Design

Design

Generator (GEN) design





The main components of the generator (GEN) consist of:
1. Stator
2. Rotor with slip rings
3. Integrated cooling fans
4. DC bridge
5. Charge regulator
6. Pulley (with, in certain cases, freewheel)

Generator terminals





Generator (GEN) terminals:
- #B:1. To engine control module (ECM).
- #B:2. Not used
- #A:1. B+
For 6 cylinder engines the generator's connectors are axially positioned, straight out from the rear end.

Stator




The stator is fixed and consists of grooved plates which are insulated internally and pressed together onto a fixed plate unit. The stator winding coils are positioned in the grooves. They are delta connected at 120° and provide a three phase alternating current to the rectifier bridge.

Rotor with slip rings




The rotor consists of two halves (claw-poles) which interlock. The halves are pressed onto the rotor shaft. There are twelve claws on the rotor, i.e. twelve poles. One half consists of six north poles and the other half six south poles. The excitation winding is secured on the rotor shaft between the claw-pole halves. The excitation winding (also known as rotor winding) consists of a circular coil surrounded by the claw-poles and connected to the slip rings. The charge regulator supplies a magnetic current through carbon brushes positioned against the slip rings. The higher the current in the rotor the stronger the magnet field of the rotor, and therefore the higher the current generated in the windings of the stator.

Cooling fans




The heat generated in the generator is, in principle, proportional to the current generated and must be directed away to prevent damage to the insulation and diodes. The generator (GEN) is therefore air cooled and equipped with two integrated cooling fans on the rotor shaft.

Rectifier




Alternating current (AC) is created in the generator stator windings. This must be converted to direct current (DC) before it can be used in the electrical system of the car. This conversion is made using a rectifier bridge which consists of six diodes, two diodes per phase winding.
The stator windings generate three phases and are delta connected An exciter diode is also connected to each of the three stator windings. The diodes are pressed into a diode holder.

Charge regulator (Alternator control module (ACM))

General




The generator (GEN) has a built-in charge regulator at the rear (also known as the alternator control module (ACM)). The regulator can be replaced. The carbon brushes are integrated on the slip rings on the rotor on the regulator.
For 6 cylinder engines the generator's connectors are axially positioned, straight out from the rear end.

Charge voltage
At room temperature a fully charged battery cell produces 2.12V. A 12V battery has 6 cells and therefore produces 12.72V when the battery is fully charged. The battery has an internal resistance which must be exceeded before charging can take place. At room temperature 0.2 V is required per cell, or 1.2 V for the entire battery. Therefore to charge a battery at room temperature, 13.92 V (12.72 V+1.2 V) is required. In cold conditions, the chemical reactions take place at a slower rate and the internal resistance increases. This requires a higher voltage during charging to exceed the internal resistance.
The charge regulator, depending on how it is controlled by the engine control module (ECM) and central electronic module (CEM), regulates the output voltage so that the battery receives optimal charge.

Charge regulator control




The charge regulator (also called the alternator control module (ACM)) is connected to the engine control module (ECM) and communicate via LIN communication.
The engine control module (ECM) then communicates with the central electronic module (CEM) via the controller area network (CAN) to regulate the generator (GEN).
The central electronic module (CEM) has internal functions to regulate the voltage level and the power consumption in the vehicle. The central electronic module (CEM) controls the charge regulator via the engine control module (ECM) and therefore the current/voltage generated by the generator (GEN).
The generator does not initially charge when the engine is started. The charge increases successively when the engine has been started. When there is an increase in load at the generator, the increase from 0 - 100% takes place over the course of a few seconds (this time varies slightly depending on the operating mode the engine management system is in). This is to gradually increase the load on the engine during the start-up phase and to ensure the engine starts.
If the engine speed (RPM) exceeds idle speed (the exact engine speed varies slightly depending on the mode the engine management system is in) during this delay, or if the progressive increase is interrupted, full charge is obtained immediately.
If there is no communication with the charge regulator, the charge regulator will not start charging on start-up. The charge regulator can however self magnetize the rotor and start charging. This occurs at engine speeds greater than approximately 2100 rpm. There is no charge engagement by stages with self magnetization, the generator operates at full charge immediately.
When self magnetizing has begun, the generator (GEN) also charges at engine speeds below 2100 rpm.

Freewheel




Certain generators are equipped with a freewheel between the rotor shaft and the pulley. A pulley equipped with a freewheel, can be identified by the cover installed over the pulley nut in the center. Using the freewheel, the generator rotor shaft can only rotate freely in one direction. This minimizes any jerking in the belt transmission. For V8 engines, the generator is not equipped with freewheel.