CA1119658A - Electromagnetic casting apparatus and process - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An apparatus and process for casting metals wherein the molten metal is contained and formed into a desired shape by the application of an electromagnetic field. A control system is utilized to minimize variations in the gap between the molten metal and an inductor which applies the magnetic field. The gap or an electrical parameter related thereto is sensed and used to control the current to the inductor.

Description

This application is a divisional o~ Application Ser. No, 316,547, filed November 21, 1978.
BACKGROUND OF T~IE INVENTION
This invention relates to an improved process and apparatus for electromagnetically casting metals and alloys particularly copper and copper alloys. The electro-magnetic casting process has been known and used for many years for continuously and semi-continuously casting metals and alloys. The process has been employed commer-cially for casting aluminum and aluminum alloys.
When one attempts to employ the electromagnetic casting process for casting heavier metals than aluminum such as copper, copper alloys, steel, steel alloys, nickel, nickel alloys, etc. various problems arise in controlling the casting process. In the electromagnetic casting process the molten metal head is contained and held away ~rom the mold walls oy an electromagnetic pressure which '.

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counterbalances the hydrostatic pressure of the molten metal head. The hydrostatic pressure of the molten metal head is a function of the molten metal head height and the specific gravity of the molten metal.
When casting aluminum and aluminum alloys using the electromagnetlc casting method, the molten metal head has a comparatively low density with a high surface tenslon due to the oxlde film it forms. The surface tension is additive to the electromagnetic pressure and both act against the hydrostatic pressure of the molten metal head~ A small fluctuation in the molten metal head therefore gives rise to a small difference in the magnetic pressure required for containment. For heav~er metals and alloys such as copper and copper alloys, comparable changes in the molten metal hea~ cause a greater change in hydrostatic pressure and in the reauired offsetting magnetic pressure. It has been found ~or copper and copper alloys that the change in magnetic pressure required for containment is approximately three times greater than for alu~.inum and aluminum alloys with comparable changes in molten metal head.
In order to obtain an ingot o~ uniform cross section over its full length the periphery of the ingot and molten metal head within the lnductor must remain vertical especlally near the liquid solid interface of the solidifying ingot shell. The actual location of the periphery of the ingot is a~ected by the plane over which the hydrostatic and magnetic pressures balance. Therefore, any variations ln the absolute molten metal head height cause comparable variations in hydrostatic pressure which produce surface . .

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g~;s8 undulations along the length of the ingot. Those surface undulations are very undeslrable and can cause reduced metal recovery durlng further processing.
It is apparent from the foregoing discussion that when one attempts to electro~agnetically cast such heavy metals a~d ; alloys a greater degree of control is required to obtain the desired surface shape and condition in the resulting casting.
In U.S. Patent No. 4,014,379 to Getselev a control system is described for con~rolling the current flowing through the inductor responsive to deviations in the dimensions of the liquid zone (molten metal head) of the ingot from a prescribed ~alue. In Getselev '379 the lnductor voltage is controlled to regulate the inductor current in response to measured variations ln the level of the surface of the liquid zone of the ingot. Control of the inductor voltage is achie~ed by an amplified error signal applied to the ~ield winding of a frequency changer.
A drawback of the control system described in Getselev '379 is that only changes in the molten metal head due to fluctuation of the level of the surface of ~he liquid zcne are ta~en into account. It appears that Getselev '379 has assumed that the location of the solidification front between the molten metal and the solidifying ingot shell is fixed with respect to the inductor. This is not believed to be the case in practice. Factors which tend to cause fluctuation in the ~ertical location of the solidification front lnclude ~ariation~ in casting speed, metal super heat, cooling water flow rate, cooling water application position, coollng water temperature and quality (impurity content) and inductor current amplltude and ~requency.

