Technical Trends of Transformer Core Materials

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Technical Trends of Transformer Core Materials [Copy link]

As the core component of the transformer, the materials used in the iron core play a key role in the performance and price of power transformers and electronic transformers. For example, the price of silicon steel has been rising since 2004, reaching 250-300% in early 2005, prompting the power transformer industry to hold emergency meetings twice in November 2004 and March 2005 to increase the ex-factory price of power transformers by up to 30%. Although the electronic transformer industry is not as seriously affected by the silicon steel price increase as the power transformer industry, in June 2005, the National Electronic Transformer Industry Association put forward "Suggestions on Adjusting Transformer Product Prices", and some electronic transformer companies also raised their product prices by 3-10%. From this example, it can be seen that the iron core material has a significant impact on the transformer.
1 Silicon
steel Silicon steel is the most used iron core material for power transformers and electronic transformers. The core material of power transformer is oriented cold-rolled silicon steel. Oriented and non-oriented cold-rolled silicon steel are widely used in industrial frequency electronic transformers. Special silicon steel and thin strip silicon steel are used in medium frequency electronic transformers. The frequency of silicon steel used in electronic transformers has exceeded 20kHz, and the highest frequency has reached 325kHz. Therefore, the market and technical trends of silicon steel will have a significant impact on power transformers and electronic transformers.
1.1 Technical trends of oriented cold-rolled silicon steel
The main technical requirements for oriented cold-rolled silicon steel used in power transformers are high saturation magnetic density and low loss. The technical progress in improving the performance of oriented cold-rolled silicon steel used in power transformers is:
① Reduce the thickness of steel strip. The thickness of oriented cold-rolled silicon steel used in power transformers has been reduced from 0.35mm to 0.30mm, 0.27mm, 0.23mm, and even to 0.15mm in some cases.
The 0.23mm thick oriented cold-rolled silicon steel of 23Q110 produced domestically has a saturation magnetic flux density of 1.8T, and a unit weight loss P1.7/50 of 1.10W/kg at 1.7T and 50Hz.
② Make the surface of the silicon steel strip smooth, do not form a magnesium silicate bottom layer, remove the pinning position during magnetization, improve the 180° domain wall mobility and magnetization uniformity, and then apply a tension coating. After such treatment, the loss P1.7/50 of 0.23mm thick oriented cold-rolled silicon steel drops below 0.7W/kg.
③ Adding 0.008-0.02% Bi or Pb to silicon steel can increase the saturation magnetic flux density to 1.96-1.98T.
④ Changing the annealing process and refining the magnetic domain can further reduce the loss of oriented cold-rolled silicon steel. The loss P1.7/50 of 0.23mm thick can be reduced to 0.55W/kg or even 0.45W/kg.
It can be seen that when the thickness is still 0.23mm, the performance of oriented cold-rolled silicon steel strip is greatly improved by adopting the technical improvement measures in recent years, and the loss is reduced by 40-50% compared with the domestic 23Q110 and 20-35% compared with the Japanese 23ZDkH80.
The technical requirements of oriented cold-rolled silicon steel used in electronic transformers require not only high longitudinal (oriented) saturation magnetic density and low loss, but also the characteristics of electronic transformers. For example, when used as an E-type punching core, one-fifth of the length in the E-type punching is orthogonal to the longitudinal direction, which is a transverse magnetic field. Therefore, good transverse magnetic properties are also required, but the loss is much lower than that of orientation and non-orientation. Therefore, more and more oriented cold-rolled silicon steel is used in power transformers that require high efficiency.
