Vehicle Alternator Function :

The automotive alternator is an electricity generation plant which provides power to recharge one or more vehicle batteries as well as supplying electricity for vehicle electrical systems such as lighting, wipers, heater motor, car audio ,sat nav, etc whilst the vehicle engine is running.

Vehicle alternators generate AC (Alternating current ) converted to DC (direct current) by the alternator rectifier (diode pack)

How Alternators Work

The vehicle alternator is normally engine driven via an auxilliary drive belt or sometimes direct drive and works by the principle of Magnetic induction ; When a conductor such as a wire moves through the lines of force of a magnetic field, a voltage is induced in the conductor. Whether the conductor moves in relation to the magnetic field or vice versa the principle is the same.

Alternator Rotor

The Alternator Rotor consists of a central shaft, coil winding and magnetic claw poles. The rotor assembly is supported on bearings. The alternator rotor and bearings rotate as it is driven from the engine.
Excitation current is passed to the rotor via copper slip rings which causes the electromagnet alternator rotor winding to magnetise when energised creating a magnetic field.

The alternator rotor may fail due to open or short circuit in the winding or break down of insulation. A bent or misaligned rotor shaft could cause vibration. Worn alternator bearings can also cause noise and vibration.

Alternator Excitation : Power is supplied to EXCITE the rotor to create a magnetic field. (excitation current is required to create a magnetic field unless permanent magnets are used ) Some alternators are SELF-EXCITED, this means after the rotor is initially excited a wire connected to the stator taps some of the power generated to keep the rotor excited and power generation in progress whilst the rotor is spinning. The brush pack or brushes form the means of connecting the spinning rotor copper slip rings to the  excitation power source. Worn brushes and worn slip rings can prevent excitation current passing to the rotor causing the alternator to fail to charge.

Alternator Stator

The stator normally consists of three windings of reasonably heavy gauge copper wire connected in Star or delta formation around a soft iron laminated core sandwiched between the alternator housing. Stator windings may be connected to the rectifier pack by soldering or on higher output applications by laser or resistance welding. The electrical power output of the alternator is generated in the stator as a 3 phase alternating current (a.c.)

Alternator Voltage Regulation

The alternator voltage regulator functions to limit alternator output voltage to prevent overcharging of vehicle batteries which would cause electrolyte to boil off and cell failure (the rotten egg smell). Excessive voltage also causes havoc with electronic control units ,motor speed regulation, and bulbs to blow.

Control of the alternator rotor excitation current proportionally controls magnetic field strength. Excitation field power control is used to regulate alternator output by means of the Voltage regulator.
With ECU regulated systems, the duty cycle of the excitation current can be reflected as a feedback signal to measure the amount of alternator capacity being used via the DFM signal terminal (DF Monitor).

Rectifier Diodes

A diode acts like a non return valve in an electrical circuit allowing current to flow one way.

The initial 3 phase alternating current (a.c.) output produced from the alternator stator cannot be used to recharge vehicle batteries so has to be converted to direct current (d.c.). The process of converting AC power to DC power is known as rectification. The rectification is done by the Diode pack (a diode acts like a one way valve in an electrical circuit). Car alternators are Three Phase AC generators with rectification to DC.
The DC voltage level produced by the alternator must be slightly higher than battery voltage to allow the battery to charge.
The Diode pack has a secondary function when the ignition key is off to prevent power draining from the vehicle battery via the alternator windings which would cause the battery to discharge.

The diode bridge is connected to stator wires. Jump starting, reverse polarity, and voltage spikes can cause alternator diode failure leading to the alternator not charging. Diode failure can be diagnosed by oscilloscope signal testing of the alternator with the engine running , or by multimeter diode test function. Diode packs can be supplied seperately.

Basic Vehicle Alternator Maintenance

With engine stopped - Ensure:
Exterior of alternator remains clean and dry.
Ensure the alternator is mounted securely and drivebelt correctly tensioned.
Ensure electrical terminals are clean and tight.
With ignition key on , battery charge warning light should illuminate.
With engine running battery charge warning lamp should go off.

Alternator translated : Alternateur Alternador Alternatore Dynamo Generatoren Lichtmaschine 发电机 алтернатор