How does an Alternator Work
It might come as a big surprise to some people to find out that their car can run perfectly well without a battery. Their car won’t start without the battery being connected but once started a car doesn’t need a battery to run, so long as it has a working alternator.
The alternator is connected via fan or serpentine belt to the crankcase and converts mechanical energy to electrical energy. The electric energy is necessary to provide the spark plugs with their spark, and the electricity to power the headlights, break and tail lights, interior lights as well as the radio, air conditioner, and heater fan.
Inside the alternator the pulley driven by the belt causes the rotor, which becomes an electro-magnet, to spin very quickly. The rotor becomes magnetized as a result of two slip rings attached to its shaft making constant contact with brushes containing electrical current. The rotor is situated in the center of the stator which is comprised of three coils of copper wire It is the effect of the rotor’s positive and negatively charged parts each alternately approaching and then passing the coils as the rotor is spun that produces the alternating current.
The coils within the stator are even spaced at a120 degrees apart and each produces pulse or phase of electric current per 360 degree rotations. Half of each rotation is positive and the other half negative resulting in a three phase alternating current. But the current needs to be changed to direct current and this conversion is handled by six diodes, two from each coil that are arranged in a series so that the current only flows in one direction.
The reason an electro-magnet is used as opposed to a permanent magnet is that it’s necessary to regulate the voltage output. A voltage regulator continually keeps track of the voltage leaving the alternator and when it reaches a pre-determined volt limit the regulator reduces the current to the rotor and thereby the strength of its magnetic field. With the strength of the magnetic field reduced the alternator produces less volts. If the volts leaving the alternator falls enough then the voltage regulator allows more current to the rotor to increase the magnetic field.
As stated earlier a running car can be disconnected from its battery and still run; can a car run without a working alternator? The answer is yes, but only for a short time. The car will run off the battery until it’s drained and how long that would take would depend on how many accessories were being used, whether it was day or night and the temperature (the colder the weather the more difficulty a battery has holding a charge).
The alternator in spite of its importance is one of the few automobile components that can still be changed by a weekend mechanic because it has changed little over the years.
