Alternator and Generator Theory
Back Home Up Next


This page is an explanation of the theory of operation behind the alternator and the generator. If you know how these critters work already, then this won't matter much to you. If they are nothing short of alchemy and you need to work on or need to modify your charging system - then this page is a must-read for you. I wrote this as a side-bar to my work on various projects, see the High Amp Alternators for older GM's and Alternator Conversions for GM's articles for more details on each area. Each section describes a basic component and how it works.

My experience (and thus this page) is heavily tilted toward GM vehicles, so if your manual says different things for your car, trust it instead of me. I know Ford and Chrysler are fairly close to this, but some imported models use some really weird variations on these basic systems. The basic theory is the same, but some of the wiring is, um, a bit more funky that is described here. In particular, I believe both the Ford and Chrysler alternator systems were externally regulated until well into the '80s, and neither has the remote voltage sensing feature. There are unique issues to be aware of on each one, so I'd suggest that you go read up on them elsewhere before you attempt a non-GM swap. Or, just be like me and stick a GM alternator in it even if it's not a GM. :-)

Electricity and Magnets

This stuff is basic to any kind of electrical charging system, so you should understand this first. The only test will be if you know enough to do what you want to do without messing anything up. :-)

When you put electricity (current) down a wire, the wire will have a magnetic field around it. Conversely, if you move a wire through a magnetic field, a small current (electricity) is created in the wire. The more wires you use and/or the greater the strength of the magnetic field, the greater the effect becomes. These two inverse principles are the basis for electric motors, generators, alternators, and even things like the solenoid inside of a relay. If you have one item (movement or electricity), you can convert it into the other. Also tied in here is the fact that magnets repel and attract each other - that's part of how you make an electric motor move. You can use more turns of wire (windings) to generate a stronger effect.

What about voltage vs. current? Well, current is a measure of how much stuff is flowing down a wire - kind of like the number of gallons of water that are flowing down a pipe every second. Voltage is a measure of pressure - like how many pounds per square inch (PSI) of air are in your tires. They measure different things, but they can be confusing since you can't "see" electricity.

What about AC vs. DC? These stand for Alternating Current and Direct Current. AC is the stuff used in your house. DC is the stuff used in your car and what you get out a battery. The difference is that in DC current always flows in the same direction - from positive to negative (or, if you're a real physics geek, from negative to positive) - while AC alternates the flow of current between the two directions at some rate. This rate is expressed a cycles per second, or Hz (pronounced "hurtz"). In the USA, the electricity in your house is changing directions at 60Hz - 60 times a second.

The final tid-bit of information is that when you spin wires and magnets near each other, you create AC in the wire. This is because the wire and magnets are continuously moving closer to and farther away from each other in a repeated cycle. As they move closer together, the current moves one way. As they more farther apart, the current goes the other way. If you've ever seen the typical "sine wave" graph of AC power, that exactly what I'm talking about here. This is important because you need some way to make that AC into DC to use it in your car. The process of charging AC into DC is called rectification. How you choose to do that is the key design difference between an alternator and a generator.


First up is the generator, also known as a dynamo. I explain it first because it functions in a more basic way and is easier for many people to understand. These are the original electrical generation units used on automobiles - it was much later on that alternators were invented and car manufacturers switched over to them. To understand alternators, you should make sure you have a basic understanding of generators as many of the pieces and basic theory are the same.

The generator is like an electric motor in reverse. Instead of applying electricity to it to make it spin, when you spin it, it makes electricity. It does this by spinning a series of windings of fine wire (called the armature) inside of a fixed magnetic field by connecting them to a belt and pulley arrangement on the engine. As the armature is spun by the rotation of the belt and pulley, it gets a current and voltage generated in those windings of wire. That current and voltage will be directly proportional to the speed that the armature spins and to the strength of the magnetic field. If you spin it faster, it makes more and if you make the magnetic field stronger it makes more current. The speed of the spinning is controlled by the speed of the engine - that's why you need to rev the engine up to help charge the battery faster. The magnetic field is controlled by an electro-magnet, so by changing the amount of current supplied to the electro-magnets that make up the field you control the strength of the magnetic field. This current is referred to as the "field" current and it is controlled by the regulator in response to the electrical needs of the automobile at any given time.

