Patent Publication Number: US-6987372-B1

Title: Integrated DC link choke and method for suppressing common-mode voltage in a motor drive

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/463,986 filed Jun. 18, 2003 and entitled INTEGRATED DC LINK CHOKE AND METHOD FOR SUPPRESSING COMMON-MODE VOLTAGE IN A MOTOR DRIVE, now U.S. Pat. No. 6,867,564, which is a continuation of U.S. Ser. No. 09/832,329, now U.S. Pat. No. 6,617,814 B1 filed Apr. 11, 2001 and entitled INTEGRATED DC LINK CHOKE AND METHOD FOR SUPPRESSING COMMON-MODE VOLTAGE IN A MOTOR DRIVE. 

   TECHNICAL FIELD 
   The invention relates generally to the art of electric motors and motor drives and more particularly to an integrated DC link choke and methodology for suppressing common-mode voltage and smoothing DC current in a motor drive. 
   BACKGROUND OF THE INVENTION 
   In the field of electric motors and motor drives, a motor is connected to a motor drive, which provides electrical power to the motor in a controlled fashion, wherein the motor drive may take a variety of forms, such as a current source inverter (CSI), voltage source inverter (VSI) or the like. Such motor drives are commonly employed to provide electrical power to motors. The motor drive may be employed in order to provide speed control, torque control, and/or to control other motor performance characteristics. For AC motors, electrical power is converted in the motor drive from supply power, typically AC power from a utility or other source, into DC. The DC power is then converted, for example using an inverter stage, into AC power at a controlled frequency and amplitude, which is provided to the motor windings. 
   The AC to DC converter stage in the motor drive, and/or the subsequent inverter stage, often comprise solid state semiconductor-based switching elements, such as silicon-controlled rectifiers (SCRs), gate turn-off thyristors (GTOs), gate commutated thyristors (GCTs), insulated gate bipolar transistors (IGBTs), or other switching devices. As the switching elements are activated in the AC to DC converter stage or the inverter stage, common-mode voltages are produced. This common-mode voltage appears in the output phases of the motor drive, and hence, on the motor windings. Where a neutral associated with the input power source is grounded, this common-mode voltage appears between the motor windings and ground, and may reach levels beyond the motor winding insulation ratings. Consequently, such common-mode voltages may damage the motor or decrease the life expectancy thereof. 
   Conventional approaches to addressing this motor drive common-mode voltage problem have included designing the motor winding insulation to higher voltage ratings. However, this approach provides no solution for existing motors, which do not include insulation capable of withstanding the common-mode voltage levels. Another approach has been to provide an isolation transformer between the power source and the motor drive AC power input terminals. However, such an isolation transformer must be rated to handle the common-mode voltage levels. In addition, isolation transformers add cost and occupy space in the motor drive system. Thus, there is a need for apparatus and techniques to reduce or minimize the common-mode voltages in a motor drive, which do not require isolation transformers, and which operate in association with existing motors. 
   SUMMARY OF THE INVENTION 
   The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
   The invention provides a motor drive and a choke therefor, wherein the choke comprises a magnetic core with an inner leg and two outer legs, and four coils, which may be connected in DC current paths in the motor drive. First and second differential coils are wound around first and second outer legs, respectively; and first and second common-mode coils are wound around the inner leg of the choke. The first and second differential coils smooth the DC power on a DC bus in the motor drive, and the first and second common-mode coils suppress common-mode voltages in the motor drive. The invention thus provides an integrated DC link choke, which may be employed to provide both DC current smoothing and common-mode voltage suppression in a motor drive. In addition, the invention provides for a solution to the above motor drive common-mode voltage problems, whereby motors need not include extra insulation beyond the normal motor ratings, and whereby no external isolation transformers are required for a motor drive system. 
   According to one aspect of the invention, a choke provides a differential inductance and suppresses common-mode voltages in a motor drive having an AC to DC converter stage for converting AC power to DC power on a DC bus with positive and negative DC current paths, and an inverter stage for converting DC power from the DC bus to AC motor power in a controlled fashion. The choke comprises a magnetic core having an inner leg extending between an upper member and a lower member, and first and second outer legs extending from the lower member toward the upper member. A first air gap is provided between the first outer leg and the upper member and a second air gap is provided between the second outer leg and the upper member. 
   First and second differential coils are wound around the first and second outer legs, respectively, and may be connected in the positive and negative DC current paths of the motor drive, respectively, to smooth the DC power on the DC bus. The differential coils may be wound on the core to provide a differential flux in a differential flux path through the first and second outer legs, the first and second air gaps, and the upper and lower members of the core in response to differential current in the positive and negative DC current paths of the motor drive, whereby a differential inductance is provided to smooth DC power on the motor drive DC bus. 
