High power density/limited DC link voltage synchronous motor drive

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.

BACKGROUND OF THE INVENTION

1. Field of the Invention

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.

2. Description of the Related Art

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

DETAILED DESCRIPTION

Aspects of the invention are more specifically set forth in the accompanying description with reference to the appended figures.FIG. 1is 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 system100illustrated inFIG. 1includes the following components: a power source block40; a motor drive system45with design/control optimization; an energy output system50; and individual systems and equipment55. Operation of the electrical/mechanical system100inFIG. 1will become apparent from the following discussion.

Electrical/mechanical system100may 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 block40provides electrical power to motor drive system45. Power source block40handles 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 block40may be a smoothed DC voltage. Power source block40may include transformers, rectifiers, thyristors, filters, and circuit breakers. Motor drive system45transforms electrical energy received from power source block40into mechanical energy. Motor drive system45may include electrical circuits and components, as well as magnetic components such as coils and permanent magnets. Energy output system50outputs the energy generated by motor drive system45to individual systems and equipment55. Energy output system50may include shafts, gearboxes, wheels, transmission systems, electrical sensors, and electrical circuits. Individual systems and equipment55are systems that enable functioning of services onboard a vehicle, or in a lab. Individual systems and equipment55may include a cabin air compressor, an electric generator, a set of wheels, a traction system, a braking system, etc.

FIG. 2is a block diagram of a motor drive system45with design/control optimization according to an embodiment of the present invention. Motor drive system45includes the following components: a DC-AC inverter65; inverter output filters70; and a motor75. DC power source block60provides electric DC input to DC-AC inverter65. DC power source block60belongs to power source block40. DC-AC inverter65converts the electric DC input received from DC power source block60into an electric AC voltage. DC-AC inverter65is an electrical system that may include semiconductor devices, energy storage components such as capacitors, etc. Inverter output filters70eliminate voltage-switching noise caused, for example, by long cables connecting the inverter output filters70to motor75. Inverter output filters70may also eliminate noise in systems where shielding is not preferred, such as “more electric” aircraft systems. Inverter output filters70include electrical circuits and components such as inductors and capacitors. The AC voltage output filtered by inverter output filters70is input into motor75, which outputs mechanical and electric energy. Motor75includes electronic and electromagnetic components such as electronic devices, metallic windings, and magnetic cores. Motor75may be a permanent magnet synchronous motor.

FIG. 3is a flow diagram illustrating operations performed for overall design/control optimization of a motor drive system45according to an embodiment of the present invention. The flow diagram inFIG. 3implements an optimization technique including both off-line design and on-line control for overall optimization of motor drive system45to obtain the maximum efficiency/power density. The optimization technique illustrated inFIG. 3takes 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 system45. Overall system optimization is achieved when the combination of design/control of inverter65and design/control of motor75is taken into account during the optimization process. The result is that motor drive system45is 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 system45to reach optimized performance are assessed. An analytical solution that achieves the optimization goal is then established.

Minimizing the inverter65current and motor75current in the entire specified operating range of motor drive system45is 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 (S124) and on-line control optimization (S134) is implemented to accomplish overall design/control optimization of motor drive system45.

In the off-line design optimization step S124, 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 system45parameters are selected (S125). The selected parameters are input into an optimization block where off-line optimization design constraints are applied (S126). The off-line optimization design step S126uses an optimization iteration step S142to find the optimized values of the selected parameters that minimize inverter65current (S127), minimize motor75current (S128), and bring the power factors of motor75and inverter65to unity (S129). For this purpose, combinations of motor drive system45parameters that satisfy the off-line design optimization constraints in steps S127, S128and S129are found.

Motor back electromotive force (EMF) and inductance value selection for motor75, as well as filter parameters for inverter output filters70are found in optimization step S143. Minimum inverter current is achieved when DC voltage utilization for inverter65is maximized (S130). Motor back EMF and inductance value selection for motor75are optimized to ensure minimum motor current under the constraint imposed by the DC bus voltage limitation of inverter65(S131). Filter parameters for inverter output filters70are optimized so that minimum inverter and motor currents are obtained at the same time when both inverter65and motor75operate at unity power factor (S132). Step S124for off-line design optimization of the rated operation point outputs optimized filter70parameters, and optimized motor back EMF and inductance for motor75(S133). 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 system45(S133).

