Power Transformer Construction – The Core

Power Transformer Construction - Core
Power Transformer Construction – Core (on photo transformer cores showing silicon steel top; by Megawatt)

The construction of a power transformer varies throughout the industry. The basic arrangement is essentially the same and has seen little significant change in recent years, so some ofthe variations can be discussed in this article.

The transformer core

The core, which provides the magnetic path to channel the flux, consists of thin strips of high-grade steel, called laminations, which are electrically separated by a thin coating of insulating material.

The strips can be stacked or wound, with the windings either built integrally around the core or built separately and assembled around the core sections.

Core steel can be hot or cold-rolled, grain-oriented or non-grain oriented,and even laser-scribed for additional performance.

Thickness ranges from 0.23 mm to upwards of 0.36 mm. The core cross section can be circular or rectangular, with circular cores commonly referred to as cruciform construction. Rectangular cores are used for smaller ratings and as auxiliary transformers used within a power transformer. Rectangular cores use a single width of strip steel,while circular cores use a combination of different strip widths to approximate a circular cross-section.

The type of steel and arrangement depends on the transformer rating as related to cost factors such as labor and performance.

Just like other components in the transformer, the heat generated by the core must be adequately dissipated.

While the steel and coating may be capable of withstanding higher temperatures, it will come in contact with insulating materials with limited temperature capabilities. In larger units, cooling ducts are used inside the core for additional convective surface area, and sections of laminations may be split to reduce localized losses.

The core is held together by, but insulated from, mechanical structures and is grounded to a single point in order to dissipate electrostatic buildup. The core ground location is usually some readily accessible point inside the tank, but it can also be brought through a bushing on the tank wall or top for external access.

This grounding point should be removable for testing purposes, such as checking for unintentional core grounds. Multiple core grounds, such as a case whereby the core is inadvertently making contact with otherwise grounded internal metallic mechanical structures, can provide a path for circulating currents induced by the main flux as well as a leakage flux, thus creating concentrations of losses that can result in localized heating.

The maximum flux density of the core steel is normally designed as close to the knee of the saturation curve as practical, accounting for required overexcitations and tolerances that exist due to materials and manufacturing processes.

For power transformers the flux density is typically between 1.3 T and 1.8 T, with the saturation point for magnetic steel being around 2.03 T to 2.05 T.

There are two basic types of core construction used in power transformers: core form and shell form.

In core-form construction,there is a single path for the magnetic circuit. Figure 1 shows a schematic of a single-phase core, with the arrows showing the magnetic path.

Schematic of single-phase core-form construction.
Figure 1 – Schematic of single-phase core-form construction.
Schematic of three-phase core-form construction
Figure 2 – Schematic of three-phase core-form construction

For single-phase applications, the windings are typically divided on both core legs as shown. In three-phase applications, the windings of a particular phase are typically on the same core leg, as illustrated in Figure 2.

'E'-assembly, prior to addition of coils and insertion of top yoke
Figure 3 – ‘E’-assembly, prior to addition of coils and insertion of top yoke

Windings are constructed separate of the core and placed on their respective core legs during core assembly. Figure 3 shows what is referred to as the “E” – assembly of a three-phase core-form core during assembly.

In shell-form construction, the core provides multiple paths for the magnetic circuit. Figure 4 is a schematic o fa single-phase shell-form core, with the two magnetic paths illustrated.

The core is typically stacked directly around the windings,which are usually “pancake” – type windings, although some applications are such that the core and windings are assembled similar to core form.

Schematic of single-phase shell-form construction
Figure 4 – Schematic of single-phase shell-form construction

Due to advantages in short-circuit and transient-voltage performance, shell forms tend to be used more frequently in the largest transformers,where conditions can be more severe. Variations of three-phase shell-form construction include five- and seven-legged cores, depending on size and application.

Video Illustrating The Construction of Distribution Transformer

Reference: Electric Power Transformer Engineering, published May 16, 2012 by CRC Press // chapter Power Transformers authored by H.J. Sim and S.H. Digby (Get this ebook from CRC Press)

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About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on


  1. chandu
    Aug 20, 2014

    HI sir !
    I need info how to select area of core in transformers with details please forward your solution to above mail id .
    thank you.

  2. Michael Fazzone
    Jun 27, 2014

    well detailed

  3. jithin
    Mar 13, 2014

    ohh a lot of thanks……exact details here…

  4. senthil kumar perumal
    Jan 01, 2013

    very useful and informative

  5. omeralnahi
    Aug 23, 2012

    thank u very much

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