The voltage of the generator is controlled by the number of windings in the armature. The current output varies widely from zero if the battery is perfectly charged and nothing is using any power up to the maximum rated output of the generator. The current output is controlled by the field current, but also by the speed at which the armature is spinning. This is important because a generator can only put out it's maximum rated current at or above some speed - at lower speeds the output drops off very quickly. This is why a generator-equipped car will not charge (or even maintain!) the battery at idle and this is one of the main reasons for the development of the alternator.

The current generated in the armature is AC - not DC. To get it converted to DC so it can charge your batter and run your headlights, a device called a commutator is used to "rectify" this situation. It is on the armature and has a series of contacts along it's outer surface. Two spring-loaded brushes slide on the commutator - one brush is connected to ground and the other is connected to the main output of the generator. As the armature and commutator assembly rotates, the brushes come touch the different contacts on the commutator such that the polarity of the current moving in the armature is always connected to the correct brushes. The net effect of this is that the generator output is always DC even though the current inside the armature windings is always AC.

A generator has to be "polarized" after the system is connected and before it is used. This is typically done by momentarily connecting the main output terminal of the generator to the battery with a jumper wire. This allows things to be set up so that the generator produces power of the correct polarity due to residual magnetism in the generator. For a simple visual image, imagine trying to jump start a car and reversing the jumper cables on one vehicle. It's not something you really want to do - unless of course you like sparking, arcing, and possibly burning out electrical components... This is important if you ever disconnect a generator or regulator - you must polarize it (follow the instructions in the manuals for your car!) before starting the engine.

A generator will have three connections - the field, the armature, and ground, although the ground is sometimes an "implied" connection because everything is metal and is bolted together. The field terminal is the smaller of the two main connections and is typically labeled "F". The armature is the bigger of the two main connections and is typically labeled "A" - this connections carries the main power output of the generator. Consult your manual for the specifics. All three connections go directly to the regulator and there will be a separate output on the regulator for the battery. The OEM regulator is almost always a mechanical device, although some aftermarket replacement units could be solid-state. (I don't know of any myself, but it is theoretically possible to build one.) A typical generator wiring diagram from a 1958 Buick is below for reference - click on the image to see a larger view.

(Diagram is scanned from a 1958 Buick service manual)


The more modern and more capable alternator is explained here. Every modern vehicle uses an alternator - and for good reasons. It is more complicated than a generator, but that added complexity brings a few very good features that you will most certainly want on your vehicle - mainly the fact that it will charge the battery at idle and can support the higher amperages needed to run all of the electrical equipment on a modern vehicle. Alternators tend to be more reliable than a generator and have fewer "hard to diagnose" problems as the system ages - particularly the internally regulated models. The internally regulated models are also very easy to service if something goes wrong - there is only one part to fail (the alternator itself) and replacing it is a simple 30 minute job. This all adds up to the performance and reliability that is expected in a modern vehicle.

The key different between an alternator and a generator is what spins and what is fixed. On a generator windings of wire (the armature) spin inside a fixed magnetic field. On an alternator, a magnetic field is spun inside of windings of wire called a stator to generate the electricity. This allows the wires to be directly and easily connected to their outputs without the need for sliding contacts to carry the relatively high output current. The magnetic field is still generated via electro magnets mounted on a rotor, and the relatively small field current that powers them is supplied to the rotor by two small brushes that each ride on a separate and continuous slip rings. These smooth slip rings (unlike the comparatively rough contacts on a commutator in a generator) and the fact that the relatively heavy windings are fixed instead of rotating allows the alternator to be spun to much higher speeds. This allows it to reach it's maximum output sooner and to be spun fast enough at engine idle speeds to produce enough electricity to power most (if not all) of the needs of the car without relying on the battery.

There are typically three separate windings of wire in the stator that are all set to so that the AC current that is generated is slightly out of phase in each one. The peaks and valleys of the rising and falling current do not happen at the same time, rather they are staggered a bit. This increases and smoothes the electrical output of the alternator much the same way that a 8 cylinder car runs more smoothly than a 4 cylinder one does - there are more power pulses happening in each revolution allowing more total power and better smoothness.

