The starter motor which is instrumental for initial engine rotation, consumes a large amount of battery current which is shared by other intermediate elements in the electrical circuit. It is mistakenly believed that since the battery produces the electric current distributed to the ignition system (as discussed in our earlier article), the engine should start instantaneously. But an electric current alone cannot set the crankshaft in motion since it bears a huge amount of pressure created in the combustion chambers. The starter motor uses the electric current produced by the battery and then performs its task of rotating the flywheel, which is an external rotating shoulder of an engine bolted to one end of the crankshaft. The flywheel in rotation enables piston movement within the powerplant which later functions independently only on the induction of combustible air and fuel mixtures.

Due to a heavy build and constitution, the starter motor needs a massive feed of electricity from the battery to set its internal components working. The starter motor houses internal working ancillaries such as carbon brushes hinged on a base plate, rotating armature, field coil and bendix drive. These components in coordination make engine rotation possible. Providing initial push to the engine necessitates a big jolt of power. A starter motor and its internal working parts have to be robust enough to withstand the negative thrust produced by the internal cylinder pressures on the piston.

Regarding key aspect of the electric pipeline, it is essential that the starter is connected to the positive terminal of the battery through the solenoid switch. The solenoid switch is a transitional component which facilitates metering of the incoming electric current in conjunction with actuation of the ignition switch which is connected to its positive terminal. The volume of current channelled to the starter motor via the solenoid being massive, a large diameter wire or superior gauge is engaged for transportation of the electric current. Heat produced by that voltage of electricity could burn a wire of thinner gauge and disrupt functions and continuity of the entire circuit. Also, the amount of electric current needed by the starter to set its parts working demands free flow so that the rotation of the flywheel is not interrupted. This current starts flowing in the starting circuit when the ignition switch is turned on. So, let us next focus on how a starter motor functions.

As discussed earlier, the battery current travels to the solenoid switch through a thick gauge lead wire. The solenoid switch is mounted on the starter motor. A cut-off switch stops current flow in the starter motor as the ignition switch returns to the 'off' position in the event of roll of the engine. Excess current that is stalled and accumulated in the solenoid is diverted back to the battery from the earth terminal of the solenoid. Another important function of the solenoid is that it enables movement of a lever, one end of which is connected to the solenoid and the other to the bendix drive.

Prior to engine rotation initiated by the starter motor, the electric current wired in from the battery is poured into the starter motor by the second terminal of the solenoid. This current is absorbed by the field coil and the carbon brushes. The field coil is a cladding of the inner wall of the starter motor. Permanent magnet strips are used in the construction of the field coil. A rotating component known as the armature, in form of copper windings is placed so that it is encompassed by the field coil. One end of the armature is inserted in a circular metal carbon-brush plate so that the metal portion of the armature made of copper, known as commutator, rubs against the carbon brushes hinged on the base plate and current from the solenoid is conducted to its copper windings after passing through the current conducting commutator. The tip of the armature is fitted into a circular recess in the body of the starter motor. The cavity holds a circular bush in which the tip is held tight enabling it to rotate freely without eccentricity. The other end of the armature is a protruding shaft that continues to the bendix drive. A bendix drive is a type of a pinion gear. The bendix (pinion) gear is the component which engages the teeth on the engine flywheel ring. The shaft on which the bendix drive is installed hosts splines or teeth on which the bendix moves back and forth. The movement of the bendix is facilitated by a lever whose to and fro movement is controlled by the solenoid. The armature starts rotating soon as a magnetic field is induced within the starter motor assembly due to the surge of current from the solenoid in the field coil.

As the electric current enters the solenoid and the contacts are met (or circuit is completed), the lever moves forward only to usher the bendix gear towards the flywheel ring. Following this action the toothed bendix gear on the shaft gradually clutches itself with splines on the circumference of the engine flywheel and initiates the crank spin. Once the flywheel starts rotating the crank spins in turn and so do the pistons in the engine cylinders. Due to the flywheel's weight, inertia is maintained and it continues rotating faster. As soon as the flywheel starts spinning with requisite speed, the bendix drive declutches from the flywheel.

Until the completion of this process the magnetic field in the starter motor persists due to the 'on' position of the ignition switch. As soon as the switch is released the magnetic field collapses and the starter motor stops functioning. When the saturation point of the current is reached at the solenoid it is smoothly diverted from the starter motor circuit to the ignition circuit and the rest to complete the circuit through the earth terminal.

The whole procedural explanation seems to be overly lengthy. In practical reality, the entire circuit is completed in a matter of seconds. All the inter-related functions happen so quick that one does not give a thought to the starter motor when a vehicle breaks down and instead the battery is blatantly put to blame.