|Title of Invention||
POWER SUPPLY AND METHOD FOR INDUCTIVELY HEATING OR MELTING AN ELECTRICALLY CONDUCTIVE MATERIAL
|Abstract||A rectifier/inverter power supply (10) for use with induction heating or melting apparatus includes a tuning capacitor (CI) connected across the output of the rectifier (14) and input of the inverter (20). The tuning capacitor (CI) forms a resonant circuit with an inductive load coil (L9) at the operating frequency of the inverter (20), Additionally, the load coil (L9) may comprise an active load coil connected to the out put of the inverter (20) and a passive load coil, in parallel with a resonant tuning capacitor, for an improved efficiency circuit.|
POWER SUPPLY FOR INDUCTION HEATING OR MELTING Cross Reference To Related Applications
 This application claims the benefit of U.S. Provisional Application No. 60/312,159, filed August 14,2001.
Field of the Invention
 The present invention relates to an ac power supply for use in induction heating or melting applications wherein the induction power circuit is resonantly tuned.
Background of the Invention
 FIG. 1 illustrates a conventional power supply 110 that is used in induction heating or melting applications. The power supply consists of an ac-to-dc rectifier and filter section 112, a dc-to-ac inverter section 120 and a tuning capacitor section 130. For the power supply shown in FIG. 1, a three-phase diode bridge rectifier 114 converts three-phase (A, B, C) ac utility line power into dc power. Current limiting reactor Lios smoothes out the ripple in the output dc current of the rectifier, and capacitor Cms filters the ac component from, the output dc voltage of the rectifier. The filtered dc output of the rectifier is inverted to ac by a full-bridge inverter consisting of solid state switches Sioi, S102, S103 and S104 and associated antiparallel diodes D101, D102, D103 and Dj04, respectively. Alternating turn-on/turn-off cycles of switch pairs S101/S103 and S102/S104 produce a synthesized ac inverter output at terminals 3 and 4.
 Induction load coil Lioi represents the power coil used in the induction heating or melting application. For example, in an induction furnace, load coil Lioi, is wound around the exterior of a crucible in which metal charge has been placed. In an induction heating application, a metal workpiece, such as a strip or wire, may travel through a helical winding of load coil Lioi or
otherwise be brought near to the coil to inductively heat the workpiece. Current supplied by the power supply and flowing through load coil Lioi creates a magnetic field that either directly heats the metal charge or workpiece by magnetic induction, or heats the workpiece by heat conduction from a susceptor that is heated by magnetic induction. Load coil Lioi, whether it be a single coil or an assembly of interconnected coil sections, has a very low operating power factor. Because of this, a tuning capacitor (or bank of capacitors), such as capacitor Cioi must be provided in the load coil circuit to improve the overall power factor of the load coil circuit. These tuning capacitors are a significant cost and volume component of the power supply. Therefore, there exists the need for a power supply for inductive heating or melting applications that utilizes smaller and less costly tuning capacitors.
 An objective of the present invention is to provide a power supply for inductive heating or melting applications that utilizes a capacitor connected between the output of the rectifier and the input of the inverter to form a resonantly tuned circuit with the induction load coil used in the application.
Brief Summary of the Invention '
 In one aspect, the present invention is apparatus for, and a method of, providing a power supply with rectifier and inverter sections for use with an induction load coil wherein a tuning capacitor is provided across the output of the rectifier and the input of the inverter to form a resonant circuit with the induction load coil. The induction load coil may comprise an active load coil connected to the output of the inverter, and a passive load coil connected in parallel with a capacitor to form a tank circuit. Other aspects of the invention are set forth in this specification and the appended claims.
Brief Description of the Drawings
 For the purpose of illustrating the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.
 FIG. 1 is a schematic diagram of a prior art power supply with a full-bridge inverter that is used in induction heating and melting applications.
 FIG, 2 is a schematic diagram of one example of the power supply of the present invention for use in induction heating or melting applications.
