Abstract:
An apparatus and process are provided for controlling the cross sectional temperature profile of a continuously moving workpiece with an induction coil assembly that provides for a combination of longitudinal and transverse magnetic flux field heating of the workpiece. The induction coil assembly includes means for laterally moving the workpiece in and out of the coil assembly without movement of the coil assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. Provisional Application No. 60/469,539 filed May 9, 2003, hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention generally relates to magnetic induction heating of a workpiece, and in particular, to induction heating of a workpiece moving through an induction heating coil assembly. 
   BACKGROUND OF THE INVENTION 
   The induction coil through which a moving workpiece is heated by magnetic induction is typically configured as a longitudinal flux inductor or transverse flux inductor. The workpiece may be a continuous, electrically conductive strip that passes through the induction coil. A longitudinal flux inductor is generally described as a solenoidal coil that surrounds the strip. AC current flowing through the solenoidal coil produces a magnetic field that is parallel with the longitudinal axis of the strip in regions where the field penetrates the strip. The magnetic field induces eddy currents in the continuously moving strip that heat the strip. A longitudinal flux inductor is superior to a transverse flux inductor for uniform cross sectional heating of a strip. However, when a longitudinal flux inductor is used to inductively heat a thin strip, a high frequency power supply, with attendant cost penalty, is required. Further the solenoidal configuration of a conventional longitudinal flux inductor makes it impossible to laterally move the continuous strip out from within the coil as may be desired, for example, to replace the existing inductor with a new inductor. The continuous strip must be cut to accomplish a change in inductors. 
   A transverse flux inductor can be used to inductively heat a thin strip at lower frequencies than those used with a longitudinal flux inductor. A transverse flux inductor is generally described as a pair of coils wherein the strip moves in a plane positioned between the planes in which the pair of coils are located. AC current flowing through the coils produces a magnetic field between the pair of coils that penetrates the strip and inductively heats it. Field penetration is generally orthogonal to the surface of the strip. Consequently the induced eddy currents are circulated in a plane near the surface of the strip, but not throughout the width or thickness of the strip. An additional advantage of a transverse flux inductor over a longitudinal flux inductor is that its configuration allows for lateral removal of a continuous strip from between the pair of coils. 
   In some applications, uniform cross sectional heating of the strip is not desired since the edges will cool down faster than the interior of the strip. For example, in a galvannealing process, a continuously moving strip is dipped into a liquid coating bath. The liquid coating thermally bonds with the strip after exiting from the bath. Since the edges of the strip will cool faster than the central region of the strip, the degree of bonding at the edges may vary to produce an unsatisfactory grade of galvanized product. In such cases, the edges must be scrapped from the galvanized strip product. Various types of dedicated edge heaters have been used to compensate for edge heat losses in a strip. U.S. Pat. No. 5,156,683 discloses a dedicated induction edge heater. The edge heater is preferably of the channel type, and requires the use of a mechanical drive system to reposition the edge heater as the width of the strip changes. Also a continuous strip will laterally oscillate as it moves along the heating line, so the mechanical drive system must be used to adjust the position of the edge heaters to accommodate this lateral motion. Further, in order to allow lateral removal of a continuous strip, the mechanical drive system must move at least one of the edge heaters away from the plane of the strip. 
   U.S. Pat. No. 5,837,976 discloses a coil system that allows lateral movement of a continuous strip similar to that provided by a transverse flux inductor while providing the advantages of a longitudinal magnetic flux field. The disclosed coil system comprises upper and lower coil sections that, together, form a two-turn solenoidal coil. AC current flowing through the coil sections results in a longitudinal flux field while a gap between the vertical bars or shunts connecting the two coil sections in series permits lateral movement of the strip out from the coil system. 
