JP5803711B2 - Induction heating inverter power supply and overvoltage protection method thereof - Google Patents

Description

  The present invention relates to an induction heating inverter power supply and an overvoltage protection method thereof.

  In the steel process, a step of heating a steel plate is an essential step, and an induction heating device is used, for example, when heating a steel plate in a plate. The induction heating device is a device that has a heating coil and heats a conductor plate by an eddy current induced by the heating coil. In this induction heating apparatus, an eddy current is induced in a conductor plate by an alternating magnetic field (AC magnetic field) generated from a heating coil, and Joule heat is generated in the conductor plate by this eddy current.

  As one of such induction heating devices, for example, there is a so-called transverse type induction heating device in which an alternating magnetic field is applied to a conductor plate so as to intersect the plate surface of the conductor plate to be heated substantially perpendicularly. As a method for controlling a transverse induction heating apparatus, there is a technique disclosed in Patent Document 1. In Patent Document 1, a capacitor is provided in parallel with the heating coil constituting the induction heating device to configure a parallel resonance circuit of the heating coil and the capacitor, and the heating coil is fed with power by a parallel resonance type inverter.

  On the other hand, Patent Literature 2 uses four semiconductor elements having low on-resistance that do not have reverse blocking capability, such as P-MOSFETs and transistors having reverse conducting diodes connected in parallel, but can be controlled in the forward direction. The switch can be turned on and off only by controlling the gate, and the magnetic energy of the current at the time of interruption is stored in the snubber capacitor, and it can be controlled without loss of energy by releasing it to the load side at the next turn-on. A magnetic energy regenerative bidirectional current switch (hereinafter referred to as “magnetic energy regenerative switch”) has been disclosed and is expected to be applied to efficient induction heating.

  However, since the induction heating device using the magnetic energy regenerative switch described in Patent Document 2 is a current source type power supply device as shown in FIG. 3, when the steel plate to be heated is broken or blown, it is used as a capacitor. Current continues to flow, and the voltage applied to both ends of the capacitor exceeds the limit, causing a problem that a semiconductor element provided in the power supply apparatus is destroyed.

  A general overvoltage protection circuit starts a protection operation when a threshold value is set and a voltage higher than that is detected, but it takes a considerable time from the detection of a voltage abnormality to the completion of protection. In the case of a device controlled in accordance with a switching speed of 100 kHz, the breakdown voltage can be increased to a breakdown voltage in a short time, resulting in device failure. In addition, once the induction heating power source fails, it is necessary to stop the operation for about 3 to 8 hours for the failure recovery, resulting in an increase in the operation cost.

JP 2002-313547 A JP 2000-358359 A

  The present invention has been made to solve the above-mentioned problems, and proposes an induction heating inverter power supply equipped with a function capable of protecting a capacitor and a semiconductor element provided in the induction heating inverter power supply and an overvoltage protection method thereof. There is to do.

  The inventors configured a circuit of an electric resistance and a switching element connected in parallel with the capacitor, measured the induction heating supply current and the power supply current, and determined the maximum value of the voltage generated at both ends of the capacitor from these both current values. If this voltage exceeds the limit voltage, it is assumed that the steel sheet to be heated is broken or blown, and the switching element is immediately turned on to prevent the voltage across the capacitor from exceeding the limit value. By doing so, it was found that the destruction of the semiconductor element equipped in the inverter power supply device can be avoided.