1.2 Technical Trends of Non-Oriented Cold-Rolled Silicon Steel
At present, most of the non-oriented cold-rolled silicon steel is used to manufacture motor cores, including large and medium-sized motor cores for power stations, small motor cores for industrial use, small and micro motor cores for automobiles, and small and micro motor cores for household appliances. Various motor cores have different requirements for non-oriented cold-rolled silicon steel. Some motors (such as automobile wiper motors) require small size, and the core material requires high working magnetic flux density, so non-oriented cold-rolled silicon steel with a silicon content of less than 1% has been developed. This silicon steel can also be used in electronic transformers that require small size. Some motors (such as air-conditioning compressor motors) require variable frequency speed regulation, and the working frequency is extended from 50 to 60Hz to 10 to 1kHz. At the same time, the core material is required to have a small loss P1.5/50 at low frequency and high magnetic density and a small loss P1.0/400 at medium frequency, so a 0.35mm thick non-oriented cold-rolled silicon steel with a silicon content of 1.5 to 3% has been developed. This silicon steel can also be used in high-order harmonic power frequency electronic transformers and medium-frequency electronic transformers of about 400Hz. Some motors (such as high-speed printing machine motors) have a speed of up to tens of thousands of revolutions per minute and an operating frequency of 400 to 1kHz. The core material medium-frequency loss P1.0/400 and P1.0/1K are required to be small. Therefore, non-oriented cold-rolled silicon steel with a thickness of 0.25mm, 0.2mm, and 0.15mm has been developed. This silicon steel can also be used in medium-frequency electronic transformers of 400Hz, 1kHz, or even 10kHz. The motor core is mostly made of stacked punching sheets, and is mass-produced. Therefore, the non-oriented cold-rolled silicon steel developed requires good stamping processing performance, material dimensional accuracy, and good performance consistency. It is also applicable to small electronic transformers produced by mass stamping.
The following introduces several types of non-oriented cold-rolled silicon steel developed in Japan in recent years. Although the starting point of its technical development is the motor core, it is also applicable to electronic transformers.
⑴ Non-oriented cold-rolled silicon steel with a silicon content of less than 1%
The 0.5mm thick 0.6%Si+0.3%Al silicon steel developed by Kawasaki Corporation of Japan has a magnetic density B100 of 1.95T and a loss P1.5/50 of 4W/kg. If 0.5-2% Ni is added to increase the resistivity, the loss can be further reduced, B100 is 1.96T, and P1.5/50 is about 3W/kg. For electronic transformers, the working magnetic density Bm can be 1.9T, and the volume is small.
⑵ Non-oriented cold-rolled silicon steel with a silicon content of 1.5-3.0%
increases the silicon content, increases the steel hardness, and deteriorates the processing performance. One method is to increase the Al content to control the Si alloy so that the hardness HV is less than 200 to ensure the processing performance. Because Al increases the resistivity similar to Si, but increases the steel hardness by only one-third of Si. Generally, a thickness of 0.35mm is used to reduce losses. The 0.35mm thick 2%Si+2%Al steel developed by Nippon Steel Corporation of Japan adopts this method, and the loss P1.5/50 is less than 2.1W/kg. The hardness HV is less than 200, the average grain size is about 150μm, the punching processability is significantly improved, and the yield strength is 260-370N/mm2.
⑶ Non-oriented cold-rolled silicon steel strips
can reduce the strip thickness from 0.5mm and 0.35mm to 0.2mm and 0.15mm, which can reduce the loss to one-third and one-fifth, and increase the operating frequency of motors and transformers to 400-1kHz. In recent years, a variety of new products have been developed for thin strips below 0.35mm.
The 20HTH1200 silicon steel strip developed by Nippon Steel Corporation of Japan is 0.20mm thick, with a loss P1.0/400 of 11W/kg, P1.0/1K of 21W/kg, a magnetic density B50 of 1.63T, and a yield strength of 420Mpa. 15HTH1000 silicon steel strip, 0.15mm thick, loss P1.0/400 is 10W/kg, P1.0/1k is 16.7W/kg, magnetic density B50 is 1.61T.
⑷ Adhesive coating
In recent years, Japanese companies have developed various adhesive coatings, which are applied to silicon steel strips. After punching and stacking, 0.1~1.0N/mm2 pressure is applied, and heating is 150~250℃, which can make the laminated iron core bonded together to form a whole without welding or riveting. The non-oriented cold-rolled silicon steel with this adhesive coating has good punching performance, no local strain, uniform magnetism, and low core noise, but the core can no longer be annealed to eliminate stress.
