For details, please purchase the PDF file of the mechanical worker (thermal processing) in the fourth issue of 2007 or at the bottom of the download page.
4-1 inverter selection
1. Most of the power supply devices in the induction heating equipment currently used in the domestic forging plant of the intermediate frequency inverter are static converters with the thyristor as the main component. Because it works in the intermediate frequency band, it is called the thyristor intermediate frequency inverter.
Figure 4-1 Schematic diagram of the main circuit of the IF inverter...
*This article was originally published in Mechanical Workers (Thermal Processing), 2007, Issue 4 (Total No. 547)
A parallel resonant circuit is formed by an inductor and an electric heating capacitor to inductively heat the billet. When the square wave intermediate frequency current Ia composed of the fundamental wave and the harmonic enters the parallel resonant circuit, the resonant circuit exhibits a large impedance to the fundamental current and a small impedance to the resonant current. Therefore, the square wave intermediate frequency current is under the load of the inductor. The voltage is close to a sinusoidal waveform.
2. The calculation of the main circuit parameters of the intermediate frequency inverter is based on the rectifier transformer valve side line voltage 660V, 12 pulse wave, double rectifier, rated power 2000kW, nominal frequency 200Hz, parallel resonant frequency converter as an example to introduce the main circuit parameter calculation.
1 Negative inverter output voltage (intermediate frequency voltage) Ua is ignored.
Ua = πUd / (2 cosφ) = π × 810 / (2 cos30 °) =1039V
Where: Ud - DC voltage.
Φ—— Thyristor inverter triggers the lead angle.
Table 4-1 Relationship between valve side line voltage of rectifier transformer and DC voltage of inverter
......
* The DC voltage Ud in this table is not calculated, but is simulated when the voltage drop of the rectifier transformer is 6.5% under the rated output power of the inverter.
Table 4-2 Domestic thyristor inverter trigger lead angle reference value......
4-2 Selection of rectifier transformer
1. The characteristic of the rectifier transformer is the transformer used for power supply of the rectifier device, which is called a rectifier transformer. Although the working principle of the rectifier transformer and the power transformer is the same (Faraday's law of electromagnetic induction), the structure is similar (composed of two or more windings on the common core), and the appearance is similar (both static electrical equipment). But there is still a big difference between the two:
1 The number of pulse waves (ie equivalent phase number) in one cycle of the rectifier transformer can reach 6 or 12.
......
2. Determination of the capacity of the rectifier transformer:
The empirical relationship between the capacity S of the rectifier transformer and the rated power P of the thyristor IF inverter output:
S= ≈1.25P (kVA)
Where k is the product of the efficiency of the rectifier transformer, the power factor of the rectifier transformer load, and the insurance factor, which is 0.85.
Η1: The efficiency of the thyristor intermediate frequency inverter rectifier is 0.97.
Η2: The efficiency of the thyristor IF inverter inverter is 0.97.
......
3, rectifier transformer connection group 1 6 pulse wave rectifier transformer connection group has two forms:
One: D, d0
Another: D, Yn11
......
4. Impedance voltage of the rectifier transformer The impedance voltage of several rectifier transformers commonly used in the forging factory can be selected by referring to the following table.
Table 4-4 Commonly used rectifier transformer impedance voltage selection table capacity (kVA) 630 1000 1600 2500 4000 6300 10000
Impedance voltage (%) 6 6 6.5 6.5 7 7.5 7.5~8
4-3 harmonic source and its harm
A non-linear electrical device that injects harmonic current into the utility grid is called a harmonic source. Induction heating equipment used in forging plants is a collection of multiple harmonic sources.
1 The three-phase full-controlled bridge rectifier and inverter in the thyristor type IF inverter are harmonic sources. Among them, the influence of harmonics generated by the rectifier on the power grid is the most important.
......
4-4 Harmonic effects of harmonics:
1. Increase the short circuit ratio.
The short circuit ratio refers to the ratio of the short circuit capacity of the grid to the rated power of the inverter. If harmonic suppression measures are not taken, in order to ensure the quality of the power, the rated power of the inverter must be limited, that is, the short circuit ratio is increased.
Figure 4-2 Relationship between the power of the rectifier and the short-circuit capacity of the grid and the number of rectified phases The relationship between the rectification device recommended by the UK power sector and the short-circuit capacity of the grid and the number of rectified phases (see Figure 4-2) can be used for reference.
2. Increase the voltage level of the grid side and valve side of the rectifier transformer separately, and strengthen the system's ability to withstand harmonics.
3. Using the phase shift of the rectifier transformer and increasing the number of rectified phases to suppress harmonics In general, the characteristic harmonic order h of the current of the harmonic source of the rectifier is theoretically related to the following:
h=kP±1
......
The next part of the content: steel blue brittle cutting, warm forging induction heating and sensor design and parameter calculation.
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