How Generators & Regulators Work
Double click on pictures....
Once you understand the basics of how a battery works and how it is constructed,
we can move on to the generator, which is the second most important part of the electrical system.
To sound bona fide, I might as well give you the official job description of the generator. It is “a machine that converts mechanical energy, supplied by the engine, into
electrical energy used for either recharging the battery or supplying power to the electrical system.”
While the description seems a little confusing, if you follow along a little further we
will make sense out of it all. Come on, it’ll be better than you think.
THE WORK SCHEDULE FOR THE GENERATOR FAMILY
When the engine speed is at idle or at low rpm, the generator has little or no output,
and the battery provides all the electrical energy needed for the electrical system.
When vehicle speed reaches about 20 mph or engine rpm reaches about 1200, the
generator will begin to charge. The output will help the battery with some of the electrical load. (This speed is known as the generator “cut-in” speed.)
At higher engine rpm of about 1800, the generator is capable of providing all of the
electrical current needed to run the accessories, as well as recharge the battery as
Generators will usually provide their maximum output at about 1800 to 2300 rpm
engine speed. Normally the pulley diameter of a generator is designed so the engine will spin the generator at, or close to, its ideal rpm, (the rpm at which the generator operates most efficiently.) This rpm is matched to the rpm at which the engine will spend most of its time.
IN MOST OLDER CAR APPLICATIONS, THE GENERATOR ARMATURE TURNS ABOUT TWICE FOR EVERY RPM THE ENGINE TURNS.
When a generator spins at high speeds (above 3500 rpm engine speed) the output
of the generator will actually drop off quite a bit, as the brushes are lifted off of the armature by centrifugal force. If heavier brush springs were used (a great idea), it would cause excessive brush wear at the slow speeds.
An interesting note: Did you ever wonder why over the road trucks get such long
life out of their generator brushes as compared to a car? Here are the reasons. One is the constant rpm that make it easy to match the correct engine to generator speed.
The other factor is called air gap. This is when the brushes lift off of the commutator
just slightly due to the centrifugal force. The brushes will then experience minimum wear
because the brushes are not physically touching the commutator and the loss in output will be slight.
Cars driven in town will wear out generator brushes at a much faster rate than those
that spend their life traveling up and down the highway. The same principle applies.
WHILE WE ARE ON THE SUBJECT OF BRUSHES...BUICK CARS OF THE LATE 1940’s AND EARLY 1950’s HAD AN INTERESTING SAFETY FEATURE.
They had what they called a “brush protected generator.” The “field” wire of the
charging system was routed through the ignition system. When the brushes in the generator got too “short” from wear, the field wire would “ground out” the ignition and the car would not start.
While this was a good idea in theory, it left a lot of early-day Buick owners stranded
Without warning (and very unhappy). The servicemen of the day carried a jumper wire in the tow truck. If this was the problem (a simple check), they used the jumper wire to by-pass the generator to ignition circuit. If the car started, they simply drove it back to the dealership and installed new brushes in the generator. And the customer was happily on his way.
HEY, HOW COME THERE ARE SO MANY DIFFERENT SIZE PULLEYS
USED ON THE SAME STYLE OF GENERATOR?
As we learned earlier, the pulley size is matched to the rpm at which the engine will
spend most of its time running. In-town delivery trucks had a small diameter pulley so the armature turned faster at the low engine rpm, increasing the output at the slow speeds.
All generators “make” electricity in much the same way. Let’s take a look and see
what parts make up a generator and what job each of those parts has to perform. As I
have done before, I will give you the official description of what a generator does, then
explain things in common English.
Generator operation is based on the principle of electromagnetic induction. This
means that voltage is generated when any conductor is moved at right angles through a
magnetic field. When voltage is produced in this manner, it will cause the current to flow in the conductor if that conductor is a complete circuit. Whew! Got all that? Now let’s ex-
plain that in common sense terms, starting with the internal parts.
- An armature starts out as a bare hardened steel shaft. To this shaft is
added a series or group of non-insulated copper wires wound close together. They in turn will form what is called a loop.