The process of rectifying the AC current into DC current is handled inside the alternator by something more complex than a commutator - diodes. A diode is a "solid state" device that allows current to flow in one direction only - "solid state" means it does this without any mechanical or moving parts. It relies on the different electrical properties of the materials it is made of to act as a one-way valve for current. By arranging diodes so that current from each of the three stator wires is only allowed to pass in one direction, and by connecting the three outputs together, you get a reasonably smooth and stable DC output without any moving parts. (This arrangement is typically manufactured as a single part and is referred to as the diode pack or diode trio.) This lack of moving parts makes the alternator not only very reliable - but also comparatively inexpensive to build and repair. That diode trio costs well something trivial like $1 to produce in large quantities.

Alternators do not need to be polarized after installation. You mount them to the engine, plug them in, and go. This is an advantage for not only manufacturing the car but for servicing it as well.

On externally regulated models, there are typically four connections on the alternator - the large output terminal (BAT), the ground terminal (GRD) which may be "implied" though the metal mountings of the alternator, the field connection (F), and terminal #2 on the regulator is a separate connection to one of the three poles on the stator (R). Unlike on a generator, the BAT terminal is directly connected to the battery and the rest of the cars wiring system, while only the F, R, and GRD connections will connect to the regulator. Also, terminal #3 on the regulator (if present) is connected to the main junction block for the wiring system and serves as a "remote voltage sensing" wire. Terminal #4 on the regulator will be connected via small wires to the charge indicator light on the dashboard of the car and the charge resistance wire. The regulator itself can be a mechanical or solid state device. A typical externally regulated alternator wiring diagram from a 1963 Buick is below for reference - click on the image to see a larger view.

(Diagram is scanned from a 1970 Buick service manual)


On internally regulated models, there are also four connections on the alternator, but there is no separate regulator in the system - it is inside the alternator and constructed of solid-state components. The connections here are the large output terminal (BAT), the ground terminal (GRD) which may be "implied" though the metal mountings of the alternator, and two connections typically labeled simply 1 and 2. Terminal #1 on an internally regulated alternator is the same as terminal #4 on the regulator of an externally regulated system - it connects to a small wire that is goes to the charge indicator light on the dashboard of the car and the charge resistance wire. Terminal #2 on an internally regulated alternator matches terminal #3 on an external regulator - it is connected to the main junction block for the wiring system and serves as a "remote voltage sensing wire". If you are comparing to the externally regulated wiring, then you will note that the F and 2/R wiring connections are done inside the alternator. A typical internally regulated alternator wiring diagram from a 1973 Buick is below for reference - click on the image to see a larger view.

(Diagram is scanned from a 1973 Buick service manual)


What exactly does that little black box on your inner fender do? What's the difference between internally and externally regulated alternators? The regulator does just what it's name implies - it regulates the output of the generator or alternator to the proper voltage and current by controlling the field current that is supplied.

For all generators and externally regulated alternators, the regulator is a small device mounted somewhere on the firewall or the inner fender of the car. It is connected with relatively long wires to the generator or alternator. It is usually a mechanical device that works by rapidly opening and closing the contacts of several relays to create the correct "average" voltage and to limit the current supplied to the correct amount. These mechanical regulators need periodic adjustments and can be somewhat noisy in operation. They also have moving parts that will fail after a period of time. Some later-model and aftermarket replacement regulators are solid-state devices that are quieter and longer lasting even though they look pretty much the same externally as a mechanical unit.

For internally regulated alternators, the regulator is a solid state device (no moving parts) that is mounted inside the alternator casing. These units will never need replacement separately from the alternator and will last for many, many years giving trouble-free service. There are no separate wires to run between the the two units, and there are only a few simple connections to make at the alternator itself.

