The core is the main part of the magnetic circuit in the transformer. It is usually composed of hot rolled or cold rolled silicon steel sheets with a high silicon content and a surface coated with insulating paint. The iron core and the coils around it constitute a complete electromagnetic induction system. The amount of power transmitted by the power transformer depends on the material and cross-sectional area of the iron core.
Classification of iron cores
2.1.1 wound iron core
Wound core is commonly used in small and medium-sized transformers (below 1000kVA), transformers, magnetic amplifiers and zero sequence current transformers of leakage protectors.
The materials used for wound core are ultra-thin cold-rolled silicon steel sheet with high permeability and soft magnetic strip such as permalloy. The thickness of the silicon steel sheet is 0.18~0.30; The thickness of the Permalloy strip is 0.03~0.10mm. Taking small and medium-sized transformers as an example, the use of wound core has the following advantages:
1) Under the same conditions, the no-load loss of the wound core is reduced by 7% to 10% compared with the laminated core; No-load current can be reduced by 50%~75%.
2) The wound core can be made of very thin high-permeability cold-rolled silicon steel sheets, which can produce transformers with lower losses.
3) The wound core has good processability, no shearing waste, and the utilization rate is almost 100%. It can also adopt mechanized operation, eliminating the stacking process, and the production efficiency is 5 to 10 times higher than that of the laminated core.
4) The wound core itself is a whole, does not need to be fixed by clamping support parts, and does not have a joint, so under the same conditions as the laminated core, the transformer noise can be reduced by 5~10dB.
5) The process coefficient of the wound core single-phase transformer is about 1.1; Three-phase below 1.15; For laminated iron cores, the process coefficient of small capacity is about 1.45, and the process coefficient of large capacity is about 1.15. Therefore, the wound core is particularly suitable for small and medium-sized transformers.
Definition
Laminated iron core is a key component used in power transformers, inductors, transformers and other power equipment. It is composed of multiple sheets, with high permeability and low hysteresis loss, which can effectively improve the working efficiency and performance stability of the equipment.
Structure of laminated iron core
A laminated core consists of multiple sheets, each made of a highly permeable material, such as silicon steel. These sheets are separated by insulating material to form a single structure. Laminated iron cores are usually rectangular or circular in shape to adapt to the requirements of different equipment. In the manufacturing process of laminated iron core, it is also necessary to consider factors such as the thickness of the sheet, the selection of insulation materials and the processing process to ensure its performance and reliability. The iron core constitutes a closed magnetic circuit in the transformer, and it is also the skeleton of the installation coil, which is a very important part for the electromagnetic performance and mechanical strength of the transformer. The iron core is the magnetic circuit part of the transformer, which is composed of an iron core column (winding set on the column) and an iron yoke (connecting the iron core to form a closed magnetic circuit). In order to reduce eddy current and hysteresis loss and improve the magnetic conductivity of the magnetic circuit, the iron core is made of 0.35mm ~ 0.5mm thick silicon steel sheet coated with insulating paint. The small transformer core section is rectangular or square, and the large transformer core section is stepped, which is to make full use of space.
Laminated core features
Since the core and winding of the laminated core transformer are manufactured separately, the core is stacked first, and then the upper yoke is removed, and then the core insulation and coil are fitted, and the coil and core post are supported with a brace, and finally the iron yoke is inserted to complete the assembly of the body.
The structure of the laminated core transformer has the following characteristics:
1. The clamping direction of the core is the thickness direction of the core sheet, which can clamp the core well;
2. For the double-layer cylindrical coil, the inner layer of the coil has no coil skeleton;
3. Because the upper iron yoke is removed during installation, the core column and the coil can be easily tightened with a stay;
4. The coil is wound separately, and the coil can be dipped separately after winding.
1) Three-dimensional triangular wound iron core
Three-dimensional wound core: A triangular three-dimensional arrangement of an iron core composed of three single-frame wound cores of the same geometric size.
Three-dimensional wound core transformer: distribution transformer with three-dimensional wound core as the magnetic circuit.
