Starter motor parts and functions

The starter motor is a primary component of the starting system. A starter (also self-starter, cranking motor, or starter motor) is a device used to rotate (crank) an internal combustion engine to initiate its operation under its own power. Starters can be electric, pneumatic, or hydraulic, but we will learn the electric starter motor in this post.

The starter motor drives the engine through a pinion gear that engages the ring gear on the flywheel. When you turn the ignition switch, the starter motor is energized, and the electromagnet inside the body engages. The electromagnet pushes out a rod to which the pinion gear is attached.

As the pinion gear meets the flywheel's ring gear, the starter turns. It also turns the engine over. At the same time, the other systems are turning ON, such as ignition system, fuel system, charging system, etc., so the engine can run on by its power. After its power runs the engine, the pinion gear disengages, and the starter motor is stopped.

The starter motor is built from various components were work both mechanically and electrically. In this article, we will learn all about parts of the starter motor parts and their function. Below are starter motor parts and functions.

1. Starter Solenoid Switch

The starter solenoid switch is a part of the motor starter that was commonly attached at the top of the main body of the motor starter. The starter solenoid has two functions :

As a switch to actuating a contactor coil that connects the battery to the starter motor, its function is the same as a common relay function but designed for a larger electric current.
As a driver, to drive the starter pinion gear to engage in to the flywheel ring gear.

The starter solenoid switch is divided into three parts.

a. Coils (hold in coil and pull in coil), they are works as an electromagnet to push or pull the contacting switch and the Plunger
b. Contacting switch is a switch connecting terminal 30 from battery to terminal C so the starter motor can works properly.
c. Plunger is a cylindrical metal shape with a hook where drive the overrunning clutch with a fork lever.

The coils and the contacting switch are made together as one part, but the Plunger is separated. It is attached inside the body of the coils and switch.

2. Yoke and pole core

Yoke is a starter motor part were functions as a body of the starter motor, and at once, it is used as a place to pole core and field coil winding. Yoke is made of metal with cylindrical shapes.

The Pole core is made of metal and attaches inside the yoke. Pole core is a place for field coil winding. When the field coil being a magnet, the pole core strengthens the effect of the magnetic field.

Yoke and pole core often use only in conventional starter motor types. In contrast, the other starter motor types (Planetary reduction segment and Reduction gear) use permanent and interpolar magnets.

3. Field coil

The field coil function is to generate a magnetic field using electric current flow. The field coil is made from wire were formed as coils. The field coil is attached to the pole core and surrounds it. It is placed at the inner side of the yoke.

Like the pole core, the field coil is commonly used on the conventional motor starter type. Meanwhile, the other types used permanent and interpolar magnets.

4. Armature

The armature is a power-producing component of a starter motor. Its function is to convert electrical energy to mechanical energy. Armatures used in starter motors are coils that carry current and rotate in a magnetic field.

There are three major components on the armature such as:

a. Armature coil

An armature has a continuous series of wire windings from each bar. It loops around the iron stack teeth and connects to the next bar on the commutator. The armature coil function generates a magnetic field. So, the magnetic field from the armature will drive in contradicting ways with the magnetic field from the field coil. As it moves in the opposite direction, the armature assembly will rotate.

b. Armature shaft

The armature shaft is a shaft were supports the armature coil assembly. On each side of the armature shaft, there are bushings attached. With bushings attached, the shaft will be rotating freely without any resistance.

c. Commutator

A commutator is a rotary electrical switch that periodically reverses the current direction between the rotor and the external circuit. It's a spinning contactor working with brushes to deliver the electric current from the battery.

A commutator consists of contact bars fixed to the rotating shaft. It is also connected to the armature wire windings. The winding continues to loop around the armature in the same manner. The commutator is a gate for the electric current to flow in and out from the armature windings.

5. Brushes and holder

The starter motor's brushes and holder flow the electric current to the armature assembly while it is spinning. Brushes are connected to the commutator. So, when the electric current flows, the commutator will be spinning. While the commutator spins, the brushes are still flowing the electric current without interruption from rotating movement.

Commonly, there are four brushes on the holder inside the starter motor. Two positives connected to the field coil and battery, and the two others were negative were connected to the ground.

6. Drive lever

Drive lever is a lever were connecting the plunger and the overrunning clutch. The plunger movement drives the overrunning clutch to make the pinion gear engage with the flywheel ring gear.

When the electric current has activated the hold in coil and pull-in coil, the plunger will pull in its body inside the starter solenoid. At the same time, the drive lever will push the overrunning clutch to the opposite direction to engage with the ring gear on the flywheel.

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7. Over-running clutch

Both conventional and gear reduction starter motors are fitted with a one-way, over-running clutch. The Over-running clutch prevents damage to the starter motor when the engine starts, mainly when the rotational speed of the ring gear is higher than the starter pinion gear.

Over-running clutch operation:

During engine start, the starter pinion gear drives the engine's flywheel ring gear.
Once the engine fires, the ring gear almost instantly turns faster than the starter pinion gear. Over-speeding would damage the starter motor if it not immediately disengaged from the pinion gear.
The clutch uses its wedged rollers and springs to disengage the pinion shaft from the clutch housing (which turns with the motor armature). It happens any time the pinion shaft tries to turn faster than the clutch housing.

8. Pinion gear

The pinion gear is fitted on the armature assembly, mainly in the front shaft facing the flywheel ring gear. The pinion gear function is to drive the ring gear to rotate to run the engine. Once the engine is running, the pinion withdraws.

9. Planetary gear

The planetary gear is used only in the planetary reduction starter motor type. It is fitted in front of the armature assembly. The planetary gear function allows the starter motor to operate at a higher speed than a conventional stater.

10. Gear reduction

The gear reduction component is used only in the Gear reduction starter motor type. This type is a compact, lightweight unit with high torque capacity. It is used extended gear before the pinion gear.

The reduction gears transfer this torque to the pinion gear at 1/4 to 1/3 the motor speed. The pinion gear still rotates faster than the gear on a conventional starter and has much greater torque (cranking power).

The reduction gear is mounted on the same shaft as the pinion gear. Unlike the conventional starter, the magnetic switch plunger acts directly on the pinion gear (not through a drive lever) to push the gear into mesh with the ring gear.