JPH0428194A - Parallel high-frequency induction heating method - Google Patents

Abstract

PURPOSE:To lessen magnetic-field interference between plural heating coils so as to cause each of the heating coils to surely do induction heating by setting the frequency of each of currents applied respectively to the plural heating coils wound round an object of paramagnetic body being heated to a high frequency. CONSTITUTION:Heating coils 2, 3 are respectively wound round a steel pipe 1 whereto such heat treatment as annealing or the like is given in proximity to the pipe. A high-frequency power unit 4 has an independent oscillator, and it has its input side P connected to each of commercial electric wires l and applies a high-frequency current(not less than 600Hz) to the heating coil 2 from its output side S. Also a high-frequency power unit 5 has an independent oscillator, and it has its input side P connected to each of the electric wires l and applies a high-frequency current(not less than 600Hz) to the coil 3 from its output side S. By applying the currents each having not less than 600Hz respectively to the coils 2, 3 in this manner, the electromagnetic mutual interference of the coils 2, 3 is evaded, so that each of the coils 2, 3 can efficiently do induction heating of the steel pipe even if the frequencies and phases of currents flowing respectively through the coils 2, 3 are different from one another.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a parallel high frequency induction heating method, and is useful when applied to, for example, heat treatment such as annealing of steel pipes.

<Prior Art> Conventionally, the following methods have been used to heat treat, such as annealing, a welded joint such as piping using a steel pipe.

(a) A method in which combustible gas such as acetylene or propane is combusted and the combustion flame is blown directly onto the welded joint.

(b) A method in which an electric heater is placed close to the welded joint, and the welded joint is heated by radiant heat generated from the electric heater when energized.

→ For example, by winding a conductor along the outer periphery of a steel pipe to form a heating coil, and passing a commercial current of 50Hz or 60Hz, or a high frequency current of 116 to 30Hz to this heating coil. , a method that generates an induced current in a steel pipe and heats it using the Joule heat.

Steel pipes, such as the main steam pipes of thermal power plants, etc.
In order to anneal large diameter tubes, the above method ←→, that is, the induction heating method using a heating coil is used. When using this type of induction heating method, if a single power supply does not provide enough heating power, or if there is a need to widen the heating zone, multiple power supplies can be used to supply power to multiple heating coils. I try to do that.

Here, a conventional technique for annealing using a plurality of power supplies and a plurality of heating coils will be explained with reference to FIG. As shown in the figure, two heating coils 2 and 3 are wound around a steel pipe 1.

The power supplies 6 and 7 configured with transformers receive commercial frequency power from the commercial power line j at the secondary sides U and V, respectively, and the secondary sides U, V, respectively.
An alternating current is supplied from V' to the heating coil 2°3. Therefore, an alternating magnetic field is generated in the heating coils 2 and 3, and the steel pipe 1 is heated by induction by Joule heat.

<Problem to be solved by the invention> By the way, in the prior art shown in FIG.
The respective terminals U and V on the primary side of 6 and 7 are connected to the commercial power supply with the same polarity, and the respective terminals U and V on the secondary side are connected to the heating coils 2 and 3 wound in the same direction. Must be connected with the same polarity. If one of the connection polarities is incorrect, the magnetic field generated by the heating coil 2 and the magnetic field generated by the heating coil 3 will have opposite polarities and cancel each other out.
It becomes impossible to heat the steel pipe l. Therefore, it is necessary to be extremely careful not to make a mistake in the connection polarity, but incorrect connections have sometimes occurred due to mistake.

On the other hand, AC power supplies using thyristor inverters or transistor inverters are used as induction heating power supplies, but this type of inverter type AC power supply generally has a low output frequency and performs induction heating using a single heating coil. It's just that it's being adopted. Even if two inverter-type AC power supplies of this type are used to supply current to two heating coils placed close to each other, in other words, the third
Even if an inverter-type AC power source is used in place of the power sources 6 and 7 in the figure, efficient heating cannot be achieved. This is because inverter-type AC power supplies have a built-in oscillation circuit that automatically oscillates, so even if the inverter-type AC power supplies have the same specifications, the frequency and phase of each inverter-type AC power supply will be slightly different. If the frequencies and phases differ in this way, the magnetic fields from both coils will partially cancel out, resulting in a decrease in heating efficiency.

Especially when the temperature or other conditions change, the frequency and phase of the automatic oscillation circuit built into the inverter-type AC power supply tend to fluctuate, so the output frequency and phase of the two inverter-type AC power supplies may deviate significantly. There is,
In this case, the heating efficiency will be significantly reduced.

In view of the above-mentioned prior art, the present invention provides a parallel high-frequency induction heating method that eliminates the need to match the connection polarity of power supplies when heating using a plurality of heating coils, and allows efficient heating. .

Means for Solving the Problems> The present invention to solve the above problems consists of winding a plurality of heating coils around a paramagnetic object to be heated, and individually applying 600 volts to each heating coil from a plurality of independent high-frequency power sources.
It is characterized by supplying a high frequency current of Hz or more.

