EP0294913A2

EP0294913A2 - Polyphase power supply for continuous levitation casting

Info

Publication number: EP0294913A2
Application number: EP88301503A
Authority: EP
Inventors: Paul C. Boehm; Richard A. Ranlof
Original assignee: Inductotherm Corp
Current assignee: Inductotherm Corp
Priority date: 1987-06-12
Filing date: 1988-02-23
Publication date: 1988-12-14
Also published as: EP0294913A3; JPS63317238A

Abstract

An apparatus (50) for supplying polyphase power to an induction coil (62) for magnetically levitating metal to be cast comprises an input (52) for connecting the apparatus to a source of polyphase AC power and a polyphase rectifier (54) for rectifying the AC power. A first controller (56) selectably varies the magnitude of RMS current supplied to the induction coil by controlling the electrical phasing of the rectifier. A polyphase inverter (58) operatively associated with the rectifier converts the rectified AC power to polyphase AC power and supplies the polyphase AC power to the induction coil at a preselected frequency. A second controller (60), independent of the first controller, controls the frequency of the AC power supplied to the induction coil by controlling the electrical phasing of the inverter. The induction coil has a plurality of sections (0̸1 to-Ø3), each section being wound to provide a phase rotation of a magnetic force vector over substantially the entire length of the coil to produce a continuous magnetic levitation force.

Description

Field of the Invention

The invention relates generally to the metal melting and casting art, and particularly to apparatus for supplying polyphase power to an induction coil for magnetically levitating metal for continuous casting of metal articles.

Background of the Invention

Levitation casting of continuous metal articles is a relatively new and unique continuous casting technique which uses an electromagnetic levitation field to support and contain a column of solidifying metal. By counteracting gravitation forces and hydrostatic pressure on the metal column, levitation casting completely eliminates friction and adhesion at the interface between the solidifying metal and the cooled walls of the heat exchanger. Levitation casting inherently provides high casting speed and excellent dimensional control combined with smooth continuous emergence of solidified product from the top of the casting chamber.
In addition, levitation casting causes intense electromagnetic stirring of the liquid metal, before and during solidification, which results in a homogeneous, equiaxed cast product suitable for immediate drawing or other forming operations without hot rolling or other expensive additional processing. Levitation casting is particularly well-suited for economic continuous casting of rod and other shapes from a variety of pure metals and alloys.
Levitation casting is known. In levitation casting, metal is cast in continous lengths by moving a liquid metal column to and through a forming zone in which it is progressively cooled and solidified while being subject to an electromagnetic field which reduces the force required to remove the resulting cast product from the forming zone. This effect of the electromagnetic field is accomplished by levitating and by maintaining the molten metal column out of continuous pressure contact with the walls of any containing vessel throughout the greater part of its length and particularly in that portion of it in the region where solidification occurs. Levitation is accomplished by means of upwardly travelling electromagnetic waves applied to the column so that a major portion of the column length is maintained out of continuous pressure contact and hence is essentially weightless throughout the casting operation. The levitating and maintaining effects are employed simultaneously so that a continuous column of molten metal is established and maintained essentially weightless and out of contact with physical mold structures throughout the major portion of its length.
The electromagnetic field which provides the levitating force is generated by an induction coil. The levitating force on the metal being cast depends on both the magnitude of the current induced in the metal and the frequency of the induction field. The levitating force required is a function of the mass of metal to be levitated (which is a function of the diameter of the rod being cast) and the resistivity of the metal (which is a function of the particular metal being cast). Therefore, it is necessary to control the frequency of the field and the amplitude of the induced current in the metal being cast so that the right amount of levitating force can be generated by the induction coil. It is desirable to be able to independently adjust the frequency and current of the induction coil to compensate for the mass and resistivity of the molten metal.
The present invention provides a polyphase power supply and an induction and levitation coil in which frequency and current output of the power supply are adjustable to match the resistivity and mass of molten metal being cast and produce a continuous levitating force on the metal.

