US3536983A - Frequency multiplier and stirring circuit for an induction furnace - Google Patents

Description

Oct. 21,1970

r. R. KENNEDY 3,536,983 FREQUENCY MULTIPLIER AND STIRRING CIRCUIT FOR AN INDUCTION FURNACE Filed Dec. 12. 1967 47 II v #5 43 IL lr 55 f0 f4 60 I 5 2 f0. E E v y ,K K K\ "442 62 6 l/mwme THEODORE R KEV/V50) Arid/FIVE:

United States Patent M 3,536,983 FREQUENCY MULTIPLIER AND STIRRING CIR- CUIT FOR AN INDUCTION FURNACE Theodore R. Kennedy, Willingboro, N.J., assignor to Inductotherm Corporation, Rancocas, N.J., a corporation of New Jersey Filed Dec. 12, 1967, Ser. No. 689,831 Int. Cl. H02m 5/14, 5/16; H05b 5/00 US. Cl. 321-7 10 Claims ABSTRACT OF THE DISCLOSURE Frequency multiplier with stirring circuit for an induction furnace having three transformers with saturable cores connected in Y-open delta, the furnace coil connected to the open terminals of the delta connected secondary to apply single phase, triple frequency power to the coil, and the coil also connected to the junctions intermediate the transformer secondary winding to apply triple phase, fundamental frequency power to the coil for stirring.

This invention relates to a frequency multiplier with stirring circuit for an induction furnace. More particularly, this invention relates to a circuit for supplying both single phase, triple frequency power for heating and three phase, fundamental frequency power for stirring.

Induction furnaces (coreless, channel, or otherwise), inherently stir the metal which they are heating. This stirring action is the result of the magnetic forces created by the heating currents induced in the metal. This stirring action need not be further elucidated upon since it is well described in the literature. It is known that higher frequency furnaces have less stirring action basically because the current depth (penetration) is much lower. The reduction of the stirring action is the primary reason why inexpensive methods for developing high frequency power supplies for relatively small furnaces are developed. The shear mass of large furnaces (8000-120,000 pounds) permits the use of line frequencies (60 Hz.). For smaller furnaces (50016,000 pounds) higher frequency powers are used to minimize what would otherwise be a destructive stirring force.

Since line frequencies are 50 Hz. or 60 Hz. throughout almost the entire world, it is necessary to provide auxiliary equipment for increasing the power supply fre quency for induction furnaces. This auxiliary equipment normally takes the form of a motor-generator set or a static magnetic multiplier of which the latter is generally preferred because it costs less. Magnetic multipliers depend upon the saturation of non-linear magnetic cores as a means to develop harmonic magnetizing current. Triple and higher frequency current can be derived from three phase line frequency sources by properly connecting the windings on such saturable cores. These techniques are well known and need not be described in detail. It is sufficient to indicate that two general types of widely used systems include (1) a single phase transformer having one side of its primary connected to the common connection of three star connected saturable reactors or (2) a saturable core transformer having a star connected primary and a series connected secondary.

As stated above, the significant advantage of high frequencies is the reduction in stirring forces applied to the metal. This, however, does not mean that some stirring is unwanted or even undesirable. In fact, one of the significant advantages of an induction furnace is that it inherently stirs the metal therefore providing a uniform product. The disadvantage of too much stirring is that it erodes the crucible and may result in unwanted gaseous 3,536,983 Patented Oct. 27,, 1970 occlusions in certain metals such as steel. In other instances, such as vacuum degassing processes, stirring in combination with heating is desirable.

It therefore is apparent that the real goal is induction heating combined with controlled stirring. As indicated above, good stirring is provided by relatively low frequencies. On the other hand, good heating with reduced stirring effects is provided by high frequency power supplies. The present invention is concerned with a new and unobvious circuit for obtaining these seemingly contradictory goals while at the same time retaining the benefits of static, magnetic frequency multipliers. In accordance with the present invention, a 60 cycle controlled stirring current is derived from a static frequency multiplier power supply. In accordance with the present invention, little or no circuit modifications are required in the basic static frequency multiplier, and in addition, minimal auxiliary equipment is required for regulating the fundamental frequency stirring current supplied to the furnace coil.

As specifically disclosed herein, a static frequency multiplier of the star-series type describd above is used to provide high frequency heating currents to a furnace induction coil. Stirring currents are derived from the series connected secondary by taking line frequency current from connection points between the series connected coils of the secondary. This stirring current is multiphase and can be applied to tap points on the coil for controlling the stirring action therein. If desired, the connection between the secondary and the coil can include regulatory equipment such as controlled saturable reactors which will limit the stirring power.

For the purpose of illustrating the invention, there is shown in the drawing a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangement and instrumentalities shown.

The figure in the drawing shows a schematic diagram of a frequency multiplier with stirring circuit in accordance with the present invention.

