Abstract:
A method for use with an adjustable frequency controller to deliver current to an electromagnetic load including a stator and a rotor, the method for identifying a flux current estimate and comprising the steps of (a) identifying a rated torque value; (b) providing an initial q-axis current estimate; (c) using the q-axis current estimate to identify a d-axis current estimate; (d) mathematically combining the d-axis current estimate and the q-axis current estimate to identify a torque estimate; (e) comparing the rated torque value to the torque estimate; (f) where the torque estimate is similar to the rated torque value skipping to step (i); (g) altering the q-axis current estimate; (h) repeating steps (c) through (f); and (i) storing the d-axis and q-axis current estimates as flux and torque current values for subsequent use.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     The field of the invention is motor controllers and more specifically field oriented controllers and a method and apparatus for identifying a rated flux current estimate for an electromagnetic machine during a static commissioning procedure. 
     A typical three-phase induction motor controller is provided with three phases of electrical voltage and uses the three phases to produces a rotating magnetic stator field within the stator cavity of a motor linked thereto. The stator field induces (hence the label “induction”) a rotor current within a rotor mounted in the stator cavity. The rotor current in turn generates a rotor field within the cavity. The rotor field interacts with the stator field (e.g., is attracted to or repelled by) and causes the rotor to rotate. 
     The magnitude of the attractive force between the rotor and stator fields is generally referred to as torque. Where the force between the two fields is high, the torque is high and the force that can be applied to a load is high. Where the attractive force between the stator and rotor fields is low, the torque is low and the force that can be applied to a load is also relatively low. 
     To a first approximation, the torque and speed of an induction motor may be controlled by changing the frequency of the driving voltage and thus the angular rate of the rotating stator field. Generally, for a given torque, increasing the stator field rate will increase the speed of the rotor (which follows the stator field). Alternatively, for a given rotor speed, increasing the frequency of the stator field will increase the torque by increasing the slip, that is, the difference in speed between the rotor and the stator field. An increase in slip increases the rate at which flux lines are cut by the rotor, increasing the rotor-generated field and thus the force or torque between the rotor and stator fields. 
     Referring to FIG. 1, the rotating phasor  14  of a stator magneto motive force (“mmf”) will generally form some angle a with respect to the phasor of rotor flux  18 . The torque generated by the motor is proportional to the magnitudes of these phasors  14  and  18  but is also a function of the angle α between the two phasors  14  and  18 . The maximum torque is produced when phasors  14  and  18  are at right angles to each other (e.g., α=90°) whereas zero torque is produced when phasors  14  and  18  are aligned (e.g., α=0°). The mmf phasor  14  can be usefully decomposed into a torque producing component  15  perpendicular to the phasor  18  and a flux component  17  parallel to rotor flux phasor  18 . 
     Components  15  and  17  of the stator mmf are proportional, respectively, to two stator currents i qe , a torque producing current, and i de , a flux producing current, which may be represented by orthogonal vectors in the rotating frame of reference (synchronous frame of reference) of the stator flux having slowly varying magnitudes. The subscript “e” is used to indicate that a particular quantity is in the rotating frame of stator flux. 
     Accordingly, in controlling an induction motor, it is generally desired to control not only the frequency of the applied voltage (hence the speed of the rotation of the stator flux phasor  14 ) but also the phase of the applied voltage relative to the current flow and hence the division of the currents through the stator windings into the i qe  and i de  components. Control strategies that attempt to independently control the currents i qe  and i de  are generally referred to as field oriented control strategies (“FOC”). 
     Generally, it is desirable to design field-oriented controllers that are capable of driving motors of many different designs and varying sizes. Such versatility cuts down on research, development, and manufacturing costs and also provides easily serviceable controllers. 
     While multi-purpose controllers have reduced manufacturing costs, unfortunately versatile controllers have complicated commissioning processes required to set up a controller to control a motor. Specifically, to control a motor most efficiently, the controller has to be programmed with certain motor unique operating parameters. Because manufacturers of multi-purpose controllers cannot know the specific operating parameters of the motor with which their controllers will be used, the manufacturers cannot set the parameters for the end users—the users have to set the parameters themselves. 
     After an electromechanical machine (e.g., a motor) has been manufactured, the machine is typically characterized by several maximum recommended or most efficient operating characteristics (e.g., rated operating current value, a rated voltage value, a rated rotational speed, a rated horsepower, etc.) that are determinable through various tests and procedures. These rated values are determined by manufacturers and are usually provided to end users so that the users can match machine capabilities with applications (e.g., expected loads, speeds, currents, voltages, etc.). Many of these rated values can also be used to commission a motor controller to control the associated motor. 