The loops of wire are then embedded in a series of slots in an iron core. This iron core is then attached to the armature shaft. This shaft spins and helps to generate the electrical current. As you might guess, the size of the wire and the number of wires in the loop will affect the output of the generator.
- The commutator is a series of segments or bars that are also attached to the armature shaft at the rear of the armature. It is the wire ends from the loops of the armature windings in the iron core that are attached to the commutator.
When this is done, a complete circuit is formed.
-Field coils are the windings or the group of wires that are wrapped around the pole magnet. It is the job of the field coils to take the current drawn to the pole magnet, and make it stronger. (Field coils are the windings that are attached to the
inside of the generator housing.) This increased strength in current will force even more current to be drawn to the pole magnets, which will continue to build up current. This is how the current produced by the generator is built up and increased, until it can be used by the battery and the accessories.
The ends of the armature loop are securely attached to a split ring called a commutator.
- After the generator develops the current, it is the brushes that carry the cur-
rent to the “field” circuit and the “load” circuit, so the electricity can be used by the battery and the accessories. This process is called “commutation.” The brushes will ride on the commutator segments of the armature. Brush holders hold the brushes in position by way of spring tension. Most automotive generators will contain
two brushes, one that is grounded to the frame of the generator, one that will be insulated from the frame. The insulated brush is the positive
brush and is connected to the “A” terminal of the generator, and to one end of the field coils. The other end of the field coil is connected to the insulated “F” terminal of the generator.
BEARINGS AND BUSHINGS
- At either end of a generator you will find a bushing or a bearing. They have the job of making the armature shaft run true in the housing between the field coils and pole shoes. Bushings will be made of copper or brass and are soaked in oil before they are
installed. The brass or copper bushing material is porous and able to absorb the oil like a sponge. This provides the lubrication between the shaft and the bushing. They can also be re-oiled from the oiling tube on the outside of the generator. Some heavy-duty generators will use ball bearings instead of bushings for the armature shaft to ride . This is done to support a radiator fan or other accessory.
BUILDING A WORKING GENERATOR
An assembled generator will look something
like this: The electrical rule that applies
to a generator states that “electrical
voltage will be generated when any conductor
is moved at right angles through a magnetic
To demonstrate this theory to yourself take
a simple horseshoe magnet and stand it on
its side. (It will have a north pole and a
south pole, just like in your generator.)
Now take a piece of plain copper wire and
move it back and forth between the poles of
the magnet. You will be breaking the magnetic
field, which will produce a magnetic
current inside of your wire. This is exactly
what the armature does to the field coils.
When current is produced this way, it will cause current to flow in the conductor if it
is a complete circuit. (Remember the armature with the loops of wire embedded in the
slots of an iron core? Didn’t the ends go down and connect to the commutator to form a
complete circuit?) Okay, so you’re lost...
First let’s look at a simple generator with an armature that has only one turn or loop
of wire and two pole pieces. These pole pieces (actually magnets) will always have some
“magnetism” left over from the last job they did.
However, these magnets are week because of the magnetic field between them.
(Remember these two magnets are exactly opposite of each other. That is the cause of the
weak current. They will tend to cancel out each other.)
If we place the armature between these two magnets and then spin it in a clockwise
direction, a weak voltage will be “generated.” Remember, the rule of generators says that
any current generated will flow to the conductor if it is a complete circuit. Because the
armature is a complete circuit, the current will flow to the armature and then to the field
coils where the voltage will be increased.
The rotating armature cutting through the current produced by the field coils forces
even more current through the field coils that makes still more stronger voltage. This is
how the voltage generated by the loops is increased into voltage that can be used by the
battery and the accessories.
Now, if we were to add a real armature to our generator with additional loops of
wires imbedded in an iron core and connected to the commutator, what is going to happen?
That’s right. Any voltage generator by any one loop will be added to the voltage
developed by any of the other loops. By having multiple loops, an almost constant supply
of voltage is developed, finally!
