JPS584432B2 - Metal pipe induction heating device - Google Patents

Description

[Detailed Description of the Invention] This invention continuously detects the wall thickness of a metal pipe without touching it nearby, supplies optimal power according to changes in wall thickness, and compensates for temperature changes in the pipe due to changes in wall thickness. This invention relates to a metal pipe induction heating device that constantly heats a metal pipe to a constant temperature.

Conventionally, when induction heating a metal pipe, the amount of power to be supplied to a heating coil with a standard wall thickness is determined, and that power is always supplied to heat the pipe.

However, it is known that the wall thickness of a pipe cannot be made uniform due to the manufacturing process, and that there are always variations in the wall thickness.

Now, the external dimensions of the pipe are dw (an), and the wall thickness is t (cm).
), the feed rate is S (m/see), and the specific gravity of the pipe is 7.85P/cm3 considering steel, the processing amount M per unit time is M=π(dw-t)・tX7.85X0. IXSX36
00Ckg/Hr) (1) Now assuming that the wall thickness t has increased by a factor of 10, the processing jar M' that changes is M' = π (dw - 1.1t) 1.1tX7.85X0
．． IXSX3600 (kg/Hr) (2) It will increase by almost 10 factors.

Assuming that there is no change in coil impedance, coil efficiency, or coil power ratio due to the change in wall thickness, M
', the amount of power supplied to the pipe is approximately 90 parts per unit weight.

If the pipe is heated to 1000° C. at a processing amount of M, if the amount of electric power is 90%, the temperature of the pipe will be approximately 900° C.

In reality, if the pipe wall thickness changes, the magnetic field strength inside the pipe will change, so the coil impedance Z.

X Kyle power factor cosψ. , the coil efficiency η also changes.

If the voltage now applied to the coil is EC, the electric power Pw applied to the pipe is expressed as.

Therefore, as the wall thickness changes, the coil power Pc supplied to the →il changes, and the work power Pw applied to the work changes accordingly.

Therefore, it is completely impossible to heat the pipe to a uniform temperature by keeping the voltage supplied to the coil constant when the pipe wall thickness changes.

Also, since the efficiency of the coil changes with the wall thickness, it is impossible to heat the pipe to a uniform temperature even if the power supplied to the coil is kept constant.However, this invention solves this problem. This was developed with this in mind, and the purpose is to provide a metal pipe induction heating device that can always heat a pipe to the same temperature even if the wall thickness of the pipe changes.

An embodiment of the present invention shown in the drawings will be described below.

In the figure, 1 is a transformer, 2 is an inverter, 3 is a changeover switch such as a contactor, 4 is a capacitor, and 5 is a heating coil, through which a metal pipe to be heated (hereinafter referred to as pipe) is sent in the direction shown by the dashed line.

Reference numeral 6 is an instrument transformer, and 8 is a speed detector such as a tachogenerator, which detects the feed speed of the pipe.

9 is a pipe external dimension detector; in the case of the figure, a part of the magnetic path of the AC electromagnet is made up of a part of the heated metal pipe, and by detecting the coil current of the AC electromagnet, the external shape of the heated metal pipe is determined. Know the dimensions.

7 is a pipe wall thickness detection coil, and a power source 14 separate from the heating coil 5 is provided to increase the surface power density of the pipe.
The magnetic permeability of the pipe is brought close to 1, and the wall thickness detection coil 7
The thickness of the pipe can be determined from the relationship between the current flowing through the pipe and the applied voltage.

The magnetic field strength Hr at any point inside the pipe is determined by the following equation.

Hr=HP(P-jQ)(4) However, H.

:Magnetic field strength on the pipe surface In equation (1), P and Q are shown by the following equation.

PccF(dw/s)G(t/dw)(5)QXF'(
dw/s) G'(t/dw) (6) However, F, G, F'
, G' represent a function.

dw...Pipe diameter, S...Current penetration depth t...
If the relationship between the pipe wall thickness, the current and voltage flowing through the wall thickness detection coil 7, and the pipe diameter determined by the external dimension detector 9 are known, the wall thickness t of the pipe can be determined.

In other words, the wall thickness t can be detected by determining the impedance from the relationship between the voltage and current of the wall thickness detection coil 7.

Information on the pipe wall thickness, pipe feeding speed, and external dimensions from the wall thickness detection coil 7, speed detector 8, and external dimension detector 9 is sent to the central information processing unit 13.

