by James Dreamer

Created on: June 22, 2010

The function of an alternator is to convert some of the rotational energy from the engine into electricity to insure the battery remains fully charged and provide power for the vehicle's electrical system. When operating in conjunction with a regulator, it provides the exact amount of power required under varying loads and conditions. Here is an explanation of how it performs this task.

In physics class, we learned that when a coil of wire passes through a magnetic field, electrical energy is generated. By connecting the coil to a circuit, this energy can be harnessed. The coil does not create energy, it merely converts the motion of itself in relation to the magnetic field into electricity. This is the principle on which all alternators operate.

Inside a typical automotive alternator, there are three coils wound around a circular iron core. This assembly is referred to as the stator or field. At the center of the stator is another electrical assembly called an armature. This assembly rotates with the pulley that is driven by the engine. Ironically, there are no magnets in an alternator. The magnetic field is created when the regulator applies a small current to the armature turning it into an electro-magnet. By varying this current, the regulator can control the strength of the magnetic field, thereby determining the overall power output of the alternator.

The electricity produced by an alternator is referred to as alternating current, or AC. In alternating current, the polarity is constantly shifting from negative to positive and positive to negative. The speed of this shifting is determined by the rotational speed of the armature. While AC is used in your house’s electrical system, it is incompatible with the direct current, or DC, system present in a vehicle. The positive in a DC system is always positive and the negative is always negative.

To transform the AC produced by the alternator into electricity usable by the vehicle, it must be converted to DC or "rectified". This is accomplished by the placement of diodes between the alternator’s output and the rest of the vehicle’s electrical system. A diode is a semiconductor which allows current to flow in only one direction, thus converting the AC into DC. When the AC polarity is equal to the DC polarity, the diodes allow it to pass through. When it reverses, the diodes stop the flow.

Many people have the idea that an alternator produces "free" power. This is untrue. As stated earlier, the alternator does not create energy. It merely converts the rotational power from the engine into electricity. Exactly how does this occur?

The regulator constantly monitors the electrical system’s voltage. For the battery charge to be properly maintained, this voltage must remain between 13.8 and 14.5 volts. When you turn on an electrical device (headlights, fan, etc.) the system voltage drops. As soon as the regulator detects this, it increases current to the armature. This raises the power of the magnetic field which in turn forces the alternator to produce more electricity.

So where does this extra power come from? When the magnetic field is increased the armature becomes more difficult to rotate. This requires the engine to work harder to maintain its RPMs. The only way it can accomplish this is to burn more fuel. Therefore, each electrical accessory you turn on increases fuel consumption.

Each separate device requires only a tiny amount of fuel to operate, but collectively they can cause a small reduction in fuel mileage. To get the most distance from each gallon of fuel, it is a good idea to switch off all unnecessary electrical devices to minimize the load on the alternator.

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