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2022-07-31
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The concept that power electronics technology should be re recognized in the development of high frequency field

today, one of the development directions of power electronics technology is that the working frequency is getting higher and higher, from the previous power frequency (50Hz, 60Hz), intermediate frequency (hundreds of Hz to more than 1000 Hz) to the current high frequency (thousands of Hz). There are many new products such as switching power supply, inverter welding machine and frequency converter. Their appearance fully demonstrates the advantages of the new power electronics technology, such as small volume, light weight, energy saving and material saving. However, it is found that they are not so ideal, have many faults, and are easy to burn out components

I think that when the frequency is high, some basic concepts have changed and need to be re understood. When the frequency is low, we regard some basic concepts, such as the resistance, capacitance, inductance, conductor, etc. with an active horizontal beam at the top, as ideal states, that is, the so-called parameter elements. In a lumped parameter circuit, all resistance functions are concentrated on the establishment of resistance elements. In addition, resistance is no longer considered; All electromagnetic induction phenomena are expressed by inductive elements, and other effects are not considered; All charges are stored only in capacitors. The concept of distributed parameter is different. Resistance, capacitance, inductance and wire are not pure resistance, capacitance, inductance and wire, but a complex of resistance, capacitance and inductance. Regularly replace the oil suction filter and filter element

resistance. Generally, the power resistance is a ceramic tube with resistance wire wound and wound into a spiral tube. It is not difficult to see that it also has inductance, but the dry electricity is very small, which can be generally ignored. There is also distributed capacitance between each turn of resistance wire, and its equivalent circuit is shown in Figure 1

generally speaking, the frequency used in current power electronics technology is below tens of kHz, and its distributed capacitance can be ignored. The actual equivalent circuit is shown in Figure 2. The total impedance Z is the vector sum of resistance Zr and inductive reactance ZL:

z=er+el

the figure is: z= r2+ (2 fl) 2

to illustrate the problem, give an example. Assuming r=10, l=h, f=5*104hz, the result is: z=33. Since the inductive reactance is much larger than the resistance, its phase will also change greatly

capacitance is generally rolled into a roll with two layers of metal films sandwiched with an insulating layer in the middle, and then separately. Two leads are led out from the two furthest ends of the two metal films. The equivalent circuit is shown in Figure 3 (this mainly ignores the leakage resistance of the capacitor). Its total capacitance

c=c1+c2+ +cn

its L and l'are the respective inductance of the two metal films, and

l=l1+l2+ +ln

l'=l'1+l'2+ +l'n

and l=l'

when f is low enough, 2 fl1=2 fln+2 FLN can be regarded as zero or short circuit, so the circuit in Figure 3 is equivalent to several small capacitors in parallel, and the total capacitance

c=c1+c2+ +cn

this is our general use.

when f is high enough, 2 fc1=2 fc2+ +2 FCN is very small, It can also be regarded as a short circuit. At this time, the circuit in Figure 3 becomes Figure 4. The total impedance is equivalent to an inductance, and its value is half of L

The equivalent circuit of the

inductor is shown in Figure 5

where l=l1+l2+ +ln

c1, C2 and CN are inter turn distributed capacitors. Since the frequency reached by the current power electronics technology is not high enough, the distributed capacitance can generally be ignored. However, for the occasions with high voltage, small current and high frequency, the distributed capacitance should be considered

conductor is generally considered as a connecting element. At most, it is recognized that it is a small resistance. When the frequency is high, due to the skin effect, it is recognized that its resistance is larger than its DC resistance when the frequency is high. However, this understanding is not enough. Its equivalent circuit is shown in Figure 6, that is, it is composed of R and l in series

when f is large enough, ZL >> Zr, the conductor shows inductive characteristics. As the current used in the field of power electronics technology is large and di/DT is often large, its induced electromotive force = -ldi/DT is often large, which is a factor that can not be ignored. Therefore, during high-frequency operation, the connecting sub line shall be as short and thick as possible, and shall not turn at will, so as to reduce inductance. Here, I would like to explain why a thick wire should be thick. This is because a thick wire can be regarded as several thin wires in parallel, and its inductance is formed by parallel connection of each inductance. The thicker the wire, the smaller the inductance

for long lines and short lines, we generally believe that the current and voltage at each point on a wire are the same, and they are only a function of time, that is, u=u (T), i=i (T). However, we know that the experimental machine works for customers all the year round, escorting the electric signal to propagate at the speed of light wave, which is 300000 km/S; Its wavelength =3*108/f M. if the length of a wire is equal to the wavelength, the result is as shown in Figure 7. The voltage and current at each point on the wire are obviously not equal. They are not only a function of time, but also a function of space. That is, u=u (x, t), i=i (x, t). When the wire length is 1 = 1/2, it is a good transmitting antenna. In fact, when l can be compared with, we call it a long line. When l and comparison can be ignored, we call it a short line

interestingly, when 50Hz power frequency is used for power transmission, when the transmission line is very long, we can not regard it as a short line. Power frequency wavelength =3*105/50=6000 (km). When the transmission line is one or two hundred km long, it should be considered as a long line. In order to reduce the loss, the frequency must be reduced, which explains why DC transmission is used when long-distance transmission is used

the development of power electronics technology towards high frequency is an important direction. It has brought many benefits, such as small size, energy saving, material saving, etc. but the maturity of new technology. To study goodness, we need not only to explore, but also to have a new understanding in theory

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