The gist of the present invention is as follows.
(1) A bridge circuit in which the end points of two legs are joined to each other, and each of the four branches of the bridge circuit includes a semiconductor device having no reverse blocking capability in which a switching element A and a diode are connected in antiparallel. One semiconductor device is connected in a pair so that the forward direction of the diodes in the semiconductor device faces each other with the midpoint sandwiched between one leg, and the midpoint is sandwiched between the other legs. The semiconductor devices are connected in pairs so that the forward directions of the diodes in the semiconductor devices are reversed.
A capacitor between the midpoint and midpoint of the two legs of the bridge circuit;
A circuit in which an electrical resistance and a switching element B are connected in series;
A circuit in which a thyristor converter and a reactor are connected in series,
Connected in parallel,
A three-phase AC power source is connected to the thyristor converter and power can be supplied from a terminal of the bridge.
Means for measuring a current supplied to the induction heating device (hereinafter referred to as “induction heating supply current”);
Means for measuring a current (hereinafter referred to as “power supply current”) supplied from a circuit in which the three-phase alternating current source and the thyristor converter and the reactor are connected in series;
When any two of the four semiconductor devices are turned off, the measured value of the induction heating supply current and the measured value of the power supply current are input, and any two of the four semiconductor devices are turned off. Means for calculating a maximum value of voltage generated between both ends of the capacitor (hereinafter referred to as “maximum capacitor voltage”);
Means for immediately turning on the switching element B when the maximum capacitor voltage exceeds a preset limit voltage of the semiconductor device;
An induction heating inverter power supply characterized by comprising:
(2) The induction heating inverter power source according to (1), wherein the reactor is 10 to 15 mH in the case of a 6.6 kV system.
(3) Any one of MOSFET, IGBT, GTO, and BJT is used for the switching element A, any one of MOSFET, IGBT, GTO, and BJT is used for the switching element B, and a fast recovery diode is used for the diode. The induction heating inverter power supply according to (1), wherein
(4) A bridge circuit in which the end points of two legs are joined to each other, and each of the four branches of the bridge circuit includes a semiconductor device having no reverse blocking capability in which a switching element A and a diode are connected in antiparallel. One semiconductor device is connected in a pair so that the forward direction of the diodes in the semiconductor device faces each other with the midpoint sandwiched between one leg, and the midpoint is sandwiched between the other legs. The semiconductor devices are connected in pairs so that the forward directions of the diodes in the semiconductor devices are reversed.
A capacitor between the midpoint and midpoint of the two legs of the bridge circuit;
A circuit in which an electrical resistance and a switching element B are connected in series;
A circuit in which a thyristor converter and a reactor are connected in series,
Connected in parallel,
The thyristor converter is a method for overvoltage protection of an induction heating inverter power supply, in which a three-phase AC power supply is connected and power can be supplied from the terminal of the bridge,
Measuring a current supplied to the induction heating device (hereinafter referred to as “induction heating supply current”);
Measuring a current (hereinafter referred to as “power supply current”) supplied from a circuit in which the three-phase alternating current source and the thyristor converter and a reactor are connected in series;
When any two of the four semiconductor devices are turned off, the measured value of the induction heating supply current and the measured value of the power supply current are input, and any two of the four semiconductor devices are turned off. Calculating a maximum value of voltage generated between both ends of the capacitor (hereinafter referred to as “maximum capacitor voltage”);
A step of immediately turning on the switching element B when the maximum capacitor voltage exceeds a preset limit voltage of the semiconductor device;
An overvoltage protection method for an induction heating inverter power supply, comprising:
(4) The overvoltage protection method for an induction heating inverter power source according to (4), wherein the reactor is 10 to 15 mH in the case of a 6.6 kV system.
(5) Any one of MOSFET, IGBT, GTO, and BJT is used as the switching element A, any one of MOSFET, IGBT, GTO, and BJT is used as the switching element B, and a fast recovery diode is used as the diode. The overvoltage protection method for an induction heating inverter power supply according to (4),

  In an inverter device comprising a bridge circuit having a semiconductor device in which a switching element A and a diode are connected in antiparallel in four branches, and an inverter power supply device for induction heating having a capacitor on the input side of the inverter device, Since the circuit in which the switching element B is connected in series is connected in parallel to the capacitor, the switching element B is immediately turned on when the steel sheet to be heated breaks or melts, thereby preventing the capacitor voltage from rising. As a result, the semiconductor device equipped in the apparatus can be prevented from being damaged.

