Text b dc generator (Part 1)

You should know at the outset that the DC generator is not used in modern cars. Although it is less complex than the AC generator, is output is inadequate for the high current demand of most modern electrical systems, particularly at low engine speeds.

However, the DC generator was used on all cars until Chrysler introduced the AC generator in 1960. So, if you’re restoring an older car, you must deal with a DC generator. So will kit-car builders using a VW older than 1973.

In an automobile generator, it would not be practical to rely on permanent magnets to provide the necessary magnetic field. They aren’t very strong and they tend to weaken with age. So, the DC generator has a set of electromagnets called field coils. These consist of numerous wire windings on iron-pole shoes, energized by the car’s electrical system. The battery provides the initial current to energize the field, and then the generator takes over and “feeds” itself when operating speed is high enough.

In the moving-wire example, a single wire was moved through a magnetic field to produce current. But in a generator, a single wire wouldn't generate enough current to be useful. So, a generator has a large number of individual wire loops wrapped around a laminated iron core called an armature, which is turned by a belt driven by the engine. The two ends of each wire loop are attached to copper commutator bars. The bars are arranged in a circle at one end of the armature shaft such that the bars for each loop are positioned directly opposite each other, or 1800 apart. The commutator bars are insulated from each other and from the armature shaft with pieces of mica.

Two carbon-based brushes, mounted 180° apart in the generator frame, rub against the commutator bars. This configuration allows the pair of brushes to contact the commutator bars for one wire loop at a time. And because the armature is turning rapidly, the brushes contact successive commutator pairs rapidly. Each wire loop passes its small voltage charge into a brash. Because there are so many loops, voltage flows from the generator in a practically constant stream.

Generator output is routed directly into the car’s electrical system. If there’s a heavy load at the moment, current flows through the load. If the load is light, current flows into the battery positive terminal and through its plates, recharging the battery.

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(Part 2)

But what if there's a low load and the battery is fully charged? What happens to all that current the generator is producing?

If the excess current were not controlled, the generator would continue to push it through the battery, causing the electrolyte to overheat. Excessіѵе gassing would take place inside the battery, boiling the electrolyte out of it, thus raining it. Obviously, there is a means of controlling excess generator output — this is the function of the mechanical voltage regulator.

As noted earlier, the field coils of the generator become magnets only when current flows through them. What if you could turn off the current to the coils when the battery is fully charged? And what if you could turn field current back on when system voltage starts to drop? That’s exactly what the mechanical voltage regulator does.

Current for the generator's field coils must pass through a set of movable points in the voltage regulator. When the points are closed, or touching, current flows. If system voltage gets too high — battery is fully charged — a specially calibrated electromagnet in the voltage regulator becomes strong enough to pull the points open, shutting off current to the coils. So, the field loses its magnetism and the generator stops generating. As soon as system voltage drops below the calibration point of the electromagnet, the points snap shut again, energizing the field once more. This regulation process happens constantly, at a rate of up to 200 times per second. It effectively limits the generator’s output to provide only what’s needed to power the electrical loads and maintain a full battery charge.

Strictly speaking, a voltage regulator consists only of the electromagnet and point set just discussed. These are mounted in a metal housing, which also contains a current regulator. The entire unit is commonly referred to as the voltage regulator.

The current regulator comes into play when there are many loads working and high generator output is needed. In this case, the voltage regulator slays out of action, permitting maximum generator output. But under these circumstances, it's possible for the generator to work so hard that it bums itself out. Excess current flow is the culprit. To guard against this, the current regulator — a second calibrated electromagnet and point set in the voltage-regulator housing — temporarily cuts off field current if generator output current flow is too high.

Most mechanical voltage regulators used in recent years are not adjustable, so there’s nothing you can do if the calibration of one of the internal components gets out of specification. Earlier models are adjustable, so consult the appropriate shop manual to see if yours is. AC generators (alternators) use electronic voltage regulators.