Effect〉 When the frequency of the current flowing through multiple heating coils wound around a paramagnetic object to be heated is set to 600) fz or more, the interference of the magnetic fields between the heating coils becomes extremely small, and each heating coil Reliable induction heating.

Embodiment FIG. 1 shows a first embodiment to which the method of the present invention is applied. As shown in the figure, the steel pipe 1 undergoes heat treatment such as annealing.
Two heating coils 2°3 are wound closely. The high frequency power supply 4 has an independent oscillator, and the input side P is connected to the commercial f41s7, and the high frequency current (600 Hz or more) is passed through the heating coil 2 from the output side S. The high frequency power source 5 has an independent oscillator, and the input side P is connected to the commercial power supply @1, and a high frequency current (600 Hz or more) is passed through the heating coil 3 from the output side S. When a current of 600) tz or more is passed through the heating coils 2 and 3 in this way, mutual electromagnetic interference between the heating coils 2 and 3 is eliminated (the reason will be explained later), and the current flowing through both heating coils 2 and 3 is reduced. Even if the frequencies and phases are different, induction heating can be efficiently performed for each heating coil 2 and 3. Therefore, commercial electric line i
and the connection with the high frequency power supplies 4 and 5, as well as the high frequency power supplies 4 and 5.
5 and the heating coils 2 and 3 do not need to be polarized.

Here, we will show the experimental data. tl [1 has an outer diameter of 43
0 [■], heating coils 2 and 3 (the cross-sectional area of the coil wire is 38 [n;) and 20 turns, the supply current frequency and the spacing between the heating coils 2゜3 are as shown in the table below. As a result, the steel pipe 3 could be heated efficiently and reliably by each of the heating coils 2 and 3. From this table, we can see that the supply current frequency is 6QO
It can be seen that efficient induction heating can be performed at Hz or higher.

Next, the reason why there is no mutual electromagnetic interference between adjacent heating coils when a high frequency current is passed through the heating coils will be explained. There are two reasons for this.

(it) The penetrating current that flows through the steel pipe exposed to the high-frequency magnetic field generated by the heating coil flows only through the surface portion of the steel pipe due to the skin effect, so the influence on the adjacent heating coil is reduced.

(iii) As the current frequency increases, the reversal of the magnetic domains of the steel material cannot be followed over time, and the magnetic field component that goes out to the area outside the coil width decreases.This will be explained in more detail.For example, paramagnetic If a coil is wound around a body and a direct current is passed through it, magnetizing it in one direction, and then passing a current in the opposite direction, the magnetic body will be magnetized in the opposite direction according to the hysteresis curve, but all the magnetic domains in the magnetic body will align in the opposite direction. A certain amount of time is required for this to occur.If the current is further reversed before all the magnetic domains are in the same direction, the magnetic material will be in the same state as if it were not magnetized.When high-frequency current (alternating current) is passed through the heating coil,
In such a state, when two sets of heating coils are wound around a magnetic material (m-tube), the higher the frequency, the less interference between both heating coils. The time required for the magnetic flux density to completely follow the magnetizing force of the hysteresis curve is determined by the permeability, coercive force, residual magnetism, size and thickness of the magnetic body, magnetizing force, etc. of the magnetic body.

FIG. 2 shows a second embodiment to which the method of the present invention is applied. In this embodiment, four heating coils 11, 12, 13 degrees 14 are wound around #W1, and each heating coil 11, 12, 13, 14 is connected to a high frequency power source 21, 22, 23, 24 connected to a commercial electric line 4. A high frequency current of 600 Hz or more is applied to each of the two. This embodiment also has no interference between the coils, and there are four heating coils 11, 1213.14.
This increases the heating width. Of course high frequency power supply 21-2
The connection polarity of 4 may be arbitrary.

<Effects of the Invention> Even if a plurality of heating coils are wound around the object to be heated and the heating coils are placed close to each other, the current flowing through each heating coil is a high frequency current of 6007 or more, so that each heating coil has a high frequency current of 600. Even if the flowing current frequencies and phases are different, induction heating can be performed efficiently without mutual interference between the heating coils. In other words, it is possible to perform parallel heating operation in which a plurality of heating coils are lined up and heated. At this time, the connection polarity of the primary side and secondary side of the plurality of power supplies can be set arbitrarily, and there is no need to pay attention to the polarity when connecting, which reduces the burden on the operator.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a first embodiment to which the method of the present invention is applied, FIG. 2 is a block diagram showing the second embodiment, and FIG. 3 is a block diagram showing a conventional technique. In the drawing, 1 is a steel pipe, 2. 3, 11, 12, 13 degrees, 4. 5, 21, 22, 23 degrees are power supplies, and I is a commercial electric wire. 14 is a heating coil 24 is a high frequency

Claims (1)

[Claims] Multiple heating coils are wound around a paramagnetic object to be heated, and each heating coil is individually supplied with 600 Hz from multiple independent high-frequency power sources.
A parallel high-frequency induction heating method characterized by supplying a high-frequency current of Hz or more.