Referring now to the drawing, wherein like numerals indicate like elements, there is shown a frequency multiplier with stirring circuit for an induction furnace designated generally as 10. In accordance with the present description, the induction furnace is connected to a three phase source by the multiplier circuit 10. The frequency multiplier circuit is a tripler circuit and is based upon the pronounced third harmonic component in the magnetizing current of a saturable core reactor connected to a three phase alternating current source. When the three phase alternating current source has a frequency of 60 Hz., the tripler circuit will deliver to the furnace coil alternating current power at a frequency of cycles per second. Those skilled in the art will recognize that other frequencies and other types of multiplier circuits may be used.

The drawing illustrates a frequency multiplier circuit for supplying alternating current to a load. In this case the load takes the form of an induction furnace coil 12. A single phase alternating current output having a frequency which in this case is three times the frequency of the alternating current from a polyphase source (not shown) is applied to the coil 12. The source is connected at the input or line terminals 14, 1-6 and 18. In general, the frequency multiplier system shown in the drawing includes a plurality of saturable transformers each having a primary winding and a secondary winding. A plurality of substantially linear reactors could be connected in series between the primary winding of an associated saturable transformer and one of the input terminals 14, 16 and 18. These linear reactors would stabilize the circuit. Alternatively and preferably, toroidal cores formed as described in U.S. Pat. 3,296,059 are used. These toroidal cores are designed to provide a sharp knee in the saturation curve while at the same time avoiding instability and the creation of harmonics in the transformer primary circuit. The secondary winding of the saturable transformers are connected in series relationship to load coil 12 to provide an alternating current having a frequency which is greater than that of the source current at the input terminals.

In particular, the saturable transformers 20, 22, and 24 include the primary windings 26, 28 and 30 respectively, and the secondary windings 32, 34 and 36, respectively. The primary windings 26, 28 and 30 are connected in star connection to a common point 38. Since this particular embodiment involves a three-phase source the common connection 38 may be referred to as a neutral point and the star connection may be referred to as a Y-connection.

The secondary windings 32, 34 and 36 of the transformers 20, 22 and 24 are connected in series circuit relation with one another. The series circuit is connected to output terminals 40 and 42 which in turn are connected to the input terminals of the coil 12. Since the present invention is described with reference to a three phase source, the series connection of the secondary windings 32, 34 and 36 may be referred to as an open-delta and the terminals 40 and 42 as the open terminal thereof.

Capacitors 44, 46 and 48 are connected across each of the three phases. Thus, capacitor 44 is connected across the line supplied by terminals 14 and 18. Capacitor 46 is connected across the line supplied by terminals 14 and 16. And capacitor 48 is connected across the line supplied by terminals 16 and 18. Capacitors 44, 46 and 48 as connected, will absorb a substantial portion of the nonsinusoidal voltage developed phase to phase between the input terminals 14, 16 and 18 and the primary windings 26, 28 and 30. The capacitors 44, 46 and 48, however, do no absorb the third harmonic and odd multiples thereof.

In general, the operation of the frequency multiplier circuit as thus far described provides a sinusoidal alternating current output from a three-phase or polyphase source of alternating current to the load coil 12, the frequency of the alternating current being three times as great as the frequency of the three-phase alternating current at the input terminals and being single phase rather than three phase.

The saturation of the cores 20, 22 and 23 results in the presentation of a third harmonic component in each of the three secondary coils 32, 34 and 36. The third harmonic components in each coil are in phase with each other, and therefore add to give a strong third harmonic voltage when the secondary 32, 34 and 36 are connected in series as here. The fundamental voltages and other harmonics appearing in each secondary are 120 out of phase with each other, and therefore add to zero when the secondary windings are connected in series.

As indicated just above, the fundamental frequency and all harmonics with the exception of third harmonics are 120 out of phase and cancel each other when the secondary coils 32, 34 and 36 are connected in series. This means that they will not appear across the output terminals 40 and 42. On the other hand, the fundamental and other harmonics do appear on each of the secondaries 32, 34 and 36 and can be measured. In accordance with the present invention, the fundamental or line frequency is taken off of each of the secondaries 32, 34 and 36 and used to control the stirring action of the metal contained within a crucible about which the coil 12 is wound in an induction furnace.

As shown, the junction between secondary winding 32 and secondary winding 34 is connected through the coil 52 of saturable reactor to an intermediate tap on the coil 12. In a like manner, the junction between secondary winding 34 and secondary winding 36 is connected through the coil 56 of saturable reactor 54 to a secondary intermediate tap on the coil 12. Accordingly, each of the secondary windings 32, 34 and 36 is connected to a portion of the furnace coil 12. Therefore, the fundamental or line frequency developed in each of the secondary windings is applied to the portion of the coil 12 to which it is connected. Since the line frequency is lower than the frequency applied to the coil 12 at terminals 40 and 42, it will have a much greater penetration into the metal and can be used to effectively stir it. The line frequency applied to the furnace coil 12 is of course three phase. The particular portion of the coil 12 to which it is applied so as to take best advantage of the stirring action can be determined by those skilled in the art using well known principles of magnetic stirring action.