The adhesive coating of Sumitomo Metal Corporation of Japan is coated with epoxy resin (or acrylic resin) + hardener (dense fat resin), and then SiO2 or AL2O3 is added to improve the bonding strength. The single-sided coating thickness is less than 2μm, and the shear strength is greater than 50kgf/cm2.
2 Technical trends of special silicon steel
In order to reduce the eddy current loss of silicon steel at medium and high frequencies, in addition to the development of silicon steel strips, silicon steel and gradient silicon steel with a silicon content of 6.5% were developed in the 1990s, but the hardness HV was 350 and the processing performance was poor. In recent years, Kawasaki Corporation of Japan has developed Cr-added silicon steel, the main components of which are 4.5% Si + 4% Cr, the resistivity is 82μΩcm, the same as 6.5% silicon steel, the hardness is 230, the elongation is 25%, the grain size is less than 100mm, and it can be stamped into punching sheets and riveted. The 0.10mm thick Cr-added silicon steel has a magnetic stacking B50 of 1.55T, a loss of P0.2/5k of 20W/kg; P0.1/10k of 10W/kg; P0.05/20K of 6W//kg, good wear resistance, no coating and no corrosion in salt water and moisture. The filter inductor of the switching power supply at 25kHz made of this Cr-added silicon steel has an inductance of 0.42MH and 0.32MH when the DC bias current is 0-30A, and the core loss is 22W/kg, which is smaller than that of iron-based amorphous alloy (29W/kg) and 6.5% silicon steel (36W/kg). It is also used as a double-terminal impedance element for 70kHz induction heating welding and a high-speed actuator for pulse excitation, and the effect is better than that of non-oriented cold-rolled silicon steel and 6.5% silicon steel. Based on
the silicon steel technology development situation introduced above, the relevant technical indicators are summarized in the following table:
2.1 Amorphous alloy market and technology trends
(1) Market trends of amorphous alloys
Several domestic companies are preparing to build iron-based amorphous alloy strip production lines, and the domestic technology for producing amorphous alloy strips has matured. The quality of amorphous alloy strips with a width of less than 150mm produced by Antai Technology Co., Ltd. is close to the international level. At the same time, the main raw materials of amorphous alloy strips are produced in China. When producing strips domestically, there is no need to export ore raw materials to foreign countries and then import amorphous alloy strips from foreign countries. This not only reduces round-trip transportation costs, but also reduces export and import tariffs. The production cost and price of amorphous alloy strips can be further reduced. More importantly, a complete amorphous alloy industry chain can be formed from mining, smelting ferroboron, amorphous alloy strip making, amorphous alloy core manufacturing to amorphous alloy transformer production, so as to independently and fully meet the needs of the domestic power grid for amorphous alloy distribution transformers.
(2) Technical trends of iron-based amorphous
alloys Iron-based amorphous alloys are the most widely used amorphous alloys. In order to improve the performance of distribution transformer cores made of iron-based amorphous alloys, several significant changes have recently occurred:
① The working magnetic flux density has been gradually improved. The working magnetic flux density of single-phase amorphous alloy distribution transformer has increased from 1.30T to 1.35T and recently to 1.40T; the working magnetic flux density of three-phase amorphous alloy distribution transformer has increased from 1.25T to 1.30T and recently to 1.35T, which can reduce the consumption and cost of distribution transformer core.
② The loss is gradually decreasing. Originally, the loss was less than 0.20W/kg at 1.30T and 50Hz, but now the loss is less than 0.18W/kg at 1.40T50Hz, and some can reach 0.16W/kg.
③ The core filling factor is continuously improved. From the original 0.80, it has increased to 0.86, and some have exceeded 0.90, which is close to the filling factor level of 0.1mm thick cold-rolled silicon steel core.
In this way, amorphous alloy distribution transformers show more superiority than oriented cold-rolled silicon steel distribution transformers. Taking a single-phase distribution transformer as an example, the core of an amorphous alloy distribution transformer with the same capacity of 10kVA is only about 20% heavier than that of a silicon steel distribution transformer, and its volume is about 8% larger. Considering that the price per unit weight of an amorphous alloy core is still 120% of that of oriented cold-rolled silicon steel, the total price of an amorphous alloy distribution transformer core is 144% of that of a silicon steel distribution transformer core, and the total cost of the transformer only increases by about 18%. If calculated based on the current price per unit weight of an amorphous alloy core of 40 yuan/kg, the total price of an amorphous alloy distribution transformer core is only 89% of that of a silicon steel distribution transformer core, and the total cost will be about 4% lower. Therefore, after the parameters of iron-based amorphous alloys change, the competitiveness of amorphous alloy distribution transformers and silicon steel distribution transformers will be significantly improved.