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Aluminum and aluminum alloys possess a narrow range of electrical resistivity. Therefore, in the electromagnetic casting process the depth to which eddy currents are generated in the molten metal head and solidifying ingot i5 comparatlvely uniform over a wlde range of aluminum alloys.
The depth of penetration of the electromagnetic lnduced current is a function of resistivity of the load and the frequency.
For copper and copper alloys as well as for other heavy metals and alloys there is a wide range of resistivity over the range of different alloys. There~ore, the range of f penetration of the induced current at a constant frequency fo~ such alloys is also comparatively wide as compared to aluminum. This is disadvantageous because the degree of magnetic stirring of the molten metal is a function of the penetration depth of the induced current.
For such hea~y metals and alloys in changing from one alloy to another the operating frequency must be changed to obtain the desired penetration depth for the induced ~0 current. For example, for Alloy C 510 00 the induced penetration depth would be expected to be about 10 mm at 1 kHzj 5 mm at 11 kXz and 3 mm at 10 kHz. The penetration depth commonly used in electromagnetic casting of aluminu~
alloys is about 5 mm. As compared to Alloy C 510 00, pure copper achieves a 5 mm penetration depth at 2 kHz, hal~ the frequency at which Alloy C 510 00 achieves that penetration depth. Therefore, the control system for the electromagnetic casting of metals such as copper and copper alloys must be capable of operating at a variety o~ frequencies in order to obtain the approprlate induced current penetration depth.

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l9~S~3 It is known in the art to utilize high frequency power supply equipment u~lng solid state static inverters in place I o~ motor generator sets. A particular advantage of such ,. solid state ln~erters is that the equlpment is operable j over a ~ide ~requency range.
¦ . The present inYention o~ercomes the de~lciencies described above and provides an accurate means for controllin~
the electromagnetic casting apparatus to allow casting o~
ingots o~ copper and copper base alloys and the like wlth uni~orm transverse dime.nsions o~er their length.

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SUMMARY OF THE INVENTION
Thi~ invention relates to a process and apparatus for casting metals wherein the molten metal is contained and ~ormed into a desired shape by the application o~ an ~ electromagnetic field. In particular, an inductor is used ¦ to apply a magnetic ~ield to the molten metal. The ~ield itself is created by applying an alternating current to the inductor. In operatlon, the inductor is spaced from the molten metal by a gap which extends from the surface of the molten metal to the opposing sur~ace of the inductor.
In accordance with this invention an lmproved process , and apparatus is provided wherein a control system is - utili~ed to minimize variations in the gap during operation of the casting apparatus. ~he control system includes a control circult which is connected to the power supply which applies the alternating current to the inductor. The control circuit includes circuit means for sensing variations in the gap and means responsive thereto for controlling the magnitude of the current applied to the inductor so as to minimize the gap variation.
In accordance with a preferred embodiment an electrical parameter of the inductor is measured. The particular electrical parameter which ls selected for measurement is one such as :reactance or inductance which varies wlth the magn~tude of the gap. Means are provided which are responsi~e to the measuring means ~or generating an error signal the magnitude of which is a function of the d1fference between the value of the measured electrical parameter and a predetermined value thereof. In response to the error signal, means are provlded ~or controlling the current _ applied to the inductor in a manner so as to drive the error signal towards zero, In another preferred embodiment the apparatus includes means ror sensing the magnitude of the gap and means responsive thereto for generating an error signal the magni-tude of which is a function of tlle difference between the sensed gap magnitude and a predetermined gap magnitude, In response to the error signal, means are provided for control-ling the current applied to the inductor so as to return the gap to the predetermined magnitude, The process and apparatus of this invention can be carried out using either analog or digital circuitry or com-binàtions thereof, Accordingly, it is an object of this invention to provide an improved process and apparatus for electro-magnetically casting metals and alloys.
It is a further object of this invention to provide a process and apparatus as above wherein shape perturbations in the surface of the resultant casting are minimized, It is a still further object of this invention to provide a process and apparatus as above wherein the gap between the molten metal and the inductor is sensed electric-ally and the current applied to the inductor is controlled in response thereto, - .

