The stator core stacking is done on a special device external to the stator frame. In the axial direction the stator core is subdivided into a number of partial stacks, separated by spacers. These spacers form radial cooling ducts allowing the cooling air to pass. The spacers are I-shaped, of sturdy design and arranged to minimize the air-friction losses. In order to achieve a uniform temperature distribution over the whole length of the core the thickness of the partial stacks is gradually increased towards the end stacks.