Transformer core – discover the working mechanism, types, materials, and more

Transformer core – discover the working mechanism, types, materials, and more

Transformers are indispensable components in electrical power systems, playing a vital role in the efficient transmission and distribution of electricity. At the heart of every transformer lies the core, a critical element that significantly influences the transformer’s performance and efficiency. This article delves into the intricate details of transformer cores, exploring their construction, the principles governing their function, and the techniques employed to mitigate energy losses, thereby providing a comprehensive overview of this fundamental technology. But first, let’s define what is a transformer.

What is a transformer?

Transformers are fundamental devices in the electrical power system, enabling the efficient transmission and distribution of electricity. Their ability to step up and step down voltage levels, provide isolation, and adapt to various applications makes them indispensable in modern electrical infrastructure.

How does the transformer core work?

A transformer core functions based on the principles of electromagnetic induction, where the flow of an alternating current (AC) through the primary winding generates a time-varying magnetic field around the winding. This magnetic field, in turn, induces a magnetic flux in the core, which is a crucial element in the transformer’s ability to transfer electrical energy between circuits. The induced magnetic flux is guided through the core, which is designed to provide a continuous and low-reluctance path for this flux. This pathway is essential for linking the primary and secondary windings of the transformer efficiently, ensuring that the energy transfer is both effective and minimally lossy.

Core material

Laminated Silicon Steel

Laminated silicon steel is the most commonly used material for transformer cores thanks to its advantageous properties that minimize energy losses. This type of steel is composed of thin sheets, or laminations, typically ranging from 0.2 to 0.35 mm in thickness. These laminations are coated with an insulating layer, which helps to reduce eddy current losses. By using thin, insulated laminations, the path of these currents is interrupted, thereby minimizing their magnitude and associated losses.

Silicon, typically in the range of 3% by weight, is added to the steel to improve its magnetic properties and reduce hysteresis losses. The presence of silicon increases the electrical resistivity of the steel, which further helps to limit eddy current formation. Additionally, silicon steel has a high permeability, which means it can easily support the formation of a magnetic field, making it highly efficient for use in transformers.

Grain-Oriented Electrical Steel (GOES)

Grain-oriented electrical steel (GOES) is a specialized type of silicon steel that is processed to have its grain structure aligned in a particular direction, typically the rolling direction of the steel sheet. This alignment significantly improves the steel’s magnetic properties, reducing hysteresis losses and increasing efficiency. GOES is designed to have very high permeability along the direction of the grain orientation, which allows it to carry magnetic flux more efficiently and with lower core losses.

The manufacturing process of GOES involves several stages, including hot rolling, cold rolling, and annealing. During annealing, the grains in the steel are aligned in a specific direction, enhancing its magnetic properties. This type of steel is particularly advantageous in high-efficiency transformers and is commonly used in power distribution and transmission transformers.

Amorphous Steel

Amorphous steel, also known as metallic glass, is a non-crystalline form of steel that has been rapidly cooled to prevent the formation of a regular crystal lattice. This material has exceptionally low hysteresis and eddy current losses thanks to its unique atomic structure. Amorphous steel cores are used in applications where energy efficiency is critical, such as in distribution transformers for smart grids.

Nanocrystalline Materials

Nanocrystalline materials are composed of nanoscale crystalline grains, typically iron-based alloys, and offer excellent magnetic properties. These materials have very low core losses and high permeability, making them suitable for high-frequency transformer applications, such as in power electronics and switch-mode power supplies.

Ferrite Cores

Ferrites are ceramic compounds consisting of iron oxide mixed with metallic elements like nickel, zinc, and manganese. Ferrite cores are widely used in high-frequency transformers and inductors due to their high magnetic permeability and low electrical conductivity. This combination helps to minimize eddy current losses at high frequencies.

