Abstract:
Provided is an electrical apparatus comprising a magnetic core, a conductive coil wound around at least a part of the core, a cooling element configured to receive a cooling fluid to cool the core and the coil during operation, and at least one biasing element operatively associated with the core to urge the core and the coil into engagement with the cooling element despite differential expansion or contraction of the core and the coil and manufacturing tolerances. Further provided is a method for making an electrical apparatus comprising disposing a conductive coil wound around at least a part of a magnetic core, disposing a cooling element between the core and the coil, the cooling element configured to receive a cooling fluid to cool the core and the coil during operation, and urging the core and the coil into engagement with the cooling element despite differential expansion or contraction of the core and the coil and manufacturing tolerances.

Description:
BRIEF DESCRIPTION 
       [0001]    The present invention relates generally to the field of power electronic devices and their thermal management. More particularly, the invention relates to a technique for improving cooling and heat distribution in power modules. 
         [0002]    Power electronic devices and modules are used in a wide range of applications. For example, in electric motor controllers, rectifiers, inverters, and more generally, power converters are employed to condition incoming power and supply power to devices, such as a drive motor. However, the power and signals transmitted within the electronic devices often contain undesirable characteristics that may require additional devices to reduce or filter the signals. For instance, in alternating current (AC) motor controllers, a rectifier may be used to covert the AC power to stable direct current (DC) power, and an inverter may be used to convert the stable DC power back to the AC power supplied to the motor. 
         [0003]    In a standard three phase rectifier (e.g., input converter) that uses six silicon-controlled rectifiers (SCR&#39;s) or six diodes and a filter capacitor bank, the three phase input current may contain harmonic distortions. Often, an inductor, such as a reactor or choke, may be added to the system to reduce the harmonics. For example, a reactor may be included at the input of the circuit to reduce the harmonics. Similarly, a choke may be added to buffer the capacitor bank from the AC line to reduce the harmonics. Accordingly, inductors may be useful in circuits for motor drives and other applications where characteristics of inductors are beneficial to the system. However, the design of such inductors may include inherent limitations, including the potential to build up heat within the inductor. 
         [0004]    An inductor usually includes a passive electronic device constructed of a conductive coil of material (e.g., wire or foil) wrapped around a core of air or a ferromagnetic material (magnetic core). Passing electrical current through the conductive coil generates a magnetic flux proportional to the current. The inherent resistance of this winding converts electrical current flowing through the conductive coils into heat due to resistive losses, causing a loss of inductive quality. This may be referred to as coil loss. Further, energy loss that occurs in inductors may include core losses. Core losses may be attributed to a variety of mechanism related to the fluctuating magnetic field, such as eddy loss currents and hysteresis. Most of the energy is released as heat, although some may be mechanical, potentially resulting in audible signals (“hum”). The build up of heat due to coil losses and core losses may reduce performance of the inductor, and lead to failure of the device. Similar problems may be experienced by similarly constructed devices. 
         [0005]    Accordingly, there is a need for improved techniques and cooling systems for removing heat from electronic modules and power converters. 
       BRIEF DESCRIPTION 
       [0006]    The invention provides a novel approach to power electronic device thermal management. The technique may be applied in a wide range of settings, but is particularly well-suited to inductors, and similar devices. The technique may be utilized with single coil or multiple coil inductors, such as those used in single phase alternating current power systems, three-phase alternating current systems, or direct current power systems. A presently contemplated implementation, for example, is with a reactor used in a three-phase alternating current power system. 
         [0007]    The technique relies upon a biasing element adjacent to a magnetic core of an inductor. The element may be provided between multiple core elements or pieces. The biasing element provides a biasing force to urge at least one cooling element disposed within the inductor into contact with a coil, and, where desired, into good thermal contact with both the core and the coil. The contact may close this and reduce the thermal resistance at the interfaces of the components, and thus promote heat transfer from the magnetic core and the conductive coil to the cooling element. The cooling element is configured to extract the heat from the inductor, such as via the flow of a cooling fluid. 
