CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/224,859 filed Jul. 11, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to apparatus and method for dual frequency heating, melting or stirring with electric induction power. 
     BACKGROUND OF THE INVENTION 
     Typically changes in electric induction power frequencies for heating, melting or stirring applications are achieved by using separate power supplies or varying the output frequency of a direct (DC) current to alternating (AC) current inverter by gate control of switching devices used in the inverter. 
     One object of the present invention is to provide a power supply incorporating a DC to AC inverter capable of operating at two different output frequencies by switched rearrangement of an H-bridge inverter for inductively heating, melting or stirring electrically conductive materials. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect the present invention is apparatus for, and method of, providing a dual frequency output from a DC to AC inverter for electric induction heating, melting or stirring of a composition or workpiece in an inductive load circuit. An H-bridge inverter is provided with switching arranged to reconfigure the inverter from half bridge to full bridge so that the inverter&#39;s output can be switched from high frequency to low frequency, respectively. A resonant load tuning capacitance is utilized across the input of the inverter, subsequent to the DC link input (for example from an AC utility fed rectifier) to the inverter. The inductive load circuit at the output of the inverter may be one or more induction coils surrounding a crucible in which an electrically conductive material is placed, or a susceptor, or one or more inductors used to heat treat an electrically conductive material brought within the vicinity of a magnetic flux field generated by alternating current flow in the one or more inductors. In some arrangements of the invention a tank capacitance connected across the legs of the H-bridge inverter is removed from the circuit when the inverter operates in the low frequency mode. 
     In another aspect the present invention is apparatus for, and method of, dual frequency electric induction heating, melting or stirring, of a composition or workpiece in an inductive load circuit. A dual frequency switch is inserted across a tank capacitance of a full bridge inverter having a resonant load tuning capacitance connected across the direct current input of the full bridge inverter. The dual frequency switch has a low frequency switch position and a high frequency switch position. In the high frequency switch position the tank capacitance is in the inverter circuit, and in the low frequency switch position the dual frequency switch shorts out the tank capacitance. 
     The above and other aspects of the invention are set forth in this specification and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1(   a ) illustrates one example of a power supply apparatus used in the present invention configured for inverter half-bridge series-resonance operation. 
         FIG. 1(   b ) illustrates the power supply apparatus in  FIG. 1(   a ) configured for inverter full-bridge operation. 
         FIG. 2(   a ) illustrates another example of a power supply apparatus used in the present invention configured for electric induction melting of an electrically conductive material placed within a crucible. 
         FIG. 2(   b ) illustrates the power supply apparatus in  FIG. 2(   a ) configured for electromagnetic stirring of an electrically conductive material placed within the crucible. 
         FIG. 3(   a ) illustrates another example of a power supply apparatus used in the present invention configured for inverter half-bridge series-resonance operation. 
         FIG. 3(   b ) illustrates the power supply apparatus in  FIG. 3(   a ) configured for inverter full-bridge operation. 
         FIG. 4(   a ) illustrates another example of a power supply apparatus used in the present invention configured for electric induction melting of an electrically conductive material placed within a crucible. 
         FIG. 4(   b ) illustrates the power supply apparatus in  FIG. 4(   a ) configured for electromagnetic stirring of an electrically conductive material placed within the crucible. 
         FIG. 5(   a ) illustrates another example of a power supply apparatus used in the present invention configured for inverter high frequency series-resonance operation. 
         FIG. 5(   b ) illustrates the power supply apparatus in  FIG. 5(   a ) configured for inverter low frequency series resonance operation. 
         FIG. 6(   a ) illustrates another example of a power supply apparatus used in the present invention configured for electric induction melting of an electrically conductive material placed within a crucible. 
         FIG. 6(   b ) illustrates the power supply apparatus in  FIG. 6(   a ) configured for electromagnetic stirring of an electrically conductive material placed within the crucible. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     There is shown in  FIG. 1(   a ) and  FIG. 1(   b ) one example of the present invention. Apparatus  10  comprises: AC to DC rectifier section  12  represented, for example, by a three phase rectifier with suitable AC input, for example, from a three phase (A, B, C) utility source; DC (link) section  14  represented, for example, by capacitor C f  and chokes/reactors L F1  and L F2 ; resonant load tuning capacitance section  16  represented by capacitor C TUNE ; DC to AC inverter section  18 ; and tank (resonant) capacitance section  20 , represented, for example, by capacitors C T1  and C T2 . The resonant load tuning capacitor used herein is as disclosed in U.S. Pat. No. 6,696,770 B2 (Induction Heating or Melting Power Supply Utilizing a Tuning Capacitor). Inverter AC output switching device SW INV  switches between high frequency output mode ( FIG. 1(   a )) and low frequency output mode ( FIG. 1(   b )) as further described below. 
     Referring to  FIG. 1(   a ) when switching device SW INV  is in switch position A, the inverter is configured as a half bridge series-resonance loaded (L LOAD ) inverter. The circuit for each of the two inverter branches or legs (between terminals  1  and  2 , and between terminals  3  and  4  of the inverter) are connected in parallel via interconnection with the switching device SW INV . The output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, f RES1 , which can be calculated from the following equation: 
     