The second step of design/control optimization of motor drive system45is on-line control optimization of the entire motor driving operation range (S134). In this step, the minimal current operation point for the optimized efficiency at all operating points is selected. As motor drive system45starts operation, DC bus voltage is turned on and connected to the inverter (S135). 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 (S136), inverter current, or motor current, or both, are minimized (S137), by applying a control strategy defined by Imd=0 for the motor drive system45(S138). This strategy aligns the motor current to the motor back EMF. When the AC output voltage hits the upper AC output voltage limit (S136), the constraint Imd=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 system45(S139). The inverter current and motor current are next minimized at the controlled operating point (S140). Hence, with DC bus utilization maintained at the maximum value, the inverter65current, or the motor75current, or both, are at the minimum.

FIG. 4Ais a block diagram of a motor drive system45A with design/control optimization including a LC filter70A in accordance with an embodiment of the present invention. Motor drive system45A with design/control optimization includes three main components: a three phase DC-AC inverter65A; an inverter output filter70A; and a synchronous motor75A. Inverter output filter70A is an AC differential LC filter that includes an inverter output filter inductor90with inductance L and an inverter output filter capacitor95with capacitance C. Synchronous motor75A includes a motor stator105and a rotor110. The winding of motor stator105acts as an inductor with inductance Lm. The synchronous motor75A optimized winding inductance Lmis in a specific per unit value range suited for applications in which motor drive system45A may be used. Rotor110generates a motor back electromotive force (motor back EMF) E. Three phase DC-AC inverter65A converts a DC current input Idcand voltage input Vdcinto a three phase AC waveform with inverter bridge output current Iacand inverter bridge output voltage Vac. Current Icpasses through inverter output filter capacitor95. Motor current Impasses through synchronous motor75A, and creates motor terminal voltage Vm. Inverter output filter capacitor95is in parallel with synchronous motor75A. Therefore motor terminal voltage Vmis equal to inverter output filter capacitor voltage Vc.

FIG. 4Bis a functional block diagram of a motor drive system45A with design/control optimization including a LC filter in accordance with an embodiment of the present invention.FIG. 4Billustrates all electrical components of the electrical circuit for motor drive system45A inFIG. 4A. Inverter bridge output current Iacgives rise to inverter bridge output voltage Vacand inverter output filter inductor voltage VL. Inverter bridge output current Iacis split into current Icthat creates voltage Vcon capacitor95, and motor current Imthat gives rise to voltage VLmon winding inductance Lmof motor stator105. The following mathematical relationships describe the electrical circuit of motor drive system45A, where “j” is the complex unity, and “ω” is the angular frequency of the three phase AC signal output of inverter65A:
ZL=jωL(1)
ZLm=jωLm(2)

FIG. 5is a vector diagram of the variables in a motor drive system45A for a mathematical model of the design/control optimization in accordance with an embodiment of the present invention. Motor drive system45A, 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)}mhas a d-axis component Imd(158) and a q-axis component Imq(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(160), {right arrow over (V)}Lm=jωLm{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) inFIG. 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 system45A is described by the equations below:
{right arrow over (E)}=0+jE(11)
{right arrow over (I)}m=Imd+jImq(12)
{right arrow over (V)}m=−ωLmImq+j(E+ωLmImd)  (13)
{right arrow over (I)}c=−ωC(E+ωLmImd)−jω2CLmImq(14)

FIG. 6is a vector diagram of the variables in a motor drive system45A 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=Imd+jImq(18)
{right arrow over (V)}ac=Vm=−ωLmImq+j(E+ωLmImd)  (19)
which describe the case when motor drive system45A does not include a LC filter. Thus, the results obtained in the general case when motor drive system45A includes inverter output filters are applicable to the particular case when motor drive system45A does not include inverter output filters.

FIG. 7Aillustrates off-line design optimization of the rated operation point in a motor drive system45A with design/control optimization in accordance with an embodiment of the present invention.FIG. 7Aillustrates the result of operations performed during the off-line design optimization step S124inFIG. 3, before the optimized parameters for the rated operation point were identified. The power factor for motor75A 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 inFIG. 7Ais small but not zero, the vector diagram inFIG. 7Adepicts a motor drive system45A status that is close to the optimized rated operation point but that has not yet reached the optimized rated operation point.

FIG. 7Billustrates off-line design optimization of the rated operation point in a motor drive system45A with design/control optimization in accordance with an embodiment of the present invention.FIG. 7Bis a vector diagram of the optimized rated operation point of motor drive system45A, obtained from operations performed during the off-line design optimization step S124inFIG. 3.

