Abstract:
One or more sections of a solenoidal induction coil are moved relative to the surface of a strip passing through the coil as one or more parameters of the strip change to affect the impedance of the load circuit, while the output frequency of the power supply providing power to the coil via a capacitive element is changed so that the power supply&#39;s load circuit continues to operate at substantially resonant frequency.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/757,353, filed Jan. 9, 2006, hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to electric induction heating of a strip material particularly in applications where the width of the strip material, or another parameter, changes to alter the electrical impedance of the load circuit.  
       BACKGROUND OF THE INVENTION  
       [0003]     A flexible solenoidal induction coil, when connected to an ac power supply, can be used to inductively heat a workpiece passing through the coil. The flexible coil is of particular use when the workpiece has a changing crosss sectional dimension. In this arrangement the coil can be flexed to maintain a constant distance between the coil and the cross section of the workpiece presently passing through the coil. For example if the workpiece is a camshaft, irregularly shaped cams (features of the workpiece) will be spaced apart from each other along the shaft (workpiece). As the cam shaft passes through the coil for induction heat treatment, the flexible coil can be dynamically changed in shape-by attachment to suitable linear motion actuators that alter the cross sectional shape of the coil, for example, from circular to oval, to conform to the cross sectional shape of the feature of the workpiece passing through the coil.  
         [0004]     For electric induction heating of a continuous strip material, the strip can be passed through a solenoidal coil that is powered from ac power source  112  as shown in  FIG. 1 . Capacitance of tuning capacitor C TUNE , impedance of solenoidal coil L COIL  and the resistance of the strip material  90 , which is magnetically coupled with the primary power circuit, substantially comprise the load circuit impedance. For certain applications the coil must accommodate strip materials of varying width. Rolls of materials having different widths may be sequentially fed through the coil, either as individual rolls, or with consecutive rolls of varying widths welded together at their ends to pass continuously through the coil. Power induced in the strip material is substantially equal to the electrical resistance of the material multiplied by the square of the current supplied to the coil from the ac power supply. As the width of the strip increases, the resistance of the strip increases. Since applied power is equal to the square of the current supplied to the coil multiplied by the resistance of the strip, as the resistance increases and the supplied current does not change, applied power will linearly increase, as will the product output rate (that is, weight of the strip material heated per unit time measure, for example, in Tons (T) per hour (hr)) as graphically illustrated in  FIG. 2 ( a ). In some applications there is a desire to maintain the product output rate at a constant value after a particular level of applied power is reached. As illustrated in  FIG. 2 ( b ), between increasing strip widths w 1  and w 2 , product output rate (power) increases linearly from y 1  to y 2 . If the width of the strip further increases, the output level remains constant at y 2 . To keep the output rate constant, load resistance must be kept constant.  
         [0005]     One object of the present invention is to selectively achieve a constant rate of production of inductively heated strip materials having different widths when the width of the strip changes by changing the distance between the strip and a solenoidal coil used to inductively heat the strip while keeping the load circuit operating at substantially resonant frequency by modulating the output frequency of the power supply providing power to the load circuit.  
         [0006]     Another object of the present invention is to selectively achieve a constant rate of production of inductively heated strip materials having one or more different parameters that affect the electrical impedance of the inductive heating circuit by changing the distance between the strip and a solenoidal coil used to inductively heat the strip while keeping the load circuit operating at substantially resonant frequency by modulating the output frequency of the power supply providing power to the inductive heating circuit.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     In one aspect the present invention is an apparatus and method of inductively heat treating strips when at least one parameter of the strips changes to change the impedance of the inductive load heating circuit. The apparatus comprises an ac power supply providing power to the load circuit. The load circuit comprises a capacitive element, a solenoidal induction coil having at least one flexible section, and at least one means for moving the at least one flexible section of the coil. A strip moves through the coil so that the strip is magnetically coupled with the load circuit. As the strip moves through the coil, the at least one flexible section of the coil is moved to change the load impedance. The output of the power supply is frequency modulated to change the output frequency as the at least one flexible coil section is moved so that the load circuit continues to operate at substantially resonant frequency.  
         [0008]     Other aspects of the invention are set forth in this specification and the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The foregoing brief summary, as well as the following detailed description of the invention, is better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings exemplary forms of the invention that are presently preferred; however, the invention is not limited to the specific arrangements and instrumentalities disclosed in the following appended drawings:  
         [0010]      FIG. 1  is a prior art apparatus for induction heating of a strip material.  
         [0011]      FIG. 2 ( a ) graphically illustrates the increase in applied induction power and rate of production of inductively heated strip material with the prior art apparatus as the width of the heated strip increases.  
         [0012]      FIG. 2 ( b ) graphically illustrates the increase in applied induction power, and rate of production of inductively heated strip material, as the width of the heated strip increases to a selected value, and is then maintained at a constant applied induction power and rate of production over a range of further increasing strip widths with the induction heating apparatus of the present invention.  
         [0013]      FIG. 3  is a cross sectional view of one example of the induction heating apparatus of the present invention used to inductively heat a strip having a first width, w 1 .  