   In addition, first and second common-mode coils are wound around the inner leg of the choke, which may also be connected in the positive and negative DC current paths, respectively, to suppress common-mode voltages in the motor drive. The common-mode coils provide a first common-mode flux in a common-mode flux path through the inner leg, the first and second outer legs, the first and second air gaps, and the upper and lower members of the core in response to common-mode current in the positive and negative DC current paths. The common-mode coils thus provide a common-mode inductance to suppress common-mode voltages in the motor drive. In addition, the differential coils may be wound around the outer legs of the core in order to provide a second common-mode flux in the common-mode flux path in response to common-mode current in the positive and negative DC current paths. The common-mode coils and the differential coils may thus provide a common-mode inductance to suppress common-mode voltages in the motor drive. The common-mode inductance and the differential inductance may be adjusted independently of one another according to a particular motor drive design. 
   Another aspect of the invention provides a motor drive for providing AC power to a motor in a controlled fashion. The motor drive comprises an AC to DC converter stage for converting AC power to DC power on a DC bus with positive and negative DC current paths, and an inverter stage for converting DC power from the DC bus to AC motor power in a controlled fashion. The motor drive further comprises a choke for providing a differential inductance and suppressing common-mode voltages in the motor drive. The choke comprises a magnetic core having an inner leg extending between an upper member and a lower member, and first and second outer legs extending from the lower member toward the upper member and providing a first air gap between the first outer leg and the upper member and a second air gap between the second outer leg and the upper member. First and second differential coils are wound around the first and second outer legs, respectively, and first and second common-mode coils are wound around the inner leg. The first and second differential coils are connected in the positive and negative DC current paths, respectively, to smooth the DC power on the DC bus, and the first and second common-mode coils are connected in the positive and negative DC current paths, respectively, to suppress common-mode voltages in the motor drive. 
   Yet another aspect of the invention provides a methodology by which common-mode voltages may be suppressed in a motor drive. A magnetic core is provided having an inner leg and two outer legs. First and second differential coils are wound around the first and second outer legs, respectively, and first and second common-mode coils are wound around the inner leg. The first differential and common-mode coils are connected in a positive DC current path in the motor drive, and the second differential and common-mode coils are connected in a negative DC path, so as to suppress common-mode voltages in the motor drive. 
   To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described. The following description and the annexed drawings set forth in detail certain illustrative aspects of the invention. However, these aspects are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of an exemplary motor drive for providing AC power to a motor in a controlled fashion in accordance with an aspect of the present invention; 
       FIG. 2  is a side elevation view of an exemplary choke for providing a differential inductance and suppressing common-mode voltages in a motor drive according to another aspect of the invention; 
       FIG. 3  is a side elevation view further illustrating the choke of  FIG. 2 ; and 
       FIG. 4  is a flow diagram illustrating an exemplary method of suppressing common-mode voltages in a motor drive according to another aspect of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The various aspects of the present invention will now be described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. The invention provides a choke for use in a motor drive, which comprises a magnetic core with an inner leg and two outer legs, and four coils, which may be connected in DC current paths in the motor drive. First and second differential coils are wound around first and second outer legs, respectively; and first and second common-mode coils are wound around the inner leg of the choke. The first and second differential coils smooth the DC power on a DC bus in the motor drive, and the first and second common-mode coils suppress common-mode voltages in the motor drive. In addition, the differential coils may also suppress common-mode voltages. The invention thus provides an integrated DC link choke, which may be employed to provide both DC current smoothing and common-mode voltage suppression in a motor drive. 
