Different Types of Transformer Cores Explained

The transformer is manufactured based on the principle of electromagnetic induction. The core is the skeleton and magnetic circuit of the transformer. Its role is the medium for energy conversion. Through the combination of it and the number of turns of the windings, the output voltage of the transformer can be stepped down or up.

According to different production processes, structures and materials, there are different types of transformer cores. Today, Isabel, an engineer from Jinan Omek Electric Power Co., Ltd., has summarized the standardized classification of cores for you, helping you understand and select.

Based On the Structure

Why are transformers made with different structural cores? This is a good question. Currently, there are two mainstream structural types of cores in the market – core type and shell type. These types are not designed randomly; instead, it is based on the performance requirements of the transformer, the usage environment, and the heat dissipation requirements.

(1) Shell type core

The cross-section of this core is generally rectangular. Each core column has two side yokes. Since the core is surrounded by the windings and looks like an outer shell, it is called shell type.
The chip specifications of shell-type cores are fewer, and the core is easy to fasten. The side yokes help reduce the additional losses caused by leakage flux.

Shell-type cores also have single-phase and three-phase types. The three-phase shell-type can be regarded as three independent single-phase shell-type transformers placed side by side.
Shell-type cores have one major drawback. Since the windings are surrounded by the core, the heat dissipation channels are blocked, making it difficult for the heat to escape, and thus unable to produce large-capacity and high-power models.

(2) Core type core

The construction of core type transformer has a stepped cylindrical cross-section, and the windings are wrapped around the core column, so it is called core-type, also known as inner-iron type.

This type of core has a larger number of chip specifications and higher requirements for binding and clamping. However, the windings are cylindrical, making winding easy and short-circuit stability good.

And the insulation between the windings and the core is also easy to handle, so it has been widely used.

Due to this design, the windings are exposed, and the generated heat can be easily dissipated, suitable for high-power transmission and long-term full-load operation.

If you want to know the differences between shell-type and core-type transformers (including their respective advantages, applications, and diagrams), this article will provide you with more detailed information.

Based On the Processing Technology

For the same material and structure of iron cores, if the processing techniques are different, the final losses, noise, and continuity of the magnetic circuit will be vastly different. OMEK Power currently has the following three major processing techniques, which are also the three main mainstream techniques in the market.

Traditional laminated iron core

This is the most classic and mature iron core processing technique. Silicon steel sheets are cut into standard sheet shapes and assembled layer by layer in an alternating pattern. Why are they assembled in an alternating pattern? Because this method has smaller gaps between the silicon steel sheets and lower magnetic resistance, which is conducive to increasing the magnetic flux.

Flat rolled iron core

The planar rolled iron core transformer is made by continuously rolling silicon steel sheets. The iron core has no seams, greatly reducing the magnetic resistance, and the no-load current can be reduced by 60 – 80%. Continuous rolling fully utilizes the orientation of the silicon steel sheets, reducing the no-load loss by 20 – 35%. After the iron core is annealed, its magnetic conductivity can be restored to the original level before machining.

3D rolled iron core

The spherical rolled iron core transformer is also an energy-saving power transformer. Its three-phase magnetic circuit is completely symmetrical, and the three-phase no-load current is completely balanced. The magneticization direction of the three-dimensional spherical rolled iron core is completely consistent with the rolling direction of the silicon steel sheets, and there are no overlapping seams between the iron core layers, with uniform magnetic flux distribution everywhere in the magnetic circuit, and no obvious high-resistance areas or distortion of the magnetic flux density at the seam areas.

Based on magnetic conductive materials

The material of the core directly determines the loss, frequency compatibility, cost and service life of the transformer.

Silicon steel sheet (LAMINATION)

The common core material of transformers is silicon steel sheet. This is because silicon steel sheet has good magnetic properties and low iron loss.

Silicon steel sheet is made of low-carbon steel containing silicon (usually 3% – 4%). The addition of silicon can significantly increase the resistivity of the material, thereby reducing eddy current loss.

According to the different silicon content, silicon steel sheets can be classified as low silicon steel and high silicon steel. The silicon content of low silicon steel sheets is generally below 0.5%, while that of high silicon steel sheets is between 0.5% and 4.5%.

High silicon steel sheets have higher magnetic permeability, but their mechanical strength and toughness are poorer. Therefore, in practical applications, the appropriate type of silicon steel sheet needs to be selected according to the specific requirements of the transformer.

The core made of silicon steel sheet can be divided into different shapes: EI type, UI type, C type, and mouth type.

Amorphous alloy core

In addition to silicon steel sheet, the core material of transformers also includes amorphous alloy. Amorphous alloy has extremely low iron loss and excellent performance at high frequencies.

Amorphous alloy transformers are energy-saving transformers. Taking iron-based amorphous alloy as an example, it has characteristics such as large magnetic permeability, small coercive force, high saturation magnetic induction intensity, small magnetic hysteresis loss and eddy current loss.

The no-load loss is only 1/5 – 1/3 of that of ordinary silicon steel sheet core, with outstanding energy-saving effect.

Which type of transformer core is most efficient?

To evaluate the efficiency of a transformer core, the most important aspects are to measure its no-load loss and load loss. The lower the overall loss, the higher the efficiency. As a company with a 20-year history of transformer production, we can confidently inform you that among all types of transformer cores, the amorphous alloy solid rolled core is the most efficient type, followed by the silicon steel solid rolled core.

One reason is that the no-load loss of amorphous alloy material is only 1/3 to 1/5 of that of ordinary silicon steel.

The second reason is that the solid rolled core structure design ensures a continuous magnetic path and complete symmetry of the three-phase magnetic path, thereby reducing losses in the process.

The combination of these two advantages makes the amorphous alloy solid rolled core the most energy-efficient combination for energy-saving distribution transformers at present.