Abstract:
Improved voltage stiffness in the output voltage of an inverter used for a motor drive is obtained by a minor voltage feedback loop connecting the output of the inverter to a three-phase input in a vector domain.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     --  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     --  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to motor drives and in particular to an improved inverter used in such motor drives.  
         [0004]     Motor drives are used to control the speed, torque, or other operating characteristics of AC induction motors.  
         [0005]     In a typical large motor drive, three-phase power from a power line is rectified and filtered to provide a source of DC power. The DC power is provided to an inverter which converts the DC power back again to synthesized AC power that is used to drive the motor. By changing the frequency of the synthesized AC power, the speed or torque of the induction motor may be affected.  
         [0006]     In operation, a controller within the motor drive receives a command signal from a user, for example, torque or speed, and provides the inverter with input signals indicating the desired characteristics of the synthesized AC power needed to achieve that torque or speed. The inverter receives the input signals and converts them to gate pulses driving solid state semiconductor switches, such as insulated gate bipolar transistors (IGBT), rapidly between on and off states in a class D or switching mode. The result is a duty cycle or pulse width modulated pulse train whose average voltage or current mirrors that of the desired synthesized AC power. Such switched operation is power efficient because it runs the solid state switching devices principally in a low power dissipation region where the solid-state devices are either fully conductive or non-conductive.  
         [0007]     Despite the efficiency of such inverters, the voltage output of such inverters may not be an accurate representation of the input inverter signals particularly at lower voltages. Much of this accuracy problem appears to result from a non-linearity of the characteristics of the switching devices. For example, a variable delay in switching speed in the devices will affect the amplitude and phase of the synthesized waveform.  
         [0008]     One method of addressing inverter voltage inaccuracy is by modeling the nonlinearity of the switching devices and building an inverse model into the circuitry that generates the gate pulses for the switching devices. To the extent that such nonlinearities may vary from switching device to switching device, this approach requires a cumbersome adjustment of the model used in each inverter.  
       SUMMARY OF THE INVENTION  
       [0009]     The present inventor has determined that modeling nonlinearities of the switching devices can be avoided by the introduction of a minor voltage feedback loop from the output of the inverter to its input. Although current feedback loops are known for the purpose of providing current controlled outputs for inverters, adding of a minor voltage feedback loop significantly improves voltage accuracy beyond that provided by current feedback.  
         [0010]     Specifically then, the present invention provides a motor drive having a controller converting command signals to inverter-input signals received by an inverter to produce three-phase motor drive signals. A voltage feedback loop acquires voltage feedback signals measuring the voltage of the three-phase motor drive signals and provides the voltage feedback signals to the controller to correct the inverter-input signals, adjusting the voltage of the three-phase motor drive signals to conform with the inverter input signals.  
         [0011]     Thus it is one object of the invention to provide a simple mechanism for correcting for nonlinearities in the inverter that does not require a modeling of inverter nonlinear characteristics.  
         [0012]     The controller may include control logic receiving the command signals to produce the inverter input signals as a vector described by a q-component and a d-component; the vector may be received by a first transform means converting the vector to the three-phase sinusoidal signals. The voltage feedback loop may include a second transform means converting the voltage feedback signals to a feedback q-component and a feedback d-component and summers subtracting the feedback q-component from the q-component and subtracting the feedback d-component from the d-component, prior to the q-component and d-component being received by the first transform means.  
         [0013]     Thus it is another object of the invention to provide the minor loop voltage feedback at a point of constancy in the error signal allowing improved feedback loop closure. The voltage minor loops are intentionally placed in the synchronously rotating reference frame so that PI regulators can be used to reduce the steady state errors close to zero.  
         [0014]     The feedback loop may further include a proportional-integral controller positioned between the summer and the transform means for modifying the q-component and the d-component by an integral and proportional factor.  
         [0015]     Thus it is another object of the invention to compensate for the nonlinearities of the switching devices of the inverter using standard proportional/integral controller factors.  
         [0016]     The inverter may be a switched output amplifier.  
         [0017]     Thus it is another object of the invention to provide a control loop suitable for use with a switched output amplifier.  
         [0018]     The output stage of the inverter may use insulated gate bipolar transistors.  
         [0019]     Thus it is another object of the invention to provide a system that works with the nonlinearities of commonly used solid state switching devices.  
         [0020]     The system may further include a current feedback loop accepting feedback current from the three-phase motor drive signals and providing the current feedback signals to the controller to produce corrected inverter input signals.  
