When a current is supplied to the solenoid it engages the plunger.
This moves the lever fork and in turn pushes the pinion on the starter motor drive shaft.
The movement also closes high current contacts (brushes) for the starter motor which then begins to turn.
Together with the rotation and engagement of the pinion to the engines flywheel the starting process is complete.
When the ignition switch is released and the current is no longer supplied to the starter motor the solenoid retracts. This moves the pinion away from the ring gear and the starter motor becomes inoperative.
Excessive cranking or vehicle faults resulting in the starter motor remaining engaged can result in excessive heat generation and the premature failure of the starter motor and/or its components.
ARMATURE, COMMUTATOR AND BRUSHES
The armature is mounted on the drive shaft and bearings for support. It is a laminated soft iron core which is wrapped with numerous conductors loops or windings. The commutator is a section of the shaft at the rear of the housing on which the brushes run to conduct electricity. Current flowing through the windings and the commutator bars crate a magnetic field which rotates the armature.
SOLENOID, PLUNGER AND LEVER FORK
The solenoid contains of two coils of wire that are wrapped around a moveable core. The solenoid acts as a switch to close the electrical connection and connects the starter motor to the vehicles battery. The process allows the solenoid to push forward the plunger connected to the lever fork which engages the pinion.
The pinion is a unique combination of a gear and springs. Once the starter is engaged the gear is extended into the gearbox housing and is engaged with the flywheel. This spins the engine to begin the combustion process.
The housing (yolk assembly) holds the starter fields in the housing with screws. This can consist of two to four field coils connected in series. Energised by the battery this converts the coils into an electromagnet which then turns the armature. The later type starter motors are fitted with permanent magnets in the housing. When the armature coils are powered a magnetic field is created around the armature. The left side of the armature is pushed away from the left magnet and drawn towards the right (also known as North and South magnetisation), causing rotation.