   It is one object of the present invention to provide a means for inductively heating a continuously moving workpiece, such as a strip, that allows for controlled edge heating of the strip without movement of the induction heating coils for strips of varied widths, while allowing unrestricted lateral removal of the continuous strip from within the induction heating coil assembly. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, the present invention is an apparatus for, and method of, inductively heating a strip moving through an induction coil assembly wherein the induction coil assembly comprises a first coil assembly and a second coil assembly. The first coil assembly is arranged and supplied with ac current to produce a substantially longitudinal magnetic flux field that inductively heats the strip uniformly across its cross section as it passes through the first coil assembly. The second coil assembly is arranged and supplied with ac current to produce a substantially transverse magnetic flux field that inductively heats the strip non-uniformly across its cross section as it passes through the second coil assembly. One example of this aspect of the invention is shown in FIG.  1 . The control of the first and second coil assemblies are cooperatively arranged to provide a selective cross sectional heating profile of the strip as it passes through the induction coil assembly. 
   In another aspect, the present invention is an apparatus for, and method of, inductively heating a strip moving through an induction coil assembly wherein the induction coil assembly comprises upper and lower coil sections through which the strip moves (see e.g. FIG.  2 ). AC current is supplied to the upper and lower coil sections by two high frequency inverters and one low frequency inverter that are connected to the upper and lower coil sections by a network of inductive and capacitive circuit elements (see e.g. FIG.  3 ). The high frequency inverters are arranged to supply a high frequency ac current to the induction coil assembly to create a longitudinal flux magnetic field that inductively heats the strip uniformly across its cross section as it passes through the induction coil assembly. The low frequency inverter is arranged to effectively supply a low frequency ac current to the coil assembly to create a transverse flux magnetic field that inductively heats the strip non-uniformly across its cross section as it passes through the induction coil assembly. An interconnecting network of impedances between the ac outputs of the inverters and the terminals of the induction coil assembly allows the flow of low and high frequency currents through the induction coil assembly, and blocks low frequency current flow to the ac output terminals of the high frequency inverters, and blocks high frequency current flow to the ac output terminals of the low frequency inverter. 
   Other aspects of the invention are set forth in this specification. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. 
       FIG. 1  is a diagrammatic view of one example of an apparatus of the present invention for induction heating of a workpiece. 
       FIG. 2  is an isometric view illustrating one example of an arrangement of an induction coil assembly used with an apparatus of the present invention for induction heating of a workpiece. 
       FIG. 3  is a diagrammatic view of one example of a power circuit for use with the induction coils illustrated in FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring now to the drawings, wherein like numerals indicate like elements, there is shown in the  FIG. 1 , one example of the induction heating coil assembly  10  of the present invention. Induction heating coil assembly  10  comprises first coil assembly  12  and second coil assembly  14 . First coil subassembly  12  comprises upper coil section  16  and lower coil section  18  as disclosed in U.S. Pat. No. 5,837,976, which is incorporated herein in its entirety. Gap h 1  separates first coil assembly  12  into two parts and allows workpiece  24  to be laterally removed from within the coil. A non-limiting configuration of the workpiece is an electrically conductive strip. Second coil assembly  14  comprises a first coil  20  and second coil  22  which form a coil pair on opposing sides of workpiece  24 . Gap h 2  separates the first and second coils and allows the strip to be laterally removed from within the pair of coils. In this non-limiting example, the strip moves sequentially through the second coil assembly and first coil assembly in the direction indicated by the arrow. In other examples of the invention movement may be sequentially through the first coil assembly and then the second coil assembly. 
   First inverter  26  supplies single phase ac power to terminals  28   a  and  28   b  of upper coil section  16 . Second inverter  30  supplies single phase ac power to terminals  32   a  and  32   b  of lower coil section  18 . A common first rectifier  34  supplies dc power from a suitable ac source, such as utility power, to both the first and second inverters. The dc inputs to inverters  26  and  30  are arranged in series as shown in  FIG. 1  so that approximately one-half of the dc output voltage of the rectifier is applied equally across the inputs of each of the first and second inverters. AC currents from the first and second inverters to the upper and lower coil sections, respectively, are of substantially equal magnitudes and 180 electrical degrees out of phase to establish substantially equal magnitude current flows a and b, as indicated by the arrows in  FIG. 1 , through the coil sections. These current flows create a longitudinal magnetic flux field that achieves substantially uniform cross sectional induction heating of the strip. 