     
    
     
       DRAWINGS 
         [0008]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0009]      FIG. 1  is a diagrammatical overview of an exemplary power electronic circuit implementing inductors, including a three-phase reactor and a choke, in accordance with aspects of the invention; 
           [0010]      FIG. 2  is an illustration of an exemplary embodiment of the three-phase reactor of  FIG. 1 ; 
           [0011]      FIG. 3  is an illustration of an assembled magnetic core piece of the reactor of  FIG. 2 ; 
           [0012]      FIG. 4  is an illustration of assembled components of the reactor of  FIG. 2 ; 
           [0013]      FIG. 5  is an illustration of an exploded view of the a conductive coil, cooling element and support of the reactor of  FIG. 4 ; 
           [0014]      FIG. 6  is an illustration of an exploded view of the magnetic core and biasing element of the reactor of  FIG. 2 ; and 
           [0015]      FIG. 7  is an illustration of a top view of a portion of the reactor of  FIG. 2 , including one conductive coil. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Various electronic circuits benefit from the use of inductors. Although inductors are useful for filtering, smoothing or otherwise conditioning power signals, inductors, including reactors, chokes, transformers, and the like, generally produce heat due to core and coil losses. Heat may degrade the performance of the inductor, or may cause degradation and premature failure of the device. Accordingly, the following embodiments provide a system and method to remove thermal energy from the core and the coils of an inductor. In certain embodiments, a cooling element is disposed adjacent to a core, such as between the core and the coil of an inductor such that it may absorb the heat generated by the inductor. In a presently contemplated embodiment, the core includes multiple core pieces that are urged outward by a biasing element disposed between the core pieces. Urging the core pieces outward promotes contact between the core pieces and a cooling element located proximate to the core. Contact between the surface of the core and the surface of the cooling element may reduce the thermal resistance across the interface to promote heat transfer between the core and the cooling element. Accordingly, the effectiveness of the cooling element may be improved. Similarly, the biasing element may also urge the cooling element into contact with surfaces of the conductive coil. This improved contact may reduce the thermal resistance between the conductive coil and the cooling element, and increase the effectiveness of the cooling element to remove heat from the conductive coil. The system and technique are generally applicable to similarly constructed systems that may benefit from improved surface contact between components. 
         [0017]      FIG. 1  illustrates an exemplary embodiment of a power circuit  10  including two inductors. In the illustrated embodiment of  FIG. 1 , the power circuit  10  may be provided as part of a power module, such as for a motor drive. The power circuit  10  is adapted to receive three-phase power from a line side  12  and to convert the input power to an output power delivered at a load side  14 . It should be noted that this particular circuit of  FIG. 1  is merely one example of an environment that this invention may be usefully employed. 
         [0018]    In the embodiment illustrated in  FIG. 1 , the power circuit  10  includes a rectifier  16  defined by an array of six diodes  18 , although SCRs or other power electronic devices may be used in place of diodes. The diode array converts three-phase AC input power to DC power that is applied to a DC bus  20 . The power circuit  10  also includes a capacitive filter  22  formed from a capacitor bank. The capacitive filter may be desired to smooth ripple current on the DC bus, for instance. Further, an inverter  24  is formed by an array of switches  26  and associated fly-back diodes  28 . The inverter may include high-speed transistors as switches to apply a pulse width modulated (PWM) waveform to the load side  14  to power a motor, for instance. 
         [0019]    Standard motor drives that are configured to draw from the power circuit  10  may include “six pulse” drives that have a non-linear load. These drives tend to draw current only periodically during positives and negatives during loses of input power. Because the current wave-form is not perfectly sinusoidal the current may contain undesired harmonics. For instance, with a standard three-phase rectifier using six diodes  18 , or SCRs, and a capacitive filter  22 , as depicted in  FIG. 1 , the three-phase input current may contain an increased amount of harmonic distortion. The harmonic distortion may be reduced with the addition of inductors, such as reactors and chokes, to the power circuit  10 . 