   Referring now to  FIG. 1 , an exemplary motor drive  2  is illustrated for providing AC electric power to an electric motor  4  in a controlled fashion, wherein the motor  4  may be an induction or other type of AC motor. Electric power is provided to the drive  2  from a three phase input power source  6  comprising AC power sources  7   a ,  7   b , and  7   c , interconnected in a “Y” arrangement, with a grounded neutral. Three phase AC power from the input power source  6  is provided to an AC to DC converter stage  8  comprising switching elements S 1 , S 2 , S 3 , S 4 , S 5 , and S 6 , which may be selectively actuated by control signals (not shown) thereto in order to generate DC power on a DC bus  10  having a positive DC current path  12  and a negative DC current path  14  in a manner known in the art. DC power from the bus  10  is then converted to AC motor power via an inverter stage  20  comprising switching elements S 7 , S 8 , S 9 , S 10 , S 11 , and S 12 . The switching elements S 7 –S 12  may be activated according to control signals (not shown) in a manner known in the art in order to provide AC power of appropriate amplitude and phase to the motor  4  in a controlled fashion. 
   The motor drive  2  further comprises a choke  30  for providing a differential inductance to smooth DC power on the bus  10 , and also to suppress common-mode voltages in the motor drive  2  in accordance with an aspect of the invention. The choke  30  comprises first and second differential coils A and B connected in the positive and negative DC current paths  12  and  14 , respectively, to smooth the DC power on the DC bus  10 . In addition, choke  30  comprises first and second common-mode coils C and D connected in the positive and negative DC current paths  12  and  14 , respectively, to suppress common-mode voltages resulting from switching of one or more switching elements S 1 –S 12  in the motor drive  2 . Coils A and C are connected in series in the positive DC current path  12  with terminal A 1  of coil A connected to switches S 1 –S 3  of the AC to DC converter stage  8 , terminal A 2  of coil A connected to Terminal C 2  of coil C, and terminal C 1  of coil C connected to switches S 7 –S 9  of the inverter stage  20 , whereby DC current Id flows through the coils A and C in the positive DC current path  12  of the DC bus  10 . 
   Coils B and D are connected in series in the negative DC current path  14  with terminal B 1  of coil B connected to switches S 4 –S 6  of the AC to DC converter stage  8 , terminal B 2  of coil B connected to Terminal D 2  of coil D, and terminal D 1  of coil D connected to switches S 10 –S 12  of the inverter stage  20 . Thus connected, DC current Id flows through the coils B and D in the negative DC current path  14  of the DC bus  10 . By the inclusion of the choke  30 , the motor drive  2  provides for smoothing of ripple current on the DC bus  10  via a differential inductance provided to the DC current Id by the differential coils A and B, as well as suppression of common-mode voltages via a common-mode inductance provided by the differential coils C and D. In this regard, it will be noted that common-mode current I CM  flows through the positive and negative current paths  12  and  14  in the directions indicated in  FIG. 1 . In addition, the first and second differential coils A and B may provide further suppression of common-mode voltages in the motor drive  2 . 
   Other forms and topologies of motor drives are possible within the scope of the present invention, including the exemplary motor drive  2  of  FIG. 1  as well as other motor drives not illustrated and described herein. For example, the motor drive  2  may be modified by inclusion of line inductors between the single phase power sources  7   a ,  7   b , and  7   c , and the switching elements S 1 , S 2 , S 3 , S 4 , S 5 , and S 6  of the AC to DC converter stage  8 , output capacitors connected between the motor phase windings and a common neutral, and/or a capacitance connected across the DC bus  10 . 
   Referring now to  FIGS. 2 and 3 , an exemplary choke  130  is illustrated, which may be employed to provide a differential inductance and suppress common-mode voltages in a motor drive in accordance with another aspect of the invention. For example, the choke  30  may be employed in a motor drive, such as drive  2  of  FIG. 1 , having an AC to DC converter stage for converting AC power to DC power on a DC bus with positive and negative DC current paths, and an inverter stage for converting DC power from the DC bus to AC motor power in a controlled fashion, although the choke  130  finds application in association with motor drives and power converters apart from those illustrated and described herein. 
   The choke  130  comprises a magnetic core  132  having an inner leg  134  extending between an upper member  136  and a lower member  138 . While the exemplary choke core  132  is illustrated as having a generally rectangular profile, core profiles and orientations other than those illustrated and described herein are contemplated as falling within the scope of the invention, for example, rounded profiles, and the like. The core  132  may be made of any suitable material for creating a choke, as is known in the art. A first outer leg  140  extends from the lower member  138  toward the upper member  136  with a first air gap  142  therebetween, and a second outer leg  144  extends from the lower member  138  toward the upper member  136  with a second air gap  146  between the second outer leg  144  and the upper member  136 . 
