   (12)  {right arrow over (V)} m   =ωL   m   I   mq   +j ( E+ωL   m   I   md )  (13)    {right arrow over (I)}   C   =−ωC ( E+ω   L   m   I   md )− jω   2   CL   m   I   mq   (14)                        I   →     ac     =       [       I   md     -     ω   ⁢           ⁢     C   ⁡     (     E   +     ω   ⁢           ⁢     L   m     ⁢     I   md         )           ]     +     j   ⁡     (       I   mq     -       ω   2     ⁢     CL   m     ⁢     I   mq         )                     =         I   md     ⁡     (     1   -       ω   2     ⁢     L   m     ⁢   C       )       -     ω   ⁢           ⁢   CE     +       jI   mq     ⁡     (     1   -       ω   2     ⁢     CL   m         )                       (   15   )                         V   →     L     =         -   ω     ⁢           ⁢     L   ⁡     (       I   mq     -       ω   2     ⁢     CL   m     ⁢     I   mq         )         +     j   ⁢           ⁢   ω   ⁢           ⁢     L   ⁡     [       I   md     -     ω   ⁢           ⁢     C   ⁡     (     E   +     ω   ⁢           ⁢     L   m     ⁢     I   md         )           ]                       =       (         -   ω     ⁢           ⁢     LI   mq       +     ω   ⁢           ⁢   L   ⁢           ⁢     ω   2     ⁢     CL   m     ⁢     I   mq         )     +     j   ⁡     (       ω   ⁢           ⁢     LI   md       -       ω   2     ⁢   LCE     -     ω   ⁢           ⁢   L   ⁢           ⁢     ω   2     ⁢     L   m     ⁢     CI   md         )                       (   16   )                         V   →     ac     =     -                   ω   ⁢           ⁢     L   m     ⁢     I   mq       -     ω   ⁢           ⁢     LI   mq       +     ω   ⁢           ⁢   L   ⁢           ⁢     ω   2     ⁢     CL   m     ⁢     I   mq         )     +     ⁢                         j   [     E   +     ω   ⁢           ⁢     L   m     ⁢     I   md       +     ω   ⁢           ⁢     LI   md       -       ω   2     ⁢   LCE     -     ω   ⁢           ⁢   L   ⁢           ⁢     ω   2     ⁢     L   m     ⁢     CI   md         )     ]                         =     -               ω   ⁢     (       L   m     +   L     )     ⁢       I   mq     ⁡     [     1   -       L       L   m     +   L       ⁢     ω   2     ⁢     CL   m         ]         +     ⁢                       j   ⁡     [       E   ⁢     (     1   -       ω   2     ⁢   LC       )       +       ω   ⁡     (       L   m     +   L     )       ⁢     I   md       -     ω   ⁢           ⁢   L   ⁢           ⁢     ω   2     ⁢     L   m     ⁢     CI   md         ]                           =     -               ω   ⁢     (       L   m     +   L     )     ⁢       I   mq     ⁡     [     1   -       L       L   m     +   L       ⁢     ω   2     ⁢     CL   m         ]         +     ⁢                       j   ⁡     [       E   ⁡     (     1   -       ω   2     ⁢   LC       )       +       ω   ⁡     (       L   m     +   L     )       ⁢       I   md     ⁡     (     1   -       L       L   m     +   L       ⁢     ω   2     ⁢     CL   m         )           ]                             (   17   )               
         [0045]      FIG. 6  is a vector diagram of the variables in a motor drive system  45 A for a mathematical model of the design/control optimization with zero inverter output filter inductance and capacitance according to an embodiment of the present invention. When L=0 and C=0, the relationships (15) and (17) become: 
 {right arrow over (I)} ac   ={right arrow over (I)}   m   =I   md   +jI   mq   (18)    {right arrow over (V)}   ac   ={right arrow over (V)}   m   =−ωL   m   I   mq   +j ( E+ωL   m   I   md )  (19)  
 which describe the case when motor drive system  45 A does not include a LC filter. Thus, the results obtained in the general case when motor drive system  45 A includes inverter output filters are applicable to the particular case when motor drive system  45 A does not include inverter output filters. 
 