   Third inverter  36  supplies single phase ac power to first and second coils  20  and  22  that form the coil pair of the second coil assembly. The first output of third inverter  36  is connected to terminals  38   a  and  38   c  of the coil pair, and the second output of the third inverter is connected to terminals  38   b  and  38   d  of the coil pair. The ac current supplied from third inverter  36  to coils  20  and  22  are substantially equal magnitude currents a and b, as indicated by the arrows in  FIG. 1 , through the coil pair. These current flows create a transverse magnetic flux field around the coil pair that can be used to control the induction heating of the edges of strip  24 . In this example of the invention, first rectifier  34  also supplies dc power to third inverter  36 . In other examples of the invention, a separate rectifier may be provided for supplying dc power to the third inverter. 
   With this arrangement, substantial control of the overall cross sectional temperature of the continuous strip as it exits induction heating coil assembly  10  can be accomplished by controlling the current outputs of first and second inverters  26  and  30 , and temperature at the edges of the strip, relative to the temperature at the central region of the strip, can be accomplished by controlling the current output of third inverter  36 . 
   The above example of the invention utilizes a first coil assembly  12  as disclosed in U.S. Pat. No. 5,837,976 to produce a longitudinal magnetic flux field for uniform cross sectional heating of workpiece  24 . In other examples of invention, first coil assembly  12  may comprise other types of coil arrangements that produce a longitudinal magnetic flux field for uniform cross sectional heating of the workpiece. 
   In some processing lines, the line length available for an induction coil assembly is limited. This is particularly the case in retrofit of existing processing lines. The available line length may not provide sufficient space for separate coil assemblies  12  and  14  as shown in FIG.  1 . FIG.  2  and  FIG. 3  illustrate another example of an induction heating coil assembly of the present invention wherein the advantages of both longitudinal and transverse flux induction heating can be accomplished in a single, compact coil assembly. In this arrangement, as shown in  FIG. 2 , the induction coil assembly  11  comprises upper coil section  15  and lower coil section  17 . Upper coil section  15  has end terminals  1  and  2 , and lower coil section  17  has end terminals  3  and  4 . 
     FIG. 3  illustrates one example of a diagrammatic power supply circuit for providing induction heating current to induction coil assembly  11 . In  FIG. 3  the upper and lower coil sections  15  and  17  are schematically illustrated as inductive elements L c . That is, upper coil section  15  is represented by inductive elements L c  on either side of continuous strip  24 , and lower coil section  17  is represented by inductive elements L c  on either side of continuous strip  24 . 
   Low frequency (LF) inverter  40  supplies ac current to the terminals of the upper and lower coil sections via inductive circuit elements as shown in FIG.  3 . The first output terminal of LF inverter  40  is connected by inductive elements L 1  and L 2  to terminals  1  and  2  of upper coil section  15 , respectively, and the second output terminal of LF inverter  40  is connected by inductive elements L 3  and L 4  to terminals  3  and  4  of lower coil section  17 , respectively. 
   High frequency (HF) inverter  42  supplies ac current to the terminals of the upper coil section via capacitive circuit elements as shown in FIG.  3 . The first output terminal of HF inverter  42  is connected by capacitive element C 1  to terminal  1  of upper coil section  15 , and the second output terminal of HF inverter  42  is connected by capacitive element C 2  to terminal  2  of upper coil section  15 . High frequency (HF) inverter  44  supplies ac current to the terminals of the lower coil section via capacitive circuit elements as shown in FIG.  3 . The first output terminal of HF inverter  44  is connected by capacitive element C 3  to terminal  3  of lower coil section  17 , and the second output terminal of HF inverter  44  is connected by capacitive element C 4  to terminal  4  of lower coil section  17 . 
