Starter Motor

I recently had to replace a starter motor, so I got a chance to look the interior of the starter motor assembly.    The starter motor assembly is consisted of a DC brushed motor, a solenoid and gears.   

The DC motor has series field winding: the input positive battery coming through the solenoid switch first goes into the field winding, then to a pair of copper brushes.  The negative terminal is the chassis connecting to the other brushes.  

The solenoid serves two functions: switching the battery power to the motor and pushing the gear to engage the engine flywheel.  Because of the large starting current (hundreds of amperes),  heavy gauge wires and large copper contact are used.   The solenoid has two coils: push-in coil that is connected to the motor positive terminal (the output of the switch) and the holding coil connected to the chassis.   This is a clever design: before the motor power switch is closed, the push-in coil is energized through the motor windings.  Once the motor power switch is closed; the push-in coil becomes inactive and a lower power holding coil keeps the solenoid in place.

The motor shaft is geared to the output the starter.  The solenoid has a spring loaded pin that pushes the gear to engage the flywheel.  There is also a clutch that prevents the flywheel to backdrive the motor once the engine has started.