Abstract:
An improved choke assembly for a power electronics device is provided. More specifically, a choke assembly with improved protection from environmental conditions such as dirt and water is provided. An improved choke assembly may include an insulative housing for an inductor coil that seals the inductor coil from the environment.

Description:
BACKGROUND 
     The invention relates generally to the field of power electronic devices such as those used in power conversion or for applying power to motors and other loads. More particularly, the invention relates to devices such as motor drives with an improved choke which provides improved protection from the environment. 
     In the field of power electronic devices, a wide range of circuitry is known and currently available for converting, producing and applying power to loads. Depending upon the application, such circuitry may convert incoming power from one form to another as needed by the load. In a typical arrangement, for example, constant (or varying) frequency alternating current power (such as from a utility grid or generator) is converted to controlled frequency alternating current power to drive motors, and other loads. In this type of application, the frequency and voltage of the output power can be regulated to control the speed of the motor or other device. Many other applications exist, however, for power electronic circuits that convert alternating current power to direct current power, or vice versa, or that otherwise manipulate, filter, or modify electric signals for powering a load. Circuits of this type generally include rectifiers (converters), inverters, and power conditioning circuits. For example, a motor drive will typically include a rectifier that converts AC voltage to DC. Inverter circuitry then converts the DC voltage into an AC voltage of a particular frequency desired for driving a motor at a particular speed. Often, power conditioning circuits, such as a choke and/or a bus capacitor are used to remove unwanted voltage ripple on the internal DC bus. Depending on the power load, the power conditioning circuits, such as the choke, may conduct very high levels of current and generate significant levels of heat. 
     To dissipate the heat generated by the circuitry of the motor drive, the motor drive unit will typically include a cooling channel that conducts cooling air through a heatsink thermally coupled to the semiconductor circuits described above. To make efficient use of the space within the motor drive unit, the choke is usually deployed within this cooling channel. Furthermore, the motor drive may be deployed such that the cooling channel is exposed outside of the equipment cabinet. Thus, the choke may be subject to dust and water. 
     Therefore, it may be advantageous to provide a motor drive unit with an improved choke that is protected from the environment. In particular, it may be advantageous to provide a choke with improved protection from water and dust. 
     BRIEF DESCRIPTION 
     The present invention relates generally to a choke configuration that addresses such needs. One embodiment of the present invention employs a container configured to hold an inductor coil and seal the inductor coil from the outside environment, while still allowing the inductor coil to be disposed about a magnetic core. Although the present invention is described, for convenience, in relation to a motor drive application, it will be appreciated that chokes fabricated in accordance with present techniques may be used in any choke related application, such as electrical power transmission and telecommunications, for example. 
    
    
     
       DRAWINGS 
       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: 
         FIG. 1  is a diagrammatical representation of an exemplary motor drive circuit employing an improved choke in accordance with one embodiment of the present invention; 
         FIG. 2  is a perspective exploded view of an exemplary motor drive unit employing an improved choke in accordance with one embodiment of the present invention; 
         FIG. 3  is a perspective view of the improved choke shown in  FIG. 2 ; 
         FIG. 4  is a perspective exploded view of the improved choke shown in  FIG. 2  providing additional details regarding the construction of the improve choke; 
         FIG. 5  is a cross section of an exemplary inductor coil shown in  FIG. 4  providing additional details regarding the construction of the improved choke; and 
         FIG. 6  is a flow chart of an exemplary method of fabricating the improved choke in accordance with certain embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagrammatical representation of an exemplary motor drive circuit  10  employing an improved choke configuration in accordance with present embodiments. The motor drive circuit  10  includes a three phase power source electrically coupled to a set of input terminals  12 ,  14  and  16  that provides three phase AC power of constant frequency to a rectifier circuitry  18 . In the rectifier circuitry  18 , a set of six diodes  34  provide full wave rectification of the three phase voltage waveform. Each input terminal entering the rectifier circuitry  18  is coupled between two diodes  34  arranged in series, anode to cathode, which span from the high side  38  of the DC bus  36  to the low side  40  of the DC bus  36 . Also coupled to the DC bus  36  is a choke  20  with improved techniques for protection from the environment that will be explained further below. The choke  20  may include inductors  42  that are coupled to either the high side  38  or the low side  40  of the DC bus  36  and serve to smooth the rectified DC voltage waveform. Capacitors  44  link the high side  38  of the DC bus  36  with the low side  40  of the DC bus  36  and are also configured to smooth the rectified DC voltage waveform. Together, the inductors  42  and capacitors  44  serve to remove most of the AC voltage ripple presented by the rectifier circuitry  18  so that the DC bus  36  carries a waveform closely approximating a true DC voltage. It should be noted that the three-phase implementation described herein is not intended to be limiting, and the invention may be employed on single-phase circuitry, as well as on circuitry designed for applications other than motor drives. 
