Abstract:
A transverse flux induction heating apparatus adjusts the level of edge heating of a workpiece by changing the pole pitch of induction coils forming the apparatus to provide a more uniform transverse temperature of the workpiece. Changes in the operating frequency of the induction power supply and in the distance between induction coils and workpiece are not required to adjust edge frequency heating. The pole pitch, and therefore, the level of edge heating can be continuously changed, or conveniently adjusted prior to a production run, in a high speed continuous heat treatment process for a workpiece.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S Provisional Application No. 60/259,578, filed Jan. 3, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention generally relates to transverse flux induction heating and more particularly to transverse flux induction heating with induction coil turns having an adjustable coil pitch.  
           [0004]    2. Description of Related Art  
           [0005]    A conventional transverse flux induction apparatus  100  is shown in exploded view in FIG. 1. The apparatus includes a coil pair comprising a first and second coil,  112  and  114 , respectively, configured as two-turn coils. Transverse (substantially perpendicular to the longitudinal direction of workpiece  120 , as indicated by the arrow labeled “X”) segments and longitudinal (approximately parallel with the longitudinal direction of workpiece  120 ) segments of each coil form a generally rigid and continuous coil. The pole pitch, τ, is fixed for each turn of the two-turn first and second coil segments. A magnetic flux concentrator  116 , shown as laminated steel plates, surrounds the first and second coils generally in all directions except for coil surfaces that face workpiece  120 , which is a continuous metal workpiece (such as a metal strip) that will be inductively heated as it passes between the coil pair. For clarity of coil arrangements in FIG. 1, the concentrator for coil  112  is shown in broken view and the concentrator for coil  114  is not shown. In this exploded view, coil gap, g c , is exaggerated. In typical applications, the coil gap is generally only larger than the thickness, d s , of the workpiece as to allow unobstructed travel of the strip between the coils. When in-phase ac electric power is applied to the terminals of the first and second coil sections (that is, for example, instantaneously positive power to terminals  1  and  3 , and instantaneously negative power to terminals  2  and  4 ), the current flowing through the first and second coils establish a common magnetic flux that passes perpendicularly through the workpiece as illustrated by the exemplary dashed flux line in FIG. 1, with the arrows indicating the direction of the flux.  
           [0006]    [0006]FIG. 2 is a graph plotting the temperature across the transverse of a workpiece. Transverse points on the workpiece (x-axis) are normalized with 0.0 representing the center of the transverse and +1 and −1 representing the opposing edges of the transverse. Curve  81  in FIG. 2 is a plot of the typical cross sectional temperature distribution for a workpiece that is inductively heated by the common magnetic flux established in a conventional transverse flux coil pair. If the workpiece enters the transverse flux induction apparatus  100  with its edges at temperatures lower than the temperature at the center of the workpiece, this effect could be used to an advantage to more evenly heat the workpiece across its width or transverse. However, if the workpiece enters the apparatus with a uniform temperature across its transverse, the edges will be overheated. For this condition, it would be ideal to inductively heat the workpiece uniformly across its transverse, as indicated by line  82  in FIG. 2. The frequency of the power source can be varied to some extent to compensate for the edge overheating effect, at the expense of a significant increase in the cost of the power supply. Alternatively, discrete edge heaters, in addition to a main induction heating apparatus, can be used to compensate for this non-uniform cross sectional heating. See, for example, U.S. Pat. No. 5,156,683 entitled  Apparatus for Magnetic Induction Edge Heaters with Frequency Modulation . However, this approach requires additional equipment and a more complex control system.  
           [0007]    Therefore, there exists the need for a transverse flux induction heating apparatus and method that will provide a quick and efficient method of reconfiguring the coil pair to provide a variable degree of heating across the cross section of a workpiece, including selective edge heating, without changing the frequency of the induction power source or adding separate edge heaters.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    In one aspect the present invention is a transverse flux induction heating apparatus and method that allows continuous adjustment of the operating pole pitch for a coil pair used in the apparatus to heat the transverse of the workpiece to a substantially uniform temperature. These and other aspects of the invention are set forth in the specification and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.  
         [0010]    [0010]FIG. 1 is an exploded perspective view of a conventional prior art transverse flux induction heating apparatus.  
         [0011]    [0011]FIG. 2 is a graph of typical (non-uniform) and ideal (uniform) cross section temperature distributions of a workpiece inductively heated with a transverse flux induction heating apparatus.  
         [0012]    [0012]FIG. 3 is an exploded perspective view of one example of a transverse flux induction heating apparatus of the present invention with its pole pitch adjusting apparatus removed.  
