WO1988003351A1

WO1988003351A1 - Coil-free pulsed-operation power source for inductive heating

Info

Publication number: WO1988003351A1
Application number: PCT/HU1987/000047
Authority: WO
Inventors: László KOLLER; György TEVAN
Original assignee: Budapesti Mu^"Szaki Egyetem
Priority date: 1986-10-24
Filing date: 1987-10-23
Publication date: 1988-05-05
Also published as: EP0327561A1; HU196683B

Abstract

Pulsed power source for inductive heating, consisting essentially of the main circuit. In the latter, the first input terminal (1) is connected to the negative pole of the DC power supply and to the cathode of the charging thyristor (T2), whereas the second input terminal (2) is connected to the positive pole of the DC power source, to the discharging thyristor anode (T1) and to the cathode of the discharge diode (D1). The first output terminal of the main circuit is common with the second input terminal (2), which is connected to the inductor (I) provided with an insert (B), and the other terminal of the inductor (I) forms the second output terminal (3). The latter is connected via the condensor (C) to the nodal point (4) which is connected to the charging thyristor (T2), to the discharging diode (D1) and to the cathode of the discharging thyristor (T1). The power supply source described includes also the necessary regulating, control and protection elements required for starting, operating and stopping.

Description

Pulse-mode power source without choke coil for inductive heating

The invention relates to a simply constructed, pulse-operated feed source with thyristors that serves for inductive heating and does not contain a choke coil in the main circuit. The space requirement of this supply source is generally much less compared to the previously common, thyristorized inverters with an oscillating circuit, the mass is smaller, the design is cheaper, the effectiveness is higher, and a much more flexible operation can be achieved with it. A further cost saving can be achieved in that, in the majority of cases, there is no need to change the voltage of the supply source supplying the direct voltage, since the power of the supply source according to the invention is continuously increased from zero to the nominal value by the ignition of the thyristor present in the oscillation circuit can be changed.

In certain technological applications of inductive heating, it seems to be expedient or is it inevitable that the electrical energy taken off from the alternating current network has been converted into energy with a higher frequency. Recently, this task has been solved more and more with semiconductor food sources. In the medium frequency range, those thyristorized Distribution sources with an oscillating circuit, which generate an approximately sinusoidal inductor voltage. Such food sources are e.g. B. described in the following specialist books: Berkovics, Ivanszkij et al .: high frequency inverter with thyristors, Energia Verlag Leningrad, 1973; Haumann, Stumpe: Thyristortechnik, Technischer Verlag, 1971. 5.4.3. Inverters with vibration circuits, pages 200-208. The disadvantage of these inverters is that their cut-off frequency is not high enough, flexible operation cannot be ensured, and power regulation is difficult, and the occasion is also difficult.

The latest research - after recognizing that you can also heat with a non-sinusoidal inductor current - has focused on the development of various pulse-powered sources. The most important advantage of this inverter, in addition to its simplicity and cheapness, is that a higher cut-off frequency can be achieved with an open thyristor with the same release time. Another advantage is the simple power control and the immediate reason. On the other hand, the greatest disadvantage of this solution is that, on the one hand, the loss which arises in the choke coil in the main circuit and, on the other hand, the direct current component present in the inductor current reduce the effectiveness. Such a switch is for example in the article of the magazine Electronique et Microelect ronique Industrielles. 1976/236 - Peter: Realization d'un onduleur 40 kHz a thiristor rapidly; Koller et al. Power source with pulse operation for inductive heating, HU-PS 186043/1982 / described. The object of the invention was to create a pulsed supply source that retains or increases all the advantages of the known pulsed supply sources, but does not have their disadvantageous characteristics, that is, a choke coil is not included and no direct current component is produced in the inductor current . In this way, a higher efficiency can be achieved and better parameters can be achieved not only with pulse sources, but with all thyristorized supply sources.

The invention is explained in more detail using an advantageous embodiment, with the help of the accompanying drawings; show it:

1 shows the circuit arrangement of the main circuit of the feed source according to the invention, FIG. 2 shows the time functions of the inductor current i and the capacitor voltage u _c during the function of the feed source in the stabilized

Status.

