Alternators: Explained

Automotive charging systems consist of three components- the alternator, battery and the voltage regulator. The alternator works along with the battery to convert mechanical energy into electrical energy to power the electrical components of a car such as the instrumental panel, interior and exterior lights and fan, among others. Alternators are located near the front of the engine and are propelled by the crankshaft. They are mounted using brackets screwed to specific areas of the engine; one of the brackets is fixed permanently, and the other is adjustable. Alternators are small and lightweight and have aluminium housing. Aluminium is used because it does not magnetise and dissipates the heat produced by converting it to electricity. An alternator is made up of three parts- the rotor, stator and diode, and operates in a sequence. The rotor consists of a group of magnets located in a shell of copper wires called the stator. Once the car battery is powered, the V-belt causes the rotor to spin quickly to convert mechanical energy to electrical energy; a process known as electromagnetism. The electricity generated is conducted using the copper wires through the diode and changes the electricity from AC (alternator current) to DC (direct current).

Types of Alternators

They are classified based on factors such as output power, construction, efficiency, field of regulation and type of vehicle.

Output Power

Older vehicles that require minimal lighting will use an alternator that produces 30 Amps only while ordinary passenger cars and light trucks are fitted with devices rated at 50-70 amps like the Brise 12V Lucas ACR replacement alternator. Devices with higher ratings also come in handy given the increasing load of modern electrical systems of vehicles, e.g., electrical powering and air conditioning infotainment systems, among others. Larger vehicles like buses, emergency cars or heavy equipment require devices with ratings as high as 300 A while semi-trucks use an alternator with an output of 140 A.


As earlier highlighted, the AC produced in the stationary stator is converted to DC by the diodes. The alternator fitted on regular passenger vehicles use claw-pole field construction that features a shaped iron core on the rotor to produce multi-pole field using a single coil winding. Its stator windings and field are cooled by axial air flow and produced using an external fan attached to the drive belt pulley. Modern cars, however, use a compact alternator layout that has an improved air cooling system. Enhanced cooling allows the machine to generate more power. The casing has radial venting slots on each end that enclose the fans.


The efficiency of an alternator depends on the fan's bearing loss, copper loss, cooling loss and voltage drop on the diode bridge. The rate decreases dramatically when driving at high speed due to fan resistance. At medium speed, a modern alternator has an efficiency of 70-80%.

Type of Vehicle

An alternator that is used on a hybrid vehicle is different from that used on regular cars. Usually, hybrid vehicles replace the separate construction of the starter motor and alternator with a combined motor/generator to start the internal combustion engine and power all the electrical systems of the car. Some cars like the Toyota Prius have more than one M/G where one system operates as the generator and the other functions as a motor to create an electromechanical path for the engine power to drift to the wheels.





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