# Patent

## Title
Method of fabricating a choke assembly

## 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.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a Divisional of application Ser. No. 12/241,361, filed Sep. 30, 2008, now U.S. Pat. No. 8,125,304, entitled “Power Electronic Module with an Improved Choke and Methods of Making Same” in the name of John R. Brubaker et al.


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 58 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 62 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 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.

## Claims

1. A method for fabricating a motor drive, comprising:
coupling rectifier circuitry configured to be coupled to an AC power source to inverter circuitry configured to generate drive signals for driving a motor via a DC bus; and
coupling a choke assembly to the DC bus, the choke assembly comprising a bobbin, a preformed inductor coil wound around the bobbin, a preformed insulative container comprising an outer wall, an integral base and a hollow projection extending from the base of the container and creating an internal space between the outer wall of the container and the projection, the bobbin fitting over the hollow projection and the space configured to receive the inductor coil on the bobbin, the projection forming a passageway through the container, an annular cover disposed over the container and configured to seal the inductor coil inside the container, and a magnetic core extending through the hollow projection.

2. The method of claim 1, wherein the magnetic core comprises an E-shaped magnetic core having a center projection and two side projections, wherein the container, coil and cover are disposed over one of the side projections, and an I-shaped core disposed over the annular cover.

3. The method of claim 1, wherein the container is filled with a potting material to encase the inductor coil within the potting material.

4. The method of claim 1, wherein the choke assembly is coated with varnish.

5. The method of claim 1, wherein a first end of an electrical conductor is coupled to the preformed inductor coil and a second end of the electrical conductor is coupled to a lead, and the lead extends through at least one opening in the cover and an interface between the at least one opening and the lead is sealed.

6. The method of claim 1, wherein the magnetic core comprises two pieces, a first piece being inserted through the projection, and a second piece being magnetically coupled to the first piece after insertion of the first piece through the projection.

7. A method for fabricating a motor drive, comprising:
coupling rectifier circuitry configured to be coupled to an AC power source to inverter circuitry configured to generate drive signals for driving a motor via a DC bus; and
coupling two distinct portions of a choke assembly to the DC bus, each portion comprising a bobbin, a preformed inductor coil wound around the bobbin, a preformed insulative container comprising an outer wall, an integral base and a hollow projection extending from the base of the container and creating an internal space between the outer wall of the container and the projection, the bobbin fitting over the hollow projection and the space configured to receive the inductor coil on the bobbin, the projection forming a passageway through the container, an annular cover disposed over the container and configured to seal the inductor coil inside the container, and a magnetic core extending through the hollow projection.

8. The method of claim 7, wherein within each portion the core comprises an E-shaped magnetic core having a center projection and two side projections, wherein the container, coil and cover are disposed over one of the side projections, and an I-shaped core disposed over the annular cover.

9. The method of claim 7, wherein within each portion the container is filled with a potting material to encase the inductor coil within the potting material.

10. The method of claim 7, wherein the portions are coated with varnish.

11. The method of claim 7, wherein within each portion a first end of an electrical conductor is coupled to the preformed inductor coil and a second end of the electrical conductor is coupled to a lead, and the lead extends through at least one opening in the cover and an interface between the at least one opening and the lead is sealed.

12. The method of claim 7, wherein within each portion the magnetic core comprises two pieces, a first piece being inserted through the projection, and a second piece being magnetically coupled to the first piece after insertion of the first piece through the projection.

13. The method of claim 7, wherein at least a portion of the magnetic core is disposed within each of the two distinct portions of the choke assembly.