It is the induction heating inverter power supply of this invention. It is operation | movement of the induction heating inverter power supply of this invention, Comprising: (a) It is a figure which shows the case where an overvoltage arises. It is an operation | movement of the induction heating inverter power supply of this invention, Comprising: (b) It is a figure which shows the condition at the time of normal time which an overvoltage does not produce. It is an induction heating inverter power supply not equipped with the protection circuit of the present invention. FIG. 4 is a diagram illustrating the flow of current immediately after the gate signals are turned on for the semiconductor devices U1 and Y4 and the gate signals are turned off for the semiconductor devices V2 and X3 in the apparatus shown in FIG.

  The embodiment of the present invention is an induction heating inverter power supply shown in FIG.

  As shown in FIG. 1, it has a bridge circuit in which the end points of two legs are joined by terminals 13 and 14, and each of the four branches of the bridge circuit has a reverse connection in which a switching element A and a diode are connected in antiparallel. It is based on a bridge circuit in which one semiconductor device having no blocking capability is connected. The semiconductor devices U, V, X, and Y that do not have the reverse blocking capability in which the switching element A and the diode are connected in antiparallel are 1, 2, 3, and 4, which are one in each of the four branches of the bridge circuit. A connected circuit is configured.

  A pair of semiconductor devices U and V are connected to one leg of the bridge circuit so that the forward direction of the diodes in the semiconductor device faces each other with the middle point 11 therebetween, and the other leg includes Two semiconductor devices X and Y are connected in pairs so that the forward directions of the diodes in the semiconductor device are reversed with respect to the point 12. This bridge circuit constitutes an inverter device.

Between the midpoint 11 of one leg and the midpoint 12 of the other leg, 1) a circuit having a capacitor C _MERS 5 and 2) a circuit in which an electric resistance R _b 6 and a switching element B7 are connected in series, 3 ) A total of three circuits of a thyristor converter 8 and a reactor L _dc 9 connected in series are connected in parallel. A thyristor converter 8 is connected to a three-phase AC power source 10. In the present invention, a current measuring device 16 for measuring a current flowing through a circuit in which 3) the thyristor converter 8 and the reactor L _dc 9 are connected in series is further installed.

  When the inverter power supply of the present invention is used for induction heating, an induction heating coil is connected to two end points (terminals 13 and 14) obtained by joining the end points of two legs of the bridge circuit, and induction heating of the steel sheet is performed. Thereby, electric power is supplied from the terminals 13 and 14 to the induction heating device 20. In the present invention, a current measuring device 15 for measuring the current supplied from the terminals 13 and 14 is further installed.

The current supplied from the terminals 13 and 14 is measured by the current measuring device 15, and the current supplied from the three-phase AC power supply 10, the thyristor converter, and the reactor L _dc 9 is measured by the current measuring device 16.

The measured values of the current measuring devices 15 and 16 are sent to the capacitor maximum voltage calculator 40 from moment to moment, and the maximum capacitor voltage, that is, the capacitor C is used by using the current value when any one of the four semiconductor devices is turned off. The estimation calculation of the maximum value of the voltage generated across _MERS 5 is started. When the estimated value of the maximum capacitor voltage exceeds the breakdown voltage of the semiconductor element, an ON signal (gate signal of the switching element B) is output from the capacitor maximum voltage calculator 40, and the switching element B is turned on.

  Here, the reactor is preferably 10 to 15 mH in the case of the 6.6 kV system. This is because if the reactor is less than 10 mH, a circuit in which the reactor and the thyristor converter are connected in series cannot be regarded as a current source as shown on the left side of FIG. On the other hand, since it costs more than necessary to manufacture a reactor of more than 15 mH, the upper limit is set to 15 mH.

  Further, it is desirable to use any one of MOSFET, IGBT, GTO, and BJT as the switching element A, and any one of MOSFET, IGBT, GTO, and BJT as the switching element B. This is because a switching speed of 30 kHz to several hundred kHz is necessary.

When the estimated value of the capacitor voltage exceeds the breakdown voltage of the semiconductor element, the electric resistance R _b 6 has the same breakdown voltage as the capacitor in order to instantaneously turn on the switching element B7 to form a short circuit, and 30 ( A low value of about Ω) to 100 (Ω) is desirable.