Since the line or fundamental frequency applied to the coil 12 is to be used for stirring and not for heating, it is desirable to limit the power thereof. This is accomplished by the saturable reactors 50 and 54 which effectively chop the voltage of the applied line frequency so as to limit the power. Adjustment of the stirring attion is provided by applying direct current voltages E to the coils 58 and 60 which saturate the cores 50 and 54. Adjustment of the voltage E applied to coils 58 and 60 results in an adjustment of line frequency voltage applied to the coil 12 in accordance with well known principles of saturable reactors. The capacitors 62, 64 and 66 are connected between phases as shown and cooperate with the saturable reactors 50 and 54 to control the stirring voltages applied to the coil 12 by absorbing unwanted harmonics.

The power factor of the coil 12 may be controlled by the adjustable capacitance 68. During operation, the coil 12 is maintained at necessary power factor by the capacitance 68 to attain required multiple frequency.

In accordance with the foregoing, a circuit has been provided for simultaneously supplying high frequency and line frequency currents to an induction furnace for both heating and controlled stirring.

I claim:

1. A circuit for providing fundamental and multiple frequency power to a load, comprising a plurality of saturable reactors, one of said reactors for each phase of a multiphase source, a primary winding for each saturable reactor, a secondary winding magnetically coupled to each of said saturable reactors, said secondary windings being connected in series to obtain single phase electrical power at a harmonic frequency of the source frequency, and conductive means connected between said secondary winding and said load for obtaining multiphase electrical power at the source frequency.

2. A circuit in accordance with claim 1 wherein said primary saturable reactors are connected in star connection.

3. A circuit in accordance with claim 1 wherein capacitors are connected between each phase of the primary saturable reactors.

4. A circuit in accordance with claim 1 wherein a coil for an induction furnace is connected as the load to the open terminals of the series connected secondary windings whereby said harmonic frequency power may be applied to said coil, and said conductive means also being connected to said coil to apply electrical power at the source frequency.

5. A circuit in accordance with claim 1 wherein said conductive means includes controlled, saturable reactors.

6. A circuit for providing fundamental and triple frequency power to an inductive load comprising three primary saturable reactors connected in star connection, each of said primary saturable reactors being adapted to be connected to one phase of a three phase source, capacitors connected phase-to-phase between the primary saturable reactors for providing a low impedance path for harmonic currents, said primary saturable reactors having saturable cores for developing harmonic components of the fundamental frequency of the source, a secondary winding magnetically coupled to each of said saturable reactors, said secondary windings being connected in series to provide single phase electrical power at three times the fundamental frequency of the source, an inductive load connected to the open terminals of the series connected secondary windings whereby said triple frequency power is applied to said inductive load, and conductive means connected from junctions between said secondary windings to said load whereby three phase electrical power at the source frequency is applied to said coil.

7. A circuit in accordance with claim 6 wherein said inductive load is the coil for an induction furnace, where by triple frequency heating power is applied to said furnace coil and three phase source frequency stirring power is also applied to said furnace coil.

8. A circuit in accordance with claim 6 wherein said conductive means includes controlled, saturable reactors.

9. A circuit for providing fundamental and multiple frequency power to an inductive load comprising a plurality of primary saturable reactors connected in star connection, each of said primary saturable reactors being adapted to be connected to one phase of a multiphase source, capacitors connected in phase-to-phase between the primary saturable reactors for providing a low impedance path for harmonic currents, said primary saturable reactors having saturable cores for developing harmonic components of the fundamental frequency of the source, a secondary winding magnetically coupled to each of said saturable reactors, said secondary windings being connected in series to provide single phase electrical power at a multiple of the fundamental frequency of the source, an inductive load connected to the open terminals of the series connected secondary windings whereby said multiple frequency power is applied to the inductive load, and conductive means connected from junctions between said secondary windings to said load whereby multiple phase electrical power at the source frequency is applied to the load.

10. A circuit in accordance with claim 9 wherein said inductive load is the coil for an induction furnace.

References Cited UNITED STATES PATENTS 3,040,231 6/1962 Biringer 3217 3,099,784 7/1963 Forsha et al. 32l-7 3,188,550 6/1965 Hauck 321-68 3,382,311 5/1968 Rydinger et al. 13-26 3,414,659 12/1968 Kennedy 1326 J D MILLER, Primary Examiner G. GOLDBERG, Assistant Examiner U.S. C1. X.R.

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