Starter Motor Function: Types, Function, and Why It Is Important

starter motor function

Ever wonder what is a starter motor? In a nutshell, the starter motor working principle is quite simple. This is a device that helps you start your car engine. It is composed of several components including an electric motor, a solenoid starter switch, and a set of connecting wires. As soon as you turn your key to start your engine, the starter motor function is to engage with the flywheel and crankshaft of your vehicle. This is what makes it possible to start your engine.


Starter Motor Function

Basic Operation of Starter Motors

Let’s get more in-depth about the starter motor parts and function. There are several parts that make up a starter motor. These include the solenoid starter switch, which is located in the engine bay. It is a device that activates the relay when you turn your key and push down on your clutch pedal. The relay then sends power to the electric motor, which spins the flywheel of your vehicle until it starts spinning fast enough to run on its own.For parts and components, the starter motor has an armature, field coils, solenoid, and gears. The armature is a cylindrical metal piece that houses permanent magnets. The field is a coil of wire wrapped around the armature, and it generates a magnetic force when current runs through it. The solenoid is an electromagnet that is activated by pressing your clutch pedal, which turns on the power to the starter motor relay.Can you run your car without a starter motor? Unfortunately, having a failing starter motor can be a huge problem, especially in cold weather conditions. You may not be able to start your car at all, or it could take quite some time to get it going. Thus, it is important to have your car checked if you suspect that your starter motor is not working properly.

Different Types of Starter Motor

A starter motor is an essential component of an internal combustion engine, and there are several types of starter motors that are commonly used in vehicles today, each with their own unique features and benefits.

Direct Drive Starter Motor

First on our list the Direct Drive Starter Motor. This type of starter motor is commonly found in smaller engines where the engine itself is mounted directly to the transmission. This allows for a very compact design, but it also has some drawbacks. Direct-drive starter motors are not able to provide as much power as other types of starter motors, and they are usually only found in applications where space is limited.Pros:

  • High Reliability: They are extremely reliable because there are fewer parts, which means less chance of mechanical failure.
  • Fast and Efficient: Fewer moving parts reduce energy lost through friction, so start the engine faster and more efficiently.
  • Lightweight & low maintenance: Its small size and light weight make it ideal for small engines. And due to its simplicity, it requires less maintenance.


  • Higher initial cost: These starter motors can be somewhat more expensive than other types of starter motors, making them less attractive to some customers.
  • Lower torque: Lower torque than other starter motors, less suitable for starting an engine with a high compression ratio.
  • Limitations: Unsuitable for some engines due to low flexibility in terms of speed and torque. And its durability is weak, so it can not be used in harsh environments or used frequently.

Planetary Gear Reduction Starter Motor

Another type of starter motor is the Planetary Gear Reduction Starter Motor. This type of starter motor is commonly found in larger engines where space is not an issue. The engine itself is mounted to the transmission via a drive shaft, which then connects to a gear reduction unit that uses planetary gearsets to lower the speed of the engine by several hundred RPMs.Pros:

  • High Torque: Their high torque allows them to start the engine more efficiently, especially in cold weather conditions.In this case, the thicker oil puts extra stress on the starter motor, and its high torque output ensures it overcomes the extra resistance and starts smoothly.
  • Durability: Its biggest advantage is that it is durable, so it has a longer service life. And can withstand a lot of wear and tear, making it ideal for high mileage vehicles or those used in demanding conditions.
  • Compact Size: Especially suitable for vehicles with limited space due to its compact size. Plus, smaller size means more fuel efficiency, making it a popular choice for eco-conscious consumers looking to reduce their carbon footprint.


  • More expensive: Because it contains more parts and a more complex design, it is more expensive to manufacture. At the same time, the weight will be relatively heavy, which may increase the overall weight and fuel consumption of the vehicle.
  • Requires more maintenance: Since it has more components, it requires more maintenance and care. They require the lubricant and other parts of the planetary gears to be checked and replaced frequently to keep them functioning properly.
  • Gear Noise: Since the planetary gear reduction starter motor uses a gear reducer, it will generate a lot of noise and vibration when starting. This could cause discomfort to the driver and vehicle occupants.

Permanent Magnet Starter Motor

Designed to provide synchronous speed operation from a supply of constant voltage, Permanent Magnet Starter Motors are common in smaller and mid-sized engines. This type of starter motor has magnets that are directly embedded in the rotor iron so that when the current is applied to the windings, a magnetic field is created. The rotor then revolves within this field and starts turning at the same speed as the engine.Pros:

  • Light weight and small size: Due to the use of permanent magnet materials, it is lighter and takes up less space than other types of starter motors.
  • High Power-to-Weight Ratio: It also has a high power output, resulting in a higher power-to-weight ratio relative to its own weight.
  • Efficient and reliable starting performance: The permanent magnet starter motor has efficient and reliable starting performance, and its torque output can meet most starting needs.


