Abstract:
A motor drive system is disclosed that includes a power input configured to receive alternating current (AC) power and a rectifier having a switching frequency selected to convert the AC power to direct current (DC) power. The motor drive unit also includes an input filter circuit connected between the power input and the rectifier and configured to suppress frequency harmonics across a range of harmonics. Additionally, the motor drive unit includes a block filter circuit connected between the power input and the rectifier and configured to substantially block frequency harmonics associated with the switching frequency of the rectifier. Furthermore, the motor drive unit includes an inverter configured to receive the DC power from the rectifier and convert the DC power to a series of pulses configured to drive a motor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention relates generally to motor drive systems and, more particularly, to a system and method for suppressing harmonics injected onto AC power lines when operating a motor drive unit. 
   Often, power delivered from a power source or supply is not properly conditioned for consumption. For example, power plants are linked to power consuming facilities (e.g., buildings, factories, etc.) via utility grids that are designed to be extremely efficient at delivering massive amounts of power. To facilitate efficient distribution, power is delivered over long distances as fixed frequency three-phase alternating current (AC) power. As such, the power must typically be converted or “conditioned” prior to consumption. 
   For example, motors and their associated loads are one type of common inductive load employed at many consuming facilities that require power conditioning. When a motor is the consuming point, power “conditioning” systems are utilized to convert the fixed frequency AC power delivered over utility grids to a form suitable for driving the motor. To this end, power conditioning for motor systems typically include AC-to-DC (direct current) rectifiers that convert the utility AC power to DC power applied to positive and negative DC buses (i.e. across a DC link). The power distributed across the DC buses is then converted, for example, by use of an inverter, to AC power designed to drive the motor. 
   Specifically, referring to  FIG. 1 , the present invention will be described with respect to a motor system  10 . The motor system  10  generally includes a power supply  12 , a motor drive unit  14 , and a motor  16 . The power supply  12  provides power to the motor drive unit  14  that, in turn, converts the power to a more usable form for the motor  16  that drives an associated load  18 . 
   The motor drive unit  14  includes a variety of components, such as a rectifier  20 , an inverter  22 , and a controller  24 . During operation, the power supply  12  provides three-phase AC power, for example, as received from a utility grid over transmission power lines  26 . However, it is also contemplated that the power supply  12  may deliver single-phase power. The rectifier  20  is designed to receive the AC power from the power supply  12  and convert the AC power to DC power that is delivered to positive and negative DC buses  28 ,  30  of a DC link  32 . Specifically, the rectifier  20  includes a plurality of switches that are controlled by the controller  24  using pulse-width modulation (PWM) to rectify the AC power received from the AC power lines  26 . 
   The DC power is delivered from the rectifier  20  over the positive and negative DC buses  28 ,  30  to the inverter  22 . The inverter  22  includes a plurality of switching devices (e.g., IGBTs or other semiconductor switches) that are positioned between the positive and negative buses  28 ,  30  and controlled by the controller  24  to open and close specific combinations of the switches to sequentially generate pulses on each of the supply lines  34  to drive the motor  16  and, in turn, the load  18  through a drive shaft  36 . 
   While PWM control of the rectifier  20  is an efficient means for converting the AC power received from the power lines  26  to DC power, high-frequency harmonics can be injected onto the power lines  26  as a result. These injected high-frequency harmonics can adversely affect operation of the motor drive unit  14  and other systems connected to the power supply  12 . 
   As a result, a filter  38  is often arranged between the power supply  12  and the motor drive unit  14 . Typically, an inductor L 1  is arranged to act as a filter to suppress high-frequency harmonics injected onto the power lines  26  by operation of the motor drive unit  14 . However, in many cases, the inductor L 1  is unable to sufficiently attenuate the high-frequency harmonics. 
   Accordingly, an additional inductor L m  and a capacitor C 1  are sometimes included to construct an LCL filter. While the LCL filter is typically insufficient to suppress all harmonics from being injected onto the power lines  26 , it provides increased attenuation over the simple inductor L 1  filter, such that the harmonics injected are tolerated by the system. 
   However, as current power regulations, such as IEEE 519, become more and more stringent, the amount of high-frequency harmonics tolerated under the regulations decrease. Furthermore, since harmonics regulations vary between countries, the amount of harmonics tolerated under these regulations varies by country. Accordingly, motor drive units must be tailored to meet the requirements of each country or must be designed to meet the most stringent of harmonics regulations. 
   It would be desirable to have a system and method for suppressing or substantially eliminating harmonics injected onto AC power lines when operating a motor drive unit. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention overcomes the aforementioned drawbacks by providing a block filter designed to substantially block harmonics associated with a rectifier switching frequency of a motor drive unit. When coupled with an LCL filter, the combined filter and block system substantially reduces or blocks harmonics from being injected onto the supply lines from the power supply, including specific frequency harmonics associated with the switching frequency component of the PWM rectifier. 
   In accordance with one aspect of the present invention, a motor drive system is disclosed that includes a power input configured to receive alternating current (AC) power and a rectifier having a switching frequency selected to convert the AC power to direct current (DC) power. The motor drive unit also includes an input filter circuit connected between the power input and the rectifier and configured to suppress frequency harmonics across a range of harmonics. Additionally, the motor drive unit includes a block filter circuit connected between the power input and the rectifier and configured to substantially block frequency harmonics associated with the switching frequency of the rectifier. Furthermore, the motor drive unit includes an inverter configured to receive the DC power from the rectifier and convert the DC power to a series of pulses configured to drive a motor. 
