Basic Principle of High Frequency Welding

The so-called high frequency is relative to the frequency of the alternating current of 50 Hz, and generally refers to a high frequency current of 50 kHz to 400 kHz. When high-frequency currents pass through metal conductors, they produce two peculiar effects: skin effect and proximity effect. High-frequency welding uses these two effects to welded steel pipe. So what are these two effects?

The skin effect means that when the alternating current of a certain frequency passes through the same conductor, the density of the current is not uniformly distributed over all the cross sections of the conductor, and it concentrates on the surface of the main conductor, that is, the current density of the current on the surface of the conductor is large. The density inside the conductor is small, so we call it the "skin effect". The skin effect is usually measured by the penetration depth of the current. The smaller the penetration depth value, the more significant the skin effect. This penetration depth is proportional to the square root of the resistivity of the conductor and inversely proportional to the square root of the frequency and permeability. In layman's terms, the higher the frequency, the more concentrated the welded steel pipe current is on the surface of the steel plate; the lower the frequency, the more dispersed the surface current. It must be noted that although steel is a conductor, its magnetic permeability will decrease with increasing temperature. That is to say, when the temperature of the steel sheet rises, the magnetic permeability will decrease and the skin effect will decrease.

Proximity effect means that when a high-frequency current flows in opposite directions in two adjacent conductors, current will flow to the close edges of the two conductors. Even if the two conductors have a shorter side, the current does not follow. For shorter routes, we call this effect: "proximity effect." The proximity effect is essentially due to the effect of the inductive reactance, which plays a dominant role in the high frequency current. The proximity effect increases as the frequency increases and the spacing of adjacent conductors approaches. If a core is added around the adjacent conductor, the high frequency current will concentrate more on the surface of the workpiece.

These two effects are the basis for achieving high frequency welding of metals. High-frequency welding uses the skin effect to concentrate the energy of the high-frequency current on the surface of the workpiece; the proximity effect is used to control the position and range of the high-frequency current flow path. The speed of the current is very fast, it can heat, melt and join the adjacent steel plate edges in a short time.

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