First, the attraction coil also has another function, which is to ensure the smooth combination of the starter gear and the flywheel when the starter electromagnetic switch is attracted. The second reason is that the attraction switch alone cannot complete the entire working process of the starter. Working principle: (1) When the starter switch is just turned on, the current in the attraction coil passes through the starter armature winding and then grounds. The holding coil is directly grounded. At this time, the holding coil and the attraction coil generate magnetic flux in the same direction. After the magnetic flux is strengthened, a strong attraction is generated to attract the movable iron core to move until the moving and static contacts (main contacts) are closed. At this time, the two ends of the attraction coil are short-circuited by the main contacts, and no current flows through the attraction coil. The active iron core is only maintained in the attracted state by the current in the holding coil. Since the moving and static iron cores are in contact at this time, the magnetic resistance is very small, and the attraction generated by the smaller magnetic flux in the holding coil can maintain the active iron core in the attracted state. Due to the closure of the main contacts, the battery voltage is directly applied to the starter, and the starter drives the engine. (2) When the starter switch is disconnected after the engine starts, the current supplied to the attraction coil and the holding coil is cut off. However, the main contacts are still closed at this time. The current flows from the main contact to the attraction coil, and then through the holding coil to the ground. That is, when the starting switch is disconnected, the same current flows through the attraction coil and the holding coil, but the current in the attraction coil changes direction, while the current direction in the holding coil remains unchanged. Since the two coils have the same number of turns, their magnetic potentials are the same but in opposite directions, and the magnetic fluxes cancel each other out, that is, the synthetic magnetic field is zero. Under the action of the return spring, the moving iron core returns to its original position and the main contact is disconnected.