   In accordance with another aspect of the present invention, a motor drive unit is disclosed that includes an input configured to receive AC power. A filter and block circuit are included that are configured to filter the AC power and that include an input inductor (L 1 ), a main inductor (L m ), a main capacitor (C m ), a block inductor (L t ), and a block capacitor (C t ) arranged in an LCL-plus-C circuit. The motor drive unit also includes a rectifier configured to receive filtered AC power from the filter and having a switching frequency selected to convert the AC power to DC power and an inverter configured to receive the DC power from the rectifier and convert the DC power to a series of pulses configured to drive a motor. At least one of the block inductor and block capacitor are configured to substantially block harmonics associated with the switching frequency of the rectifier. 
   In accordance with yet another aspect of the invention, an input filter is disclosed that is configured to be connected to a power supply line to receive AC power and deliver the AC power to a rectifier. The rectifier includes a plurality of switches controlled by at least one switching signal having a switching frequency to convert the AC power to DC power and deliver the DC power to an inverter. The inverter is configured to convert the DC power to pulses configured to drive a motor. The input filter includes a main inductor configured to receive the AC power and reduce harmonics along the power supply line, a block inductor connected to the main inductor, and a block capacitor connected to the main inductor. At least one of the block inductor and the block capacitor are arranged in a block circuit configured to substantially reduce harmonics associated with the switching frequency. 
   Various other features of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
       FIG. 1  is a schematic diagram of a motor system and associated traditional harmonic filter circuit; 
       FIG. 2  is a schematic diagram of a motor system, associated traditional harmonic filter, and block filter circuit in accordance with the present invention; 
       FIG. 3  is a circuit diagram of a filter system including a block filter circuit configuration and equivalent circuit in accordance with the present invention; 
       FIG. 4  is a circuit diagram of another filter system including another block filter circuit configuration and equivalent circuit in accordance with the present invention; 
       FIG. 5  is a circuit diagram of yet another filter system including another block filter circuit configuration and equivalent circuit in accordance with the present invention; 
       FIG. 6  is a partial, simplified circuit diagram showing that the use of fixed component percentages allows motor drive units with differing power ratings to have similar current total harmonic distortion values. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to  FIG. 2 , a block filter  40  has been added to the motor system  10  as part of the filter  38  between the power supply  12  and the rectifier  20  of the motor drive unit  14 . That is, the block filter  40  has been added to the filter  38  and is arranged between the point of common connection (PCC) between the supply lines  26  and the input to the motor drive unit  14 . As will be described below, this filter configuration  38  including the block filter  40  will be referred to as an LCL-plus-C circuit. As will be described, the block filter  40  has an inductive reactance that is substantially equal to its capacitive reactance at the switching frequency of the rectifier. 
   As illustrated, it is contemplated that the main inductor (L m ) and block filter  40  may be formed as an integrated package that may be readily retrofitted into a traditional filter system  38 . However, as will be described, it is contemplated that the main inductor L m , and block filter  40  may be integrated with the other components of the filter  38 . 
   Referring now to  FIGS. 3-5 , a variety of configurations for the main inductor L m  and block filter  40  of  FIG. 2  are illustrated. Specifically,  FIG. 3  shows a cascaded block filter  40   a  coupled with the above-described LCL filter to form an LCL-plus-C circuit.  FIG. 3  also includes an equivalent circuit  42  of the cascaded block filter  40   a . Additionally,  FIG. 4  shows the above-described LCL filter coupled with a direct-coupled block filter  40   b  to form another LCL-plus-C circuit configuration.  FIG. 4  further shows an equivalent circuit  44  of the direct-coupled block filter  40   b . Finally,  FIG. 5  shows the above-described LCL filter coupled with a transformer block filter  40   c  to form still another LCL-plus-C circuit configuration and an equivalent circuit  46  of the transformer block filter  40   c.    
   As illustrated in  FIG. 3 , the cascaded block filter  40   a  can be readily separated from the main inductor L m . In this regard, the cascaded block filter  40   a  can be used to retrofit a traditional LCL filter to create an LCL-plus C circuit. On the other hand, as illustrated in  FIGS. 4 and 5 , the block filters  40   b ,  40   c  can be integrated with the main inductor L m  by sharing a common core. In the configurations illustrated in  FIGS. 4 and 5 , the main inductor L m  has two windings for one phase. The main winding forms L m  and the secondary winding forms L t . As such, the direct-coupled block filter  40   b  and the transformer block filter  40   c  have identical equivalent circuits  44 ,  46 . However, as will be shown below, the value of the capacitor C t  in the transformer block filter  40   c  configuration shown in  FIG. 5  can typically be reduced over that required in the direct-coupled block filter  40   b  configuration shown in  FIG. 4 . On the other hand, the direct-coupled block filter  40   b  will typically have a higher Q factor than the transformer block filter  40   c.    
   In any case, each of the block filter configurations  40   a - 40   c  operates as an LC resonator that can be configured to have substantially similar admittance and frequency characteristics. Specifically, the resonant frequency of the LCL-plus-C circuit of  38  configured with the cascaded block filter  40   a  of  FIG. 3  is given by: 
   
