Functions and Classification of Transformer Cores

1. Functions of Transformer Cores

Practical transformers always operate under alternating current (AC) conditions. Power losses occur not only in the resistance of the windings but also in the core due to AC magnetization. The power loss within the core is commonly referred to as "iron loss" (or core loss), which is caused by two main factors: "hysteresis loss" and "eddy current loss." Hysteresis loss is the energy dissipated during the magnetization process due to the hysteresis phenomenon. The magnitude of this loss is proportional to the area enclosed by the material's hysteresis loop. Because silicon steel has a narrow hysteresis loop, using it as the core material significantly reduces hysteresis loss and substantially minimizes heat generation.

Since silicon steel has these advantages, why isn't the core made from a solid block of silicon steel, but instead processed into thin sheets? This is because a laminated core reduces another type of iron loss—eddy current loss. During operation, the alternating current in the windings generates an alternating magnetic flux. This changing magnetic flux induces currents within the core. These induced currents circulate in planes perpendicular to the direction of the magnetic flux and are therefore called eddy currents. Eddy current loss also causes the core to heat up. To minimize this loss, the transformer core is constructed by stacking silicon steel sheets that are insulated from each other. This forces the eddy currents to flow through narrow loops with a smaller cross-sectional area, thereby increasing the electrical resistance of the eddy current path. Additionally, the silicon in the steel increases the material's electrical resistivity, which further helps to reduce eddy currents. For transformer cores, 0.35 mm thick cold-rolled silicon steel sheets are generally selected. They are cut into rectangular strips according to the required core dimensions and then stacked to form a "figure-eight" or rectangular frame shape. In theory, the thinner the silicon steel sheets and the narrower the strips, the better the effect in reducing eddy currents. This not only reduces eddy current loss and lowers the temperature rise but also saves material. However, in actual manufacturing, these favorable factors are not the only considerations. Making the sheets too thin and narrow would significantly increase labor hours and reduce the effective cross-sectional area of the core. Therefore, when manufacturing silicon steel cores, one must weigh the pros and cons based on specific circumstances to select the optimal dimensions.

Transformers are manufactured based on the principle of electromagnetic induction. Two windings are wound around a closed core limb: the primary winding and the secondary winding. When an AC voltage is applied to the primary winding, an alternating current flows through it, establishing a magnetomotive force (MMF). Under the influence of this MMF, an alternating main magnetic flux is generated in the core. This main magnetic flux passes through and links both the primary and secondary windings. Due to electromagnetic induction, electromotive forces (EMF) are generated in both windings. The reason it can step up or step down voltage is explained by Lenz's Law: the magnetic flux generated by the induced current always opposes the change in the original magnetic flux. When the original magnetic flux increases, the magnetic flux produced by the induced current opposes it. In other words, the induced magnetic flux in the secondary winding is opposite to the main magnetic flux produced by the primary winding, resulting in a lower-level alternating voltage across the secondary winding. Thus, the core serves as the magnetic circuit of the transformer, while the windings constitute its electrical circuit.

2. Classification of Transformer Core Configurations

Shell-Type and Core-Type Cores

The part of the core around which the windings are wound is called the "core limb," while the part that does not have windings but serves solely as a magnetic path is called the "yoke." A core is referred to as "shell-type" when the core surrounds the windings, and "core-type" when the windings surround the core limb. Shell-type and core-type configurations each have their distinct characteristics, but the manufacturing processes they dictate are significantly different. Once a specific structure is selected, it is difficult to switch to another. In China, most transformer cores utilize a stacked core-type design.

Silicon steel sheets are used for low-frequency transformers and come in various types. Based on the manufacturing process, they can be divided into:
A: Annealed (black sheets)
N: Non-annealed (white sheets)

Based on their shape, they can be classified into: EI-type, UI-type, C-type, and Square (Mouth)-type.

