Starter Motor || How the starting systems work?

 What is starter Motor?

The starter motor is just like an electric motor that cranks the engine to start. It consists of a powerful DC (Direct Current) electric motor and the starter solenoid that is attached to the motor. In most of the vehicles, the starter motor is bolted to the engine or transmission.

The starter motor is powered by the battery either 12 V or 24 V depending upon the vehicle. In case of passenger cars it is 12v and in commercial vehicles it is 24V. To turn over the engine, the starter motor requires high electric current, which means the battery has to have sufficient power. If the battery is discharged, the lights in a vehicle might not work, but it won't be enough power (current) to turn over the starter motor. When starting a vehicle with a fully discharged battery, there is a single click or nothing happens at all. The starter motor doesn't run, even when there is a 12-Volt power at the starter control terminal.  Another symptom is when the starter motor runs, but fails to turn over the engine. Often, this might cause a loud screeching noise when starting the vehicle. Of course, this may also cause damage to the teeth on the ring gear of the flex plate or flywheel.

 

Why does starter motor need starter solenoid?

As we discussed earlier to run the engine, starter motor drags high current from battery at a rate of 250 to 400 amps depending upon the vehicle. To deal with such high current is hazardous and also it may damage switching device by producing sparks while switching directly. Also it needs oversized cables to be fitted from battery to switch and then to starter motor. To avoid this problem, starter solenoid is first actuated by the help of cranking switch. When the solenoid gets actuated, the plunger connects starter coil with the battery cable. By doing so switching action remains safe.

Main components of starter motor

There are four major components are found in starter motor. They are:

1.      Starter solenoid

2.      Battery cable

3.      Over running clutch

4.      Pinion

 1.      Starter solenoid

A typical starter solenoid has one small connector for the starter control wire and two large terminals: one for the positive battery cable and the other for the thick wire that powers the starter motor itself.

The starter solenoid works as a powerful electric relay. When activated, through the control terminal, the solenoid closes with the high current electric circuit and sends the battery power to the starter motor. At the same time, the starter solenoid pushes the starter gear forward to mesh with the ring gear of the engine flex plate or flywheel.

2.      Battery Cable:

As we know, the starter motor requires high electric current to turn over the engine. That's why it's connected to the battery with thick cables (As shown in figure). The negative (ground) cable connects the negative battery terminal to the engine cylinder block, or transmission, close to the starter. The positive cable connects the positive battery terminal to the starter solenoid. Usually, a poor connection at one of the battery cables may cause the starter motor not to work.

When you turn the ignition key to the START position, or press the START button, if the vehicle is in Neutral, the battery voltage goes through the starter control circuit and activates the starter solenoid. As the starter solenoid actuates, it powers the starter motor. At the same time, the starter solenoid pushes the starter gear forward to mesh it with the engine flywheel (flex plate in an automatic transmission). The flywheel is attached to the engine crankshaft. The starter motor provides turning moment to the engine crankshaft allowing the engine to start. As soon as the driveshaft reaches its required rpm, the system disengages the starter.

3.      Over running clutch (What happens when engine rotates at higher RPM?)

Overrunning clutches, sometimes called freewheel clutches. These are used to protect starter motor when drive shaft starts rotating at a higher rpm than motor. When the engine gets started, drive shaft starts rotating at higher rpm and the mechanically arranged spring loaded balls (as shown in fig.) helps motor shaft idle. There are many different types of over running clutch available in the market sometimes it is sun and planetary gear arrangements. It may vary as per the mechanical designs.

How the starting systems work?

The starter motor typically has four field windings (field coils) attached to the starter motor housing from the inside. The armature (the rotating part) is connected through the carbon brushes in series with the field coils. At the front end of the armature, there is a small pinion gear that is attached to the armature through an overrunning clutch. When the driver turns the ignition key or presses the Start button, the solenoid winding gets energized. The solenoid plunger moves in the direction of the arrow (As shown in figure) and closes the solenoid contacts. This connects the battery power to the starter motor (field coils and armature). At the same time, the plunger pushes the starter gear forward through the lever. The gear then engages with the ring gear of the flexplate or the flywheel and turns it over. The flywheel is attached to the engine crankshaft. Most of the starter problems are caused by worn out or burned solenoid contacts, worn brushes and a commutator and worn armature bushings. The symptom of worn out solenoid contacts is when the solenoid clicks but the starter motor doesn't run. When the starter brushes are worn out, the starter motor doesn't make any noises. When front and rear armature bushings wear out, the armature rubs against the field shoes causing the starter motor to run slow and noisy. Many modern starter motors have small ball bearings instead of the bushings.