     Other operating characteristics cannot be determined until after a motor is linked to a load and, thereafter, are identified by performing some commissioning procedure. For example, a stator resistance r s  and a leakage inductance L σ  are determinable via various commissioning procedures. 
     One other operating parameter that is necessary for efficient and effective FOC is the rated flux or d-axis current value (and related q-axis current value) which depends in part on specific motor design and other operating parameters and hence cannot be provided by a controller manufacturer. To identify a rated flux current value, commissioning procedures have been developed that require rotation of the motor rotor while different current levels are injected into the motor windings so that a flux saturation curve can be generated. In some applications rotor rotation prior to motor operation is unacceptable. 
     Where rotor rotation prior to operation is unacceptable, some processes have been devised for estimating a saturation curve while the motor is at stand still. Unfortunately, the commissioning processes that are used to generate saturation curves while a motor rotor is stationary are not very accurate and the end result is typically poor motor starting performance. 
     Thus, there is a need for a process whereby a relatively accurate rated flux estimate can be identified during a static commissioning procedure (i.e., prior to motor rotation/operation). 
     BRIEF SUMMARY OF THE INVENTION 
     It has been recognized that several of the rated motor operating parameters that are typically provided by motor manufacturers and several other operating parameters that can be derived during static commissioning procedures can be used in an iterative fashion to identify a relatively accurate flux current estimate for use in starting a motor from standstill. More specifically, a stator resistance value r s  and a leakage inductance value L σ  can be identified using stationary commissioning procedures. Thereafter, a motor torque current (i.e., a q-axis current) can be assumed and used along with the stator resistance r s  and leakage inductance L σ  values and rated motor voltage, rated current and rated speed to identify a flux value aligned with the d-axis. Next, the flux value and a set of the other parameters identified above can be mathematically combined to generate a torque estimate. Continuing, a rated motor speed and rated horse power can be used to identify a motor rated torque value. The torque estimate is compared to the rated torque estimate and the q-axis motor torque current assumption is altered as a function of the difference between the estimated and rated torques. 
     The process described above is repeated until the torque estimate is within a tolerable range of the rated torque value. Once the torque estimate is within the tolerable range of the rated torque value, the d and q-axis current values arc stored as rated flux and torque current values. In at least some embodiments convergence on the rated torque value expedited by altering the q-axis torque current assumption (i.e., the q-axis current value) as a function of the magnitude of the difference between the torque estimate and the rated torque value. 
     These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a cross-sectional schematic view of an induction motor showing instantaneous locations of a rotor flux, a stator mmf and the torque and flux components of the stator mmf; 
     FIG. 2 is a schematic diagram illustrating processor modules assembled to perform an inventive method according to the present invention; 
     FIG. 3 is a flow chart illustrating one method according to the present invention; 
     FIG. 4 is a sub-process that may be substituted for a portion of the process of FIG. 3; 
     FIG. 5 is a graph illustrated tolerance modifications as an estimated torque value converges on a rated torque value according to one inventive embodiment; and 
     FIG. 6 is a graph illustrating the results of a static commissioning procedure according to the present invention and a flux current value identified via a rotational commissioning test as a point of reference. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A. Theory 
     There are several fundamental equations that are routinely used in the FOC art to describe AC motors. The fundamental equations can be used in conjunction with rated values and operating parameters that can be derived from rated values and during static commissioning procedures to identify the flux current in an iterative fashion. More specifically, after a rated torque for a specific motor is identified, a relatively high torque producing q-axis current value is assumed and an estimated torque for a specific motor that would be produced if the high q-axis torque were used to drive the motor can be identified. Next, the torque estimate and rated torque are compared and the q-axis current estimate is altered until the resulting torque estimate is similar to the rated torque value. One of the intermediate values identified during the torque estimating process is the rated flux current value. Once the torque estimate is similar to the rated torque value, the flux current or d-axis current value is stored for subsequent use. 
     Several fundamental AC motor equations form the basis for identifying the rated flux current value for a specific motor according to the present invention. The fundamental equations are as follows. First, a rated motor torque T r  can be expressed as: 
     