         [0013]    [0013]FIG. 4 is a graph of typical cross section temperature distributions of a workpiece inductively heated with one example of a transverse flux induction heating apparatus of the present invention.  
         [0014]    [0014]FIG. 5( a ) is a top view of one example of a transverse flux induction heating apparatus of the present invention.  
         [0015]    [0015]FIG. 5( b ) is a cross sectional view of one example of a transverse flux induction heating apparatus of FIG. 5( a ) as indicated by section line A-A in FIG. 5( a ). 
     
    
     DETAILED DESCRIPTION  
       [0016]    There is shown in FIG. 3, FIG. 5( a ) and FIG. 5( b ), a first example of the transverse flux induction heating apparatus  10  of the present invention. The apparatus  10  includes a coil pair comprising a first and second coil,  12  and  14 , respectively, that is used to inductively heat a workpiece  20 , such as a metal strip, passing between the first and second coils. In this particular example of the invention, a two-turn coil arrangement is used. A single-turn coil pair, more than two-turn coil pair arrangements, or multiple coil pairs can be used without deviating from the scope of the invention. Each turn of the first and second two-turn coils comprises two transverse coil segments, for example, segments  40  and  42 , and segments  41  and  43 , for the two coil turns making up second coil  14 . All transverse coil segments are arranged substantially perpendicular to the longitudinal direction of the workpiece and are generally longer than the width (transverse) of the workpiece. The longitudinal distance between corresponding pairs of transverse coil segments that comprise a coil turn represents the pole pitch, τ, for each coil turn. The pole pitch for each turn making up the first coil is substantially the same as the pole pitch for each corresponding turn making up the second coil. Further corresponding transverse segment pairs (i.e.,  50  and  40 ;  52  and  42 ;  51  and  41 ; and  53  and  43 ) of first coil  12  and second coil  14  lie substantially in a plane perpendicular to the longitudinal direction of the workpiece (indicated by an arrow labeled “X” in FIG. 3) so that the created flux remains substantially perpendicular to the surface of the workpiece.  
         [0017]    Each turn of the first and second coils has an adjustable coil segment that connects together two transverse coil segments of a turn to complete a coil turn, and connects the two coil turns that make up the first or second coil. For example, adjustable coil segments  45 ,  46  and  47  join transverse coil segments  40  and  42 ,  41  and  43 , and  41  and  42 , respectively, for second coil  14 . Each adjustable coil segment is generally oriented in the longitudinal direction of the workpiece  20 . Each adjustable coil segment may be a flexible cable or other flexible electrical conductor that is suitably connected (connecting element  70  diagrammatically shown in the figures) at each end to a transverse coil segment. Any electrically conducting material and arrangement, including multiple interconnecting sliding partial segments, may be used for each adjustable coil segment as long as it can maintain electrical continuity in a coil turn as the pole pitch is changed as further described below.  
         [0018]    Further, in applications where the first and second coils are water-cooled by circulating cooling water through hollow passages in the first and second coil segments, the adjustable coil segments can be used as convenient connection points to the supply and return of a cooling medium, such as water.  
         [0019]    Magnetic flux concentrators  16   a  and  16   b  (formed from high permeability, low reluctance materials such as steel laminations) generally surround transverse coil segments  52  and  53 , and  50  and  51 , respectively, of the first coil in all directions except for the coil surfaces facing workpiece  20 . For clarity of coil arrangements in FIG. 3, the concentrators for coil  12  is shown in broken view and the concentrators for coil  14  are not shown. In this exploded view, coil gap, gc, is exaggerated. In typical applications, the coil gap is generally only larger than the thickness, d s , of the workpiece as to allow unobstructed travel of the workpiece between the coils. When terminals  1  and  3  are connected (either directly or indirectly by, for example, a load matching transformer) to the first output terminal of an ac single-phase power source, and terminals  2  and  4  are connected to the second output terminal of the power source, the currents flowing through the first and second coils establish a common magnetic flux that passes perpendicularly through the workpiece as illustrated by the exemplary dashed flux line in FIG. 3, with the arrows indicating the direction of the flux when the current at terminals  1  and  3  is instantaneously positive and the current at terminals  2  and  4  is instantaneously negative.  
