JP5354652B2 - Induction heating power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an induction heating power supply apparatus increasing effective power to a heating coil and shortening dissolution time by automatic impedance matching of an inverter in accordance with load impedance. <P>SOLUTION: In the induction heating power supply apparatus, a forward converter converting DC connected to a commercial power supply into AC is connected to an inverter constituted of a switching element through a DC reactor, and DC is converted into AC so as to supply it to a load. When measured impedance is out of upper/lower limits which are previously set, a self control frequency and a gate lap time at that time are stored. A system is switched from a self control operation to a non-self-control operation and a non-self-control frequency is raised or dropped in accordance with impedance. Loss is compared with reference loss data and lap time is adjusted. The measured non-self-control frequency is compared with the set upper/lower limits and then restoration to the self-control operation is determined. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

The present invention relates to an induction heating power supply device having an improved mechanism for turning on a gate of a switching element of an inverse converter.

Generally, a high frequency inverter for induction heating is a power supply device for induction heating, and its main circuit connects a forward converter connected to a commercial power supply to an inverse converter composed of switching elements via a DC reactor, From here, the power corresponding to the load resonance frequency is supplied to the heating coil. Induction heating is one of the electric heating methods in which a material to be heated is mainly put into a heating coil and electric power is supplied to the heating coil to generate an eddy current in the material to be heated to melt and heat.

The heating and melting conditions of the material to be heated are constantly changing, and the resonance frequency and load impedance fluctuate accordingly.However, an inverter that operates in synchronization with the load resonance frequency always has high efficiency and stable power supply. Required. Since the inverter is a current source inverter, the maximum power can be applied to the heating coil when the load impedance that changes depending on the melting conditions in the heating coil is equal to the output impedance of the inverter. State.

However, as the load fluctuates, current precedes when the load impedance is low, and conversely, voltage precedes when the load impedance is high, resulting in a problem that the power applied to the heating coil decreases. In particular, melting of the cooling furnace material at the start of induction furnace operation has a large load impedance variation, and impedance matching caused by this greatly varies. If impedance matching is automatically obtained according to the above, the dissolution time can be shortened.

As a method of taking impedance matching, there are a method of changing the tap voltage of the matching transformer in the matching device, and a method of changing the load resonance frequency. Here, as a method of changing the load resonance frequency, There is a method of changing the inductance of the heating coil. However, this method is not practical because the inductance of the heating coil cannot be easily changed.

There is also a method of changing the total capacity of the power factor adjusting capacitor. However, in the case of automatic adjustment, there is a problem that a switch is required and the price increases. There is also a method to change the output frequency of the inverter, but the conventional inverter cannot be operated in other ways due to restrictions on the commutation allowance time of the inverter, and basically the inverter operates according to the load resonance frequency. Therefore, the load resonance frequency cannot be adjusted from the inverter side.

JP-A-10-241846

The present invention improves the above-mentioned problem, and by changing the load resonance frequency by automatically variably controlling the output frequency in accordance with the load impedance, and automatically taking impedance matching, the active power to the heating coil is obtained. Thus, an induction heating power supply device in which the melting time is shortened is provided.

A power supply device for induction heating according to claim 1 of the present invention connects a forward converter for converting alternating current connected to a commercial power supply to direct current through a direct current reactor to an inverse converter composed of switching elements. In the induction heating power supply device that converts DC to AC and supplies the load to the load,
(1) an impedance measuring circuit that measures load impedance during self-limiting operation; and (2) a self-limiting frequency storage circuit that stores a self-limiting frequency at this time when the measured impedance deviates from a preset upper and lower limit value; (3) When the measured impedance deviates from the preset upper and lower limit values, a lap time storage circuit for storing the gate lap time at this time, and (4) the measured impedance is set to the preset upper and lower limit values. A self-control operation stop circuit that stops the self-control operation when it is off, (5) another control operation start circuit that starts the other control operation by setting the stored self-control frequency and gate lap time, and (6) measurement when the load impedance exceeds the upper limit, and the frequency falling circuit for lowering the other system frequencies, (7) the measured the load impedance is less than the lower limit value And (8) a wrap for measuring the power loss and adjusting the wrap time of the gate when the loss is large compared to the power loss data corresponding to the output power. A time adjustment circuit, (9) an other control operation stop circuit for stopping the other control operation when the measured frequency deviates from a preset upper and lower limit value, and (10) after the other control operation is stopped, the operation is switched to the self operation. And a self-limiting operation start circuit.

