Power transformer stacked iron cores generally fall into two structural types: core-type and shell-type. In China, the core-type is predominantly used, where both the limb and yoke lie within a single plane — known as a planar iron core. The lamination method is used for assembly: limbs and edge columns are secured by binding, while upper and lower yokes are clamped in place.

1. Classification by Phase Count and Limb Configuration

Based on phase count and limb form, iron cores are classified as: single-phase double-limb, three-phase three-limb, single-phase three-limb, single-phase four-limb, single-phase five-limb, and three-phase five-limb, among others. The yoke-to-limb cross-sectional ratio and diameter ranges for each type are specified in standard reference tables.

2. Classification by Clamping Structure

The clamping structure — comprising clamps, tie plates, and accessories — holds the entire iron core assembly as a rigid unit. Based on clamping configuration, iron cores are classified into:

• Standard (Conventional) Series: Includes clamps, tie plates, and accessories such as base feet, top beams, support plates, side beams, tensioning devices, and binding straps.

• P-Series: No clamps or tie plates; the core laminations are held together only by binding straps and temporary clamping beams during assembly.

• G-Series: No clamps; adds horizontal tie plates connected to axial tie plates. Clamping system includes tie plates, connecting beams, and binding straps.

3. Classification by Limb Lamination Shape

3.1 Solid-Limb (Full-Sheet) Structure

Each limb lamination is a single uncut sheet — the most commonly used configuration in practice.

3.2 Split-Limb (Puzzle-Cut) Structure

Used in large-capacity three-phase transformer cores to optimize material utilization. Typically applied when limb lamination width exceeds 600 mm. The puzzle-cut joints alternate left and right in a cyclic pattern. Note: this structure introduces high magnetic-flux saturation zones and increased no-load losses and no-load currents.

3.3 Frame Structure

The limb laminations are split into two equal halves with inter-frame insulation or oil channels placed between them. All laminations share equal widths and are trapezoidal — no corner-cut waste is generated. This gives the frame structure high material utilization and low iron core cost.

4. Core Body

The iron core body consists of the limb and the yoke. Limb cross-sections are multi-step circular; yoke cross-sections may be multi-step circular, elliptical, D-shaped, or T-shaped. Standard lamination thicknesses: 0.23 mm, 0.27 mm, 0.30 mm, and 0.35 mm. At joint areas, full 45° oblique joints and step-lap splicing are used.

5. Clamping System

The clamping system integrates the entire iron core into a rigid unit. Key components:

• Clamps & Tie Plates: Plate-type or spliced-type clamps; one-piece or multi-plate tie plates.

• Clamp-to-Plate Connection: Hanging blocks, hanging bolts, and hanging spindles.

• Tie Straps & Binding Straps: Pre-formed epoxy-glass non-woven tapes, steel tie straps, or tie rods.

• Body Positioning: Lower — dowel pins with positioning cups. Upper — pin-cup, pin-flange, or tank lid-to-clamp bolt assemblies.

6. Insulation Structure

Poor iron core insulation compromises transformer safety. Core insulation covers:

• Inter-Lamination Insulation: Oil channels (spacer-strip or ceramic-disc type) and frame insulation (full-board or segmented type).

• Lamination-to-Structure Insulation: Clamp insulation, tie plate insulation, strap/rod insulation, pad foot insulation, connecting beam insulation, and insulating blocks.

Note: Insulating sleeves between tie straps/rods and support bases must withstand both compressive and bending forces. Some manufacturers use composite molded insulating sleeves to prevent cracking and insulation failure.

7. Grounding Structure

• Core Grounding: Connects inter-stage oil channels and frame spaces into an integrated unit, brought out via cable and a dedicated grounding bushing.

• Clamp Grounding: Clamps, tie plates, and accessories are connected via cable and copper strips, brought out via a clamp grounding bushing. Side yoke binding buckles require 2 mm wax thread serial connection.