         [0046]      FIG. 7A  illustrates off-line design optimization of the rated operation point in a motor drive system  45 A with design/control optimization in accordance with an embodiment of the present invention.  FIG. 7A  illustrates the result of operations performed during the off-line design optimization step S 124  in  FIG. 3 , before the optimized parameters for the rated operation point were identified. The power factor for motor  75 A is equal to the cosine of the angle between the motor voltage {right arrow over (V)} m , and the motor current {right arrow over (I)} m , Power Factor=cos(Θ). Maximum power factor is reached when cos(Θ)=1, that is when Θ=0. Since the angle Θ in  FIG. 7A  is small but not zero, the vector diagram in  FIG. 7A  depicts a motor drive system  45 A status that is close to the optimized rated operation point but that has not yet reached the optimized rated operation point.  
         [0047]      FIG. 7B  illustrates off-line design optimization of the rated operation point in a motor drive system  45 A with design/control optimization in accordance with an embodiment of the present invention.  FIG. 7B  is a vector diagram of the optimized rated operation point of motor drive system  45 A, obtained from operations performed during the off-line design optimization step S 124  in  FIG. 3 .  
         [0048]     The identification of optimized parameters for the rated operation point done in step S 124  in  FIG. 3 , is achieved using equations (11)-(17). The vector diagram in  FIG. 7B  exhibits unity power factors for motor  75 A and inverter  65 A, as the angles between {right arrow over (I)} m  ( 252 ) and {right arrow over (V)} m  ( 258 ), and {right arrow over (I)} ac  ( 256 ) and {right arrow over (V)} ac  ( 262 ) respectively, are zero. The resulting right angles between {right arrow over (V)} m  ( 258 ) and jωL m  {right arrow over (I)} m  ( 260 ), {right arrow over (V)} ac  ( 262 ) and jωL {right arrow over (I)} ac  ( 264 ), and {right arrow over (I)} m  ( 252 ) and {right arrow over (I)} C =jωC{right arrow over (V)} m  ( 254 ) lead to the following relationships: 
 
 E   2   =V   m   2 +ω 2   L   m   2   I   m   2   (20) 
 
 V   m   2   =V   ac   2 +ω 2   L   2   I   ac   2   (21) 
 
 I   ac   2   =I   m   2 +ω 2   C   2   m   2   (22) 
 
 which, by mathematical manipulation, lead to a dependence of I m  on L, C, E, and L m  of the form:  
               I   m   2     =           E   2     ⁡     (     1   -       ω   4     ⁢     L   2     ⁢     C   2         )       -     V   ac   2             ω   2     ⁢     L   2       +       ω   2     ⁢     L   m   2       -       ω   6     ⁢     L   2     ⁢     L   m   2     ⁢     C   2                   (   23   )             
 
 A dependence of I ac  on L, C, E, and L m  can be similarly found from the equation:  
                 I   ac   2     =         I   m   2     +       ω   2     ⁢     C   2     ⁢     V   m   2         =       I   m   2     +       ω   2     ⁢       C   2     ⁡     (       V   ac   2     +       ω   2     ⁢     L   2     ⁢     I   ac   2         )       ⁢           ⁢   as           ⁢     
     ⁢       I   ac   2     =         I   m   2     +       ω   2     ⁢     C   2     ⁢     V   ac   2           1   -       ω   4     ⁢     C   2     ⁢     L   2                     (   24   )             
 
 The goal of off-line design optimization is to reach the point shown in equations ( 23 ) and ( 24 ), so that both the inverter  65 A and the motor  75 A are optimized for minimal current with unity power factor. Inverter bridge output voltage V ac  is determined by the DC bus voltage input limit at inverter  65 A. Output filter parameters L and C, motor back EMF E, and inductance L m  need to be optimized for minimal current of the overall system during the optimization iteration process S 142  shown in  FIG. 3 . Equations ( 20 ), ( 21 ), and ( 22 ) are useful for calculating the minimal inverter and motor currents at an optimized rated operation point. The inverter  65 A and motor  75 A are both running at unity power factor and with minimum current at the optimized rated operation point of motor drive system  45 A. Significant to this off-line design optimization process is the consideration of a wide variety of parameters, as shown in step S 124  in  FIG. 3 . Step S 124  in  FIG. 3  drives off-line design optimization for parameters Vac_max, L, C, E, and L m  in the off-line design optimization of motor drive system  45 A implemented in a motor controller system. 
 