     An inverter  24  is coupled to the DC bus  36  and generates a three phase output waveform at a desired frequency for driving a motor  32  connected to the output terminals  26 ,  28  and  30 . Within the inverter  24 , two switches  46  are coupled in series, collector to emitter, between the high side  38  and low side  40  of the DC bus  36 . Three of these switch pairs are then coupled in parallel to the DC bus  36 , for a total of six switches  46 . Each switch  46  is paired with a flyback diode  48  such that the collector is coupled to the anode and the emitter is coupled to the cathode. Each of the output terminals  26 ,  28  and  30  is coupled to one of the switch outputs between one of the pairs of switches  46 . The driver circuitry  50  signals the switches  46  to rapidly close and open, resulting in a three phase waveform output across output terminals  26 ,  28  and  30 . The driver circuitry  50  is controlled by the control circuitry  52 , which responds to the remote control and monitoring circuitry  54  through the network  56 . 
     Turning to  FIG. 2 , a perspective view of an exemplary motor drive unit  58  employing an improved choke configuration in accordance with one embodiment is shown. Many of the circuit components depicted in  FIG. 1 , including the choke  20 , will typically generate significant amounts of heat, which can lead to component failure due to overheating. Therefore, the motor control circuit  10  may be packaged within a unit that includes a system for enhancing the heat dissipating properties of the motor control circuit  10 . Accordingly, the motor drive unit  58  may include a frame  60  that defines a cooling channel  62  which is thermally coupled to the electrical components discussed in  FIG. 1 . The motor drive unit  56  also includes a set of fans  64  to provide a flow of cooling air through the cooling channel  62 . The switches  46 , diodes  34 , capacitors  44 , driver circuitry  50  and controller circuitry  52  are situated adjacent to the cooling channel  58  on the opposite side of the barrier  66  from cooling channel. The barrier  66  protects the motor drive circuitry from exposure to harmful environmental conditions while allowing heat from the circuitry to pass through the barrier into the cooling channel. In this way, the flow of cool air forced through the cooling channel  62  by the fans  64  draws heat from the circuitry. 
     Also included in the motor drive unit  58  is a heat sink  68 , which is thermally coupled to the barrier  66  inside the cooling channel  62 . The fans  64  blow cooling air through the heat sink  68 , thereby increasing the transfer of heat from the electrical components to the cooling air. 
     In some embodiments, the cooling channel may be subject to harsh environmental conditions. For example, the motor drive unit  58  may be mounted such that the front side of the motor drive unit sits inside a cabinet that provides access to the controls and electrical inputs and outputs of the drive unit  58 , while the backside of the motor drive unit sits outside of the cabinet. In this case, although the circuitry on the front side of the motor drive unit is protected from the environment by the barrier  66 , the cooling channel  62  is exposed to the environment. Additionally, to make efficient use of the space within the cooling channel, the choke  20  may also be situated within the cooling channel  62 . Therefore, the choke will be exposed to the environment as well. Therefore, to prevent electrical failure of the choke  20 , the choke  20  is sealed to provide protection against dust and water, as described below. A cover  69  may be secured over the frame  60 . 
