Field of the Invention
[0001] 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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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
[0007] 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.
[0008] 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
[0009] 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
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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.
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.