In this article, we investigate rotor design of claw-pole alternators for automobile applications to increase output and decrease core loss. First, the electromagnetic field distribution is calculated by the three-dimensional (3-D) nonlinear time-stepping finite-element method, which considers eddy currents generated at a solid rotor core and a laminated stator core. From the results, a novel rotor design is proposed. Experimental verification is carried out to confirm the advantage of the proposed design. The mechanism of output increase and loss reduction by the proposed alternator is revealed by analyzing detailed 3-D flux paths in the rotors.