The identification of optimized parameters for the rated operation point done in step S124inFIG. 3, is achieved using equations (11)–(17). The vector diagram inFIG. 7Bexhibits unity power factors for motor75A and inverter65A, 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 jωLm{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:
E2=Vm2+ω2Lm2Im2(20)
Vm2=Vac2+ω2+L2Lac2(21)
Ica2=Im2+ω2C2Vm2(22)
which, by mathematical manipulation, lead to a dependence of Imon L, C, E, and Lmof the form:

Im2=E2⁡(1-ω4⁢L2⁢C2)-Va⁢⁢c2ω2⁢L2+ω2⁢Lm2-ω6⁢L2⁢Lm2⁢C2(23)
A dependence of Iacon L, C, E, and Lmcan be similarly found from the equation:
Iac2=Im2+ω2C2Vm2=Im2+ω2C2(Vac2+ω2L2Iac2) as

Ia⁢⁢c2=Im2+ω2⁢C2⁢Va⁢⁢c21-ω4⁢C2⁢L2(24)
The goal of off-line design optimization is to reach the point shown in equations (23) and (24), so that both the inverter65A and the motor75A are optimized for minimal current with unity power factor. Inverter bridge output voltage Vacis determined by the DC bus voltage input limit at inverter65A. Output filter parameters L and C, motor back EMF E, and inductance Lmneed to be optimized for minimal current of the overall system during the optimization iteration process S142shown inFIG. 3. Equations (20), (21), and (22) are useful for calculating the minimal inverter and motor currents at an optimized rated operation point. The inverter65A and motor75A are both running at unity power factor and with minimum current at the optimized rated operation point of motor drive system45A. Significant to this off-line design optimization process is the consideration of a wide variety of parameters, as shown in step S124inFIG. 3. Step S124inFIG. 3drives off-line design optimization for parameters Vac_max, L, C, E, and Lmin the off-line design optimization of motor drive system45A implemented in a motor controller system.

FIG. 7Cillustrates an exemplary result of off-line design optimization iteration process of the rated operation point in a motor drive system45A with design/control optimization in accordance with an embodiment of the present invention.FIG. 7Cshows an example of a “run” from the motor back EMF optimization process performed during the off-line design optimization step S124inFIG. 3. Similar “runs” can be performed for optimization of parameters L, C, Lm, or other motor drive system parameters or combination of parameters. The circled region316on the graph is the optimized rated operation point where unity power factors for motor75A and inverter65A have been achieved. The optimized motor back EMF E value is found on the x-axis in circled region316. Graphs310and312are graphs of motor75A and inverter65A power factors, PF(M) and PF(Inv), which attain the value of 1 (unity) at the optimized rated operation point316. Graphs300and304are graphs of inverter65A current Iacand motor75A current Im. Graph302is the inverter bridge output voltage Vac, which is a constant determined by the maximum capability of the inverter65A as well as by the limit of DC bus voltage input to inverter65A. Graph314is the output power Pout, which is also a constant determined by the required specified power of motor drive system45A at the rated condition. Graphs306and308represent the d-axis and q-axis components of motor75A current, Imdand Imq. The squared sum of graphs306and308gives graph304for current Imof motor75A.

Off-line design optimization of the motor drive system45A for “rated operation” as in FIGS.7B–FIG. 7Cenables maximum efficiency at the rated design operating point. At another operating point, motor drive system45A 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 system45A operating points to the maximum efficiency/power density.

FIG. 8Ais 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 system45A with design/control optimization according to an embodiment of the present invention.FIG. 8Ais a vector diagram obtained from operations performed during the on-line control optimization step S134inFIG. 3before the AC output voltage limit for inverter65is reached. In this situation, the control of Imd=0 sets the minimum motor current, and consequently the inverter current operation point. When Imd=0, equation (12) {right arrow over (I)}m=Imd+jImqindicates that {right arrow over (I)}mis along the q-axis and is parallel to {right arrow over (E)}=0+jE. The vector diagram inFIG. 8Ais obtained based on equations (11)–(17). As it can be seen inFIG. 8A, angle Θ1(354) between {right arrow over (I)}m(350) and {right arrow over (V)}m(352) can never be zero because {right arrow over (V)}mincludes the component jωLm{right arrow over (I)}m. Therefore the power factor for motor75A cannot become unity when the AC output voltage is within its lower and upper limit values.

FIG. 8Bis 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 system45A with design/control optimization according to an embodiment of the present invention.FIG. 8Bis a vector diagram obtained from operations performed during the on-line control optimization step S134inFIG. 3after the inverter AC output voltage limit has been reached.FIG. 8Billustrates on-line control optimization achieved based on equations (11)–(17) for the entire operating range, so that both inverter65A and motor75A are optimized for the minimal current. The maximum DC bus voltage utilization is maintained, and the motor75A current and inverter65A currents are minimized. The following equations can be derived from equations (11)–(17) for on-line control optimization:

Imd=Vac_q-Eω*(1-KLC)ω*(Lm+L)*(1-KL⁢KLmC)(32)
Table 1 shown inFIG. 11includes 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 Imcurrent component Imd.