Nickel-Iron Alloys (Permalloy)

Nickel-iron alloys, such as Permalloy, are used in specialized transformer cores that require high magnetic permeability and low coercivity. These alloys are often used in sensitive applications, such as audio transformers and instrumentation transformers, where signal integrity is paramount.

Types of transformer cores

The shape of the transformer core significantly influences its efficiency, size, and application suitability. Common core shapes include:

E-I Cores

E-I cores are constructed from E-shaped and I-shaped laminations. The E-shaped laminations form the central part and limbs of the core, while the I-shaped laminations complete the magnetic circuit. This configuration is easy to assemble and provides good performance for a wide range of applications. E-I cores are commonly used in distribution transformers and small to medium-sized power transformers.

C Cores

C cores are made from strips of steel wound into a coil and then cut into a C shape. This design offers advantages in terms of ease of assembly and reduced magnetic leakage. C cores are often used in applications where compact size and high efficiency are required, such as in audio transformers and high-frequency transformers.

Toroidal Cores

Toroidal cores are doughnut-shaped and provide a continuous magnetic path with minimal air gaps. The windings are evenly distributed around the core, which reduces magnetic leakage and improves efficiency. Toroidal transformers are known for their compact size, high efficiency, and low electromagnetic interference (EMI). They are used in various applications, including power supplies, audio equipment, and medical devices.

Shell-Type Cores

Shell-type transformers are characterized by a design where the core surrounds the windings. The primary and secondary windings are placed on a central limb, which is enclosed by two outer limbs, forming a shell-like structure. This configuration provides better mechanical strength and enhanced short-circuit resistance, making shell-type transformers more robust against electrical and mechanical stresses. The design also facilitates better cooling, as the core can dissipate heat more efficiently. These advantages make shell-type cores particularly suitable for low-voltage and high-current applications, such as furnace transformers, welding transformers, and certain types of distribution transformers where durability and cooling efficiency are critical.

Core-Type Cores

Core-type transformers feature a design where the windings are wrapped around two vertical limbs of the core, connected by horizontal yokes at the top and bottom. This arrangement allows for easier insulation and cooling, as the windings are more accessible and can be adequately separated and ventilated. Core-type transformers are well-suited for high-voltage applications, such as in power transmission and large power distribution systems. The robust and straightforward design of core-type transformers also makes them easier to manufacture and maintain, contributing to their widespread use in various electrical power applications where high voltage and reliable performance are required.

H Cores

H cores are a specific type of transformer core that uses two C-shaped cores combined to form an H-shaped structure. This design creates an efficient magnetic path with low magnetic leakage, as the magnetic flux is well-contained within the core. The H core’s compact design also helps to minimize core losses, making it highly efficient for power conversion. H core transformers are commonly used in high-power applications where efficient energy transfer and compact size are essential. Their design is particularly beneficial in high-frequency transformers, where reducing core losses and maintaining a stable magnetic field are critical for performance.

Cut-Core Transformers

Cut-core transformers utilize a design where a continuous strip of core material is wound into a shape and then cut to form two halves. These halves are then assembled around the windings, making it easy to place the windings without threading them through the core. This design facilitates easy assembly and disassembly, which is advantageous for applications requiring maintenance or replacement. Cut-core transformers are known for their low core losses and high efficiency, making them ideal for high-frequency applications such as switch-mode power supplies and power electronics. The precise cutting and joining of the core material ensure minimal air gaps and efficient magnetic flux containment, enhancing the overall performance of the transformer.

Loses in transformer cores

Transformer cores experience two primary types of losses: hysteresis loss and eddy current loss. Both of these losses contribute to the overall inefficiency of a transformer, causing energy to be dissipated as heat. Understanding these losses and the methods to mitigate them is crucial for designing efficient transformers.

Hysteresis Loss

Hysteresis loss occurs due to the inherent properties of the magnetic core material. As alternating current (AC) flows through the transformer’s windings, the core material undergoes repeated cycles of magnetization and demagnetization. This process follows a hysteresis loop, which is a graphical representation of the relationship between the magnetic field strength (H) and the magnetic flux density (B) in the core material.