Accordingly, the invention relates - in accordance with the objective - to a pulse source for inductive heating; the main circuit, which is essential, contains a charge thyristor, a charge thyristor, a discharge thyristor, a discharge diode and a capacitor, furthermore the main circuit is connected with its input terminals to a direct voltage supply source, with the output terminals to an inductor provided with an insert; the feed source according to the invention can be characterized in that - as can be seen from FIG. 1 - the first input terminal 1 is located in the main circuit is connected to the negative pole of the DC voltage supply source and the cathode of the charge thyristor T2, while the second input terminal 2 connects the positive pole of the DC voltage supply source and the anode of the discharge thyristor and the cathode of the discharge diode Dl, the first output terminal of the main circuit is common to the second input terminal 2 connected to one terminal of the inductor I provided with the insert 8; the other terminal of the inductor I forms the second output terminal, which is connected to the node 4 via the capacitor C, and the node 4 is connected to the charge thyristor 2 and the anode of the discharge diode D1, and to the cathode of the discharge thyristor T1.

As already mentioned, the main circuit shown in Figure 1 incorporates the essence and novelty of the invention. This main circuit is connected to control, regulating and protective elements which are essential for the operation of the feed source according to the invention. With regard to the fact that the elements mentioned do not affect the essence of the invention and, furthermore, are known and equivalent elements, they are not detailed here. The structure is legally discussed in a depth that is absolutely necessary to know the function of the apparatus according to the invention.

The input of the main circuit according to Figure 1 is connected to the DC voltage source. In the majority of cases, this DC voltage source can be formed by a rectifier, the terminals of which are connected to a smoothing capacitor. The rectifier itself can be implemented in numerous versions: with one or three phase bridge circuit, with diode, possibly with thyristor, etc. In general, the last-mentioned more expensive solution does not have to be applied to the feed source according to the invention, since the continuous regulation of the power is solved by the inverter itself. The output of the main circuit is connected to the inductor I, which is in a magnetic coupling with the insert to be heated. The inductor can not only be connected directly to the output of the inductor - as shown in FIG. 1 - it is also possible to implement the connection with an inserted matching transformer.

Satisfying the requirements in terms of control, regulation or protection may require that the anode of the charge thyristor T2 be connected to the positive pole of the DC voltage supply source, the cathode to the second input terminal 2 / in this case the connection between hears + Pole and terminal 2, the negative pole of the DC power supply connects to node 4. For a similar reason, the capacitor C and the inductor I can change places. The main circuit according to FIG. 1 remains unchanged due to the possible changes. The mode of operation of the apparatus according to the invention is as follows:

Let us assume that the apparatus is switched off; the capacitor C is not charged, the discharge thyristor T1 and the charge thyristor T2 are in the closed state. As the first step of the event, the first input terminal 1 and

Junction 4 bridged with the resistance of the size R _i . Via this resistor, the capacitor C is charged aperiodically to the supply voltage U _o if the value of R _i is chosen so that the inequality

exists, R _s and L _s are at the terminals of the inductor I, C _S at the terminals of the capacitor C measurable values of the effective resistance, the inductance and the capacitance with a sinusoidal supply with natural frequency f _S. When writing down the inequality / 1 /, we neglected the effect of the smoothing capacitor at the terminals of the DC voltage supply unit, although this effect largely exceeds the value C _S , aperiodic charging takes place with two time constants, and four times the value of the larger time constant can Charging can be regarded as practically finished / the voltage of the capacitor has reached the value U /. The resistance of the value R _{i is then} switched off, as a result of which the charge thyristor T2 separates the negative pole of the DC voltage supply source from the node 4 repeatedly. As the next step, with the aid of a first pulse generator with the appropriate power, needle-like voltage pulse between node 4 and the discharging control electrode 5 / the control electrode of the discharge thyristor T1 / is switched on. If this pulse has a positive polarity with respect to the node 4, the discharge thyristor T1 biased in the forward direction with the voltage U _o begins to conduct, whereupon in the lossy oscillation circuit formed by the inductor I and the capacitor C, a decaying current oscillation with the period duration of T = l / f _S arises.