       
           T   r =(5250HP r )/(0.739RPM r )  (1) 
       
     
     Where HP r  is a rated horse power value and RPM r  is a rated rotor speed value in rotations per minute. 
     Second, the electromagnetic torque equation for an AC motor can be expressed as: 
     
       
           T   e =0.75 P (λ de i qe −λ qe   i   de )  (2) 
       
     
     where: 
     P=# motor poles; 
     i qe =motor current aligned with the q-axis, typically reflecting motor torque; 
     i de =motor current aligned with the d-axis, typically motor flux; 
     λ qe =motor flux aligned with the q-axis; and 
     λ de =motor flux aligned with the d-axis. 
     For rotor FOC, the flux component λ qe  can be redefined in steady state as follows: 
     
       
         λ qe   =L   σ   i   qe   (3) 
       
     
     where 
     L σ =transient inductance 
     Third, as well known in the FOC art, the production of any given set of currents i qe  and i de  requires that the stator be excited with voltages V qe  and V de  as follows: 
     
       
           v   qe =r s   i   qe +ω e λ de   (4) 
       
     
     
       
           v   de =r s   i   de −ω e λ qe   (5) 
       
     
     where 
     v qe , v de =terminal voltages; 
     r s =stator resistance; and 
     ω e =electrical field frequency. 
     Fourth, the d-axis current and q-axis voltage components i de  and v qe , respectively, can be expressed by the following equations: 
     
       
           i   de =( i   r   2   −i   qe   2 ) 1/2   (6) 
       
     
     
       
           v   qe =( v   r   2   −v   de   2 ) 1/2   (7) 
       
     
     where 
     i r  is the rated motor current; 
     v r  is the rated motor voltage. 
     Prior to starting the inventive iterative commissioning procedure, five required operating characteristics include the rated current i r , the rated voltage v r , the rated torque T r , the stator resistance value r s  and the transient inductance value L σ . The rated current i r  and rated voltage v r  values are typically provided by the motor manufacturer (e.g., are referred to as “name plate” values). 
     In addition, motor manufacturers routinely provide a rated or name plate horse power value HP r  and a rated motor speed value RPM r . The rated horse power HP r  and speed RPM r  values can be plugged into Equation 1 above to identify the rated torque value T r . 
     The industry has developed several processes to determine the stator resistance r s  and the transient inductance L σ  values during static commissioning procedures. For example, U.S. Pat. No. 5,689,169 which is titled “Transient Inductance Identifier for Motor Control” teaches one method for determining the leakage inductance. Hereinafter it will be assumed that each of the stator resistance r s  and the transient inductance L 94   values have been determined. 
     With the stator resistance r s , transient inductance L σ , rated voltage v r , rated current i r  and rated torque T r  values determined, the following steps can be performed to identify the rated d-axis flux current value i de  for the motor. 
     First, a relatively high torque producing q-axis current value i qe  is assumed so that the resulting estimated torque value T est  should be extremely high. For instance, the q-axis current value i qe  may initially be assumed to be equal to or slightly less than the rated motor current i r . Next, Equation 6 above is used to identify the d-axis current i de  and Equation 7 is used to identify a q-axis voltage value v qe . Continuing, Equation 4 is rewritten as: 
     
       
         λ de =(v qe −r s i qe )/(ω e )  (8) 
       
     
     The d-axis flux component λ de  is determined by solving Equation 8. Next, Equations 2 and 4 are combined to yield the following equation: 
     
       
           T   e =0.75 P(λ   de   i   qe   −L   σ   i   qe   i   de )  (9) 
       
