Electric heating – Inductive heating – With power supply system
Reexamination Certificate
1999-03-01
2001-04-03
Walberg, Teresa (Department: 3742)
Electric heating
Inductive heating
With power supply system
C333S032000
Reexamination Certificate
active
06211498
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to induction heating systems, and more particularly, to apparatus and methods for delivering optimum power to a workpiece over a wide range of operating conditions.
2. Description of Related Art
Induction heating systems heat an electrically conductive workpiece by magnetically inducing eddy currents therein. Electrical resistance in the eddy current paths in the workpiece cause I
2
R losses, which in turn heat the workpiece.
One type of induction heating system includes a power supply inverter, which has an AC voltage output having a desired frequency of operation. The output of the inverter is usually connected through a step-down transformer to a pair of power supply output terminals, across which is connected the series combination of a series inductor and a resonant tank circuit. The tank circuit includes a work coil in parallel combination with a resonance capacitor. The work coil, in operation, is. placed in proximity with the workpiece, and creates the oscillating magnetic field which induces the eddy currents in the workpiece.
Depending on the application, a wide variety of different operating conditions may be desired. For example, different applications may require different frequencies of operation. Frequencies commonly used for induction heating range anywhere from approximately 10 kHz to approximately 400 kHz. Different applications can also require different voltages across the work coil. Additionally, depending on the configuration and composition of the workpiece, the power factor of the energy delivered to the work coil could also vary widely.
Most induction heating systems are designed for a particular application. For example, a system designed to heat automobile bodies for the purpose of drying paint that has been applied to the surface, need only be designed to operate at one particular frequency, voltage and power factor. It is desirable, however, to provide a general-purpose induction heating system which can be used in a wide variety of applications, under a wide variety of different circumstances. For example, it would be desirable to permit a user to select the operational frequency over the full range of typical frequencies, 10 kHz-400 kHz. Adjustability within this large range of frequencies, spanning a range of 40:1, is extremely difficult to support. Even a range of 50 kHz-400 kHz (8:1) is very difficult to support. It is desirable to provide a system which supports a large range of operating conditions.
In addition, systems which do support a range of operating conditions typically require an operator to tune the system prior to operation. Tuning procedures for such systems are typically complicated and require a technical understanding of the principles under which the induction heating system operates. Accordingly, skilled or trained operators are usually required to operate induction heating systems intended to support a variety of operating conditions. It is therefore desirable to provide an induction heating system and method which simplifies the tuning process.
SUMMARY OF THE INVENTION
According to the invention, roughly described, induction heating apparatus has a series inductor L
s
between an AC source and a parallel tank circuit. The AC source has a variable frequency inverter, and an output transformer which has a leakage inductance, viewed from the secondary, no larger than
L
l
max
=
V
L
min
V
p
min
PF
min
2
π
Nf
max
P
max
,
(
1
)
where
V
Lmin
is a desired minimum permitted rms voltage across the tank circuit,
V
pmin
is a desired minimum rms input voltage to the output transformer,
N is the primary:secondary turns ratio of the output transformer,
PF
min
is a desired minimum permitted power factor, measured at the input of the transformer (ignoring the effect of the magnetizing inductance),
f
max
is a desired maximum frequency of operation, and
P
max
is a desired maximum power output into the induction heating coil.
The output transformer achieves such a low leakage inductance because of its construction as inner and outer hollow windings disposed substantially coaxially with each other, the inner winding being electrically continuous through T turns, and the outer winding having S electrically broken longitudinal segments through the T turns, S>1 . All of the outer winding segments are connected in parallel with each other. The inner and outer windings can be made of braided stranded wire, instead of solid wire or solid tubes, and the insulation between them is made very thin. If necessary to also reduce inter-winding capacitance, the transformer can further include a core.
In another aspect of the invention, a very simple tuning procedure is set forth for tuning an induction heating system which has a series inductor between an AC source and a parallel tank circuit. The tuning procedure involves first selecting a preliminary series inductance and a preliminary resonance capacitance. The operator then operates the system at low power, increasing the resonance capacitance if the system is operating at a frequency that is higher than desired, and decreasing resonance capacitance if the system is operating at a frequency that is lower than desired. Once the frequency is acceptable, the operator then operates the system at fill power, increasing the series inductance if the system is current limiting, and decreasing the series inductance if the system is resonance limiting. When the series inductance is acceptable, the system is ready for use.
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Koertzen Henry W.
Patridge Donald F.
Fliesler Dubb Meyer & Lovejoy LLP
Powell Power Electronics, Inc.
Van Quang
Walberg Teresa
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