1.Ferrite cores selection

1) Length of ferrite cores

In theory and practice, in order to "eliminate" the welding efficiency loss caused by the diverging magnetic field of the ferrite cores, the length of the ferrite cores is usually 15~40mm too long to reserve the length of the diverging magnetic field of the ferrite cores. The higher the hf power, the larger the pipe diameter, the thicker the pipe wall, and the larger the values of G and L. However, longer is not always better. Too long is not only ineffective, but also increases the difficulty and cost of placing magnetic rods.

G should not be too long, because the welded pipe is an irregular round pipe after welding, and the inner diameter of the pipe cavity with internal burrs will be smaller, theoretically at least 1~2mm smaller than the blank to be welded. If there are too many ferrite cores in front, the risk and probability of pulling the ferrite cores during the movement of the blank will increase.

2) O.D. of ferrite cores

When selecting the outer diameter of the magnetic bar, it should be as large as possible under the premise of ensuring the cooling effect. If the coolant pressure of the magnetic bar is greater than 0.15mpa, it can be larger; otherwise, it should be smaller. For details, please refer to the following formula：d=k（D－2t）

In the formula, d and D are the outer diameter of magnetic rod and welded tube respectively, and T is the wall thickness. D is the maximum diameter of a bundle of magnetic rods when the combination form of magnetic rods is bare rods with multiple branches or bare rods with clusters. When the magnetic rods are used in insulation bushing, d is the outer diameter of insulation bushing. K is the magnetic bar diameter selection coefficient, k=0.8~0.9, when the coolant pressure is large, the coolant pressure is small, when the insulation sleeve is used, k is large, the cut tube blank is large, the uncut tube blank is small; When the welding joint is firm and smooth, take a large value, and vice versa.

3)Notice

（1）Maximize cross section to improve welding efficiency.

（2）The maximum surface area ensures that the magnetic bar is fully cooled.

（3）The distance between any point of the magnetic bar (radial) and the heat dissipation surface should not be more than 20mm, to avoid heat dissipation blind area, resulting in partial magnetic loss.

（4）Geometric dimensions to be serialized, so that users have more options according to the specifications of welded pipe.

2. Impedor installation

1) Installation Position

The installation position of impedance device has an important effect on whether it can fully play its function.

Starting from the law of electromagnetic induction, the installation position of the impedords should follow the following principles: one end of the impedor should exceed the induction coil, and a section beyond the induction coil should be appropriately long (about the length of a coil). Specifically, if the impedor device is long enough, the midpoint of the coil can be taken as the center position of the impedor device, so that one end of the impedor device beyond the center line of the extrusion roll 3-6mm, and another section beyond the induction coil a longer distance. If the impedor device is not long enough, its center should be appropriately moved to the heating area, to ensure that one end of the impedor device beyond the center line of the extrusion roll, but also pay attention to make it must exceed the other end of the induction coil.

2) Installation requirements

The installation of impedor should first be firm to avoid vibration of impedor in the welding process. The position of installation should be so that the axis of the impedor and rolling center line, and as close to the solder joint as possible.

3) Water cooling

The purpose of water cooling is to keep each part of the impedance device at a low and constant temperature in the high-frequency welding process, so that the impedance device has a high permeability and remain unchanged. To achieve this goal, attention must be paid to two issues:

One is to have enough water, which requires enough water pressure and appropriate large water cross - sectional area

Second, it should have high heat dissipation efficiency. Even if the water flow is very high, if the heat dissipation efficiency of the impedance is very low, it still cannot reach the purpose of water cooling. In order to improve the heat dissipation efficiency of the impedance, it is best to arrange the water channels of the impedance evenly.

4) Notice

（1）When installing the impedor, the front end shall not exceed the center line of the extrusion roll too far, generally controlled between 3~6mm.

（2）When adjusting the impedance, do not touch the edge of the tube tube, otherwise it is easy to be carried away by the tube tube, of course, the distance should not be too large

（3）In the process of winding impedance device, attention should be paid not to wrap too thick, not too fine, according to the diameter of the steel pipe, the wall thickness and the size of the inner diameter to determine properly, to ensure that the middle water cooling gap, and there are measures to prevent breakage.

（4）In the production process, the position of the impedance device should be checked regularly and whether it is taken away or broken, and the wiredrawing or sundries hanging on the impedance device should be removed frequently

（5）In the normal production process, short ferrite cores can be attached to both sides of the inductor to improve the welding efficiency (there are two functions: one is to strengthen the proximity effect of the inductor's lower tube edge, reduce its eddy current loss, increase the welding current, and then improve the welding efficiency; Second, the induced current is concentrated in the upper half of the tube blank, which can reduce the loss of the back of the tube.)