         [0049]      FIG. 7C  illustrates an exemplary result of off-line design optimization iteration process of the rated operation point in a motor drive system  45 A with design/control optimization in accordance with an embodiment of the present invention.  FIG. 7C  shows an example of a “run” from the motor back EMF optimization process performed during the off-line design optimization step S 124  in  FIG. 3 . Similar “runs” can be performed for optimization of parameters L, C, L m , or other motor drive system parameters or combination of parameters. The circled region  316  on the graph is the optimized rated operation point where unity power factors for motor  75 A and inverter  65 A have been achieved. The optimized motor back EMF E value is found on the x-axis in circled region  316 . Graphs  310  and  312  are graphs of motor  75 A and inverter  65 A power factors, PF(M) and PF(Inv), which attain the value of 1 (unity) at the optimized rated operation point  316 . Graphs  300  and  304  are graphs of inverter  65 A current I ac  and motor  75 A current I m . Graph  302  is the inverter bridge output voltage V ac , which is a constant determined by the maximum capability of the inverter  65 A as well as by the limit of DC bus voltage input to inverter  65 A. Graph  314  is the output power P out , which is also a constant determined by the required specified power of motor drive system  45 A at the rated condition. Graphs  306  and  308  represent the d-axis and q-axis components of motor  75 A current, I md  and I mq . The squared sum of graphs  306  and  308  gives graph  304  for current I m  of motor  75 A.  
         [0050]     Off-line design optimization of the motor drive system  45 A for “rated operation” as in  FIG. 7B - FIG. 7C  enables maximum efficiency at the rated design operating point. At another operating point, motor drive system  45 A will deviate from the optimized minimal current operation if the motor drive system control is not optimized. A design-compatible on-line control methodology follows the off-line design optimization process. The on-line control methodology is used to optimize motor drive system  45 A operating points to the maximum efficiency/power density.  
         [0051]      FIG. 8A  is a vector diagram for on-line control optimization of the motor driving operation point before the AC output voltage limit has been reached in a motor drive system  45 A with design/control optimization according to an embodiment of the present invention.  FIG. 8A  is a vector diagram obtained from operations performed during the on-line control optimization step S 134  in  FIG. 3  before the AC output voltage limit for inverter  65  is reached. In this situation, the control of I md =0 sets the minimum motor current, and consequently the inverter current operation point. When I md =0, equation (12) {right arrow over (I)} m   =I   md +jI mq  indicates that {right arrow over (I)} m  is along the q-axis and is parallel to {right arrow over (E)}=0+jE. The vector diagram in  FIG. 8A  is obtained based on equations (11)-(17). As it can be seen in  FIG. 8A , angle Σ 1  ( 354 ) between {right arrow over (I)} m  ( 350 ) and {right arrow over (V)} m  ( 352 ) can never be zero because {right arrow over (V)} m  includes the component jωL m  {right arrow over (I)} m . Therefore the power factor for motor  75 A cannot become unity when the AC output voltage is within its lower and upper limit values.  
         [0052]      FIG. 8B  is a vector diagram for on-line control optimization of the motor driving operation point after the AC output voltage limit has been reached in a motor drive system  45 A with design/control optimization according to an embodiment of the present invention.  FIG. 8B  is a vector diagram obtained from operations performed during the on-line control optimization step S 134  in  FIG. 3  after the inverter AC output voltage limit has been reached.  FIG. 8B  illustrates on-line control optimization achieved based on equations (11)-(17) for the entire operating range, so that both inverter  65 A and motor  75 A are optimized for the minimal current. The maximum DC bus voltage utilization is maintained, and the motor  75 A current and inverter  65 A currents are minimized. The following equations can be derived from equations (11)-(17) for on-line control optimization:  
                 I   mq     =         P   out       E   ω       ⁢           ⁢   where       ⁢     
     ⁢       E   ω     =     E   *     ω     ω   rated                   (   25   )                 K   L     =     L       L   m     +   L               (   26   )             
  K   LmC =ω 2   L   m   C   (27) 
   K   LC =ω 2   LC   (28)    V   ac     —     d =−ω*( L   m   +L )* I   mq *(1− K   L   K   LmC )  (29)                V   ac     =       V   ac_max     =         V   ac_limit     *     M   max       =         V   dc         3     *     2         *     [     1   -     2   *     (       T   dead     +     T   minimal       )     *     f   SW         ]                   (   30   )                 V   ac_q     =         V   ac_max   2     -     V   ac_d   2                 (   31   )                 I   md     =         V   ac_q     -       E   ω     *     (     1   -     K   LC       )           ω   *     (       L   m     +   L     )     *     (     1   -       K   L     ⁢     K   LmC         )                 (   32   )               
 Table 1 shown in  FIG. 11  includes detailed definitions of the variables present in the above equations. Equation ( 32 ) shows that the condition of maximum DC bus voltage utilization control can be achieved by controlling the d-axis I m  current component I md . 
 
