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Starter Motor Definition and Function

The starter motor plays a crucial role in your car. Every time you start the car, the motor turns and sets the engine running. But that only happens when you have a working starter. To ensure a healthy starting system, you need to be conversant with how a starter motor works. That’s what you will be learning about shortly.

Among the topics we will be discussing are; starter motor meaning, operation, and the different types. We will also be looking at the starter motor cost and why prices are different on the auto parts market. Before all that, the starter motor definition and where to find it in a vehicle.

What is Starter Motor?

The starter motor is a type of high-torque motor used to start vehicle engines. It’s also called a cranking motor, starting motor, or simply starter. The motor is commonly used in vehicles with internal combustion engines. These engines require an external force to turn them over and start the combustion cycle. The starter motor performs that function perfectly.

The starter motor is one of the parts that make the starting system. Other starter system components include the ignition switch, starter relay, neutral safety switch or clutch switch, flywheel ring gear, and the battery. These parts are connected by wires and other electrical components to make the starting circuits.

The starter motor itself is an assembly of mostly metal parts. It basically consists of an electric motor, a drive mechanism to transfer motion to the engine, and a solenoid to act as a switch and actuator. To increase torque, a gear reduction mechanism is often added to the starter motor assembly.

The starter motor used in automobiles is usually mounted to the engine or transmission. However, it has to be positioned in a way that makes engaging with the ring gear on the flywheel easy. Different engagement methods are used, as we will be seeing shortly. For now, let’s see why vehicles need a starter

Starter Motor Function

What is the purpose of a starter motor or why do you need a motor to start your car engine? There are two reasons. For one, the engine cannot start the combustion process on its own, and while at zero rpm. It has to be turned over by an external force. In most vehicles, this force comes from a small but powerful motor called the starter motor. The starter motor in car starter systems performs two functions:

First, it converts the electric energy from the battery into mechanical energy and second, it provides the mechanism to engage with the flywheel crank the engine. Without the motor’s input, you would have a difficult setting the engine in motion.

The starter motor is a small component yet capable of turning the engine. To enable that, the motor uses a small gear that offers a large gear reduction ratio when it engages with the flywheel. The ratio varies across the engines but generally lies between 15:1 and 20:1 for most applications. Away from the starter motor purpose in a car, let’s now see how the motor works.

starter internal motor structure and the flywheel to the right

How a Starter Motor Works

As already mentioned, the starter motor assembly contains several parts besides the motor. There’s usually a solenoid mounted to the top and a pinion at the end of the shaft- sometimes alongside a gear reduction mechanism. The motor itself operates in the same way as any other motor. The only difference is in the way it starts and the added mechanism to help transfer motion.

Here is the starter motor working principle explained in steps.

  1. The motor contains an armature rotating inside a magnetic field. The armature shaft ends in a pinion and, in some types of the starter, a set of gears. When your turn on the ignition key of press the engine starting button, a current flows through the ignition circuit and activates the starter relay. The relay on the other hand, sends power to a part of starter motor called starter solenoid.

  2. The starter motor solenoid acts as a large switch or relay and sends current from the battery directly to the motor. The solenoid also moves a lever, helping to push a pinion gear to mesh with the ring gear on the flywheel or flexplate. These two events happen almost concurrently. The motor then rotates the flywheel, turning the engine over and starting it.

  3. When you release the ignition key, the ignition circuit stops sending current and the starter relay disconnects power to the starter solenoid. With the starter solenoid having no current flowing, it loses its magnetism and releases the plunger.

  4. The plunger returns to its former position, disconnecting the motor circuit and pulling back the pinion drive gear from the flywheel or flex plate. At this time, the engine can run on its own. There are several ways that manufacturers ensure the starter motor engagement and disengagement happens smoothly. They include the following.

In some types of the motor, the principle of inertia is employed to make the pinion travel along a shaft with helical splines. As soon as the engine starts, the flywheel spins at a speed higher than the pinion, thereby sending it back along the splined shaft- and out of engagement. A powerful spring on the shaft absorbs the shock of engagement and disengagement.

In most modern starters, a solenoid is used to move the pinion gear to mesh with the flywheel or flexplate. To prevent violent engagement, the motor does not rotate at a high speed initially. It only speeds up once the pinion has engaged the flywheel. In these types of starters, a one-way clutch is also installed to prevent the flywheel from turning (and damaging) the motor.

this image shows the main parts of a starter motor assembly

Starter Motor Parts and Function

The starter motor in car starting systems, as we have seen, is not just an electric motor. It contains parts that help it to start the engine and is often a complex piece of car part. Basically, the starter assembly is made up of a DC motor, a drive mechanism to transmit motion to the flywheel, and a solenoid in most types of the component. The different starter motor parts names and their functions are explained below.

Starter Motor Housing

This is the outer casing and usually made from metal. The housing protects the different internal components from the effects of corrosive materials, impact and other forms of damage. In most types of the starter, the casing splits into two pieces held together by long bolts.

a starter motor housing with the coils inside

Starter Motor Field Coil/Magnet

The field coils are the electromagnets that provide the magnetic flux to turn the armature. They mount to the housing and consist of copper wires around magnetic cores or pole shoes. In some types of starters, permanent magnets are used instead of field coils.

Starter coils

Starter Motor Armature

Also known as the rotor, the armature is the part of starter motor that turns when the motor is in operation. It’s made up of these components: copper wire windings around a magnetic core, a commutator to conduct current to the coils, and a shaft to hold the parts as the armature rotates. The shaft ends in a bearing to smoothen rotation and a pinion drive gear in some starters.

a starter motor armature showing the commutator

Starter Motor Brushes

The starter brushes are arranged around the commutator and held in brush holders. They connect to the circuit that brings current to the motor. The brushes are responsible for transferring the current to the armature coils or rotor. They are usually made of carbon or graphite and prone to wear over time.

