Abstract:
System and method for enhancing the torque output of a field oriented induction motor including a controller having a plurality of predetermined control parameters operable for processing input signals to generate output signals. The plurality of predetermined control parameters are dependent upon the nature of the input signals and the operational state of the motor. A sensor system is operable for communicating feedback signals related to the output signals and the operational state of the motor from the motor to the controller.

Description:
BACKGROUND OF INVENTION  
         [0001]    The present invention relates generally to systems and methods for directing and controlling the operation of an induction motor and, more specifically, to systems and methods for enhancing the torque output of a field oriented induction motor operating in an overmodulation/field-weakening state.  
           [0002]    he “fuel” powering a field oriented induction motor is current. This current may be divided into two components, torque current and flux current. Torque current may be viewed as that component of the current which generates motive force, or torque. Flux current may be viewed as that component of the current which generates magnetic flux in the rotor. Shaft torque and rotor flux are related, with shaft torque being proportional to the product of rotor flux times torque current.  
           [0003]    Typically, the torque current and slip frequency of a field oriented induction motor, operating in a normal modulation state, are calculated using simple control parameters or equations which include two parts. The first part of these equations, commonly referred to as the feed forward or open loop portion, takes into account variables such as the reference or commanded torque, the flux feedback, the torque current, and the flux current. The second part of these equations, commonly referred to as the feedback or closed loop portion, takes into account variables such as the torque feedback and direct-axis back-EMF voltage feedback, as communicated to a proportional and integral controller (PI controller).  
           [0004]    [t high speed, it is desirable for a field oriented induction motor to operate in an overmodulation/field-weakening state, maximizing the available torque. However, in such a state, feedback oscillates, current waveforms become non-sinusoidal, and the system becomes generally unstable. Thus, in an overmodulation/field-weakening state the equations discussed above break down, resulting in inaccurate torque current and slip frequency calculations, and diminished torque output.  
         SUMMARY OF INVENTION  
         [0005]    The present invention overcomes the above problems and provides systems and methods for enhancing the torque output of a field oriented induction motor operating in an overmodulation/field-weakening state. Specifically, the present invention provides systems and methods for enhancing the conversion from a reference torque command to a processed torque current or slip frequency command.  
           [0006]    In one embodiment, a system for enhancing the torque output of a field oriented induction motor includes a controller having a plurality of predetermined control parameters operable for processing input signals to generate output signals, the plurality of predetermined control parameters dependent upon the nature of the input signals and the operational state of the motor, and a sensor system operable for communicating feedback signals related to the output signals and the operational state of the motor from the motor to the controller.  
           [0007]    In another embodiment, a method for enhancing the torque output of a field oriented induction motor includes, using a plurality of predetermined control parameters, processing input signals to generate output signals, the plurality of predetermined control parameters dependent upon the nature of the input signals and the operational state of the motor, and communicating feedback signals related to the output signals and the operational state of the motor from the motor to a controller. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    [0008]FIG. 1 is a schematic/functional block diagram of one embodiment of a system for enhancing the torque output of a field oriented induction motor operating in an overmodulation/field-weakening state, the system including a controller utilizing a plurality of control parameters;  
         [0009]    [0009]FIG. 2 is a schematic/functional block diagram illustrating the operation of the system of FIG. 1;  
         [0010]    [0010]FIG. 3 is a plot of the relationship between torque current and flux current, and the contribution of each to phase current, under normal modulation conditions;  
         [0011]    [0011]FIG. 4 is a plot of the relationship between torque current and flux current, and the contribution of each to phase current, under overmodulation/field-weakening conditions; and  
         [0012]    [0012]FIG. 5 is a flow chart of one embodiment of a method for enhancing the torque output of a field oriented induction motor operating in an overmodulation/field-weakening state, the method including utilizing a plurality of control parameters. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring to FIG. 1, one embodiment of a system  10  for directing and controlling the operation of a field oriented induction motor  12  includes a controller  14  operable for converting a reference command  16  into a processed command  18  for directing and controlling the operation of the motor  12 . The reference command  16  may be, for example, a torque command (TorqueRef). The processed command  18  may be, for example, a torque current command (i q ) or a slip frequency command (slipFrequency). The system  10  also preferably includes a sensor system  20  for measuring the actual or estimated operation of the motor  12 . The sensor system  20  may measure, for example, torque (TorqueFb), flux (fluxFb), or direct-axis back-EMF voltage (emf_d_Fb). The sensor system  20  converts these measurements into feedback signals  22  which are communicated to the controller  14 . The controller  14  then compares the feedback signals  22  to the reference command  16 , generates an error signal, and adjusts the processed command  18  accordingly, generating a modified processed command  24 . The modified processed command  24  may then be used for directing and controlling the operation of the motor  12 . In this manner, the operation of the motor  12  is directed and controlled such that it operates in accordance with the reference command  16 .  
