Abstract:
A method and apparatus control a power converter ( 20 ) of an AC motor drive system ( 10 ). The method and apparatus: generate a field current regulating signal to control a field current component flowing from the power converter ( 20 ) to the AC motor ( 30 ), thereby achieving field current regulation; generate a torque current regulating signal to control a torque current component flowing from the power converter ( 20 ) to the AC motor ( 30 ), thereby achieving torque current regulation, the torque current regulation having lower priority than the field current regulation; and execute a close-loop field weakening control scheme, which generates a field weakening control command as a function of the difference between a torque current regulation voltage demand and voltage available for torque current regulation. The field current regulating signal is adjusted in accordance with the field weakening control signal to selectively reduce back EMF of the AC motor ( 30 ), thereby enabling the step of generating a toque current regulating signal to achieve a toque current component needed to drive the AC motor ( 30 ) at a desired speed despite a limitation on DC voltage available to the power converter ( 20 ).

Description:
FIELD OF THE INVENTION 
   The present invention relates to inverter control in an AC motor drive system, and more particularly to a method and apparatus for controlling field weakening in an AC motor drive system. 
   BACKGROUND OF THE INVENTION 
   Voltage source inverter feed AC motor drives have become increasingly popular in general industrial applications, as well as in transportation vehicles such as electrical propulsion systems. In such applications, a wide operating speed range above the base speed (e.g., a high speed cruise) is often required. The recently emerging “more electrical” aircraft concept has also created more demand for AC motor drives in aerospace applications, such as for supplying engine starter, fan, and pump loads. Because of the limited DC bus voltage on aircraft and high output power rating requirement, some of these drive systems must be designed to operate at field weakening mode even at the rated operating point to achieve maximum voltage/current utilization and high efficiency operation. This makes field weakening control a critical part of the motor controller design. 
   When motor speed is lower than base speed, the inverter can provide enough voltage to support motor back EMF, so that field weakening is not required. When motor speed is higher than base speed, however, motor back EMF will exceed the inverter output voltage capability unless field weakening is applied. Thus, field weakening must be implemented to reduce the effective back EMF to achieve high-speed operation above base speed. 
   One basic field-weakening technique, such as the one applied in U.S. Pat. No. 6,407,531 issued to Walters et al. on Jun. 18, 2002, relies on a look up table. This kind of technique, however, requires that a large quantity of data be created off line and stored in the memory to achieve optimal field weakening control under any DC link voltage and any load conditions. Furthermore, sufficient margin must be factored in for parameter variation and the extra voltage needed during transition state. As such, the inverter output voltage capability cannot be fully utilized, which is a significant drawback for aerospace applications because it is directly related to the inverter size and weight. 
   Another approach to flux weakening is to calculate, on-line, the field weakening current from motor equations. Such an approach is described in U.S. Pat. No. 5,739,664 issued to Deng et al. on Apr. 14, 1998 and U.S. Pat. No. 6,504,329 issued to Stancu et al. on Jan. 7, 2003. These approaches, however, are very sensitive to uncertainties related to the system parameters and equations will be very complex for systems with an AC side output LC filter. A sufficient margin must be factored in to ensure stable system operation even with parameter variation. Thus, inverter output capability cannot be fully utilized. 
   A known field weakening control scheme is a close loop method, which does not use machine parameters for calculations in the field weakening operation. This control scheme should be able to adjust field-weakening current automatically during transient and steady state according to DC link voltage and load conditions. U.S. Pat. No. 5,168,204 issued to Schauder on Dec. 1, 1992, U.S. Pat. No. 6,288,515 issued to Hiti et al. on Sep. 11, 2001 and the paper authored by J. H. Song, J. M. Kim and S. K. Sul, entitled “A New Robust SPMSM Control to Parameter Variations in Flux Weakening Region,” Proc. IECON&#39;96, pp. 1193–1198, 1996, provide techniques that possess these features. These techniques adjust field-weakening current according to the inverter output voltage amplitude. There is no need for machine parameters but the choice of parameters in the field-weakening loop is still critical for the stability of the system. Because such techniques are close-loop based, during transition both d-axis and q-axis current loops lose control due to the shortage of voltage and over modulation will also occur, which will cause high frequency resonance for systems with AC side output LC filters. Unfortunately, many drive systems in aerospace applications require LC filters for the tough EMI requirements and the long cable between inverter and motor. 