         [0053]     Two optimization control rules can be derived for on-line control optimization, from equation (32). On-line optimization control rule I is detailed in the equations below:  
         if   ⁢           ⁢     I   md       =           V   ac_q     -       E   ω     *     (     1   -     K   LC       )           ω   *     (       L   m     +   L     )     *     (     1   -       K   L     ⁢     K   LmC         )         ≥   0         
 that is, if  V   ac     —     q   −E   ω *(1 −K   LC )≧0 
 
that is, if √{square root over (V ac     —     max   2   −V   ac     —     d   2 )}− E   ω *(1− K   LC )≧0 
 
then select I md =0.  (33) 
 
 On-line optimization control rule II is detailed in the equations below:  
         if   ⁢           ⁢     I   md       =           V   ac_q     -       E   ω     *     (     1   -     K   LC       )           ω   *     (       L   m     +   L     )     *     (     1   -       K   L     ⁢     K   LmC         )         &lt;   0         
 that is, if  V   ac     —     q   −E   ω *(1 −K   LC )&lt;0 
 
that is, if √{square root over ( V   ac     —     max   2   −V   ac     —     d   2 )}− E   ω *(1 −K   LC )&lt;0 
 
 then select  
               I   md     =           V   ac_q     -       E   ω     *     (     1   -     K   LC       )           ω   *     (       L   m     +   L     )     *     (     1   -       K   L     ⁢     K   LmC         )         .             (   34   )             
 
 In the case of a motor drive system without output filters, as shown in  FIG. 6 , equations (25)˜(29) can be simplified as follows:  
               I   mq     =       P   out       E   ω               (   35   )             
  K   L   =K   LmC   =K   LC =0  (36) 
 
 V   ac     —     d   =−ω*L   m   *I   mq   (37) 
 
               V   ac     =       V   ac_max     =         V   ac_limit     *     M   max       =         V     d   ⁢           ⁢   c           3     *     2         *     [     1   -     2   *     (       T   dead     +     T   minimal       )     *     f   sw         ]                   (   38   )             
 V ac     —     q =√{square root over ( V   ac     —     max   2   −V   ac     —     d   2 )}  ( 39 )  
               I   md     =         V   ac_q     -   E       ω   *     L   m                 (   40   )             
 
 On-line optimization control rules I and II for a motor drive system without output filters are the same as the rules for a motor drive system with output filters described by equations (33) and (34). 
 