This current is conducted in one direction by the discharge thyristor T1 and in the other direction by the discharge diode D1. The prerequisite for vibration is the existence of inequality

During the current oscillation, the voltage of the capacitor C also performs a decaying oscillation; after the period T has elapsed, the voltage U ₁₁ <U _o remains, ie part of the energy has been lost. With regard to the fact that during the period corresponding to the period T between the node 4 and the second input terminal 2, a short circuit is practically to be measured / the discharge thyristor T1 or the discharge diode D1 conduct continuously /, the charge thyristor T2 with the voltage U _{o is} in the forward direction biased. After the period T has elapsed, the residual voltage of the capacitor C in the value of U ₁₁ suddenly appears between the node 4 and the second input terminal 2. With this voltage jump, a second pulse generator is controlled, which switches the first input terminal 1 and the charge control electrode 6 when the voltage U _{11 occurs} , with a corresponding power, a needle-like voltage pulse. Compared to the first input terminal 1, this voltage pulse has a positive polarity, as a result of which the charge thyristor T2, which is biased in the forward direction with the voltage / U _o -U ₁₁ /, begins to conduct, while the decaying oscillation lasts for half a period / T / 2 / the capacitor

C is charged via the inductor I to a voltage of U _ml > Uo. From the moment of the zero crossing of the charging current, a voltage in the blocking direction / U _ml U _o / reaches the charge thyristor T2, which has a relatively high value and thus effectively accelerates the release of the charge thyristor T2. The capacitor C remains in its state charged to the voltage U _ml until the remainder of the pulse generator after Expiry of the pulse train time T _i emits a newer voltage pulse between node 4 and the discharging control electrode. If the relation

T _i 1.5 T + tq _2/3 /

exists, where tq ₂ denotes the release time of the charge thyristor T2, the apparatus will operate continuously and the stabilized state will be established. The relation / 3 / can also be maintained automatically with the first pulse generator, in such a way that the generator between the second input terminal 2 and the node 4 perceives the voltage jump occurring per period time T when the current of the discharge diode D1 ceases, and in accordance therewith the pulses delivers. Of course, the prerequisite for the function of the feed source according to the invention is that for the release time tq _{1 of} the discharge thyristor T1 the relation c / V

exist. If thyristors with the same free time

/ tq ₁ = tq ₂ /, the condition T _i / T = 2. must be met.

In this regard, we show in FIG. 2 the time sequences of the current / i / of the inductor and of the voltage / u _c / of the capacitor C during the function in the stabilized state, with a phase factor cosφ = 0.3 which characterizes the system of the inductor insert represents, where

/ 5 /

In the stabilized state, the greatest voltage of the capacitor C is somewhat lower than that in the Occasion for the maximum value U _ml . It can be confirmed that the ratio U _ml / U _o is smaller than 1.4 depending on the change in the factor cos φ. In the case shown in FIG. 1, the pulse generator simply doubles the perceived time T and delivers the control pulses at intervals T ₁ = 2T. If the device is dimensioned in this way, it outputs the nominal power, which theoretically can be continuously reduced to zero if the pulse sequence time starting from the value T _i = 2T is extended by the external intervention in the pulse generator.

The other method for power regulation is that the value T _i / T = const. keeping the first pulse generator off or on periodically. is switched on / pulse group control /. After switching off, the capacitor C remains in the state charged to the voltage U _m , which means that when the pulse generator is switched on, the repeated starting from the stabilized state is continued.

Checking the harmonic content of the inductor current / i / according to FIG. 1, it can be determined that the frequency, which is decisive from the point of view of inductive heating, of the feed source characterized by the ratio value T _i / T = 2 has the value 0.8 f to be regarded as the mean value _S at cosφ = 0.3 / at cos φ = 0.05 the value is 0.85.f _S 'at cos φ = 0.6 the decisive frequency is 0.77 f _S /. This means that 0.4 T time is available to free the thyristors, which is longer than the time to free the thyristors of the pulsed supply source according to the patent HU-PS 186043, but it is also much longer / 3.2 times / than the value relating to the inverters in the bridge circuit.

This means that with the feed source according to the invention - with thyristors with the same free time - systems with 3.2 times higher cut-off frequency than with the inverters in a bridge circuit can be brought about. By checking the inductor current, it can be demonstrated that a DC component never occurs during the operation of the feed source according to the invention, regardless of the values cos φ and T _i ./T.