   SUMMARY 
   According to one aspect, the present invention is a method of controlling a power converter of an AC motor drive system, the method comprising: generating a field current regulating signal to control a field current component flowing from the power converter to the AC motor, thereby achieving field current regulation; generating a torque current regulating signal to control a torque current component flowing from the power converter to the AC motor, thereby achieving torque current regulation, the torque current regulation having lower priority than the field current regulation; and executing a close-loop field weakening control scheme, which generates a field weakening control command as a function of the difference between a torque current regulation voltage demand and voltage available for torque current regulation, wherein the field current regulating signal is adjusted in accordance with the field weakening control signal to selectively reduce back EMF of the AC motor, thereby enabling the step of generating a toque current regulating signal to achieve a toque current component needed to drive the AC motor at a desired speed despite a limitation on DC voltage available to the power converter. 
   According to another aspect, the present invention is directed to a power converter controlling apparatus for controlling a power converter of an AC motor drive system, the controlling apparatus comprising: a field current controller for generating a field current regulating signal to control a field current component flowing from the power converter to the AC motor, thereby achieving field current regulation; a torque current controller for generating a torque current regulating signal to control a torque current component flowing from the power converter to the AC motor, thereby achieving torque current regulation, the torque current regulation having lower priority than the field current regulation; and a field weakening controller for executing a close-loop field weakening control scheme, which generates a field weakening control command as a function of the difference between a torque current regulation voltage demand and voltage available for torque current regulation, wherein the field current controller adjusts the field current regulating signal in accordance with the field weakening control signal to selectively reduce back EMF of the AC motor, thereby enabling the torque current controller to output a torque current regulating signal to achieve a torque current component needed to drive the AC motor at a desired speed despite a limitation on DC voltage available to the power converter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, wherein: 
       FIG. 1  is a block diagram illustrating an AC motor drive system to which principles of the present invention may be applied; 
       FIG. 2  is a general block diagram illustrating functional components of an inverter control unit according to an embodiment of the present invention; and 
       FIG. 3  illustrates, in greater detail, elements of the inverter control unit of  FIG. 2  in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In one aspect, an embodiment of the present invention described below applies field weakening control in an AC motor drive system to maximize efficiency and power density by fully utilizing available DC bus voltage and minimizing the inverter current dynamically, and to ensure stable system operation when in a voltage limiting mode. In one embodiment, field weakening is initiated at a point that is adjusted, “on line,” based on DC link voltage, which is typically not fixed in certain environments, such as on aircraft. In one implementation, transition to field weakening is achieved automatically and smoothly, without the need for a look-up table or knowledge of system parameters. In an embodiment of the present invention described below, a field weakening reference current is always maintained under the voltage limit condition, while a torque reference current is controlled with the limit of available DC bus voltage and the voltage that has already been used for generating the required field generating current. Thus, the field current demand has higher priority than the torque current under the limitations of both DC link voltage and inverter maximum current. In this way, a stable field is always guaranteed, which is a basic condition of a stable operation for a motor drive system. 
     FIG. 1  illustrates an exemplary voltage source inverter feed vector controlled AC motor drive to which principles of the present invention may be applied. As illustrated, the AC motor drive system  10  includes: a DC link voltage input  15 ; a voltage source inverter  20  receiving DC power from the DC link voltage input  15 ; a pulse width modulation unit  35 , which controls gating of the voltage source inverter  20  (e.g., utilizing a configuration of insulated-gate bipolar transistors (IGBTs)) so that the inverter  20  converts supplied DC voltage to multi-phase AC power; an AC motor  30 , which is supplied multi-phase AC power via the inverter  20 ; and a control unit  100  for generating direct-, quadrature-axis inverter command reference voltages (V d *, V q *). An LC filter  25  may be included on the AC side of the inverter  20 . 