         [0054]      FIG. 8C  illustrates the optimized inverter AC voltage vector {right arrow over (V)} ac  over the whole operating range with on-line control optimization of a motor drive system  45 A with design/control optimization in accordance with an embodiment of the present invention. The graph in  FIG. 8C  plots the q-axis component of V ac , V ac     —     q , versus the d-axis component V ac     —     d . Both components increase from zero value at zero speed (point A in  FIG. 8C ) to higher values with higher speed. From point A onwards to point B, V ac     —     q  is increasing faster than V ac     —     d . At point B in  FIG. 8C , the maximum V ac  value V ac     —     max  corresponding to maximum DC bus voltage utilization, has been reached. From point B onwards to point C, V ac  stays constant at V ac     —     max . Since V ac   2 =V ac     —     d   2 +V ac     —     q   2 , V ac     —     q  decreases while V ac     —     d  slowly increases from point B to point C in  FIG. 8C . Point C represents the optimized inverter AC voltage for inverter  65  at the optimized motor driving rated operation point when DC bus limit is met. Hence, curve AB in  FIG. 8C  shows the operating range for V ac  when on-line optimization control rule I described in equation (33) is applied, and curve BC shows the operating range for V ac  when on-line optimization control rule II described in equation (34) is applied. Curve BC can continue beyond point C. The portion beyond point C would correspond to a motor drive system operating range with higher speed than the speed at the rated operating point.  
         [0055]      FIG. 8D  illustrates an exemplary result of the on-line control optimization process of the motor driving operation point in a motor drive system  45 A with design/control optimization in accordance with an embodiment of the present invention.  FIG. 8D  shows an example of a “run” from the whole operating point optimization process of motor drive system  45 A performed during the on-line control optimization step S 134  in  FIG. 3 . The circled region  428  on the graph is the optimized rated operating point at which the DC bus voltage limit is met, representing the point where the unit power factors for motor  75 A and inverter  65 A, and the minimum inverter and motor currents have been achieved. Graphs  426  and  424  are graphs for motor  75 A and inverter  65 A power factors PF(M) and PF(Inv), which attain the value of unity at the optimized rated operation point  428 . Graph  416  is the voltage V ac , which reaches V ac     —     max  at point B (corresponding to point B in  FIG. 8C ), after which is remains constant. Point C on V ac  graph  416  in  FIG. 8D  corresponds to point C in  FIG. 8C . Graph  418  represents the d-axis motor current I md , which is zero until V ac  reaches V ac     —     max  (point B), and increases afterwards, as dictated by the application of on-line control optimization rules I and II from equations (33) and (34). Graph  410  represents the q-axis component I mq  of motor  75 A current I m . I mq  increases continuously with rotational speed as the power of motor drive system  45 A increases. The I mq  increase follows the general relationship between I mq  and P out  described in equation (25). Graphs  420  and  422  represent motor and inverter currents I m  and I ac , which attain minimal values at the controlled operating point. At the same time, unity power factors for inverter and motor are achieved in the optimized operation point region  428 . Graph  414  is the motor back EMF E, which increases proportionally with rotational speed.  
         [0056]      FIG. 9  is a schematic of a complete system for design/control optimization of a motor drive system  45 B in accordance with an embodiment of the present invention. A system for design/control optimization of motor drive system  45 B includes an off-line optimization design unit  502 , and an on-line optimization control unit  503 . Off-line optimization design unit  502  performs optimization iterations that determine and adjust motor drive system functional parameters to optimize motor drive system design. Off-line optimization design unit  502  then outputs the adjusted motor drive system design parameters. When the off-line design optimization process is completed, the motor drive system is physically designed and built, with motor, inverter, and filters exhibiting parameter values output by the off-line optimization design unit  502 . When the motor drive system is ready for testing, the on-line optimization control unit  503  is activated. On-line optimization control unit  503  starts the operation of the inverter  65 B with a DC input voltage within a specified range with an upper limit value. The on-line optimization control unit  503  also starts control of motor  75 B rotor speed from zero to its functional upper limit listed in the motor  75 B specification. On-line optimization control unit  503  receives feedback information from synchronous motor  75 B, inverter output filters  70 B, and three phase DC-AC inverter  65 B. On-line optimization control unit  503  then performs on-line optimization of motor drive system  45 B. For this purpose, on-line optimization control unit  503  applies on-line optimization control rules I and II described in equations (33)-(34) and sends control signals to three phase DC-AC inverter  65 B.  
         [0057]      FIG. 10A  is a functional block diagram of an off-line optimization design unit  502  for design/control optimization of a motor drive system  45 B in accordance with an embodiment of the present invention. Off-line optimization design unit  502  collects an extensive range of parameters from motor drive system  45 B in inverter parameters level I unit  504 , inverter parameters level II unit  505 , motor parameters level I unit  508 , motor parameters level II unit  509 , output filters parameters unit  507 , and off-line design parameters input unit  506 . The definitions and relationships between parameters shown in  FIG. 10A  are detailed in Table 1 shown in  FIG. 11 . For exemplification, table 1 also shown typical values for parameters used in the design of the 100 KW/540V aircraft cabin air compressor motor drive system.  
         [0058]     The parameters collected by units  504 ,  505 ,  506 ,  507 ,  508 , and  509  are embedded into a smaller number of directly related parameters that may include L and C related to inverter output filters  70 B, E and Lm related to motor  75 B, and V ac     —     max  related to inverter  65 B. The resulting parameters are directed to the off-line design optimization iteration unit  510 . Off-line design optimization iteration unit  510  performs iterations that optimize the values of L, C, E and L m  to achieve the optimization goal of unity power factor for both inverter and motor at the rated operating point. The unity power control is set by power factor optimization unit  511 . When off-line design optimization is complete, off-line optimization design unit  502  outputs optimized L, C, E and L m  values.  
         [0059]      FIG. 10B  is a functional block diagram of an on-line optimization control unit  503  for design/control optimization of a motor drive system  45 B in accordance with an embodiment of the present invention. The goal of on-line control optimization is minimal current operation in the whole operating range of motor drive system  45 B. On-line control optimization starts with parameter information from the optimized rated operation point determined in the off-line optimization process. On-line control optimization is designed to accommodate drift of the operating point. Such drift of the operating point may occur due to variations of DC bus voltage or load conditions of motor drive system  45 B. As shown in  FIG. 10B , input parameters selection unit  512  for on-line control optimization selects input parameters in three steps.  
         [0060]     First, parameters from off-line design optimization parameter retrieval unit  513  are stored for the feed forward calculation of I ac     —     q  in Iac_q_ref calculation unit  518 , and of I ac     —     d  in Iac_d_ref calculation unit  519 . The calculation of I ac     —     q  and I ac     —     d  is done according to the following equations derived from equation (15): 
 
