Abstract:
The present invention relates to a method of controlling a power converter ( 20 ) of a synchronous machine system ( 10 ), the method comprising sampling phase-current values between the power converter ( 20 ) and the synchronous machine ( 30 ); selecting a reference frame; regulating a current vector to align with the selected reference frame, the selected reference frame having a direct-axis component and a quadrature-axis component; estimating rotor speed and position as a function of instantaneous power; adjusting the selected reference frame, based on estimated rotor position, to synchronize the selected reference frame with a magnetic axis of the rotor, thereby generating a synchronized floating frame; and applying the synchronized floating frame to control the power converter ( 20 ). The present invention also related to a power converter controlling apparatus ( 100 ) for controlling a power converter ( 20 ) of a synchronous machine system ( 10 ) without use of a machine position sensor.

Description:
RELATED APPLICATION  
       [0001]     This application claims priority under 35 USC §119(e) of Provisional Application No. 60/557,710 filed Mar. 31, 2004, the entire contents of which are herein incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to electrical power conversion, and more particularly to a controller for a power converter, such as an inverter of a synchronous AC motor drive system.  
       BACKGROUND OF THE INVENTION  
       [0003]     A synchronous AC motor typically utilizes rotor position sensors to provide information regarding the position of the motor&#39;s rotor with respect to the motor&#39;s stator windings. Such positional information allows for proper conversion of power that is supplied to the stator windings. Rotor position sensors such as Hall effected devices are typically mounted in the stator, proximate the stator winding, to provide intelligence regarding rotor position. Such rotor position sensors, however, can be unreliable due to mechanism alignment problems and temperature incompatibility problems between the stator windings and electronic components such as the Hall effect devices. Moreover, the rotor position sensors can be difficult to mount to the motor during motor assembly, especially for multi-pole motors. In multi-pole motors, the electrical misalignment angle is equivalent to the angular mechanical misalignment angle multiplied by the number of pole pairs.  
         [0004]     Due these and other drawbacks, sensorless techniques have been developed to determine rotor position. One sensorless rotor position detection technique observes back EMF voltages at the stator windings. Another technique, which applies a floating frame control (FFC) scheme, has been described by Huggett et al. in U.S. Pat. No. 6,301,136, which in hereby incorporated herein by reference in its entirety. In the FFC scheme, motor phase-current is detected directly and used to estimate rotor speed/position, and also to control the reactive current to zero. More specifically, sensorless rotor speed/position detection is combined with current control to achieve a closed-loop equilibrium condition in which an inverter voltage vector (V ωt ) finds a position that results in a zero direct-axis current component value. Under this condition, a reference frame (floating frame) is synchronized with the magnetic axis of the rotor and can be used to control power conversion.  
         [0005]     Such control results in unity power factor during steady state operation, which is an advantage for high power inverter design. Although the FFC scheme disclosed in U.S. Pat. No. 6,301,136 is effective in many applications and conditions, the speed/position estimation in the FCC scheme is embedded in the direct-axis current regulator, which makes the loop tuning sensitive in some applications.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention relates to a method and apparatus for controlling a power converter of a synchronous machine system, without the use of dedicated rotor position sensors. In one aspect, the present invention is a power converter control method comprising: sampling phase-current values between the power converter and the synchronous machine; selecting a floating reference frame; regulating a current vector to align with the reference frame, the reference frame having a direct-axis component and a quadrature-axis component; estimating rotor speed and position as a function of instantaneous power; adjusting the selected reference frame, based on estimated rotor position, to synchronize the selected reference frame with the magnetic axis of the rotor, thereby generating a synchronized floating frame; and applying the synchronized floating frame to control the power converter.  
         [0007]     In another aspect, the present invention is a power converter controlling apparatus for controlling a power converter of a synchronous machine system, the controlling apparatus comprising a current controller for generating power converter command signals by: sampling phase-current values between the power converter and the synchronous machine; selecting a reference frame; regulating a current vector to align with the reference frame, the reference frame having a direct-axis component and a quadrature-axis component; adjusting the selected reference frame, based on estimated rotor position, to synchronize the selected reference frame with the magnetic axis of the rotor, thereby generating a synchronized floating frame; and applying the synchronized floating frame to control the power converter. The controlling apparatus further comprises a rotor position estimator for generating the estimated rotor position as a function of instantaneous power.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  illustrates a synchronous machine drive system to which principles of the present invention may be applied to control power conversion;  
         [0009]      FIG. 2  is a general block diagram of a power conversion controller in accordance with an embodiment of the present invention;  
         [0010]      FIG. 3  is a block diagram illustrating, in greater detail, functional elements of the power conversion controller of  FIG. 2  in accordance with an embodiment of the present invention;  
