Distributed-Gap Wound Cores for Pole-Mounted Transformers: Features and Benefits
Sep 25, 2025
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Pole-mounted distribution transformers are expected to run for decades on a pole, often in conditions where nobody looks at them. Their no-load loss, sound level and temperature rise are decided largely by the core, which is why the way the magnetic circuit is assembled matters as much as the grade of silicon steel used in it.
What a Pole-Mounted Transformer Does
A pole-mounted transformer is a distribution unit installed on a utility pole. It steps voltage down from a medium-voltage distribution line, typically 11 kV or 33 kV, to the low voltage used by households and businesses, commonly 400/230 V. The core and windings form the active part, and the core design directly sets no-load loss, sound level and temperature rise. Design and testing are carried out against the applicable distribution transformer standards, including the IEC 60076 and GB 1094 series requirements.
Shell type: one or more closed cores surround the windings.
Core type: a rectangular or E-shaped magnetic circuit encloses the windings.
Toroidal type: a ring-shaped core with a continuous magnetic path and no joints.
Wound core with distributed gap: a continuously wound strip core in which the required cut gap is divided into several segments.
How a Distributed-Gap Wound Core Is Built
A wound core is produced by continuously winding a strip of grain-oriented silicon steel (CRGO) into a spiral and then cutting the winding to create the air gap required by the magnetic design. In a distributed-gap version the gap is split into several segments around the core rather than concentrated in one place. Winding and cutting are the main operations, so no stamping dies are needed and gap positions can be arranged to suit the winding layout. Distributing the gap evens out magnetic reluctance, avoids the flux crowding and extra loss of a single large gap and preserves the smooth flux path that oriented strip is chosen for.
Key Features and Benefits
| Feature | Benefit |
|---|---|
| Low loss | The magnetic quality of the oriented strip is largely retained, because the core is not punched and there is no punching-induced degradation |
| Straightforward manufacturing | No dies or stamping tooling; winding and cutting dominate, which shortens the lead time for new sizes |
| Flexible gap positions | Gap segments can be placed to match the winding arrangement and to simplify assembly |
| Optional annealing | Cores can be supplied with or without annealing to balance magnetic performance against cost |
| Higher operating temperature | Because the core is not resin bonded, there is no resin degradation limit to observe at elevated temperature |
| Custom cross section | Dimensions, cross-sectional area and CRGO grade can be matched to the transformer rating |
Typical Specification Range
Wound cores are quoted by size, cross-sectional area, input current, output power and no-load voltage, and the material grade is chosen to satisfy the required quality and cost level. Representative combinations are listed below; other dimensions and cross sections are produced to drawing.
| Size (mm) | Cross-sectional area (mm²) | Input current (A) | Output power (mW) | No-load voltage (V) |
|---|---|---|---|---|
| 55 x 65 x 5 | 25 | 10 | 25.6 | 5.19 |
| 55 x 75 x 10 | 100 | 5 | 45 | 8.6 |
| 55 x 75 x 12 | 120 | 5 | 54 | 10.3 |
| 55 x 78 x 12 | 144 | 5 | 52 | 12.5 |
| 48 x 68 x 15 | 150 | 5 | 49 | 9.64 |
| 55 x 75 x 20 | 200 | 3 | 40 | 8.1 |
| 86 x 107 x 20 | 210 | 5 | 72 | 8.0 |
| 55 x 78 x 28 | 336 | 5 | 118 | 14.1 |
| 76 x 106 x 27 | 405 | 5 | 88 | 11.7 |
| 64 x 106 x 20 | 420 | 5 | 163 | 19.5 |
| 58 x 89 x 40 | 620 | 5 | 200 | 18.4 |
Why Distributed Gaps Suit Distribution Duty
In a pole-mounted transformer the core stays energised around the clock, so no-load loss accumulates even with no customer load connected. A concentrated gap raises excitation current and local loss, which appears as extra temperature rise and noise. Spreading the gap keeps flux density more uniform, holds no-load loss closer to the expectation set by the strip grade and reduces the excitation that radiates as sound from the tank or enclosure.
Rated capacity and voltage class of the transformer, together with the winding arrangement.
Required core outer dimensions and cross-sectional area, taken from the mechanical design of the unit.
CRGO grade, for example conventional CGO or high-induction Hi-B, and whether annealing is required.
Target no-load loss performance and the loss class to be demonstrated in the test report.
Order quantity, packing method and delivery schedule, so that winding and cutting capacity can be planned.
FAQ
Q: What is the difference between a distributed-gap wound core and a conventional gapped core?
A conventional design places the whole air gap in one location, while a distributed-gap core divides the same total gap into several segments around the magnetic circuit.
Q: Which material is used for these cores?
Continuously wound grain-oriented silicon steel strip, with conventional CGO or high-induction Hi-B grades selected according to the required loss level.
Q: Does the core need annealing?
Not always. Cores can be supplied with or without annealing, so the choice is made by balancing magnetic performance against cost for the specific design.
Q: Why can it operate at a higher temperature than a bonded core?
Because no resin bonding is used, there is no resin that can degrade at elevated temperature, so the temperature limit is set by the insulation system rather than by the bonding agent.
Q: Are dimensions and cross sections customised?
Yes. Dimensions, cross-sectional area, gap layout and material grade are matched to the transformer rating, and other specifications are produced on request.
Q: How is the finished core verified?
Cores are checked dimensionally, and magnetic performance is confirmed by test so that the measured loss can be compared with the value agreed for the design.