JP2955735B2 - Parallel type inverter for induction heating equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a parallel inverter, and more particularly to a parallel inverter suitable for use in an induction heating device.

[0002]

2. Description of the Related Art A parallel type inverter is used as one type of power supply for an induction heating device. In a parallel type inverter used in this induction heating device, a load is formed by an inductor (heating coil) for heating an object to be heated, and a tank composed of the inductor and a power factor compensation capacitor (also serving as a commutation capacitor). The circuit is configured to supply AC input power at a frequency tuned to the tank circuit.

[0003]

In the parallel inverter used in the induction heating apparatus described above, one parallel inverter supplies AC power to a plurality of inductors, that is, a plurality of tank circuits at the same time. May be desirable in terms of production efficiency.

However, when a plurality of tank circuits are operated in parallel by a parallel inverter, a series resonance occurs between a plurality of tank circuits connected in parallel with respect to a harmonic component contained in an output current of the parallel inverter, and this harmonic current Is superimposed on the output voltage of the parallel inverter. As a result, the commutation allowance time of the thyristor constituting the parallel type inverter is shortened, and the commutation allowance time becomes shorter than the turn-off time of the thyristor, which may cause a commutation failure of the thyristor. Due to the circuit configuration, there has been a problem that failure of commutation of the thyristor results in a short circuit of the power supply, which may lead to a serious accident.

The present invention has been made in view of such circumstances, and it is intended to operate in a stable state without causing commutation failure even when performing parallel operation on a plurality of tank circuits connected in parallel. It is an object of the present invention to provide a parallel-type inverter capable of performing the following.

[0006]

The parallel type inverter according to the present invention comprises a plurality of tank circuits each composed of an inductor as a load and a power factor compensating capacitor connected in parallel, and the plurality of tank circuits connected in parallel are connected to the plurality of tank circuits. A parallel inverter for supplying AC power, wherein an inductance is connected in series between one output terminal of the parallel inverter and an input terminal of each of the plurality of tank circuits. The series impedance of the series resonance circuit formed by the shunt is set to a constant that is inductive to the harmonic component of the output current of the parallel inverter.

[0007]

In the above-structured parallel inverter, AC power is supplied from the parallel inverter to the plurality of tank circuits connected in parallel at an inverter frequency equal to the parallel resonance frequency of each tank circuit. Since the output current of the parallel inverter is a trapezoidal wave (a symmetrical wave), it includes odd harmonic components. When a plurality of tank circuits are connected in parallel, a parallel resonance circuit is formed by shunting each tank circuit at the same time, and this generates parallel resonance with respect to a harmonic component.

However, by connecting an inductance to each of the tank circuits and each of the tank circuits and one output terminal of the inverter for these harmonics (for example, the third or higher harmonics), And a series resonance circuit with a shunt of the corresponding tank circuit, and the series impedance for harmonics is set to be inductive. No longer resonates with the waves. That is, no parallel resonance occurs between the plurality of tank circuits with respect to the harmonic component of the output current of the inverter.

Therefore, the harmonic component of the output current is not superimposed on the output voltage of the parallel inverter, so that the commutation failure of the thyristor does not occur, and the parallel inverter is stable against a plurality of tank circuits connected in parallel. Can be driven.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. Prior to the description of the embodiments of the present invention, a problem that occurs when a plurality of tank circuits connected in parallel by a conventional parallel inverter are operated in parallel will be specifically described.

FIG. 2A shows the configuration of a parallel inverter used in an induction heating device. In the figure, the AC power supplied from the three-phase AC power is converted into DC power by the converter 1, is supplied to the inverter section constructed by thyristor TH ₁ to TH ₄ through the DC reactor L _o.

In the inverter section, each of the thyristors TH _{1 to} T
The firing angle of H ₄ is controlled by a gate pulse given from a control circuit (not shown), and the output voltage _vo and the output current _io
Comprising AC power is supplied to the tank circuit composed of inductor L ₁ and the power factor compensating capacitor C ₁ serving as a heating coil for heating an object. Incidentally, the power factor compensating capacitor C ₁ is also acts as a commutation capacitor.

The waveforms of the output voltage _vo and the output current _io of the parallel type inverter shown in FIG. 2A are as shown in FIG. That is, the output voltage (load voltage) v _o is the power factor compensating capacitor C _1, and oscillates at the parallel resonance frequency f _o determined by the inductor L _1, the constants C _1, L ₁ as normal Q increases To be selected, the waveform is approximately sinusoidal.

On the other hand, the output current i _o is a symmetrical wave and a trapezoidal wave in which both positive and negative waves are symmetric with respect to the intersection with the horizontal axis. Therefore, the output current i _o consists only of odd harmonics that do not include a DC component.