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

Imd=Vac_q-Eω*(1-KLC)ω*(Lm+L)*(1-KL⁢KLmC)≥0
that is, ifVac—q−Eω*(1−KLC)≧0
that is, if √{square root over (Vac—max2−Vac—d2)}−Eω*(1−KLC)≧0
then select Imd=0.  (33)
On-line optimization control rule II is detailed in the equations below:
if

Imd=Vac_q-Eω*(1-KLC)ω*(Lm+L)*(1-KL⁢KLmC)<0
that is, if Vac—q−E*(1−KLC)<0
that is, if √{square root over (Vac—max2−Vac—d2)}−Eω*(1−KLC)<0
then select

Imd=Vac_q-Eω*(1-KLC)ω*(Lm+L)*(1-KL⁢KLmC).(34)
In the case of a motor drive system without output filters, as shown inFIG. 6, equations (25)˜(29) can be simplified as follows:

Imd=Vac_q-Eω*Lm(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).

FIG. 8Cillustrates the optimized inverter AC voltage vector {right arrow over (V)}acover the whole operating range with on-line control optimization of a motor drive system45A with design/control optimization in accordance with an embodiment of the present invention. The graph inFIG. 8Cplots the q-axis component of Vac, Vac—q, versus the d-axis component Vac—d. Both components increase from zero value at zero speed (point A inFIG. 8C) to higher values with higher speed. From point A onwards to point B, Vac—qis increasing faster than Vac—d. At point B inFIG. 8C, the maximum Vacvalue Vac—maxcorresponding to maximum DC bus voltage utilization, has been reached. From point B onwards to point C, Vacstays constant at Vac—max. Since Vac2=Vac—d2+Vaac—q2, Vac—qdecreases while Vac—dslowly increases from point B to point C inFIG. 8C. Point C represents the optimized inverter AC voltage for inverter65at the optimized motor driving rated operation point when DC bus limit is met. Hence, curve AB inFIG. 8Cshows the operating range for Vacwhen on-line optimization control rule I described in equation (33) is applied, and curve BC shows the operating range for Vacwhen 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.

FIG. 8Dillustrates an exemplary result of the on-line control optimization process of the motor driving operation point in a motor drive system45A with design/control optimization in accordance with an embodiment of the present invention.FIG. 8Dshows an example of a “run” from the whole operating point optimization process of motor drive system45A performed during the on-line control optimization step S134inFIG. 3. The circled region428on 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 motor75A and inverter65A, and the minimum inverter and motor currents have been achieved. Graphs426and424are graphs for motor75A and inverter65A power factors PF(M) and PF(Inv), which attain the value of unity at the optimized rated operation point428. Graph416is the voltage Vac, which reaches Vac—maxat point B (corresponding to point B inFIG. 8C), after which is remains constant. Point C on Vacgraph416inFIG. 8Dcorresponds to point C inFIG. 8C. Graph418represents the d-axis motor current Imd, which is zero until Vacreaches Vac—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). Graph410represents the q-axis component Imqof motor75A current Im. Imqincreases continuously with rotational speed as the power of motor drive system45A increases. The Imqincrease follows the general relationship between Imqand Poutdescribed in equation (25). Graphs420and422represent motor and inverter currents Imand Iac, 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 region428. Graph414is the motor back EMF E, which increases proportionally with rotational speed.

FIG. 9is a schematic of a complete system for design/control optimization of a motor drive system45B in accordance with an embodiment of the present invention. A system for design/control optimization of motor drive system45B includes an off-line optimization design unit502, and an on-line optimization control unit503. Off-line optimization design unit502performs optimization iterations that determine and adjust motor drive system functional parameters to optimize motor drive system design. Off-line optimization design unit502then 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 unit502. When the motor drive system is ready for testing, the on-line optimization control unit503is activated. On-line optimization control unit503starts the operation of the inverter65B with a DC input voltage within a specified range with an upper limit value. The on-line optimization control unit503also starts control of motor75B rotor speed from zero to its functional upper limit listed in the motor75B specification. On-line optimization control unit503receives feedback information from synchronous motor75B, inverter output filters70B, and three phase DC-AC inverter65B. On-line optimization control unit503then performs on-line optimization of motor drive system45B. For this purpose, on-line optimization control unit503applies on-line optimization control rules I and II described in equations (33)–(34) and sends control signals to three phase DC-AC inverter65B.