 FIG. 3 is a waveform diagram illustrating the inverter's output voltage and current for one example of the power supply of the present invention.
 FIG. 4 is a waveform diagram illustrating the voltage across a tuning capacitor and the current through a line filtering reactor used in one example of the power supply of the present invention.
 FIG. 5 is a waveform diagram illustrating the voltage across, and current through, a switching device used in the inverter in one example of the power supply of the present invention.
 FIG. 6 is a schematic diagram of another example of the power supply of the present invention for use in induction heating or melting applications.
 FIG. 7 is a vector diagram illustrating the advantages of an induction heating or melting system with the power supply of the present invention used with the load coil system illustrated in FIG. 6.
Detailed Description of the Invention
 Referring to the drawings, wherein like numerals indicate like elements, there is shown in FIG. 2 an illustration of one example of power supply 10 of the present invention for use in induction heating or melting applications. Ac-to-dc rectifier and filter section 12 includes an ac-to-dc rectifier. A multi-phase rectifier, in this non-limiting example of the invention, a three-phase diode bridge rectifier 14 is used to convert three-phase (A, B, C) ac utility line power into dc power. Optional current limiting reactor Lg smoothes out the ripple from the output dc current of the rectifier. Section 16 of the power supply diagrammatically illustrates coil tuning capacitor Ci, which can be a single capacitor or a bank of interconnected capacitors that form a capacitive element.
 In FIG. 2, the dc output of the rectifier is supplied to input terminals 1 and 2 of a full-bridge inverter in inverter section 20. The inverter consists of solid state switches Si, S2, S3
and S4 and associated antiparallel diodes Dj, D2, D3 and D4, respectively. Alternating turn-on/turn-off cycles of switch pairs Sr/Sj and S2/S4 produce a synthesized ac inverter output at terminals 3 and 4. A preferred, but not limiting, choice of component for the solid state switch is an isolated gate bipolar transistor (IGBT), which exhibits the desirable characteristics of power bipolar transistors and power MOS-FETs at high operating voltages and currents. In one example of the invention, the inverter employs a phase-shifting scheme (pulse width control) relative to the turn-on/turn-off cycles of the two switch pairs whereby variable overlapping on-times for the two switch pairs is used to vary the effective RMS output voltage of the inverter.
 Induction load coil L9 represents the power coil used in the induction heating or melting apparatus. The capacitance of capacitor Ci is selected to form a resonant circuit with the impedance of load coil L9 at the operating frequency of the inverter, which is the switching rate of the switch pairs used in the inverter. Consequently, a tuning capacitor is not required at the output of the inverter. Selection of available circuit components may not allow operation exactly at resonance, but as close to resonance as is achievable with available components. The ac current flowing through induction load coil L9 from the output of the inverter magnetically couples with an electrically conductive material, which may be, for example, a conductive metal or a susceptor.
 FIG. 3 through FIG. 5 illustrate the performance characteristics for power supply 10 of the present invention as shown in FIG. 2 with input utility line power (A, B, C) of 480 volts line-to-line, 60 Hertz, and inverter 20 operating at an output frequency of 60 Hz. For this particular non-limiting example: Lg is selected as 5,000 \xli (for an impedance of 3.77 ohms at the rectifier ripple output frequency of 120 Hz); Cj is selected as 5,000 ^F (for an impedance of 0.27 ohms at the rectifier ripple output frequency of 120 Hz); and L9 is selected as 1,000 \xU (for an impedance of 0.38 ohms at the inverter output frequency of 60 Hz). Not shown in FIG. 2, but used in this sample analysis is a resistance of 0.16 ohms for induction load coil L9. Operating the C1/L9 circuit at resonance for the output frequency of inverter 20 results in a substantially sinusoidal inverter output voltage, Vout, and output current, Iout (at terminals 3 and 4), as graphically illustrated in FIG. 3. FIG. 4 graphically illustrates that the voltage across capacitor Ci, namely Vci, is driven to its limiting lower value of zero volts as a result of capacitor C\ being in resonance with coil L9 at the ripple frequency of 120 Hz. VCi is the applied voltage to the input of inverter 20 (at terminals 1 and 2). FIG. 4 also illustrates the ripple current, IL8, through reactor
Ls. The impedance of reactor L8 is generally selected to be much greater than the impedance of Ci to block feedback of harmonics from the inverter circuit to the rectifier's power source. FIG. 5 graphically illustrates the voltage, Vs, across one of the solid state switches in inverter 20, and the current, Is, through one of the switches at maximum power output when there is zero overlap angle between Vs and Is. Switching device turn-off at zero volts for Vs when dc ripple has reached zero (e.g., at 240.0 milliseconds (ms) in FIG. 4 and FIG. 5), will minimize switching loses. Additionally, since switching commutation occurs at zero voltage in this example, any spikes due to stray circuit inductance will be significantly less than in a conventional inverter having low ac ripple current in the dc link voltage. This specific example is provided to illustrate the practice of the invention, which is not limited to the specific elements and values used in this example.
