Starter Motors
As the name suggests, the purpose of a starter motor is to rotate the engine until it ignites the fuel and rotates of its own accord.
Pre-Engaged Starter Motors
The car ignition switch closes an electrical circuit in the actuator or starter solenoid. The solenoid uses an electro-magnetically controlled linear armature to first engage the starter motor with the ring gear flywheel of the engine drive gears.
Once engaged, the solenoid armature continues to the end of its travel making an electrical contact to send electricity into the starter motor, which then turns the engine.
In other words, the starter motor is pre-engaged before it starts to turn the engine.
The drive gear system has an internal one-way clutch to protect the starter motor. The clutch slips once the engine is turning faster than the starter motor and it disengages. The drive gear allows a small low inertia starter motor to be used. This turns faster than old style motors using less current and reducing weight.
Starter Motor Construction
An electrical motor uses the repulsion of magnetic fields to turn an armature and provide mechanical power from electrical energy.
One magnetic field is formed by passing an electrical current through a wire coil on the armature. Electrical contact is made with the coils as they rotate by having conductive brushes push against a commutator that rotates with the coils on the armature.
The motor housing provides the second magnetic field either using permanent magnets mounted within the motor housing or by having field coils wound around pole shoes that generate a temporary field by electro-magnetism and channel it for optimal motor efficiency.