     
     Equation 9 is solved to determine the torque estimate value T est . Torque estimate value T est  is then compared to rated torque value T r  (see again Equation 1 above). Where torque estimate value T est  is similar to rated torque value T r , the flux current value i de  and the torque current value i qe  used in Equation 10 are stored as a relatively accurate estimates for subsequent use. Where the torque estimate value T est  is substantially greater than the rated value T r , the torque producing q-axis current estimate i qe  value is reduced and the calculations above are repeated. 
     In some embodiments reductions in the q-axis current value i qe  may be linked to the magnitude of the difference between the torque estimate and the rated torque value so that the flux current converges on an acceptable and relatively accurate value more quickly. For example, in at least one embodiment a torque tolerance value is identified and the torque current value reduction is tied to the torque tolerance value where, each time through the torque estimation loop, if the difference between the torque estimate value and the rated torque value is less than the tolerance value, the tolerance value is divided by a factor of 2. Here, the loop may be repeated until the torque tolerance value is less than a minimum tolerance value T min  which is a small fraction of the rated torque value. For instance, the initial torque tolerance value may be 25% of the rated torque value and the minimum tolerance value T min  may be 2.5% of the rated torque value. 
     B. Hardware and Method 
     Referring now to the drawings and, more specifically, referring to FIG. 2, therein is illustrated a schematic diagram of processor modules corresponding to functions that are performed according to at least one embodiment of the present invention. While illustrated as separate processor modules, it should be appreciated that the modules in FIG. 2 may be comprised in a single microprocessor that can perform the inventive iterative algorithm to be described herein for identifying a rated motor flux current value and a corresponding rated torque value. The processor modules in FIG. 2 include a rated torque determiner  22 , a torque tolerance determiner  24 , a d-axis or flux current component determiner  26 , a d-axis voltage determiner  28 , a q-axis voltage determiner  30 , a d-axis flux determiner  32 , a torque estimate determiner  34 , a first comparator  36 , a q-axis current reducer  38 , a d-axis voltage comparator  46 , a q-axis voltage comparator  48 , a second comparator  42 , a torque tolerance reducer  40  and a memory  44 . 
     Referring also to FIG. 3, a method  50  performed by the processor modules illustrated in FIG. 2 is schematically represented. FIGS. 2 and 3 will be described together. Beginning at block  54 , the nameplate or rated horsepower value HP r  and rated motor speed RPM r  are provided to the rated torque determiner  22 . In addition, at block  54 , the rated current i r  and the rated voltage v r  are provided to the d-axis current determiner and the q-axis voltage determiner  26  and  30 , respectively. Moreover, an initial torque tolerance multiplier ε is provided by a system user or by a preprogramrned processor to the torque tolerance determiner  24  while the number of poles P that characterize the specific motor for which the controller is to be provided is commissioned to the torque estimate determiner  34 . At block  52  in FIG. 3, a static commissioning procedure like any of the several procedures known in the prior art is used to identify each of a transient inductance value L σ  and a stator resistance value r s . 
     Referring still to FIGS. 3 and 4, at block  60 , the rated torque determiner  22  solves Equation 1 above to identify the rated torque value T r . The rated torque value T r  is provided to each of the first comparator  36  and the torque tolerance determiner  24 . At block  56 , a q-axis current value i qe  is assumed to be relatively large. In this case, the q-axis current value i qe  is assumed to be equal the rated current value i r . At block  58 , the torque tolerance determiner  24  multiplies the scalar ε by the rated torque value T r  to generate a torque tolerance value T tol  which is provided to first comparator  36 . 
     Next, at block  62 , a d-axis or flux current value i de  is determined by solving Equation 6 above and the flux current i de  is provided to each of the d-axis voltage determiner  28  and the torque estimate determiner  34 . At block  66 , the d-axis voltage determiner  28  solves Equation 5 above where value L σ  i qe  is substituted for the q-axis flux value λ qe  (see again Equation 3). The d-axis voltage value identified by determiner  28  is provided to each of the q-axis voltage determiner  30  and to the d-axis voltage comparator  46 . 
     At block  68 , the d-axis voltage comparator  46  compares the absolute value of the d-axis voltage value to the rated voltage v r . Where the absolute value of the d-axis voltage v de  is greater than the rated voltage v r , comparator  46  causes q-axis current reducer  38  to reduce the value of the q-axis current by some quantum and control passes back up to determiner  26  where the modules described above repeat the process with a reduced q-axis current value. The q-axis current reduction step is represented by block  64  in FIG.  3 . Where the absolute value of the d-axis voltage v de  is less than the rated voltage v r , control passes to block  80  where the q-axis voltage determiner  30  solves Equation 7 above to identify the q-axis voltage value v qe . The q-axis voltage value v qe  is provided to each of the q-axis voltage comparator  48  and to the d-axis flux determiner  32 . 
