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How to reduce the loss of the transformer core

2019-01-19 15:25:13

To design a high-efficiency, low-loss transformer, you must start with the transformer core. Reducing the loss of the core can reduce the size of the transformer, and the windings and outer casing of the outer casing must be reduced, so the copper loss will be greatly reduced. The stress remaining after the core material has been processed destroys the arrangement of the original atomic crystals, so it is necessary to give appropriate heat treatment to restore the original characteristics, which are summarized as follows:



■Select low-loss core material: The improvement process of core material characteristics is (1) directional silicon steel sheet (2) high magnetic permeability (HI-B) directional silicon steel sheet (3) high magnetic conductive laser treatment (HS) - LS) directional silicon steel sheet (4) Amorphous Alloy core. The designer can select the most suitable core material to make the transformer according to actual needs.
■The core structure with small magnetic reluctance is selected: after the silicon steel sheet is assembled into iron cores, the loss will be greater than that of the silicon steel sheet, and the increase ratio will vary depending on the iron core construction method. In principle, the less the number of seams, the lower the magnetic resistance and the smaller the iron loss. Only considering the quality without considering the cost, such as advanced audio, instruments, etc., it is recommended to use a seamless R-Core or ring-shaped core, or even a ring-shaped core with a circular cross-sectional area, to achieve the minimum iron loss. If seams cannot be avoided, please consider the "stacked core" with only one seam as shown in Figure 4. For large transformers, consider the "V-notch stepped core" shown in Figure 9.
■ Annealing heat treatment: The steel mill processes the “directional iron core silicon steel sheet” by cold rolling extension. In the key process of rolling thinning, heat treatment must be carefully applied to ensure the magnetization characteristics of the alloy material. Depending on the composition of the material, the quality control heat treatment temperature range is 900~1200 °C, and the material must be placed in a low carbon steel heat treatment furnace filled with inert gas (Inert Gas) or hydrogen. After the transformer production plant finishes the processing of the rolled or rolled silicon steel sheet, the processing will leave a lot of stress, so it must be annealed at 800~875 °C, and must be placed in a hydrogen furnace. Because the hydrogen content (purity of 97% or more) in the hydrogen furnace is difficult to control, if it is improperly controlled, it may cause an explosion accident, and most of them have been changed to vacuum furnaces. Some silicon steel sheet suppliers also provide shaped silicon steel sheets that have been annealed, but must be confirmed in advance according to the above methods to ensure the quality of the transformer. If the quality requirements of the transformer are not so high based on economic considerations, the annealing heat treatment temperature can be lowered to 700~750 °C, and the annealing temperature is maintained in this range, so that most of the properties of the directional silicon steel sheet can be achieved, although Can't get the best features, but can save a considerable amount of money. In other occasions (for example, when the frequency of use is high), it is necessary to use a very thin silicon steel sheet. This silicon steel sheet must be processed through a special processing process, which is very expensive, so it is recommended to use nickel-iron alloy (Nickel-Iron Alloy). Materials can achieve the same purpose, but at a lower cost. After the iron core of the transformer is finished by annealing and heat treatment, it must not be impacted by external force or dropped to the ground. Even the winding work must be very careful, so as to avoid the deformation of the iron core and the deterioration of characteristics, so the predecessor must not be inadvertent!



Method of reducing copper loss
The resistance of the winding accounts for more than 50% of the copper loss of the transformer. Therefore, to reduce the copper loss, the following methods should be considered:
■ Increase the cross-sectional area of ​​the wire: In addition to carefully selecting the copper wire with high material purity, it is advisable to increase the cross-sectional area of ​​the wire to reduce the DC resistance of the wire.
■ Reduce the number of coil turns: Use a core material with a relatively high magnetic permeability or use a seamless coil core to reduce the number of winding turns and reduce the copper loss.
■Reducing drift loss: The cross-linking between the conductor and the leakage flux will cause a hole flow loss inside the conductor. Therefore, the core structure with low magnetic flux leakage should be carefully selected. Leakage flux will also flow outside the coil. For the metal parts such as clips and casings, it is better to change the material of the parts that are affected by the magnetic leakage to non-magnetic materials to reduce the loss. The magnetic leakage can be blocked by a "shell shield" mounted between the coil and the outer casing. The type and shape of the shielding plate of the outer casing are many. According to the material, it can be divided into three types: copper plate, aluminum plate and silicon steel sheet. Experience shows that the copper plate has the best barrier effect, but the price is high, usually in the flat outer casing side plate. Silicon steel sheets are used on the board, and copper or aluminum sheets are often installed at corners or corners where bending is required.

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