Classification of Transformer Cores

The part of the core where the windings are mounted is called the limb (or core column), while the part that does not carry windings and only serves as a magnetic path is called the yoke. When the core surrounds the windings, it is called a shell-type transformer; when the windings surround the core limbs, it is called a core-type transformer. Both shell-type and core-type have their own characteristics, but the manufacturing process determined by the core structure differs greatly. Once a certain structure is selected, it is very difficult to switch to another. Most transformer cores in China adopt the stacked core-type structure.

Single-phase and Three-phase Cores
There are five types of single-phase cores: single-phase two-limb stacked core, single-phase single-limb core with side yokes (four-limb), single-phase two-limb stacked core, and single-phase radial stacked core. (注：原文此处列举略有重复，按原意翻译)
There are four types of three-phase cores: three-phase three-limb stacked core, three-phase side-yoke five-limb core, three-phase double-frame stacked core, and three-phase reactor stacked core.

Three-dimensional (Spatial) and Planar Types
In a three-dimensional core, the limbs and yokes are not in the same plane. Types include radial, involute, and symmetrical. Due to the relatively uniform magnetic flux distribution, losses can be reduced. In a planar core, the limbs and yokes are in the same plane, offering high mechanical strength and good manufacturability.

Stacked Cores and Wound Cores
Most cores are stacked cores, made by stacking laminations. Wound cores come in various forms. The air gap between the limb and yoke of an involute core is relatively large, which affects the no-load current, so the capacity cannot be made too small. However, since leakage flux enters the lamination plane perpendicularly, affecting additional losses, the lamination width should not be too large, meaning the capacity cannot be made too large either.

The core is mainly composed of the core body, fasteners, and insulating parts:
① Core body: The magnetic conductor, made of electrical steel sheets.
② Fasteners: Clamps, bolts, glass fiber binding tape, steel binding tape, spacers, etc.
③ Insulating parts: Clamp insulation, insulating tubes and pads, grounding tabs, and feet, etc.