         [0020]    A reactor may be added at the line side  12  or DC bus of a power circuit  10  to reduce harmonics. This reactor or inductor reduces the rate of change of current. It may force the capacitive filter  22  to charge at a slower rate drawing current over a longer period of time. In one embodiment of the power circuit  10 , an inductor  30 , may be configured as an input reactor  32  to reduce the harmonics. As illustrated in  FIG. 1 , the reactor  32  is located between the line side  12  and the rectifier  16 . In this embodiment, the reactor  32  includes three coils, wherein each coil is configured to receive power from one conductor of the three phase conductors of the line side  12 , and to transmit the power to a respective phase input of the rectifier  16 . In this configuration, the reactor  32  may reduce harmonics and limit the peak current into the rectifier  16  and the capacitive filter  22 . 
         [0021]    In other configurations (not shown), the power circuit  10  may include a reactor  32  located between the inverter  24  and the load side  14 . In such a configuration, the reactor  32  may buffer the current at the load side  14 , such as the current input to a motor drive. 
         [0022]    The illustrated embodiment of the power circuit  10  also includes a DC choke  34 . The choke  34  is located on the DC bus  20 , between the rectifier  16  and the capacitive filter  22 . The choke  34  may help to buffer the capacitive filter  22  from the AC line and to reduce harmonics. The choke  34  may protect the power circuit  10  against a current surge. However, the choke  34  may not protect the rectifier  16  from a voltage spike, as the choke  34  is located downstream of the rectifier  16 . 
         [0023]    Embodiments of the power circuit  10  may include a single inductor, such as the reactor  32  at the line side  12 , the load side  14 , or the choke  32 . Other embodiments may include various combinations of the three, as depicted in  FIG. 1 . 
         [0024]    As mentioned previously, the reactor  32  and the choke  34  are both forms of inductors. Accordingly, the characteristics of such inductors may be critical to the operation in which they are installed, such as power circuit  10 . Such inductors generally include a passive electrical device that is employed in an electrical circuit for its property of inductance. Inductance (measured in Henries) is an effect which results from the magnetic field that forms around a current carrying conductor. An inductor typically consists of a coil of conducting material (e.g., conductive coil or wire or foil) wrapped around a core. The core typically comprises air or a ferromagnetic material (magnetic core). Electrical current passed through the conductive coil creates a magnetic flux field proportional to the current. A magnetic core is a key component of higher power inductors, as the magnetic core increases the strength and effect of the magnetic field produced by the electric current passed through the conductive coil. 
         [0025]    Configurations and the design of inductors may vary based on specific applications. For example, inductors may include a single conductive coil disposed about a singe magnetic core. In other embodiments, inductors may include multiple conductive coils, each wound about a portion of the magnetic core. For example, the reactor  32  may include a total of three conductive coils (one for each conductor of three-phase power from the line side  12 ) wrapped about a magnetic core. Other inductors may include two or more conductive windings about a magnetic core, wherein the conductive coils are magnetically coupled to form a transformer. 
         [0026]    The inherent resistance of inductor coils converts a portion of electrical current flowing through the conductive coils into thermal energy (heat), causing a loss of inductive quality. This may be referred to as coil loss. Further, an inductor may experience energy loss attributed to a variety of mechanisms related to the fluctuating magnetic field, such as eddy loss currents and hysteresis. This form of energy loss may be referred to as core losses. Most of the energy due to coil losses and core losses is released as heat. Accordingly, heat may build up within the inductor if it is not dissipated or removed. Unfortunately, the build up of heat within the inductor may reduce performance of the inductor, and/or lead to failure of the device. 
         [0027]    Turning now to  FIG. 2 , an inductor  30  in accordance with an embodiment of the present technique is illustrated. The inductor  30  has a magnetic core  36 , conductive coils  38 , and cooling elements  40 . More particularly, the inductor  30  includes the magnetic core  36  surrounded by three conductive coils  38 , with two cooling elements  40  disposed between each conductive coil  38  and the magnetic core  36 , resulting in a total of six cooling elements  40  for the particular embodiment illustrated. 
         [0028]    The overall design of the inductor  30  may be varied to meet specific applications and the desired performance. For example, as illustrated in  FIGS. 2 and 3 , the magnetic core  36  includes a “figure-eight” shaped geometry. In this configuration, each leg  42  of the magnetic core  36  may be surrounded by a conductive coil  38  to form the inductor  30 . However, the geometry of the magnetic core  38  may be varied depending on the application. For example, other embodiments of the magnetic core  36  may have “I”, “C,” “E,” toroidal, planar, or pot shaped geometries, and so forth. The magnetic core  36  may also include a geometry formed from a combination of shapes. For example, the figure-eight shape of  FIG. 2  may include an “I” shaped piece and an “E” shaped piece, or two “E” shaped pieces, combined to for the single magnetic core  36 . 
