RU2768380C1 - Frequency multiplier current inverter - Google Patents

Abstract

FIELD: electrical engineering.

SUBSTANCE: invention is a current inverter with frequency multiplication, relates to the field of electrical engineering and metallurgy, namely to power supplies based on semiconductor frequency converters for induction heating and melting of metals. The current inverter consists of a single-phase valve bridge, the DC clamps of which are connected through a filter choke to a DC voltage source, and the AC clamps are connected to the heater inductor through an L-shaped capacitive voltage divider, consisting of a parallel capacitor and connected to one of its plates of a series capacitor. The new is that the current inverter is equipped with additional series capacitors connected in series with the inductor and connected to the second plate of the parallel capacitor, and the outputs of the inductor are connected through the contacts of the power contactor with the opposite plates of the parallel capacitor and power two-pole contactors, which, when switched on, switch the initially connected in series with the inductor, the capacitors are parallel to the input capacitor, and in the off position of the contactors, all the capacitors of the oscillatory circuit are connected in series and the voltage from the inverter is applied to the input capacitor of the circuit.

EFFECT: proposed current inverter allows switching the operating mode from one operating frequency to the operating frequency multiplication mode and vice versa while maintaining the matching of the parameters of the inverter and the load, which ensures the optimization of the operating modes of the induction heater at different stages of heating and melting metals.

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Description

The present invention relates to the field of electrical engineering and metallurgy, namely, to power supplies based on semiconductor frequency converters for induction heating and metal melting.

Induction heating and melting of metals by high-frequency currents provides high efficiency of heat treatment of metals, has a high efficiency compared to other types of heat treatment of products. When melting metals in induction crucible furnaces (ITF), the choice of the frequency of the supply current will be different at certain stages of metal melting. At the stage of heating the charge material, it is advisable to choose an increased frequency of the supply current, and at the stage of technological processing of liquid metal, it is advisable, when the required melt temperature is reached, to reduce the supply frequency of the inductor during current stabilization. In this case, the power transferred to the load decreases, and the metal flow rate in the crucible increases. In this regard, the power supply must provide the possibility of changing the frequency of the inductor current at different stages of melting, based on the criteria for the most efficient implementation of the processes of heating and melting metal.

Furnace and heater inductors are electrical loads with a low power factor, which can be increased by connecting either parallel or series compensation capacitors. In this case, an oscillatory circuit is formed, which has a high quality factor.

As a power source, semiconductor frequency converters based on autonomous current inverters (AITs) are currently widely used. Since the load parameters during heating and melting of metal vary widely, especially ferromagnetic metals, it is more expedient to use circuits of series-parallel and parallel-series current inverters in frequency converters [1]. A series-parallel inverter differs from a parallel current inverter in that its output circuit is a series-parallel circuit, and the resonant frequency of the circuit is determined by the inductance of the inductor and the capacitance of a capacitor connected in parallel with the inductor. In this case, the operating frequency of the inverter is set approximately equal to the resonant frequency of the load circuit, to which the reactive power of the inductor is compensated. A parallel-series inverter is different in that the output circuit of the inverter is a parallel-series oscillatory circuit, and the resonant frequency of the load circuit is determined by the capacitance of the series-connected parallel (input) and series (pass) capacitors. In this case, both capacitors are used to compensate for the reactive power of the inductor in the operating modes of the inverter.

During induction heating of ferromagnetic materials, the choice of the frequency of the inductor current depends on the heating temperature, as it rises, a significant change in the electrical characteristics occurs (the load parameters change most significantly with the loss of ferromagnetic properties when the Curie temperature is reached). In this case, the required value of the inductor current frequency, at which induction heating will be effective, will increase by 2-3 times [2]. A change in the electrophysical characteristics during the heating of ferromagnetic materials also affects the depth of current penetration into the heated material, which significantly affects the power distribution over the cross section of the heated workpieces. Therefore, induction heating of massive ferromagnetic workpieces, from the point of view of ensuring the requirements for the allowable temperature difference during the heating process, it is advisable to conduct it up to the temperature of loss of magnetic properties (Curie temperature) at a lower frequency, and then at an increased frequency. In this case, the power is distributed more evenly over the cross section of the workpiece, which directly affects the temperature distribution over the cross section of the heated workpiece, and makes it possible to exclude the occurrence of unacceptable temperature drops over the cross section of the heated workpiece at the initial stage of heating.

