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Car alternator basics

Inside the alternator, the coils along with the magnetic field produce voltage (which rises and falls). As the voltage rises and falls, the polarity changes. This produces a current that reverses direction, or an alternating current (AC). 

Household electricity used to operate your stove, fridge and coffeemaker is exactly the same. 

The big dilemma is that automobile electrics or the battery can’t use AC. These elements all function on direct current (DC). Obviously, some method to convert the AC output to DC has to be included in an alternator system. A solid-state device called a rectifier accomplishes this. 

Today, most rectifiers are integrated with the alternator. However, in some early examples, the rectifier was a separate component.

Alternator voltage regulator
A regulator looks after voltage regulation of the alternator. Early regulators were stand-alone devices, but later-model examples are contained within the alternator. In any case, voltage regulators are most often solid-state devices (which translates into no moving parts).

Typically, they never need replacement.

Additionally, there is no additional wiring harness routed to an external regulator, which means fewer connections and fewer wires. 

The alternator regulator’s role
It’s been said that the alternator regulator is basically the “gatekeeper” for the alternator, and that’s a good description.

The alternator regulator takes an electrical system reading, constantly monitoring the voltage level of the system. When more power is needed, the regulator delivers it to the battery (typically providing it with approximately 14.2 volts to keep it topped up). Too much voltage and the regulator “closes the gate.”

This is what keeps the battery from either becoming drained or overcharged and boiling. 

On the nose of the alternator, you’ll find the drive pulley (virtually all alternators are driven by way of some form of belt, off the nose of the engine crankshaft).

Some pulleys are configured for v-belts (older cars and trucks) while others are set up for serpentine belts (newer vehicles). On the many alternators, you’ll find a fan is sandwiched between the drive pulley and the frame of the alternator. The purpose is to cool the works during operation. 

Alternator housing
The actual frame or “housing” of the alternator is most often a two-piece affair. They’re regularly manufactured from die-cast aluminum. Aluminum is used for a number of reasons: It’s lightweight, nonmagnetic and provides good heat dissipation.

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When you examine a housing, note that it often has holes where air can pass through it. When an alternator is given the task of charging a heavily depleted battery, it can definitely get hot.

The basic frame or housing of the alternator most often consists of two aluminum castings. You can see here how the two components are physically bolted together

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Large cooling holes are common on the body of an alternator, because the assembly heats up under load. The bigger the load, the more the temperature rises. Early alternators incorporated dedicated cooling fans mounted behind the drive pulley.

Bearings that support the spinning rotor shaft are mounted in the respective halves of the housing. Most often, the rear bearing is lightly pressed to fit into the back of the housing.

In some cases, the front bearing can be pressed into the front half of the housing; in other applications, it can be pressed directly onto the rotor shaft. In all cases, the bearings are sealed, factory-lubed affairs.

Alternators typically make use of a robust mount system. Mounting points are typically in several locations on the alternator (keeping in mind the loads they experience from the drive belts). They often mount directly to the engine cylinder head.

On vehicles with v-belt drive systems, the mounts often have some means of adjusting tension to the drive belt. On the other hand, serpentine drive systems most often incorporate some form of automatic tensioning device. 

For a closer look at alternators, check out more photos:

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Alternator assemblies mandate big, robust mounting arrangements. The loads placed upon the alternator, by way of the drive belt, are often considerable. Here, this GMC pickup uses an incredibly large (and hefty) aluminum mount.

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Most modern alternators are driven by way of a serpentine belt. In years gone by, the alternator was most often driven by a v-belt. That arrangement usually mandated belt-checking and tightening procedures. While a serpentine belt can still wear out, most are equipped with built-in tensioning devices. As a result, you never have to adjust the belt tension.

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Some cars actually have alternators that are extremely difficult to reach, to the point where servicing can prove very expensive (sometimes the engine has to be removed to access the alternator). Here, the alternator is easily accessible, mounted on the nose of the engine.

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