How to choose pressboard strip thickness for a transformer winding
A practical guide to picking strip thickness and width for axial ducts, packing and clamping in distribution transformers.
Pressboard strips do three jobs inside a winding: they form axial cooling ducts, they pack gaps so the winding sits tight, and they carry clamping force at the ends. The right thickness depends on which job the strip is doing.
Strips for axial cooling ducts
Duct strips sit between the winding cylinder and the first layer of conductor, running the full height of the winding. Their thickness sets the width of the oil channel. For small distribution transformers (up to about 250 kVA) duct strips of 4 mm to 6 mm are common. Larger units use 6 mm to 8 mm or even laminated strips. A wider duct cools better but takes space and increases the winding diameter.
Strips for packing
Packing strips fill the small gaps that remain after winding so nothing can move in service. Thin strips of 1 mm to 3 mm are usual, often used in combination to reach an exact dimension. Keep a range of thicknesses in stock so the fitter can make up any gap.
Strips for clamping
At the top and bottom of the winding, strips and blocks transfer the clamping force from the pressure ring into the winding. These see the highest load, so use thicker strips (6 mm to 10 mm) or Permawood blocks for larger units.
Width
Width is usually set by the number of strips around the circumference. More, narrower strips give more even support; fewer, wider strips are quicker to fit. Widths of 15 mm to 40 mm cover most distribution transformers.
A simple rule of thumb
| Duty | Typical thickness | Typical width |
|---|---|---|
| Axial duct, up to 250 kVA | 4 – 6 mm | 15 – 25 mm |
| Axial duct, 315 kVA – 2.5 MVA | 6 – 8 mm | 20 – 40 mm |
| Packing | 1 – 3 mm | as needed |
| End clamping | 6 – 10 mm or Permawood | 25 – 50 mm |
These are starting points. Your design engineer's drawing always takes priority, and we cut to whatever it specifies.