The closest in technical essence to the claimed invention is a parallel-series current inverter (see L.1 page 57, Fig. 25) which is taken as a prototype.

The problem of this prototype is the impossibility of autonomous operation in dual-frequency mode. And for this mode, manual (mechanical) connection of additional compensating capacitors with a capacity several times greater than the capacity of the main compensating capacitors is necessary. As a result, the weight and size indicators of induction installations are significantly increased. In addition, when a block of additional compensating capacitors is connected, there is a mismatch between the output characteristics and the parameters of the load circuit, which, most often, leads to a shutdown of the metal melting process in the IHF.

In induction and melting furnaces, in order to accelerate the processes of technological processing of liquid metal at the final stages of melt processing (cleaning of metal from non-metallic inclusions, alloying), it is not necessary to transfer high power to the melt, since the required temperature of the melt has already been reached, but it is required to maintain or slightly increase the speed of its movement . This can be achieved by significantly reducing the frequency of the inductor current [3]. To change the operating frequency of the inverter, it is necessary to change the resonant frequency of the load circuit, which can be reduced by increasing the capacitance of the compensating capacitors when connecting some of them. Therefore, in order to maintain optimal operating modes in the metal heating cycle, it is necessary to connect and disconnect additional capacitors with a capacity 4–9 times greater than the capacity of the main compensating capacitors.

To eliminate these shortcomings, the authors propose a current inverter with frequency multiplication.

The essence of the invention lies in the fact that the current inverter for power supply by a current of one frequency with switching to the frequency multiplication mode contains a single-phase valve bridge, the DC clamps of which are connected through a filter choke to a DC voltage source, and the AC clamps are connected to the heater inductor through the G- shaped capacitive divider, consisting of a parallel capacitor and a serial capacitor unit connected to one of its plates.

What is new is that power two-pole contactors and additional blocks of series capacitors are introduced into the inverter circuit, connected in series with the inductor and connected to the second plate of the parallel capacitor, and the inductor leads are connected through the contacts of the power contactor with the opposite plates of the parallel capacitor.

Let us give an example of the operation of an inverter circuit with a frequency tripling. Figure 1 shows a diagram of such a current inverter, which indicates:

1-4 - single-phase bridge valves;

5 - filter choke;

6 - parallel capacitor;

7 - series capacitor;

8 - additional series capacitor;

9 - inductor;

10, 11 - power contactor.

Consider the operation of the inverter when the power contactor is turned on, when contacts 10, 11 are closed. In this case, all three capacitors 6, 7, 8 are connected in parallel to the inductor and the capacitance of the compensating capacitor becomes equal to

If the capacitors have the same capacitance, then

,

,

and the resonant frequency of the load circuit has a minimum value

.

.

равно выходному напряжению инвертора

, а величина тока определяется полным сопротивлением индуктора на низкой частотеInductor voltage

equal to inverter output voltage

, and the magnitude of the current is determined by the impedance of the inductor at a low frequency

.

.

In the off position of the power contactor 10, 11, when its power contacts are open, the capacitance of the compensating capacitor decreases and is determined by the capacitances of the series-connected capacitors 6, 7, 8. With the same capacitance of the capacitors

,

,

in this case, the resonant frequency of the load circuit increases in comparison with the resonant frequency of the circuit with parallel connection of all capacitors in accordance with the formula

.

.

The voltage on the inductor also increases, since capacitors 7, 8 are connected in series with the inductor. In this case, the equivalent capacitance of the series capacitor is

,

,

. Напряжение на индукторе может быть определено с учетом коэффициента передачи емкостного делителя какand the capacitance of the parallel capacitor

. The voltage across the inductor can be determined taking into account the transfer coefficient of the capacitive divider as

,

,

that is, it is tripled.