   The system  10  includes a transforming unit  60  for transforming multi-phase line current values I a , I b , I c  to d-q reference frame quantities I q , I d  (e.g., using well known Clark and Park transforms), which are input to the control unit  100 . An additional transform unit transforms voltage reference signal V d *, V q * output by the control unit  100  to multi-phase voltage commands V a , V b , V c  or stationary stator frame voltage commands V α , V β . A speed sensor or speed estimator  50  determines rotor positioning/speed of the rotor of the AC motor  30 . 
   Vector control or field-oriented control is one technique used in motor drives to control the speed and torque of AC motors. The control is conducted in a synchronous reference frame, i.e., the d-q frame. With this technique, motor stator current is resolved into a torque producing (q-axis) component of current, I q , and a field producing (d-axis) component of current, I d , where the q-axis leads the d-axis by 90 degrees in phase angle. The terminal voltage of the inverter is also resolved into the d-axis and q-axis components. As shown in  FIG. 1 , the phase angle of the synchronous reference frame can be from a speed/position sensor or from a speed estimator (i.e., for a speed sensorless controlled system). The q-axis current reference is typically output by a speed controller or a torque controller. The d-axis current reference is typically output from a field-weakening controller. The error signals between reference current and actually detected current are fed into a regulator to create inverter output voltage reference signal or modulation index. 
   In the system  10  illustrated in  FIG. 1 , it should be apparent to those of ordinary skill in the art that different PWM techniques may be used to generate PWM gatings. The DC link voltage input  15  may be from a DC power supply or battery unit. The torque current reference could be from a speed controller, a torque controller or a torque current profile. The current regulation performed by the control unit  100  could be applied with or without feedforward terms. The voltage source inverter  20  could be any topology inverter that converts DC voltage to variable frequency and amplitude AC voltage. The system  10  could be with or without the AC side output LC filter  25 . The AC motor  30  could be permanent magnet, wound-field, synchronous reluctance motor or induction motor. The control unit  100  may be implemented using digital signal processing circuitry, analog circuitry, application specific integrated circuit(s) (ASIC), various combinations of hardware/software, etc. 
   The achievable output voltage and current of the inverter  20  are determined by the physical power ratings of the inverter  20  and the motor  30  and DC link voltage input  15 . This relationship is illustrated mathematically as follows:
 
 V   d   2   +V   q   2   ≦V   max   2   ,I   d   2   +I   q   2   ≦I   max   2  
 
where V max  and I max  are maximum inverter voltage and current. As explained in detail below, embodiments of the present invention provide a voltage limit mechanism and a close-loop field weakening control loop. No lookup table and no system parameters are required in the field-weakening loop.
 
     FIG. 2  is a general block diagram illustrating functional components of the control unit  100  in accordance with an embodiment of the present invention. As illustrated, the control unit  100  includes: a d-axis (field) current regulating unit  110 ; a q-axis (torque) current regulating unit  120 ; and a field weakening control unit  140 . The control unit  100  further includes: a d-axis voltage reference limiter  130 , which limits a d-axis current regulator output voltage (V d ′) generated by the d-axis current regulating unit  110 , thereby outputting the d-axis inverter command reference voltage (V d *); and a q-axis voltage reference limiter  160 , which limits a q-axis regulator output voltage (V q ′) output by the q-axis current regulating unit  120 , thereby outputting the q-axis inverter command reference voltage (V q *). The control unit  100  further includes: a q-axis voltage limit calculator  150 ; and a q-axis current component limit calculator  152 . 
     FIG. 3  is a block diagram illustrating, in greater detail, an arrangement for the control unit  100  according to an embodiment of the present invention. As illustrated, the d-axis current regulating unit  110  includes: a summing element  112  for summing a field weakening reference signal (I d ″) output by the field weakening control unit  140  and the rated field current (I dr ), thereby generating a d-axis current reference signal (I d *); a comparing element  114  for generating an error signal based on the difference between the d-axis current reference signal (I d *) and the detected d-axis inverter output current (I d ); and a regulator  116  for generating the d-axis current regulator output signal (V d ′) based on the error signal. 