In order to be able to compare the utilization of the thyristors in the different inverters, we introduce an utilization factor:

where P is the power of the source,

U _{z is} the maximum voltage on the thyristors in the

Blocking direction, I _eff denote the root mean square of the current flowing through the thyristors. The quantity after the product U _z .I _eff is called the power of the thyristors. The utilization factor of the thyristors of the feed source according to the invention is practically identical to that of the pulse operation feed source according to the patent HU-PS 186043. The utilization factor for the thyristors of the inverters in the bridge circuit is smaller at cos φ <0; 5, larger at cos φ> 0.5 / approximately 0.5 times at cos φ = 0.1, approximately at cos φ = 0.3 0.8 times, at cos φ = 0.8 approximately 1.2 times /. In contrast, increased the price of the thyristors increases with the reduction in the release time more than with the increase in the power. Taking into account the 3.2 times the equivalent cut-off frequency of the cut-off frequency according to the invention, even with cos φ = 0.1, a much less expensive thyristor can be used to implement a pulse-operated supply source without a choke coil.

As we have already mentioned, the supply source according to the invention, which operates in pulsed mode, contains - in contrast to the previous solutions - no choke coil. The inductor I itself takes on the role of the inductor, in which the loss - which occurs as a result of the current flowing through the charging of the capacitor C - is converted into useful heat. It can be confirmed that that

If the power loss of the choke coil is omitted - also taking into account other sources of loss - in the majority of cases that occur in practice, the efficiency of the feed source according to the invention exceeds all the efficiencies of the inverters that have been customary up to now. Compared to the inverters in bridge connection, the increase in efficiency can also reach 5%. Compared to the inverters in pulse mode according to the patent HU-PS 186043, the increase in efficiency is even greater. One of the reasons for this is that the inductor current of the inverter according to the invention contains no DC component.

It is clear from what has been said that the feed source according to the invention is capable of adapting flexibly and between wide limits to the change in the parameters / R _S and L _S / which can be measured at the terminals of the inductor I, until the relations / 2 /, / 3 / and / 4 / exist. The majority In practical cases, the inductance L _{S that can be} measured at the terminals of inductor I, compared with the empty (without insert) inductors not provided with insert 8, decreases due to the de-energizing effect of the eddy currents induced in insert B. Therefore, if a given inductor I is dimensioned to a state that represents the greatest load, ie the relations / 2 /, / 3 / and / 4 / exist, these will also be the case of any decrease in the load, ie also be present with an empty inductor, the apparatus remaining operational. For a given application B, if the apparatus is dimensioned taking into account the inductor I - which means the lowest inductance L _S - it will also work with an increasing L _S value. From what has been said, it can be seen that the feed source dimensioned for an inductor I with the lowest possible inductance and for the insert B with the greatest load remains operable with any system of the inductor insert, which in comparison with all the known inverters offers a high degree of flexibility brings itself. For the sake of good order, however, it should be noted that the nominal power supplied by the supply source decreases approximately with the square root of the ratio of the inductance increase. If this power is not sufficient, the frequency of the pulse groups has to be increased due to the increased frequency. The supply source according to the invention can be switched off either by switching off the supply voltage or by the first pulse generator. In both cases, there is a residual voltage on the capacitor C. the capacitor C must be discharged.

Compared to the previous thyristorized inverters, the pulsed supply source without choke coil has the following advantages:

- the space requirement and the mass are smaller,

- the execution is cheaper,

- in general, the efficiency is greater,

- more flexible operating characteristics, - the possibility of continuously regulating the - output without changing the supply voltage,

- With thyristors with the same release times, the cut-off frequency is higher / compared to the inverters in bridge connection about 3.2 times /, compared to the inverters in bridge connection, there are the following advantages:

- Both the structure and the occasion are simpler, compared with the inverter in pulse mode according to the patent HU-PS 186043: - The inductor current has no DC component.

Claims

Claim:

Power source in pulse mode without a choke coil for inductive heating, the main circuit of which contains a charge thyristor, a discharge thyristor, discharge diode and capacitor, furthermore the main circuit with its input terminals is connected to the DC voltage source, with the output terminals to an inductor provided with an insert, characterized in that in the main circuit the first input terminal (1) connects to the negative pole of the DC voltage supply source and the cathode of the charging thyristor (T2), while the second input source (2) connects the positive pole of the DC voltage supply source, the anode of the discharge thyristor (T1) and the cathode of the discharge diode ( D1) is connected, the first output terminal of the main circuit being common to the second input terminal (2) with which the inductor (I) provided with the insert (B) is connected to one of its terminals and the other terminal of the Inductor (I) forms the other output terminal (3), which is connected to the node (4) via the capacitor (C), the node (4) in turn is connected to the charging thyristor (T2), the anode of the discharge diode (D1) , and the cathode of the discharge thyristor (T1) connected.