A power supply device for induction heating according to claim 2 of the present invention is characterized in that, in claim 1, the load is an induction melting furnace or an induction heating furnace.

A power supply device for induction heating according to claim 3 of the present invention is characterized in that, in claim 1, the switching element is formed of an IGBT, a power MOSFET, or a power transistor.

According to the induction heating power supply device of the first aspect of the present invention, the variable operation is switched from the self-regulating operation in which the inverter operates in synchronization with the frequency of the load resonance circuit to the other braking operation in which the inverter is operated at the frequency set by the inverter. By controlling the load resonance frequency, the effective power to the heating coil can be increased by automatically taking impedance matching to shorten the melting time.

In addition, the induction heating power supply device according to claim 2 can be widely applied not only to the induction melting furnace but also to a heating device that inserts a material to be heated such as a plate material or a bar material into a coil and performs induction heating. it can.

According to the power supply device for induction heating according to claim 3, a power MOSFET or a power transistor can be used in addition to the IGBT as a switching element constituting the inverse converter.

Hereinafter, an embodiment of the present invention will be described in detail with reference to FIGS. FIG. 1 shows a power supply device for induction heating. In the figure, 1 is a commercial power source, 2 is a forward converter connected to the commercial power source 1, 3 is a DC reactor, 4 is a reverse converter, and 5 is for power factor improvement. A capacitor, 6 is a heating coil, 7 is a circulating circuit, 8 is a DC voltage detection circuit, 9 is a control circuit, 10 is a control device, 11 is a current transformer for measurement, 12 is a measurement transformer, and 13 is an inverter.

The forward converter 2 includes forward converter thyristors 21 to 26, and is a device that generates a DC voltage from the three-phase commercial power supply 1. The DC voltage changes according to the gate timing set by the control circuit 9. The DC reactor 3 removes ripples of the DC voltage generated by the forward converter 2 and also generates excess power factor adjusting capacitors 5 and heating coils 6 that are loads of the inverter 4 and the inverter. This suppresses a rapid increase in current.

The reverse converter 4 includes reverse converter IGBTs 41 to 44 and reverse converter reverse blocking diodes 45 to 48, and supplies power to the heating coil 6 by turning on and off the IGBTs 41 to 44 according to the load resonance frequency. Is. Since the IGBTs 41 to 44 that output this power do not have reverse blocking capability, the reverse blocking diodes 45 to 48 prevent the current from flowing back through the IGBTs 41 to 44 when the IGBTs 41 to 44 are turned off.

Further, since the induction heating device is a low power factor device in the delay direction, the power factor improving capacitor 5 has an effect of improving the delay power factor of the heating coil 6 in the forward direction. Further, when the material to be heated is put into the coil, the heating coil 6 is supplied with electric power from the inverter, thereby generating an eddy current in the heated material in the furnace, and by Joule heat generated by the eddy current, The material to be heated is heated and dissolved.

The recirculation circuit 7 includes a current-limiting reactor 71 and a high-speed thyristor 72 for recirculation circuit for performing recirculation operation. When the inverter 13 stops, the high-speed thyristor 72 is turned on, and the forward converter 2 changes from direct current to alternating current. By operating in the reverse converter region, the power stored in the main circuit is regenerated to the power system side. The current limiting reactor 71 prevents the current from rising suddenly and damaging the thyristor 72 when the circulating circuit high-speed thyristor 72 is turned on.