     At decision block  82 , the q-axis voltage comparator  48  determines whether or not the sum v qe −r s i qe  is greater than zero. Where the sum v qe −r s i qe  is not greater than zero, comparator  48  again causes q-axis current reducer to reduce the q-axis current value and begin the process described above with a different and smaller q-axis current value. However, at block  82 , where sum v qe −r s i qe  is greater than zero, control passes to block  84  where the d-axis flux determiner  32  determines the d-axis flux value λ de  by solving Equation 8 above. Determiner  32  provides the d-axis flux value λ de  to torque estimate determiner  34 . Estimate determiner  34  solves Equation 9 at block  70  and provides a torque estimate T est  to first comparator  36 . 
     First comparator  36  compares the difference between the torque estimate T est  and the rated torque value T r  to the torque tolerance value T tol  and, where the difference is greater than the torque tolerance value T tol , control passes to q-axis torque reducer  38  which, again, reduces the q-axis torque value i qe  and causes the process as described above to be repeated. Where the difference value at block  72  is less than the torque tolerance value T tol , control passes to second comparator  42  which compares the torque tolerance value T tol  to the minimum torque tolerance value T min  (e.g., 2.5% of the rated torque value). Where the torque tolerance value T tol  is greater than the torque tolerance minimum value T min  control passes to torque tolerance reducer  40  which, as its label implies, reduces the torque tolerance value T tol  at block  76 . 
     In the example illustrated, the torque tolerance value T tol  is reduced by dividing that value by 2 at block  76 . After the torque tolerance reducer  40  reduces the torque tolerance value, control again passes to the q-axis current reducer  38  which again reduces the q-axis current value prior to causing the process described above to be repeated. At block  74 , when the torque tolerance value T tol  is less than or equal to the torque tolerance minimum value T min , second comparator  42  stores the d and q-axis current values i de  and i qe , respectively, in memory  44  for subsequent use. 
     Referring now to FIG. 4, an exemplary q-axis current reduction step  90  which may be substituted for block  64  in FIG. 3 is illustrated. Generally, the reduction step includes multiplying an instantaneous q-axis current value i qe  by the sum          (     1   -       T   tol       T   r         )     .                          
     In this manner, the q-axis current value i qe  is reduced to a greater extent when the torque tolerance value T tol  is large and, as the torque tolerance value T tol  is reduced and approaches the torque tolerance minimum value T min , the reduction in q-axis current value i qe  is similarly reduced. By tying the reduction in q-axis current to the magnitude of the torque tolerance value, the algorithm converges more rapidly on the rated flux current value i de  as desired. 
     C. Simulation and Results 
     Various simulations have been performed using the inventive method and apparatus and the initial results have been encouraging. FIG. 5 illustrates how the torque tolerance value is reduced during loop iterations until a minimum tolerance level is reached. In the example, the initial torque tolerance value is 25% of the rated value. As illustrated, after two iterations the estimated torque value T est  is within 25% of the rated value and therefore the tolerance value is divided by 2(e.g., T tol =12.5%). After an additional 8 iterations the estimate value T est  is within 12.5% of the rated value and thus the tolerance value is again divided to 2 (e.g., T tol =6.25%). The process is repeated until, after 12 additional iterations, the tolerance value T tol  is reduced to a point below the 2.5% value and the process is completed. 
     In FIG. 6, a flux current value i de  identified via an exemplary static commissioning procedure according to the present invention is illustrated along with a flux current value i der  identified via a rotational commissioning test. The final value identified using the static commissioning flux methodology was 3.05 Arms which, after approximately 50 iterations was very close to the rotational commissioning result of approximately 2.8 Arms 
     It should be understood that the methods and apparatuses described above are only exemplary and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall under the scope of the invention. For example, in some embodiments the tolerance value may simply be set to the minimum tolerance value T min  and additional iterations will be required to generate the rated flux value. In addition, the q-axis torque current value assumption need not converge as a function of the magnitude of the tolerance value T tol . Moreover, where the q-axis torque value does converge as a function of the magnitude of the tolerance value, other converging algorithms are contemplated that trade off speed of convergence and potential overshoot of the most accurate estimation of the rated flux current value. 
     To apprise the public of the scope of this invention, the following claims are made.