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 within the iron core as it is magnetized by the alternating current. These power losses in the iron core are generally referred to as "core losses," which are caused by two factors: hysteresis loss and eddy current loss. Hysteresis loss occurs during the magnetization process due to magnetic hysteresis, and its magnitude is proportional to the area enclosed by the material's hysteresis loop. Silicon steel has a narrow hysteresis loop; therefore, using it for transformer cores results in lower hysteresis losses, significantly reducing heat generation.

Given these advantages of silicon steel, why is it processed into laminations rather than used as a solid block? This is because laminated cores help minimize another type of core loss: eddy current loss. During operation, the alternating current in the windings generates alternating magnetic flux, which induces currents within the iron core. These induced currents circulate in planes perpendicular to the direction of the magnetic flux and are thus called eddy currents. Eddy current losses also cause the core to heat up. To reduce these losses, the transformer core is assembled from mutually insulated silicon steel sheets. This forces the eddy currents to flow through narrow, elongated loops with smaller cross-sectional areas, thereby increasing the electrical resistance along their path. Additionally, the silicon content in the steel increases the material's electrical resistivity, further reducing eddy currents.

For transformer cores, 0.35mm thick cold-rolled silicon steel sheets are typically selected. Based on the required core dimensions, they are cut into rectangular strips and stacked into "E-I" or square ("□") shapes. In theory, to minimize eddy currents, thinner sheets and narrower strips yield better results. This not only reduces eddy current losses and temperature rise but also saves on silicon steel materials. However, in practical manufacturing, other factors must be considered. Using excessively thin or narrow sheets would drastically increase labor hours and reduce the effective cross-section of the core. Therefore, when manufacturing transformer cores from silicon steel, engineers must weigh the pros and cons based on specific conditions to select optimal dimensions.

Transformers are built upon the principle of electromagnetic induction. Two windings—a primary winding and a secondary winding—are wound around closed iron core limbs. When an AC voltage is applied to the primary winding, an alternating current flows, establishing a magnetomotive force (MMF). Under the influence of this MMF, an alternating main magnetic flux is generated within the core. This main flux simultaneously passes through both the primary and secondary windings. Due to electromagnetic induction, electromotive forces (EMFs) are generated in both windings. The mechanism of stepping up or stepping down the voltage can be 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 flux increases, the induced flux acts in the opposite direction. This means the induced magnetic flux in the secondary winding opposes the main flux produced by the primary winding, resulting in a transformed AC voltage across the secondary winding. Thus, the iron core serves as the magnetic circuit of the transformer, while the windings form its electrical circuit.

2. Classification of Transformer Core Structures

Shell-Type and Core-Type Iron Cores
The part of the iron core that houses the windings is called the "core limb," while the part that does not house windings and solely serves as a magnetic path is called the "yoke." If the iron core encloses the windings, it is termed a shell-type core; if the windings enclose the core limbs, it is termed a core-type core. While both have distinct characteristics, the manufacturing processes dictated by their structural designs differ significantly, making it difficult to switch between them once chosen. In China, most transformer cores utilize the stacked core-type structure.

Silicon steel is commonly used for low-frequency transformers. Based on manufacturing processes, they are divided into:
A. Annealed (Black Sheets)
N. Non-annealed (White Sheets)

Based on shape, they are categorized into EI-type, UI-type, C-type, and Square (□) types.

• Square (□) Type: Often used in high-power transformers, they offer excellent insulation, easy heat dissipation, and a short magnetic path. They are primarily used for transformers rated above 500~1000W.

• CD Type: Consists of two C-type silicon steel sheets. For CD-type transformers with identical cross-sectional areas, a taller window height equates to greater power capacity. Since coils can be installed on both sides of the core, the total number of turns can be distributed across two bobbins. This reduces the average turn length per bobbin, lowering copper losses. Furthermore, placing symmetrical coils on separate bobbins achieves perfect symmetry.

• ED Type: Composed of four C-type silicon steel sheets, ED-type transformers have a flat, wide profile. Under the same power rating, they are shorter but wider than CD-types. Because the coils are installed in the center of the silicon steel with an external magnetic path, leakage flux and overall interference are minimal. However, since all coils are wound on a single thick bobbin, the average turn length is longer, resulting in higher copper losses.

• C-Type: Transformers made from C-type cores are compact, lightweight, and highly efficient. From an assembly perspective, C-type components are few and highly versatile, leading to high production efficiency. However, processing C-type silicon steel involves numerous complex steps and requires specialized equipment, making them currently more expensive.

• E-Type (Shell-Type / E-I Type): Its main advantage is that the primary and secondary coils share a common bobbin, yielding a high window space factor (the ratio of the net cross-sectional area of the copper wire to the window area). The silicon steel forms a protective shell around the windings, preventing mechanical damage. It also offers a large heat dissipation area and low stray magnetic fields. However, it suffers from higher leakage inductance and greater susceptibility to external magnetic interference. Additionally, due to the longer average perimeter of the windings, E-I type transformers require more copper wire for the same number of turns and core cross-section.

Thickness and Stacking Methods:
Common silicon steel thicknesses are 0.35mm and 0.5mm. Assembly methods include interleaved stacking and butt-stacking. Interleaved stacking alternates the open ends of the sheets on opposite sides. Though tedious, it minimizes air gaps and magnetic reluctance, maximizing magnetic flux, making it ideal for power transformers. Butt-stacking places E-sheets and I-sheets on opposite sides, leaving an air gap (adjusted via paper inserts) to prevent saturation caused by direct current (DC)..

3. Coil Types
Coils/cores are divided into three categories:
A. Toroidal Core: Assembled from O-shaped laminations or wound from silicon steel strips. Winding is quite challenging for this type.
B. Rod Core.
C. Drum Core.

3. Classification of Transformer Core Materials

1. High-Frequency Category: Iron Powder Cores & Ferrite Cores
Ferrite cores are used in high-frequency transformers. They are ceramic materials with a spinel crystalline structure, composed of iron oxide and other divalent metal compounds (e.g., kFe₂O₄, where 'k' represents metals like Manganese (Mn), Zinc (Zn), Nickel (Ni), Magnesium (Mg), or Copper (Cu)).
Common combinations include MnZn, NiZn, and MgZn series. These materials possess high permeability and impedance, operating effectively from 1kHz to over 200kHz.

• Ferrite Core Frequency Range: 18 kHz ~ 1 MHz and above.

2. Low-Frequency Category: Silicon Steel & Amorphous Alloys

• Silicon Steel Core Frequency Range: 5 Hz ~ 1 kHz. Standard silicon steel transformers typically operate at 50Hz. While higher frequencies can improve efficiency depending on the Gauss rating of the silicon steel, audio applications exceeding 20kHz are impractical. The recommended range is 50-60Hz, though they can operate normally between 50-200Hz.

• Amorphous Core Frequency Range: 2 kHz ~ 13 kHz.