FIG. 10Ais a functional block diagram of an off-line optimization design unit502for design/control optimization of a motor drive system45B in accordance with an embodiment of the present invention. Off-line optimization design unit502collects an extensive range of parameters from motor drive system45B in inverter parameters level I unit504, inverter parameters level II unit505, motor parameters level I unit508, motor parameters level II unit509, output filters parameters unit507, and off-line design parameters input unit506. The definitions and relationships between parameters shown inFIG. 10Aare detailed in Table 1 shown inFIG. 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.

The parameters collected by units504,505,506,507,508, and509are embedded into a smaller number of directly related parameters that may include L and C related to inverter output filters70B, E and Lmrelated to motor75B, and Vac—maxrelated to inverter65B. The resulting parameters are directed to the off-line design optimization iteration unit510. Off-line design optimization iteration unit510performs iterations that optimize the values of L, C, E and Lmto 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 unit511. When off-line design optimization is complete, off-line optimization design unit502outputs optimized L, C, E and Lmvalues.

FIG. 10Bis a functional block diagram of an on-line optimization control unit503for design/control optimization of a motor drive system45B 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 system45B. 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 system45B. As shown inFIG. 10B, input parameters selection unit512for on-line control optimization selects input parameters in three steps.

Eω=E*ωωrated(43)
In the equations above, Eωand E are the motor back EMFs at current speed ω and at rated speed ωratedfrom off-line design optimization. The presence of inverter LC output filters70B calls for the feed-forward compensation performed in units518and519.

Secondly, measurement feedbacks from measurement/feedback unit514for DC voltage Vdc, rotor position θ and speed ω are sent to the AC voltage limitation calculation Vac_max unit524. The components Iac_A, Iac_B and Iac_C of measured three phase current inverter {right arrow over (I)}ac—abcare sent to the abc/dq transformation unit515which obtains the dq current components Iac_d and Iac_q. The dq current components Iac_d and Iac_q are later used by regulation units520and521.

Thirdly, the input parameters from input parameters selection unit512are sent to system level command unit516which controls ωref, hence driving the commanded operating point of motor drive system45B. Speed command ωrefand measured speed ω are then sent to the speed regulation unit517, which generates the motor torque command Imq_ref.

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 unit524calculates the AC voltage limit based on the measured Vdc, as shown in equation (30). Unit525then uses equation (31) to calculate the Vac—qlimitation due to the DC voltage limitation. For the case when Imd=0, simplified equation (17) can also be used to obtain
Vac—q—demand=Eω*(1−ω2LC)=Eω*(1−KLC)  (44)
Equation (44) gives the demanded Vac_q value when control Imd=0 applies.

On-line control optimization rules I and II described by equations (33) and (34) are applied forward in units525,526,527,528and529. Using equation (44) and on-line control optimization rule I, units525,526and528implement on-line control optimization of motor drive system45B as described below:

if √{square root over (Vac—max2−Vac—d2)}−Eω*(1−KLC)≧0

that is, if √{square root over (V2ac—max−V2ac—d)}−Vac—q≧0,

then set Imd—ref=0.

Using on-line control optimization rule II, units525,526,527and529implement on-line control optimization of motor drive system45B as described below:

if √{square root over (Vac—max2−Vac—d2)}−Eω*(1−KLC)<0

that is, if √{square root over (Vac—max2−Vac—d2)}−Vac—q—demand)<0

that is, if √{square root over (V2ac—max−V2ac—d)}−Vac—q<0,

then use √{square root over (V2ac—max−V2ac—d)}−Vac—qas the input to regulate Imd—refto the expected value

Vac_max2-Vac_d2-Eω*(1-KLC)ω*(Lm+L)*(1-KL⁢KLmC),
in order to keep the maximum AC voltage operating with the limited DC voltage value. Unit527can 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 (V2ac—max−V2ac—d)} and Vac—q, or a combination of both rule II and error analysis.

Iac—q—ref and Iac—q—ref generated by the Iac_q_ref calculation unit518and Iac_d_ref calculation unit519are sent to Iac_q regulation unit520and Iac_d regulation unit521respectively. Units520and521compare optimized operating parameters of motor drive system45B with actual operating parameters of motor drive system45B and perform appropriate regulation of motor drive system operating parameters. Outputs of units520and521are sent to the dq/abc transformation unit522and then next to the inverter Pulse Width Modulation (PWM) control unit523. The commanded PWM gating is then added to DC-AC inverter65B to complete the on-line optimization control process.

FIG. 11is 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 system45.

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.