AC losses in rotating and reciprocating superconductors and normal conductors

Abstract

The generation of an alternating voltage by varying the velocity with which a conductor cuts a uniform magnetic field should not be accompanied by the production of ac losses. Experiments to check this theory are described for both superconductors and normal conductors. Samples of each type of conductor were rotated at 3000 rpm and then reciprocated at the same frequency, and the losses measured by noting the increase in power ΔP required by the driving motor when the uniform magnetic field was switched on.Significant values of ΔP were recorded for both the normal and superconducting samples when rotated, even in the lowest flux densities (similar 0·1 T). When reciprocated, however, it was not possible to detect ΔP for either type of sample, even in B=5 T. This means that the copper losses were less than 0·01% and the Nb-Ti losses less than 0·1% of the respective rotational losses under the same electromagnetic conditions; it also implies that the superconductor losses were less than those appropriate to the superficial transport current density (5·5×104 A m−1) that might be used in a large generator employing the reciprocating principle.