In each coil, the electromotive force is proportional to the sine value of the coil angle (the rotation speed is certain), but unlike the alternator, it has a brush, which can convert negative current (reverse current) into positive current, so the generated current has a certain direction, but the current is still large and small. Generally, DC motors have many coils that are angled to each other, so that the angles of the coils are different at the same time, and the electromotive force is large and small, but the sum tends to be stable.