Meanwhile, anode voltage V _th of the waveform of the thyristors TH ₁ to TH ₄ parallel type inverter shown in FIG. 2 (1) is as shown in FIG. 2 (3), the commutation margin time T
_c is determined by the time t ₂ when a reverse bias voltage is applied from time t ₁ to a forward current flowing through the thyristor becomes zero on the thyristor becomes zero. As is well known, in order for the inverter circuit to operate stably without causing a commutation failure, the turn-off time of the thyristor is assumed to be T _off.
c > T _off must be satisfied.

By the way, as shown in FIG. 3A, consider a case where a plurality of (two in this embodiment) tank circuits 11 and 12 connected in parallel by a conventional parallel type inverter 10 are operated in parallel. Here, the tank circuit 11 includes an inductor L ₁
And a power factor compensating capacitor C ₁ and a tank circuit 1
2 consists of inductor L ₂ and the power factor compensating capacitor C ₂ Prefecture.
T is a transformer.

FIG. 3B is a circuit diagram showing the circuit shown in FIG. 1A in consideration of the leakage inductance of the transformer T. In FIG. 1, when the tank circuits 11 and 12 connected in parallel by the parallel inverter 10 are operated in parallel, the harmonic components of the output current of the parallel inverter 10 are connected in parallel between the capacitor C ₁ , the leakage inductance L _r , and the capacitor C _2. resonance occurs, flows in a loop shown by the dotted line resonant current i _n is shown. As a result, the parallel inverter 1
0 to the output voltage v _o, the harmonic resonance current i _n higher-order distortion voltage v _in is superimposed with the (FIG. 4 (1)), high by harmonic resonance current i _n to the anode voltage V _th of the thyristor to order distortion voltage v _in is superimposed (Fig. 4 (2)), the commutation margin time T when the higher-order distortion voltage v _in accordance with high-order resonance current does not overlap
_The commutation allowance time becomes shorter as T _c 'as compared with _c .

In this case, as described above, if T _c ′ <T _{off in} relation to the turn-off time T _{off of the} thyristor, commutation fails.

Next, FIG. 1 shows the configuration of an embodiment of a parallel inverter according to the present invention. In FIG. 1, a parallel inverter according to the present invention includes an inverter body 10 including a main circuit of the inverter, commutation capacitors C ₁ and C _2, and harmonic current suppressing inductance elements L _s1 and L _s2 . The tank circuit 21 from the inverter 10 consists of inductor L ₁ and the power factor compensating capacitor C ₁ Tokyo via the inductance element L _s1, also inductor L ₂ and the power factor through the transformer T ₁ and the inductance element L _s2 output voltage v _o to the tank circuit 22 consisting of the compensation capacitor C ₂ Prefecture, AC power of the output current i _o is supplied.

Here, the constants of L _s1 and L _s2 of the inductance element are such that the series impedance of the series resonance circuit formed by the shunt of the corresponding tank circuits 21 and 22 is a harmonic of the output current of the parallel type inverter. It is set to a constant that is inductive to the components.

Next, a method of determining the constants of the inductance elements L _s1 and L _s2 in the circuit configuration of FIG. 1 will be described with a specific example. On flights declaration of description, it is assumed that to ignore the leakage inductance of the transformer T _1.

[0022] (1) inductance L _1, L ₂ of the decision heating coil of the power factor constant of the compensation capacitors C _1, C ₂ are determined from the heating conditions, therefore, each of the constants of the capacitors C _1, C _2, induction It is determined by the parallel resonance frequency of the coils (heating coils) L ₁ and L ₂ . For example, the frequency of the output voltage to the inverter applied to the heating coil, in other words, the tank circuit 2
Assuming that the resonance frequencies f _{0 of} 1, 22 are f ₀ = 1000 (Hz),
The resonance frequency f ₀ is f ₀ = １ ／ π√ (LC) (where L,
C is a constant of an inductance and a capacitor constituting the tank circuit. )

[0023]

## EQU1 ## C ₁ (or C ₂ ) = ｛1 / (2π × 1000) ² ×
L ₁ (or L ₂ )｝ (F). The optimum value of the frequency f ₀ is determined by the processing amount of the induction heating device and the diameter of the object (bar) to be heated.
The DC resistance of the inductance element is ignored.

(2) Determination of Constants of Inductance Elements L _s1 and L _{s2 Since} the harmonic frequency output from the parallel inverter is 3f ₀ or more, it is necessary to prevent parallel resonance from occurring at a frequency of 3f ₀ or more. There is. Further, since the inductive reactance is larger than the capacitive reactance for the harmonic frequency, the inductance shunt of each tank circuit can be ignored. Therefore, the added inductance element L _s1
Series resonance order of the capacitor shunt C ₁ corresponding tank circuit 21 and is set to be 3f ₀ or less, similarly series resonance with the capacitor shunt C ₂ of the tank circuit 22 corresponding to the inductance element L _s2 The order is set so as to be 3f ₀ or less.