Summary of the Invention

The present invention is an apparatus for supplying polyphase power to an induction coil for magnetically levitating metal. The apparatus comprises input means for connection to a source of polyphase AC power and polyphase rectifier means for rectifying the AC power. A first control means selectably varies the magnitude of RMS current supplied to the induction coil by controlling the electrical phasing of the rectifier means. A polyphase inverter means is operatively associated with the rectifier means for converting the rectified AC power to polyphase AC power having a preselected frequency and supplying the polyphase AC power to the induction coil. A second control means independent of the first control means controls the frequency of the AC power supplied to the induction coil by controlling the electrical phasing of the inverter means.
The present invention also includes levitation casting apparatus for magnetically levitating metal and comprises a conduction and levitation coil and power supply means for supplying polyphase power to the coil. The power supply means has input means for connection to a source of polyphase AC power, polyphase rectifier means for rectifying the AC power, first control means for selectively varying the magnitude of RMS current supplied to the induction coil by controlling the electrical phasing of the rectifier means, polyphase inverter means operatively associated with the rectifier means for converting the rectified AC power to polyphase AC power having a preselected frequency and supplying the polyphase AC power to the induction coil, and second control means independent of the first control means for controlling the frequency of the AC power supplied to the induction coil by controlling the electrical phasing of the inverter means. The coil has a plurality of sections, each section being wound to provide a phase rotation of a magnetic force vector over substantially the entire length of the coil to produce a continuous magnetic levitation force.

Description of the Drawings

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

Figure 1 illustrates one form of levitation casting apparatus in which the present invention is used.
Figures 2A and 2B are a simplified block diagram of the invention.
Figure 3 is a simplified schematic diagram of a current limiting circuit used in the present invention.
Figure 4 is a timing diagram showing wave forms generated by the circuit shown in the block diagram of Figure 2A and 2B.