         [0014]      FIG. 4  is a cross sectional view of one example of the induction heating apparatus of the present invention used to inductively heat a strip having a second width, w 3 , which is greater than the width of the strip in  FIG. 3 , prior to adjustment of one or more flexible sections of the induction coil.  
         [0015]      FIG. 5  is a cross sectional view of one example of the induction heating apparatus of the present invention used to inductively heat a strip having the second width, and after adjustment of the one or more flexible sections of the induction coil to reduce the resistance of the primary load circuit. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]     Referring now to the drawings, wherein like numerals indicate like elements, there is shown in  FIG. 3  through  FIG. 5  one example of the induction heating apparatus of the present invention. In  FIG. 3  power supply  12  supplies ac power to solenoidal induction coil  14 . Strip  16   a  (shown in dashed outline) passes through the coil and is heated by electric induction when ac current from the power supply flows through the coil to establish a magnetic field that couples with the strip. A means for moving the one or more sections of the coil is provided so that the distance between at least one of the surfaces of the strip and the one or more coil sections, for example, d 1  in  FIG. 3 , can be selectively changed. The means for moving the one or more coil sections may comprise a manual mechanism, an actuator  20 , as shown in the figures, or other suitable device. The actuator may be, for example, an electrically or hydraulically powered linear actuator.  
         [0017]     Power supply  12  outputs variable frequency ac power and can be an ac inverter fed from a dc rectifier having an input from utility power. Tuning capacitor  18  forms a resonant load circuit with solenoidal coil  14  and the equivalent electrical impedance of strip  16   a  by magnetic coupling with the primary load circuit. The output frequency of the power supply is selected so that the load circuit comprising the tuning capacitor, the induction coil and impedance of the strip reflected into the load circuit by magnetic coupling, which, in combination, is referred to as combined load impedance Z load , operates substantially at resonant frequency.  
         [0018]     In  FIG. 3 , strip  16   a  having width w 1 , is inductively heated with the coil in a first position as shown in the figure. This physical configuration results in a first load circuit impedance, Z load1 , which requires the power supply to operate with an output frequency, f 1 , so that the load circuit is operating substantially at resonance.  
         [0019]     In  FIG. 4 , strip  16   b  having width w 3 , which is greater than width w 1 , of strip  16   a  in  FIG. 3 , is inductively heated by passing the strip through coil  14 . If the tuning capacitance and inductance of the coil in the load circuit remain the same, load circuit impedance will change to a second value of load circuit impedance, Z load2 , since the impedance of the strip reflected into the primary load circuit by magnetic coupling increases. If the output current of the power supply remains the same, applied power, as graphically shown in  FIG. 2 ( a ), will continue to increase, as will the rate of production of the strip.  
         [0020]     With the induction heating apparatus of the present invention, as shown in  FIG. 5 , actuators  20  are used to move one or more sections of coil  14  to a second position that is farther away (distance d 2 ) from a surface of the strip than distance d 1  in  FIG. 3 , which will reduce the impedance of the strip reflected into the primary load circuit by magnetic coupling. With suitable movement of the coil and change (modulation) in output frequency of the power supply, applied power and rate of production, can be maintained constant, for example, between strip widths w 2  and W 4  as graphically shown in  FIG. 2 ( b ). For example, with the induction heating device of the present invention, the output frequency of power supply  12  can be changed to f 2 , which is the resonant frequency with the coil in the position shown in  FIG. 5 .  
         [0021]     Suitable feedback means, such as but not limited to, sensing of the actual position of the coil, or electrical sensing of instantaneous load power, can be used to adjust the output frequency of the power supply so that the load circuit is powered at resonant frequency as the position of the coil changes. A processing system comprising a computer executing a program to control the applied power to the load circuit may be used with suitable input and output devices to control the movement of the coil and output frequency of the power supply as the width of the strip changes.  
         [0022]     In the above examples of the invention, changing of the width of the strip represents one parameter that will change the electrical impedance of the load circuit when the parameter changes. Other such parameters are, for example, the composition of the strip material and the composition of any coating on the strip as it passes through the solenoidal coil. In other examples of the invention, the induction heating apparatus of the present invention may be used to increase and decrease the applied power and rate of production of inductively heated strip as one or more of such parameters changes over a range by changing the position of the coil and modulating the output frequency of the power supply as described above.  
         [0023]     Solenoidal coil  14  may comprise a singular coil that is flexible for movement between positions. In other examples of the invention the coil may comprise a number of sections, one or more of which may be flexible with means for moving the flexible coil section from one position to another. Coil  14  may comprise other arrangements, such as but not limited to, multiple coils, so long as at least one section of a coil can be moved to change the load impedance. While the above non-limiting example of the invention illustrates moving opposing coil sections, other examples of the invention include arrangements with one or more moveable coil sections not necessarily symmetrically arranged about the strip.  
         [0024]     The above examples of the invention have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the invention has been described with reference to various embodiments, the words used herein are words of description and illustration, rather than words of limitations. Although the invention has been described herein with reference to particular means, materials and embodiments, the invention is not intended to be limited to the particulars disclosed herein; rather, the invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims. Those skilled in the art, having the benefit of the teachings of this specification, may effect numerous modifications thereto, and changes may be made without departing from the scope and spirit of the invention in its aspects.