         [0021]     Thus it is another object of the invention to provide a feedback voltage loop that works with existing current control loops in a motor drive.  
         [0022]     The motor drive may further include a command feedback loop accepting feedback signals from a feedback sensor physically communicating with the motor and providing the command feedback signals to the controller to adjust the operation of the motor to better conform to the command signal.  
         [0023]     Thus it is another object of the invention to provide a voltage feedback loop that works within standard feedback provided for motor drive systems.  
         [0024]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0025]      FIG. 1  is a block diagram of a motor drive suitable for use with the present invention having a DC power supply, an inverter producing three-phase motor drive signals and a controller receiving command signals to produce inverter input signals;  
         [0026]      FIG. 2  is a detailed block diagram of the controller of  FIG. 1  showing control logic producing a vector inverter input signal (rotating framework) converted to a three-phase inverter input signal (stationary framework) and the implementation of a voltage feedback loop for controlling the voltage of the three-phase motor drive signals by modifying the vector inverter input signal;  
         [0027]      FIG. 3  is a set of graphs on common time axes showing desired and actual three-phase motor drive signals shifted in phase, voltage error in the stationary framework, and voltage error in rotating framework; and  
         [0028]      FIG. 4  is a figure similar to that of  FIG. 3  showing desired and actual three-phase motor drive signals shifted in amplitude, voltage error in stationary framework, and voltage error in rotating framework. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]     Referring now to  FIG. 1 , a motor drive  10  may receive a command signal  22  from a user or an external device providing a desired motor torque, motor speed, motor position or the like for control of a motor three-phase induction motor  16 . The motor drive  10  accepts AC power from a three-phase line  12  and produces synthesized three-phase motor drive signals  14  for the induction motor  16  according to the command signal  22 .  
         [0030]     The three-phase line  12  is received by a power supply  24  within the motor drive  10 , the power supply  24  rectifying and filtering the three-phase power of the line  12  to provide a source of DC power  26 . The DC power  26  is then provided to an inverter  28  having solid state switching devices  30  (only one shown for simplicity) such as insulated gate bipolar transistors (IGBT) which modulate the DC power  26  to synthesize the three-phase motor drive signals  14  according to inverter input signals  32  received from a controller  34 . Generally, the inverter input signals  32  are three sinusoidal waveforms, one for each phase, varying in shape, phase, and frequency according to the demands of the command signal  22 .  
         [0031]     As is understood in the art, the inverter  28  may be operated in a switched mode in which the switching devices  30  are switched between ON and OFF states rapidly at a frequency higher than that of the frequency of the three-phase motor drive signals  14 . The duty cycle of the switching of the switching devices  30  provides for the desired average voltage and current needed by the output signal. Generally, however, the three-phase motor drive signals  14  are three different high frequency square waves, one associated with each power phase.  
         [0032]     The three-phase motor drive signals  14  may be received by current sensors  36  and voltage sensors  38  providing current feedback signals  40  and voltage feedback signals  42  to the controller  34 . The current feedback signals  40  may provide a measurement of current in each of the three conductors providing the three-phase motor drive signals  14  or in two of the conductors with the current in the third conductor deduced. The three-phase motor drive signals are also provided to the motor  16  which may be attached to an encoder or other feedback sensor  18  providing a motor feedback signal  20 . The controller  34  receives the feedback signals  40 ,  42 , and  20  and the command  22  to generate the necessary inverter input signals  32 .  
         [0033]     Referring now to  FIG. 2 , the command signal  22  may be received at control logic  46  within the controller  34 , the control logic  46  also receiving the motor feedback signal  20 . The control logic  46  is constructed according to techniques known in the art to produce a current vector  48  having a d-term i qs   e  and a q-term i ds   e  describing an in-phase (d) and quadrature (q) component of the vector. As is generally understood in the art, the current vector  48  is a vector having direction and magnitude in a rotating framework keyed to a frequency θ e  of the three-phase motor drive signals  14 . Thus, θ e  is equal to the rotational rate of the motor θ r  plus the slippage θ s . The two values i ds   e  and i qs   e  uniquely describe in static form a set of three time varying sinusoidal signals separated by 120 degrees and forming the basis for the inverter input signals  32 . This representation of three sinusoidal signals will be termed a rotating framework representation whereas the three sinusoidal waveforms will be termed a stationary framework representation. Transformation from a rotating framework representation to a stationary framework representation and vice versa is well known in the art. The super script e denotes that the quantity is in synchronous reference frame, subscript s denotes that the quantity is a stator quantity.  