More specifically, the inductance elements L _s1 and L _{s1
When s2} is inserted, shunt impedances Z ₁ and Z ₂ are

## EQU2 ## Z ₁ = j (nω ₀ L _s1 −1 / (nω ₀ C ₁ ))

## EQU3 ## Z ₂ = j (nω ₀ L _s2 −1 / (nω ₀ C ₂ )), and L _s1 and L _s2 whose series impedance is less than or equal to third order and whose parentheses are positive are selected. As a result, for the third and higher harmonics, the value in the parentheses becomes positive and the inductive property is obtained. Therefore, any series resonance circuit constituted by the inductance elements L _s1 and L _s2 connected in series to each tank circuit 21 and 22 becomes inductive to harmonics, and the shunt of the plurality of tank circuits 21 and 22 is obtained. Can prevent parallel resonance.

As described above, the output current i _o of the parallel inverter is a trapezoidal wave, and is composed of odd harmonics (fundamental waves, third, fifth, seventh,...). The lower harmonic component is third order. Therefore, the constants of the inductance elements L _s1 and L _s2 may be determined so as to have the capability of suppressing the third or higher harmonic components. In practice the inductance element L _s1 and the capacitor C ₁ is for this, or the inductance element L _s2 and the series resonance frequency f _o by the series circuits of a capacitor C _2, of the lower third harmonic second harmonic The frequency can be selected, for example, f _o = 2f.

Inductance element L_s, Configure the tank circuit
The resulting power factor compensating capacitor C, inductor L, and
Inductance element (L_s1Also
Is L s_Two) And the tank circuit 21 (or 22) in series
Of the embodiment obtained by putting specific numerical values into the equivalent circuit of
Fig. 4 shows the characteristic diagram of
Child (L_s1Or L_s2The figure below shows the characteristic diagram of the conventional circuit without
It is shown in FIG. In the embodiment shown in FIG.
Higher-order parallel resonance due to each shunt in the tank circuit is avoided.
On the other hand, in the case of FIG.
You can see that it is shaking.

[0028] Since the harmonic content of the output current i _o of the inverter by adding this manner the inductance element L _s1, L _s2 in the present embodiment is not superimposed on the output voltage v _o of the inverter, the harmonic of the output current i _o Commutation failure due to wave components is eliminated, and the parallel inverter can be operated in a stable state.

[0029]

As described above, according to the present invention, in a parallel inverter in which a plurality of tank circuits connected in parallel are operated in parallel, one output terminal of the inverter is connected to each of the plurality of tank circuits. An inductance is connected in series between the input terminal and the input terminal. The inductance is such that the series impedance of the series resonance circuit formed by the shunt of the corresponding tank circuit induces the harmonic component of the output current of the parallel inverter. Is set to a constant
The parallel type inverter can be operated in a stable state with respect to a plurality of parallel-connected tank circuits without causing a thyristor commutation failure.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of an embodiment of a parallel inverter according to the present invention.

FIG. 2 is a circuit diagram illustrating a configuration of a basic circuit of a parallel inverter, a waveform diagram illustrating an output voltage and an output current, and a waveform diagram illustrating an anode voltage of a thyristor of the parallel inverter.

FIG. 3 is a circuit diagram showing a connection relation of a parallel type inverter when a plurality of tank circuits are operated in parallel with a load, and a circuit diagram showing the leakage inductance of a transformer thereof.

4 is a waveform diagram showing a state in which a harmonic component of an output current is superimposed on an output voltage of the conventional parallel inverter shown in FIG. 5, and a harmonic component of an inverter output current on an anode voltage of a thyristor of the inverter; FIG. 9 is a waveform diagram showing a state in which are superimposed.

FIG. 5 is a frequency-impedance characteristic diagram of the parallel inverter according to the embodiment of the present invention.

FIG. 6 is a frequency-impedance characteristic diagram of an inverter when a plurality of tank circuits are operated in parallel from a conventional parallel inverter.

[Explanation of symbols]

_{DESCRIPTION OF SYMBOLS} 1 Converter 10 Parallel type inverter 11 Tank circuit 12 Tank circuit 20 Inverter main body 21 Tank circuit 22 Tank circuit L _s1 inductance element L _s2 inductance element

Claims (1)

(57) [Claims] 1. A parallel inverter for connecting a plurality of tank circuits each including an inductor as a load and a power factor compensating capacitor in parallel, and supplying AC power to the plurality of tank circuits connected in parallel, An inductance is connected in series between one output terminal of the parallel inverter and an input terminal of each of the plurality of tank circuits, and the inductance is connected in series with a series resonance circuit formed by a shunt of the corresponding tank circuit. A parallel inverter for an induction heating device, wherein an impedance is set to a constant that is inductive with respect to a harmonic component of an output current of the parallel inverter.