Process features: The entire iron core is made of three identical single frames, and the three core columns of the iron core are arranged in an equilateral triangle. Each single frame is made of a number of trapezoidal material belts successively wound. The cross section of the single frame after winding is close to semi-circular, and the cross section after splitting is very close to the whole circle quasi-polygon. The trapezoidal material belt of different sizes of the single frame is wound by the special folding line cutting machine. This type of cutting processing can be done without material processing, that is, when cutting, the material utilization rate is 100%.
2) Laminated iron core
Laminated iron core: It is composed of longitudinal shear production line and transverse shear production line, and the silicon steel strip is processed into a certain shape of silicon steel sheet, and then the silicon steel sheet is stacked in a certain way.
Laminated core has three disadvantages:
There are air gaps formed by many joints in the magnetic circuit, which increases the magnetic resistance of the magnetic circuit, thereby increasing the loss and no-load current.
The direction of the magnetic circuit in some places is inconsistent with the direction of the high magnetic permeability of the silicon steel strip.
The lack of tightness between the slices not only reduces the lamination coefficient, but more importantly, increases the noise.
Effect of process on loss
Longitudinal shear and transverse shear produce increased mechanical stress loss
The direction of the magnetic circuit in the corner is inconsistent with the direction of the magnetic conductivity, which greatly increases the loss
Joints increase the loss, especially the increase in no-load current
The process coefficient is 1.15 ~ 1.3
3) The influence of the structure on the magnetic circuit
In the traditional stack core with air gap, the coupling magnetic circuit between the A-C phase is obviously 1/2 longer than the magnetic circuit of the A-B phase and the B-C phase, so the magnetic circuit is unbalanced, and the magnetic resistance of the A-C phase is larger. When a three-phase voltage is applied to the transformer, the core produces a three-phase balanced magnetic flux φA, φB, and φC.
When the magnetic flux of the three-phase balance passes through the unbalanced magnetic circuit, the magnetic voltage drop of the A and C phases is large, which affects the three-phase voltage balance. This imbalance in the magnetic circuit is an insurmountable structural defect for planar transformers.
4) Flat wound iron core
Flat wound core: A flat arranged iron core consisting of one or more single frames with wound cores.
Process characteristics: The flat wound core is first wound two smaller inner frame, after the combination of two inner frames that have been wound, and then wound a larger outer frame in its external composition, the three core columns of the flat wound core are arranged in a plane.
Flat wound core structure defects
The same as the flat wound core and the laminated core, the three core columns are arranged in a plane, so that the magnetic circuit length of the three core columns is inconsistent: the magnetic circuit length of the middle column is short, the magnetic circuit length of the two side columns is longer, and the average magnetic circuit length is about 20%, resulting in a large difference in the no-load loss of the three core columns, the no-load loss of the middle column is low, and the no-load loss of the two side columns is large, resulting in a three-phase imbalance.
Single-phase core has a single two-column laminated core. There are five kinds of single-phase single-column side-yoke type four-column core, single-phase double-column type laminated core and single-phase radiant type laminated core. There are four kinds of three-phase core: three-phase column laminated core, three-phase side-yoke five-column core, three-phase double-frame laminated core and three-phase reactor laminated core.
The iron core consists of two parts: an iron core column and an iron yoke. The core column is covered with winding, and the iron yoke connects the core column to form a closed magnetic circuit. The core plan of the transformer is shown in Figure 1, Figure 1a is a single-phase transformer, and Figure 1b is a three-phase transformer, the core structure can be divided into two parts, B is the part of the coil, called the core column. A is used to close the part of the magnetic circuit, called the yoke. The single-phase transformer has two core columns, and the three-phase transformer has three core columns.
