Mobil 1 Logo with trademark
Mobil 1 RacingTM
Car Care for a Better Ride

My Mobil
Discover the Benefits of Membership
Forgot your username or password?

YouTubeMobil 1 Merchandise
Mobil 1 Rewards
How Alternators WorkNotes From The Road

Alternating Currents
By Wayne Scraba/

Understanding Your Car’s Alternator - 

It’s no secret all cars need electricity to run. The battery provides the power, but it’s only good for a few miles before it tires. Keeping the battery topped up is the job of the alternator. An alternator works on the same principles as an old-fashioned generator, however the mechanics differ. Rather than having a fixed magnetic field along with rotating armature coils, an alternator makes use of fixed coils within the case. The magnetic field is created in the rotating shaft in the center of the alternator.

Inside the Alternator
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. But the big dilemma is that automobile electrics or the battery can’t use AC. These elements all function on DC (direct current). 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.
A regulator looks after voltage regulation of the alternator. Early regulators were standalone 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 Regular’s Role 
It’s been said that the regulator is basically the “gate keeper” for the alternator, and that’s a good description. The 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. When you examine a housing, you’ll note 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 Alternator Housing
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.
Alternator Cooling Holes
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; but, 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). Often they 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:
An Alternator Assembly
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.
Serpentine Belt on an Alternator
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.
Alternator Mounting
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.
Copyright 2005-2014 Exxon Mobil Corporation. All Rights Reserved. ExxonMobil Home | Site Map | Contact Us | Privacy | Legal