When I was a lad at school, we were taught that iron cores in AC applications were lossy and so were made from laminations that were insulated from one another by a coating on the iron. Indeed transfomers held together with bolts had fibre washers under the heads. If these washers were left out things got quite hot. Similar precautions were seen on some motors too.
However in the last year or so, servicing various AC items I've seen both transformers and motors with a neat welded line joining all the laminations together as well as uninsulated bolts. Yesterday, and AEG washer motor had 4 such welds evenly spaced around the stack of laminations
Can anyone explain a) why this is done and b) why it does not cause excessve loss and hence overheating