The DC voltage detection circuit 8 is connected between P and N of the inverse converter 4. In the inverse converter 4, one IGBT 41 and 44 or 42 and 43 and the other IGBT 42 and 43 or 41 and 44 are alternately arranged. In operation, when one is turned on and the other is turned on at a predetermined timing, a reverse voltage generated by commutation and current interruption is detected. The detected reverse voltage is output to the control circuit 9.

The control circuit 9 protects the main circuit element from the operation / stop of the inverter 13, the timing setting for starting the IGBT of the thyristor of the forward converter 4 and the reverse converter, and the overvoltage and overcurrent.

The control device 10 performs operation / stop and output adjustment of the wave induction furnace system. The load impedance is set from the measurement signals of the measurement current transformer 11 and the measurement transformer 12, and the load impedance is set. When the impedance deviates from the upper and lower limit impedance, the inverter 13 is switched from the self-limiting operation that operates in synchronization with the frequency of the load resonance circuit to the other control operation that operates at the frequency set by the inverter 13. The control frequency is variably controlled.

The circuit configuration of the control device 10 is an impedance that measures current and voltage with a measuring current transformer 11 and a measuring transformer 12, and measures load impedance (% R) during self-control operation as shown in FIG. A measurement circuit 15 is connected to an impedance comparison circuit 16 that compares the measured impedance with preset upper and lower limit values. When the measurement circuit 15 deviates from the upper and lower limit values, a self-control frequency storage circuit 17 that stores the self-control frequency at this time; A gate lap time storage circuit 18 for storing the gate lap time at this time is connected.

Further, a self-control operation stop circuit 19 and a separate control operation start circuit 20 are connected to the gate lap time storage circuit 18. Further, an impedance comparison circuit 21 that compares the measured impedance with a preset upper limit value is connected to the other braking operation start circuit 20, and an impedance comparison circuit 22 that compares the measured impedance with a lower limit value is further connected. The impedance comparison circuit 21 is connected to an other damping frequency lowering circuit 23 for lowering the other damping frequency, and the impedance comparison circuit 22 is connected to an other damping frequency raising circuit 24 for raising the other damping frequency.

The impedance comparison circuit 22 is connected to a loss measurement circuit 25 and a loss comparison circuit 26 that compares the measured loss and reference power loss data corresponding to the output power. When the loss is large, the lap time is adjusted. A lap time adjustment circuit 27 is connected. The loss comparison circuit 26 is connected to an other frequency control circuit 28, and further connected to another frequency comparison circuit 29 for comparing the measured frequency with a preset upper / lower limit value. The other-system operation stop circuit 30 for stopping the other-system operation is sometimes connected. In addition, a self-control operation start circuit 31 for switching to self-control operation is connected to the other control operation stop circuit 30.

The operation of the induction heating power supply device having the above configuration will be described. As shown in FIG. 1, an inverter 2 connected to a commercial power source 1 converts an AC voltage into a DC voltage, and through a DC reactor 3, IGBTs 41 to 44 for the reverse converter and reverse blocking diodes 45 to 45 for the reverse converter. 48, the DC voltage is converted into a low frequency or high frequency voltage by the inverter 4 and this electric power is supplied to the heating coil 6 to generate an eddy current in the material to be heated in the furnace. The material to be heated is heated and dissolved by the generated Joule heat.

FIG. 3 shows the change in characteristics of the load impedance (% R) and the resonance frequency F due to melting of the cold material in the induction furnace. Assuming that the load impedance at the rated power / rated voltage is 100% R (= V ² / P), the load impedance is low impedance at the initial stage of melting, but increases as the temperature of the material to be heated rises to a certain constant value. If it rises to 100%, it will fall to 100%, and all the materials to be heated will be melted and the melting will be completed.

