Starter Motor Working Principle

Armature assembly
The starter is an important component for starting the engine. It converts electrical energy into mechanical energy and turns the crankshaft of the engine through the starting mechanism to help the engine start. Understanding the structure and working principle of a starter is essential for maintaining and repairing a car.
The starter motor has a complex structure, including components such as motors, gears and clutches. The information on the structure and working principle of the starter described in this article can be helpful to automotive engineers, car owners, and those who are interested in automotive mechanics.

Starter Structure

  1. Armature assembly

The armature assembly consists of armature core, armature winding, commutator and armature shaft.
  • Armature core: In order to reduce eddy current loss and hysteresis loss, it is made of laminated low carbon steel plates.
  • Armature winding: It is composed of many coils, and the two ends of each coil are respectively connected to two commutator segments. After the brushes are placed on the commutator, the closed loop composed of winding elements is divided into positive and negative brushes. A number of parallel branches are connected to the external circuit through brushes.
Commutator: The function of the commutator is to inject current into the rotating armature winding.
Armature assembly
  1. Electromagnetic switch

The electromagnetic switch is used to push out the drive gear to mesh with the flywheel ring, and also has the function of a switch. When the static contact and the movable contact of the electromagnetic switch are closed, the main circuit of the starter is connected to make the armature of the starter rotate. It consists of an attraction (series) coil, a holding (parallel) coil, a static iron core, a moving iron core, a return spring, a static contact and a moving contact.
When the start switch is just turned on, the current in the attracting coil is grounded through the field winding and armature winding of the starter, and the holding coil is directly grounded. At this time, the attracting coil generates magnetic flux in the same direction. After the magnetic flux is strengthened, a strong attractive force is generated to attract the iron core to move until the dynamic and static contacts are closed.
At this time, the two ends of the attracting coil are short-circuited by the main contact, and there is no current passing through the attracting coil, and only the current in the retaining coil maintains the attracting state of the moving iron core. Because the moving and static iron cores are in contact at this moment, the reluctance is very small, and only by the attractive force generated by the less magnetic flux in the holding coil, the pulling state of the moving iron core can be maintained. Due to the closure of the main contact, the battery voltage is directly applied to the starter to rotate the starter armature, and the movement of the switch moving iron core pulls the shift fork to extend the drive gear to start the engine.
Electromagnetic switch
  1. Shell assembly

The shell of the starter is an important part, which consists of the shell itself, the magnetic field coil and the magnetic pole. Its main function is to protect the internal parts of the starter from external damage, and it can also play a role in heat insulation. The magnetic field coil and magnetic poles can generate magnetic flux and form a magnetic circuit, so that the starter can generate enough torque to drive the engine to rotate.
Shell assembly
  1. One-way clutch

Sectional structure of one-way clutch

   Sectional structure of one-way clutch

   Roller state when torque is transmitted

   Roller state when torque is transmitted

Roller state during idling

       Roller state during idling
The starter torque is transmitted from the armature shaft through the drive gear to the engine flywheel. For example, after the engine is started, when the flywheel of the engine drives the drive gear to rotate, the centrifugal force of high-speed rotation causes the armature coil to be thrown out, and the commutator is thrown away.
For example, when the engine speed is 1000 rpm, the starter gear speed is 10000 rpm (the engine flywheel and starter gear speed ratio: 1:10). As the engine speed increases, the speed of the starter will rise higher. This situation will not only increase the sound of the starter, but also cause serious damage to the starter.
When the starter is used to drive the engine, the armature drives the outer ring to rotate. Due to mechanical resistance after the drive gear meshes with the flywheel, the inner ring rotates slowly, thus forming a speed difference between the inner ring and the outer ring.
The column squeezes to the narrow part of the oblique cavity of the outer ring and the inner ring to lock the outer ring and the inner ring. Torque is transferred directly from the outer ring to the inner ring, so the drive gear rotates with the inner ring as well.
After the engine starts, its flywheel will drive the drive gear to rotate at a high speed, and the roller will move to the wide part of the inclined cavity while rotating. Therefore, the inner ring integrated with the drive gear is in an idling state, and the torque will not be transmitted to the starter side. In the idling state, the pressure is not applied to the teeth of the drive gear and the flywheel, and the drive gear with the helical spline is easy to disengage.
  1. Drive gear

1) The meshing between the driving gear and the flywheel is electromagnetic meshing. After power on, the electromagnetic switch pulls the shift fork to pull the drive gear forward.
2) The drive gear is disengaged from the flywheel. After the engine has started and the electromagnetic switch is disconnected, the electromagnetic switch return spring will push the shift fork to return the drive gear. A helical spline on the check shaft will facilitate disengagement of the drive gear.
Drive gear
  1. Brush assembly

The brushes of the starter motor are made of carbon and metal materials. When the starter motor is working, the brushes will contact the commutator surface on the armature to introduce current into the armature winding. The armature winding is the main motor part of the starter. It is composed of multiple turns of windings. When the current passes through the windings, a rotating magnetic field will be formed on the armature, thereby driving the rotation of the engine.
The commutator is an important part of the starter. It is located on the armature and can automatically change the direction of the current according to the positive and negative directions of the current, so that the direction of the magnetic field in the armature winding is always consistent with the contact surface of the brush. .
This technology of automatically changing the direction of the current is called “reversing”, and it is one of the keys to the normal operation of the starter. Since the brushes generate friction when they contact the surface of the armature, the quality and design of the brushes are also very important, directly related to the life and reliability of the starter.
Brush assembly
Understanding the structure of a starter motor is very important to understanding how it works. A starter is a mechanical device made up of several components, each of which has a different function, but are closely related and work together. After understanding the structure of the starter, we will introduce the working principle of the starter in detail, so that you can have a deeper understanding of the working method of the starter.

Starter Motor Working Principle

Now that you have a basic understanding of how a starter motor works, let’s take a closer look at how it works. A starter motor relies on the interaction of its various components to generate the power needed to start the engine. By understanding how the different parts work together, you can know how to troubleshoot and fix starter motor problems. So, let’s explore the intricacies of how a starter motor works and how it makes a car’s engine do its thing.
  1. Press the start button: When the car’s start button is pressed, a circuit sends current to the starter’s motor and relay.
  2. Relay Action: A relay is a switch that controls the flow of electrical current. When the start button is pressed, the relay closes the circuit, sending current to the starter’s electric motor.
  3. Motor starting: The current passes through the relay and enters the electric motor of the starter. A motor has a series of electrical coils that generate a strong magnetic field when current flows through them. This magnetic field causes the rotor inside the motor to spin.
  4. Gear Meshing: As the motor spins, the gear on its output shaft will mesh with the flywheel gear on the engine. This process sets the flywheel into motion, which starts the engine’s compression-explosion cycle.
  5. Automatic cut-off: When the engine starts, the relay of the starter will disconnect the connection between the electric motor and the electric circuit. At the same time, a spring mechanism in the starter will unhook the motor’s gear from the engine’s gear.
  6. Charging: When the engine is started, a circuit sends current to the car’s alternator to charge the battery. This process can provide enough electrical energy for the next engine start.
In summary, a car starter converts electrical energy into mechanical energy to provide enough torque for the internal combustion engine to start and run. The working principle of a starter is relatively simple, but its importance cannot be underestimated because a good starter ensures that the engine starts smoothly, thus providing good performance and reliability to the vehicle.


After reading this article, you should be able to understand how a car starter is built and how it works, and the role each component plays in the process. This will help you better understand how your car’s starter works and allow it to be properly maintained and serviced when needed.
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