Self-control for maximum output current
A characteristic of the alternator is that the output current flows almost constant when it exceeds a certain speed (self-control operation).
Therefore, when applying a large load exceeding the maximum output current, the voltage generated drops. Another characteristic of the alternator is that output current is smaller when heated than when cold because the resistance value in each component changes depending on the temperature even when the speed does not change.
If the v-ribbed belt slips, the alternator speed is lower than usual and electricity generated decreases, causing the battery to go dead.
If the electricity consumed is larger than that generated, the electricity charged in battery is consumed, causing the battery to go dead.
When it turns at a low speed (when engine idles), the electricity generated is small. Therefore, when many electric devices such as heater and headlight are operating, electricity of the battery is consumed. If this condition continues for a long time, battery could go dead.
The Alternator consists of the following components.
Pulley with one-way clutch
2.Drive end frame, rear end frame
The end frames have two functions: to support the rotor and to act as a mount to the engine. Both end frames have several air passages to improve cooling efficiency.
The stator is pressure-fitted to (integral with) the drive end frame. The rectifier, brush holder, IC regulator, etc., are bolted to the back of the rear end frame.
6.Rear end frame
11.Rear end cover
(1)Rotor is a turning magnet inside the stator coil that produces the magnetic field to generate an electromotive force in the stator coil. The coil is wound around six pairs (12 poles) of pole core (magnetic pole) and it becomes electromagnetic when current flows.
As the current flowing into the rotor becomes larger, the electromagnetic force becomes stronger.
(2)At both sides of the rotor, a fan is installed to cool the rotor coil, the stator coil and the rectifier to make them below the limit temperature by sucking the air from the ventilation of the end frame by rotor’s revolution.
2.Brush and slip ring
(1)These parts generate magnetic field by making current flow into the rotor coil, and are installed at the back end of the rotor.
(2)Generally, a brush made of metal graphite is used to decrease the electric resistance and contact resistance, and the brush also resists wear well.
(1)Stator generates three-phase alternating current by the change of the magnetic flux caused by the rotor revolution. The stator consists of stator core and stator coil and is built into the drive end frame as a unit.
Winding of stator coil
Stator coil consists of three pairs of coils. The point where three ends meet is called neutral point.
(2)Since stator generates much more heat than the other components in the alternator, the heat-resisting shield is used to wire the coils.
(1)Rectifier performs the full-wave rectification to convert the three-phase alternating current generated at the stator coils into direct current using six diodes (or eight diodes with neutral-point diodes).
(2)Rectifier consists of terminal (output terminal), holder fin (heat-releasing fin) and diodes, and the two-layer construction of the holder fin improves the heat radiation and downsizes the rectifier.
The diode used for rectification produces heat when current flows. However, because element of diode itself (semi-conductor) is weak against heat, heating causes the insufficient rectification function. Therefore, holder fin (heat-releasing fin) is required to have large area as much as possible to release heat.
(1)Construction of IC regulator
IC regulator mainly consists of hybrid IC, heat-releasing fin and connector.
A smaller size is achieved by using hybrid IC.
(2)Types of IC regulator
Battery sensing type
This type of IC regulator detects the battery voltage by terminal S (battery sensing terminal) and regulates the output voltage to the specified value.
Alternator sensing type
This type of IC regulator detects the inner voltage of the alternator and regulates the output voltage to the specified value.
(3)Function of IC regulator
<1>IC regulator has the following functions.
Warning when the alternator stops generating and charging condition is abnormal.
<2>IC regulator warns by lighting up the charge warning light when the following problems are detected.
Open or short in the rotor coil
Disconnection of terminal S
Disconnection of terminal B
Overvoltage (battery voltage rises by short between terminal F and terminal E)
(4)Characteristics of IC regulator
Battery load characteristics
There is little or no change in output voltage (no more than 0.1 to 0.2 V) with changes in alternator speed.
External load characteristics
Output voltage becomes lower as the load current increases. Voltage variation, even at the rated load, or maximum output current of the alternator, is between 0.5 V and 1 V.
If a load exceeding alternator’s capacity is applied, the output voltage will suddenly drop.
The output voltage generally becomes lower as the temperature rises.
Since the output voltage drops at high temperature (e.g. in summer) and rises at low temperature (e.g. in winter), proper charging suited to the battery’s characteristics is performed at all times.