         [0029]    The magnetic core  36  may comprise various materials suitable for use in an inductor  30 . In one embodiment, the magnetic core  36  may be formed from copper, aluminum, or steel. For instance, the magnetic core  36  may include conductive “tape” wrapped to form the body of the magnetic core  36 . Other embodiments may include various materials as well as other techniques to form the core. For instance, iron may be used as to form a unitary magnetic core  36 . The magnetic core  36  may also include iron alloyed with silicon, for example. Other materials used to form the magnetic core  36  may include carbonyl iron, ferrite ceramics, and so forth. 
         [0030]    Further, various forming techniques, such as lamination and the like, may be employed to form the magnetic core  36 . Laminating multiple pieces to form the magnetic core  26  may aid in the reduction of undesired eddy currents. 
         [0031]      FIG. 4  is an illustration of an assembled conductive coil  38  and cooling element  40 . This is representative of one of the three conductive coils  38  and one of the three pairs of cooling elements  40  depicted in  FIG. 2 . Similarly,  FIG. 5  is an illustration of the assembly of  FIG. 4 , exploded to provide an improved view of the conductive coil  38  and the cooling elements  40 . 
         [0032]    The conductive coil  38  includes various features that may be desired for use within in the inductor  30 . In one embodiment, the conductive coil  38  includes a coil of material disposed about a central region  44 . As depicted, the central region  44  includes an opening configured to accommodate at least a portion of the magnetic core  36 . Further, the central region  44  provides a location to dispose the cooling elements  40 . For example, cooling element  40  may be disposed at both ends of the conductive coil  38 , as depicted. 
         [0033]    The conductive coil  38  also includes leads  46  configured to connect to other conductors, such as one of the three conductors at the line side  12 , and one of the three conductors output to the rectifier  16 , as depicted in  FIG. 1 . The leads  46  provide for the flow of current through the conductive coil  38 . Accordingly, the inductor  30 , as depicted in  FIG. 2 , may include a total of six leads  46  (two at each of three conductive coils  38 ). Each lead is configured for connection to an input or an output of the three conductors in a three-phase power system. The conductive coil  38  may include any number of coil turns or wraps around the central region, as desired by a specific application. 
         [0034]    The conductive coil  38  may be composed of various materials. In one embodiment, the conductive coil  38  may include copper, aluminum or steel windings. In other embodiments, the conductive coil  38  may comprise other conductive materials suitable for use in the inductor  30 . 
         [0035]    The cooling element  40 , as depicted in  FIGS. 2 ,  4 , and  5 , may take a variety of shapes and configurations to provide for the removal of heat from components of the inductor  30 , including the magnetic core  36  and the conductive coil  38 . For instance, each of the depicted cooling elements  40  has a semicircular shape, including a curved surface  48  and a generally flat surface  50 . In a presently contemplated embodiment, a surface, such as the curved surface  48 , may have a shape configured to conform to a curvature at an end turn of the conductive coil  38 . For example, the cooling elements  40  may be disposed within a conductive coil  38  that has been formed prior to placement of the cooling elements  40 . In another embodiment, the conductive coil  38  may conform to the shape of the cooling element  40 . For instance, forming the conductive coil  38  may include fixing the cooling elements  40  in a position and subsequently wrapping the windings of the conductive coil  38  about the cooling elements  40 . The generally shared profile at each end turn may promote contact of the conductive coil  38  and the cooling element  40  such that thermal energy may be more efficiently transferred between the conductive coil  38  and the cooling element  40 . For example, disposing the conductive coil  38  and the cooling element  40  such that they are proximate to one another along the curved surface  48  may reduce thermal resistance across that interface, and, thus, promote the transfer of thermal energy (heat) between the conductive coil  38  and the cooling element  40 . Thus, heat from the conductive element  38  may be more efficiently removed by the cooling element  40 . 