As you might guess, the strength of the magnetic field, the number of conductors on
the armature, and the speed at which the armature is turned will affect the output of the
generator. Just like the internal parts of a battery, all of these things are matched to the
OK, SO OUR GENERATOR IS CHARGING. WHAT HAPPENS IF WE SPIN THE ARMATURE
REALLY FAST TO PROVIDE A HIGH OUTPUT FOR A HEAVY ELECTRICAL LOAD?
Right. Things are going to get hot, in part because of the resistance or electrical
friction and in part due to the mechanical friction. What will happen to our generator then?
The high heat can melt the “varnish” and damage the insulation used to hold the
loops or conductors in the armature slots. Also, the soldered connections of the armature
coils and the commutator bars will melt from the heat. When this happens, it is commonly
called “throwing the solder” out of the generator. Besides losing all of the solder, the bars
of the commutator separate from the shaft that holds everything together; in simple terms,
everything just flies apart, and the generator is ruined.
To prevent this damage, a current regulator is necessary. Just as it sounds, a current
regulator limits the amount of current the generator is allowed to produce for both the
electrical demand of the accessories, and the safe limit of current the generator can produce
without damage to the generator itself.
Another source of internal heat that has to be dealt with is called “iron loss.” The
iron core of the armature will act as a large electrical conductor, and will “cut” magnetic
“lines” of energy as the armature spins. As a result, the armature core itself will generator
unwanted current. The current developed by the core of the armature is mixed with the
current developed by the regular conductors of the armature.
This creates excess heat inside of the generator
that is not wanted. To overcome this problem, the iron
core of an armature is made up of thin sections of steel
material that is laminated together. By doing this the
lamination or varnish will act as an insulator and help to
prevent the flow of core current to the regular conductor
of the armature.
Last on our parts list is the fan. It is mounted
behind the pulley and has the job of keeping the generator
cool. In some heavy-duty applications, the fan gets a
little help from the engine intake where some of the air
intake from the engine is used to cool the generator.
To overcome the problem of excess heat, the iron
core of an armature is composed of thin sections of
steel material that is laminated together.
WE NOW KNOW THE PARTS OF A GENERATOR AND WHAT EACH PART DOES. WE ALSO KNOW
ABOUT PULLEY SIZE AND COOLING THE GENERATOR. NEXT ON OUR LIST ARE “REGULATOR.”
TIME-SAVING TIP: Before we go on, here’s a quick reminder to check the wiring between the regulator and
the generator. While this sounds like no-brainer, you would be surprised how many times the wiring gets
switched by accident. So as a simple review, and in order for everything to work properly, things should be
connected as follow.
1 The positive wire from the generator will be connected from a post on the generator (marked either
“B” for battery or “A” for armature) and should be connected to the (armature) terminal of the regula
2 The field terminal of the generator should connect to the field terminal of the regulator.
3 And finally, the wire that travels down from the amp gauge in the dash to the regulator should
connect to the “BATT” terminal of the regulator.
Did you know...The amp gauge only tells you what is going into or what is being
drawn out of the battery? It is not connected directly to the generator to tell you if the
generator is actually charging or not charging as commonly believed. (This idea comes
from the belief that the generator has to be working if the gauge shows a charge.)
Volt meters are also sometimes used instead of an ammeter in the dash of a vehicle
to show the condition of the charging system. Volt meters became quite common in the
1960s with the introduction of alternator charging systems.
This was done so no “heavy” current had to be carried up to the dash. By using a
volt meter, a much smaller gauge of wire could be used with less danger of electrical fire
when a wire shorted out under the dash. With a volt meter, only minimal amps were on
hand, as opposed to an amp gauge where all of the generator’s output passed through the
THERE IS SOME ARGUMENT OVER WHICH GAUGE IS MORE ACCURATE IN READING
THE TRUE CONDITION OF AN ELECTRICAL SYSTEM. HERE IS THE DIFFERENCE;
YOU CAN THEN DECIDE FOR YOURSELF.
