Basic principles

7.28.2 Basic principles

When the alternator voltage is less than the bat- tery (engine slow or not running for example), the direction of current flow is from the battery to the vehicle loads. The alternator diodes prevent current flowing into the alternator. When the alternator output is greater than the battery volt- age, current will flow from the alternator to the vehicle loads and the battery.

It is clear therefore, that the alternator output

Figure 7.67 Bosch compact alternator

voltage must be above battery voltage at all times when the engine is running. The actual voltage

The main component of the charging system is used is critical and depends on a number of the alternator and on most modern vehicles, with

factors.

the exception of its associated wiring, it is the only The main consideration for charging voltage component in the charging system. Figure 7.67

is the battery terminal voltage when fully charged. shows an alternator in common use. The alternator

If the charging system voltage is set to this value generates AC but must produce DC at its output

then there can be no risk of overcharging the terminal as only DC can be used to charge the bat-

battery. This is known as the constant voltage tery and run electronic circuits. The output of the

charging technique. The figure of 14.2 ⫾ 0.2 V alternator must be a constant voltage regardless of

is the accepted charging voltage for a 12 V engine speed and current load.

system. Commercial vehicles generally employ The charging system must meet the following

two batteries in series at a nominal voltage of criteria (when the engine is running):

24 V; therefore the accepted charge voltage ● supply the current demands made by some or

would be doubled. These voltages are used as all loads;

the standard input for all vehicle loads. For the

Engine systems 165

purpose of clarity the text will just consider a

A further three positive diodes are often

12 V system. included in a rectifier pack. These are usually The other areas for consideration when deter-

smaller than the main diodes and are only used to mining charging voltage are any expected volt

supply a small current back to the field windings drops in the charging circuit wiring and the oper-

in the rotor. The extra diodes are known as the ating temperature of the system and battery. The

auxiliary, field or excitation diodes. voltage drops must be kept to a minimum but it is

When a star wound stator is used the addition important to note that the terminal voltage of the

of the voltages at the neutral point of the star is in alternator may be slightly above that supplied to

theory 0 V. In practice, however, due to slight inac- the battery. Figure 7.68 shows the basic principle

curacies in the construction of the stator and rotor of an alternator.

a potential develops at this point. By employing two extra diodes, one positive and one negative connected to the star point the energy can be col-

lected. This can increase the power output of an In order to full wave rectify the output of a three

7.28.3 Rectification of AC to DC

alternator by up to 15%.

phase machine six diodes are needed. These are Figure 7.69 shows the full circuit of an alter- connected in the form of a bridge, which consists

nator using an eight diode main rectifier and three of three positive diodes and three negative diodes.

field diodes. The voltage regulator, which forms The output produced by this configuration is

the starting point for the next section, is also shown compared to the three phase signals.

shown in this diagram. The warning light in an alternator circuit, in addition to its function in warning of charging faults, also acts to supply

the initial excitation to the field windings. An alternator will not always self-excite as the resid- ual magnetism in the fields is not usually enough

Stator

to produce a voltage which will overcome the 0.6

Rotor

or 0.7 V needed to forward bias the rectifier diodes. A typical wattage for the warning light

bulb is 2 W. Many manufacturers also connect a

Three phase

AC output

resistor in parallel with the bulb to assist in exci- tation and allow operation if the bulb blows. The charge warning light bulb is extinguished when the alternator produces an output from the field diodes as this causes both sides of the bulb to take on the same voltage (a potential difference

Figure 7.68 Basic alternator principle

across the bulb of 0 V).

Figure 7.69 Complete internal alternator circuit

166 Advanced automotive fault diagnosis