         [0020]    As shown in FIG. 5( a ) and FIG. 5( b ), mounting means  60  are provided and attached either directly or indirectly to each of the four magnetic flux concentrators,  16   a ,  16   b ,  16   c  and  16   d , and its associated transverse coil segments, namely  52  and  53 ,  50  and  51 ,  42  and  43 , and  40  and  41 , respectively. Mounting means  60  provides means for attachment of a pole pitch adjusting apparatus  62  as shown in FIG. 5( a ) and FIG. 5( b ) (not shown in FIG. 3 for clarity). The pole pitch adjusting apparatus provides the means for changing the coil pitch, τ, between transverse coil segments of each coil turn. In the present example, the pole pitch adjusting apparatus can be jack screws that are either manually or automatically operated by remote control. Further, while two jack screws are used in the present example other arrangements and configurations of pole pitch adjusting apparatus are contemplated as being within the scope of the present invention. The adjustable coil segments,  55 ,  56  and  57  in the first coil  12 , and  45 ,  46  and  47  in the second coil  14 , allow the jack screws to move the transverse coil segments of the first coil  12  and the second coil  14  closer to each other (smaller pole pitch) or farther away from each other (larger pole pitch) in the longitudinal direction of the workpiece. Further in the preferred example of the invention, movement of corresponding transverse segments of the first and second coils is synchronized so that the pole pitch for each turn making up the first coil remains substantially the same as the pole pitch for the corresponding turn making up the second coil.  
         [0021]    [0021]FIG. 4 illustrates the general effect that a change in pole pitch has on the cross sectional heating temperature profile for the induction heating apparatus of the present invention. In FIG. 4, the x-axis represents the normalized width (transverse) of a workpiece from its center (point 0.0 on the x-axis) to its edges (points ±1.0 on the x-axis). The y-axis represents the normalized transverse temperature of a workpiece having a normalized temperature of 1.0 at its center (point 0.0).  
         [0022]    The equivalent depth of induced current penetration, Δ o , in meters, is defined by the following equation:  
         Δ   0     =     503   ·           ρ   s     f     ·       g   c       d   s                                   
 
         [0023]    where ρ s =the resistivity of the workpiece (in Ω·m);  
         [0024]    f=the frequency (in Hertz) of the induction power source;  
         [0025]    g c =the distance between the first and second coils; and  
         [0026]    d s =the thickness of the workpiece.  
         [0027]    In the present invention, for a given workpiece with a substantially constant resistivity and thickness, the distance between the first and second coils, g c , and the frequency of the induction power source are kept substantially constant. Curves  91 ,  92 ,  93  and  94  in FIG. 4 represent four different cross sectional heating temperature profiles for a workpiece inductively heated by the apparatus of the present invention. Curves  91  through  94  are a parametric set of curves that are defined by the relationship  
         τ     Δ   0       =   k                         
 
         [0028]    where k=constant.  
         [0029]    As the coil pitch, τ, increases for a substantially constant Δ o , the cross sectional heating of the workpiece generally progresses from that shown in curve  91 , through curves  92  and  93 , and to curve  94 . For example, for one particular substantially constant set of the four variables used to determine Δ o , the four curves in FIG. 4 are parametric representations where the following mathematical relationship is maintained between τ and Δ o :  
                                                   Curve   k = τ/Δ o                             91   0.5           92   1.0           93   2.0           94   3.0                      
 
         [0030]    Thus, with Δ o  (depth of current penetration) held substantially constant, as the coil pitch, τ, increases, edge heating correspondingly increases from that shown in curve  91  to that shown in curve  94 . For example, if higher edge heating of the workpiece is desired when pole pitch is currently set to achieve the cross sectional temperatures in the workpiece illustrated in curve  92 , the pole pitch could be increased so that the cross sectional temperatures in the workpiece illustrated in curve  93  is achieved without changing the distance between the first and second coils and the frequency of the power source.  
         [0031]    In the present example, a pluuality of temperature sensors  80 , such as pyrometers, sense the temperatures across section (transverse) of workpiece prior to its entry into induction heating apparatus  10 . The values of the sensed temperatures are used as an input to a means (such as an electronic processor) for determining a pre-heat cross section temperature profile of the workpiece. Thus any non-uniform transverse temperature distribution of the workpiece will be sensed prior to the workpiece moves through the transverse flux induction coil. The processor will then determine a transverse heating profile that will inductively heat the workpiece to a more uniform transverse temperature distribution. The processor will determine an appropriate pole pitch setting to achieve the more uniform cross sectional heating temperature of the workpiece, with appropriate inductive edge heating of the workpiece in apparatus  10 . Processor determination of the adjustment of the pole pitch setting can be based upon a set of data curves similar to those in FIG. 4, as modified for a specific application, that can be stored in a database accessible to the processor.  
         [0032]    Alternatively, the pole pitch may be manually adjusted at the start of a production run to achieve a desired cross sectional heating temperature of the workpiece, with appropriate inductive edge heating of the workpiece, prior to passing the workpiece between the coil pair of the heating apparatus of the present invention. In some applications, a pole pitch range of a few inches will be sufficient to provide a suitable control range of variable edge heating.  
         [0033]    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.