60/269,666, filed February 16,2001.

Field of the Invention [0002] The present invention is in the technical field of inductively heating and stirring electrically conductive molten materials wherein the heating and stirring can be accomplished simultaneously.

Background of the Invention [0003] It is well known in the art to melt an electrically conductive material, such as a metal, to heat the molten metal (or melt), and to hold the melt at a temperature by placing the metal in an induction furnace or holding crucible and magnetically coupling the metal to an ac magnetic field. The field is produced in one or more induction coils surrounding the crucible by the flow of ac current from a power source. To maintain sufficient electromagnetic stirring, the electrical frequency of the current is reduced as the furnace capacity increases and the applied ac induction power (and current) increases. For example, a furnace with a melt capacity of 35,000 pounds (16 tonnes) of iron has an optimal power supply frequency of approximately 150 Hz, whereas a furnace with a melt capacity of 5,000 pounds (21/4 tonnes) of steel has an ideal power supply frequency of approximately 600 Hz.

[0004] It is also well known that a melt subjected to an ac magnetic field will move when eddy currents generated in the melt by the applied field produce a flux field that opposes the applied magnetic field. Generally, fields produced by higher frequency currents will result in little stirring action and fields produced by lower frequency currents will result in preferred electromagnetic stirring motions with circular-like flow streams through the melt. Further the turbulence of the flow will increase as the magnitude of the applied field (supplied current) is increased.

[00051 For some melt compositions and applications, the pre-selected frequency of a single ac power supply may provide both heating and stirring actions that are sufficient for the process. In other applications, separate heat and stir frequencies may be used.

There are numerous prior art approaches to applying ac power to a melt at two different frequencies to achieve the heating and stirring functions. Earlier approaches focused on using switching arrangements that alternatively isolated heating and melting power sources from the induction coil sections. Switching arrangements are disadvantageous in that they do not allow for simultaneous heating and stirring of the melt and require additional system components.

[0006] Later approaches focused on system topologies that simultaneously applied heating power (operating at a pre-selected heat frequency) and stirring power (operating at a pre-selected stir frequency). A significant technical problem to be overcome in these systems is adequate electrical isolation between the simultaneously connected heating and stirring ac power supplies. Failure to provide this isolation when electronic ac power sources are used can result in component malfunction or failure in a power supply that has its output connected to a second power supply operating at a different output voltage and/or frequency.

[0007] One solution to this technical problem is identified in U. S. Patent No.

5,012,487, entitled Induction Melting (the 487 patent). FIG. 1 is a simplified schematic that represents the prior art teachings of the 487 patent. In FIG. 1 an electrostatically screened three-phase transformer 126, having primary windings 124 and secondary windings 128, is used to provide stirring power to three coil sections, 114a, 114b and 114c, that make up an induction coil for an induction melting vessel. Stirring power is provided from a 50 Hz, three-phase power source 120 (utility service power). The transformer also uses a tertiary three-phase winding 127 that feeds a three-phase delta- connected power factor correction arrangement (not shown in the simplified schematic).

Capacitors 138a, 138b and 138c are connected to the three coil sections as shown in FIG. 1. The high voltage single-phase output of the heating power source 136, operating in the frequency range of 150 Hz to 10 kHz, provides heating power to the coil sections through the capacitors. By selecting the impedance of the capacitors, the coil sections and the secondary of transformer, so that the resultant L-C series circuit is at resonance for the operating frequency of the heating power supply, heating power is

transferred from the heating power supply to the coil sections. The 50 Hz stirring power source, operating at off-resonant frequency, is impeded from being applied to the input terminals of the heating power source 136 by the tuned series-resonant circuit.

Conversely, heating power is blocked from the stirring power source since the secondary windings of transformer 126 are effectively in parallel at the operating frequency of the heating power source.

[0008] There are a few disadvantages to the circuit arrangements disclosed in the 487 patent. Power transformer 126 is an expensive component with voltage tap changers (not shown in the simplified schematic) and the tertiary winding as further described in the 487 patent. Further the operating frequency difference between the heat power source and the stir power source must exceed a certain range for the series resonant circuit to operate effectively. This is particularly problematic for large capacity induction melting vessels.