~9~iS8 In accordance with a particular en~odiment of the invention there is provided an apparatus for treating castable materials comprising: induction means for apply-ing a magnetic field to said material and means for apply-ing an alternating current to said induction means to gener-ate said magnetic field; the improvement wherein said apparatus further includes:- means for sensing the voltage applied to said induction means 90~ out of phase to the current ap~lied to said induction means and for generating a phase sensitive vol-tage signal corresponding thereto, means for sensing the current applied to said induction means-for generating a voltage signal corresponding thereto, and divider circuit means for dividing said phase sensitive voltage signal by said voltage signal corres-ponding to said current for generating a signal correspond-ing about to the reactance of said induction means, These and other objects will become more apparent from the following description nd drawings.

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~9~S8 BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schemattc representatlon of an electro-magnetic casting apparatus ln accordance wlth the present invention;
Figure 2 is a block diagram o~ a control system in accordance with one embodiment of this invention;
Figure 3 is a block diag:ram o~ a control system in accordance with another embodiment of this invention; and Flgure 4 ls a block diagram of a control system in accordance with a dlfferent embodiment of this invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
' Referring now to Figure 1 there is shown by way of example an electromagnekic casting apparatus of this invention.
The electromagnetic casting mold 10 is comprised of an inductor 11 which is water cooled; a coolin~ mani~old 12 for applying cooling water to the peripheral surface 1~ o~ the metal being cast C; and a non-mag~etic screen 14. Molten metal is continuously introduced lnto the mold 10 during a ; 20 casting run~ in the normal manner using a trough 15 and down spout 16 and conventional molten metal head control. The - inductor 11 is excited by an alternating current from a power source 17 and control system 18 in accordance with thls invention.
The alternating current in the inductor 11 produces a magnetic field whlch interacts with the molten metal head 19 to produce eddy currents therein. These eddy currents in turn interact wlth the magnetic ~ield and produce forces which apply a magnetic pressure to the molten metal head 19 to contain it so that it solidifies in a desired ingot -cross section.

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An air gap d exists during casting, between the molten metal head 19 and the inductor 11. The molten metal head 19 is formed or molded into the same gene-ral shape as the inductor 11 thereby providing the desired ingot cross section.
The inductor may have any deslred shape lncluding clrcular or rectangular as required to obtain the desired ingot C cross section.
The purpose of the non-magnetic screen 14 is to flne tune and balance the magnetic pressure with the hydrostatlc 1~ pressure of the mol~en metal head 19. The non-magnetic screen 14 may compr~se a separate elemenk as shown or may, if ' i desired be incorporated as a unitary part of the manifold ~or applying the coolant.
Initially, a conventional ram 21 and bottom block 22 is held in the magnetic containment zone of the mold 10 to allow the molten metal to be poured into the mold at the start o~ the casting run. The ram 21 and bottom block 22 are then uniformly withdrawn at a desired cas~ing rate.
Solidification of the molten metal which is magnetically contained in the mold 10 is achieved by direct application of water from the cooling manifold 12 to the lngot surface 13. In the embodiment which is shown in Figure 1 the water is applied to the ingot surface 13 within the confines of the inductor Il. The water may be applied to the ingot surface 13 above, within or below the inductor 11 as desired.
If desi~ed any of the prlor art mol~ constructions or other known arrangements of the electromagnetic casting apparatus as described in the Background of the Invention could be employed.

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The present lnvention is concerned with the control of `the casting process and apparatus 10 in order to provide cast ingots~ which have a substantially uniform cross section over the length o~ the ingot and which are formed of metals and alloys such as copper and copper base alloys. This is accompli~hed in accordance wlth the present invention by sensing the electrical properties of' the inductor ll which are a function of the gap '7d" between the inductor and the load, which is the ingot C and molten metal head 19.
It has been found in accordance with this invention that the inductance of the inductor ll during operation is ., a function of the gap '1d". The following equation is an e~pression of the relationship which is believed to exist between the inductance of the inductor and the gap spacing:
Li = kd(2DC-d) (1) where:
Li = inductance of the inductor;
Dc = the inductor diameter;
d = the inductor-ingot separation (air gap);
; ~0 k = a f'actor taking into account the geometrical parameters of the system including the level of the surface 23 of the molten metal head 19; the level of the solldification front 24 with respect to the inductor ll; the electrlcal conductivlty '.
' of the metal belng cast; and the current i f'requency.
"k" is d.etermined empirically by measuring the inductance for a known i.nductor diameter and inductor ingot separation and solving f'or "k" in equation {l)~ The factor "k" does not vary wlth gap spacing "d". 1'k" varles only slightly with --lQ-9004-~B

