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 (GEN) terminals:
- #A:1 (B+)
- #B:1 (L)
- #B:2 (DF), only diesel engines D5252T.
On D5252T diesel engines the wiring for the L signal and DF signal are mounted in a common two pin connector.
Ground terminal via the cylinder block.
Stator
The stator is secured and consists of grooved plates which are insulated and pressed onto a secured 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 DC bridge.
Rotor with slip rings
The rotor consists of two halves (claw-poles) which engage each other. 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 winding which is connected to the slip rings and surrounded by the claw-poles. The charge regulator supplies a current to the rotor winding through the brushes which lie against the slip rings. The greater the current in the rotor the stronger the magnetic field in the rotor becomes, and thus a greater current is generated in the stator windings.
Cooling fans
The heat distributed in the generator (GEN) is, in principle, proportional to the current generated and must be directed away so that the insulation and diodes are not damaged. The generator (GEN) is therefore air cooled and equipped with two integrated cooling fans on the rotor shaft.
Rectifier
The current created in the generator (GEN) stator winding is AC and must be converted to DC before it can be used in the electrical system of the car. The conversion (rectifying) occurs using a DC bridge consisting of six diodes, two diodes per phase windings.
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
General
The charge regulator is mounted at the rear edge of the generator (GEN). The role of the charge regulator is to constantly maintain the generator current at engine speeds, depending on the load and speed of the generator.
If the voltage produced exceeds the preset desired value depending on the load, the charge regulator reduces or cuts the magnetic current to the rotor. This, in turn, lowers the magnetic field. The voltage from the stator windings decreases. When the voltage is below the desired value, the current increases through the rotor so that magnetization increases as does the generator voltage until the desired value is exceeded again. This process is repeated continuously.
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 overcome for charging to occur. At room temperature 0.2 V per cell or 1.2 V for the entire battery is required. 13.92 V (12.72 V+1.2 V) is required to charge a battery at room temperature. In cold weather conditions the chemical reactions are slower and the inner resistance becomes higher. This requires a greater voltage when charging to overcome the internal resistance.
The charge regulator controls the output voltage, depending on the engine compartment temperature, following a predetermined curve, see the illustration.
The charge regulator calculates the engine compartment temperature using integrated semi-conductor electronics. In cold conditions the charge regulator allows a higher voltage. At higher temperatures the voltage is lowered. This ensures full battery charging regardless of the temperature. A battery must have a greater charge voltage at lower temperature for charging to occur.
L-signal for the central electronic module (CEM)
The charge regulator is connected to the central electronic module via an L-signal.
The L-signal is used to pre-magnetize the generator rotor and to transmit signals to the central electronic module telling the central electronic module that the generator is charging or not when the engine is running.
When the engine is starting, the generator (GEN) does not charge. When the engine speed (RPM) passes approximately 550 rpm a delay of approximately 3 seconds occurs. After the delay a successive increase of charging begins. When the generator (GEN) is at full load the increase ranges from 0-100% for a period of 6 seconds.
If the engine speed (RPM) passes approximately 1300 rpm during the delay or the successive increase breaks the increase, full charging is obtained immediately. This is to successively increase the load on the engine during the start-up phase and to ensure the engine starts.
If the L-signal is missing the generator will not start charging on start-up. Charge regulator can self magnetize the rotor and start charging in this way. This only happens at speeds in excess of approximately 2300 rpm. There is no successive charge engagement with self magnetization, the generator operates at full charge immediately.
DF-signal to the engine control module (ECM)
For diesel engine D5252T the charge regulator also has a DF-connection which is connected to the engine control module (ECM). The signal transmits information to the control module about the current output voltage from the generator. When charging, there is a pulsed signal between Ulow (approximately 0 V) and Ubat (approximately 12 V) with a secure frequency and variable pulse ratio. The pulse ratio changes when charging increases or decreases. The control module uses this signal to control engagement and disengagement of the glow plugs for the engine coolant heater.
For further information, see Design and Function, Diesel engines.
Freewheel
Certain generators are equipped with a freewheel between the rotor shaft and the pulley. The cover over the pulley nut in the center of the pulley indicates whether a freewheel is installed. Using the freewheel, the generator rotor shaft can only rotate freely in one direction. This means that any jerking in the belt transmission is minimized.