A large scale horizontal axis rotary electrical machine has a stator with a laminated annular core (14) resting upon an open-topped support structure (12). At one position along its length the core is axially fixed to the support structure but is otherwise free to move axially on the structure to accommodate differential thermal movements. Clamping means (40,42) for the core laminations incorporate springs (44) that allow the core to expand axially. The stator windings are axially slidable in slots in the laminated core and are held by respective winding end supports (54,56) spaced from the ends of the core, whereby differential thermal expansion is accommodated between the core and the windings. By adoption of the open-topped support structure, assembly of the core can be accelerated by lowering individual lamination stampings or pre-assembled biscuits of such stampings onto the structure while the structure remains in a horizontal position.