|The Stovebolt Page||
6-Volt DC Charging Systems
By Jerry Herbison
Herbison is from Dellrose, Tennessee currently operating "Action Enterprises,"
a racing engine and gunsmithing business located at his farm in rural south
central Tennessee. Jerry spent 15 years as a high school auto mechanics instructor
and has ASE certifications, Master Technician certification in Automotive, Heavy
Truck, Auto Body, Engine Machining, and Advanced Engine Performance. He grew
up around a large independent automotive shop in Nashville in the 1950's, and
has been associated with the auto repair trade since then. He's worked as an
industrial maintenance technician, aircraft electronics technician (US Air Force),
industrial electrician, welder, machinist, truck driver, and short-order cook!
Jerry's putting together a series of articles on charging systems each dealing with a separate area of inspecting and maintaining the oldtime DC systems. He has a pretty extensive library of research material and specifications on these systems, some going back into the 1940's. His father, who went into business in 1945, is still living, and is a deep well of information on the older systems. He was the chief automotive instructor at Nashville Auto-Diesel College for several years in the 1960's and opened the high school shop where Jerry ended up teaching in 1971.
Ever since the electric starter was introduced by Cadillac in the early 1900's, automobiles have needed a battery, and a way to keep it charged. Before that, the only electrical system most cars had was a magneto ignition system which was self-contained. The starting chores were handled with a hand crank, and lighting was either by kerosene lamps, or an acetylene gas system. The development of the lead-acid storage battery into a portable unit, rather than the bell-jar laboratory item seen in the old horror movies made it possible to have a ready source of electricity in a car, and ushered in the era of electrical systems of ever-increasing complexity. With few exceptions, the early systems were 6-volt, and used a DC generator for recharging the battery. In 1957, all American car manufacturers standardized their electrical systems at 12 volt, negative ground to accommodate aftermarket manufacturers, and to take advantage of the new solid-state electronics, which did not work well in positive-ground systems. The alternator came on the scene soon after, and the old DC generator began a slow decline into obscurity. Let's look at some of the differences in DC generator and alternator systems, and how to keep that 6-volt Stovebolt generator operating at peak efficiency.
The basic difference in generators and alternators is the way the charging current is produced. The generator produces charging current in the armature, the rotating part of the unit. To get the current from the armature to the battery, a set of individual contacts are built onto the armature, called a "commutator." Carbon brushes conduct the charge current to the "armature" terminal of the generator, where it's routed to the voltage regulator, and eventually, to the battery. This system is necessary because the current tries to reverse its direction every half-turn of the armature, and AC can't be used to charge a battery. Here's a picture of a DC generator for a Chevy Corvair. It's a 12-volt unit, but the 6V type will be almost identical in appearance.
Conducting heavy current through brushes creates a certain amount of arcing, so generators are limited in their capacity to develop heavy charge current. Also, they don't develop much current flow at low speed. If the pulley on a generator is small enough to spin it fast at idle speed, it will throw the windings out of the armature at highway speed due to centrifugal force. The DC generator has some definite limitations, but it served well in the days of low-speed high-torque engines and simple electrical systems.
The alternator solves a lot of the problems associated with the generator by creating the charge current in the stationary alternator housing, and using the rotating part, called a "rotor" to generate the magnetism needed to induce a current. It's also a 3-phase unit, with three separate windings, each developing part of the current. Since only a small current is necessary to power the rotor, and it doesn't have to be interrupted by a commutator, much higher current can be created, and the alternator can be driven to higher rotating speeds without damage. The disadvantage is, it creates alternating current, not direct, and the output must be "rectified," changed to DC, before being used to charge the battery. This is done with six solid-state "diodes," mounted in the alternator housing. This provides a stable, high-amperage current to the battery, keeping it charged better than a DC generator, with less need for periodic maintenance.
Alternators are also self-regulating where current output is concerned, because the internal resistance of the stator windings keeps the alternator from overloading itself when a heavy current demand is placed on it. A generator will self-destruct under high current load, unless a current regulator is used to limit its output.
The electromagnetism in a generator is developed by the "field" coils, the windings in the generator housing. Anytime current passes through a wire, a magnetic field is created, and when the wire is wound into a coil, the magnet gains strength. For even more magnetism, the coil can be wound around an iron or steel core, called a "pole shoe." Most generators have two, or sometimes four field coils, and create a magnetic field around the armature. Here's a pic of a field frame, showing the field coils and a pole shoe.