     
       
         
           
             
               
                 
                   f 
                   res 
                 
                 = 
                 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                     
                   
                   · 
                   
                     
                       
                         
                           
                             L 
                             1 
                           
                           + 
                           
                             L 
                             m 
                           
                           + 
                           
                             L 
                             t 
                           
                         
                         
                           
                             
                               L 
                               1 
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   L 
                                   m 
                                 
                                 + 
                                 
                                   L 
                                   t 
                                 
                               
                               ) 
                             
                           
                           ⁢ 
                           C 
                         
                       
                     
                     . 
                   
                 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 1 
               
             
           
         
       
     
   
   Similarly, the resonant frequency of the LCL-plus-C circuit of  38  configured with the direct-coupled block filter  40   b  of  FIG. 4  or the transformer block filter  40   c  of  FIG. 5 , which have identical equivalent circuits  44 ,  46 , is given by: 
   
     
       
         
           
             
               
                 
                   f 
                   res 
                 
                 = 
                 
                   
                     1 
                     
                       2 
                       ⁢ 
                       π 
                     
                   
                   · 
                   
                     
                       
                         
                           
                             L 
                             1 
                           
                           + 
                           
                             L 
                             m 
                           
                         
                         
                           
                             L 
                             1 
                           
                           ⁢ 
                           
                             L 
                             m 
                           
                           ⁢ 
                           C 
                         
                       
                     
                     . 
                   
                 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 2 
               
             
           
         
       
     
   
   The specific component values of the block filters  40   a ,  40   b ,  40   c , are selected based on the specific switching frequency of the rectifier  20  of the motor drive unit  14  of  FIG. 2 . In particular, the value of capacitor C t  is selected to block the switching frequency component of injected harmonics. With respect to the direct-coupled block filter  40   b , capacitor C t  is selected by: 
   
     
       
         
           
             
               
                 
                   
                     C 
                     t 
                   
                   = 
                   
                     1 
                     
                       
                         
                           ( 
                           
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               f 
                               sw 
                             
                           
                           ) 
                         
                         2 
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             L 
                             m 
                           
                           + 
                           
                             L 
                             t 
                           
                           - 
                           
                             2 
                             ⁢ 
                             M 
                           
                         
                         ) 
                       
                     
                   
                 
                 ; 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
         
       
     
   
   where f sw  is the switching frequency of the rectifier  20 , k is the coupling factor of L m  and L t  windings, and M is the mutual inductance given by:
 
 M=k√ {square root over ( L   m   L   t )}  Eqn. 4.
 
