Yamaha EF4500iSE Theory & Diagnostics Manual page 43

Generator Diagnostics
That DC current then flows through the Main Field
Coil, the last section of the rotor, and turns it into a
north and a south pole magnet.
The magnet spinning inside the sub-coil will then
produce about 4-6 volts AC in the sub-coil.
That AC current is fully rectified by diodes within the
AVR circuit board. This increased DC current then
flows back through the Exciter Field Coil, Exciter
Armature Coil, Main Field Coil and Sub-Coil, which
causes their voltage to rise and in turn causes the
voltage in the Main Coil to rise to the rated output.
There are only three extra components in this type
of generator. They are the AVR circuit board, Exciter
Field Coil and the Exciter Armature Coil.
The AVR circuit board makes it possible to hold a
more constant voltage even if there is some fluctua-
tion in the engine RPM.
This is achieved by two voltage detecting taps on
the main stator coil that are connected to the AVR
board.
When the voltage in the main stator coil reaches the
rated output, but is about to exceed it because of
increasing engine RPM, the AVR board detects that
voltage increase and reduces the amount of output
voltage from itself to the exciter field coil.
The voltage reduction to the exciter field coil in turn
reduces it's output voltage through the rotor and
thus reduces the voltage from the main coil. This
process is continuously repeated to maintain a con-
stant voltage.
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