Basic Structure Of Single Phase Overhead Distribution Transformers

1. Core: Magnetic Circuit & Mechanical Skeleton

1.1 Core Function & Material Requirements

• Core Role: Two-in-one function—forms a closed magnetic circuit for magnetic flux transmission, and serves as the mechanical support for windings.
• Material Requirements: High magnetic permeability, minimal hysteresis loss and eddy current loss.

1.2 Core Materials (Evolution & Current Status)

• Standard Thickness: 0.35mm (meets low-loss design requirements).

1.3 Core Structure Classification

• Core-Type Transformer (Corrected from “Cardioid”)
  • Structure: Windings are placed on core columns, forming a “windings surround core” layout.
  • Advantage: Simple structure, easy winding installation and insulation.
  • Application: Dominates domestic power transformers (most common structure).
• Shell-Type Transformer
  • Structure: Windings are sleeved on the middle core column, forming a “core surrounds windings” layout.
  • Application: Special scenarios requiring compact structure (e.g., small power supplies).

1.4 Core Production Process Classification

• Laminated Core
  • Manufacturing Steps: Punch and cut silicon steel sheets → stagger and insert into pre-insulated windings → clamp with fixtures.
  • Key Requirement: Minimize air gaps at seams to reduce magnetic resistance and iron loss.
• Rolled Core (Wound Core)
  • Structural Feature: Integral winding of silicon steel strips, no obvious seams.
  • Advantage: Lower no-load loss, higher magnetic permeability (compared to laminated cores).

2. Winding (Coil): Electrical Circuit Core

2.1 Winding Function & Materials

• Core Role: Transmits electrical energy via electromagnetic induction—high-voltage windings connect to the grid, low-voltage windings supply loads.
• Materials:
  
  • Small-capacity transformers: Insulated enameled round copper wire.
  • Medium-to-large capacity transformers: Flat copper wire or flat aluminum wire (better current-carrying capacity and heat dissipation).

2.2 Winding Classification (by Position & Shape)

2.2.1 Concentric Winding (Mainstream)

• Structure: High-voltage and low-voltage windings are concentrically sleeved on core columns.
• Layout Rules:
  
  • Default: Low-voltage winding inside (facilitates insulation with the core), high-voltage winding outside.
  • Exception: Low-voltage, high-current large-capacity transformers—low-voltage winding outside (thick leads are easier to route).
• Key Design: Gap between windings acts as an oil channel (for oil-immersed transformers) or insulation layer, enhancing heat dissipation and insulation.
• Subtypes by Winding Method: Cylindrical, spiral, continuous (simple structure, easy manufacturing).
• Application: Core-type transformers—adopted by nearly all domestic power transformers.

2.2.2 Overlap Winding (Pie Winding)

• Structure: High-voltage and low-voltage windings are divided into multiple wire pies, alternately arranged along the core column height.
• Layout Rule: Low-voltage wire pies placed on top and bottom layers (simplifies insulation).
• Key Advantages: Small leakage reactance, high mechanical strength, convenient lead routing.
• Application: Low-voltage, high-current transformers (e.g., large-capacity furnace transformers, resistance welding machine transformers).