The frequency-response method is well established in the analysis of control systems and networks, but has hitherto not been extensively used for analysing rotating machines. The method is of particular value When the magnetic circuit of the machine contains unlaminated iron in which eddy currents are set up when the flux changes. The paper describes a method of calculating the performance of the machine, and shows how the effect of the eddy currents can be taken into account.Several methods of calculating the appropriate frequency-response curves have been worked out and experimentally verified. The application of the frequency-response curves to determine the performance characteristics is illustrated by calculations and tests for several simple transient conditions.