4
SECTION 2
PRINCIPLE OF OPERATION
The permanent magnet generator provides power for
excitation of the exciter field via the AVR which is the
controlling device governing the level of excitation provided
to the exciter field. The AVR responds to a voltage sensing
signal derived, via the isolating transformer, from the main
stator winding. By controlling the low power of the exciter
field, control of the high power requirement of the main field
is achieved through the rectified output of the exciter
armature.
The simple control principle described above hides the
complexity of the electronic AVR which drives it. The AVR
senses true rms voltage on three phases ensuring close
regulation even with a generator output distorted by non-
linear load e.g. thyristor controlled DC drive. In addition it
detects engine speed and provides variable levels of voltage
fall off with speed, below a pre-selected speed (Hz) setting,
preventing over-excitation at low engine speeds and
softening the effect of load switching to relieve the burden on
the engine.
It also provides over-excitation protection during fault
conditions, and over-voltage protection, with provision for
operating an external circuit breaker, and additional
adjustable elements are incorporated to shape and optimise
the heavy load switching capability of the generating set, by
varying the generator performance during load switching to
match the engine performance.
The detailed function of the AVR circuits and their
adjustment are covered in the load testing section 4.7.
In addition the AVR incorporates circuits which, when used
in conjunction with accessories, can provide for parallel
operation either with 'droop' or 'astatic' control, short circuit
current limit, and VAr/PF control, during parallel operation.
Function and adjustment of the accessories which can be
fitted inside the generator terminal box are covered in the
accessories section of this book.
Separate instructions are provided with other accessories
available for control panel mounting.
