Page 287 - Introduction to Marine Engineering

Electrical equipment 261

lagging or leading power factors which have a value less than one. The
power consumed is the product of current, voltage and power factor.
The alternating current generator supplying a load has a voltage drop
resulting from the load. When the load has a lagging power factor this
voltage drop is considerable. Therefore the exciter, in maintaining the
alternator voltage, must vary with the load current and also the power
factor. The speed change of the prime mover must also be taken into
account.
Hand control of excitation is difficult so use is made of an automatic
voltage regulator (AVR). The AVR consists basically of a circuit fed
from the alternator output voltage which detects small changes in
voltage and feeds a signal to an amplifier which changes the excitation to
correct the voltage. Stabilising features are also incorporated in the
circuits to avoid 'hunting' (constant voltage fluctuations) or overcorrect-
ing. Various designs of AVR are in use which can be broadly divided
into classes such as carbon pile types, magnetic amplifiers, electronic
types, etc,
The statically excited alternator has a static excitation system instead
of a d.c. exciter. This type of alternator will more readily accept the
sudden loading by direct on-line starting of large squirrel cage motors.
The static excitation system uses transformers and rectifiers to provide
series and shunt components for the alternator field, that is, it is
compounded. Brushes and sliprings are used to transfer the current to
the field coils which are mounted on the rotor. The terminal voltage
from the alternator thus gives the no-load voltage arid the load current

Cooler

Air
circulation

Slip rings
Heater
Figure 14.7 Alternator construction