  Furthermore, it is desirable to use a fast recovery diode as the diode. This is because a high-frequency current of several tens of kHz to several hundreds of kHz flows through the diode, so it is necessary to be fast both in turn-off and on. If a diode that is not fast in both turn-on and off is used, it is always in a conductive state and does not function as a diode. Because.

(Operation of induction heating inverter power supply)
The operation of the induction heating inverter power supply will be described with reference to FIG.

FIG. 2B shows the operation when the steel plate to be heated does not break or blow. Here, the gate signals of the semiconductor device U1 and the semiconductor device Y4 are simultaneously turned ON, and the gate signals of the semiconductor device V2 and the semiconductor device X3 are simultaneously turned OFF.
Further, the gate signals of the conductor device U1 and the semiconductor device Y4 are simultaneously turned OFF, and the gate signals of the semiconductor device V2 and the semiconductor device X3 are simultaneously turned ON.

The semiconductor device is repeatedly turned on and off at a predetermined cycle. At this time, a pulsating flow as shown in the figure is obtained in the voltage V _c of the capacitor C _MERS 5. Since the steel sheet does not break or melt, no abnormal voltage is generated in the voltage V _c of the capacitor C _MERS 5. As a result, the gate signal of the switching element B does not rise.

FIG. 2 (a) shows the operation when the steel sheet to be heated is broken or melted. That is, the plate breakage occurred at the time indicated by the downward arrow immediately after the gate signals of the semiconductor device U1 and the semiconductor device Y4 were simultaneously turned ON and the gate signals of the semiconductor device V2 and the semiconductor device X3 were simultaneously turned OFF. In the case of the conventional apparatus, the voltage V _c of the capacitor C _MERS 5 becomes a voltage as shown by the broken line in FIG. 2A, and the peak voltage exceeds the limit voltage. Capacitors and semiconductor elements are destroyed.

On the other hand, when the plate breaks, each current value increases as indicated by broken _lines in “coil current i _load (induction heating supply current)” and “power supply current i _dc ” in FIG. Therefore, in the present invention, the induction heating supply current and the power supply current are measured, and the maximum value of the voltage generated at both ends of the capacitor is calculated from these both current values in the capacitor maximum voltage calculator 40. When the estimated value of the maximum capacitor voltage exceeds the breakdown voltage of the semiconductor element, the capacitor maximum voltage calculator 40 immediately outputs the gate signal (ON signal) of the switching element B, and the switching element B is turned on. As a result, both ends of the capacitor C _MERS 5 are substantially short-circuited, and the voltage V _c follows the course shown by the solid line in FIG. When the voltage V _c of the capacitor C _MERS 5 is not short-circuited at both ends, it becomes a voltage like a dotted line and reaches a voltage at which the semiconductor element is destroyed. In the present invention, the semiconductor element is prevented from being destroyed. .

  The breakdown voltage of the semiconductor element is set in advance.

(Voltage behavior at both ends of C _MERS 5 when steel sheet breaks or melts)
As illustrated in FIG. 2 (a), the inventors believe that the case where the estimated value of the maximum capacitor voltage is estimated to exceed the breakdown voltage of the semiconductor element is considered to be a case where the steel sheet is broken or blown. Found.

In such a case, for example, the time point after the gate signals of the semiconductor device U1 and the semiconductor device Y4 are simultaneously turned from OFF to ON and the gate signals of the semiconductor device V2 and the semiconductor device X3 are turned from ON to OFF is taken as an example. The behavior of the voltage across the capacitor C _MERS 5 from the time will be described.

As shown in FIG. 4, immediately after the gate signals are turned on for the semiconductor devices U1 and Y4, both semiconductor devices are turned on, and the gate signals are turned off for the semiconductor devices V2 and X3. Flowing. At this time, the current i _dc flowing from the current source 30 is I _dc which is a direct current.

