Dovetail spacers explained: how radial cooling ducts are built
What dovetail spacers and strips do, why the dovetail shape matters, and how to specify them.
In a disc or helical winding, each turn or disc has to be separated from the next by a gap that oil can flow through. These horizontal gaps are called radial ducts. Dovetail spacers are the small blocks that create them.
Why the dovetail shape
A plain block would slip out as the winding is wound and clamped. A dovetail spacer has a tapered key on its back that slots into a matching groove in a dovetail strip running up the winding. Once in the strip, the spacer cannot fall out, and a row of spacers stays exactly in line from top to bottom of the winding.
How many spacers
Spacers are placed at regular intervals around the circumference, usually 8 to 24 per layer depending on winding diameter. The number of layers equals the number of discs. A 100 kVA distribution transformer may use a few hundred spacers; a large power transformer uses thousands.
What to specify
- Spacer thickness (sets the duct height): commonly 3 mm to 6 mm
- Spacer width and length: matched to the conductor width
- Dovetail profile: standard, or to your drawing
- Matching dovetail strip dimensions
- Quantity, in kg or pieces
Material
Both spacer and strip are made from electrical-grade pressboard so they dry and oil-impregnate along with the rest of the winding. The pressboard must be dense enough that the dovetail does not crush under clamping force.
We supply spacers and matching strips together so the fit is right first time.