   As illustrated, the q-axis current regulating unit  120  includes: a speed or torque control element  122  for generating a speed or torque control loop output signal (I q ′); a q-axis torque current reference signal limiter  124  for limiting I q ′, thereby outputting the q-axis current reference signal (I q *); a comparing element  126  for comparing the q-axis current reference signal (I q *) with a detected q-axis inverter output current (I q ) to generate an error signal; and a regulator  128  for generating the q-axis current regulator output signal (V q ′) based on the error signal. 
   The field weakening control unit  140  includes: an absolute value calculator  142  for calculating the amplitude of the q-axis current regulator output signal V q ′; a comparing element  144 , which produces a q-axis voltage error signal as the difference between the output of absolute value calculator  142  and the output of the q-axis voltage limit calculator  150 ; a polarity calculator  145  for determining the polarity (sign) of the error signal generated by the comparing element  144 ; a regulator  146  for regulating a field weakening control signal based on the output of the polarity detector  145 , thereby generating a field weakening current reference signal (I d ′); and a limiter  148  for limiting the field weakening current reference signal (I d ′) based on a field weakening current limit (I dl ), thereby generating the field weakening reference signal (I d ″) that is output to the d-axis current regulating unit  110 . 
   In the embodiment illustrated in  FIG. 3 , the d-axis voltage reference limiter  130  includes a max voltage calculator  132  for calculating maximum voltage available to the inverter  20  as a ratio (K) of the DC link voltage (V dc ) from the DC link voltage input  15 ; and a limiter  134  for limiting the d-axis current regulator output signal (V d ′) based on V max , thereby outputting the d-axis inverter command reference voltage (V d *). This embodiment assumes the DC link voltage V dc  changes dynamically. It should be recognized, however, the principles of the present invention are applicable to the simpler case in which V dc  is fixed. In such as case, it is not necessary to dynamically calculate V max , which instead will be a constant value. 
   Although elements of the control unit  100  are shown as discrete elements, it should be recognized that this illustration is for ease of explanation and that functions of these elements may be combined in the same physical element, e.g., in the same microcontroller or in one or more application-specific integrated circuits (ASIC). Additional aspects of the operation of the elements illustrated in  FIGS. 2 and 3  will become apparent from the following description. 
   The d-axis current regulating unit  110  generates the d-axis current reference signal I d * based on the result of the field weakening control loop implemented by the field weakening control unit  140 . The q-axis current regulating unit  120  generates the q-axis current reference signal I q * from the speed or torque control loop output signal I q ′ (from the speed or torque control unit  122 ) through the limiter  124 , which applies a limit level (√{square root over (I max   2 −I d   *2 )}) calculated by the q-axis current component limit calculator  152 . I max  is the maximum current the inverter  20  can provide. Both of the d-axis and q-axis current reference signals I d * and I q * are compared, by comparing elements  114 ,  126 , respectively, with detected d-axis and q-axis inverter output current signals I d  and I q  to produce current error signals. The current error signals are fed into d-axis and q-axis current regulators  116 ,  128  to generate d-, q-axis current regulator output signals V d ′ and V q ′. The d-axis current (field generating component of the stator current) regulator output signal V d ′ is sent to the limiter  134 , with limit level ±V max , to create final d-axis inverter command reference voltage V d *. V max  is the maximum voltage the inverter  20  can create, which is proportional to the DC link voltage V dc . The ratio K between V dc  and V max  depends on the PWM method adopted. Since V d ′ is usually far away from V max , d-axis current actually can be controlled without voltage limit. In this way the d-axis current is always under control, i.e., a solid air gap flux can be achieved in transient and steady state, which is advantageous to motor stable operation. 
   The q-axis current (torque generating component of the stator current) regulator output signal V q ′ is sent to the limiter  160 , with the limit level ±√{square root over (V max   2 −V d   *2 )} calculated by the q-axis voltage limit calculator  150 , to create the final q-axis inverter command reference voltage V q *. This limit mechanism  160  can ensure that there is no over modulation during transient and steady state, if required, which can prevent high frequency resonance for the drive systems with AC side output LC filters  25 . 