The central information processing unit 13 calculates the electric power required to heat the pipe and determines the coil voltage EC to be supplied to the heating coil 5.

A signal is sent from the central information processing unit 13 to the gate of the rectifying side thyristor of the inverter 2, and the output voltage of the inverter 2 is controlled to an optimal value.

In that case, the voltage transformer 6 feeds back to the central information processing unit 13 to check whether EC is the optimum value.

In addition, in order to maintain the output frequency of the inverter 2 at an optimal value, the frequency is also fed back to the central information processing unit 13 by the voltage transformer 6, and if the output frequency is not at the optimal value, a switch is used to change the total capacitance of the capacitor 4. Adjust by adding or removing 3.

10.11 is a current transformer, and 12 is a voltage transformer.

The following describes how the central information processing unit 13 performs calculations to determine the EC.

Now, wall thickness t (Cm) pipe diameter dw (cni) feed speed S
(r/min) information is sent.

Inner diameter dc (an) of heating coil 5, coil length tc (cm
), the number of coil turns Nc, and the temperature rise curve of the pipe are stored in the central information processing unit 13 as initial conditions.

Therefore, the coil impedance Zo and coil power ratio cosψ of the heating coil 5.

, the coil efficiency η can be calculated.

The unit weight M1 of the pipe is 7.85 g/cm3, and the throughput M2 is M2=M1-s. x60 (ky/Hr) (8) Now, the net amount of electricity required to heat from room temperature to any temperature is PN
shall be.

For example, when heating from room temperature to 1000℃, PN is PN = 0.201 [III/kg) (9) The work power to be input into the pipe ~ is Pw - PN - M2 [KW] (10) The coil power PC is Therefore, even if the three pieces of information, namely wall thickness, pipe diameter, and feeding speed change, the coil voltage EC required to heat the coil to the required temperature can be given by:

In the above example, the output voltage of the inverter 2 is changed, and the power input inverter 2 is applied with the optimum power necessary for the heating coil 5.
It is also possible to apply the optimum power by changing the oscillation frequency of the oscillation frequency.

The impedance 2 of the heating coil 5 is mostly reactance.

Therefore, the following equation approximately holds true. (However, they are different in a strict sense.

)ZQ仄f(13) Now if the coil voltage is EC As shown in equation (3), Pw is Therefore It is expressed as

When a self-excited inverter is used as the inverter 2, its oscillation frequency f is L: Inductance of the coil 5 C: Capacity of the power factor compensation capacitor 4 Therefore, from equations (15) and (16), PwX√C (17) ( As can be seen from equation 17), even if the output voltage of the inverter 2 is constant, the output power can be controlled by changing the capacitance of the capacitor 4.

As mentioned when formula (13) was derived, the coil power Pw is C
It is not exactly proportional to 1/2.

Since f affects the current penetration depth and is also related to the resistance of the heating coil, strictly speaking, the relationship is complicated, but by switching the capacitance of the capacitor 4 with the changeover switch 3, the optimum value can be determined. It can be seen that it is possible to obtain coil power.

In the above description, as a thickness detection method, a voltage was applied to the thickness detection coil 7, and the thickness was detected from the relationship between the voltage and current.

In addition, an AC electromagnet was used to detect the external dimensions of the pipe, but
In any of the above, the effects of the present invention will not be hindered in any way even if other methods are used that continuously and accurately know the wall thickness and external dimensions.

As described above, according to the present invention, even if the wall thickness of the pipe changes, induction heating can always be automatically performed to a predetermined temperature.

[Brief explanation of drawings]

The figure is a block diagram showing one embodiment of the present invention. In the figure, 2 is an inverter, 3 is a changeover switch, 4 is a capacitor, 5 is a heating coil, 6 is an instrument transformer, 7 is a wall thickness detection coil, 8 is a feed rate detector, 9 is an external dimension detector, 13 is the central information processing unit.

Claims (1)

[Claims] 1 It has a heating coil that inductively heats the metal pipe to be heated, and a wall thickness detection coil that is excited by a power source different from the heating coil, and the thickness detection coil is determined by determining the impedance from the relationship between the current and voltage of this detection coil. A metal pipe induction heating apparatus comprising: a wall thickness detector for detecting thickness; and a control means for controlling power supplied to the heating coil according to the output of the wall thickness detector.