Remote Voltage Sensing

Both regulator styles can have what is known as a "remote voltage sensing" feature on them - many thanks to the explanations on the MAD Enterprises site for finally making this clear enough to me so I could explain it here. They have details on the remote sensing feature, 1-wire vs. 3-wire alternators, and a great description of a typical musclecar-era Chevy charging system. The details are interspersed throughout those documents, but together they provide very valuable insight into how the typical alternator-based charging system works, and how to modify your charging system to work correctly using an alternator. All internally regulated systems come with the remote voltage sensing feature, but not all externally regulated systems do. Basically, the remote sensing wire should be connected to the main junction point for the entire electrical system. This is because the voltage at the place this wire is connected to will be maintained at the proper level. If this connection is at the alternator or regulator, then that's where the maximum voltage will be with lower voltage out in the rest of the electrical system. If you connect this wire to the main junction point, then the main junction point will have the proper voltage. The difference that results from this can be very noticeable, especially in cars with the battery mounted somewhere besides the engine compartment. A 1V drop is common between the alternator output and the main junction point in many cars, so if you have 14V at the alternator and only 13V at the junction point, you may not be doing much better than 12V by the time you get to the actual devices that need to use that voltage. In this theoretical 1V drop scenario, by connecting the remote sensing wire to the main junction point, you will have 15V at the alternator (yes, 15V - it's OK and desired here), 14V at the junction point, and then 13V at the accessories.

Dashboard Indicator Light

If you have an alternator and are using the factory style indicator light on your dashboard, it is a pretty helpful thing. It helps kick-start the alternator into working at idle speeds when you first start the car, and it tells you if the alternator is putting out less voltage than the battery has in it, indicating a problem. The light is connected on one side to the field current system inside the alternator and to a switched ignition power source on the other side. When you turn the key on but have not started the car yet, the field acts as a ground and power flows through the light and out to ground - lighting the bulb so you know it works. Once you start the car, the voltage at the field is powered internally by the output of the alternator. If this value is exactly the same as the battery voltage, then you have the exact same voltage on each side of the indicator light and they balance each other out - kind of like a tug of war in reverse. If all goes well, the light never comes on, and you drive happily around knowing all is well with your alternator. If the output of the alternator should drop due to a slipping/broken belt or due to certain kinds of electrical faults inside the alternator itself, there will be less voltage on the field side of the light and more voltage on the switched ignition side of the light. The result is that some amount of electricity will flow through the light and into the field and the light will glow proportional to that voltage difference. This is how a slipping belt or an overloaded alternator will cause the light to glow very dimly, while a full-on failure will cause the light to glow very brightly. Note that if you disconnect (or forget to connect) the wire at the alternator, the light will never come on and the alternator will not charge properly.

The dashboard indicator light circuit also typically has an extra wire with a calibrated resistance in it. This wire is run in parallel to the indicator light and has about a 10ohm resistance. It's purpose is to allow slightly more current to flow to the alternator field current system at initial start-up to make sure the alternator begins producing power as soon as the engine starts. About 1 amp total current is flowing to the field current between the light and the resistance wire, with the resistance wire supplying about 3/4 of an amp. This extra resistance wire does not affect the functionality of the indicator light in any way.

NOTE: I've been informed by my readers that a Radio Shack 10 ohm 10 watt 10% wire wound ceramic resistor (part #271-132) has worked well on their GM vehicles. Use caution if you decide to do custom wiring work with resistors as they can get hot and melt stuff.

Conversions and Customizations

Many "hot rod" style conversions use a modified internally regulated alternator to eliminate the two small wire connections and only leave the single large BAT connection to be hooked up. This is usually referred to as a "one wire" alternator - you only have to run one wire to it instead of the usual three wires. In this conversion, the dashboard indicator light is eliminated entirely, the field terminal is connected to the BAT terminal internally, and the connection to the other terminal is made inside the alternator. Conceptually, this conversion works like a factory system without the indicator light on the dashboard and with the remote voltage sensing wire connected to the back of the battery. There are several major drawbacks to this setup. One is that you have to to rev the engine up to approx 1100rpm once after the engine is first started for the alternator to begin charging - the alternator has to reach a high enough RPM so that it "self-excites". Another is that the field connection inside the alternator can allow a small current draw while the vehicle is not running, and this can cause a dead battery if the car is stored for a period of time. Lastly, you do not have the advantage of the remote voltage sensing feature and that means poor electrical system performance - dim headlights, slow wipers, and various other maladies. There are some great details on this at the MAD Enterprises website - check out their articles on the remote sensing feature, 1-wire vs. 3-wire alternators, and a great description of a typical muscle car-era Chevy charging system for more details.

