How a Alternator works

- a simple explanation?

  1   switch ON

When you switch 'on' a red lamp on the dash board lights, providing current throught the lamp to the alternator rotor. This current in the rotor establishes a (small) magnetic field across the rotor.

  2   ROTATION

As the engine spins the rotors magnetic field passes the stator windings such that the stator windings see a north pole followed by a south pole repedativly. This induces a voltage in the stator windings that similary changes from positive to negative. [Note that the induction only occures when the field changes.]

  3   RECTIFICATION

This stator voltage is sent through the rectifier that connects the negative voltage from the stator to the battery negative, and the positive voltage from the stator to the battery positive. The rectifier only performs this connetion when the alternator voltage exceeds the battery voltage. Once connected the alternator starts to supply current to the battery.
The amount of this current is proportional to the current in the rotor. [The regulator requires an excess of about 0.7 volts before it will pass significant current. This voltage drop times the current going through the rectifier produces power that is lost through heat. In other words it will get hot, hotter with the more work it does.]

  4   REGULATION

The voltage regulator limits the battery voltage by reducing the rotor current. The dash board lamp can only supply a small current, not enought to realise the full potential of the alternator. So some extra diodes in the rectifier take some of the output power of the stator to supply rotor current via the regulator. This extra diode supply also enables the red dash board lamp to turn off.

      Faults

  Sympton 1 Red dash board lamp not lit at switch on. The oil lamp is lit.
If both lamps are out, then you probably have not switched the bike on!!! Check the kill switch, ignition switch or fuses.

For bmw airhead riders - probably an open circuit rotor.   :(
Others - Open circuit lamp
bad connection from lamp to regulator, or rotor.
regulator or diode board faulty.

     Fault finding for airhead owners

Turn the engine off, and remove the front engine cover to reveal the alternator and rectifier. This is the least amount of work for the maximum axcessablity that you can do.

red dash board lamp not lit at switch on.
Do this	Check This reaction	If reaction is true this is the fault
Short both rotor wires together	Lamp lights	Rotor or brushes open circuit
Short both rotor wires to ground	Lamp lights	Rotor wire to ground is open
Put both rotor wires back on brushes
Disconnect Blue (small) wire to Diode Board	Lamp lights	Diode Board Faulty. Rare! This fault would cause large currents to flow in the rotor.
Reconnect Blue (small) wire to Diode Board
Short Blue (small) wire on Diode Board to Ground	Lamp lights	Wire to rotor is open cct. Or regulator is faulty, Remove regulator and place a short from the regulator's blue wire terminal to both the other regulator terminals in turn. If lamp lights in both cases then the regulator is faulty. If not then the respecive wires need to be checked.
Replace lamp with working lamp (oil or neutral lamp?)	New Lamp lights	Old lamp faulty
Check battery voltage on lamp	No battery voltage	Open cct to battery, check wires/switches/connectors to battery using voltmeter
Check Battery voltage on Blue wire at Diode Board	No voltage	Open cct to lamp, check wires/switches/connectors using voltmeter

  Sympton 2 Red dash board lamp does not go out with engine spining

rectifier failure
rotor shorted
shorted wire on rotor supply
regulator failure

     Fault finding for airhead owners

Turn the engine off, and remove the front engine cover to reveal the alternator and rectifier. This is the least amount of work for the maximum axcessablity that you can do.

red dash board lamp always lit
Do this	Check This reaction	If reaction is true this is the fault
Remove both wires from the rotor	Lamp goes out	Rotor or brushes shorted to ground
Disconnect Blue (small) wire from diode board	Lamp goes out	Diode Board has short.
Reconnect Blue (small) wire to diode board
Remove regulator	Lamp still lit	short in wire from lamp to regulator/diode Board
Replace regulator	Lamp lights	Regulator faulty
Put both rotor wires back on

 

KEY
cct   Circuit
Short cct   Circiut is connected by 0 (or close to 0) ohms
Open cct   Circuit is disconnected by infinite (or close to infinite) ohms
Ground     Chassis, Earth or battery negative terminal. All the same really.
Switch 'ON'   Turn ignition 'on' so that the bike would normally start if you pressed the starter button.

I have assumed that only one fault exists, if you have two faults at the one time ... well your in trouble. Best to fix faults as they occure! Or you have some bad karma. Either way you will need mre help than I have provided here.