     Turning to  FIG. 3 , an exemplary choke  20  that provides improved protection from the environment is shown. The choke  20  may include an E-shaped core element  70  coupled to an I-shaped core element  72  with brackets  74 . The two inductor coils  42  are mounted to the outside arms of the E-shaped core element  70 . Together the core elements  70  and  72  provide for inductive coupling between the inductor coils  42 . The level of coupling may be determined by the spacing between the E-shaped core element  70  and the I-shaped core element  72 , which may be set by the brackets  74 . Additionally, brackets  74  may also include mounting holes  76  for attaching the choke to the motor drive unit  58 . The choke  20  may also include the high-side bus leads  78  and the low-side bus leads  80 , which couple each respective inductor  42  to the high-side  38 , or the low-side  40  of the DC bus  36 . As will be described further below with respect to  FIG. 4 , the inductor coils  42  are held within a protective container  82  that seals the inductor coils  42  from the magnetic core and outside environment. For convenience, the present application describes the use of an E-I lamination, however, this is not intended to be a limitation of the present invention, and it will be understood that other embodiments may include any suitable type of lamination shape, such as a U-I lamination, E-E lamination, and C-core lamination, for example. Furthermore, in some embodiments, the choke  20  may include one or more than two inductor coils  42 . For example, a choke  20  fabricated in accordance with disclosed techniques may be deployed in a three-phase input or output line reactor. 
     Turning now to  FIG. 4 , an exploded perspective view of an improved choke  20  is shown in accordance with an embodiment. As can be more easily seen in  FIG. 4 , the E-shaped core element  70  includes a center projection  86  and two side projections  88  on which the inductor coils  42  are mounted. The container  82  is open at the top and includes side walls  92 , base  94 , and center member  96 , which projects longitudinally from the base of the container to at least the open top of the container  82 , forming a sort of donut-shaped container volume. The container  82  may form a unitary piece and may be formed from any suitable plastic or other non-conductive material. In embodiments, the cover  102  is injection molded from a polyethylene terephthalate such as Rynite®. 
     The inductor coils  42  may be formed with any suitable conductor, such as aluminum or copper wire or sheets. In some embodiments, inductor coils  42  may be formed by winding the conductor around a bobbin  100 . Furthermore, the conductor may be insulated to prevent the loops of conductor from shorting to each other. The diameter of the inductor coils  42  and the number of windings of the conductor will, in part, determine the inductance of the choke. The gauge of the wire or thickness of the sheet will determine the power handling. The bobbin  100  may be made of any suitable plastic or other non-conductor and may be dimensioned to fit over the center member  96 . The high-side bus leads  78  and low-side bus leads  80  are electrically coupled to the respective ends of the inductor coils  42 , as will be described further below, with respect to  FIG. 5 . The assembled inductor coils  42  are positioned within the container  82  around the center member  96 . 
     On top of the container  82  is a cover  102  that seals the inductor coils  42  inside the container  82 . As with the container  82 , the cover  102  may be formed from any suitable plastic or other non-conductor. In embodiments, the cover  102  is injection molded from polyethylene terephthalate. The cover may provide openings  104  which allow the bus leads  78  and  80  to pass through the cover  102 . In some embodiments, the openings  104  may be raised cylindrical openings configured to provide a pressure seal against the leads  78 ,  80  and provide a surface over additional protection may be applied, as will be described further below, with respect to  FIG. 5 . In some embodiments, the container  82  may be filled with a potting material to provide additional environmental protection as well as thermal conductivity. 
     Over the cover  102  is the I-shaped core element  72 , which is coupled to the E-shaped core element  70  via the mounting holes  76 . The I-shaped core element completes the magnetic circuit between the two inductor coils  42 , providing a desired level of mutual inductance between the inductors  42 . Furthermore, the mutual inductance may be adjusted by controlling the air gap between the E-shaped core element  70  and the I-shaped core element  72 . The air gap is controlled by the length of the bracket  74 . As with the E-shaped core element, the I-shaped core element may include any form of magnetic material, such a ferromagnetic material. The I-shaped core element  72  may be held in position on the brackets  74  via fasteners (not shown) received in the apertures  106  of the I-shaped core element  72 . 
     Turning now to  FIG. 5 , a partial cross-section of the assembled inductor coil  42  of  FIG. 4  is shown. As shown in  FIG. 5 , the bus leads  78  and  80  include electrical conductors  108  surrounded by an insulator  110 . The bus leads  78  and  80  project from the container  82  through the raised cylindrical openings  104 , which may be tapered to provide pressure against the insulator  110 . At the end of the conductor  108  inside the container  82 , the insulator  110  is stripped from the conductor  108  and the conductor  108  is electrically coupled to the inductor coil  42  by any suitable method, such as soldering, for example. In the embodiment shown, inductor coil lead  114  is crimped and soldered to the conductor  108  at the connection point  112 . Additionally, where the insulator  110  is stripped from the conductor  108 , the bus lead may be wrapped with electrical tape  116  to provide additional protection. 