 FIG. 6 illustrates a second example of the present invention. In this example, the load coil consists of an active coil L| and at least one passive coil L2. Coils Li and L2 may be wound in one of various configurations, such as sequentially or overlapped, to accomplish mutual magnetic coupling of the coils as further described below. Coil Li is connected to the output of inverter 20. Coil L2 is connected in parallel with resonant tuning capacitor C2 to form a parallel tank resonant circuit. Coil La is not physically connected to coil Li. The parallel tank resonant circuit is energized by magnetically coupling coil L2 with the magnetic field generated in coil Li when current supplied from the output of inverter 20 flows through coil Li.
 The benefit of separate active and passive coils can be further appreciated by the vector diagram shown in FIG. 7. In the figure, with respect to the active coil circuit, vector OV represents current l\ in active coil Li as illustrated FIG. 6. Vector OA represents the resistive component of the active coil's voltage, I1R1 (R| not shown in the figures). Vector AB represents the inductive component of the active coil's voltage, 01L1I1 (where 00 equals the product of 2n and f, the operating frequency of the power supply). Vector BC represents the voltage, a>MI2, induced by the passive coil L2 onto active coil L|. The half-wave ripple voltage VCi across capacitor Ci and the switching function of the two switch pairs S1/S3 and S2/S4 produce the effect of a pseudo capacitor Ci1 connected in series with Li that would result in a sinusoidal voltage at terminals 5 and 6 in FIG. 6. Vector CD represents the voltage, IJADCI1, that would appear across this pseudo series capacitor C\. Vector OD represents the output voltage, Vinv» of the inverter (terminals 3 and 4 in FIG. 6).
 With respect to the passive coil circuit, vector OW represents current I2 in passive coil L2 that is induced by the magnetic field produced by current I*. Vector OF represents the resistive component of the passive coil's voltage, I2R2 (R2 not shown in the figures). Vector FE represents the inductive component of the active coil's voltage, c*L2I2. Vector EG represents the voltage,
 The active coil circuit is driven by the voltage source, Vinv, which is the output of inverter 20, while the passive coil loop is not connected to an active energy source. Since the active and passive coils are mutually coupled, vector BC is added to vector OB, V'LOAD, which represents the voltage across an active induction load coil in the absence of a passive capacitive load coil circuit, to result in vector OC, VLOAD* which is the voltage across an active load coil with a passive capacitive load coil circuit of the present invention. The resultant load voltage, VLOAD, has a smaller lagging power factor angle,
|Indian Patent Application Number||464/CHENP/2004|
|PG Journal Number||12/2012|
|Date of Filing||04-Mar-2004|
|Name of Patentee||INDUCTOTHERM CORP|
|Applicant Address||10 INDEL AVENUE, RANCOCAS, NEW JERSEY 08073, USA|
|PCT International Classification Number||H02M5/458|
|PCT International Application Number||PCT/US02/25414|
|PCT International Filing date||2002-08-12|