PI Manufacturing AC generators - Manual (Page 7)

SECTION 2
PRINCIPLE OF OPERATION
2.1 SELF-EXCITED AVR CONTROLLED
The main stator provides power for excitation of the exciter
field via the SX460 (SX440 or SX421) 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 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 SX460 or SX440 AVR senses average voltage on two
phases ensuring close regulation. In addition it detects engine
speed and provides 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.
The SX421 AVR in addition to the SX440 features has three
phase rms sensing and also provides for over voltage
protection when used in conjunction with an external circuit
breaker (switchboard mounted).
2.2 PERMANENT MAGNET GENERATOR (PMG)
EXCITED - AVR CONTROLLED GENERATORS
The permanent magnet generator (PMG) provides power for
excitation of the exciter field via the AVR (MX341 or MX321)
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 an isolating transformer in the
case of MX321 AVR, 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 PMG system provides a constant source of excitation
power irrespective of main stator loading and provides high
motor starting capability as well as immunity to waveform
distortion on the main stator output created by non linear
loads, e.g. thyristor controlled dc motor.
The MX341 AVR senses average voltage on two phases
ensuring close regulation. In addition it detects engine speed
and provides an adjustable 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 which acts following a time delay, to de-
excite the generator in the event of excessive exciter field
voltage.
The MX321 provides the protection and engine relief features
of the MX341 and additionally incorporates 3 phase rms
sensing and over-voltage protection.
The detailed function of all the AVR circuits is covered in the
load testing (subsection 4.7).
2.3 AVR ACCESSORIES
The SX440, SX421, MX341 and MX321 AVRs incorporate
circuits which, when used in conjunction with accessories, can
provide for parallel operation either with 'droop' or 'astatic'
control, VAR/PF control and in the case of the MX321 AVR,
short circuit current limiting.
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.
2.4 TRANSFORMER CONTROLLED GENERATORS
The main stator provides power for excitation of the exciter
field via a transformer rectifier unit. The transformer combines
voltage and current elements derived from the main stator
output to form the basis of an open-loop control system, which
is self regulating in nature. The system inherently
compensates for load current magnitude and power factor
and provides short circuit maintenance in addition to a good
motor starting performance.
Three phase generators normally have a three phase
transformer control for improved performance with
unbalanced loads but a single phase transformer option is
available.
No accessories can be provided with this control system.