[0009] Therefore, there exists the need for apparatus for and method of simultaneously induction heating and stirring a melt from two separate power supplies, without the use of isolation transformers or switches, wherein the frequency of stir power supply (and induced stir field) is less than the frequency of the heat power supply (and induced heat field), particularly when the frequency of the heat power supply is close in frequency of the stir power supply.

Brief Summary of the Invention [0010] In one aspect, the invention is apparatus for and method of simultaneous induction heating and stirring of an electrically conductive material in a vessel having at least one set of three interconnected induction coil sections disposed around the vessel.

Inductive heating of the electrically conductive material is accomplished by applying single-phase ac power across the coil sections via one or more tuning capacitors and stirring of the electrically conductive material is accomplished by applying three-phase ac power to the coil sections via one or more inductors. The capacitive heating circuit and the coil sections operate at or near a first resonant point and the inductive stir circuit and the coil sections operate at or near a second resonant point to block power transfer between the sources of the single-phase and three-phase ac power.

[0011] These and other aspects of the invention are set forth in the specification and claims.

Brief Description of the Drawings [0012] 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.

[0013] FIG. 1 is a simplified schematic of a prior art arrangement for achieving simultaneous induction heating and stirring of a melt in an induction melting vessel.

[0014] FIG. 2 is a simplified single-line schematic diagram of one example of an arrangement for simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention.

[0015] FIG. 3 (a) is an elementary schematic diagram of one example for simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention using a voltage-fed full bridge converter as the single-phase heating power source and a three-phase dc-to-ac inverter as the three-phase stirring power source wherein the induction coil sections disposed around the vessel are connected in an open-delta configuration relative to the three-phase stirring power source.

[0016] FIG. 3 (b) is an elementary schematic diagram of another example for simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention using a voltage-fed half bridge converter as the single-phase heating power source and a three-phase dc-to-ac inverter as the three-phase stirring power source wherein the induction coil sections disposed around the vessel are connected in an open-delta configuration relative to the three-phase stirring power source.

[00171 FIG. 4 is a first graphical illustration of the output current from a pulse width modulated (PWM) power supply used as a three-phase power source for electromagnetic stirring in the present invention.

[0018] FIG. 5 is a second graphical illustration of the output current from a pulse width modulated (PWM) power supply used as a three-phase power source for electromagnetic stirring in the present invention.

[0019] FIG. 6 (a) is an elementary schematic diagram of another example for simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention using a voltage-fed full bridge converter as the single-phase heating power source and a three-phase dc-to-ac inverter as the three-phase stirring power source wherein the induction coil sections disposed around the vessel are connected in a wye configuration relative to the three-phase stirring power source.

[0020] FIG. 6 (b) is an elementary schematic diagram of another example for simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention using a voltage-fed half bridge converter as the single-phase heating power source and a three-phase dc-to-ac inverter as the three-phase stirring power source wherein induction coil sections disposed around the vessel are connected in a wye configuration relative to the three-phase stirring power source.

[0021] FIG. 7 schematically illustrates one method of using transformers for changing the output characteristics of a single-phase heating power supply or a three-phase stirring power supply used in examples of the invention.

Detailed Description of the Invention [0022] There is shown in FIG. 2 a simplified single-line schematic diagram of one example of the simultaneous induction heating and stirring apparatus 10 of the present invention. Single-phase heating source 12 is any type of source that will provide induction heating power to induction coil Ll. The coil surrounds a heating vessel or crucible (not shown in the drawing) containing an electrically conductive molten material, or melt. The induction heating power can be used to melt electrically conductive material in the vessel, as well as keep it at a desired temperature once the material has been melted, and while additional material is added to the melt. Therefore, the term"heating"as used herein also encompasses induction heating power for melting material in the vessel. The preferred, but non-limiting, frequency range for a power source that is used to heat the electrically conductive material is from approximately 100

Hz to 100 kHz. Cl represents one or more tuning capacitors that are used to improve the power factor of the C1-L1 series circuit. Power source 16a represents one phase of a three-phase stirring power source. The three-phase source is any type of source that can provide electromagnetic stir power to induction coil Ll. As further described below, a suitable, but non-limiting, range of output frequency for the stirring power supply is between 1 Hertz and approximately 100 Hertz.

[0023] Referring to the example of FIG. 2, for a heating power source 12 operating at a frequency, fh, of 160 Hertz, and an induction coil Ll having an inductance (Ll) equal to 50ho0-6 Henries, the capacitance (Cl) of capacitor Cl, which forms a series resonant circuit with coil Ll, can be calculated from the equation: where T = 2Bfh. The equation leads to a value of approximately 20 mFarads for Cl.