When it is less than 100% R as in the initial stage of melting, the relationship between the load impedance and the applied voltage / current is such that the current flows easily as shown in FIG. Do not exceed the rated current by suppressing it. On the contrary, when the melting material is insufficient, or when the material to be heated is equal to or higher than the Curie point temperature, the magnetic flux does not easily pass, so that a high impedance state is obtained and the current hardly flows and the voltage increases.

In the state where the self-sustained operation that operates in synchronization with the frequency of the load resonance circuit is performed in this way, the current and voltage are measured by the measurement current transformer 11 and the measurement transformer 12, and as shown in FIG. During the self-limiting operation, the load impedance is constantly measured by the impedance measuring circuit 15. When the measured impedance is connected to an impedance comparison circuit 16 that compares a preset upper limit value (for example, 120% R) and a lower limit value (for example, 80% R), and when the measured impedance deviates from the upper and lower limit values, the self-control frequency at this time Is stored in the self-limiting frequency storage circuit 17, and the gate lap time at this time is stored in the gate lap time storage circuit 18. Thereafter, if the frequency is rapidly changed, there is a possibility of unnecessary stop, so that the self-control operation stop circuit 19 is activated to stop the self-control operation in order to reduce the impact.

Next, the other braking operation start circuit 20 operates and the other braking operation is started. At this time, the impedance measured by the impedance comparison circuits 21 and 22 is compared with the upper and lower limit values. If the upper limit value is exceeded, the other control frequency lowering circuit 23 lowers the other control frequency. If it is less than the lower limit value, the other frequency raising circuit 24 raises the frequency.

Induction heating generates eddy currents in a material to be heated by electromagnetic induction and heats them with Joule heat. However, eddy currents are larger on the surface of a material to be melted and tend to decrease toward the center of the material. The higher the value, the more prominent. Assuming that the current density flows uniformly on the surface of the material to be heated, the current width δ (penetration depth) is expressed by the following equation.
δ = 5.03√ (ρ / μr · f) Equation 1
Where ρ: resistivity of the material to be heated (μΩ · cm), μr: relative permeability, f: frequency

Therefore, by increasing the frequency, the penetration depth becomes shallow, and magnetic flux concentrates on the surface of the material to be heated, so that it is difficult for current to flow through the entire material to be heated, resulting in a high impedance state. On the contrary, the penetration depth is deepened by lowering the frequency, and the magnetic flux is concentrated at the center of the material to be heated. That is, the load impedance can be adjusted by changing the load resonance frequency.

Since the other damping frequency is different from the original resonance frequency, the loss increases. At the same time as changing the load resonance frequency, the loss is measured and the wrap time of the gates of the IGBTs 41 to 44 for the inverters 4 of the inverter 4 is adjusted. First, the loss is measured by the loss measurement circuit 25, and this is compared with the reference power loss data corresponding to the output power by the loss comparison circuit 26. When the loss exceeds the reference power loss value, the lap time adjustment circuit 27 Adjust the lap time of the gate. In order to suppress loss to a certain level due to this frequency difference, it is effective to adjust the wrap time of the gate of the inverse converter 4 by varying it.

Further, the other control frequency is measured by the other control frequency measuring circuit 28, and the upper and lower limit values set in advance are compared by the other control frequency comparison circuit 29. Stop operation. After the other control operation is temporarily stopped, the self-control operation is restarted by the self-control operation start circuit 31. If it is within the upper and lower limit values, the process returns to the impedance comparison circuit 21 and the adjustment is performed again.

In other words, when the other control frequency is changed and the loss does not increase, the other control frequency is further changed. However, if the other control frequency exceeds the upper limit frequency (upper limit value F: 105% F), or the lower limit frequency. When it becomes less than (lower limit F: 50% F), the device is temporarily stopped and switched to the self-control operation.

Therefore, by increasing the frequency by other control operation, what was originally in a low impedance state becomes a high impedance direction in which current can be suppressed, so that current limit operation is prevented and active power is increased. Conversely, by decreasing the frequency, what was originally in a high impedance state becomes a low impedance direction in which a current can flow, so that the effective power increases.