Square-type silicon steel sheets are commonly used in high-power transformers. They offer excellent insulation performance, ease of heat dissipation, and a short magnetic path. They are primarily used for transformers with power ratings above 500–1000 W and high-power transformers. A set of silicon steel sheets composed of two C-type sheets is called a CD-type core. For power transformers made with CD-type sheets, under the same cross-sectional area, the higher the window, the greater the transformer's power capacity. Since coils can be installed separately on both sides of the core, the number of coil turns can be distributed across two bobbins, resulting in a shorter average turn length per bobbin and reduced copper loss. Furthermore, if two symmetrical coils are wound on separate bobbins, perfect symmetry can be achieved.

A set of silicon steel sheets composed of four C-type sheets is called an ED-type core. Transformers made with ED-type cores have a flat, wide profile. Under the same power rating, ED-type transformers are shorter and wider than CD-type transformers. Because the coils are installed in the center of the silicon steel sheets with an external magnetic path, they exhibit low leakage flux and minimal interference to surrounding components. However, all coils are wound on a single thick bobbin, resulting in a longer average turn length and higher copper loss.

C-type cores offer superior performance, resulting in transformers that are compact, lightweight, and highly efficient. From an assembly perspective, C-type cores require very few parts and have high versatility, leading to high production efficiency. However, the machining process for C-type silicon steel sheets is complex and requires specialized equipment, making their current cost relatively high.

E-type silicon steel sheets, also known as shell-type or figure-eight sheets, have the primary advantage of sharing a single bobbin for both primary and secondary windings, yielding a high window space factor (Space Factor Km: the ratio of the net cross-sectional area of the copper wire to the window area). The silicon steel sheets form a protective shell around the windings, preventing mechanical damage. They also provide a large heat dissipation area and minimize magnetic field dispersion. However, they exhibit higher leakage inductance between the primary and secondary windings and greater susceptibility to external magnetic interference. Additionally, due to the longer average perimeter of the windings, E-type transformers require more copper wire for the same number of turns and core cross-sectional area.

Common thicknesses for silicon steel sheets are 0.35 mm and 0.5 mm. There are two assembly methods for silicon steel sheets: interleaving and butt-stacking. The interleaving method alternates the open ends of the sheets on opposite sides. This method is more tedious but results in smaller air gaps and lower magnetic reluctance, which is beneficial for increasing magnetic flux; therefore, it is widely used in power transformers. The butt-stacking method is commonly used in applications with direct current (DC). To prevent saturation caused by DC current, an air gap must be left between the silicon steel sheets. In this method, E-sheets and I-sheets are placed on opposite sides, and the air gap between them can be adjusted using paper shims.

3. Coil Core Types

These are divided into three types:
A. TOROIDAL CORE: Made by stacking O-type laminations or by winding silicon steel sheets. Winding wire on this type of core is very difficult.
B. ROD CORE: A rod-shaped iron core.
C. DRUM CORE: A drum-shaped iron core.

4. Classification of Transformer Core Materials

1. High-Frequency Category: Ferrite Cores
Ferrite cores are used in high-frequency transformers. They are ceramic magnets with a spinel crystalline structure, composed of iron oxide and other divalent metallic compounds, such as kFe₂O₄ (where k represents other metals). Currently commonly used metals include manganese (Mn), zinc (Zn), nickel (Ni), magnesium (Mg), and copper (Cu). Common combinations include the Manganese-Zinc (MnZn) series, Nickel-Zinc (NiZn) series, and Magnesium-Zinc (MgZn) series. These materials possess high permeability and impedance characteristics, with an operating frequency range from 1 kHz to over 200 kHz.
Ferrite core frequency range: 18 kHz to over 1 MHz.

2. Low-Frequency Category: Silicon Steel Sheets and Amorphous Alloys
Silicon steel core frequency range: 5 Hz to 1 kHz.
Transformers using silicon steel typically operate at 50 Hz. However, in terms of efficiency, a slightly higher frequency may yield better results, depending on the Gauss rating of the silicon steel sheet. While audio frequencies can reach above 20 kHz, this is not practical. It is recommended to operate at 50–60 Hz, though they can function normally within a frequency range of 50–200 Hz.
Amorphous core frequency range: 2 kHz to 13 kHz.