  1. Mechanism
    • When the magnetic field in the core reverses direction with each cycle of AC, the magnetic domains within the core material realign themselves. This realignment process requires energy because of the internal friction and resistance encountered by the magnetic domains.
    • The energy required to reorient the magnetic domains with each cycle is not fully recovered when the magnetic field returns to zero. This energy loss manifests as heat within the core.
  2. Factors Affecting Hysteresis Loss
    • Core Material: Different materials have different hysteresis characteristics. Soft magnetic materials with narrow hysteresis loops, such as grain-oriented electrical steel (GOES), exhibit lower hysteresis losses compared to hard magnetic materials.
    • Frequency: Hysteresis loss increases with the frequency of the alternating current. This is because the core material undergoes more cycles of magnetization and demagnetization per unit time.
    • Magnetic Flux Density: Higher magnetic flux densities result in larger hysteresis loops, which correspond to higher hysteresis losses.
  3. Reduction Techniques
    • Material Selection: Using materials with lower coercivity and narrower hysteresis loops, such as GOES, helps to reduce hysteresis losses.
    • Optimal Design: Designing the transformer to operate within the optimal range of magnetic flux density minimizes hysteresis losses.

Eddy Current Loss

Eddy current loss is caused by circulating currents induced within the core material by the alternating magnetic field. These currents create their own magnetic fields that oppose the original magnetic field, leading to energy dissipation as heat.

  1. Mechanism
    • When an alternating magnetic field passes through a conductive material, such as the steel core of a transformer, it induces circulating currents known as eddy currents.
    • According to Faraday’s Law of Induction, any change in the magnetic flux through a conductor induces an electromotive force (EMF) within the conductor. This EMF drives the eddy currents, which flow in closed loops within the plane of the laminations.
  2. Factors Affecting Eddy Current Loss
    • Core Material: The electrical resistivity of the core material influences eddy current formation. Materials with higher resistivity generate lower eddy currents.
    • Lamination Thickness: Thinner laminations increase the resistance to eddy current flow, reducing their magnitude and associated losses.
    • Frequency: Eddy current loss increases with the frequency of the alternating current, as higher frequencies induce stronger eddy currents.
  3. Reduction Techniques
    • Lamination: The core is constructed from thin, insulated laminations of steel. These laminations are stacked together, with the insulating coating preventing eddy currents from circulating between them. This increases the effective resistance to eddy current flow, thereby reducing losses.
    • Material Selection: Using materials with higher electrical resistivity, such as silicon steel, helps to minimize eddy current formation.
    • Core Design: Optimizing the design of the core to minimize the cross-sectional area exposed to the changing magnetic field reduces eddy current losses.

Combined Impact of Hysteresis and Eddy Current Losses

Both hysteresis and eddy current losses contribute to the overall core losses in a transformer, which can significantly impact the transformer’s efficiency. Core losses are generally categorized under no-load losses because they occur even when the transformer is not supplying load current. These losses result in heat generation, which can affect the transformer’s temperature and necessitate cooling mechanisms to maintain safe operating conditions.

Conclusion

Transformer cores are fundamental to the efficient operation of transformers, with their design and material choices significantly impacting performance. Understanding the mechanisms of magnetic flux, core materials, and the types of core designs is crucial for optimizing transformer efficiency and reliability. By mitigating losses such as hysteresis and eddy current losses, engineers can ensure that transformers operate effectively, contributing to the stable and efficient distribution of electrical power. This knowledge is vital for advancing electrical infrastructure and meeting the growing demands of modern power systems.

At TTES, we have over 100 combined years of experience manufacturing, maintaining, and repairing transformers. If you’re looking for industry-leading lead times of just 20 weeks on average, don’t hesitate and reach out to us for a free quote!

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