How a Starter Motor Works: A Detailed Insight

Car Starter Motor

The starter motor is a small but powerful electric motor used to convert electrical energy into mechanical (rotational) energy in an IC engine to start the engine. Getting started in the car is a complex procedure. The car starter motor is designed to work in a precise way to start the car. However, starting the car involves many more systems than the starting system.

In my last article, I explained the 11 complete and detailed car starter motor parts and functions, and now on this page, you will learn the starter motor working operation in deep detail. This article is going to explain comprehensively and detailed way on how a car starter motor works in easy-to-understand language.

Related Post: Car Starting System, Diagram, Working, Components, Functions

How A Starter Motor Works

Starter Motor
Starter Motor

The car’s electric starter motor working principle is based upon converting electrical energy into mechanical energy to make the engine run.

A starter motor and solenoid are tightly integrated pairs and the heart of the starting system. It is used to start the vehicle engine, without the starter motor, the vehicle will not move. To get the engine going on, it must be cranked at some speed to complete the combustion cycle and get the engine working.

As you know that the flywheel is very hard to rotate. The high inertia of the heavy crankshaft assembly prevents the flywheel from turning. That is why the starter motor requires a very high amount of electric current. It consumes 12 volts of DC power, which draws through a thick cable from the battery.

The starter motor is earthed via the case itself. The negative side of the starter motor is connected to the metal case of the starter motor which is mounted via bolts to the engine. Clearly, the engine is connected to the battery negative terminal through the chassis.

For initial cranking, a powerful starter motor is required to crank the engine up to a speed sufficient to run smoothly on minimum RPM. And the starter motor is only used to start the engine as soon as the engine has started and runs on its own power, then it is not needed and burdened until the next start.

Starter Solenoid Terminals
Starter Solenoid Terminals

When you turn the ignition key or press the start button, the current flows to the “S” terminal of the solenoid, which is usually a pin-type or a small stud-type terminal fitted on the back of the solenoid cap. Also the “S” terminal of the solenoid consists of two types of windings pull-in and hold-in windings.

Starter Motor Solenoid Pull-in and Hold-in Winding
Starter Motor Solenoid Pull-in and Hold-in Winding

When these windings are energized, create an electromagnetic field. This electromagnetism attracts the rod called a plunger inside the solenoid. This does two functions simultaneously. One is, the pinion gear moves towards the flywheel by lever-fork, which is attached to the end of the plunger.

The second function is, it closes two heavy contacts that are placed at the other end of the solenoid plunger. Now here is the interesting part.

The meshing of the pinion gear and flywheel occurs just before the starter motor turns ON. This is done to prevent any damage to the teeth of either the pinion gear or flywheel. These types of starter motors are called Pre-Engaged Starter Motors. In a pre-engaged starter motor, the full power is not applied until the pinion gear fully meshes with the flywheel ring gear.

This pre-engage feature comes in both types of Gear Reduction and Direct Drive starter motors. I know that’s a lot to take in, but bear with me, I will cover a lot more interesting points.

Motor Coil Rotation Due to Electromagnetism
Motor Coil Rotation Due to Electromagnetism

As the motor is fed with current by the solenoid, the current flows in the direction of the brushes to the commutator and then to the armature winding. The armature is fitted in the center of the starter motor (Here a coil is shown in the above picture, as you know armature is the collection of several coils). By flowing current in the armature windings, electromagnetism is produced. Electromagnetism means North and South poles.

This N-pole and S-pole react with the already available North and South poles of the permanent magnet or field coil. As you know like polarities repel and opposite polarities attract each other. When the like polarities of armature and field coil or permanent magnet face each other, it repels, and the opposite polarities attract each other.

The reaction between the North and South Poles of both components causes the rotation of the armature. This rotation is then transferred to the engine flywheel by the armature shaft.

A planetary gear set or gear reduction assembly is installed on the armature shaft to increase the torque of the starter motor. The planetary gear set reduces the output speed of the pinion gear with a 4:1 ratio, which means with each rotation of the pinion gear, the armature rotates four times.

As the armature turns, the pinion gear also turns the flywheel, this makes the engine suck in air and fuel in the cylinder. Meanwhile, the ignition system is activated, and the current is sent to the spark plug wires to ignite the fuel in the cylinder. As a result, the car starts over. However, if the engine starts over and the pinion gear mistakenly remains intact with the flywheel,

So, it is possible the flywheel would spin the starter motor too fast which can cause damage to the starter motor. For this reason, an overrunning clutch is designed to slip the pinion gear from excessive turning. The overrunning clutch is located on the armature shaft just before the pinion gear. It allows the pinion gear to be spun freely when the flywheel runs faster than the pinion gear.

In the end, when the engine starts over, and combustion is possible, the driver releases the key switch, which de-energizes the starter solenoid. The return spring inside the solenoid moves back the plunger to its normal rest position, which de-meshes the pinion gear from the flywheel and also, discontinues the battery current from the starter motor.

These are the operations that occur in a starter motor when you turn the key switch. Hope you got everything.

Related Post: 11 Parts Of Car Starter Motor & Functions + Working

Frequently Asked Questions (FAQs)

What happens if you wire a starter wrong?

If a starter is wired incorrectly, it can cause a number of problems. The most common problem is that the engine will not start. The current will not flow through the starter correctly and the engine will not start. This is because the starter needs a certain amount of current to flow through it in order to work properly. If the current is not flowing through the starter correctly, then the engine will not start. In some cases, a short circuit can occur and the starter can overheat and gets damaged.

How many wires go to a car starter?

There are a few different types of car starters, so the number of wires will depend on the specific starter. Generally speaking, however, most car starters will have at least three wires. Two of these wires will be connected from the solenoid to the battery, and the other one will be connected from the solenoid to the starter motor.
Starter Solenoid Terminals


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