         [0034]     The values i ds   e  and i qs   e  are provided to the non-inverting input of summers  50  and  52 , respectively.  
         [0035]     Referring still to  FIG. 2 , the voltage feedback signals  40  are received by a sampling circuit  60  which samples the square wave waveform of the three-phase motor drive signals  14  at a high rate and then averages the samples over a window to produce three-phase average voltage feedback signals  62 . Each of these signals is essentially sinusoidal and varying in phase from one another by 120 degrees per the three-phase motor drive signals  14 .  
         [0036]     The three-phase average current feedback signals  62  are received by a 3-2 transformer  64  which converts the stationary framework representation of the feedback to a rotating framework representation using the value θ e  which may be produced by the control logic  46  according to methods well known in the art. The result is a single current feedback vector  67  also having i ds   e  and i qs   e  components. These i ds   e  and i qs   e  components of the current feedback vector  67  are provided to the inverting inputs of summers  50  and  52  and serve to correct the current vector  48  producing modified current vector  66  so as to bring the value of current of the three-phase motor drive signals  14  into closer alignment with the current vector  48  generated from the command  22 .  
         [0037]     The i ds   e  and i qs   e  components of the modified current vector  66  are received by proportional/integral controllers  68  and  70 , respectively, which multiply the i ds   e  and i qs   e  components by a proportional factor and sum that to a time integral of the i ds   e  and i qs   e  components times an integral factor, as is understood in the art, to produce a voltage vector  72 , also having a v ds   e  and v qs   e  component. These v ds   e  and v qs   e  components are provided to summers  75  and  76 , respectively, at their non-inverting inputs.  
         [0038]     The voltage feedback signals  42  may be received by a sampler  78  similar to sampling circuit  60  providing a high-speed sampling of the voltage square wave of the three-phase motor drive signals  14  that is averaged to provide a set of three sinusoidal voltage feedback waveforms  80 . These wave forms  80  are received by 3-2 transformer  82  similar to that of 3-2 transformer  64  also receiving a θθ e  value to provide a feedback voltage vector  84  represented as two components v ds   e  and v qs   e  component. These components are provided, respectively, to the inverting inputs of summers  75  and  76 . The outputs of summers  75  and  76  provide a modified voltage vector  74  provided, respectively, to the proportional/integral controllers  86  and  88  similar to the proportional/integral controllers  68  and  70  described above.  
         [0039]     The outputs of the proportional/integral controllers  86  and  88  together form an error vector  90  providing a correction to the vector  48  intended to bring the current and voltage of the three-phase motor drive signals  14  of the inverter  28  into better conformity with the vector  48 .  
         [0040]     The error vector  90  is provided to a 2-3 transformer  92  which operates substantially in the opposite manner as 3-2 transformers  64  and  82  to produce three-phase signals  94  being essentially sinusoidal signals  96  that are provided to the inverter  28 .  
         [0041]     As is understood in the art, the inverter  28  takes the sinusoidal signals  96  and produces the necessary gate drive signals to produce the three-phase motor drive signals  14 .  
         [0042]     The components of the controller  34  may be implemented in discrete circuitry or may be implemented as a program running on a processor within the controller  34  or by combinations of these approaches or other techniques well known in the art.  
         [0043]     Referring now also to  FIG. 3 , voltage vector  72  of  FIG. 2  (v*) may have a slight phase difference with respect to voltage v of three-phase motor drive signals  14 . In stationary framework, the difference between these waveforms v* and v, indicated by voltage v e  is a sinusoidal voltage with a frequency substantially equal to θ e . In the rotating framework dimensions of d and q, however, the error voltage (for example, v ds   e  is a substantially constant value  102 ) (here shown as the vertical or q axis difference between the two phases representing v and v*). Accordingly, a proportional/integral controller may provide through its integral term more accurate reduction in this error.  
         [0044]     Similarly, referring to  FIG. 4 , a slight amplitude difference between v and v* provides a voltage error signal v e  that is sinusoidally varying rendering the use of an integral correction problematic for the control of this error. Nevertheless, the d (and q) component of the error is a relatively constant value that may be controlled using an integration. In this way, nonlinearities in the switching devices  30  that produce amplitude or phase shifts can be corrected through the use of a voltage feedback loop.  
         [0045]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.