Why is the square silicon steel sheet used as the transformer core?

- Sep 28, 2020-

Transformers are currently used more in power transmission and instrumentation equipment (switching power supplies). In the two industries, the shapes are basically similar, and the materials are obviously different. Most of the magnetic cores of power transmission transformers are silicon steel sheets, characterized by BS saturation magnetic induction intensity of about 2T (Tesla); and instrumentation (switching power supply) transformers are ferrite core materials, characterized by BS saturation magnetic induction The intensity is about 0.3T (Tesla).

Prerequisite: Our transformer is working in AC, and AC has a high or low frequency, and the transformer core cannot be saturated during operation.

1. In the early days, when transmitting power, when the transformer was working at power frequency, the frequency of 50/60hz was relatively low, but the working current was relatively large. The loss is mainly concentrated in copper loss, and due to the large current, the saturation magnetic induction intensity of the transformer core material is required to be relatively large. Finally, it was found that the material of silicon steel sheet as the magnetic core of the power transmission transformer can satisfy the requirement that the magnetic core is not saturated under low frequency and high current working mode. Therefore, silicon steel sheets are generally used in power transmission transformers because the magnetic saturation is relatively large.

2. Around the 1980s (to be verified), with the increase of electrical equipment, many different power sources are needed and the requirements for power quality of equipment are getting higher and higher. The development of power supply has changed from the original transformer rectification or transformation, to the linear power supply, and then to the switching power supply that has developed rapidly in China.

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The transformer in the switching power supply is characterized by the fact that the operating current becomes very small compared to the power transmission. But in order to improve the efficiency of the power supply and reduce the size of the power supply. The operating frequency of the switching power supply transformer is from 20K-200K. If the previous silicon steel sheet is used, its resistivity is very small (small resistance). When working in high frequency mode, eddy current loss is the main factor, and the actual heat is serious, which affects the use of silicon steel sheet in switching power supplies. Thus, ferrite core materials appeared. Its characteristic is that the magnetic saturation intensity is not high, but for the switching power supply, it is basically not saturated. The key advantage is that the resistivity is much higher than that of the silicon steel sheet (the resistance becomes larger), the eddy current loss is reduced, and the heating is no longer so serious. And there is basically no problem in actual use.

3. Why are the shapes of power transmission transformers and instrumentation transformers mostly square? Look at the red magnetic field lines below. It can be known that the main purpose of the design as a square or a circle is to provide a loop for the magnetic field lines.

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