Figure 1 The core plane of the transformer
Because the magnetic flux in the transformer core is an alternating magnetic flux, in order to reduce the eddy current loss, the transformer core is generally made of silicon steel sheets with large resistivity into a certain size of iron chip, the silicon steel sheets composed of the iron core are cut into the required shape and size, and then the punching sheet is combined in the overlapping way. Figure 2a shows the iron core of a single-phase transformer, each layer consisting of 4 punching pieces. Figure 2b shows the iron core of the three-phase transformer, each layer is composed of 6 pieces, and the combination of each two layers of the chip applies a different arrangement to stagger the joints of each layer of the magnetic circuit. This assembly method is called overlapping assembly, and this assembly can avoid eddy current flow between the steel sheet and the steel sheet. And because each layer of punching is interwoven, fewer fasteners can be used to make the structure simple when pressing the iron core. During assembly, the punching plates are first stacked to form a whole iron core, and then the lower iron yoke is clamped, the upper iron yoke punching plate is removed to expose the core column, the prefabricated winding is placed on the core column, and finally the extracted upper iron yoke punching plate is inserted.
Figure 2 Overlap assembly of transformer core
The part of the clad winding in the iron core is called the “core column”, and the part of the non-clad winding that only plays the role of the magnetic circuit is called the “iron yoke”. Where the iron core surrounds the winding, it is called shell type; Where the winding surrounds the core column is called the core type. Shell type and core type have their own characteristics, but the transformer manufacturing process determined by the iron core is very different, and it is difficult to turn to a structure once a certain structure is selected. Most of the transformer core in our country adopts stacked core type.
According to the arrangement of the winding in the iron core, the transformer is divided into core type and shell type. The difference is mainly in the distribution of the magnetic circuit, the yoke of the shell transformer core surrounds the coil, the core transformer core is mostly in the coil, only part of the iron yoke outside the coil, which is used to form the magnetic circuit.
Heat dissipation of iron core
When the transformer is in normal operation, the iron core will generate heat due to the existence of iron loss, and the greater the weight and volume of the iron core, the more heat will be generated. Transformer oil temperature above 95 degrees is easy to age, so the temperature of the core surface should be controlled below this temperature as far as possible, which requires the heat dissipation structure of the core to quickly dissipate the heat of the core. The heat dissipation structure is mainly to increase the heat dissipation surface of the iron core. The heat dissipation of the iron core mainly includes the heat dissipation of the iron core oil channel and the heat dissipation of the iron core airway.
In oil-immersed transformers with large capacity, oil slots are often arranged between the laminates of the iron core to enhance the heat dissipation effect. The oil tank is divided into two kinds, one is parallel to the silicon steel sheet, and the other is vertical to the steel sheet, as shown in Figure 4. The latter arrangement has better heat dissipation effect, but the structure is more complex.
In the dry transformer core is air cooling, in order to ensure that the core temperature does not exceed the allowable value, often installed in the core column and iron yoke air duct.
core noise
The transformer will produce noise during operation. The source of the transformer body noise is the magnetostriction of the silicon steel sheet of the iron core, or the noise of the transformer core is basically caused by magnetostriction. The so-called magnetostriction refers to the increase of the size of the silicon steel sheet along the direction of the magnetic induction line when the iron core is excited; The size of the silicon steel sheet decreases in the direction perpendicular to the magnetic inductance line, and this size change is called magnetostriction. In addition, the structure and geometric size of the iron core, the process of iron core processing and manufacturing will have a certain degree of impact on its noise level.
The noise level of the iron core can be reduced by the following technical measures :(1) The use of high-quality silicon steel sheets with small magnetostrictive ratio ε value. (2) Reduce the magnetic flux density of the core. (3) Improve the structure of the iron core. (4) Select a reasonable core size. (5) Adopt advanced processing technology.
In the normal operation of the transformer, the electric field formed between the charged winding and the lead wire and the fuel tank is an uneven electric field, and the iron core and its metal parts are in the electric field. Because the potential of electrostatic induction is different, the suspension potential of the iron core and its metal parts is not the same, and when the potential difference between the two points is able to break down the insulation between them, spark discharge is generated. This discharge can break down the oil of the transformer and damage the solid insulation. To avoid this, both the core and its metal components must be reliably grounded.