Newly installed starter motor brushes

Starter Motor Bendix

a starter motor Bendix drive and its parts

This is the name given to the part that transfers the motor rotation to the flywheel ring gear. The starter Bendix primarily consists of a pinion gear and a method to push the gear toward the flywheel. In some starters, the Bendix uses inertia to engage with the flywheel. In a majority of starters today, the Bendix is a solenoid-operated mechanism. These starter types also include an over-running clutch to prevent the engine from driving the motor.

Starter Motor Solenoid

Also called the starter solenoid, this is the part used in newer starter types operate the Bendix drive. The solenoid also switches the starter motor circuit, connecting the motor and the battery directly. It’s one of the most important parts of the starter assembly. Inside the solenoid housing are several components. These include a set of coils (pull-in and hold-in coil), a plunger, and contacts.

a starter solenoid detached from the main starter assembly

Starter Motor Lever

The starter motor lever is the forked arm found in solenoid-operated starter motors. The lever moves the pinion drive to engage the flywheel, and back once the engine can run on its own. The solenoid is responsible for moving the fork back and forth during the engine starting session.

the starter motor lever is the forked part

gear reduction starter type

Types of Starter Motors

There several starter motor types. Their classification is mainly based on the type of magnet used, the method to engage or disengage the pinion with the flywheel, and the mechanism to increase torque. Some are commonly used in older cars while others are mostly found in newer vehicles. The types of starters are explained below.

Based on Type of Magnet

Permanent Magnet Starter

In this type of starter motor, a series of permanent magnets are used to rotate the armature. Permanent magnet starter motor advantages include their reliable operation, which makes the suit a wide range of applications. A permanent magnet starter motor is also usually compact since there are no coils inside except those of the armature. To multiply torque, these starters commonly employ a gear reduction mechanism.

Field Coil Starter

A field coil starter motor uses electromagnets instead of permanent magnets. These starters are usually large as they have to pack a long coil of wire wound on a magnetic core. Field coil starters are quite powerful, though, and often used without a gear reduction drive. Unlike those that use permanent magnets, field coil starter motors strain the battery and drain it too quickly.

Based on Engagement Method

Inertia Starter

The inertia type of starter motor is a traditional design but still used in some automobiles. It consists of an electric motor and a splined shaft. A pinion gear is inserted on the shaft and free to move. The starter motor depends on the principle of inertia to engage and disengage with the flywheel, making it a simple starter.

During operation, the shaft spins as soon as the motor receives current. However, due to inertia, the heavy pinion does not spin with the shaft at once. Instead, it slowly moves along the shaft to toward the flywheel. A powerful spring behind the pinion helps to ensure a smooth engagement.

When the engine starts turning on its own, the pinion cannot keep up with the speed. Instead, it’s thrown out of engagement with the flywheel and back along the splined shaft of the motor. An inertia starter motor engages and disengages violently and is known to wear the drive gear quickly, sometimes even breaking it.

Pre-engaged Starter

Most cars today use the pre- engaged starter motors. The starter consists of an electric motor but with a more effective solenoid-driven pinion. The construction basically consists of the motor, a solenoid mounted to the top of the motor and a fork or lever to push the pinion to engage with the flywheel. An over-running clutch is also used to prevent the flywheel from driving the motor once the engine starts.

The operation of the pre-engaged starter motor has already been explained. Turning the engine starting key or pushing the START button activates the solenoid which, in turn pulls a plunger. The plunger closes the motor circuit while at the same time moving a lever and pushing the pinion to mesh with the flywheel gear.

Cutting off the current to the solenoid has an opposite effect. The motor stops spinning and the pinion withdrawal. Pre-engaged starter motors provide a smoother engagement and disengagement. There’s less wear on the pinion gear. Other advantages include the reliable operation.

Based on Torque Multiplication

Gear Reduction Starter

The gear reduction starter motor is named so for having a gear setup to increase torque. The pinion gear in these types of starter motors is not installed on the armature shaft. Instead, the armature contains a gear that turns another set of gears to provide an increase in torque. The pinion is then mounted in a separate assembly.

The working of the gear reduction starter does not differ from that of the others. However, it’s usually more powerful compared to the types mentioned, and usually more compact. Because of the gear reduction design, these starters do not drain the battery as quickly as the other types and are often used in demanding applications.

the price of starter motor depends on its type and brand

Starter Motor Price: How much does a Starter Motor Cost?

The cost of a starter for a car engine varies widely. Reasons include the different types of the starter, the different applications or vehicles and different manufacturer prices. The cost of a starter motor also depends on whether you are looking for an OEM or aftermarket type of component. In most cases, the aftermarket starter will be cheaper.

That said, the starter motor price ranges from $100 to $200 for aftermarket types and up to $500 for OEM models. Expect to pay more if your engine is large or if you own a high-end car. That’s because such engines need a heavy-duty starter.

When buying your starter motor, be sure to purchase the one that fits your type of car. That’s easy enough even when shopping for the part online. You only need details such as the car make, model and the year it was manufactured. Other than that, you also need to consider the brand and manufacturer’s reputation on the market, among other factors.


The starter motor performs an important function in your vehicle. It ensures the car starts when you want it to by turning over and cranking the engine. As described in this chapter, different types of the starter motor use various components to ensure sufficient torque and proper engagement with the flywheel. Over time, though, these starter motor components wear or break down, causing the starter to fail. Find out what cause starter issues and the ways to correct then in the next chapter.

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