両辺を時間で微分し、かつＬ_hで割り、

の関係を用いると、

と変形できる。また、 先に述べたように、ｉ_dcはＩ_dcという一定値であるから、

と、さらに変形できる。この微分方程式をｉ_load（ｔ）＋Ｉ_dcについて解くと、

が得られる。ただし、ｉ_loadoffとは、半導体デバイスＶ，ＸのゲートをＯＦＦした瞬間に流れる電流である。この式は、

と書き直すことができる。このとき、Ｃ_MERSの両端の電圧は、

以上から、Ｃ_MERS両端の電圧の最大値Ｖ_{c_peak}は、

と、算出される。 Further, when the induction heating device and the heated steel plate 20 are expected from the power supply device, the inductance L _h and the electric resistance R _h are known and the electric resistance R _h is assumed to be negligibly small. At this time, the current i _dc from the current source and the current i _load flowing through the induction heating device follow the following differential equation.

Differentiate both sides by time and divide by L _h

Using the relationship

And can be transformed. In addition, as described above, since i _dc is a constant value of I _dc ,

And can be further transformed. Solving this differential equation for i _load (t) + I _dc

Is obtained. However, i _loadoff is a current that flows at the moment when the gates of the semiconductor devices V and X are turned off. This formula is

Can be rewritten. At this time, the voltage across C _MERS is

From the above, the maximum voltage V _{c_peak} across C _MERS is

And calculated.

That is, the maximum value V _{C_peak} of the voltage across the C _MERS is calculated from the measured value of the current I _dc from the current I _Loadoff a current source flows through the steel plate 20 is induction heating device and the heating by using the Equation 8] it can.

Now, when the steel sheet to be heated is broken or melted during induction heating, the resistance value of the steel sheet decreases rapidly and the inductance value of the steel sheet increases. The impedance Z _load of the induction heating device 20 is determined from the resistance value and inductance value of the heating coil and the steel plate, and is expressed by Z _load = [R _h ² + (ω _LC · L _h ) ² ] ^1/2 . Here, R _h is a resistance value between the heating coil and the steel plate, and L _h is an inductance value between the heating coil and the steel plate.

When the frequency is several tens (kHz) or more, R ² << (ω _LC · L _h ) ² , so Z _load ≈ω _LC · L _h , and the heated steel sheet breaks or blows In this case, the voltage V _load of the induction heating device 20 increases as the inductance value Z _load of the steel plate increases from V _load = Z _load · I _load . Along with this, the current I _load also increases as shown in FIG. As a result, I _loadoff, which is a current that flows at the moment when the gates of the semiconductor devices V and X are turned off, also increases when the heated steel sheet breaks or melts.

Further, the current i _dc flowing from the current source 30 is a direct current I _dc , and when the steel plate to be heated is broken or blown, the resistance value of the steel plate rapidly decreases as shown in FIG. Therefore, I _dc increases.

Thus, when the steel plate to be heated is broken or blown, I _{dc jumps} up and I _loadoff increases. The inventors have found that this can be observed with the current measuring device 15 or the current measuring device 16.

Therefore, the capacitor maximum voltage calculator 40 obtains from the current measuring device 16 the instruction value I _loadoff of the ammeter when two of the four semiconductor devices obtained from the current measuring device 15 that measures the supply current are turned off. Using both measured values of the current value I _dc supplied by the thyristor converter, the maximum voltage generated at both ends of the capacitor after two of the four semiconductor devices are turned off is calculated, and this is a preset breakdown. When it is determined that the voltage is exceeded, the circuit element can be prevented from being destroyed by short-circuiting the switching element B7.

  The results of evaluating the examples of the present invention and the comparative examples are shown in Table 1. Inventive Examples 1 to 7 were satisfactory with no circuit breakage even when the plate was broken or melted. However, in the case of Comparative Examples 1 and 2, when the plate breakage or fusing occurred, the circuit breakage occurred and was defective.