         [0036]    Similarly, a surface of the cooling element  40  may be configured to contact other heat generating components, including the magnetic core  36 . For instance, the flat surface  50  of the cooling element  40  is generally shaped to provide contact between the magnetic core  36  and the cooling element  40 . Contact between the flat surface  50  of the cooling element  40  and a surface of the magnetic core  36  may enable a more efficient transfer of thermal energy (heat) between the magnetic core  36  and the cooling element  40 . Thus, heat from the magnetic core may also be more efficiently removed by the cooling element  40 . 
         [0037]    Further, the cooling element  40  may include various features configured to provide for the transfer of heat from components of the inductor  30  to the cooling element  40 . For instance, the cooling element  40  may comprise a thermally conductive material, such as aluminum. In certain embodiments, the body of the cooling element  40  may include various channels configured to circulate a cooling fluid through the cooling element  40 . The circulation of a cooling fluid may help to remove heat from the cooling elements  40  and, thus, promote heat exchange between the cooling element  40  and components of the inductor  30 . For example, the inductor  30  depicted in  FIGS. 4 and 5  includes coolant inlets  52  and outlets  54  configured to receive coolant from an external source, such as a fluid pump (not shown.) In a diversely contemplated embodiment, coolant enters the cooling element  40  via the coolant inlet  52 , passes through cooling channels internal to the cooling element  40 , and exits from the cooling element  40  via the cooling outlet  54 . The circulation of cooling fluid through the cooling element  40  provide for an increased rate transfer of thermal energy from other components of the inductor  30 , such as the conductive coil  38  and the magnetic core  36 . 
         [0038]    The cooling fluid may include any gas or liquid capable of being passed through the cooling element  40  and including thermal properties beneficial to absorbing heat from the body of the cooling element  40 . For example, the cooling fluid may include a water based liquid or an oil. 
         [0039]      FIGS. 4 and 5  also depict a support  56  disposed between each of the cooling elements  40 . The support  56  may be included to provide for spacing of the cooling elements  40 . For example, the support  56  includes a plate of material fastened to each of the cooling elements  40  via fasteners  58  disposed through holes  60  in the support  56 . This illustrates each set of cooling elements  40  includes two supports  56  that are fastened to the sides of the cooling elements  40 . In this configuration, the conductive coil  38  may be wrapped around the cooling elements  40 , with the supports  56  acting to maintain the open central region  44 . Maintaining the central region  44  may provide a location to assemble the magnetic core  36  or other components of the inductor  30 , for instance. The size, shape, and method of fastening the support  56  may be varied to accommodate applications. 
         [0040]    In other embodiments, the support  56  may be a temporary component. For example, the support  56  may be included for assembly and placement of the cooling elements  40  and removed during assembly or prior to use of the inductor  30 . 
         [0041]    As mentioned previously, cooling of the inductor  30  may be provided via the cooling elements  40 . The cooling elements  40  may be disposed proximate to the magnetic core  36  and/or the conductive coil  38  to remove thermal energy from the inductor  30 . To promote the transfer of heat, the inductor  30  may include areas in which each cooling element  40  contacts the components to be cooled, such as the conductive coil  38  and the magnetic core  36 . Good thermal contact between the surface of the cooling elements  40  and other components reduces thermal resistance across the interface to enable more efficient conduction of thermal energy between the components to the cooling element  40 . 
         [0042]    In design and assembly, components of the inductor  30  may generally include some surface contact with the cooling element  40 . Even with good manufacturing tolerance, each of the components may experience expansion and contraction due to fluctuations in temperature during operation. The expansion of contraction in size may reduce or eliminate contact between components and the cooling element  40 . This concern may become more prevalent due to use of different materials for each component and the differing coefficients of thermal expansion for each material. 
