FRP Dog Bone Insulation For Cast Resin Transformer Winding

FRP Dog Bone Insulation for Cast Resin Transformer Winding

In a cast resin (vacuum-cast dry-type) transformer, the winding is encapsulated in epoxy resin - so every spacer embedded inside the coil must survive vacuum casting, resin exotherm, and decades of thermal cycling without creating voids, cracks, or partial discharge. The FRP dog bone insulation is engineered for exactly this job: its dumbbell-shaped, fiberglass-reinforced plastic profile holds the conductor layers at a precise, constant distance, forms uniform vertical cooling ducts, and remains dimensionally stable while the epoxy cures around it.

Available in Class F (155°C) and Class H (180°C) systems, this dog bone spacer is the dedicated insulation component for cast resin transformer winding - combining dielectric insulation, mechanical support, and process reliability in one pultruded profile.

Why Cast Resin Windings Need a Dedicated Dog Bone Spacer

A cast resin coil is not an ordinary winding - it is a winding that gets permanently locked inside resin. That changes the demands on every internal spacer:

Resin shrinkage - during cure, epoxy shrinks and compresses the coil; the spacer must resist deformation and keep the conductor gap constant.

Casting exotherm - resin generates heat as it cures; the spacer must tolerate elevated process temperature at its rated class.

Thermal-expansion mismatch - the spacer's CTE must be close to the resin and copper to avoid interface stress and micro-cracks.

Partial discharge (PD) risk - any void or air pocket at a spacer/conductor interface becomes a PD site; the dog bone profile promotes clean, void-free resin flow.

Cooling-duct formation - the encapsulated winding has no open air path, so the spacer must create the vertical ducts that carry heat away.

Short-circuit forces - the cured coil must resist radial and axial forces; the spacer is part of that mechanical chain.

A general-purpose spacer is not enough - this is why a purpose-built cast resin dog bone exists.

Product Benefits 

Electrical & dielectric benefits

High dielectric strength for reliable layer-to-layer and turn-to-turn insulation coordination (confirm value).

Void-free interfaces - the smooth, waisted dog bone profile lets epoxy flow evenly and wet out, reducing partial-discharge initiation sites.

Stable dielectric properties under humidity - low moisture absorption keeps insulation resistance steady in tropical and coastal service.

Non-magnetic - no eddy-current loss introduced into the magnetic circuit.

Mechanical & structural benefits

High compressive and flexural strength from continuous glass-fiber reinforcement - supports the coil during winding and curing.

Minimal creep - holds conductor spacing for the transformer's full service life.

Short-circuit withstand - helps the cured coil resist radial/axial electromagnetic forces.

Dimensionally stable - resists resin-shrinkage pressure without distortion.

Thermal & casting-process benefits

Class F (155°C) / Class H (180°C) - matched to cast resin winding hot-spot temperatures.

Withstands casting exotherm - performs through vacuum casting and resin cure.

CTE matched toward resin and copper (confirm) - reduces interface stress and micro-cracking.

Creates uniform vertical cooling ducts - improves heat dissipation in the encapsulated winding.

Vacuum-process compatible - low outgassing, suitable for vacuum casting.

Economic & assembly benefits

Pultruded, cut-to-length - supplied to your coil height, reducing winding waste and assembly time.

Lightweight and easy to handle - faster coil building on the winding table.

Custom profiles - dog bone geometry, thickness and length produced to your drawing or sample.

Predictable, repeatable quality - consistent profile means consistent, repeatable windings batch after batch.