the height "h" of the molten metal head so long as the metal surface 23 ls maintained in the viclnity of the top of the inductor 11.
Therefore, it is apparent that the inductance of the inductor-ingot system is a function of the gap spacing "d"~
The lnductance is related to t~le reactance of the inductor-ingot system by the equation:
X1 = 2~ f Li (2) where:
Xi = inductive reactance (ohms)g Li = inductance (henrys);
j f = frequency (hertz).
The air gap "d" between the inductor 11 and the metal load 19 imposes the reactive load Xi on the electrical power supply feeding the inductor. The macnitude of th~s induc~ive reackance "Xi" is a function of the current frequency "f", the size of the air gap "d", the inductor turns and the inductor height. Both the reactance '~Xi" and the inductance 'l~i are relatively independent of the alloy being cast as co~p2red to resistance o The combination of the inductor 11 and the metal load 19 which it surrounds imposes a resistive load as well on the electrical power supply feeding the lnductor. The magnitude of the resistive load is a function of the geometry (size) of the inductor 11 and the metal load 19 and the resistivities of both. The combination of the resistive and reactlve loads described above results in a total impedance ~Zili through which the containment current "I"
must pas~. This total impedance is deflned in ohms as:
-Zl~J ~Ri~ +(2~f Li)2 (3) where: Zl ~ i~pedance (ohms); Ri = resistance (ohms);
f = frequency (hertz) and Li = inductance (henrys).

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~ ariation in load cross section namely the cross sectlon of the molten metal head 19 will result in changes in the electrical loading of the inductor 11. I~ a constant voltage is applied across the inductor 11 as in Getselev '379, the containment process balances the hydrostatic pressure of the molten metal head 19 and the magnetic pressure of the electromagnetic forces to provide inherent control character-istics. Accordingly, an increase in molten metal head will tend to overcome the magnetic pressure and result in a larger ingot section. This in turn will reduce the gap "d" or ingot-inductor separation and thereby lower the impedance Zi~ and inductance "L1" of the system. Getselev '379 suggests this effect is based on a change in resistance associated with the increasing size of the ingot. However, it is believed that impedance rather than resistance is the controlling property. The inductor current amplitude ' and, hence, the induced current amplitude is increased thereby in accordance with the equation:

Ii = Vi (4) ~0 where:
Ii ~ the current, Vi = the ~oltage; and Zi = the impedance;
so that the in~ot reverts to its original size.
Inasmuch as thls is a dynamic process, shape perturbations or undulations will be formed in the resultant ingot surface 13. It is anticipated that such perturbations would occur i.n characteristic time periods on the order of a second. In order to counteract these effects by electrical c;S~

control means the response rate of the power supply 17 and control system 18 should be considerably more rapid. Ac-cordingly, a response tlme of 100 milliseconds or less is desirable.
As described above, induclance or reactance o~ the loaded lnductor 11 are ~unctions o~ the gap size "d'l. In the prior art approach of the Getselev '379 patent a constant voltage is maintained across the inductor and a corrective voltage responsive to the height of the surface of the molten metal head ls employed to control the inductor current. In contrast thereto, in accordance with the present inven~ion, an electrical property of the casting apparatus 10 which is a ~unction o~ the gap "d" between the molten metal head 19 and interior surface and the inductor 11 is sensed and a signal representative thereo~ is generated.
Responsive to the gap signal the power supply 17 output is controlled to provide an appropriate frequency, voltage and current so as to maintain the gap "d" substantially constant.
It ls the current applied to the inductor 11 which is '0 t~e prlncipal factor in generating the electromagnetic pressure. That current is a function of the applied vol~age and the impedance of the loaded inductor which in turn is a function of frequency and inductance. It is possible in accordance wi.th the present invention to control the applied current by acl~ustment of the voltage output o~ the power supply 17 at a constant ~requency or by ad~ustment of the frequency o~ the power supply 17 at a constant voltage or by ad~ustment of the ~requency and voltage in combination.