 I   ac     —     d     —     ref   =I   md     —     ref (1−ω 2   L   m   C )−ω CE   ω   (41) 
 
 I   ac     —     q     —     ref   =I   mq     —     ref (1−ω 2   CL   m )  (42) 
 
               E   ω     =     E   *     ω     ω   rated                 (   43   )             
 
 In the equations above, E ω  and E are the motor back EMFs at current speed ω and at rated speed ω rated  from off-line design optimization. The presence of inverter LC output filters  70 B calls for the feed-forward compensation performed in units  518  and  519 . 
 
         [0061]     Secondly, measurement feedbacks from measurement/feedback unit  514  for DC voltage Vdc, rotor position θ and speed co are sent to the AC voltage limitation calculation Vac_max unit  524 . The components Iac_A, Iac_B and Iac_C of measured three phase current inverter {right arrow over (I)} ac     —     abc  are sent to the abc/dq transformation unit  515  which obtains the dq current components Iac_d and Iac_q. The dq current components Iac_d and Iac_q are later used by regulation units  520  and  521 .  
         [0062]     Thirdly, the input parameters from input parameters selection unit  512  are sent to system level command unit  516  which controls ω ref , hence driving the commanded operating point of motor drive system  45 B. Speed command ω ref  and measured speed ω are then sent to the speed regulation unit  517 , which generates the motor torque command Imq_ref.  
         [0063]     The motor magnetic current command Imd_ref is also generated, based on the on-line control optimization rules I and II described by equations (33) and (34). For this purpose, AC voltage limitation calculation Vac_max unit  524  calculates the AC voltage limit based on the measured Vdc, as shown in equation (30). Unit  525  then uses equation (31) to calculate the V ac     —     q  limitation due to the DC voltage limitation. For the case when I md =0, simplified equation (17) can also be used to obtain 
 