         [0043]    In the illustrated embodiment, the inductor  30  includes a magnetic core  36  and a biasing element  62  configured to urge the components of the inductor  30  into good thermal contact with the cooling element  40 . As depicted in  FIG. 6 , the magnetic core  36  includes a first piece  64  and a second piece  66  with the biasing elements  62  disposed between the two pieces  64  and  66 . The first piece  64  and second piece  66  may be configured to be positioned or mated together to form the magnetic core  36 , as depicted in  FIG. 1 . The two pieces  64  and  66  may include two complementary pieces that are symmetrical or generally symmetrical, as depicted. In other embodiments, the first piece  64  and the second piece  66  may include any shape and design configured to accommodate a specific application. For example, the two core pieces  66  and  64  may be varied in thickness, or may include any of the core geometries described previously. 
         [0044]    The biasing element  62  may include a component, mechanism or material capable of being disposed between the two pieces  64  and  66  of the magnetic core  36 , and providing a biasing force to the pieces. The biasing element  62  exerts a force on the core pieces  64  and  66  after completion of assembly and closes any gap between the core  36 , coil  38  and cooling element  40  due manufacturing tolerances. When the reactor is in operation and warms up, the biasing element  62  exerts a force between the core  36  and coil  38  and closes any gap that is developed between the core  36 , cooling element  40  and core  36  due to thermal expansion mismatch between the components. This ensures improved thermal contact between the core  36 , coil  38  and cooling element  40 . As depicted, the biasing element  62  may include one or a plurality of corrugated sheets of material disposed at various locations between the faces of the two pieces  64  and  66  of the magnetic core  36 .  FIG. 6  illustrates two biasing elements  62  located symmetrically about the edges of the pieces  64  and  66  of the magnetic core  36 . Other embodiment may include a single biasing element  62  or a plurality of biasing elements  62  disposed between the two pieces  64  and  66 . In certain embodiments, the biasing element  62  may be pre-compressed during manufacturing. For example, the biasing element  62  may be compressed during assembly of the core  36  such that the biasing element  62  provides a constant reactive force against the pieces of the core  64  and  66 . 
         [0045]    Further embodiments may include alternate forms of the biasing element  62 . For example, the biasing mechanism  62  may include a beveled washer, a linear spring, and the like. Other embodiments may include a mechanically flexible material that is configured to provide a reactive force. For example, the biasing element  62  may include a rubber or resilient material disposed on at least one of the faces of the two pieces  64  and  66 , such that the material provides a biasing force when the two pieces  64  and  66  are compressed together. 
         [0046]    Turning now to  FIG. 7 , the top view of a portion of the inductor  30 , including the magnetic core  36 , biasing elements  62 , a single conductive coil  39 , and cooling elements  40  is depicted. The biasing elements  62  are disposed between the first piece  64  and the second piece  66  of the magnetic core  36 . Accordingly, the biasing element  62  may provide a biasing force in the direction of the arrows  70 . The force may urge the first piece  64  and the second piece  66  in the direction of the arrows  70  to increase contact between the magnetic core  36  and the cooling elements  40  at a core/cooling interface  72 . Accordingly, the thermal resistance between the core/cooling interface  72  may be reduced, thereby, promoting the efficient transfer of thermal energy from the magnetic core  36  to the cooling elements  40 . 
         [0047]    Further, the biasing force provided by the biasing element  62  may urge the cooling elements  40  and the conductive coil  38  into contact. For example, the force in the direction of arrows  70  may be transmitted from the core  36  to cooling elements  40 , and, thus, the cooling elements  40  may be displaced in the direction of arrow  70 . The force and displacement on the cooling elements  40  may act to create or increase the contact between the surface of the cooling elements  40  and the surface of the conductive coil  38  at a coil/cooling interface  74 . Accordingly, the thermal resistance between the coil/cooling interface  74  may be reduced, thereby promoting the efficient transfer of thermal energy from the conductive coil  38  to the cooling elements  40 . 
         [0048]    In one embodiment, the inductor  30  may include the support  56  (See  FIG. 4 ) configured to allow increased movement of the cooling element  40 . For example, if the support  56  remains in the inductor  30 , the holes  60  may be increased in diameter relative to the fasteners  58 , or may include a slot, such that the cooling element  40  is capable of displacing as the other components contract and expand. Further, such an embodiment may account for variations in the coefficient of thermal expansion for the support  56  relative to other components of the inductor  30 . 
         [0049]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. For example, the described system may be employed for heating elements, and or may be employed in similar systems that desire urging components into contact. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.