A consequence of the projected trend in turbine-generator design towards larger airgaps, airgap windings without slots, and superconducting windings, is a progressive change in the nature of the machine from ‘iron-cored’ to ‘air-cored’.A simple analysis is presented which enables the main design and performance paramaters of both types of machine to be calculated and compared. A design philosophy for air-cored machines is outlined, and a number of favourable and adverse design features are revealed. In a companion paper the analysis of fields and inductances in air-cored and iron-cored synchronous machines is discussed.