Further, for resonance at 160 Hertz, the reactive impedance, Xu, of coil Ll will be approximately 0.05 ohms (from the equation XLI = TLI) and the reactive impedance, Xcl, of capacitor Cl will be approximately 0.05 ohms (from the equation Xcl-l/TCl).

Coil resistance is represented by resistive element Rl. A typical value of induction coil resistance, Rlheat, as reflected in the coil Ll load, is approximately 10 percent of the reactive impedance of coil Ll. Therefore, Rlheat is approximately equal to 0.005 ohms.

For a magnitude of heating power equal to 5 megawatts (5tl 06 W), the current that the L1-C1 resonant circuit will draw from heating power supply 12 is approximately 31,500 amperes, as calculated from the equation: I = p Rl heat [0024] For a stirring power source operating at a frequency, fs, of 2.5 Hertz, the resistance, R1stìr7 of induction coil Ll at 2.5 Hertz can be calculated from the equation: fs Rlstir = Rl heat V f : h as approximately 0.00062 ohms. At the stir frequency of 2.5 Hz, the reactive impedance of coil LI will be approximately 0.00079 ohms, and the reactive impedance of Cl will be approximately 3.2 ohms. The output of stirring power source 16 is adjusted so that the induction coil Ll draws approximately one-half of the heating current. For this

example, the stir current, Astir, will be approximately 8,000 amperes. Stir power, Pstir, can be calculated from the equation: as 40 kilowatts, or 0.8% of heating power. Inductor L2, in the line of the stirring power source 16, is selected to have a relatively high impedance with respect to the impedance of induction coil Ll. In this example, the inductor, L2, is selected as 4810-3 Henries, which is eighty times the inductance of coil Ll. At 160 Hertz, the reactive impedance of inductor L2 can be calculated as approximately 4.0 ohms. At 2.5 Hertz, the reactive impedance of inductor L2 can be calculated as approximately 0.006 ohms. The resistance of inductor L2 is ignored since it is significantly smaller in value than the reactance of the inductor.

[0025] The following table summarizes the approximate impedance of each passive circuit component for the present example: Impedance (ohms) at Impedance (ohms) at Heat Frequency (160 Hz) Stir Frequency (2.5 Hz) Capacitor Cl 0.05 3.2 Coil Ll 0.05 0.00079 Coil Resistance RI 0.005 0.00062 Inductor L2 4.0 0.006

As illustrated by the impedance values in the above table for the circuit shown in FIG. 2, the CL-LI-RI series circuit offers a relatively low impedance path to the output current from heating power source 12. Conversely, inductor L2 effectively blocks current from the heating power source 12 from flowing through stirring power source 16. The L2-Ll-Rl series circuit offers a relatively low impedance path to the output current from stirring power source 16, whereas capacitor Cl effectively blocks current from the stirring power source 16 from flowing through heating power source 12.

[0026] The following table summarizes the contributions of the heating and stirring power sources to the voltage across, current through, and power used in coil LI : Contribution from Heating Contribution from Stirring Power Source (160 Hz) Power Source (2.5 Hz) Coil Ll Current 31,500 8,000 (amperes) Coil Ll Voltage 1, 700 11 (volts) Power, Ll Coil 5,000 40 (kW)

Coil Ll voltage is calculated from the product of the magnitude of coil LI current and the magnitude of coil Ll impedance (reactive and resistive) for the appropriate power source.

[0027] Consequently, the heating power source 12 supplies 31,500 amperes to coil Ll and approximately 425 amperes (determined by dividing coil Ll voltage for heating power source 12 by the impedance of inductor L2 at heat frequency) to the input of stirring power source 16. Stirring power source 16 supplies 8,000 amperes to coil LI, and approximately 3.4 amperes (determined by dividing coil Ll voltage for stirring power source 16 by the impedance of capacitor Cl at stir frequency) to the input of heating power source 12. The approximately 425 amperes imposed on the input of the stirring power source 16, which can be a solid state, pulse width modulated supply as further described below, is deemed an acceptable current level that will not impact the performance of the stirring power source. Similarly the approximately 3.4 amperes imposed on the input of the heating power source 12, which can be a solid state, series-resonant power supply as further described below, is deemed an acceptable current level that will not impact the performance of the heating power source.

