Abstract:
A method and apparatus for regulating a harmonic current component of an inverter fed motor drive system comprises measuring a phase current of the motor, substantially eliminating the fundamental component of the phase current, demodulating the phase current having the fundamental component substantially eliminated to produce a first current signal and providing the first current signal to a PWM control input of an inverter drive of the inverter fed motor drive system to affect the inverter output voltage by driving a first harmonic current of the motor substantially to zero thereby to achieve harmonic current control. More accurate, estimated motor voltages can also be provided by the system.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]    This application claims the benefit and priority of U.S. Provisional Application No. 60/330,980 filed Nov. 6, 2001 and entitled CURRENT RIPPLE REDUCTION BY HARMONIC CURRENT REGULATION, the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
       1. Field of the Invention  
         [0002]    The present invention relates to a method of suppressing current ripple in an inverter fed motor drive system.  
         2. Description of the Related Art  
         [0003]    High performance inverter fed AC motor drives require smooth torque control. The motor torque generated by an inverter fed AC drive contains various harmonic torque components.  
           [0004]    These harmonic torque components are consequences of inverter-induced harmonics and motor-induced harmonics. Examples of inverter-induced harmonics include inverter dead-time, sensor offset, PWM chopping, etc. Examples of motor-induced harmonics include winding slots and symmetry, etc. The n th  harmonic torque component mainly is generated by the (n+1) th  and (n−1) th  harmonic currents interacting with the fundamental flux of the motor.  
           [0005]    The low frequency torque ripples are considered the most crucial because they can transmit to the drive load, causing speed fluctuation, and in some cases damaging the drive load. The 6 th  harmonic torques, due to inverter blanking time (dead time), are of particular concern.  
           [0006]    The consequence of inverter dead-time is torque ripple. Most of the prior art schemes address the problem by compensation of inverter voltage error. Due to the complex, non-linear behavior of the dead-time effect, however, a complete compensation of the dead-time effect may not be achieved. Moreover, the prior art schemes do not focus directly on torque ripples.  
           [0007]    It would be beneficial to provide a direct approach to harmonic torque elimination that addresses not only the torque harmonic problem caused by inverter dead time, but also problems caused by various other imperfections, such as motor space harmonics, which lead to the generation of harmonic torques. Further, it would be beneficial to provide a method of suppressing the n th  harmonic torque by eliminating the (n+1) th  and (n−1) th  harmonic currents.  
           [0008]    Furthermore, it would also be useful to be able to derive more accurate estimated motor voltages from the controller command voltages, particularly to provide better lower operating speed encoderless low cost drivers.  
         SUMMARY OF THE INVENTION  
         [0009]    A method and apparatus are provided for suppressing current ripple in an inverter fed motor drive system. The method and apparatus can be used to eliminate harmonic torque currents in AC motor drive systems.  
           [0010]    In accordance with the invention, a method is provided for regulating a harmonic current component of an inverter fed motor drive system comprising, measuring a phase current of the motor, substantially eliminating the fundamental component of the phase current, demodulating the phase current having the fundamental component substantially eliminated to produce a first current signal; and providing the first current signal to a PWM control input of an inverter drive of the inverter fed motor drive system to affect the inverter output voltage by driving a first harmonic current of the motor substantially to zero thereby to achieve harmonic current control.  
           [0011]    Further, in accordance with the invention, apparatus is provided for regulating a harmonic current component of an inverter fed motor drive system comprising a sensor measuring a phase current of the motor, a circuit substantially eliminating the fundamental component of the phase current, a demodulator to demodulate the phase current having the fundamental component substantially eliminated to produce a first current signal and a circuit for injecting the first current signal to a PWM control input of an inverter drive of the inverter fed motor drive system to affect the inverter output voltage by driving a first harmonic current of the motor substantially to zero thereby to achieve harmonic current control.  
           [0012]    In more detail, the harmonic torque is suppressed by a new control method that eliminates harmonic currents which contribute to the undesirable torque harmonics. The method includes measuring the motor phase currents, and removing the fundamental current component by current command. The remaining harmonic currents, which contribute to the torque ripple of concern, are discriminated by demodulation and controlled to zero by way of current regulation. For example, to eliminate the 6 th  harmonic torque, the (n+1) th  and (N−1) th  harmonic currents are reduced by the current regulation technique of the invention.  