   In this non-limiting example of the invention, rectifier  46  provides dc output power to LF inverter  40  from a suitable ac source, and rectifier  48  supplies dc power from a suitable source to both HF inverters  42  and  44 . The dc inputs to inverters  42  and  44  are arranged in series so that approximately one-half of the dc output voltage of rectifier  48  is applied equally across the input of the two HF inverters. 
   The ac current output of LF inverter  40  has a substantially lower frequency than the ac current outputs of HF inverters  42  and  44 . For example, the LF inverter may operate at an output current frequency of 10 kHz and the HF inverters may operate at an output current frequency of 100 kHz. Further the ac current outputs from HF inverters  42  and  44  are of substantially equal magnitudes and  180  electrical degrees out of phase. 
   The capacitance of capacitive elements C 1 , C 2 , C 3  and C 4  is selected so that their impedance at the frequency of the substantially equal-magnitude output currents, I hf , of HF inverters  42  and  44  is low, and the inductance of the inductive elements L 1 , L 2 , L 3  and L 4  is selected so that their impedance at the frequency of I hf  is high. In this arrangement, I hf , as indicated by the arrows in  FIG. 3 , flows through the upper and lower coil sections. The high impedance inductive elements block the flow of I hf  from the ac output terminals of LF inverter  40 . I hf  creates a magnetic flux field around coils L c  that is substantially directed along the longitudinal axis of continuous strip  24  and inductively heats the strip as it passes through coil heating assembly  11  substantially uniformly across its cross section. 
   The inductance of the inductive elements L 1 , L 2 , L 3  and L 4  is selected so that their impedance at the frequency of the output current, I lf , of LF inverter  40  is low, and the capacitance of capacitive elements C 1 , C 2 , C 3  and C 4  is selected so that their impedance at the frequency of I lf  is high. In this arrangement, I lf , as indicated by the arrows in  FIG. 3 , flows through the upper and lower coil sections. The high impedance capactive elements block the flow of I lf  to the ac output terminals of HF inverters  42  and  44 . Current I lf  creates a magnetic flux field around coils L c  that is substantially directed along the transverse axis of continuous strip  24  and inductively heats the strip as it passes through coil heating assembly  11  non-uniformly across its cross section. 
   In this example of the invention, longitudinal and transverse flux field induction heating are achieved simultaneously with a single coil assembly. Control of the overall cross sectional temperature of the continuous strip as it exits induction heating coil assembly  11  can be accomplished by controlling the ac current outputs (magnitude, phase and/or frequency) of the LF and HF inverters. 
   Typically capacitances of all capacitive elements, C 1 , C 2 , C 3  and C 4 , Will be the same, and the inductances of all inductive elements, L 1 , L 2 , L 3  and L 4 , will be the same. Further the capacitances of the capacitive elements may be selected to form resonant circuits with coils L c  to maximize power transfer from the HF inverters to coils L c . 
   In all examples of the invention, rectifiers may utilize non-controllable rectification components, such as diodes. Rectifiers may also utilize controllable rectification components, such as silicon controlled rectifiers (SCR), in which case dc output control, if desired, can be achieved by control of the rectification components. 
   While a particular arrangement of rectifiers supplying dc power to the inverters in each example of the invention is illustrated, other arrangements of rectifiers, including different quantities and types, including single and three phase, are within the scope of the invention. Further use of either current fed or voltage fed inverters with the induction coil assemblies of the present invention is within the scope of the invention. Further the induction coil assemblies may be suitably modified by one skilled in the art for use with other types of inverter, including single and three phase, without deviating from the scope of the invention. While all coils are shown as single turn coils in the examples of the invention, coil assemblies with other number of turns and arrangements are contemplated as being within the scope of the invention. The examples of the invention include reference to specific electrical components. One skilled in the art may practice the invention by substituting components that are not necessarily of the same type but will create the desired conditions or accomplish the desired results of the invention. For example, single components may be substituted for multiple components or vice versa. 
   The foregoing examples do not limit the scope of the disclosed invention. The scope of the disclosed invention is further set forth in the appended claims.