Description of the Invention

Referring now to the drawings, wherein like numerals indicate like elements, there is shown in Figure 1 one embodiment of a levitation casting apparatus 10 in which the present invention may be used. Apparatus 10 comprises a levitation casting section designated generally by numeral 12. Levitation casting section 12 is mounted on and extends vertically upwardly from a base 14. Molten metal to be cast is supplied, as will be more fully described, to a vertical delivery tube 16 which delivers molten metal to a polyphase levitation coil and heat exchanger assembly 18. Vertical delivery tube 16 is provided with a conventional induction heating coil 20 which keeps the molten metal at the required casting temperature.
Levitation coil and heat exchanger assembly 18 comprises a polyphase levitation coil, to be described in greater detail, and also includes upper and lower annular plenums and a cylindrical section fitted around the liner of the levitation coil and heat exchanger assembly in contact with the outer surface thereof. Although for clarity this structure is not illustrated in Figure 1, it is well known in the art. Liquid coolant, for example, tap water, is continuously delivered from a source into the upper plenum and flows through the cylindrical section and is withdrawn through the lower plenum to a drain, carrying with it the heat absorbed through the levitation coil and heat exchanger assembly 18 from the liquid metal and the freshly solidified continuous casting therein. As those skilled in the art will understand from U.S. patent 4,414,285, the molten metal will solidify at a location within levitation coil and heat exchanger assembly 18.
Solidified continuous casting emerges from the upper end of levitation coil and heat exchanger assembly 18 into a cooling chamber (not shown), which contains a plurality of pairs of counter-rotating pinch rollers which convey the continuously cast product to a location where it can be cut to length, coiled, or otherwise further processed.
Still referring to Figure 1, the apparatus 10 includes a suitably configured housing which contains coreless induction furnace 22. Coreless furnace 22 comprises a refractory crucible 24 surrounded by an induction heating coil 26. Coreless furnace 22 is mounted on a base 28. Molten metal may be added to crucible 24 from a conventional melting furnace 30 by means of crucible inlets 32.
Molten metal 34 in crucible 24 is supplied to levitation casting section 12 through a launder tube or conduit 36, which connects crucible outlet 38 to the vertical delivery tube 16. As with vertical delivery tube 16, conduit 36 is provided with a conventional induction heating coil 40 to keep the molten metal at the required temperature for casting.
The level of molten metal 34 in crucible 24 is controlled by displacer mass 42. Displacer mass 42 may be composed of a refractory, conductive material so that it can be inductively heated. Suitable materials include, but are not limited to, graphite, carbon-bonded silicon carbide and refractory metals. Displacer mass 42 may be lowered into and raised out of molten metal 34 to vary the level of the molten metal within crucible 24. Since the molten metal within crucible 24 is in hydrostatic communication with the molten metal within levitation coil and heat exchanger assembly 18, the hydrostatic head produced by the level of molten metal 34 within crucible 24 will cause the molten metal within levitation coil and heat exchanger assembly 18 to seek the same level a the molten metal in crucible 24. By controlling the level of molten metal 34 within crucible 24, a constant flow of metal and a controllable level of molten metal in levitation casting section 12 can both be achieved.
Although the foregoing description of apparatus 10 is sufficient to understand the present invention, a more detailed description of apparatus 10 may be found by referring to co-pending patent application Serial No. 925,013, filed October 30, 1986, and assigned to the same assignee as the instant application.
Referring now to Figures 2A and 2B, there is shown in block diagram form an apparatus 50 according to one embodiment of the present invention by which the required polyphase power can be supplied to the levitation coil. Apparatus 50 comprises six major elements, namely input means 52, polyphase rectifier means 54, first control means 56, polyphase inverter means 58, second control means 60 and induction and levitation coil 62, as well as a number of secondary elements, all of which are discussed in greater detail below.
Input means 52 may be any conventional means for connecting apparatus 50 to a source of polyphase AC power, typically a three phase AC power source of 50 Hz or 60 Hz, such as is commonly supplied by the local electrical utility company. However, although the invention is illustrated with reference to a conventional three phase AC power source, it should be understood that any conventional polyphase AC power source can be utilized without departing from the invention.
The polyphase AC input is preferably, but not necessarily, coupled to rectifier means 54 by means of line disconnect and transient protection circuitry 64. Line disconnect and transient protection circuitry 64 may comprise, for example, a circuit breaker and a fuse on each power line to act as disconnect and fault protection devices to protect apparatus 50 from unexpected power surges or other potentially hazardous electrical faults. In addition, if desired, conventional RC snubber circuits and MOVs may be provided on the power lines for transient protection due to power source disturbances.
Rectifier means 54 is an SCR phase control bridge whose purpose is to produce and regulate DC current to inverter means 58. Rectifier means 54 comprises six conventional phase controlled SCRs. Although omitted from Figure 2A for clarity, each SCR may be shunted by conventional RC snubber circuits for transient and dv/dt protection. The phase control signals for the individual SCRs of rectifier means 54 are provided by first control means 56, in a manner to be described.
Preferably, but not necessarily, the rectifier means 54 is operatively coupled to inverter means 58 by means of current limiting and discharge circuitry 66, shown in more detail in Figure 3. Current limiting and discharge circuit 66 comprises a conventional current limiting reactor 68 connected in series in the positive output line between rectifier 52 and inverter 58. Current limiting reactor 68 provides filtering to provide constant current to inverter 58 and also acts to limit current rise in the event of a fault condition. This protects rectifier 52 and allows sufficient time for the circuit breakers in transient protection circuit 64 to clear before a fuse blows. Current limiting circuit 66 also comprises discharge diodes to provide reverse voltage protection in the event of a power interruption. An RC snubber circuit 74 is provided to suppress high voltage transients generated by inverter 58. In addition, a discharge resistor 76 is provided to furnish a path for discharging the snubber capacitor.
Referring now to Figure 2B, inverter means 58 is preferably a three-phase variable high frequency auto-sequential commutated inverter. Inverter 58 will produce a three-phase, 120°-shifted sinusoidal current from 1,000 to 3,000 Hz. Inverter 58 comprises two complete inverters paralleled on both the DC input and AC output sides. Each of the six sets of SCRs, diodes, commutating capacitors and di/dt reactors in each half of inverter 58 has a complementary member in the remaining half. The halves are paralleled to provide the necessary current. Although omitted for clarity from Figure 2B, each SCR and diode is preferably shunted with an RC snubber circuit to provide dv/dt and transient suppression.
