A planetary gear mechanism includes a sun gear, a planet gear, a ring gear, and a planet carrier. An input member couples an electric motor to the ring gear or the sun gear. An output member is coupled to the planet carrier. A first clutch couples/decouples the input member and the output member. A selectable one-way clutch is disposed between a stationary member and the other of the ring gear and the sun gear. In a first operating mode, the selectable one-way clutch allows the other of the ring gear and the sun gear to rotate in a forward moving direction but prevents the other of the ring gear and the sun gear from rotating in a rearward moving direction. In a second operating mode, the selectable one-way clutch prevents the other of the ring gear and the sun gear from rotating in the forward moving direction.