The electric motor ac is made of stator and rotor. In the model, the stator is electromagnet and the rotor is coil. The stator and rotor use the same power supply, so the direction of the stator and rotor in the current is always synchronized. Two copper rings are matched with the corresponding two brushes so that the current source of the energy can be constantly sent into the coil. The benefit of this design is to prevent the two power cords from intertwining together. If you use two wires to supply power to the coil, the two power cords will be intertwined because the coil keeps rotating. The current in the coil is alternating current, so it could be zero current sometimes. But the zero-current time is really too short compared to the time with the current. Besides, the coil has quality and inertia, so the coil will not stop with the inertia.