Dual frequency heating, melting and stirring with electric induction power

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.

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'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.

DETAILED DESCRIPTION OF THE INVENTION

There is shown inFIG. 1(a) andFIG. 1(b) one example of the present invention. Apparatus10comprises: AC to DC rectifier section12represented, for example, by a three phase rectifier with suitable AC input, for example, from a three phase (A, B, C) utility source; DC (link) section14represented, for example, by capacitor Cfand chokes/reactors LF1and LF2; resonant load tuning capacitance section16represented by capacitor CTUNE; DC to AC inverter section18; and tank (resonant) capacitance section20, represented, for example, by capacitors CT1and CT2. 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 SWINVswitches between high frequency output mode (FIG. 1(a)) and low frequency output mode (FIG. 1(b)) as further described below.

Referring toFIG. 1(a) when switching device SWINVis in switch position A, the inverter is configured as a half bridge series-resonance loaded (LLOAD) inverter. The circuit for each of the two inverter branches or legs (between terminals1and2, and between terminals3and4of the inverter) are connected in parallel via interconnection with the switching device SWINV. The output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, fRES1, which can be calculated from the following equation:

For example if the inductance of the equivalent load inductor LLOADis 500 microhenries and the capacitance of each tank capacitor CT1and CT2is equal to 100 microFarads, half-bridge resonant frequency, fRES1, will be 500 Hertz.

Referring toFIG. 1(b) when switching device SWINVis in switch position B, the inverter is configured as a full H-bridge inverter. Inductor load LLOADis connected in the diagonal (across terminals5and6of the inverter) of the two inverter branches via interconnection with switching device SWINV. The output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, fRES2, which can be calculated from the following equation:

For example if the inductance of the equivalent load inductor LLOADis 500 microhenries and the capacitance of tuning capacitor CTUNEis equal to 10,000 microFarads, full bridge resonant frequency, fRES2, 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 inFIG. 2(a) andFIG. 2(b) one application of the power supply apparatus of the present invention. Two power supplies10aand10bare utilized with each power supply connected to separate load inductors24aand24b(counter-wound coils in this example as diagrammatically indicated by the “dot” convention) surrounding crucible22, in which an electrically conductive material can be placed, for example, metal charge in solid or solid/molten combination. Power supplies10aand10bare partially illustrated inFIG. 2(a) andFIG. 2(b) for convenience and are similar to the power supplies shown inFIG. 1(a) andFIG. 1(b). InFIG. 2(a), both inverters operate in half-bridge (high frequency), output phase synchronized, resonant mode to inductively melt solid metal in the crucible. InFIG. 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 inFIG. 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, inFIG. 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) inFIG. 2(b).

Although two coils are shown inFIG. 2(a) andFIG. 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 inFIG. 3(a) andFIG. 3(b) another example of the present invention, which is similar to the example inFIG. 1(a) andFIG. 1(b) except that inverter switch SW1INVincludes switching contacts C1and C2for switching tank capacitors CT1and CT2out 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 CTUNEand the tank capacitors. As with the example of the invention inFIG. 1(a) andFIG. 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 inFIG. 1(a) andFIG. 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 inFIG. 3(a) andFIG. 3(b).

FIG. 4(a) andFIG. 4(b) illustrate a heating and stirring application of the circuit arrangement shown inFIG. 3(a) andFIG. 3(b) that is similar to the heating and stirring application of the circuit arrangement shown inFIG. 2(a) andFIG. 2(b) except for the modified inverter switch SW1INV.

There is shown inFIG. 5(a) andFIG. 5(b) another example of the present invention where a full bridge resonance mode is used for both high frequency mode, with tank capacitor CTin the circuit as shown inFIG. 5(a), and low frequency mode, with tank capacitor CTshorted out by inverter switch SW2INVas shown inFIG. 5(b).

For the high frequency mode inFIG. 5(a), the output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, fRES1, which can be calculated from the following equation:

For low frequency mode inFIG. 5(b), the output frequency of the inverter will be near resonance, that is within plus or minus 20 percent of actual resonant frequency, fRES2, which can be calculated from the following equation:

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 inFIG. 5(a) andFIG. 5(b).

In some applications of the invention shown inFIG. 5(a) andFIG. 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) andFIG. 6(b) illustrate a heating and stirring application of the circuit arrangement shown inFIG. 5(a) andFIG. 5(b) that is similar to the heating and stirring application of the circuit arrangement shown inFIG. 2(a) andFIG. 2(b).

A susceptor vessel may be used in some examples of the invention in lieu of crucible22, 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 inFIG. 5(a) andFIG. 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 LLOADso 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.