Eddy-current effects in solid, unslotted iron rotors

A turboalternator, when undergoing speed oscillations or when running out of synchronism, experiences damping torques caused by eddy currents set up in the solid iron of the rotor. Previous attempts at estimating these torques have inadequately represented the effects of nonlinearities in the iron, and of return paths for the eddy currents at the ends of the rotor. An analysis is made in this paper of the relative distribution of currents in the stator winding and in the rotor iron, and of the resultant distribution of air-gap flux. An expression for the eddy-current torque is obtained for the case of an unslotted rotor rotating at a steady slip speed. Results obtained from two test machines confirm the theory for the 2-pole and 4-pole cases.