An amp gauge will tell you the amount of amps passing into or being drawn out of
the battery. The volt meter, on the other hand, will tell you the “pressure” behind the
EXAMPLE: If the electrical load is light, and there is not much resistance, in theory
a problem can occur with the alternator’s output; however, it will not show up on the voltage
gauge because the amp demand id low, so the pressure will remain strong. But when
the electrical load increases, then the voltage will drop, exposing the problem. From a
safety standpoint, some engineers believe the minimal amps is better.
This problem will occur more often with cars of the 1960s and with alternator charging
systems. (We will get into alternators in an upcoming chapter). An alternator can have
a blown diode that will take away (in most cases) one-third of the alternator’s charging
ability. But if the amp load is light, the voltage will not drop until the amp load increases.
1 The cut-in speed of a generator is the rpm that a generator begins to provide an output, typically
about 1200 rpm vehicle engine speed or about 20 mph.
2 Generator pulley diameter is determined by the rpm at which the vehicle engine spends most of its
time, and the rpm at which the generator operates most efficiently. In other words, the goal is to
spin the generator at the rpm it is most efficient while the vehicle engine is running at the rpm it is
3 Throwing the solder out of a generator means that because of high rpm and excessive heat caused
by a high amp load, the solder that holds the segments to the armature has melted. The centrifugal
force of the spinning armature has caused the segments to break away from the armature. In short,
you have just toasted your generator.
Keeping Track of The Generator’s Output
Now that we understand how a generator manufactures electricity, we need to figure
out how to control the output of current from the generator. As we said in the last chapter
this is done by the use of a voltage regulator.
Let’s start at the beginning and see how this happens. Inside the voltage regulator
is a set of contact points, much like those found in an ignition distributor. To these contact
points is connected a wire from the field coils. When the points in the regulator open and
close it will start and stop the flow of current to the field coils, battery, and accessories.
NOW THAT WE HAVE A BRIEF UNDERSTANDING OF WHAT THE REGULATOR DOES, LET’S
TAKE A FEW MINUTES AND TALK TERMINAL. THE TERMINAL ON A REGULATOR ARE CLEARLY
MARKED. BUT SOMETIMES THEY STILL DON’T MAKE SENSE...UNTIL NOW.
BATT - This is the battery terminal. This terminal connects the voltage regulator to the amp gauge in
the dash, on its way to the battery.
GEN or ARM - This terminal is always connected to the armature post on the generator.
F or FLD - This terminal is always connected to the field post of the generator.
IGN - This is a terminal used mainly before the war (1944). This terminal was used on the early
regulators that controlled the voltage of the entire electrical system at the ignition switch. In the old days, it
was believe that controlling the voltage at the ignition switch was the best way to furnish an even voltage to
the entire electrical system. Later on, the voltage was controlled by the battery terminal of the regulator and
this ignition terminal disappeared.
WHAT IF YOU REPLACE ONE OF THESE OLD STYLE REGULATORS
WITH THE NEWER STYLE THAT DOES NOT HAVE THE IGNITION TERMINAL?
It will not be necessary to have this terminal and with the replacement regulator you
won’t use it, but the ignition wire will still be “hot,” so you need to fold it back into the original
harness and wrap it with black electrical tape in order to insulate the end well to prevent
a short. In case someone wants to do a 100 percent restoration in another lifetime,
everything else will match up perfectly for them.
Meanwhile, when our original generator begins charging and produces enough
current to begin recharging the battery, it will travel up through the regulator to the contact
points. Beside the contact points is a shunt coil. A shunt coil is made up of many windings
of a fine wire that is shunted (wire connecting two points in an electric circuit that has the
ability to turn away part of the circuit) across the generator. The current here is not allowed
When the voltage is strong enough, the magnetism developed from this shunt coil
will close the contact points and allow the current to pass through the series windings and
on to the battery and accessories.