         [0011]      FIG. 4  is a flow diagram illustrating a floating frame synchronizing operation in accordance with an embodiment of the present invention; and  
         [0012]      FIGS. 5A-5C  are vector diagrams illustrating the concept of floating frame synchronizing in accordance with principles of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0013]     Embodiments of the present invention are more specifically set forth in the following description, with reference to the appended drawings. In the following description and accompanying drawings like elements are denoted with similar reference numbers. Further, well-known elements and related explanations are omitted so as not to obscure the inventive concepts presented herein.  
         [0014]     In accordance with aspects of the present invention, a FFC-based control apparatus measures multi-phase line current, calculates a reference frame that synchronizes with rotor position/speed based on vector control and sensorless rotor position/speed estimation. In accordance with an implementation of the present invention, instantaneous power, including imaginary power (O) and real power (P), is calculated to determine rotor position/speed. The synchronized reference frame is used to control a power converter. The instantaneous power floating frame controller can drive a synchronous machine without the use of rotor position sensors.  
         [0015]      FIG. 1  illustrates a synchronous motor system  10  to which principles of the present invention may be applied to perform sensorless power converter control. The synchronous motor system  10  includes the following main components: a synchronous motor  30 ; a multi-phase power converter  20 ; a power source  40 ; a PWM (Pulse Width Modulation) Generator and switch driver  50 ; and a controller  100 . The synchronous motor  30  may be a three-phase permanent magnet synchronous motor (PMSM), although principles of the present invention are not limited to such an environment. The power converter  20  may be an inverter for converting DC power from power source  40  into three-phase AC power, e.g., utilizing a configuration of insulated-gate bipolar transistors (IGBTs) under control of the PWM Generator and switch driver  50  (pulse width modulation (PWM) control). The controller  100  controls the power inverter  20  via the PWM Generator and switch driver  50  so that the power converter  20  outputs the desired multi-phase AC power to the stator windings of motor  30 . Thus, during operation of the synchronous motor  30 , the power converter  20  converts DC power from the power source  40  into multi-phase AC power and supplies such multi-phase AC power to stator windings of the motor  30 , creating a rotating magnetic field that interacts with the rotor&#39;s magnetic field to create torque. Thus, proper control of the power converter  20 , as a function of rotor position/speed, is necessary to generate a rotating magnetic field that results in efficient motor function, particularly for a variable speed drive system.  
         [0016]      FIG. 2  is a block diagram illustrating elements of the controller  100  according to an embodiment of the present invention. In the embodiment of  FIG. 2 , the controller  100  includes: a current controller  110  for producing voltage commands (V a , V b , and V c ); and a speed/position estimating unit  130  for generating a speed estimate ω est , and a position estimate θ est . Although the current controller  110  and the speed/position estimating unit  130  are shown as discrete elements, it should be recognized that this illustration is for ease of explanation and that the 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).  
         [0017]      FIG. 3  is a block diagram illustrating functional components of the current controller  110  and the speed/position estimating unit  130  in greater detail, in accordance with one embodiment of the present invetion. In the embodiment illustrated in  FIG. 3 , the controller  110  includes a Clarke transform unit  122  for transforming multi-phase line current values I a , I b , I c  into direct and quadrature components of a stationary stator frame α, β; a Park transform unit  124  for calculating rotating reference frame quantities I q  and I d ; a reactive current proportional integral (PI) controller  116  for generating a voltage reference V d  as a function of I d ; an I q  reference value generating PI controller  112 , which generates a torque current reference based on the difference between a speed command ω ref  and estimated rotor speed ω est ; a torque current PI controller  114  for generating a quadrature component voltage reference V q  for torque control; an inverse Park transform unit  118  for transforming voltage references V d  and V q  into αβ quantities; and an inverse Clarke transform unit  120  for transforming the αβ quantities into three-phase voltage commands V a , V b , and V c . It will be recognized that the configuration of the current controller  110  is consistent with a conventional arrangement for vector control of a PMSM.  
         [0018]     The speed/position estimating unit  130  includes an instantaneous power calculation unit  132 ; a power factor angle calculator unit  134  for calculating the power factor angle; a speed estimating PI controller  136  for calculating estimated speed ω est  based on the power factor angle; and an integrator  138  for calculating position θ est  based on estimated speed ω est .  
         [0019]     Both estimated speed ω est  and position θ est  are fed into the current controller  110 , which performs vector control to generate voltage commands V a , V b , V c . Operation of the current controller  110  and the speed/position estimating unit  130  will next be described with reference to the flow diagram of  FIG. 4  and the vector diagrams of  FIGS. 5A-5C .  
         [0020]     Initially, multi-phase line current values I a , I b , and I c  are obtained (step S 302 ) and fed into the Clarke transform unit  122 , which calculates stationary reference frame values I α , I β  (step S 304 ) by calculating:  
               I   α     =       1   3     ⁢     (       2   ⁢           ⁢     I   a       -     I   b     -     I   c       )                       I   β     =         3     3     ⁢       (       I   b     -     I   c       )     .         ⁢                     
 
