Yes Mark, 'mechanical' could be highly relevant, along with thermal considerations, especially if the primary is wound first (as in IE transformers).
A few thoughts: if the secondary is supplying a substantial current, the primary winding will obviously heat up & the copper will try to expand. That heat cannot escape easily. All that expansion and subsequent shrinking (as it cools down after power off), could lead to wire fracture, especially if the wire is thin and initially wound tightly around the small radius curves of the core. The secondary winding is on the outside, usually thicker copper wire, greater radius of curvature: so far less chances of similar breakage.
Does some of that sound feasible to you? Or is the cause far more obvious: some toroid-wound transformers are simply cheap 'n' nasty.
Al.
A few thoughts: if the secondary is supplying a substantial current, the primary winding will obviously heat up & the copper will try to expand. That heat cannot escape easily. All that expansion and subsequent shrinking (as it cools down after power off), could lead to wire fracture, especially if the wire is thin and initially wound tightly around the small radius curves of the core. The secondary winding is on the outside, usually thicker copper wire, greater radius of curvature: so far less chances of similar breakage.
Does some of that sound feasible to you? Or is the cause far more obvious: some toroid-wound transformers are simply cheap 'n' nasty.
Al.
