|The tasks of a generator|
|To convert mechanical into electrical energy|
|To provide current while driving|
|To ensure that the battery stays charged|
|Generator RPM often higher than crankshaft RPM.|
The generator should weigh only as little as possible and nevertheless, supply with a very high efficiency, and constant tension, even at idling speed, all consumers in the automobile with sufficient current. In contrast
to the DC-alternator, it charges even during idling, it also has uninterrupted collector-rings, which results in much less wear and tear as far as the carbon brushes are affected. In addition, the regulator can be kept
simple because a current limitation is no longer necessary. After a certain amount of current flow, the stator iron core is magnetically sated.
A magnetic field with 6 alternately following north- and south poles, rotates with an armature. This field is substantially supported by a winding, which through two slip rings (on the bottom right in the figure 'Rotor with
claws poles'), and carbon brush conductors, is connected with the negative pole or the voltage regulator. The tension build-up can be controlled in three coils, each offset by 120
in the casing (stator).
|Regulation through field current:||Armature winding (rotor)|
|Creation in:||Stator coil (stator)|
These windings are connected at the ends with each other (star-connection). At the other ends a three-phase (AC) current develops at first. Therefore, each of these ends is connected to the on-board electrical
system via a minus- and plus performance-diode. A DC current with as little pulsation as possible is produced.
Basically, there are two methods of regulating the current flow to the field coil-windings. In the customary method a regulator, together with the alternator and the on-board electrical system forms a control circuit. The
actual value is taken from the output D+. Should the tension here exceed a certain value, the current to the field coil windings is, with a slight delay, switched off through DF. In the range of milliseconds, the current to
D+ now drops, causing the current in the rotor windings to switch on again. Thus, it goes to and fro, and can be seen on the oscilloscope as a THD (total harmonic distortion).
The current for the field coils can be drawn, up to max. of 5A, through three additional diodes (exiter diodes). Newer systems get by without this by intergrating the voltage regulation into the
energy management. Since the customary regulator ignores the condition of the battery, this could cause a maintenance-free battery to gas, which doesn't do it any good. Sometimes, e.g., in thrust operation, the
alternator should indeed, be stressed to it's maximum. The energy management is more important for the preservation of the starting readinesss than for the economy.
Nowadays, alternators are almost always driven by broad drive-belts and with higher performance output, are liquid cooled
(pictures above), in top-of-the-range models it is
also for reasons of noise reduction. Previously the fan-wheel was placed immediately behind the belt-drive. It is the only direction-bound part with arrow. Nowadays, there may be two, and they are placed
inside, directly in front of the heat-producers, one of them is directly in front of the diode plate.
Disconnecting the battery when the engine is running endangers the semiconductor components and the control devices. 01/10
The principle of generator - magnetic field direction - direction of movement - electricity customer - direction of electron flow direction Lorentz force - if it does exactly, should vary the length of the red arrow.