For details, please purchase the PDF file of the mechanical worker (thermal processing) in the second issue of 2007 or at the bottom of the download page.
2-1Changes in physical properties during heating of metal billets
The resistivity and relative permeability of the billet are important for frequency determination and sensor parameter design.
The figure below shows the relationship between the relative magnetic permeability μr and the temperature T of the steel resistivity Ï2 and a certain magnetic field strength.
Figure 2-1 Electrical resistivity and relative permeability of steel
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*This article was originally published in Mechanical Workers (Hot Processing), Issue 2, 2007 (Total Issue 545)
1 Cold gauge specification of the billet from the initial temperature to the Curie temperature. At this time, both Ï2 and μr are variables. This region is a ferromagnetic material region, the average temperature t = 650 ° C, Ï2 may be 0.6 × 10 -6 Ω m, μr > 1.
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2-2 choice of current frequency
The frequency of the billet induction heating is determined according to the following two principles:
1 The electrical efficiency of the inductor is not less than 5% of the limit value: m2 ≥ 2.5.
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Select the frequency should pay attention to:
1 If a piece of equipment heats a variety of blanks, the diameter of the blank should be within the frequency range specified by the formula. If it is not satisfied, more than two frequencies can be used to satisfy each other.
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2-3 sensor efficiency and power
The power delivered by the grid to the induction heating device consists of two parts: one is the power loss of the power supply system (intermediate frequency converter, busbar, electric heating capacitor, etc.), and the other part is the electrical loss, heat loss and heating of the inductor in the inductor coil. Average effective power. The rated power distribution of the IF inverter is shown in Figure 2-2. We refer to the latter part as the rated power of the intermediate frequency inverter.
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2-4 heating time
In the longitudinal magnetic field, the circular section metal blank, regardless of the current frequency, the effective heating layer depth of the current penetration is always close to 0.4 times the billet radius, ie Δ2 = 0.4R2. The continued heating from the effective heating layer to the core of the blank must rely on the heat transfer of the metal itself. This is how the heart temperature difference (radial temperature difference) during induction heating is generated. Therefore, under the condition of △2≥0.4R2, the reasonable selection of the frequency (see Equation 2-2, 2-3) will minimize the distance between the surface of the billet and the core.
The shortest heating time tk is obtained from the special solution of the differential equation of heat conduction:
Tk = Ï„ (2-14)
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The above is the determination of the heating time of the equal-pitch sensor, and the heating time can be greatly shortened by using the variable-turn distance sensor heating. The so-called variable pitch method means that the axial width (pitch distance) of the copper tube (generally rectangular cross-section copper tube) wound around the inductor coil is narrower, the discharge end is wider, and generally divided into two or three. level. This design is due to the same current flowing through all the coils, and the magnetic field strength and unit power are maximal at the feed end of the inductor, causing the temperature to rise quickly.
The surface temperature of the billet is raised to the final temperature by 10% to 30% of the total heating time, that is, within 10% to 30% of the total length of the inductor. Due to the large temperature difference between the cores, the heat transfer rate of the metal is fast, and rapid heating is achieved. .
Calculation method of variable pitch distance induction heating time:
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The next part of the brief introduction: "Induction heating current frequency, power, heating time determination and solenoid sensor parameter calculation (below)." Including: the design of the solenoid sensor parameters, calculation examples and the promotion of parameter design.
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