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~ 9 ~8 Referring now to Figures 1 and 2 there is shown by way o~ example a control circult 18 for controlling the power supply 17 of the electromagnetic casting apparatus 10. The purpose of the control circult is to insure that the gap lldlt is maintained substantially constant so that only minor variations3 if any, occur therein. By minimizing any variation in the gap "d" shape per~urbations in the surface 13 of the casting C will be minimi~ed.
The inductor 11 is connected to an-electrical power supply 17 which provides the necessary current at a desired frequency and voltage. A typical power supply circuit may be ~, considered as two subcircuits 25 and 26. An external circuit

2~ consists essentially of a solid state generator providing an electrical potential across the load or tank circuit 26 which includes the inductor 11. This latter circu~t 26 except ~or the ~nductor 11 is sometimes referred to as a heat s~ation and includes elements such as capacitors and trans~ormers.
In accordance with this in~ention the generator circuit `O 2~ is preferably a solid state inverter. A solid state inverter is preferred because it is possible to provide a selectable ~requency output over a range of frequencies.
This in turn makes it possible to control the penetration depth of the current in the load as described above. ~oth the solid stat;e inverter 25 and the tank circuit 26 or heat station may be of a conventional design. The power supply 17 is providel~ with front end DC voltage control in order to separate the ~oltage and frequency functions o~ the supply.
In accordance with the present inventlon changes in electrical parameters of the inductor-ingo~ system are - , ~ . . . . .

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sensed ln order to sense changes in the gap "d". Any desired parameters or e~ignals which are a function of the gap "d"
could be sensed. Preferably, in accordance with this invention the reactance of the lnductor 11 and its load is used as a controlling parameter and most preferably the inductance of the inductor ancL its load is used. Both o~
these parameters are a functic)n of the gap between ~he inductor 11 and the load 19. However, if desired, other parameters whlch are affected by the gap could be used such as impedance and power. Impedance is a less desirable parameter because it is also a function of the resistive ( load which changes with the diameter of the load (ingot) in a generally complex ~ashion.
The reac~ance of the inductor 11 znd load 19 may be sensed as in Figure 2 by measuring the voltage across the inductor 11 90 out of phase to the current and dividing that signal by the current measured in the inductor. For a fixed frequency mode of operation ~he reactance will be directly proportional to the inductance, as in equation (2) above.
Therefore, for a fixed ~requency mode the measured reactance is ~ function of the gap '~d" in accordance with equation (1) above. If the frequency is not fixed during operation, then it is preferably to determine the inductance of the inductor 11 and its load 19 which can be done by dividing the reactance by a factor comprising 2 ~ ~.
Re~erring again to Figure 2, the control circuit 18 described therein is principally applicable to an arrangement wherein the frequency of the power supply 17 during operation is maintained fixed at some preselected frequency.
Therefore, with this control circuit 18 it is only necessary 9004 I~B

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to measure a change in the reactance of the inductor 11 and load lg to obtain a signal indlcative of a change in gap "d".
The output waveform of solid state power sources 17 contains harmonics. The amplitude of these harmonics relative to the fundamental frequency will depend on a large number of factors, such as ingot type and diameter, and the characterlstics of power-handling components in the power source (e.g. the impedance matching transformer). The intended in-process electrical parameter measurement preferably should be done at the fundamental frequency so as to eliminate errors due to harmonics admixture.
! A current transformer 27 senses the current in inductor 11. A current-to-voltage scaling resistor network 29 generates a corresponding voltage~ This voltage is fed to a phase-locked loop circuit 30 which "locks" on to the fundamental of the current waveform and generates two sinusoidal phase reference outputs, with phase angles o 0 and 90 with respect to the current fundamental. Using the 0 phase reference, phase-sensitive rectifier 31 derives the _0 fundamental frequency current amplitude. The 90 phase reference is applied to phase-sensitive rectifier 28 which derives the fundamental voltage amplitude due to inductive reactance. The voltage si~nals from 28 and 31 which are properly scaled are then fed to an analog voltage divider 32 whPrein the voltage from rectifier 28 is divided by the voltage from rectifier 31 to obtain an output signal which is proportional to the reactance of the inductor 11 and load lg. The output si~nal of the di~ider 32 is applied to the inverting input of a differential amplifier 33 operating in a llnear mode. The non-inverting input of ~he amplifier 33 ~. ~

