The internal combustion engine is not capable of self-starting. Automotive engines (both spark-ignition and diesel) are cranked by a small but powerful electric motor. This motor is called a cranking motor, starting motor, or starter.

The battery sends current to the starter when the operator turns the ignition switch to start. This causes a pinion gear in the starter to mesh with the teeth of the ring gear, thereby rotating the engine crankshaft for starting.

The typical starting circuit consists of the battery, the starter motor and drive mechanism, the ignition switch, the starter relay or solenoid, a neutral safety switch (automatic transmissions), and the wiring to connect these components.

STARTER MOTOR
The starting motor (fig. 2-37) converts electrical energy from the battery into mechanical or rotating energy to crank the engine. The main difference between an electric starting motor and an electric generator is that in a generator, rotation of the armature in a magnetic field produces voltage. In a motor, current is sent through the armature and the field; the attraction and repulsion between the magnetic poles of the field and armature coil alternately push and pull the armature around. This rotation (mechanical energy), when properly connected to the flywheel of an engine, causes the engine crankshaft to turn.

Starting Motor Construction
The construction of the all starting motors is very similar. There are, however, slight design variations. The main parts of a starting motor are as follows:

ARMATURE ASSEMBLY- The windings, core, starter shaft, and commutator assembly that spin inside a stationary field.

COMMUTATOR END FRAME- The end housing for the brushes, brush springs, and shaft bushings.

Figure 2-37.- Typical starting motor.

PINION DRIVE ASSEMBLY- The pinion gear, pinion drive mechanism, and solenoid.

FIELD FRAME- The center housing that holds the field coils and pole shoes.

DRIVE END FRAME- The end housing around the pinion gear, which has a bushing for the armature shaft.

ARMATURE ASSEMBLY.- The armature assembly consists of an armature shaft, armature core, commutator, and armature windings.

The armature shaft supports the armature assembly as it spins inside the starter housing. The armature core is made of iron and holds the armature windings in place. The iron increases the magnetic field strength of the windings.

The commutator serves as a sliding electrical connection between the motor windings and the brushes and is mounted on one end of the armature shaft. The commutator has many segments that are insulated from each other. As the windings rotate away from the pole shoe (piece), the commutator segments change the electrical connection between the brushes and the windings. This action reverses the magnetic field around the windings. The constant changing electrical connection at the windings keeps the motor spinning.

COMMUTATOR END FRAME.- The commutator end frame houses the brushes, the brush springs, and the armature shaft bushing.

The brushes ride on top of the commutator. They slide on the commutator to carry battery current to the spinning windings. The springs force the brushes to maintain contact with the commutator as it spins, thereby no power interruptions occurs. The armature shaft bushing supports the commutator end of the armature shaft.