An AC generator for vehicles includes a field rotor, a stator disposed around said field rotor and a frame for supporting the rotor and the stator. The stator has a laminated core having a plurality of slots, aluminum conductors housed in the slots, and an electric insulator. The aluminum conductor is constituted by in-slot portion and a crossover portion for connecting an in-slot portion in one slot to another in-slot portion in a different slot to provide a winding as a whole. The frame is provided with a plurality of windows substantially in a whole periphery thereof at portions encircling the crossover portions and substantially in an outer diametrical direction opposite to the crossover portions.