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is connected to an ad~ustable voltage source 34. The output of ampllfier 33 is fed to an error signal amplifler 35 to provide a voltage error signal which is applied to the power supply e~ternal circuit 25 in order to provlde a ~eedback control thereof. Amplifier 35 pre~erably also contains frequency compensatlon circuits for ad~usting the dynamic behavior of the overall feedback loop.
The error signal from the differential amplifier 33 is proportional to the variation in the reactance of the inductor 11 and load 19 and also corresponds in sense or polarity to the direction of the variation in the reactance. The ad~ustable voltage source provides a means for ad~usting the gap '~d" to a desired set point. The ~eedback control system 18 provides a means for driving the variation in the gap "d"
to a minimum value or zero. The control system 18 described by reference to Figure 2 ls principally applicable in a mode of operation wherein the ~requency once set is held constant though it is not necessarily limited to that mode of operation particularly for small changes in frequency.
~o Filtering circuits other than a phase-locked loop circuit 30 may be used to extract the fundamental frequency component. For example, both current and voltage waveforms can be examined at 0 and 90 with respect to an arbitrary phase reference, such as may be extracted from the inverter drive circuitry of the power supply 17. These in-phase (0) -and quadrature components (90) can then be combined vectori-ally to yield voltages proportional to the fundamental frequenc~ and current through the inductor 11.
The circuit of Figure 2 could be modified as in Figure

3 wherein like circult elements ha~e the same reference -17~
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numerals as in Figure 2 and operate in the same manner. In i the circuit I8' of Figure 3 the frequency of the current applled to the inductor ll is sensed and a voltage signal proportionate thereto is generated by a ~requency to voltage con~erter 36 connected to the output of the current to voltage scaling circuit 29. The output of the converter 36 is properly scaled to the output of the divlder 32 by scaling circuit 37. A second analog voltage divider 38 is provided 1 for dividing khe c,utput of the first voltage divlder 32 by - lO the proportionate voltage from the ~requency to voltage converter 36. The output signal of the ~econd divlder 38 approximates the inductance of the inductor ll and load l9 and thereby allows the control system 18' to operate even in a variable Lrequency mode of operation.
The approaches to the control systems 18 and l81 of this invention ~hich have been described thus far have employed analog type circuitry. If desired, however, in accordance wit~. this invention even greater flexibility of control can be accomplished by utilizing a digltal control 0 system 18" as exemplified by the block circuit diagram of Figure 4. ~he power supply 17 including the external circuit _ and tank circuit 26 are essentially the same as described by reference to Figures 2 and 3.
In this embodiment, a dif~erential amplifier 39 is utilized to sense the voltage across the inductor ll. A
current transformer 27 ls utilized to sense the current ln the lnductor ll. The output of the differential amplifier i is fed to a ~'ilter circuit F for extracting the fundamental frequency. The output o~ filter F is fed to a frequency~
voltage converter 40. The output signal of the frequency/

