How does an Alternator work?

How does an Alternator work

An alternator’s basic function in the automotive world today, is to supply power for the vehicle’s various electrical loads while maintaining a stable voltage to charge the battery. All alternators operate on a 12 volt system, and charge at 14-15 volts to maintain the battery’s integrity. The overall strength of an alternator or it’s ability to deliver power is rated in amperage (amps).  As you turn on an accessory (like air conditioning) the voltage regulator senses the drop in voltage and tells the alternator to put out more amps so that the battery stays at a stable 14-15 volts DC. Thus, when the battery voltage drops, the alternator amp output increases to compensate. There are a few basic electrical principles that need to be reviewed so that a good comprehension of the alternator can be achieved. The first principal is called induction. The second is called rectification.


Induction is an electrical principal that is the basis for all generators and transformers. It means that a current is caused to flow in a copper wire when a magnet passes near it, but does not come in contact with it. The stronger the magnet, the more current. Now ,picture 10 magnets on a wheel, spinning past a coil of wire. The spinning magnets in an alternator are electromagnets. The more current that runs through the electromagnets, the more current is produced in the pick up coils (stator) on the outside diameter of the spinning magnets. So if you want a lot of current, put a lot of current in the magnets. If you want a little current, put a little current in the spinning electromagnets. The brushes ride on a commutator looking devise called a slip ring. Brushes are how the current can continuously be supplied to the rotor (the spinning electro-magnets). The voltage regulator is directly in control of the supply of current to the brushes. This is how the output of the alternator is regulated to maintain a steady voltage. When the voltage regulator senses a drop in voltage, it supplies more current to the rotor (spinning magnets) through the brushes. When the regulator senses a rise in voltage, it supplies less current to the magnets which allows the voltage to become lower.

Problem is : an alternator generates alternating current. AC cannot be stored or used by a battery. Only stabilized direct current can be stored for later use by a battery. This is where diodes come in. Diodes change AC to DC by being arranged in a rectifier. To understand this, let’s think about the diagram of AC house current. 110 volt alternating current, as in it’s definition, alternates constantly from positive 110vac to negative 110vac. The power companies in our continent generate this power at 60Hz. This means 60 times per second (frequency) the voltage changes from 110v positive to 110v negative. When graphed it looks like a sign wave with perfect curves that look like waves, waving 110v up, then 110v down, 60 times per second.  Thus AC current has no polarity like a battery (DC) which has a true positive and a true negative. There are 2 types of diodes: positive and negative. A diode functions electrically like a check valve (one way valve) does in a plumbing system. One positive diode and one negative diode placed together on a heat sync work respectively to remove out the pulsating negative and positive variations and put through a direct straight line voltage (DC). This is called full wave rectification. An alternator typically does this 3 times because there are three separate phases to generate currents. This requires, of course, 6 diodes (3 pos. and 3 neg.)arranged on the rectifier or diode pack.

↑ Back to Top