By variably controlling the load resonance frequency in this way, the load impedance can be arbitrarily changed, and impedance matching with the power supply device is automatically taken to increase the effective power to the heating coil, thereby shortening the melting time. Became possible.

In the above description, the case where the present invention is applied to an induction melting furnace has been described. However, the present invention can also be applied to a heating apparatus that inserts a material to be heated such as a plate material or a bar material into a coil and performs induction heating. In this case, by adjusting the load impedance% R by controlling the frequency without using the tap switch of the matching transformer, impedance matching with the inverter can be automatically taken, and the effective power to the heating coil can be reduced. The heating time can be shortened by increasing. Further, when a magnetic material such as iron is heated from a low temperature, the magnetic properties change (referred to as a magnetic transformation point). When the heating material is iron, the magnetic transformation point is around 750 ° C. By controlling the frequency of the load impedance% R that changes greatly before and after the magnetic transformation point, the power can be stably supplied.

It is a circuit diagram of the power supply device for induction heating by one embodiment of the present invention. It is explanatory drawing which shows the structure of the control apparatus of the power supply apparatus for induction heating shown in FIG. It is a graph which shows the relationship between load impedance and a resonant frequency characteristic. It is a graph which shows the state of load impedance by the change of an applied voltage and an applied current.

1 Commercial power supply
2 Forward converter
3 DC reactor
4 Inverter
5 Power factor improving capacitors
6 Heating coil
7 Circulation circuit
8 DC voltage detection circuit
9 Control circuit
10 Control device
11 Current transformer for measurement
12 Measuring transformer
13 Inverter
21-26 Inverter forward converter thyristor
41-44 IGBT for Inverter
45-48 reverse blocking diode for reverse converter
15 Impedance measurement circuit
16 Impedance comparison circuit
17 Self-limiting frequency memory circuit
18 Gate lap time memory circuit
19 Self-control operation stop circuit
20 Other system operation start circuit
21 Impedance comparison circuit
22 Impedance comparison circuit
23 Other frequency reduction circuit
24 Other frequency increase circuit
25 Loss measurement circuit
26 Loss comparison circuit
27 Lap time adjustment circuit
28 Other frequency measurement circuit
29 Other frequency comparison circuit
30 Other control stop circuit
31 Self-control operation start circuit

Claims (3)

A forward converter that converts alternating current connected to a commercial power source into direct current is connected to an inverse converter composed of switching elements via a direct current reactor, where direct current is converted into alternating current and supplied to a load. In the heating power supply,
(1) an impedance measurement circuit that measures load impedance during self-control operation;
(2) When the measured impedance deviates from the preset upper and lower limit values, a self-control frequency storage circuit that stores the self-control frequency at this time;
(3) A lap time storage circuit that stores the lap time of the gate at this time when the measured impedance deviates from the preset upper and lower limit values;
(4) a self-limiting operation stop circuit that stops the self-control operation when the measured impedance deviates from the preset upper and lower limit values;
(5) Other control operation start circuit for starting the other control operation by setting the stored self-control frequency and the gate lap time;
(6) a frequency lowering circuit for lowering the other control frequency when the measured load impedance exceeds an upper limit value;
(7) When the measured load impedance is less than the lower limit , a frequency raising circuit that raises the other control frequency;
(8) A lap time adjustment circuit that measures power loss and adjusts the wrap time of the gate when the loss is large compared to power loss data according to output power;
A lap time adjusting circuit that adjusts the lap time of the gate when the loss is large;
(9) Other control operation stop circuit that stops the other control operation when the measured frequency deviates from the preset upper and lower limit values;
(10) A self-control operation start circuit for switching to self-control operation after stopping other control operation;
An induction heating power supply device comprising: 2. The induction heating power supply device according to claim 1, wherein the load is an induction melting furnace or an induction heating furnace. 2. The induction heating power supply device according to claim 1, wherein the switching element is formed of an IGBT, a power MOSFET, or a power transistor.