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.
Generator terminals
Generator (GEN) terminals:
- #A:1. B+
- #B:1. To central electronic module (CEM) or engine control module (ECM).
Alternator structure, engine B8444S
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.
Alternator connections, engine B8444S
Generator (GEN) terminals:
- #A:1. B+
- #B:1. To engine control module (ECM)
- #B:2. Not used
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 connected to each of the three stator windings. The six rectifier diodes are bridge connected. The diodes are pressed into a diode holder.
Charge regulator (Alternator control module (ACM))
General
For B8444S
The generator (GEN) has an inbuilt 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.
Charge voltage
At room temperature a fully charged battery cell produces 2.12 V. A 12 V battery has 6 cells and therefore produces 12.72 V 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 central electronic module (CEM), regulates the output voltage so that the battery receives optimal charge.
If the charge regulator is connected to the engine control module (ECM), it will use the commands from the engine control module (ECM) and central electronic module (CEM) to regulate output voltage so that the battery is charged optimally.
Charge regulator control
The above illustrations depict the two different types of connections for the alternator's regulator.
Depending on the model year and type of engine management system, the charge regulator (also known as the alternator control module (ACM)) is connected to either the central electronic module (CEM) or the engine control module (ECM). It communicates via LIN communication. For information on connection, see the illustration above and the wiring diagram.
If the charge regulator is connected to the engine control module (ECM), the engine control module (ECM) then communicates with the central electronic module (CEM) regarding alternator control via the controller area network (CAN).
The central electronic module (CEM) has built-in functions for regulating voltage level and current consumption in the vehicle. The central electronic module (CEM) controls the charge regulator either directly or via the engine control module (ECM). In this manner, it also controls the current/voltage produced by the alternator.
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
Some alternators are fitted with freewheels between the rotor shaft and the pulley. However, it is possible to see that the pulley is fitted with freewheels from the cover fitted over the pulley's nut in the centre. The freewheels allow the alternator's rotor shaft to rotate freely in one direction. This keeps any jerking in the belt transmission to a minimum. For engine B8444S, the alternator is not fitted with freewheels.