       
         
           
             
               
                 
                   
                     f 
                     
                       RES 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       1 
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                           
                             
                               L 
                               LOAD 
                             
                             · 
                             
                               ( 
                               
                                 
                                   C 
                                   
                                     T 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                                 + 
                                 
                                   C 
                                   
                                     T 
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     2 
                                   
                                 
                               
                               ) 
                             
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       1 
                       ) 
                     
                   
                   ] 
                 
               
             
           
         
       
     
     For example if the inductance of the equivalent load inductor L LOAD  is 500 microhenries and the capacitance of each tank capacitor C T1  and C T2  is equal to 100 microFarads, half-bridge resonant frequency, f RES1 , will be 500 Hertz. 
     Referring to  FIG. 1(   b ) when switching device SW INV  is in switch position B, the inverter is configured as a full H-bridge inverter. Inductor load L LOAD  is connected in the diagonal (across terminals  5  and  6  of the inverter) of the two inverter branches via interconnection with switching device SW INV . The output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, f RES2 , which can be calculated from the following equation: 
     
       
         
           
             
               
                 
                   
                     f 
                     
                       RES 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     
                       1 
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                           
                             
                               L 
                               LOAD 
                             
                             · 
                             
                               C 
                               TUNE 
                             
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       2 
                       ) 
                     
                   
                   ] 
                 
               
             
           
         
       
     
     For example if the inductance of the equivalent load inductor L LOAD  is 500 microhenries and the capacitance of tuning capacitor C TUNE  is equal to 10,000 microFarads, full bridge resonant frequency, f RES2 , will be 70 Hertz. 
     In half-bridge mode, power output from the inverter is controlled by changing the inverter operating frequency and pulse width modulation. In full bridge resonance mode, power output from the inverter can be controlled solely by pulse width modulation. 
     There is shown in  FIG. 2(   a ) and  FIG. 2(   b ) one application of the power supply apparatus of the present invention. Two power supplies  10   a  and  10   b  are utilized with each power supply connected to separate load inductors  24   a  and  24   b  (counter-wound coils in this example as diagrammatically indicated by the “dot” convention) surrounding crucible  22 , in which an electrically conductive material can be placed, for example, metal charge in solid or solid/molten combination. Power supplies  10   a  and  10   b  are partially illustrated in  FIG. 2(   a ) and  FIG. 2(   b ) for convenience and are similar to the power supplies shown in  FIG. 1(   a ) and  FIG. 1(   b ). In  FIG. 2(   a ), both inverters operate in half-bridge (high frequency), output phase synchronized, resonant mode to inductively melt solid metal in the crucible. In  FIG. 2(   b ) both inverters operate in full-bridge (low frequency) resonant mode to electromagnetically stir molten metal at a reasonably low frequency without excess agitation. The outputs of the two inverters in  FIG. 2(   b ) are arranged to be 90 degrees out-of-phase to produce a running electromagnetic field that induces a unidirectional stirring pattern in the molten metal as shown, for example, in  FIG. 2(   b ); changing the phase shift from plus 90 degrees to minus 90 degrees will reverse the direction of electromagnetic stirring (represented by arrows and dashed lines) in  FIG. 2(   b ). 
     Although two coils are shown in  FIG. 2(   a ) and  FIG. 2(   b ) any number of multiple coils with appropriate phase shifting between coils may used in other examples of the invention to achieve electromechanical stirring in full-bridge resonant mode. 
     A seven-to-one change in resonant frequency between half-bridge and full-bridge modes is a typical range in frequency change for the power supply apparatus of the present invention. 
     There is shown in  FIG. 3(   a ) and  FIG. 3(   b ) another example of the present invention, which is similar to the example in  FIG. 1(   a ) and  FIG. 1(   b ) except that inverter switch SW 1   INV  includes switching contacts C 1  and C 2  for switching tank capacitors C T1  and C T2  out of the circuit when operating in the full bridge resonance (low frequency) mode. This arrangement can be advantageous to avoid ringing in the circuit between tuning capacitor C TUNE  and the tank capacitors. As with the example of the invention in  FIG. 1(   a ) and  FIG. 1(   b ), in half-bridge mode, power output from the inverter is controlled by changing the inverter operating frequency and pulse width modulation, and in full bridge resonance mode, power output from the inverter can be controlled solely by pulse width modulation. As with the example of the invention in  FIG. 1(   a ) and  FIG. 1(   b ), a seven-to-one change in resonant frequency between half-bridge and full-bridge modes is a typical range in frequency change for the power supply apparatus of the present invention shown in  FIG. 3(   a ) and  FIG. 3(   b ). 
       FIG. 4(   a ) and  FIG. 4(   b ) illustrate a heating and stirring application of the circuit arrangement shown in  FIG. 3(   a ) and  FIG. 3(   b ) that is similar to the heating and stirring application of the circuit arrangement shown in  FIG. 2(   a ) and  FIG. 2(   b ) except for the modified inverter switch SW 1   INV . 
     There is shown in  FIG. 5(   a ) and  FIG. 5(   b ) another example of the present invention where a full bridge resonance mode is used for both high frequency mode, with tank capacitor C T  in the circuit as shown in  FIG. 5(   a ), and low frequency mode, with tank capacitor C T  shorted out by inverter switch SW 2   INV  as shown in  FIG. 5(   b ). 
     For the high frequency mode in  FIG. 5(   a ), the output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, f RES1 , which can be calculated from the following equation: 
     