[0028] FIG. 3 (a) illustrates another example of simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention wherein three induction coil sections 14a, 14b and 14c are interconnected to form a three-phase, delta-configured impedance network. Terminals la and 4a of coil sections 14a and 14c, respectively, are not connected together. Therefore the circuit arrangement of the induction coil sections will be referred to as an open delta, three-phase impedance network. FIG. 3 (a) illustrates one non-limiting example of how

the three coil sections may be arranged around vessel 11 that contains the electrically conductive material. Capacitor C2 is selected to form series circuit with induction coil segments 14a, 14b and 14c that operates at or near resonance when connected to the heating power source. In this example, the single-phase ac heating power source is a voltage-fed, full bridge converter 12a utilizing an ac-to-dc rectifier section 21 that has an input from three-phase ac supply lines 20. Output terminals of the power supply's full bridge converter are designated Tll and T12. Capacitor Cll and inductor Lll filter the dc power output from the rectifier section. The filtered dc power is inverted to variable ac power in inverter section 22 of the converter. Capacitor C12 is connected between open delta terminal 4a of the three-phase impedance network and one output terminal, Tll, of the single-phase ac supply. The second output terminal, T12, of the single-phase ac supply is connected to open delta terminal la of the three-phase impedance network. In this configuration, ac current that is supplied from the single-phase ac heating power source and flows through the coils sections creates a magnetic field that magnetically couples with the electrically conductive material inside the vessel to heat the material. The capacitance of capacitor C12 is selected to form a series resonant circuit with the three coil sections and to provide a relatively high impedance to the output of the three-phase stirring supply which operates at a stir frequency lower than the frequency of the heating power supply.

[0029] Stirring power source 16a can be a three-phase dc-to-ac inverter that utilizes solid state switching topologies, including power transistors such as an Insulated Gate Bipolar Transistor (IGBT). Although a separate rectifier assembly could be used as an input to stirring power source 16a, in this particular example, rectifier assembly 21 also provides dc input to the stirring power source's inverter via interconnecting dc output positive bus DC1 and negative bus DC2. Each output line (T31, T32 and T33) of the three-phase inverter supply is connected to an end terminal of coil segments 14a, 14b and 14c via inductors L2a, L2b and L2c, respectively. Inductors L2a, L2b, and L2c, are power inductors (typically, but not limited to, metal core design) with approximately the same inductance, which is much greater than the inductance of a coil section. In this configuration, ac current that is supplied from the three-phase ac stirring power source and flows through the coils sections creates a magnetic field that magnetically couples with the electrically conductive material inside the vessel to electromagnetically stir the

material. The inductances of inductors L2a, L2b and L2c are selected to form a resonant circuit with the three coil sections and to provide a relatively high impedance to the output of the single-phase heating supply that operates at a higher frequency. The output frequency of the stirring power source 16a will generally be less than the output frequency of the heating power source. The magnitude and frequency of the three-phase ac output, from the stirring power source 16a can be electronically adjusted by controlling the gate timing of the power transistors with circuitry known in the art. The frequency and magnitude of stirring current drawn from stirring power source 16a can be varied to achieve different stirring patterns while a melt is simultaneously heated.

Generally the frequency of the stirring current will affect the magnetic stirring pattern and the magnitude of the stirring current will affect the intensity of the stirring action.

As illustrated in FIG. 4 and FIG. 5, if the stirring power source 16a operates as a PWM power supply, changing the pulse width and frequency of the output supply current pulses as illustrated by curves 40a and 40b, will result in changes of the effective magnitude and frequency of output stirring current as illustrated by curves 42a and 42b.

[0030] FIG. 3 (b) illustrates another example of simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention. In this example, single-phase ac heating power supply is a voltage fed half bridge converter 12b with half bridge inverter section 22a. Capacitors C12a and C12b, having approximately the same capacitance, replace capacitor C12 in FIG. 3 (a) The output from the half bridge inverter,. The capacitors are connected in series across the positive and negative dc buses, DC1 and DC2, respectively, of the heating power supply. In this configuration, the output terminals of the heating power supply are designated as terminals Tlla and T12a, with terminal Tlla at the center of the half- bridge circuit, and terminal T12a at the common connection between capacitors C12a and C12b. Open-delta terminal 4b is connected to terminal Tlla and open-delta terminal lb is connected to terminal T12a. Otherwise, this example of the invention is similar to the previous example illustrated in FIG. 3 (a).

