Air cooling devices driven rotational power of electric motors to expand air to thereby produce and supply cooling air. An air cooling device includes a power transmission device for increasing a rotational driving force of an expander that is disposed between the expander housing and an electric motor and for separating the expander from the electric motor. The power transmission device generates rotational magnetic field by receiving rotational power from the electric motor or receiving rotational power from the electric motor and external electric power. The rotating magnetic field generates a rotational force by interacting with magnetic field, based on attraction and repulsion forces. The air cooling device increases rotation speed and force when it transmits the power. The air cooling device increases a flow rate, and expands air adiabatically, thereby supplying cold air with high air density.