   Likewise, with respect to the cascaded block filter  40   a  and the transformer block filter  40   b , capacitor C t  is selected by: 
   
     
       
         
           
             
               
                 
                   C 
                   t 
                 
                 = 
                 
                   
                     1 
                     
                       
                         
                           ( 
                           
                             2 
                             ⁢ 
                             π 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               f 
                               sw 
                             
                           
                           ) 
                         
                         2 
                       
                       ⁢ 
                       
                         L 
                         t 
                       
                     
                   
                   . 
                 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 5 
               
             
           
         
       
     
   
   It is contemplated that when extended to a three-phase system, the above-described system may be embodied using three separate single-phase main inductors L m . On the other hand, it is contemplated that a single three-phase main inductor L m  may also be used. 
   A few additional considerations aid in guiding the selection of L 1 , L m , and L t . That is, since the high frequency current going through L 1 , especially switching frequency components, is suppressed by the block circuit  40   a ,  40   b , or  40   c , the cost of including L 1  is relatively low because the current waveform is close to sinusoidal. Likewise, since only high frequency current is seen by L t , the wire gauge of the coil can be relatively high, depending on the turn ratio of the coil in L t  and L m , which reduces implementation costs. Further cost and size savings can be realized when selecting the capacitance value, which can be relatively low, for example, 0.035 μF for 20 HP motor drive units. Furthermore, since the high frequency current seen by L m  is relatively high, the inductor design of L m  should provide a relatively low core flux density or low high-frequency loss core, such as a ferrite or other inductor having a ferromagnetic compound core. 
   The specific implementations of the above-described block filters  40   a ,  40   b ,  40   c  may be readily scaled to a desired power rating. Specifically, the percentage of harmonic components permitted by the block filters  40   a ,  40   b ,  40   c  are determined by the percentage value relationship of L 1 , L m , and C m  . . . Referring now to  FIG. 6 , the filter configurations described above have been simplified to an LCL configuration for the purpose of illustrating that the use of fixed component percentages allows motor drive units with differing power ratings to have similar current total harmonic distortion values. Specifically, the percentage of harmonic components permitted can be significantly controlled by the percentage value relationship of L 1 , L m , and C m , regardless of power ratings, as follows: 
   
     
       
         
           
             
               
                 
                   
                     
                       
                         i 
                         1 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       I 
                       n 
                     
                   
                   = 
                   
                     
                       3 
                     
                     ⁢ 
                     
                       
                         1 
                         
                           
                             
                               p 
                               
                                 L 
                                 1 
                               
                             
                             · 
                             
                               p 
                               
                                 L 
                                 m 
                               
                             
                             · 
                             
                               
                                 
                                   p 
                                   c 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     s 
                                     
                                       ω 
                                       n 
                                     
                                   
                                   ) 
                                 
                               
                               3 
                             
                           
                           + 
                           
                             
                               ( 
                               
                                 
                                   p 
                                   
                                     L 
                                     1 
                                   
                                 
                                 + 
                                 
                                   p 
                                   
                                     L 
                                     m 
                                   
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 s 
                                 
                                   ω 
                                   n 
                                 
                               
                               ) 
                             
                           
                         
                       
                       · 
                       
                         
                           
                             V 
                             2 
                           
                           ⁡ 
                           
                             ( 
                             s 
                             ) 
                           
                         
                         
                           V 
                           n 
                         
                       
                     
                   
                 
                 ; 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 6 
               
             
           
           
             
               
                 
                   
                     
                       
                         i 
                         2 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       I 
                       n 
                     
                   
                   = 
                   
                     
                       3 
                     
                     ⁢ 
                     
                       
                         
                           
                             
                               p 
                               
                                 L 
                                 1 
                               
                             
                             · 
                             
                               
                                 
                                   p 
                                   c 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     s 
                                     
                                       ω 
                                       n 
                                     
                                   
                                   ) 
                                 
                               
                               2 
                             
                           
                           + 
                           1 
                         
                         
                           
                             
                               p 
                               
                                 L 
                                 1 
                               
                             
                             · 
                             
                               p 
                               
                                 L 
                                 m 
                               
                             
                             · 
                             
                               
                                 
                                   p 
                                   c 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     s 
                                     
                                       ω 
                                       n 
                                     
                                   
                                   ) 
                                 