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~, voltage converter 40 comprises a signal "~" proportionate to the frequency of the applled current. The output of the dif~erential ampll~ier 39 is a:Lso applied as one input to an AC power meter 41. The other lnput thereto comprlses the current signal sensed by the current trans~ormer 27 as flltered by ~llter circuit F' which extracts the fundamental , frequency. The AC power meter 41 provides output signals i proportional to the RMS voltage "V", the RMS current "I" and the true power "kW" applied to the lnductor 11.
The frequency output signal "f" from the converter 40 and the voltage "V" current "I" and power "kW" signals from the AC power meter 41 are fed to an analog to digital converter 42 which converts them into an appropriate digital ~orm. The output of the analog to digital converter is ~ed to a computer 43 such as a mini-computer or microprocessor as, ~or example, a PDP-8 with Dec Pack manufactured by Digital Eauipment, Inc. The computer 43 is programmed to use the valueso~ ~requency "f", voltage "V", current "I"
and power "kW" which are fed to it to compute the respective 0 values of apparent power "kVA", phase angle '~a ", impedance "Z", reactance "X", and inductance ~'L". The computer can be programmed to calculate these parameters using the following relationships: kVA = ~-I, a = COS~l lkW ~, Z = V~I, X - Z
~kVA
sin a and L = X/(2 ~ ~). Each o~ the a~orenoted relationships is well kno~l and allows the computation o~ the inductance o~
the inductor--load in operation. -A~ter calculating the ~ inductance the computer 43 then calculates the gap "dc"
¦ using formula (1) above. The computer 43 then compares the calculated gap ~dc~ to a predetermined gap setting "d" in its memory and gener~tes a preprogr~mmed error signal correspondin~ to the difference between "d" and "dc'~. The error slgnal is then fed to a digital to analog converter 44 i to con~ert the error signal into analog form. One output j signal of the digital to analog converter 44 is applied to I a voltage controller 45 and another output s~gnal thereo~
ls applied to a frequency controller 46. The outputs of ! the voltage 45 and frequency 46 controllers are each respectively tied to the power supply l7 to feedback to the power supply the error signals for ad~usting the current in the inductor to compensate for the gap varlation so as to ~ drive the ~ariation toward zero.

i ~ The control 5ystem 18" which has ~ust been described - can be operated in any of three modes o~ operation. It can operate in a fixed ~requency mode wherein only the voltage is changed to adjust the current applied to the inductor ll. In this mode of operation the frequency controller 46 would be rendered inoperative and it is posslble to compute a correction or error signal from the computed value of reactance "X" rather than having to compute the induc~ance "L" slnce they would be directly proportional.
The control system 18" of Figure 4 can also be operated in a fixed voltage mode wherein only frequency is varied in order to control the inductor ll current. In this mode of operation the voltage controller 45 would be rendered inoperative and only the frequency controller would apply an error signal to the power supply. Finally, digital operation as exempli~ied in Flgure 4 is amenable to varylng both the frequency and voltage in order to control the inductor ll current. In ~hls mode, both the voltage 45 and ~requency 46 controllers would be operative.

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9004-~3 s~ ' While the operation of the control system 18" of Figure

4 has been de~cribed by reference to compari~on of a sensed gap magnltude to a predetermined gap magnitude ~or generatlng i an error signal, it could also be operated ln a fashion I similar to that described by reference to Flgures 2 and 3.
i For example~ instead of computing the sensed gap magnltude lt could merely compute sensed reactance or inductance in accordance with the abo~e equations and compare the computed value of reactance or induc~ance to some preprogrammed preset ~alue thereo~ and generate a preprogrammed error signal in response to the ~ariation from the preset value.
This approach would advantageously reauire less computation . than ~he approach wherein the sensed gap magnitude is calculated.
The control circuit 18" described by reference to Figure 4 is desirable because of the very high speed with which the computations and correction signals can be generated by the computer ~.3 and the high degree of sensitivity and flexibility associated with the use of digital circuitry and computer programming.
Whlle a phase-locked loop clrcuit is preferred for use as a filter 303 F and F', to extract the fundamental frequency of the sensed slgnal, any desired filtering circuit could be used for that purpose.
The apparatus 10 o~ this invention can be utilized without the need to sense the top surface 23 of the liquid metal head 19. This is the case because the parameters which are used are functions of the gap spaced "d" and are not greatly affected by the height 'th" of the molten metal head 19. If deslred, however, for the purpose of fine .- :