         [0021]     Next, the Park transform unit  124  calculates rotating reference frame values I d  and I q  (step S 306 ) by calculating: 
 
 I   d   =I   α  cos θ est   +I   β  sin θ est  
 
 I   q   =−I   α  sin θ est   +I   β  cos θ est  
 
         [0022]     The stationary reference frame α, β and the selected rotating reference frame q est  and d est  can be seen in the vector diagrams  5 A- 5 C. The difference between I d  and a reference value (e.g., 0) is input to the reactive current PI controller  116  to generate a voltage reference V d  to minimize such an error (i.e., closed-loop control). As is known in the art, vector control for controlling the reactive current I d  to zero aligns the current vector I ωt  with the floating reference frame, as illustrated for example in  FIG. 5B . The torque current PI controller  114  generates voltage reference V q  as a function of the difference between I q  and the  I   q  reference (I q-ref ), which is based on the difference between a speed command value ω ref  and the estimated rotor speed ω est . The inverse Park transform unit  118  converts V d  and V q  into the stationary frame V α  and V β  by calculating: 
 
 V   α   =V   d  cos θ est   −V   q  sin θ est  
 
 V   β   =V   d  sin θ est   +V   q  cos θ est  
 
         [0023]     The inverse Clarke transform unit  120  performs an inverse Clarke transform to generate command voltages V a , V b , and V c .  
                 V   a     =     V   α       ⁢                         V   b     =       -     1   2       ⁢     (       V   α     -       3     ⁢     V   β         )                     V   c     =       -     1   2       ⁢     (       V   α     +       3     ⁢     V   β         )                 
 
         [0024]     The instantaneous power calculator  132  of the speed/position estimating unit  130  calculates imaginary power (Q) and real power (P) by calculating: 
 
 P=V   q   I   q   +V   d   I   d  
 
 Q=V   q   I   d   −V   d   I   q  
 
         [0025]     The power factor angle calculator  134  calculates the power factor angle by calculating: 
 
θ= Arctg ( Q/P ) 
 
         [0026]     As seen for example in  FIG. 5A , the power factor angle represents the angular difference between the voltage vector V ωt  and the current vector I ωt , which is minimized for proper alignment of the floating frame. The speed estimating PI controller  136  determines ω est  based on Δθ, for example by applying a phase lock loop transfer function:  
         ω   est     =             K   PLL     ⁢     T   PLL     ⁢   S     +   1           T   PLL     ⁢     S   2       +       K   PLL     ⁢     T   PLL     ⁢   S     +   1       ⁢     ω   V           
 
 where: K PLL , T PLL  are the gain and time constant of the speed estimator PI that has transfer function as  
           K   PLL     ⁡     (     1   +     1       T   PLL     ⁢   S         )       ;       
        ω v  is voltage vector rotating speed; ω r  is rotor rotating speed, in steady state ω v =ω r .        
 
         [0028]     As illustrated in  FIG. 3 , ω est  is used as the feedback of rotor speed for the vector control performed by the current controller  110 . The integrator  138  determines θ est  by integrating θ est . The estimated rotor position angle θ est  is also fed back to the current controller  110 .  
         [0029]     As described above, an embodiment of the present invention applies a sensorless technique for determining rotor speed and position based on instantaneous power in a FFC scheme. As illustrated for example in  FIGS. 5B and 5C , floating frame synchronization is achieved by first aligning the current vector I ωt  with a selected floating reference frame, which is achieved by applying vector control in the current controller  110  so that the reactive current I d  is minimized (step S 308 ). Next, the current vector I ωt /floating frame is aligned with the voltage vector V ωt  to achieve unity power factor (i.e., efficient floating frame control of the power converter  20 ) (step S 310 ). As illustrated for example in  FIG. 5C , the current vector I ωt  is aligned with the voltage vector V ωt  by minimizing the power factor angle Δθ in the speed/position estimating unit  130 . By effectively de-coupling these two steps of achieving a floating reference frame that is synchronized with rotor speed/position, tuning of the controller  100  is simplified.