Advanced Maintenance Electrician Technology

Author: Chief Editor: Bian Yin
Publisher:
Publish Date: 2002-01-01
Features: This book is based on the "Occupational Skill Qualification Specification for Maintenance Electricians" of the People's Republic of China and is a vocational skill training textbook suitable for advanced maintenance electricians. The book consists of nine chapters, covering topics such as transformers, practical electronics, motor drives, automatic control, analysis and troubleshooting of complex electrical equipment, and the principles and applications of programmable controllers. The textbook adheres to the principle of training according to job requirements, strictly limiting the content within the scope of the "Specification," while ensuring knowledge coherence and focusing on practical skills. It aims to be concise, targeted, typical, and highly practical.
Excerpt: Training requirements include understanding the structure and manufacturing of transformers, major overhaul and commissioning procedures, and the process calculations for transformer rewinding or re-winding.
Section: Transformer Structure and Manufacturing
1. Core Structure and Manufacturing Process
1. Core Structure
The core serves as the path for magnetic flux, providing a low-reluctance loop for the flux. To minimize energy loss in the core, it is made of silicon steel sheets with thicknesses of 0.35–0.5 mm, stamped into various sizes, insulated with varnish on both sides, and stacked according to specific rules. The core is both the magnetic circuit of the transformer and the skeleton of the housing. It consists of core columns, yokes, and clamping devices. The part where windings are placed is called the core column. The part connecting the core columns to form a closed magnetic circuit is called the yoke. The clamping device uses through bolts to securely fasten the core column and yoke, supporting and clamping the windings, fixing leads, and setting tap changers. Transformers are classified into shell-type and core-type designs. In shell-type transformers, the windings surround the core, while in core-type transformers, the core surrounds the windings. Shell-type transformers are commonly used in small devices, such as power supplies for recorders and small control transformers. Power transformers mostly adopt a three-phase core-type structure. Figure 1-1 illustrates the arrangement of three-phase windings on a three-phase transformer core.
To prevent electrostatic induction from causing potential differences on the core and other metal components during transformer operation, leading to ground discharge, the clamping device, core, and other metal components must be reliably grounded. However, only one grounding point is allowed on the core. If there are two or more grounding points, they may form a closed loop, causing circulating currents and localized overheating.
2. Core Manufacturing Process
The manufacturing process of the core mainly includes steps such as silicon steel sheet cutting, punching, deburring, annealing, painting, stacking, finishing, and core performance testing. The additional losses in a transformer's no-load loss are largely determined by the level of core manufacturing technology.
(1) Cutting and Punching
The length and width of the stacked sheets are determined by the core dimensions. It is essential to consider the arrangement of two or three sheets per stack and staggered seams, selecting the fewest sheet specifications while ensuring tight inter-sheet seams. For stacked sheets of ordinary hot-rolled silicon steel, which has non-directional magnetic properties, they are typically cut into rectangular shapes. For stacked sheets of cold-rolled silicon steel, which has directional magnetic properties, diagonal seams (with a 45° angle) are used to minimize the impact of inconsistent flux direction and steel sheet rolling direction.