Industrial electric heating furnaces – Induction furnace device – For furnace regulating
Reexamination Certificate
2002-01-08
2003-04-01
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Induction furnace device
For furnace regulating
C373S147000, C373S151000
Reexamination Certificate
active
06542535
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electric induction furnaces and more particularly to induction furnaces having improved efficiency coil systems.
BACKGROUND OF THE INVENTION
Electric induction furnaces are used to heat and melt metals and other electrically conductive materials. An induction furnace utilizes an induction coil that is powered from an ac power source. Alternating current flowing through the coil creates a magnetic field that is applied to the electrically conductive charge placed inside of the furnace's crucible. Eddy currents induced by the field in the charge can be used to heat, melt and superheat the charge. The magnetic coupling between the induction coil and charge is analogous to a magnetic transformer coupling. However an induction coil has a much higher leakage inductance than the leakage inductance of a magnetic transformer. Consequently an induction furnace's power factor is extremely low, typically ranging from 0.08 to 0.15, lagging and, therefore, an extremely inefficient load.
The conventional coreless induction furnace consists of a copper water-cooled helical coil with a ceramic crucible containing the charge. Alternating current in the coil generates a magnetic field that induces current into the conductive charge. As illustrated in FIG.
1
(
a
), the induction furnace
100
can be viewed as a loosely coupled transformer where the turns of the primary coil are magnetically coupled to a single turn formed by the conductive melt
102
. In the figure, I
c
represents the coil current and I
m
represents the current in the molten bath. Therefore, it can be assumed that the ratio of the current induced into the melt to the current in the coil approximates the number of coil turns. Maximum current density is induced on the circumference of the melt, with current exponentially decaying into the melt depth towards the center of the crucible.
The rate of decay is defined by a constant, namely the depth of current penetration into the metal, &Dgr;
m
, as defined (in meters) by the following equation:
Δ
m
=
2
·
ρ
m
μ
O
·
μ
m
·
f
=
503
ρ
m
f
where: &rgr;
m
=resistivity of the molten metal (in ohms/m);
&mgr;
o
·&mgr;
m
=the product of absolute and relative permeability (with &mgr;
o
=4&pgr;×10
−7
and &mgr;
m
, the relative permeability of the metal, in H/m); and
f=the frequency of the coil current (in Hertz).
Induction furnaces are usually designed to satisfy the condition that the depth of current penetration into the metal is much less than the radius of the melt (&Dgr;
m
<<r
m
).
The conventional shape of the melt is cylindrical. Most of the induced current is flowing in the outer layer of the melt with thickness equal to the depth of penetration, &Dgr;
m
. The resistance, R
m
, (in ohms) of this layer can be estimated by the following equation:
R
m
=
ρ
m
·
2
π
·
r
m
h
m
·
Δ
m
=
0.0125
·
r
m
h
m
f
ρ
m
where
R
m
=resistance of the melt (in ohms);
r
m
=radius of the melt;
h
m
=height of the melt;
&rgr;
m
, &Dgr;
m
and f are as previously defined.
Induction furnaces are principally single-phase devices. The supplied electric power is typically distributed over balanced three-phase lines. For optimal operation, induction furnaces operate at frequencies typically in the range of 100 to 10,000 Hertz. These frequencies are needed to maintain an optimal &Dgr;
m
/r
m
ratio for electromagnetic stirring of molten metal in the furnace.
Solid state power converters generate the power at required frequency, voltage and current for induction furnaces. These converters utilize power semiconductors (such as SCR, IGBT or IGCT topologies). The solid state static power converter resolves the phase balancing problem. Input 3-, 6-, or 12-phase line voltages are rectified before being inverted into a single-phase medium-frequency electrical current. Full-wave rectification of multi-phase line voltage produces a low harmonic distortion on feeding electrical lines, thus eliminating the need for line filters. As illustrated in FIG.
1
(
b
), the power converter consists of three major sections:
an ac to dc rectifier and dc filter;
a dc to ac medium-frequency inverter; and
a bank of tuning capacitors.
Power supplied to the furnace is controlled automatically by varying the commutation timing of the inverter's solid state switching components. This timing determines the operating frequency, phase and amplitude of the furnace current.
There are two conventional implementations of static solid-state power converters, namely a current-fed inverter with a parallel capacitor bank and a voltage-fed inverter with series capacitor bank. FIG.
2
(
a
) illustrates a furnace system utilizing a current-fed converter. FIG.
2
(
b
) illustrates a furnace system utilizing a current-fed converter with series/parallel tank capacitors. FIG.
2
(
c
) and FIG.
2
(
d
) illustrate furnace systems utilizing a voltage-fed converter in full bridge and half configurations, respectively. Each of these power supply topologies comprises a rectifier and filter section
110
; a solid state inverter section
120
; and a tuning capacitor section
130
. While the generally recognized symbol for an SCR is used in these set of figures, other solid state switching devices can be utilized in these applications.
In the current-fed inverter, as illustrated in FIG.
2
(
a
), the power factor correction capacitor bank is usually connected in parallel to the furnace coil. The term “capacitor bank” is used here to designate one or more capacitors connected in series or parallel to be the equivalent circuit as shown in the figures. Both the capacitor bank and the coil are connected into the diagonal of a full-bridge inverter. This connection allows the reactive component of the coil current to bypass the inverter's solid state switching components. However, the inverter is exposed to the full furnace voltage. The values of inverter voltage may be higher or lower than the dc voltage on the rectifier. Therefore, dc rectifier and inverter sections must be decoupled by reactors. The reactors supply the inverter with constant dc current. They are acting as a filter and reservoir of energy. The inverter converts dc current into square wave current that is supplied to a parallel resonant circuit.
The furnace power in current-fed inverter systems is controlled by varying both inverter timing and dc voltage. When inverter voltage falls below dc rectifier potential, the output power cannot be controlled by variation in inverter commutation frequency alone. Additional control of the injected dc current is carried out by regulating the conduction phase angle of the rectifier SCRs. Such regulation will introduce distortion into the feeding electrical line unless filters are provided.
The main advantage of the parallel resonant inverter is that only part of the coil current is passed via solid state switching devices, thus saving the number of semiconductor devices. The inverter controls only part of the coil current. This, however, limits the controllability of the inverter. Using the smoothing dc reactors as temporary energy accumulators causes difficulties in starting the inverters. The energy in the reactors is kinetic (analogous to the energy of a flywheel)—it exists only when the dc current flows from the rectifier to the inverter. To accumulate the necessary energy in the smoothing dc reactor, a special starter network is used. When the parallel inverter is stopped, the energy from this reactor is expended using the solid state switches of the inverter as a crow bar circuit.
The advantage of lower current in the inverter solid state switching devices is offset by a high voltage to which these devices are exposed. This often requires stacking the devices in series, which in turn necessitates special dynamic voltage dividers. For small current-fed inverters connected to standard
Fishman Oleg S.
Mortimer John H.
Nadot Vladimir V.
Peysakhovich Vitaly A.
Hoang Tu Ba
Inductotherm Corp.
Post Philip O.
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