Charging system operation (Electrical)

CHARGING SYSTEM OPERATION

The charging system is an alternator that contains a rectifier pack and regulator to maintain a constant direct current (d.c.) voltage in the system. The alternator has a fixed coil wound stator in which a field coil rotor rotates. Slip rings conduct current to and from the field coils via two carbon brushes. The unit is machine sensed, the regulator senses output voltage and regulates this to a maximum of 14 volts. The alternator is belt driven from the crankshaft and cooled by a fan mounted behind the pulley.

When the ignition is switched on, a small current flows through the ignition warning light then to the field windings, partially magnetising the rotor and then passes to earth via the brushes and regulator. The warning light circuit is complete and the bulb glows. When the engine is started, the magnetised rotor turns within the stator windings generating 3 - phase alternating current (a.c.) and voltage that rises rapidly with rotor speed. The rotor produces a.c. by virtue of the magnetic field of the rotor relative to the stator. The field diodes in the rectifier pack convert the full wave a.c. current into d.c. Output current from the field diodes supplements the initial current flowing through the field windings, causing an increase in the magnetic influence of the rotor resulting in self - excitation of the alternator. The field current increases with rotor speed and thus increases generated current and voltage until the alternator is fully excited.

When the voltage applied to the alternator side of the warning light exceeds battery voltage the warning light is extinguished indicating that the alternator is developing battery - charging current. The regulator functions as an electronic control switch on the earth side of the field coils, rapidly switching the earth circuit OFF and ON to maintain the maximum voltage and thus the current to safe limits.

When the battery is in a low state of charge or the current draw from electrical units causes voltage drop, the alternator automatically charges at its maximum rate (dependent on speed) until 14 volts is reached. When the demand on the alternator falls,the current output is reduced.

STARTER MOTOR OPERATION

The motor is a four pole series wound four brush unit with a moulded commutator on the end of the armature. The field windings are continuous wound, one end is connected to the yoke while the other end is attached to one pair of brushes. The remaining pair of brushes are connected to the main feed terminal. The brush assembly is fully insulated.

Engine cranking is by pre - engaged starter motor, engagement with the ring gear is controlled by a solenoid operated switch and the drive gear includes a one - way clutch to prevent armature overspeeding when the engine fires. When the ignition switch is turned to position THREE the starter solenoid, which contains two sets of contacts, is energised and moves its plunger to fully engage its pinion with the gear ring. The two sets of contacts close simultaneously connecting all four field coils to the battery. Should tooth to tooth abutment occur, the solenoid plunger compresses the driver engagement spring inside the plunger which causes the first stage of the contacts to close and connect one field coil to the battery. The starter armature will now turn slowly and pressure of the drive engagement spring, combined with the driver helix, causes the pinion to move into full engagement and the second stage contacts close. The remaining three field coils are now connected to the battery. When the solenoid is switched off, the drive gear retracts and opens the solenoid switch.