Operation of inverter 58 will now be described. The SCRs are gated in response to control signals from second control means 60. (Generation of the control signals will be described below.) For startup, SCR 6 and SCR 1 are gated simultaneously. This charges the commutating capacitor C1 connected between SCR 6 and SCR 4, with the side of capacitor C1 connected to SCR 6 being positive. The commutating capacitor C2 between SCR 1 and SCR 5 is also charged, with the side of capacitor C2 connected to SCR 5 being positive. Current flows from the positive DC line through SCR 6, diode D6 and their di/dt reactor 78 to transformer connection T1, through the load (levitation coil 62), from transformer connection T3 and through SCR 1, diode D1 and their di/dt reactor 80 to the negative DC line.
Sixty electrical degrees later, SCR 4 is gated. This causes commutating capacitor C1 to put a reverse voltage across SCR 6, which turns it off. This also charges commutating capacitor C3 connected between SCR 4 and SCR 2, with the side of C3 connected to SCR 4 being positive. Current now flows from the positive DC line through SCR 4, diode D4 and their di/dt reactor 82 to transformer connection T2, through the load, through transformer connection T3 and through SCR 1, diode 1 and their di/dt reactor 80 to the negative DC line.
Sixty electrical degrees later, SCR 5 is gated. This causes commutating capacitor C2 connected between SCR 1 and SCR 5 to put a reverse voltage across SCR 1, which turns it off. This also charges commutating capacitor C4 connected between SCR 5 and SCR 3, with the side of C4 connected to SCR 3 being positive. Current now flows from the positive DC line through SCR 4, diode D4 and their di/dt reactor 82 to transformer connection T2, through the load, through transformer connection T1, and through SCR 5, diode D5 and their di/dt reactor 84 to the negative DC line.
Sixty electrical degrees later, SCR 2 is gated. This causes commutating capacitor C3 between SCR 4 and SCR 2 to put a reverse voltage across SCR 4, which turns it off. This also charges commutating capacitor C5 connected between SCR 2 and SCR 6, with the side of C5 connected to SCR 2 being positive. Current now flows from the positive DC line through SCR 2, diode D2 and their di/dt reactor 86 to transformer connection T3, through the load, through transformer connection T1, and through SCR 5, diode D5 and their di/dt reactor 84 to the negative DC line.
Sixty electrical degrees later, SCR 3 is gated. This causes commutating capacitor C4 between SCR 5 and SCR 3 to put reverse voltage across SCR 5, which turns it off. This also charges commutating capacitor C6 connected between SCR 3 and SCR 1, with the side of capacitor C6 connected to SCR 1 being positive. Current now flows from the positive DC line through SCR 2, diode D2 and their di/dt reactor 86 to transformer connection T3, through the load, through transformer connection T2, and through SCR 3, diode D3 and their di/dt reactor 88 to the negative DC line.
Sixty electrical degrees later, SCR 6 is gated. This causes commutating capacitor C5 between SCR 2 and SCR 6 to put a reverse voltage across SCR 2, which turns it off. This also charges commutating capacitor C1 connected between SCR 6 and SCR 4, with the side of capacitor C1 connected to SCR 6 being positive. Current now flows from the positive DC bus through SCR 6, diode D6 and their di/dt reactor 78 to transformer connection T1, through the load, through transformer connection T2 and through SCR 3, diode D3 and their di/dt reactor 88 to the negative DC line.
Sixty electrical degrees later, SCR 1 is gated. This causes commutating capacitor C6 connected between SCR 3 and SCR 1 to put a reverse voltage across SCR 3, which turns it off. This also charges commutating capacitor C2 connected between SCR 1 and SCR 5, with the side connected to SCR 5 being positive. Inverter 58 is now back at the starting condition, and gating of the SCRs proceeds sequentially as described above until the inverter is turned off.
Although omitted for clarity from Figure 2B, those skilled in the art will recognize that blocking diodes may be provided in inverter 58 to prevent the commutating capacitors C1 through C6 from being discharged through the load.
Preferably, the outputs of inverter 58 are connected to induction coil 62 by impedance matching transformers 90. The impedence matching transformers may be conventional variable voltage tapped wye-connected isolation transformers to match the load impedance of induction coil 62 and to isolate induction coil 62 from inverter 58.
The magnitude of RMS current furnished to induction coil 62 is controlled by the phasing of the SCRs in rectifier 54. Phasing signals to the SCRs in rectifier 54 are generated by first control means 56, which is a conventional three-phase gating circuit which produces three sets of output pulses that are 120 electrical degrees apart. These pulses control the phasing of the rectifier by phase proportioning the SCRs within the rectifier. The output gate pulses are phase shifted in proportion to an input control reference signal from a manual control potentiometer 92 or a computer-generated reference signal from a computer control system 94, which may be selected by switch 96. The output gate pulses are regulated by an RMS-to-DC feedback circuit 98 which senses the RMS value of the inverter output current by means of current sensing transformer 100. A current meter 102 may be provided to display and/or record the value of the sensed current. First control circuit 56 maintains constant current to the levitation coil, which is necessary for a given casting rate for the particular metal and rod diameter being cast. If the casting parameters are changed, the regulated current may be adjusted by means of potentiometer 92 or computer control system 94 for optimum levitation current.
First control means 56 can also limit the input line current to a maximum predetermined level by limiting the phase angle of the gate pulses. A signal proportional to the value of the input line current is generated by current sensing circuit 104 from the output of current sensing transformer 106 on one of the AC input lines to rectifier 54. In the event of a fault condition on the load side of rectifier 54, the SCRs within rectifier 54 may be turned off by inhibiting or clamping the gate pulses.
The frequency-variable gate pulses necessary to control inverter 58 are generated by second control means 60. The frequency of the gate pulses is variable from 1 Khz to 3 Khz. The frequency is proportional to an input reference signal produced by either a manual control potentiometer 108 or the computer control system 94. The reference control signal can be selected by means of a switch 110. The reference control signal is applied to a voltage controlled oscillator (VCO) 112. If desired, appropriate conventional frequency limiting circuitry 114 may be provided to limit the minimum and maximum VCO output frequency. A frequency meter 116 may be provided on the output of the VCO to display and/or record the output frequency.
The output of VCO 112 controls gate pulse sequencing circuits 118, which divides the VCO output into six phase-shifted firing sequence pulses. These pulses are directed to firing circuits 120, which drive SCR firing modules 122. SCR firing modules 122 provide the appropriate signal levels for the inverter 58 and isolate inverter 58 from second control means 60.
Induction and levitation coil 62 may consist of three, four, six or more sections. These sections are wound to provide the correct phase rotation of the magnetic force vector in the coil to produce a continuous electromagnetic levitating force on the casting being produced. For example, coil 62 may comprise six sections, wound and connected as shown in Figure 2B. The three phase output of inverter 58, designated 0̸1, Ø2 and Ø3, is applied to coil 62 to generate six phases, Ø1, Ø2, Ø3 and -Ø1, -Ø2 and -0̸3, each phase being 60 electrical degrees apart, as shown in Figure 4. This results in a magnetic force vector in the coil with a continuous phase rotation to provide a continuous upward levitating force on the metal being cast.
Since the current in coil 62 and the frequency of the current can be independently preselected, the present invention enables a wide variety of rod sizes and materials to be easily cast and permits quick changeover from metal to metal and diameter to diameter.