Since when switching the power contactor, the capacitance of the compensating capacitor changes by 9 times, and the resonant frequency of the load circuit changes by 3 times, the operating frequency of the inverter will also change by about three times, and the operating frequency of the inverter will also change by about three times. Therefore, the impedance of the inductor changes by about a factor of three, and the magnitude of the inductor current remains unchanged, since the voltage across the inductor also changes by a factor of three. Thus, when switching from one mode to another, the current frequency changes by about three times, and its value remains unchanged, while the output power of the inverter changes by about 1.7 times, since the active component of the resistance of the inductor with metal changes when the frequency changes. . Therefore, in the attached inverter, the goal is achieved - tripling the frequency of the inductor current while maintaining the matching of the parameters of the inverter and the load.

In FIG. 2 shows a diagram of a current inverter with frequency multiplication, in which it is indicated:

1÷4 - valves of single-phase inverter bridge;

5 - filter choke;

6 - parallel (input) capacitor;

7.1÷7.N, 8.1÷8.N - feed-through condensers in blocks 10.1÷10.N;

9 - inductor;

11.1÷11.N - power contactors;

B-1÷B-N - blocks of compensating capacitors.

This inverter circuit allows you to increase the frequency of the load circuit several times, depending on the number of blocks of compensating capacitors. With open contacts of power contactors, all compensating capacitors are connected in series and the total capacitance of the compensating capacitor with the same capacitances of all capacitors (C) is equal to

,

,

where N is the number of circuits switched by means of contactors (the number of two-pole contactors in the circuit).

When the contacts of the power contactors are closed, all compensating capacitors are connected in parallel to the inductor and the total capacitance is equal to:

.

.

Therefore, the rate of change of the compensating capacitor is defined as

.

.

The measurement range of the resonant frequency of the load circuit is approximately equal to:

.

.

Thus, the frequency of change in the operating frequency of the inverter is determined by the number of switched circuits of capacitors N, which can be determined based on the requirements of the technological process of melting or heating the metal.

When switching power contactors, the voltage multiplication factor on the inductor changes

.

.

If all capacitors in the circuit have the same capacitance:

,

,

where n is the total number of capacitors in the circuit.

Thus, the claimed technical result - an increase in the efficiency of induction heating and melting of metals is achieved by optimizing the heating modes by switching from the mode of operation at a high current frequency to the mode of operation at a low frequency at different stages of the metal melting cycle. Since the resistance of the inductor increases with the frequency of the current and the voltage across the inductor proportionally multiplies, the magnitude of the current in the inductor remains unchanged when switching power contactors. Thus, the parameters of the inductor and the inverter are automatically coordinated, which makes it possible to avoid overloading the equipment during the transition from the heating mode at a high frequency to the metal mixing mode at a low frequency and vice versa. As a result, the above problem of the prototype can be solved by applying the proposed technical solution.

Sources of information

1 Gitgarts D.A., Ioffe Yu.S. New power sources and automation of induction installations for heating and melting. Moscow: Energy, 1972.

2 Induction heating units: Uch. allowance for universities / A.E. Slukhotsky, V.S. Nemkov, N.A. Pavlov, A.V., Bamuner, ed. A.E. Slukhotsky. Leningrad: Energoatomizdat. Leningrad. dept. 1981.

3 Weinberg A.M. Induction melting furnaces: Uch. allowance for universities. M.: Energy. 1967.

Claims (1)

A frequency multiplication inverter containing a single-phase valve bridge, the DC terminals of which are connected through a filter choke to a DC voltage source, the AC terminals are connected to the heater inductor through an L-shaped capacitive voltage divider, consisting of input and feed-through compensating capacitors, characterized in that it is equipped with two-pole power contactors and additional feed-through compensating capacitors connected in series with the inductor to the input compensating capacitor and switched using power contactors in parallel with the input capacitor, and the inductor leads are connected to the opposite leads of the feed-through compensating capacitors.

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