Abstract:
An apparatus and process are provided for multi-frequency induction heating of a workpiece. Switching devices are used to selectively add, remove or reconfigure capacitive elements in the circuit to change the circuit&#39;s resonant frequency, and consequently, the frequency of induction heating or melting power transfer from the power supply to the load.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   FIELD OF THE INVENTION 
   The present invention relates to induction heating or melting wherein multiple frequencies are used to heat or melt a workpiece by electric induction. 
   BACKGROUND OF THE INVENTION 
   Multi-frequency induction heating and melting is known in the art. See for example U.S. Pat. No. 2,444,259, which is titled Method of High Frequency Induction Heating. Different frequencies result in different depths of induced eddy current heating in the workpiece, susceptor, or electrically conductive load placed in a crucible; the higher the frequency, the lower the effective induced eddy current depth of the current in the workpiece, susceptor or electrically conductive load. Multi-frequency currents may be applied (1) simultaneously or sequentially and (2) to a single or multiple induction coils that are disposed around the workpiece, susceptor or crucible in which the electrically conductive material is placed. 
   For an electrically conductive material, such as a metal composition that is placed in a crucible, the combination of low and high induction frequencies may be desirable to melt the metal at a high frequency and to stir the metal at a low frequency. 
   For the geometry of some workpieces, the combination of low and high induction heating frequencies is desirable. For example to metallurgically harden gear teeth it is known that a relatively low frequency (e.g., 3 kilohertz to 10 kilohertz) with relatively deep penetration of the induced eddy current into the gear is preferred to preheat the gear while a relatively high frequency (e.g., 30 kilohertz through 100 kilohertz) with relatively shallow penetration of the induced eddy current into the gear is preferred for final induction heating. 
   One objective of the present invention is to provide induction power at multiple frequencies to a workpiece with efficient transfer means between the multiple frequencies. 
   BRIEF SUMMARY OF THE INVENTION 
   In one aspect, the present invention is an apparatus for, and method of, inductively heating or melting a workpiece at two or more frequencies by switching tuning capacitive elements into and out of an inverter circuit, or rearranging the tuning capacitive elements in the inverter circuit, to provide power at different frequencies to an induction load coil. 
   In another aspect, the present invention comprises an inverter circuit having first and second branches and a diagonal connected between the first and second branches. Commutation devices, at least one resonance capacitive element, and at least one tuning capacitive element are disposed in the inverter circuit. At least one switch is disposed in the diagonal of the inverter circuit, along with at least one induction load coil. The at least one switch is used to selectively insert or remove one or more of the at least one tuning capacitive elements in the inverter circuit, or to rearrange the at least one tuning capacitive element and the at least one resonance capacitive element in the inverter circuit, whereby the resonant frequency of the inverter circuit is changed to inductively heat or melt a workpiece at different frequencies when the workpiece is positioned adjacent to the magnetic field created by the flow of ac power through the at least one induction load coil. 
   Other aspects of the invention are set forth in this specification and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The figures, in conjunction with the specification and claims, illustrate one or more non-limiting modes of practicing the invention. The invention is not limited to the illustrated layout and content of the drawings. 
       FIG. 1  is a simplified schematic illustrating one example of the multi-frequency induction heating or melting apparatus of the present invention. 
       FIG. 2  is a simplified schematic illustrating another example of the multi-frequency induction heating or melting apparatus of the present invention. 
       FIG. 3  is a simplified schematic illustrating another example of the multi-frequency induction heating or melting apparatus of the present invention. 
       FIG. 4(   a ) is a simplified schematic illustrating another example of the multi-frequency induction heating or melting apparatus of the present invention. 
       FIG. 4(   b ) is a simplified schematic illustrating another example of the multi-frequecny induction heating or melting apparatus shown in  FIG. 4(   a ) wherein the switch associated with the tuning capacitor comprises a diode bridge and switching element. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates one example of the induction heating or melting apparatus of the present invention. In this example of the invention, resonant bridge inverter  10  comprises commutation devices, resonance capacitors, tuning capacitors and switching elements. Substantially inductive load coil LC is powered by the inverter. Commutation device SD 1  with antiparallel diode D 1 , and commutation device SD 2  with antiparallel diode D 2  comprise the first branch of the inverter. The second branch of the inverter comprises resonance capacitors RC 1  and RC 2 . Tuning capacitors TC 1  and TC 2 , along with switches S 1 , S 2  and S 3  and the load coil, form the diagonal of the inverter, which is connected between the midpoints of the first and second branches. Inductive load coil LC, although shown with an inductor symbol, also exhibits circuit resistance. 
   In other examples of the invention the inductive load coil may be otherwise arranged. For example, coil LC may be replaced by a primary transformer coil that is magnetically coupled to a secondary transformer coil, which serves as the coil around which the workpiece is disposed. 
   In  FIG. 1  an ac magnetic field is established around load coil LC by flowing ac current supplied by the inverter operating with an output frequency, F, through the coil. Switching elements, namely switches S 1 , S 2  and S 3 , provide a means for reconfiguring the capacitance of the inverter by selectively inserting, removing or reconnecting the tuning capacitors in the active circuit. A workpiece or susceptor may be brought into the vicinity of the magnetic field to inductively heat the workpiece or susceptor, or the load coil may be place around a crucible in which an electrically conductive load, such as a molten metal, has been placed, to inductively heat, melt and/or stir the load. 
   For convenience of reference, the term “workpiece” is used to refer to a workpiece for heating, a susceptor or an electrically conductive material placed in a crucible. Further the term “surrounding the workpiece” with reference to one or more induction load coils of the present invention includes arrangements wherein the workpiece is positioned so that the magnetic field created by the flow of ac current through the one or more induction load coils penetrates the workpiece. 
   Ideally for maximum transfer of power from the output of the inverter to load coil LC, the output frequency of the inverter should be at or near resonance. For an LC-circuit, resonant frequency, F, is calculated from the formula:
 