I personally do not recommend the "one wire" conversions - the dubious improvement in under hood aesthetics just isn't worth it. Your neighbors will probably not appreciate you revving your car up to 1100 rpm each morning at 7am before you head out to work and it makes your otherwise cool ride annoying to drive. Many "tamer" drivers (like your wife, if she's anything like mine) will often start the car and drive for some time before making it to 1100 rpm for the first time. During that entire time, she would be draining the battery if the vehicle was using a "one wire" conversion - and that's not cool. It is very simple to hook up the extra wires for the indicator light and the remote voltage sensing feature. It also makes the car much more pleasant to drive - you have one less thing to worry about when you just want to get in the car and go. In addition, many of the vehicles that use the 1-wire conversion tend to be specialty use vehicles and thus do get stored for a long time between uses, so the battery drain could be an issue. If you do go that route, consider a battery disconnect or some form of "battery maintainer" to keep your battery charged between vehicle uses. Lastly, the problems with reduced voltage as a result of not having the remote voltage sensing feature can be a very big deal.

If you are still pondering a "one-wire" conversion, it should be noted that you can partially eliminate the second wire by using a short pigtail to hook it directly to the BAT connection on the alternator. If you examine the diagram above for an internally regulated alternator, you will see that this wire eventually ties back into the wire that is attached to the BAT terminal anyway. When I originally wrote this webpage I was not aware of the remote voltage sensing feature and the possible issues with connecting the remote sensing wire directly to the alternator output, and now that I am, I believe I understand a few problems I was seeing in the operation of my 1973 Electra. For the record, I'm going to be making some changes to the electrical systems in my vehicles now that I understand this - I think it's that big of a deal now that I understand how this all works.

A final word of caution is to think twice (and then think about it again) before deviating from the way the factory did things if you want to customize your charging system. Modern factory charging systems are amazingly reliable and trouble-free. There is a reason the factory did what they did. Adding those extra lengths of wire probably costs them about $1 a car - and although that may not sound like much, when you make a million cars, $1 per car is a $1,000,000 less in potential profits. That's some serious money - and that's just for a few pieces of wire. (This detail is why the factory goes crazy trying to save every penny possible when building the car - it really adds up fast and they like making all the money they can.) Also, when you make a million cars and find out something is wrong with them (fire hazard, doesn't always charge the battery, etc.) it ends up being very expensive - both in real dollars and from a public-relations perspective - to repair the problem. Melding two factory-style systems to upgrade your vehicle to newer standards is a worthy goal and often a very easy thing to do - just be sure you get all the details right so you can enjoy your vehicle for many trouble-free miles to come. Take the time to make sure each wire you put into the car or change the function of (aka, push more current through it) is up to the task you are placing before it.


Theory Applied

Just to give you a taste of what this can apply to, in addition to my cars, I've corresponded with many folks who have worked on all kinds of things and used this page to help fix problems and get and keep them going. One gentleman who used this page to help repair his 1950's era bulldozer with a gear-driven generator. It was so large, heavy, and powerful, that a belt simply wouldn't do the trick! Another resurrected a 1940 FarmAll H tractor when he found it with an ancient alternator conversion with no wires left installed. Another was just trying to keep his 1967 Piper Cherokee airplane going and fix an intermittent overcharging problem. One gentleman wrote in to say the page helped him properly wire up the alternator the was the main power source in his engine-driven Lincoln welder! The specific application may vary from small car to big car, to 6 ton bulldozer, to tractor, to plane, to a engine-driven welder, to anything that needs to convert mechanical energy into electrical energy. No matter what the application, the basic theory is still the same. Learn the theory and you can get a handle on almost anything with an alternator or a generator.

1950EraBulldozer.jpg (146170 bytes) 1950EraBulldozerGenerator.jpg (74573 bytes) (Photos courtesy of Bill Doherty.)

1940FarmallHTractor_before.jpg (126663 bytes) 1940FarmallHTractor_after.jpg (110062 bytes) (Photos courtesy of Lee Darvial.)

Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? You can email Mike or Debbie.

Pretty much everything on this website is copyrighted, if you want to use something, ask first.

Page last updated 01/01/2010 03:20:57 PM