An adaptor is disclosed for connecting a rotating electrical machine 12 to a prime mover 10. The adaptor comprises at least one air outlet. The adaptor is arranged to provide an increase in a cross-sectional area of air flow towards the outlet. In this way part of the dynamic pressure can be converted into additional static pressure rise through steady expansion of the air flow cross-section. This in turn may help to improve the overall amount of cooling air passing through the machine for a given fan input power.