   The field weakening control unit  140  is part of an outer q-axis voltage regulation loop to generate a field-weakening current reference signal. The goal of this loop is to output a signal that allows the d-axis current regulating unit  110  to adjust the d-axis current reference signal I d * to make the amplitude of the output signal of the q-axis current regulator |V q ′| lower or equal than limit level √{square root over (V max   2 −V d   *2 )}. In this way, there is √{square root over (V d   ′2 +V q   ′2 )}≦V max  i.e., d-axis and q-axis current loops can be fully controlled without voltage limit. To achieve this, the comparing element  144  of the field weakening control unit  140  compares the amplitude of the q-axis current regulator output signal |V q ′| with √{square root over (V max   2 −V d   *2 )} to produce a q-axis voltage error signal. This error signal, or the sign of this error signal as determined by the polarity calculator  145  (optional), is fed into the regulator  146  to generate the field weakening current reference signal I d ′. I d ′ is sent into the limiter  148  with limit level −I dl  to 0 to create the field weakening reference signal I d ″ that is sent to the d-axis current regulating unit  110 . I dl  is the field weakening current limit to prevent deep demagnetization of rotor permanent magnets. The sum of the field weakening reference signal I d ″ and the rated field current I dr , as calculated by the summing element  112 , is the final d-axis current reference signal I d *. 
   For an induction motor, the rotor magnetizing field is excited by stator current, I dr  is the rated field current. When motor speed is lower than base speed, I d ″ is zero and I d * will be I dr . After motor speed is higher than base speed, I d ″ is a negative value and I d * will be lower than I dr . Field will be weakened to lower down back EMF to achieve field weakening operation. For synchronous motors, rotor magnetizing field is excited by the rotor itself. I dr  is set to zero. When motor speed is lower than base speed, I d ″ is zero and I d ′ will be zero i.e., no field weakening is applied. After motor speed is higher than base speed, I d ″ is a negative value and I d * will also be negative, i.e., field weakening will be applied. If the error signal output by the comparing element  144  of the field weakening control unit  140  is used by the regulator  146  to generate field-weakening current, the field weakening control loop parameters will be important for the stability of the system. Better dynamic performance, however, can be achieved. For the system with lenient dynamic performance requirements, the sign of the error signal for field weakening current adjustment, as calculated by the polarity detector  145  of the field weakening control unit  140 , is more preferable because the tuning of the field weakening control is simplified. 
   In the above-described embodiment, the field weakening reference current is always maintained under the voltage limit condition, while the torque reference current is controlled with the limit of available DC bus voltage and the voltage that has already been used for generating the required field current. Thus, the field current demand has higher priority than the torque current under the limitations of both DC link voltage and inverter maximum current. In this way, a stable field is always guaranteed, which is a basic condition of a stable operation for a motor drive system. The above-described embodiment achieves this effect by applying the following logic: 
   1) First, the field weakening I d * is only limited by I dl , which is maximum allowable field weakening defined by the system. 
   2) Second, V d ′ required by I d * is only limited by V max , which is defined by the system (max. available DC bus). 
   3) Then, the torque I q * is limited by √{square root over (I max   2 −I d   *2 )}, where I max  is defined by the inverter capability. 
   4) Fourth, V q * required by I q * is limited by √{square root over (V max   2 −V d   *2 )}. 
   5) Field weakening close loop control is applied. 
   As described above, the embodiment illustrated in  FIG. 3  assumes that the DC link voltage V dc  changes dynamically. It should be recognized, however, the principles of the present application are applicable to the simpler case in which V dc  is fixed. In such as case, it is not necessary to dynamically calculate V max , which instead will be a constant value, and there is no need for DC link voltage detection. In such an implementation, a maximum modulation index (d max ) for the inverter can be adopted for control, instead of the dynamically changing V max . In this scheme, the output signals of the d-axis and q-axis current regulating units,  110 ,  120 , are inverter modulation index signals d d ′ and d q ′, instead of V d ′ and V q ′. The limits imposed on d d ′ and d q ′ are d max  and √{square root over (d max   2 −d d   2 )}, respectively, instead of V max  and √{square root over (V max   2 −V d   *2 )}. The field weakening control error signal is √{square root over (d max   2 −d d   2 )}−|d q ′| instead of √{square root over (V max   2 −V d   *2 )}−|V q ′|. Other aspects remain the same. This simplified embodiment may be applied, for example, to a battery fed system.