Starter Motor Construction | Two-Winding and Four-winding Starter Motor.

Starter motor construction and the construction of an engine generator are very similar. In this article, I will discuss the Starter motor construction and the types of starter motors depending on the construction.

Starter Motor Construction

Starter Motor Construction

The mechanical construction of the motor is very similar to the generator, but their functions are entirely different. Whereas the function of the generator is to generate voltage when conductors are moved through a magnetic field, the function of the motor is to develop a torque or twisting effort when the current is passed through the conductors held in an area. The generator is a shunt wound but the starter motor is a series wound. 
The starter motor brushes are made of low-resistance material such as copper instead of carbon as in the case of the generator.  The main parts of the starter motor are casing, armature, commutator, filed windings, brushes, poles, and terminals. A drive mechanism is provided at the end of the armature shaft, by means of which the motor starts the engine
According to the starter motor construction, These are of two types-
  1. Two-winding starter motor.
  2. Four-winding starter motor. 

Two-Winding Starter Motor Construction

Two-Winding Starter Motor Construction


As you see in Fig, a motor uses two-field winding. The current from the battery divides when it enters the motor, each branch leading to separate field winding. From the fields, the current is led to the commutator of the armature through the two insulated brushes. The current in the armature creates simultaneously four poles adjacent to the four field poles to produce the attractive and repulsive forces that turn the armature. The armature current returns to the battery through the two grounded brushes. 

Four-Winding Starter Motor Construction

Four-Winding Starter Motor Construction


This Fig shows a starter motor with four field windings. It is used in large engines in order to develop more torque. It operates in the same manner as the two-winding starter motor. 
This is used in large vehicles because the weight of the crankshaft and the rotational movement of these mechanical component needs more torque to operate. The four winding can better distribute the power and produce more torque for the engine operation.

Series Wound Motor

Series Wound Motor


The series may be classified as a series wound, shunt wound, compound wound, and separately excited. Series Wound motors are used in automobiles for cranking the engine.
A series wound motor has its field coil in series with the armature so that all the current flows through the field as well as the armature. The field windings consist of a few turns of thick copper wire on each pole and the windings on all the poles are connected in the series. The current is large with a heavy load and small with a light load. The resistance of the field winding is purposely made low so that the loss of voltage and the field are low. The speed of the motor varies with change in load. It has high speed at light load and low speed at high load. The efficiency is high with a wide range of speeds. They have the advantage of a strong starting torque, and hence they are used in automobiles for cranking the engine. 

Anatomy of a Starter Motor

A starter motor is a crucial component in an internal combustion engine’s ignition system. Its primary function is to initiate the engine’s rotation by turning the crankshaft. It allows the engine to start. In the starter motor, the electrical energy is converted into mechanical energy. Below is an overview of the key parts and their functions in the anatomy of a starter motor:

1. Housing

Housing means the home for every material. The outer casing of the starter motor is typically made of durable metal, such as aluminum or cast iron. It serves to protect the internal components from dust, debris, and damage while also providing structural support. The casing is made to absorb the external shock coming from the outside.

2. Field Windings

Inside the housing, there are field windings. The field windings are sets of copper wires. When an electrical current passes through these windings, it creates a magnetic field. This magnetic field interacts with the armature to produce rotational motion. This rotational movement helps the engine to start.

3. Armature

The armature is a cylindrical core with wire windings. It is connected to the engine’s crankshaft. The field windings generated a magnetic field that rotated the armature. This rotation initiates the engine’s crankshaft movement.

4. Solenoid

The solenoid is a cylindrical component. It is mounted on top of the starter motor. It acts as a switch that engages the starter motor with the engine’s flywheel or flexplate. When the ignition key is turned to the “start” position, the solenoid sends a high-current electrical signal to the starter motor. It’s a one-way passage to send the signal.

5. Starter Drive

The starter drive also known as the Bendix drive, is a gear mechanism. It is connected to the armature shaft. It is designed to engage with the engine’s flywheel when the starter motor is activated. Once engaged, it transfers the rotational motion from the starter motor to the engine’s crankshaft. It gets the signal from electric media and acts as a mechanical actuator.

6. Brushes and Commutator

Inside the armature, there are brushes that come into contact with a segmented ring called the commutator. These components ensure a continuous electrical connection to the armature windings as they rotate, allowing the motor to function properly.

7. Mounting Bolts

Starter motors are secured to the engine block or transmission housing using mounting bolts. These bolts keep the starter motor in place and ensure proper alignment with the engine’s flywheel or flexplate. These are specially used to hold the starter motor.

8. Electrical Terminals

Starter motors have electrical terminals for connecting the battery’s positive and negative terminals, as well as the control signal from the ignition switch. These connections allow the starter motor to receive the necessary electrical current to operate.


The starter motor is responsible for receiving the current and turning it into mechanical power through its components. 
In this article, I have discussed the Starter motor construction and based on the construction the types of the starter motor.
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