  • Limited torque output: Compared to other types of starter motors, permanent magnet starter motors have a limited torque output, which may prevent the engine from starting reliably under certain conditions.
  • Easy to demagnetize in high temperature environments: Permanent magnet materials are prone to demagnetization in high temperature environments, which may cause performance degradation or complete failure of permanent magnet starter motors.
  • Difficulty in troubleshooting unusual faults: Troubleshooting unusual faults in permanent magnet starter motors can be difficult compared to other types of starter motors.

Inertia Starter Motor

Fitted with a Bendix gear , the inertia starter motor is a type of electric motor that uses a large flywheel and centrifugal force to start an engine. The mechanism is often used in older cars, boats, and other vehicles that require starting assistance.Pros:

  • Simple design: Compared with other types of starter motors, the inertia starter motor has a simpler design and is easier to manufacture and maintain.
  • Cost Effective: Due to their simple design, inertia starter motors are relatively inexpensive to manufacture and maintain compared to other types of starter motors.
  • High durability: The inertia starter motor is durable and can withstand the test of various harsh environments.
  • Provides High Torque Output: The inertia starter motor is capable of providing high torque output for reliable starting performance.


  • Bulky: Inertia starter motors are bulky compared to other types of starter motors, which can add to the overall weight and fuel consumption of your vehicle.
  • Noise and Vibration: Inertia starter motors can produce loud noise and vibration when starting, which can be uncomfortable for the driver and vehicle occupants.
  • Requires regular maintenance: An inertia starter motor requires regular maintenance and servicing to keep it functioning properly. This can add to vehicle repair costs and downtime.
  • Insufficient torque output: Inertia starter motors may have low torque output relative to other types of starter motors, which may prevent the engine from starting reliably under certain conditions.

Hydraulic Starter Motor

As the name suggests, a hydraulic starter motor uses a pressurized liquid to get the engine started. The most common type is an electric starter motor that’s fitted with an internal pump that pushes fuel from the fuel tank into the carburetor when it’s activated. Marine, oil and gas, and agricultural engines typically use hydraulic starter motors.Pros:

  • High torque output: The hydraulic starter motor can provide high torque output to ensure reliable starting performance.
  • Suitable for harsh environments: Hydraulic starter motors are suitable for harsh environments such as marine or industrial applications.
  • Low Power Consumption: The relatively low power consumption of hydraulic starter motors reduces energy consumption.
  • Provides smooth and consistent starting performance: The hydraulic starter motor provides smooth and consistent starting performance, allowing the engine to start steadily.


  • Large and heavy: Compared to other types of starter motors, hydraulic starter motors are bulky and heavy, which can increase overall vehicle weight and fuel consumption.
  • Complex design and high cost: The design of the hydraulic starter motor is relatively complex, and the manufacturing cost is also high. A separate hydraulic power source is also required, adding to the complexity and maintenance costs of the overall system.
  • Other Possible Issues: Hydraulic starter motors may face leaks and other hydraulic system issues, which can affect the reliability and safety of the vehicle.

Why Choosing the Right Starter Motor Functionality is Important

Starter motor types are specifically designed to cater to different applications. Thus, it is critical to choose the right starter motor for your vehicle. Different starter motors have different power levels and can be used to start engines that are larger or smaller than the original design.For instance, a high-powered electric starter motor may not be able to start a large engine, while an even more powerful hydraulic version would be needed. On the other hand, a gasoline-powered starter motor may not be able to start a small engine, so an electric version would be better suited.Moreover, the starter motor must be compatible with the battery voltage and must have enough power to rotate the crankshaft if it is used in a diesel engine. Additionally, when choosing a starter motor for use in a gas-powered vehicle, make sure that it can deliver enough power to spin over the engine at its correct speed.


The starter motor is an important part of any internal combustion engine and understanding its function and different types is important in choosing the correct starter motor for specific needs. Whether you choose a direct drive or planetary reduction starter motor, it is important to ensure it is in good working order for optimum performance and reliability. Regular maintenance and timely replacement of a failed starter motor can save you time, money and hassle in the long run.

Table of Contents
Contact Us
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

Leave a Comment

Your email address will not be published. Required fields are marked *


Leading Way In Solenoid Technology

Tiger Zhu - Founder

Request A Quote

Tell us your needs, let T&X® make it.
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.