 V   ac     —     q     —     demand   =E   ω *(1−ω 2   LC )= E   ω *(1− K   LC )  (44) 
 
 Equation (44) gives the demanded Vac_q value when control I md =0 applies. 
 
         [0064]     On-line control optimization rules I and II described by equations (33) and (34) are applied forward in units  525 ,  526 ,  527 ,  528  and  529 . Using equation (44) and on-line control optimization rule I, units  525 ,  526  and  528  implement on-line control optimization of motor drive system  45 B as described below: 
 
if √{square root over ( V   ac     —     max   2   −V   ac     —     d   2 )}− E   ω *(1− K   LC )≧0 
 
that is, if √{square root over ( V   ac     —     max   2   −V   ac     —     d   2 )}− V   ac     —     q     —     demand )≧0 
 
that is, if √{square root over ( V   2   ac     —     max   −V   2   ac     —     d )}− V   ac     —     q ≧0, 
 
then set I md     —     ref =0. 
 
 Using on-line control optimization rule II, units  525 ,  526 ,  527  and  529  implement on-line control optimization of motor drive system  45 B as described below: 
 
if √{square root over ( V   ac     —     max   2   −V   ac     —     d   2 )}− E   ω *(1− K   LC )&lt;0 
 
that is, if √{square root over ( V   ac     —     max   2   −V   ac     —     d   2 )}− V   ac     —     q     —     demand )&lt;0 
 
that is, if √{square root over ( V   2   ac     —     max   −V   2   ac     —     d )}− V   ac     —     q &lt;0, 
 
 then use √{square root over (V 2   ac     —     max −V 2   ac     —     d )}−V ac     —     q  as the input to regulate I md     —     ref  to the expected value  
                 V   ac_max   2     -     V   ac_d   2         -       E   ω     *     (     1   -     K   LC       )           ω   *     (       L   m     +   L     )     *     (     1   -       K   L     ⁢     K   LmC         )         ,       
 
 in order to keep the maximum AC voltage operating with the limited DC voltage value. Unit  527  can be implemented with feed-forward structure based on on-line control optimization rule II equations (34), or with feedback structure based on the error between √{square root over (V 2   ac     —     max −V 2   ac     —     d )} and V ac     —     q , or a combination of both rule II and error analysis. 
 
         [0065]     I ac     —     q     —   ref and I ac     —     q     —   ref generated by the Iac_q_ref calculation unit  518  and Iac_d_ref calculation unit  519  are sent to Iac_q regulation unit  520  and Iac_d regulation unit  521  respectively. Units  520  and  521  compare optimized operating parameters of motor drive system  45 B with actual operating parameters of motor drive system  45 B and perform appropriate regulation of motor drive system operating parameters. Outputs of units  520  and  521  are sent to the dq/abc transformation unit  522  and then next to the inverter Pulse Width Modulation (PWM) control unit  523 . The commanded PWM gating is then added to DC-AC inverter  65 B to complete the on-line optimization control process.  
         [0066]      FIG. 11  is a table containing typical values for design and operating parameters of the motor drive system used in the 100 KW/540V aircraft cabin air compressor system. Parameters in Table 1 can be used to better understand equations that describe design and control optimization of motor drive system  45 .  
         [0067]     The proposed invention presents a new overall design/control concept for a motor drive system to achieve an overall system optimization for maximum efficiency in the entire motor drive system operating range, under various inputs and load conditions. The technique described in the current invention can be applied to a motor drive system including various types of inverter output filters, as well as to a motor drive system without inverter output filters. The technique described in the current invention can be applied to synchronous motors with limited DC bus voltage.