         [0014]    The controller  14 , which may include a computer, a programmable logic unit, or any other suitable device capable of receiving operational inputs and processing them to generate operational outputs, may include a plurality of predetermined control parameters  26  related to torque current  28 , slip frequency  30 , etc. With respect to torque current  28 , in one embodiment of the present invention, a first predetermined control parameter  26  may be defined by the following equation:  
           i   q   =TorqueRef /(3 fluxFb )+ PI _Controller( TorqueRef−TorqueFb ),  (1)  
         [0015]    where TorqueRef is a commanded torque value or reference command, fluxFb is the flux feedback, and TorqueFb is the torque feedback. The PI controller is a proportional and integral controller, as discussed above.  
         [0016]    The above equation (1) applies only while the motor  12  is operating in a normal modulation state. At high speeds, once a state of overmodulation is reached, the torque feedback (TorqueFb) is no longer smooth and the motor phase current is no longer a pure sinusoidal waveform. Thus, in this overmodulation state, it is desirable that the conversion from a reference or commanded torque (TorqueRef) to a torque current i q  depends primarily upon the forward value (TorqueRef/(3fluxFb)) of the torque current i q  due to the fact that the output of the PI controller is oscillated with the feedback torque TorqueFb while the torque current i q  must remain stable for enhanced control. To overcome this problem, a second predetermined control parameter  26  may be used to generate a processed command  18 ,  24 . For the second predetermined control parameter  26 , the gain of the PI controller may be reduced to about ¼ th  to about {fraction (1/20)} th , and more preferably to about {fraction (1/10)} th , of the value typically used during overmodulation operation; that is, during normal modulation operation.  
         [0017]    [0017]FIG. 2 illustrates the operation of the system  10  (FIG. 1) discussed above. Once a state of overmodulation is reached, the torque feedback (TorqueFb) is no longer smooth and the motor phase current is no longer a pure sinusoidal waveform. Thus, the feedback portion  70  of the system  10  may become unstable. In this overmodulation state, it is desirable that the conversion from a reference or commanded torque (TorqueRef or T*)  72  to a torque current i q  or IQ* 74  depends primarily upon the feed forward portion  76  of the system  10  due to the fact that the output of the PI controller  78  is oscillated with the feedback torque TorqueFb while the torque current i q  must remain stable for enhanced control. To overcome this problem, the gain of the PI controller  78  may be reduced to about ¼ th  to about {fraction (1/20)} th , and more preferably to about {fraction (1/10)} th , of the value typically used during overmodulation operation; that is, during normal modulation operation.  
         [0018]    Referring again to FIG. 1, with respect to slip frequency  30 , in another embodiment of the present invention, a first predetermined control parameter  26  may be defined by the following equation:  
           slipFrequency =(1 /T   r )( i   q   /i   d )+ PI _Controller(− emf   —   d   —   Fb ),  (2)  
         [0019]    where i d  is the flux current and emf_d_Fb is the direct-axis back-EMF voltage feedback. T r  is a time constant. Under normal modulation conditions, the ratio of torque current i q  to flux current i d  is fixed, with torque current i q  equal to flux current i d . FIG. 3 illustrates the relationship between torque current i q    90  and flux current i d    92 , and the contribution of each to phase current i s    94 , in a normal modulation state. Referring again to FIG. 1, once the motor  12