                               
                               3 
                             
                           
                           + 
                           
                             
                               ( 
                               
                                 
                                   p 
                                   
                                     L 
                                     1 
                                   
                                 
                                 + 
                                 
                                   p 
                                   
                                     L 
                                     m 
                                   
                                 
                               
                               ) 
                             
                             · 
                             
                               ( 
                               
                                 s 
                                 
                                   ω 
                                   n 
                                 
                               
                               ) 
                             
                           
                         
                       
                       · 
                       
                         
                           
                             V 
                             2 
                           
                           ⁡ 
                           
                             ( 
                             s 
                             ) 
                           
                         
                         
                           V 
                           n 
                         
                       
                     
                   
                 
                 ; 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 7 
               
             
           
           
             
               
                 
                   
                     
                       
                         i 
                         c 
                       
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                     
                       I 
                       n 
                     
                   
                   = 
                   
                     
                       
                         
                           i 
                           1 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                       
                         I 
                         n 
                       
                     
                     - 
                     
                       
                         
                           i 
                           2 
                         
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                       
                         I 
                         n 
                       
                     
                   
                 
                 ; 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 8 
               
             
           
           
             
               
                 
                   
                     ω 
                     res 
                   
                   = 
                   
                     
                       
                         
                           
                             p 
                             
                               L 
                               1 
                             
                           
                           + 
                           
                             p 
                             
                               L 
                               m 
                             
                           
                         
                         
                           
                             p 
                             
                               L 
                               1 
                             
                           
                           · 
                           
                             p 
                             
                               L 
                               m 
                             
                           
                           · 
                           
                             p 
                             c 
                           
                         
                       
                     
                     · 
                     
                       ω 
                       n 
                     
                   
                 
                 ; 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 9 
               
             
           
         
       
     
   
   where p L1  is the percentage value of L 1 , p Lm  is the percentage value of L m , p c  is the percentage value of C, V n  is the rated line voltage, I n  is the rated line current, ω n  is the rated line angular frequency, and ω res  is the resonance angular frequency. The inductance/capacitance bases are: 
   
     
       
         
           
             
               
                 
                   
                     R 
                     n 
                   
                   = 
                   
                     
                       V 
                       n 
                     
                     
                       
                         3 
                       
                       · 
                       
                         I 
                         n 
                       
                     
                   
                 
                 ; 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 10 
               
             
           
           
             
               
                 
                   
                     
                       C 
                       n 
                     
                     = 
                     
                       1 
                       
                         
                           R 
                           n 
                         
                         · 
                         
                           ω 
                           n 
                         
                       
                     
                   
                   ; 
                 
                 ⁢ 
                 
                   
 
                 
                 ⁢ 
                 and 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 11 
               
             
           
           
             
               
                 
                   L 
                   n 
                 
                 = 
                 
                   
                     
                       R 
                       n 
                     
                     
                       ω 
                       n 
                     
                   
                   . 
                 
               
             
             
               
                 Eqn 
                 . 
                 
                     
                 
                 ⁢ 
                 12 
               
             
           
         
       
     
   
   It should be noted that for simplicity, the above calculations have neglected the resistance of the filter. However, the inclusion of inductor resistance and damping resistance does not alter the fact that, as illustrated above, by maintaining fixed percentages of L 1 , L m  and C, total harmonic distortions of i 1 , i 2 , and i c  can be controlled across a variety of motor drives with a variety of power ratings. 
   Similarly, as illustrated below, the percentage value of C t  does not vary with power ratings: 
                     p     C   t       =         (       ω   n       ω   sw       )     2     ⁢     1     λ   ·     p     L   m               ;           Eqn   .           ⁢   13               
where ω sw  is the angular switching frequency of the rectifier of the motor drive and is equal to f sw  multiplied by 2π. Additionally, λ is the ratio of L t  to L m , which, as described above, is typically a fixed ratio.
 
   When compared to a traditional LCL filter, such as illustrated in  FIG. 1 , the above-described filter systems including the block filters  40  to form an LCL-plus-C circuit are able to significantly suppress current harmonics with switching frequency current at the PCC. For example, residual high frequency harmonics may be suppressed to significantly less than 0.5 percent of peak phase current. Since the above-described filter system significantly reduces the harmonic current at the switching frequency at the rectifier input, the current total harmonic distortion at the rectifier input is also reduced, even though some increase in the second and third order switching frequency harmonics may be experienced. 
   The present invention has been described in terms of the various embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention. Therefore, the invention should not be limited to a particular described embodiment.