   The choke  130  further comprises a first differential coil A wound around the first outer leg  140 , and a second differential coil B wound around the second outer leg  144 . The differential coils A and B, as well as the exemplary common-mode coils C and D described hereinafter, may be made of any suitable material as is known for conducting current, and may include appropriate insulation. Furthermore, the number of turns of the various coils may be determined according to a particular application, such as a specific motor drive or power conversion system. The differential coils A and B may be used for smoothing DC current in a motor drive as illustrated and described in greater detail hereinafter. For instance, the first and second differential coils A and B may be connected in positive and negative DC current paths, respectively, in a motor drive (e.g., motor drive  2  of  FIG. 1 ) in order to smooth DC current in the drive. In addition, the choke  130  comprises first and second common-mode coils C and D, respectively, wound around the inner leg  134 . The common-mode coils C and D may provide suppression or reduction in common-mode voltages in a motor drive in accordance with the invention. For example, the first and second common-mode coils C and D may be connected in the positive and negative DC current paths, respectively, to suppress common-mode voltages in the motor drive. 
   The coils A, B, C, and D of the choke  130  may be connected in a motor drive in a fashion similar to that of choke  30  in the motor drive  2  of  FIG. 1  in order to reduce common-mode voltages and smooth DC current therein. Thus, a differential DC current Id may flow into a terminal A 1  of coil A, from terminal A 2  of coil A into a terminal C 2  of coil C, and out of terminal C 1  of coil C. For instance, the terminals A 2  and C 2  may be connected together, with terminals A 1  and C 1  connected in a positive DC current path of a DC power bus in a motor drive (e.g., path  12  of drive  2 ). Similarly, a DC current Id may flow in a return or negative current path (e.g., path  14  of drive  2 ) into a terminal D 1  of coil D, from a terminal D 2  of coil D into a terminal B 2  of coil B, and out of a terminal B 1  of coil B. Thus, the terminals D 2  and B 2  may be connected together, with terminals D 1  and B 1  connected in a negative or return DC current path of a DC power bus in a motor drive. 
   Thus connected, the choke  130  advantageously provides a differential inductance to smooth DC current in a motor drive DC bus, as well as a common-mode inductance to suppress common-mode voltages in the motor drive. Where the number of turns of coils A, B, C, and D are the same, the common-mode inductance is about 2.25 times the differential inductance. In addition, the common-mode and differential inductances may be independently adjusted for a particular motor drive or other power conversion application, for example, through adjusting the relative number of turns in the coils A, B, C, and D. As illustrated in  FIG. 2 , the first and second differential coils A and B provide differential fluxes Φ d-A  and Φ d-B  in a differential flux path through the first and second outer legs  140  and  144 , the first and second air gaps  142  and  146 , and the upper and lower members  136  and  138  of the core  132  in response to a differential current Id in the positive and negative DC current paths. It will be noted that the first and second common-mode coils C and D are wound around the inner leg  134  so as to provide substantially zero differential flux in the core  132  in response to differential current Id, wherein the differential fluxes Φ d-C  and Φ d-D  related thereto in the inner leg  134  cancel each other. 
   Referring also to  FIG. 3 , the first and second common-mode coils C and D further provide a first common-mode flux comprising Φ CM-C  and Φ CM-D  in a common-mode flux path through the inner leg  134 , the first and second outer legs  140  and  144 , the first and second air gaps  142  and  146 , and the upper and lower members  136  and  138 , respectively, of the core  132  in response to common-mode current I CM  in the positive and negative DC current paths of the motor drive. In this case, it is noted that whereas the differential current Id flows into terminal D 1  and out of terminal B 1  (e.g.,  FIGS. 1 and 2 ), the common-mode current I CM /2 flows into terminal B 1  and out of terminal D 1  (e.g.,  FIGS. 1 and 3 ). In addition, the first and second differential coils A and B provide a second common-mode flux comprising Φ CM-A  and Φ CM-B  in the common-mode flux path in response to common-mode current in the positive and negative DC current paths. 
   As illustrated in  FIGS. 2 and 3 , therefore, the common-mode coils C and D and the differential coils A and B provide a common-mode inductance to suppress common-mode voltages in the motor drive, whereas the differential coils A and B provide a differential inductance to smooth the DC current on the DC bus. Due to the winding of the common-mode coils C and D around the inner leg  134 , substantially no differential inductance is provided by the coils C and D. 
   The exemplary integrated choke  130  of  FIGS. 2 and 3  has been found to provide significant advantages in suppressing common-mode voltages in motor drives. In this regard, the ratio between the common-mode inductance provided by coils A, B, C, and D to the differential inductance provided by coils A and B facilitates the use of the choke  130  in suppressing common-mode voltages while smoothing DC power in the motor drive DC bus. The integration of the four windings A, B, C, and D, moreover, provides for induction ratios (e.g., common-mode to differential) higher than those attainable using separate differential and common-mode chokes with an equivalent number of turns. 
   For example, referring to  FIG. 2  and assuming each of the coils A, B, C, and D comprises an integer number N turns, the differential inductance Ld in the exemplary choke  130  is given by the following equation (1):
 