           [0013]    Outputs can be provided from the motor drive to provide more accurate estimates of motor voltage, as a result of the ripple reduction, without requiring expensive encoders.  
           [0014]    Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    [0015]FIG. 1 is a functional block diagram showing the integration of the method and apparatus of the present invention into a harmonic current regulator in an inverter fed motor drive system.  
         [0016]    [0016]FIG. 2 is a functional block diagram showing the method and apparatus for harmonic current regulation according to the present invention, and in particular, applied to the 6 th  harmonic torque.  
         [0017]    [0017]FIG. 3 is the functional block diagram shown in FIG. 1 showing how voltage estimation is accomplished. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    Referring to FIG. 1, the present invention provides a motor control technique that is preferably implemented in firmware using e.g., a DSP (digital signal processor) or an FPGA type device. The method can be integrated into any current regulated vector control drive, shown generally at  30 , operating a rotating field machine, e.g., an AC motor  40 , via a PWM inverter bridge  50 . The control drive  30  is modified in accordance with the invention. The invention resides in coupling a harmonic current regulator  5  to the vector control drive, operated as described herein. The vector control drive  30  is modified by providing inputs iq*, id*, ωe, Theta and phase currents in, iv to the harmonic current regulator  5 . Outputs  250  of the harmonic current regulator  5  are provided to respective summing stages  35  provided in output lines from conventional vector rotator  37 . Control elements  37 ,  39 ,  41 ,  43 ,  45 ,  47  and  49  of vector control drive  30  are conventional.  
         [0019]    The following definitions are utilized throughout:  
                                                   Term   Definition                           Del_Comp   feed back delay compensation           id*   flux current command           iq*   torque current command           iu   phase current           iu_H   phase current (fundamental removed)           iv   phase current (120 Deg lagging iu)           iv_H   harmonic phase current (fundamental removed)           N   harmonic number           Theta   field angle for vector drive (integration of We)           Theta_N   harmonic field angle for vector demodulation           Uu_C   harmonic current regulator output (u phase)           Uv_C   harmonic current regulator output (v phase)           Uw_C   harmonic current regulator output (w phase)           ωe   Inverter fundamental frequency                      
 
         [0020]    Although the invention is applicable to the reduction of any harmonic torque, i.e., the n th  harmonic torque, it will be described herein in detail with relation to reducing the 6 th  harmonic torque.  
         [0021]    According to a preferred embodiment of the present invention, the method for harmonic torque (e.g. 6 th  harmonic) reduction caused by inverter dead-time involves extraction of the n−1 th  (5th) and n+1 th  (7th) harmonic current components from the feedback current and regulating these currents to zero. FIG. 2 shows the method of harmonic current regulation. FIG. 2 shows the harmonic current regulator  5  of FIG. 1 in greater detail. The harmonic current regulator comprises a fundamental component removing stage  100 . This stage comprises a vector rotator  10  that receives the commands id* and iq*. The outputs of the vector rotator  110  are supplied to respective summing stages  112  and  113  which each generate a difference signal with the respective phase currents iu and iv, thereby generating the current signals iu_H and iv_H, each with the fundamental removed. The signals ωe and Theta are also provided as shown. A delay compensation signal Del_Comp is provided by delay stage  118 . The signal Del_Comp is subtracted from Theta in stage  116  and applied to the vector rotator  110 .  
         [0022]    The phase currents (iu and iv) shown at  10  are measured by an appropriate current sensor at  11  and the, fundamental component is removed by stage  100  from the torque and current command signals (iq* and id*) shown at  20 . The torque and current command signals iq* and id* are well known and are readily available as shown at  20  in conventional current regulated vector drives, shown generally by reference numeral  30  in FIG. 1. The harmonic currents can be extracted without the removal of the fundamental component. However, removal of the fundamental component allows extended harmonic current regulator bandwidth. The phase current in with the fundamental removed is shown at iu_H. The phase current iv 120° lagging iu with the fundamental removed is shown at iv_H.  