       
         
           
             
               
                 
                   
                     f 
                     
                       RES 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       1 
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                           
                             
                               L 
                               LOAD 
                             
                             · 
                             
                               C 
                               T 
                             
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       3 
                       ) 
                     
                   
                   ] 
                 
               
             
           
         
       
     
     For low frequency mode in  FIG. 5(   b ), the output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, f RES2 , which can be calculated from the following equation: 
     
       
         
           
             
               
                 
                   
                     f 
                     
                       RES 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                   
                   = 
                   
                     
                       1 
                       
                         2 
                         ⁢ 
                         π 
                         ⁢ 
                         
                           
                             
                               L 
                               LOAD 
                             
                             · 
                             
                               C 
                               TUNE 
                             
                           
                         
                       
                     
                     . 
                   
                 
               
               
                 
                   [ 
                   
                     equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       ( 
                       4 
                       ) 
                     
                   
                   ] 
                 
               
             
           
         
       
     
     In the high frequency resonance mode ( FIG. 5(   a )), power output from the inverter is controlled by changing the inverter operating frequency and pulse width modulation, and in the low frequency resonance mode ( FIG. 5(   b )), power output from the inverter can be controlled solely by pulse width modulation. A seven-to-one change in resonant frequency between high frequency and low frequency modes is a typical range in frequency change for the power supply apparatus of the present invention shown in  FIG. 5(   a ) and  FIG. 5(   b ). 
     In some applications of the invention shown in  FIG. 5(   a ) and  FIG. 5(   b ), optional step-down voltage transformer XFMR may be utilized to increase the impedance at the output of the inverter when the load impedance is low. 
       FIG. 6(   a ) and  FIG. 6(   b ) illustrate a heating and stirring application of the circuit arrangement shown in  FIG. 5(   a ) and  FIG. 5(   b ) that is similar to the heating and stirring application of the circuit arrangement shown in  FIG. 2(   a ) and  FIG. 2(   b ). 
     A susceptor vessel may be used in some examples of the invention in lieu of crucible  22 , to melt materials such as silicon. The half bridge, (or high frequency), mode may be used to concentrate inductive heating in the susceptor vessel to initially melt a substantially solid composition of silicon placed in the susceptor vessel by conduction and convection since solid silicon is not electrically conductive, and the full bridge, (or low frequency), mode may be used for electromagnetic stirring of at least a partially molten silicon composition in the susceptor vessel since a molten silicon composition is electrically conductive. For the example of the invention shown in  FIG. 5(   a ) and  FIG. 5(   b ) where a full bridge arrangement is used for both low and high frequency modes, the high frequency mode (tank capacitor in circuit) may be used to concentrate inductive heating in the susceptor vessel to initially melt a substantially solid composition of silicon placed in the susceptor vessel by conduction and convection since solid silicon is not electrically conductive, and the low frequency mode (tank capacitor shorted out) may be used for electromagnetic stirring of at least a partially molten silicon composition in the susceptor vessel since molten silicon composition is electrically conductive. 
     Alternatively the susceptor may be in the geometric shape of an open cylinder with the induction coils surrounding the exterior of the cylinder and a workpiece passing through the interior of the cylinder so that the workpiece absorbs heat by conduction from the inductively heated susceptor. 
     In other examples of the invention an electrically conductive workpiece may be placed within the vicinity of magnetic fields established by current flow through L LOAD  so that the workpiece may be selectively heat treated at different frequencies. 
     The present invention has been described in terms of preferred examples and embodiments. Equivalents, alternatives and modifications, aside from those expressly stated, are possible and within the scope of the invention.
A dual frequency output is provided from a DC to AC inverter. An H-bridge inverter is provided with switching arranged to reconfigure the inverter from half-bridge to full bridge so that the inverter's output can be switched from high frequency to low frequency, respectively. A resonant load tuning capacitance is utilized across the input of the inverter subsequent to the DC link input (for example from an AC utility fed rectifier) to the inverter. The inductive load circuit at the output of the inverter may be one or more induction coils surrounding a crucible in which an electrically conductive material is placed, or susceptor, or one or more inductors used to heat treat an electrically conductive material. In an alternative arrangement an H-bridge inverter is utilized in both the high and low frequency modes while a tank capacitance is in the circuit, or shorted out of the circuit, respectively, in the high or low frequency modes.