Electric power conversion systems – Current conversion – With voltage division by storage type impedance
Reexamination Certificate
2001-10-30
2002-10-15
Patel, Rajnikant B. (Department: 2838)
Electric power conversion systems
Current conversion
With voltage division by storage type impedance
C363S132000
Reexamination Certificate
active
06466467
ABSTRACT:
SUMMARY
This invention refers to an inverter or converter by current injection, provided with a circuit whose generating frequency for the alternating voltage is adjusted by the load's resonant frequency, cycle to cycle, without lags, which permits avoiding power losses when transferring energy as a result of the variation in the load's resonant frequency that necessarily occurs in all fixed frequency inverters, characterized as a frequency interlocking circuit, provided with at least one voltage scanner in the load, adapter and galvanic insulation, one lead circuit, one clipping circuit, one comparator circuit and one oscillator and coupling circuit.
SPECIFICATION
This invention refers to an inverter or converter by current injection, provided with a circuit whose generating frequency for the alternating voltage is adjusted by the load's resonant frequency, cycle to cycle, without lags, which permits avoiding power losses when transferring energy as a result of the variation in the load's resonant frequency that necessarily occurs in all fixed frequency inverters.
In general, in the field of electrical engineering, the inverter or converter is the device, equipment or electric system that is able to convert continuous power (kW) into alternate power (kva).
There are different types of converters. Depending on the type of electric power that they supply, these are classified in four categories, according to the process they carry out:
a) AC/DC rectification, a process that converts alternate current into continuous current;
b) CC/CC conversion, that converts a continuous current into another that is also continuous but has different voltage characteristics or levels;
c) DC/AC inverter, conversion of continuous current into alternate, and
d) AC/AC conversion, a process that converts one alternate current into another that is also alternate but has different voltage and/or frequency characteristics.
The transformation from continuous voltage to alternate voltage is achieved by inverting the polarity of the source on the load by using interruption and connection devices.
A source of continuous voltage, see
FIG. 1
, in this case a battery is connected to a load using a set of four switches that act in pairs, connecting alternate polar voltage periodically. The load “perceives” a voltage source in the shape of a square wave that alternates the value +Vcc and −Vcc. This is the principle of any inverter, a source of continuous power controlled by a group of switches that alternate the polarity in the load producing an alternated signal.
This is the operating principle of any inverter, a source of continuous power controlled by a set of switches that alternate the polarity in the load producing tension and alternated current in the load.
Present-day inverters or converters employ solid state elements, capable of controlling high powers that act as controlled electronic switches that periodically exchange the polarity of the continuous source on the load to a pre-assigned frequency, by means of electronic oscillatory circuits (interval timers).
The name resonant tank is given to a circuit (group of passive electric elements), formed by resistors, capacitances and inductances (RLC circuit) placed in a such a way that in order to inject current (parallel resonant) or applied voltage (series resonant) with a frequency known as resonant frequency, the impedance of the capacitance and inductance annul each other and remain limited only by the resistance of the circuit.
The circuit of the inverter that we wish to patent, refers exclusively to the case of parallel resonant tanks that are characterized because the capacitance is in parallel with the inductance and resistance as shown in FIG.
2
.
Consequently, the parallel resonant circuits produce a great alternate current circulating between the coil and the condenser when they are in resonance, limited only by the series resistance, with a small real excitation current entering the tank.
This result is very often used to produce heating by magnetic induction, because according to the Ampere Circuital Law all intensity of alternate current produces a magnetic field around it that, in turn, induces voltages in any conductor that is near it. This is the principle whereby inductive heating is produced; internal voltages are induced in a metal conductor, in the presence of an alternate magnetic field, which cause currents to circulate that produce the heating due to the Joule effect.
All the present-day inversion systems for these applications of inductive heating work at a fixed frequency, normally within the range of 200 Hz to 10 kHz, designed to generate, by the injection of a continuous current, an alternating voltage of fixed frequency to the resonance of the tank assuming an invariability in time of the tank's resonant frequency.
The specific construction of the single-phase inverters by current injection that exist in the market are manufactured using rapid power thyristors or more recently by GTO (gate turn-off). Both are solid-state devices able to rectify alternate current that conduct current in a single direction and whose conduction mode is to have a positive anode-cathode polarity and an electronic switch signal in their tripping gate.
To cut the conduction, the current must necessarily be zero, therefore a current must be injected that has the same magnitude but in the cathode-anode direction, by the voltage of the load of the tank itself (load switching) tripping the other pair of lead semiconductors with regard to the passing through zero of the voltage. Consequently, the lead time in the tripping depends on the frequency and is a very delicate value because faced with a variation in frequency the SCR (Silicon Controlled Rectifier) might not be turned of or it may be subjected to excessive voltage. All the above necessarily forces the circuits to be of a fixed frequency and higher than the resonant frequency.
The resonant inverters have a broad field of application in industry in general. They are used where a clean, rapid and efficient transfer of heat is required such as thermal treatments and fusion of metals.
The principle of energy transference is through the generation of an intense magnetic field produced by a high resonance current that circulates in the RLC tank.
The problem solved by this invention is to avoid power losses in the transfer of energy resulting from the variation of the load's resonant frequency that is produced necessarily in all fixed frequency inverters and it also allows the inverter to work in resonant frequency with any load without needing special adjustments, within the range for which it was designed.
In effect, the tank's resonant frequency (RLC circuit) depends only on the physical characteristics of its components, that is, resistance, inductance and capacitance. In the particular case of an oven, the resistance depends specially on its volume, the type of metal and its magnetic properties. The inductance will depend principally on its physical dimensions, number of windings, material to be heated or melted and temperature. On the other hand, normally, the capacitance is fixed with values that may be adjusted discretely (taps).
In particular, the temperature has an important effect on the coil's resistance and reluctance. This is specially highlighted in the case of magnetic materials that, below the Curie temperature (760° C.), have a magnetic permeability approximately 50 times greater than when they are above this temperature (magnetic permeability of the vacuum).
All the nonmagnetic metals such as copper, aluminum, certain steels, etc. present a permeability of dose to 1 or equal to 1 (permeability of the vacuum) and in these cases the reluctance varies little but the resistance, that increases as the temperature rises, does.
The coil's inductance varies with the number of turns and the diameter. Consequently, when the inverter has a fixed frequency there is one and only one coil-condenser pair that will adjust to the frequency of t
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