In turn, when the generator slows down or stops, current begins to flow in reverse
from the battery to the generator. This reverses the direction that the current travels
through the series winding. This will cause the magnetic field in the series windings to
reverse. But as we learned earlier, the magnetic current from the shunt coil is not allowed
to reverse. So instead of helping each other out, they work against each other. When this
happens, the resulting magnetic field is not longer strong enough to overcome the spring
tension on the contact points. The points are opened, stopping the flow of current to the
Just when you thought things couldn’t get any more difficult, and we had everything
figured out, there is one more factor to consider for regulator control. That is temperature
compensation. Because a cold battery is harder to charge than a warm one (due to higher
resistance), the regulator must allow for this. To do that, a regulator is built with a bi-metal
“thermostatic” hinge. What this means is the material the contact point arm is made of is
temperature-sensitive to cause the regulator to regulate to a higher voltage during colder
weather in order to charge a cold battery.
Besides the voltage being regulated, the current output (amps) of a generator is
also regulated by what is called a current regulator. The current regulator is built inside of
the voltage regulator and works in much the same way as the voltage regulator.
The main difference you will notice is that located on the inside of the voltage regulator,
the current side of the regulator is made up of wire that is thicker (heavier gauge),
and there are less turns or wraps of wire on the coil. Remember, the current regulator has
to carry all of the amps the generator is producing.
OK, DO THESE REGULATORS WORK TOGETHER OR SEPARATELY?
They are unfriendly and will never work together. One or the other will do the work
depending on the load. For instance, if the generator is spinning fast, the battery has a
good charge, but most of the accessories are turned on, then the voltage regulator is the
one doing the work.
If, on the other hand, the generator is
turning slowly, the battery is in need of a
charge, and all of the accessories are turned
on, it will be the current regulator doing the
This type of circuit where the regulator
is a part of the field circuit is called an
“A” circuit. An “A” circuit is easily identified
because the contact points are always located
after the field coils. This type of circuit is common
to the General Motors family of vehicles.
The voltage regulator and current regulator are units in the external
circuit used to “sense” either high voltage supplied to the electrical
system or high current supplied to the external loads...see diagram
OK, MY SHOP MANUAL SAYS I HAVE A “B” CIRCUIT REGULATOR.
HOW IS THAT DIFFERENT FROM AN “A” CIRCUIT REGULATOR?
A “B” circuit regulator works in much the same way that an “A” circuit type regulator
does. The only difference is the contact points are located before the field coils
instead of after. There is no advantage to either location and they both work equally well.
“B” circuit regulators are common to Ford cars
CHECKING REGULATOR OUTPUT
“SO DO YOU CHECK AND ADJUST “A” AND “B”
CIRCUIT REGULATORS THE SAME WAY?”
No, they are both checked differently. If
you have to adjust the regulator at some point
in time, it is best to follow the directions in your
shop manual. The secret is to know how the
regulator works; then reading those directions
will make sense.
This illustration shows the various factors involved
in voltage regulation and the manner in which it is
Check out the following illustrations. A simplified circuit employing both current and voltage regulators
is illustrated. The regulator or contact points are located “after” the field coils (“A” circuit). The
field current is attached to the insulated brush inside the generator.
BUT WAIT, THERE IS MORE...
The “A” and “B” circuit are by far the most common types, but not the only types of
regulator circuits. There are a few others you may encounter. They include Third Brush,
Bucking Field, and Split Field.
OK, ALL I HAVE IS A GENERAL REPAIR MANUAL,
HOW DO I KNOW IF I HAVE AN “A” CIRCUIT OR A “B” CIRCUIT REGULATOR?
Simple. All you have to do is check the connections at the brushes and the field. If
the generator field coil is connected to the insulated brush at the back of the generator,
you have an “A” circuit.
If the generator field coil lead is connected to either the grounded brush (a brush
that goes to ground) inside of the generator, or is connected to the inside of the generator
frame itself, you have a “B” circuit. From there all you have to do is follow the directions
given in the repair manual.
As seen in the diagrams above, a set of contact points is placed in series with the field coil circuit
and all field coil current passes through them. If these points were to open, current would no
longer pass through the points, but travel through a resistance to ground and then though the
ground conductor back to the ground brush of the generator.
The diagram below show the various factors involved in current regulation and the manner in
which this is done.
to be continued ......
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