 F= ½ π√{square root over (L·C)} 
 
   wherein L is the equivalent inductance of the circuit and C is the equivalent capacitance (C eq ) of the resonant circuit. 
   Referring to  FIG. 1 , preferably, but not limiting, tuning capacitors TC 1  and TC 2  will have substantially the same value of capacitance, C TC , and resonance capacitors RC 1  and RC 2  will have substantially the same value of capacitance, C RC . With this arrangement, when both tuning capacitors and both resonance capacitors are all in series (switch S 1  opened, switch S 2  at position  1  and switch S 3  at position  2 ) the equivalent circuit capacitance, C eq , at resonant frequency F, can be calculated as: 
   
     
       
         
           
             C 
             eq 
           
           = 
           
             2 
             × 
             
               
                 
                   
                     C 
                     TC 
                   
                   × 
                   
                     C 
                     RC 
                   
                 
                 
                   
                     C 
                     TC 
                   
                   + 
                   
                     C 
                     RC 
                   
                 
               
               . 
             
           
         
       
     
   
   The change in C eq , as well as the change in resonant frequency, F, as switches S 1 , S 2  and S 3  change positions, relative to the calculated C eq  above, is illustrated in the following table: 
   
     
       
             
             
             
             
           
         
             
                 
             
             
                 
                 
               Equivalent 
                 
             
             
               Positions 
               Configuration of 
               circuit 
               Resonant 
             
             
               of switches 
               circuit capacitors 
               capacitance 
               frequency 
             
             
                 
             
           
           
             
               S1 opened; 
               Parallel combination 
               C eq   
               F 
             
             
               S2 at position 1; and 
               of TC1 and TC2 in series 
             
             
               S3 at position 2 
               with parallel combination 
             
             
                 
               of RC1 and RC2 
             
             
               S1 closed; 
               RC1 in parallel with RC2 
               2 C eq   
               0.7 F 
             
             
               S2 and S3 open 
             
             
               S1 closed; 
               Parallel combination 
               4 C eq   
               0.5 F 
             
             
               S2 at position 2 and 
               of RC1 and RC2 in 
             
             
               S3 at position 1 
               parallel with parallel 
             
             
                 
               combination of TC1 
             
             
                 
               and TC2 
             
             
                 
             
           
        
       
     
   