When the armature spins in the magnetic field, the windings in the armature have a voltage induced in them, which is picked off by the brushes at the right position, and pass through the system wiring to charge the battery. Here's the armature, with the brushes in position to receive the charging current. One brush is grounded to the field frame, and the other one is attached to the "Armature" terminal of the generator.
By controlling current flow in the field windings, we control charging output. This is the job of the voltage regulator. There are two different methods of controlling field current, differing only by where the field coil receives its ground. "A" circuit generator field coils are grounded by the voltage regulator contacts. "B" circuit generator field coils are internally-grounded, and the regulator supplies voltage, not ground to the field coils. Most General Motors generators use the "A" circuit, so current from the field coils leaves the generator, and finds its ground through the points in the voltage regulator. It's important for an "A" circuit generator to have a good electrical ground between the regulator base and the generator housing. Since regulators are usually mounted on the car's body, a poor ground between the frame and the engine can create all sorts of charging system problems. A quick fix for a rusty body is to run a ground wire from the regulator base to the generator housing. I've fixed several chronic charging system problems by simply installing a ground wire!
This photo is a typical 3-unit regulator, consisting of a "cutout relay,",a "current regulator," and a "voltage regulator." There are a few different varieties, but this one's pretty typical.
The portion on the far left is the "cutout relay" used to disconnect the battery from the generator when the engine stops. This prevents the battery from discharging through the armature windings, damaging the generator and regulator in the process.
The center winding is the "current regulator," used to prevent the generator from being overloaded when it's charging a low battery. When the charge current reaches a pre-set point, a resistance is inserted into the field circuit, reducing the generator output momentarily. This process is repeated several times a second, with the regulator points opening, output going down, and the points re-closing for full output. Voltage regulation is accomplished the same way, by the "voltage regulator," the coil on the far right.
Here's a pic of the voltage and current control resistors, on the underside of the regulator. Once the current drops to an acceptable level, the voltage regulator takes over, and controls maximum charging voltage. Either the current regulator or the voltage regulator is in control, depending on current flow, not both at the same time.
Now, where does the generator get its field current in the first place? It must be "polarized." This is done by momentarily connecting battery voltage directly to the "armature" terminal, creating current flow, and magnetism, in the armature, and also the field coils. This is basically a short circuit, so don't let current flow for more than one second or so. When the voltage is removed, a small amount of magnetism remains, called "residual magnetism." Rough handling of the generator can cause residual magnetism to dissipate, so it's customary to polarize a generator after installing it. With a properly-polarized generator, there should be about 2 volts present at the armature terminal when the generator is turning, even without any wires connected. It's possible to bench-test a generator for residual voltage by spinning it with an electric drill motor or an impact wrench before installing it, or running the engine after installation before wires are installed. If no residual voltage is present, repeat the polarizing process. On a "B" circuit generator, voltage is applied directly to the "Field" terminal. (This only applies if you're working on an old Blue Oval generator, or other "B" circuit system.)
Here's a shot of the internal workings of an alternator. The first one is the rotating part, called, logically, a "rotor." It develops the magnetism necessary to create the charging current. It also has brushes, but they don't have to carry heavy current, and can be made much smaller, and last longer than generator brushes. They operate on "slip rings," so they don't carry heavy, interrupted current flow.
The slip rings are the two smooth, round items on the right, just to the left of the rear bearing.
The next illustration is the alternator housing, containing the 3-phase winding called the "stator." Its job is to create the charging current, and it does so without any movement. The brushes are visible at the rear of the housing. The voltage output of the alternator is controlled by varying the current flow through the brushes with a single-unit mechanical regulator, or a transistorized regulator on later-model units.
No cutout relay or current regulator is required on an alternator charging system.
This one is getting a little long-winded, so take a break, rest your eyes, and in the next installment, we'll discuss checking out the wiring system from the generator, through the regulator, and on to the battery. There's a simple test sequence called a "series resistance test", which will help spot high-resistance connections, bad wires, or dirty battery post connections. It's the first step in a good DC charging system checkout.
Thanks Jerry for all this hard work ... we look forward to the next installment and promise not to electrocute ourselves in the meantime. Well, we'll try ~~ Editor
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