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Distributed Gap Core, Wound Core, Unicore for Pole Mounted Transformers

Distributed Gap Core, Wound Core, Unicore for Pole Mounted Transformers


A pole-mounted transformer is a type of distribution transformer that is mounted on a utility pole. It is used to step down the voltage of electrical power from high voltage transmission lines to a lower voltage suitable for distribution to homes, businesses, and other end-users. The pole-mounted transformer typically consists of a transformer unit, which includes the core and windings, mounted on top of a pole. Pole-mounted transformers are commonly used in rural areas or areas with distributed electrical infrastructure.

These are several core types used in pole-mounted transformers, including:

  • Shell-type core: This type of core consists of two cylindrical or toroidal-shaped cores that surround the windings. It provides efficient magnetic coupling and allows for easy cooling.

  • Core-type core: This type of core has a rectangular or E-shaped core that encloses the windings. It is widely used in distribution transformers due to its compact size and high efficiency.

  • Toroidal core: This type of core is donut-shaped and provides excellent magnetic coupling, low no-load losses, and compact design. It is commonly used in single-phase pole-mounted transformers.

These are some of common core types used in pole-mounted transformers, but there maybe other variations depending on specific requirements and manufacturer preferences.

Transmart designs and manufactures two popular types cores, one is Distributed gap wound core, the other is a Folded Octagonal Core or called Unicore. Both are for pole-mounted distribution transformers, ranging from 5KVA to 500KVA,  single phase and three phase available


Distributed gap wound core is a type of core used in pole-mounted transformers. It is a variation of the shell-type core, where the windings are distributed along the core with gaps between them. This design helps to reduce losses and improve the performance of the transformer. Distributed gap wound cores are often employed in high-power distribution transformers.

The advantage of a distributed gap wound core is that it helps to reduce losses and improve the performance of the transformer. The gaps between the windings help to minimize eddy current losses and reduce the leakage flux, resulting in higher efficiency. This type of core also provides better cooling and allows for easier maintenance and repair of the windings. Additionally, distributed gap wound cores are often used in high-power distribution transformers, making them suitable for applications requiring higher power handling capabilities.

The materials commonly used for distributed gap wound cores in pole-mounted transformers are typically laminated steel or amorphous alloy. Laminated steel is a traditional choice due to its low cost, excellent magnetic properties, and easy availability. Amorphous alloy, on the other hand, is a more advanced and efficient material that offers lower core losses and improved energy efficiency. Manufacturers may choose either of these materials based on their specific requirements and cost considerations.

Manufacturing porcess of a distributed gap wound core involves several steps. Here is a general overview

The design of the core is based on the specific requirements of the transformer, such as voltage rating, power capacity, and magnetic properties.

Material selection
The appropriate material, is selected based on factors like cost, efficiency, and availability. Transmart HiB Silicon Steels list for low losses transformers.
Material (Grade)Thickness (mm)Maximum core losses @1.7T/50Hz (W/Kg)
Core winding
Our automatic winding machine produce wound cores using a continuous cutting and feeding process. As a result, A wound core with distributed gaps are produced.  Minimum strip width of the core is 40mm, maximum outside diameter of the toroidal core is 1400mm.
Core forming
After the cores are wound, they are placed in a special designed pressing system, forming the cores from toroidal shape to rectangular shape as per initial designs.
Core annealing
Wound cores will be annealed during the manufacturing process to improve their magnetic properties and reduce any residual stresses. Annealing is a heat treatment process that involves heating the wound core to a specific temperature and then slowly cooling it down. This process helps to align the magnetic domains within the core material, resulting in improved magnetic performance and reduced losses.

Transmart uses electrical steels with high magnetic permeability and C5 insulation coating to resist 820ºC of annealing temperature.

Core testing
The completed core undergoes various tests, including no-load losses, no-load current, exciting power, magnetic permeability, and dimensional accuracy, to ensure it meets the required specifications.


Transmart Industrial offers another solution for distributed gap core. It is a Folded Octagonal Core or called Unicore. The core is folded by special design machines of great precision to achieve the best no-load losses. Feutures of Transmartcores:
Low losses-near raw material magnetic properties are retained
Easy manufacturing-No tooling needed
Flexible gap positions-Easy assembly
Cores can be designed with or without annealing
Cores can be operated at higher temperature because the cores are not resin bonded

The key points for distributed gap wound cores are as follows:
  • 01
    Distributed gap
    In distributed gap wound core transformes, the core is divided into multiple sections with small air gaps distributed across the core structure. These gaps help to control the magnetic flux distribution and reduce the core losses.
  • 02
    Core construction
    The core in distributed gap wound core transformers is usually made of laminated sheets of magnetic material, such as silicon steel. The distributed gap design allows for efficient cooling of the core and minimizes eddy current losses.
  • 03
    High efficiency
    The distributed gap wound core design offers high efficiency due to reduced magnetic flux leakage, improved magnetic coupling between the windings, and controlled flux distribution. This makes them suitable for appliciations that require high energy efficiency.
  • 04
    Size and weight
    Distributed gap wound core tranformers are generally smaller and lighter compared to other types of transformers with similar power ratings. The compact design makes them suitable for space-constrained applications.
  • 05
    Low noise levels
    The distributed gap design helps to minimize vibrations and noise generated during transformer operation, resulting in quieter performance.
  • 06
    Suitable for high-frequency applications
    Distributed gap wound core transformers are often used in high-frequency applications, such as power electronics, renewable energy systems, and telecommunications. The distributed gap design helps to reduce core losses at high frequencies.

Overall, distributed gap wound core transformers offer improved efficiency, reduced losses, compact size, low noise levels, and suitability for high-frequency applications. However, their selection depends on the specific requirements and priorities of the application.

Market trends for pole-mounted distribution transformers:

1. Increasing demand for renewable energy: With the global shift towards clean and sustainable energy sources, there is a growing demand for transformers to support renewable energy generation, such as solar and wind power.

2. Smart grid integration: The integration of smart grid technologies in power transmission and distribution systems is driving the demand for transformers that are capable of handling the increased data communication and automation requirements.

3. Energy efficiency and sustainability: There is a growing focus on energy efficiency and reducing environmental impact. As a result, there is an increasing demand for transformers that are designed to be more energy efficient and comply with sustainability standards.

4. Digitalization and IoT integration: The adoption of digital technologies and the Internet of Things (IoT) in the transformer industry is driving the development of smart transformers that can monitor performance, provide real-time data, and enable remote diagnostics and maintenance.

5. Integration of renewable energy storage: As energy storage technologies continue to evolve, there is a growing need for transformers that can efficiently integrate renewable energy storage systems, such as batteries, into the power grid.

Transmart Industrial will always stay updated on market trends to align our Wound cores and Unicores with the latest industry developments. Please contact us to get your high efficiency transformer cores.
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