(3) Technical trends of nanocrystalline alloys
The iron-based nanocrystalline alloy (FeNbCuSiB) developed in the late 1980s, known as Finement, has the characteristics of relatively high saturation magnetic flux density of 1.23T, high magnetic permeability of 15X104, low medium and high frequency loss, P0.2/100k of 30W/kg, good temperature and environmental stability, etc. It is suitable as a core material with excellent comprehensive magnetic properties for medium and high frequency electronic transformers. In recent years, a series of nanocrystalline alloys have been developed by adjusting the composition ratio or adding other alloying elements. Some improve the magnetic permeability, some reduce the high frequency loss, and some reduce the residual magnetic flux density Br, so as to meet the needs of electronic transformers of different types and requirements. For example, there is a 7.2mm thick Finement nanocrystalline alloy with a loss of 11W/kg at 500kHz and 0.1T, which can be used for the core of high-frequency power transformers with an operating frequency of 30k~500kHz. For example, there is a kind of Finement nanocrystalline alloy with low residual magnetic density Br. In the relatively wide frequency band from 20k to 300kHz, the magnetic permeability is relatively high and the loss is relatively small. It can be used for common mode inductor cores.
2.2 Technical trends of soft ferrites
In the development of new materials, soft ferrites have made a series of breakthroughs recently after being silent for a period of time from 1998 to 2001.
In terms of low power loss materials, taking 100kHz X200mT and 100℃ as an example, PC40 of Japan TDK Company is 410mW/cm3, PC44 is 300mW/cm3, PC47 is 250mW/cm3, and BHI of Japan TOKIN Company is 250mW/cm3. JP4E produced by Jinning in China also reaches 300mW/cm3. In 2003, TDK of Japan developed PC95, a low-power loss material with wide temperature range. Within the range of 25-120℃, the loss is less than 350mW/cm3, and the magnetic density Bs is 540mt (25℃) and 430mt (100℃). It is currently the most excellent low-power loss material. In terms of
high-frequency transformer materials, PC50 of TDK of Japan, 7H20 of FDK of Japan, and B40 of TOKIN of Japan can all be used for 1MHz high frequency. The operating frequencies of 3F3, 3F4, 3F35, 4F1 of Ferroxcube are all over 1MHz. The operating frequencies of JP5A of Jinning in China are all 500k-1.5MHz, and the operating frequency of DMR1.2k of Benji even exceeds 3MHz, reaching 5.64MHz.
3 Conclusion
Based on the recent market and technology trends of various transformer core materials introduced above, the author puts forward several opinions for readers' reference:
① From the current domestic production volume, silicon steel ranks first, soft ferrite ranks second, and amorphous alloy ranks third. Therefore, when discussing transformer core materials, market factors should be considered. Silicon steel is not only the focus of power transformer core materials, but also the focus of electronic transformer core materials. It is not comprehensive to only pay attention to soft ferrites when discussing magnetic materials, but not to metal magnetic materials such as silicon steel and amorphous alloys.
② Regardless of the type of transformer core material, it has commodity characteristics, and the market price trend is determined by supply and demand. If the balance of supply and demand is destroyed, it will cause abnormal changes in market prices. The continuous price increase of oriented cold-rolled silicon steel is an example.
③ Market trends also affect technical trends. Market needs promote technological development. At the same time, the development of new products and new processes by technology will in turn affect market trends. It is also not comprehensive to only pay attention to technical trends and not to market trends when discussing the development trend of core materials.
④An enterprise must select projects according to market trends and strive for technological leadership according to technological trends in order to maintain continuous development towards higher and larger goals. There are such examples both at home and abroad, which are worth learning and drawing lessons from.