         [0023]    The currents (iu_H, iv_H) contain harmonics only. The fundamental component has been removed. Then the n th  harmonic component can be extracted by using a vector demodulator. In particular, since the n th  harmonic torque is mainly generated by the (n+1) th  and (n−1) th  harmonic currents interacting with the fundamental motor flux, eliminating these two harmonic currents will substantially eliminate the n th  harmonic torque. As discussed, the 6 th  harmonic torque component is particularly troublesome. To reduce it, the 5 th  and 7 th  harmonics currents are regulated, as shown in FIG. 2. Accordingly, the harmonic current regulator  5  will include a 5 th harmonic current regulator  200  and a 7 th  harmonic current regulator  300 . Each regulator  200 ,  300  has the block diagram shown in the upper portion of FIG. 2 and includes a vector demodulator  210 . The vector demodulator  210  receives the harmonic vector angle (Theta_N) for demodulation, which is generated from the fundamental field angle (Theta) as shown in FIG. 2. The field angle Theta is multiplied in a multiplier  211  by the number N of the harmonic to be reduced. In this case N=n−1. Where n=6, N=5. The signal Del_Comp is multiplied in a multiplier  213  by N. The signal from multiplier  211  is delayed by Mod 2π ( 215 ) to produce Theta_N. Theta_N and Del_Comp are summed at  217  and the output delayed by Mod 2π at  219 . Using the N th  harmonic vector angle Theta_N for the demodulation, the N th  harmonic current of current signals iu_H and iv_H will be translated to DC at the output of the vector demodulator  210 , while all other harmonics will appear as AC signals. Therefore, an integrating action by integrators  220 A,  220 B is used to force, by regulation, the DC component (translated N th  harmonic current component) to zero. The outputs of the integrators  220 A and  220 B are provided to a vector rotator  230 . The outputs of the two harmonic current regulators  200  (n−1) th ,  300  (n+1) th  are summed at  240  and the outputs  250  provided to the vector drive to be summed with the respective PWM control signals by summing stages  35 . The outputs  250  are provided to cancel the harmonic currents at the inputs to the PWM stage  49 , eventually forcing the harmonic currents, and thus the affected torque harmonic, to zero.  
         [0024]    For the harmonic current regulator  300 , N=n+1. Where n=6, N=7. The operation of regulator  300  is the same as regulator  200 .  
         [0025]    The outputs of different harmonic current regulators are combined by superposition to form three outputs (Uu_C, Uv_C, Uw_C). These are injected to the corresponding phase modulation inputs (as shown in FIG. 1) to affect the inverter output voltage for harmonic current control.  
         [0026]    The superposition of the outputs of the two current regulators  200  and  300  regulating the 5 th  and 7 th  harmonic currents, results in signals that can be injected into the control drive via the stages  35  to reduce the 6 th  harmonic torque. In the general case, the regulation of the (n−1) th  and (n+1) th  harmonic currents results in reduction in the n th  harmonic torque.  
         [0027]    Likewise, any other harmonic current components of concern can be extracted and regulated to zero by using identical structures in parallel as shown in FIG. 2. For example, the second and fourth harmonic currents can be regulated, thus reducing the third harmonic torque.  
         [0028]    Another aspect of the invention is shown in FIG. 3. Most encoderless (without shaft encoder) drive controllers require voltage feedback for back EMF control schemes. In cases where low speed (&lt;15%) operation is applied, accurate voltage information is required. However, it is in general not practical to sense motor voltages due to acquisition of high bandwidth PWM voltage signals and increased hardware cost. In most cases, the motor voltages are estimated using controller command voltages. The accuracy of using command voltages is affected by inverter nonlinearities such as dead time, device voltage drop etc. Using the harmonic current regulator of the invention to eliminate toque ripple by compensating the nonlinear effects introduced by the inverter, it is also possible to achieve a more accurate voltage estimation. As shown in the attached FIG. 3, Uv and Uw are sensed before the harmonic current regulation. Therefore these voltages better approximate motor voltages (inverter nonlinearities have been compensated) and can be used to provide better (lower operating speed) encoderless drives. FIG. 3 is identical to FIG. 1 except it shows where to tap off the estimated motor voltages.  
         [0029]    Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. Therefore, the present invention should be limited not by the specific disclosure herein, but only by the appended claims.