[0031] FIG. 6 (a) illustrates another example of simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention. This example varies from the example illustrated in FIG. 3 (a) in that the

three induction coil sections 14a, 14b and 14c are interconnected in a wye three-phase impedance network, rather than an open delta, three-phase impedance network. FIG.

6 (a) illustrates one non-limiting example of how the three coil sections may be arranged around vessel 11 that contains the electrically conductive materials. The wye three-phase impedance network has phase coil terminals Ic, 2c and 3c, and common coil terminal 4c for all induction coil sections. Capacitors C12c, C12d and C12e have one of their terminals connected to coil terminals 1c, 2c and 3c, respectively. The second terminals of all theses capacitors are commonly connected to output terminal, Tll, of the single-phase ac supply 12a. The second output terminal, T12, of the single-phase as supply is connected to common coil terminal 4c. Each of the output lines, T31, T32 and T33, of the three-phase inverter supply is connected to coil terminals le, 2c and 3c, respectively, of coil segments 14a, 14b and 14c via inductors L2a, L2b and L2c, respectively. Otherwise, this example of the invention is similar to the previous example illustrated in FIG. 3 (a).

[0032] FIG. 7 illustrates one method of providing a voltage step-up or step-down of the output of the single-phase ac supply in FIG. 6 (a) by providing a autotransformer 40 across the output terminals Tll and T12 of the supply. The autotransformer may also be replaced by a conventional four-terminal transformer. Further voltage step-up or step-down of the output of the three-phase ac supply in FIG. 6 (b) can be accomplished by using transformer elements T2a, T2b and T2c to replace inductors L2a, L2b and L2c, respectively, in FIG. 6 (a). These voltage transformations may also be provided in other examples of the invention with appropriate modifications.

[0033] FIG. 6 (b) illustrates another example of simultaneous induction heating and melting of an electrically conductive molten material in accordance with the present invention. This example varies from the example illustrated in FIG. 3 (b) in that the three induction coil sections 14a, 14b and 14c are interconnected in a wye three-phase impedance network, rather than an open delta three-phase impedance network.

Capacitors C12f, C12g and C12h have one of their terminals connected to coil terminals Ic, 2c and 3c, respectively. The second terminals of all theses capacitors are commonly connected to output terminal, T12a, of the single-phase ac supply 12a. Otherwise, this example of the invention is similar to the previous example illustrated in FIG. 6 (a).

[0034] As illustrated by the above examples, the present invention is directed to a single-phase ac heating supply connected to the vessel's induction coil impedance network by one or more capacitive elements to form an inductive heating circuit.

Components in the inductive heating circuit are selected so that the circuit is at or near resonance when driven by the heating power source operating at an inductive heating frequency. The three-phase ac stirring supply is connected to the vessel's induction coil impedance network by inductive elements to form an inductive stirring circuit.

Components in the inductive stirring circuit are selected so that the circuit is at or near resonance when driven by the stirring power supply operating at an inductive stirring frequency. Further the capacitive elements and inductive elements are selected to provide sufficient impedance to block output power from the heating supply to the stirring supply, and output power from the stirring supply to the heating supply, respectively. Generally the inductive stirring frequency is less than the inductive heating frequency. Further the stir frequency may be varied over a range to provide a varied electromagnetic stir pattern. Although this will result in some off-resonant stir circuit operation, the variance from resonance will not be sufficient to negate the impedance blocking feature of the present invention.

[0035] Other types of single-phase power supplies and three-phase power supplies power supplies can be used as heating and stirring power sources, respectively, for the disclosed invention. Other three-phase induction coil configurations may be utilized without deviating from the scope of the invention. For example, the coil sections may be physically arranged around the heating vessel to achieve a particular heating and or melting variation along the height of the molten material inside the vessel. Further, multiple three-phase induction coil configurations may be provided with connections to common (parallel) heating and/or stirring power sources, or individual heating and/or stirring power sources for each of the multiple three-phase induction coils.

[0036] 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.

[0037] The foregoing embodiments do not limit the scope of the disclosed invention.

The scope of the disclosed invention is further set forth in the appended claims.