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~ 58 tuning the apparatus 10 the upper surface 23 o~ the molten metal head 19 can be sensed in the same manner as in the Getsele~ '379 patent to generat;e a signal responslve to the height thereof, as by the use of a linear transducer 47 such as Model 350 manufactured by Trans-Tek, Inc. The output of the transducer 47 is then applied to the analog to digital converter 42 whlch converts the analog signal to a digital one. The digital molten metal head height signal is then compared by the computer 43 to a desired set value preprogrammed therein and an error signal corresponding to any difference therebetween is generated by the computer.
_. The computer 43 then combines its error signal due to gap varlation and its error signal due to head height variation and generates an appropriate combined error signal whlch is applied to control the power supply 17 in the same manner as described above.
While the load has been described above as an ingot, it could comprise any desired type o~ continuously or semi-continuously cast shape such as rods, bars, etc.
~0 Where the term inductor diameter has been employed in this application an effective induc~or diameter can be substitu~ed therefor fo~ non-circular inductors 11. The e~fective inductor diameter is computed by measuring the area defined by the inductor 11 and then computing its e~fective diameter ~rom that measured area as if it were for a circular inductor.
While the invention has been described by reference to copper and copper base alloys it is believed that the apparatus and process described above can be applied to a wide range of metals and alloys includlng nickel and nic~el -9~58 alloys, steel and steel alloys, aluminum and al~ninum alloys, etc.
The programming of the computer 43 and its memory can be carried out in a conventional manner and, therefore, such programming does not form a part of the invention herein~
While the control circuitry 18, 18', 18'~ has been described by specific reference to its application in an electromagnetic casting apparatus it is believed to have application in part or in whole to other kinds of metal treatment apparatuses wherein inductors are used to apply a magnetic field to a metal load. In particular, the circuitry for sensing the inductance in the inductor could have application, for example, in induction furnaces. -It is apparent that there has been provided in accordance with this invention an electromagnetic casting apparatus and process which fully satisfies the o~jects, means and advantages set forth hereinbefore. While the ~ -invention has been described in combination with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description.
Accordingly, it is intended to embrance all such alter-natives, modifications and variations as fall within the spirit and broad scope of the appended claims.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. In an apparatus for treating castable materials comprising:
induction means for applying a magnetic field to said material and means for applying an alternating current to said induction means to generate said magnetic field, the improvement wherein said apparatus further includes:
means for sensing the voltage applied to said in-duction means 90° out of phase to the current applied to said induction means and for generating a phase sensitive voltage signal corresponding thereto;
means for sensing the current applied to said in-duction means for generating a voltage signal corresponding thereto; and divider circuit means for dividing said phase sen-sitive voltage signal by said voltage signal corresponding to said current for generating a signal corresponding about to the reactance of said induction means.

2. In an apparatus as in claim 1 wherein said means for generating said phase reference signal with a phase angle of 90° comprises a phase-locked loop circuit.

3. In an apparatus as in claim 1 wherein means are pro-vided for sensing the frequency of said current applied to said induction means for providing a frequency signal corres-ponding thereto and means are provided for dividing said reactance signal by said frequency signal to generate a signal corresponding about to the inductance of said induction means.

4. In an apparatus as in claim 1, feedback control means receiving said reactance or inductance signal for con-trolling the output of said means for applying said alter-nating current.

5. In an apparatus as in claim 1, feedback control means receiving said reactance signal for controlling the output of said means for applying said alternating current.

6. In an apparatus as in claim 3, feedback control means receiving said inductance signal for controlling the output of said means for applying said alternating current.

7. In an apparatus as in claim 3 wherein said means for sensing said voltage 90° out of phase to said current comprises means for generating a phase reference signal with a phase angle of 90°, means for sensing the voltage applied to said induction means and means receiving said phase reference signal and said voltage signal for providing said phase sensitive voltage signal.

8. In an apparatus as in claim 1 wherein said means for generating said phase reference signal with a phase angle of 90° comprises a phase-locked loop circuit.