 Ld =( N   A   +N   B ) 2   /R= 2 N   2   /R;   (1)
 
where R is the reluctance of the core  132 , and where N A =N B =N.
 
   Referring to  FIG. 3 , and again assuming each coil A, B, C, and D has an integer number N turns, the common-mode inductance L CM-AC  produced by the coils A and C in the exemplary choke  130  is given by the following equation (2):
 
 L   CM-AC   =L   A   +L   C =3 N   2   /R+ 6 N   2   /R= 9 N   2   /R;   (2)
 
and the common-mode inductance L CM-BD  produced by the coils B and D in the choke  130  is given by the following equation (3):
 
 L   CM-BD   =L   B   +L   D =3 N   2   /R+ 6 N   2   /R= 9 N   2   /R.   (3)
 
The common-mode inductances L CM-AC  and L CM-BD  are effectively in parallel. Thus, the total common-mode inductance L CM  produced by the coils A, B, C, and D of the choke  130  is given by the following equation (4):
 
 L   CM   =L   CM-AC   //L   CM-BD =9 N   2   /R// 9 N   2   /R= 9 N   2 /2 R.   (4)
 
   Accordingly, the ratio of the common-mode inductance L CM  for the choke  130  to the differential inductance Ld is given by the following equation (5):
 
 L   CM   /Ld =(9 N   2 /2 R )/(2 N   2   /R )=9/4=2.25.  (5)
 
   It will be appreciated that other common-mode to differential inductance ratios are obtainable within the scope of the invention, for example, via adjustment of the relative number of turns in the respective coils A, B, C, and D. 
   Another aspect of the invention provides methodologies by which common-mode voltages may be reduced or suppressed in a motor drive. One such method  200  is illustrated in  FIG. 4 . While the exemplary method  200  is illustrated and described herein as a series of blocks representative of various events and/or acts, the present invention is not limited by the illustrated ordering of such blocks. For instance, some acts or events may occur in different orders and/or concurrently with other acts or events, apart from the ordering illustrated herein, in accordance with the invention. Moreover, not all illustrated blocks, events, or acts, may be required to implement a methodology in accordance with the present invention. In addition, it will be appreciated that the exemplary method  200  and other methods according to the invention, may be implemented in association with the apparatus and systems illustrated and described herein, as well as in association with other systems and apparatus not illustrated or described. 
   Beginning at  202 , a magnetic core is provided, which comprises an inner leg and first and second outer legs. The inner leg may extend between upper and lower members, and the first and second outer legs may extend from the lower member toward the upper member, with first and second air gaps in between. At  204 , first and second differential coils are wound around the first and second outer legs, respectively, of the core, after which first and second common-mode coils are wound around the inner leg at  206 . At  208 , the first and second differential coils are connected in positive and negative DC current paths in a motor drive, respectively, and at  210  the first and second common-mode coils are respectively connected in the positive and negative DC current paths. The method  200  thus provides for reducing or suppressing common-mode voltages in a motor drive by the configuration and connection of the differential and common mode coils with respect to the core and the motor drive. 
   Although the invention has been shown and described with respect to certain implementations, it will be appreciated that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure, which performs the function in the herein illustrated exemplary applications and implementations of the invention. 
   What has been described above includes examples of the present invention. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the present invention, but one of ordinary skill in the art may recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several aspects or implementations of the invention, such a feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “including”, “has”, “having”, and variants thereof are used in either the detailed description or the claims, these terms are intended to be inclusive in a manner similar to the term “comprising” and its variants.