   Therefore in this non-limiting example of the invention, induction heating or melting frequencies may be switched between F, 0.7 F, and 0.5 F with the speed of switching being dependent upon the switching speed of switches S 1 , S 2  and S 3 , which may be of any form, such as electromechanical or solid state, as required to suit a particular application. 
     FIG. 2  illustrates another example of the induction heating or melting apparatus of the present invention wherein a single switch S 4  provides a means for switching the circuit configuration of tuning capacitors TC 3  and TC 4  and resonance capacitors RC 3  and RC 4 . When switch S 4  is in the opened position as shown in the figure, the series combination of tuning capacitors TC 3  and TC 4  is in parallel with the series combination of resonance capacitors RC 3  and RC 4 , and when switch S 4  is in the closed position, the parallel combination of tuning capacitor TC 3  and resonance capacitor RC 3  is in parallel with the parallel combination of tuning capacitor TC 4  and resonance capacitor RC 4 , whereby the equivalent load circuit capacitance changes, along with the resonant frequency of the load circuit, when switch S 4  alternates between the opened and closed positions. 
     FIG. 3  illustrates another example of the induction heating or melting apparatus of the present invention. In this example, commutation device SD 1  with antiparallel diode D 1 , and commutation device SD 2  with antiparallel diode D 2  comprise the first branch of the inverter; commutation device SD 3  with antiparallel diode D 3 , and commutation device SD 4  with antiparallel diode D 4  comprise the second branch of the inverter. Tuning capacitor TC 5 , resonance capacitor RC 5 , along with switches S 5  and S 6  and the load coil, form the diagonal of the inverter, which is connected between the midpoints of the first and second branches. 
   In one non-limiting example wherein tuning capacitor TC 5  has substantially the same value of capacitance, C, as does resonance capacitor RC 5 , the change in capacitance C, as well as the change in resonant frequency, F, as switches S 5  and S 6  change positions, relative to capacitance C, is illustrated in the following table: 
   
     
       
             
             
             
             
           
         
             
                 
             
             
                 
                 
               Equivalent 
                 
             
             
               Positions 
               Configuration of 
               circuit 
               Resonant 
             
             
               of switches 
               circuit capacitors 
               capacitance 
               frequency 
             
             
                 
             
           
           
             
               S5 at position 1 and 
               TC5 in series with RC5 
               0.5 C 
               F 
             
             
               S6 opened 
             
             
               S5 at position 2 and 
               RC5 (TC5 not in circuit) 
               C 
               0.7 F 
             
             
               S6 opened 
             
             
               S5 at position 1 and 
               TC5 in parallel with RC5 
                 2 C 
               0.5 F 
             
             
               S6 closed 
             
             
                 
             
           
        
       
     
   
   There is shown in  FIG. 4(   a ) another example of the induction heating melting apparatus of the present invention. In this arrangement commutation device SD 1  with antiparallel diode D 1 , and commutation device SD 2  with antiparallel diode D 2  comprise the first branch of the inverter, and resonance capacitors RC 6  and RC 7  comprise the second branch of the inverter. Tuning capacitor TC 6 , along with switch S 7  and the load coil, form the diagonal of the inverter, which is connected between the midpoints of the first and second branches. When ac current supplied from the inverter flows through the induction load coil a magnetic field is created. A workpiece can be positioned so that the magnetic field created by the flow of ac current through the induction load coil penetrates the workpiece to inductively heat or melt the workpiece as further described above. 
   In operation switch S 7  can be opened, as shown in the figure, or closed, to either include the tuning capacitor in the circuit, or bypass the tuning capacitor out of the circuit, respectively. The circuit impedance of the load coil, the tuning capacitor (if present in the active circuit) and the resonance capacitors determines the resultant load impedance seen by the output of the inverter. Therefore shorting tuning capacitor TC 6  by closing switch S 7  will increase the circuit&#39;s equivalent capacitance and, consequently, lower the resonant frequency of the circuit. Conversely opening switch S 7  will decrease the circuit&#39;s capacitance and, consequently, increase the resonant frequency of the circuit. 
     FIG. 4(   b ) illustrates one non-limiting example of providing the switch means for switch S 7  in  FIG. 4(   a ). In  FIG. 4(   b ) the switching means for shorting tuning capacitor TC 6  is accomplished by a diode bridge that is formed from diodes BD 1 , BD 2 , BD 3  and BD 4 , and is connected across the tuning capacitor. Switch S 8 , for example, a transistor, is connected across the center of the bridge to short out the tuning capacitor when the switch is closed. 
   By way of non-limiting example, when the apparatus in  FIG. 4(   a ) or  FIG. 4(   b ) is used to achieve dual frequency induction heating of a workpiece with a low frequency of around 3 kilohertz to 10 kilohertz and a high frequency of around 30 kilohertz to 100 kilohertz, the capacitance of tuning capacitor TC 6  should be selected as around 100 times smaller than the capacitance of resonance capacitors RC 6  and RC 7 . 
   Although a resonant inverter is used in the above examples of the invention, other inverter arrangements or topologies may be used without deviating from the scope of the invention. In all examples of the invention the output of the inverter may operate at a fixed frequency or varied. The switching devices that are used in the above examples of the invention, including transistors or other solid state devices, such as the commutation devices illustrated with insulated gate bipolar transistor symbols, are exemplary and may be replaced by any other suitable switching device or element. 
   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.