1-4: Semiconductor device 5: Capacitor 6: Resistor _Rb
7: Switching element B
8: Thyristor converter 9: Reactor L _dc
10: Three-phase AC power supply 11: Midpoint of one leg of the bridge circuit 12: Midpoint of the other leg of the bridge circuit 13: Output terminal 1
14: Output terminal 2
15: Current measuring device 16: Current measuring device 20: Induction heating device and electric resistance and inductance of heated steel plate 30: Current source 40: Capacitor maximum voltage calculator

Claims (6)

A bridge circuit in which the end points of two legs are joined to each other, and each of the four branches of the bridge circuit is connected to a semiconductor device having no reverse blocking ability, in which switching elements A and diodes are connected in antiparallel. The semiconductor device is connected in pairs so that the forward direction of the diodes in the semiconductor device faces each other with the midpoint sandwiched between one leg and the semiconductor with the midpoint sandwiched between the other legs. Two semiconductor devices are connected in pairs so that the forward directions of the diodes in the device are reversed.
A capacitor between the midpoint and midpoint of the two legs of the bridge circuit;
A circuit in which an electrical resistance and a switching element B are connected in series;
A circuit in which a thyristor converter and a reactor are connected in series,
Connected in parallel,
A three-phase AC power source is connected to the thyristor converter and power can be supplied from a terminal of the bridge.
Means for measuring a current supplied to the induction heating device (hereinafter referred to as “induction heating supply current”);
Means for measuring a current (hereinafter referred to as “power supply current”) supplied from a circuit in which the three-phase alternating current source and the thyristor converter and the reactor are connected in series;
When any two of the four semiconductor devices are turned off, the measured value of the induction heating supply current and the measured value of the power supply current are input, and any two of the four semiconductor devices are turned off. Means for calculating a maximum value of voltage generated between both ends of the capacitor (hereinafter referred to as “maximum capacitor voltage”);
Means for immediately turning on the switching element B when the maximum capacitor voltage exceeds a preset limit voltage of the semiconductor device;
An induction heating inverter power supply characterized by comprising:   The induction heating inverter power supply according to claim 1, wherein the reactor is 10 to 15 mH in the case of a 6.6 kV system.   One of MOSFET, IGBT, GTO, BJT is used for switching element A, one of MOSFET, IGBT, GTO, BJT is used for switching element B, and a fast recovery diode is used for the diode. The induction heating inverter power supply according to claim 1. A bridge circuit in which the end points of two legs are joined to each other, and each of the four branches of the bridge circuit is connected to a semiconductor device having no reverse blocking ability, in which switching elements A and diodes are connected in antiparallel. The semiconductor device is connected in pairs so that the forward direction of the diodes in the semiconductor device faces each other with the midpoint sandwiched between one leg and the semiconductor with the midpoint sandwiched between the other legs. Two semiconductor devices are connected in pairs so that the forward directions of the diodes in the device are reversed.
A capacitor between the midpoint and midpoint of the two legs of the bridge circuit;
A circuit in which an electrical resistance and a switching element B are connected in series;
A circuit in which a thyristor converter and a reactor are connected in series,
Connected in parallel,
The thyristor converter is a method for overvoltage protection of an induction heating inverter power supply, in which a three-phase AC power supply is connected and power can be supplied from the terminal of the bridge,
Measuring a current supplied to the induction heating device (hereinafter referred to as “induction heating supply current”);
Measuring a current (hereinafter referred to as “power supply current”) supplied from a circuit in which the three-phase alternating current source and the thyristor converter and a reactor are connected in series;
When any two of the four semiconductor devices are turned off, the measured value of the induction heating supply current and the measured value of the power supply current are input, and any two of the four semiconductor devices are turned off. Calculating a maximum value of voltage generated between both ends of the capacitor (hereinafter referred to as “maximum capacitor voltage”);
A step of immediately turning on the switching element B when the maximum capacitor voltage exceeds a preset limit voltage of the semiconductor device;
An overvoltage protection method for an induction heating inverter power supply, comprising:   The said reactor is 10-15mH in the case of a 6.6kV system | strain, The overvoltage protection method of the induction heating inverter power supply of Claim 4 characterized by the above-mentioned.   One of MOSFET, IGBT, GTO, BJT is used for switching element A, one of MOSFET, IGBT, GTO, BJT is used for switching element B, and a fast recovery diode is used for the diode. An overvoltage protection method for an induction heating inverter power supply according to claim 4.

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