The core must be slightly grounded. When the iron core or other metal components are grounded at two or more points, a closed loop will be formed between the ground points, forming a circulation, the current can sometimes be as high as tens of amps, will cause local overheating, leading to oil decomposition, may also make the ground strip fuse, burn the core, these are not allowed. Therefore, the core must be grounded, and it must be a little grounded.
Nanocrystalline core and amorphous core in transformer
The advent of nanocrystalline and amorphous iron cores provides ideal materials for medium and high frequency transformers. With the development of industry, the operating frequency of the power supply has been increased to 20kHz, and the output power has exceeded 30kW. Traditional core materials such as silicon steel sheet have a large loss and can not meet the new requirements of power supply.
The amorphous and iron-based nanocrystalline core has the characteristics of high saturation magnetic induction, high permeability, low loss, good temperature stability, environmental protection, etc., and has important application value in high power high-frequency transformers.
Nanocrystalline materials are mainly composed of iron, chromium, copper, silicon, boron and other elements, and these specific alloy components are made into amorphous states by rapid quenching technology, and then heat treated to form nanoscale grains.
The nanocrystalline core exhibits excellent magnetic properties and temperature stability, and is particularly suitable for replacing ferrite in transformers below the frequency range of 20kHz to 50kHz.
The nanocrystalline material has a resistivity of 90 μ Ω.cm (after heat treatment) and, thanks to its nanostructure, combines the advantages of silicon steel, permalloy and ferrite.
The thickness of common iron nanocrystalline soft magnetic materials is about 30μm. Due to its brittleness and sensitivity to stress, the magnetic properties will be significantly reduced when subjected to external forces during processing and use. Therefore, the nanocrystal core is usually made into a ring or horseshoe shape and placed in a protective shell. The protective shell material will affect the heat dissipation performance of the nanocrystalline core.
The new nanocrystalline core has been applied to transformers, the thickness of nanocrystalline material is only 24μm, and the core cured after heat treatment has significant advantages over the traditional transformer core:
The new nanocrystalline core is coated with an insulating film, which achieves the required strength for winding and can be wound directly into transformers.
The cured nanocrystalline core eliminates the protective casing, providing more space for heat dissipation and improving the transformer’s operational safety.
This design reduces the influence of the protective shell material on the nanocrystalline core, and saves the structural design and forming time of the protective shell.
Nanocrystalline core design can be more flexible, offering a variety of shapes such as ring, rectangular and C-shaped core, providing more options for transformer design and subsequent winding process.
The amorphous material is produced using ultra-fast quenching technology with a cooling rate of about one million degrees per second. This technology solidifies molten steel in a single quenching into an alloy strip with a thickness of 30 microns. Due to the fast cooling rate, the metal has no time to crystallize, resulting in no grains or grain boundaries in the alloy, resulting in the formation of so-called amorphous alloys.
Amorphous metal has a unique microstructure that is different from conventional metal, and its composition and disordered structure give it many unique properties, such as excellent magnetism, corrosion resistance, wear resistance, high strength, hardness, toughness, high resistivity, high electromechanical coupling coefficient, etc.
The main components of the iron-based amorphous core are iron, silicon and boron, of which the silicon content is as high as 5.3%, and the unique structure of the amorphous state, its resistivity is 130 μΩ.cm, which is twice that of the silicon steel sheet (47 μΩ.cm).
The thickness of the ferro-based amorphous material used in the amorphous core is about 30nm, which is much thinner than the thickness of the silicon steel sheet, so the eddy current loss is small at high frequency operation. In the frequency range of 400Hz~10kHz, the loss is only 1/3~1/7 of the silicon steel sheet. At the same time, the permeability of iron-based amorphous iron core is much higher than that of traditional iron core.
Due to these advantages, the amorphous core can reduce the weight of the transformer by more than 50% and the temperature rise by 50%.
After years of development, amorphous and nanocrystalline iron cores have been widely used in high frequency transformers, current transformers, switching power supplies, electromagnetic compatibility equipment and other applications.
