CHARGING SYSTEM
Rectification
In charging system we use diodes in rectification
Diodes
A diode allows current to flow in one direction but not in the opposite
direction. Therefore, it can function as a switch, acting as either conductor or
insulator, depending on the direction of current flow. In an AC generator,
current is rectified (changed from AC to DC) through the use of diodes. The
diodes are arranged so that current can leave the generator in one direction
only (as DC). A variation of the diode is the zener diode
Zener diode;
This device functions like a standard diode until a certain voltage is reached.
When the voltage level reaches this point, the zener diode allows current to flow
in the reverse direction. Zener diodes are often used in electronic voltage
regulators. DC Rectification
Battery reverse flow when the engine is stationary, and at other times when
the alternator output voltage is lower than the battery voltage, current would
tend to flow from the battery to the alternator and discharge it. Whereas the
old dynamo charging system needed a cut-out switch in the main line to
prevent this current drain, the one-way characteristic of the diodes in an
alternator rectifier automatically prevents this happening. This inherent
feature means that the main output terminal on the alternator is subject to full
battery voltage. Therefore, when the alternator is either disconnected or
removed it must be remembered that this connection is ‘live’
VOLTAGE REGULATION
The output from an AC generator can reach as high as 250 volts if it is not
controlled. The battery and the electrical system must be protected from this
excessive voltage. Therefore, charging systems use a voltage regulator to control
the generator’s output. Voltage output is controlled by the voltage regulator as it
varies the strength of the magnetic field in the rotor. Current output does not
need to be controlled because an AC generator naturally limits the current
output. To ensure that the battery stays fully charged, most regulators are set for
a system voltage between 14.5 and 15.5 volts. Voltage output is controlled by
varying the field current
through the rotor. The higher
the field current, the higher the voltage output. By controlling the amount of
resistance in series with the field coil, control of the field current and voltage
output is obtained.
Factors Controlling Generator Output Several factors determine the total output
available from a generator other than the type of stator winding. These include:
■ The rotational speed of the rotor. Higher speeds can lead to higher output.
■ The number of windings in the rotor. Increased windings will increase output.
■ The current flow through the rotor windings. Increased current through the
rotor will increase output.
■ The number of windings in the stator. An increase in the number of windings
will increase output.
Tests carried out on the alternator parts
Continuity
Results
1. if the ohmmeter shows any resistance
Then there is continuity. But if no reading is seen then
that wire has open an open circuit (broken)
a). With diodes current should move only in one
direction.
Therefore if the reading is seen only in one direction then the
diodes are good but if the reading is not seen both then the
that particular diode has an open circuit
shorting
2. Reading should not be seen on the ohmmeter in
any case if seen.
Then the stator or rotor is shorting
1. When reading is seen in both directions then the
diode is shorting, NB For the good diode, one side
should show infinity while the other side registering
ohms.
