Alternator
The alternator is a dynamo used to convert fuel energy into electrical energy as the car motor operates. It is designed to keep the car battery fully charged. It is not designed to charge flat batteries. A drive belt, drive-chain or gear set turns the alternator rotor as you drive. The rotor turns within a stationary stator housing. Inside the alternator, a series of insulated copper windings and a set of powerful electro-magnets end up moving relative to one another. This induces a varying magnetic field within the coils. This magnetic field fluctuation converts into an alternating electrical current used to power the battery and cover the car electrical demand.
The rotor normally operates as an electro-magnet and receives the power to generate an electro-magnetic field from electrically conductive brushes contacting a slip-ring Only when the electro-magnet is switched on can the alternator produce electrical current. This allows a voltage regulator to control battery charging.
Rectifiers
Alternating current changes from positive voltage to zero to negative voltage as the rotor turns within the stator unit. If left in this form it would equally charge and drain the battery making it useless. So, this problem needs to be rectified if the electrical current is to charge the battery and supply the electrical power needed to operate the car.
This is the job of the rectifier.
The rectifier is a series of one-way diodes that allow either positive or negative voltage to pass through. An alternator normally contains 3 sets of windings and each produces its own independent alternating electrical current. The rectifier has a positive passing and negative passing diode pair for every coil in the alternator. The diodes are set up to send a positive charge to the positive battery terminal and negative charge to the negative battery terminal.
Voltage Regulator
The voltage regulator usually switches the magnetic field on at about 13.5 volts and off again at 14.5 volts to maintain a usable 14V.
Without something to control the amount of electrical energy passing into the battery, the battery would overcharge. The voltage regulator monitors the voltage or stored energy within the battery and cuts off the power controlling the electro-magnetic field to the alternator rotor when the battery is fully charged.
Self Excited Alternators
Self-excited alternators have excitation diodes fitted in the rectifier. Turning on the ignition feeds a small flow of electricity around the rotor to form a small electro-magnetic field. As the alternator starts to turn it begins generating a small electrical output that can be fed back through the excitation diodes to increase the electro-magnetic field strength generating more electrical power until within a fraction of a second the alternator is rotating at full power.
Battery Excited Alternators
The rotor receives electricity directly from the battery so battery excited alternators create the full electro-magnetic field as soon as the ignition is turned on.
Canbus Control
The car ECU has a Control Area Network (CAN) that replaces the regulation part of the rectifier system with a computer controlled Smart Charge System.
Alternator Electrical Faults
Modern alternators are now usually controlled by the ECU and can require advanced computer diagnostic tools and specialised CANBUS knowledge to identify and rectify the cause of many alternator electrical faults and battery charging problems.
Alternator Drive Belt Failure
Alternators are usually turned by a drive belt from the engine. Drive belt or drive belt bearing failure is a common cause of alternator failure on older car models. Modern cars are often fitted with a non-adjustable elasticated polymer drive belt.