Claims

1. Levitation casting apparatus for magnetically levitating metal, characterised by an induction and levitation coil (62) and power supply means (50) for supplying polyphase power to the coil, the power supply means having input means (52) for connection to a source of polyphase AC power, polyphase rectifier means (54) for rectifying the AC power, first control means (56) for selectably varying the magnitude of RMS current supplied to the induction coil by controlling the electrical phasing of the rectifier means, polyphase inverter means (58) operatively associated with the rectifier means (54) for converting the rectified AC power to polyphase AC power having a preselected frequency and supplying the polyphase AC power to the induction coil; and second control means (60) independent of the first control means for controlling the frequency of the AC power supplied to the induction coil by controlling the electrical phasing of the inverter means, the coil having a plurality of sections (0̸1 to-Ø3), each section being wound to provide a phase rotation of a magnetic force vector over substantially the entire length of the coil to produce a continuous magnetic levitation force.

2. Apparatus according to claim 1, characterised in that the first and second control means (56, 60) are responsive to separate manually-selected input signals.

3. Apparatus according to claim 1, characterised in that the first and second control means (56, 60) are responsive to separate computer-generated input signals.

4. Apparatus according to claim 1, characterised in that the first and second control means (56, 60) are selectably responsive to either separate manually-selected input signals or separate computer-generated input signals.

5. Apparatus according to any preceding claim, characterised in that the rectifier means (54) comprises an SCR phase control bridge circuit.

6. Apparatus according to any preceding claim, characterised in that the inverter means (58) comprises an autosequential commutated inverter.

7. Apparatus according to any preceding claim, characterised in that the coil (62) comprises two windings per phase, connected in opposing series connection.