     As stated above, the container  82  may be filled with a potting material  118 , such as an epoxy or other resin, which seals and electrically insulates the inductor coil  42  from the outside environment. Because the potting material  118  is more thermally conductive than air, the potting material  118  increases the transfer of heat away from the inductor coil  42 . Moreover, because the container  82  provides mechanical rigidity, the container  82  enables the use of a thin wall of potting material  118 , which also serves to increase the transfer of heat away from the inductor coil  42 . Increasing the transfer of heat away from the inductor coil  42  enables the use of a smaller gauge conductor, thereby reducing the weight, size, and cost of the inductor coil  42 . Additionally, the potting material  118  also reduces the likelihood of electrical failure of the inductor coil  42  by reducing mechanical vibration of the inductor coil  42 . 
     The potting material  118  also fastens the cover  102  to the container  82 . The cover  102  may include a lip  120  that allows the cover  102  to fit or snap into the container  82 , ensuring the proper alignment between the container  82  and the cover  102  and increasing the strength of the seal between the container  82  and the cover  102 . Additionally, a section of shrink tubing  122  may be placed around the bus lead  78  at the cylindrical opening  104 . 
     Turning now to  FIG. 6 , a method of fabricating the choke assembly illustrated in  FIG. 4  is illustrated. Process  124  begins at step  126 , in which the inductor coil  42  is formed by shaping a conductor into the form of an inductor coil  42 . In some embodiments, the conductor may be shaped by winding the conductor around a bobbin  100 , however, in other embodiments, the conductor may be shaped without the use of a bobbin. Next, at step  128 , the inductor leads  114  are coupled to the bus leads, i.e. conductor  108 . The coupling between the inductor lead  114  and the conductor  108  may be accomplished by any suitable method such as soldering, crimping, and/or the use of mechanical fasteners. Next, at step  130 , the inductor coil  42  is placed inside the container  82 . In embodiments wherein the inductor coil  42  is formed around the bobbin  100 , the bobbin  100  may be removed from the inductor coil  42  before being placed inside the container  82 . Additionally, in some embodiments, the bobbin  100  may remain in place and slide over the projection  96 . Next at step  132 , the container  82  may optionally be filled with an epoxy, resin, varnish or other potting material. Next, at step  134 , the cover  102  is placed over the container  82  before the epoxy cures. During this step, the bus leads  78  and  80  are passed through the openings  104 . Next, at step  136 , shrink tubing may optionally be positioned around bus leads  78  and  80  at the interface between the bus leads  78  and  80  and the openings  104 , and the shrink tubing may be heated to form a seal between the openings  104  and the bus leads  78  and  80 . Next, at step  138 , the inductor coils  42  inside the containers  94  may be installed over the side projections  88  of the E-shaped core element  70  and the brackets  74 . Next, at step  140 , the I-shaped core element may be attached to the E-shaped core element  70 . The spacing between the I-shaped core element  72  and the E-shaped core element  70  may be predetermined according to known inductive characteristics of such chokes. Finally, at step  142  the choke assembly may, in some embodiments, be covered with a layer of varnish. The varnish may provide an additional level of protection against dust and water, protection against corrosion, and may also serve to securely fasten the inductor coil  42  to the core element  70 , thereby minimizing vibrations. The choke  20  may then be installed within the motor drive unit  58 . 
     With the choke arrangement described above, significant protection from environmental conditions can be realized. The cup-and-bobbin style container seals electrical conductors against water and dust, protecting against electrical failure and increasing the overall safety of the device. Furthermore, chokes fabricated in accordance with disclosed techniques are easy to assemble and, therefore, cost effective. Sealing the container  82  with epoxy provides a double layer of protection and durability, and also enhances the thermal conductivity of the assembly, allowing heat to pass efficiently from the inductor coil  42  to the outside environment. Additional features, such as the cylindrical openings  104  and the shrink tubing  122  provide additional measures of protection. By providing a choke with significant protection against dust and water, the motor drive unit  58  may be mounted such that the cooling channel  62  is exposed to the environment outside of the mounting cabinet. 
     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. 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.