Page 288 - Introduction to Marine Engineering

262 Electrical equipment

provides the extra excitation to give a steady voltage under any load
condition. The careful matching of components provides a system which
functions as a self regulator of voltage. Certain practical electrical
problems and the compensation necessary for speed variation require
that a voltage regulator is also built into the system.
The brushless high speed alternator was also developed to eliminate
d.c. exciters with their associated commutators and brushgear. The
alternator and exciter rotors are on a common shaft, which also carries
the rectifiers. The exciter output is fed to the rectifiers and then through
conductors in the hollow shaft to the alternator field coils. An automatic
voltage regulator is used with this type of alternator.
The construction of an alternator can be seen in Figure 14.7. The
rotor houses the poles which provide the field current, and these are
usually of the salient or projecting-pole type. Slip rings and a fan are also
mounted on the rotor shaft, which is driven by the auxiliary engine. The
stator core surrounds the rotor and supports the three separate phase
windings. Heat is produced in the various windings and must be
removed by cooling. The shaft fan drives air over a water-cooled heat
exchanger. Electric heaters are used to prevent condensation on the
windings when the alternator is not in use.
In addition to auxiliary-engine-driven alternators a ship may have a
shaft-driven alternator. In this arrangement a drive is taken from the
main engine or the propeller shaft and used to rotate the alternator. The
various operating conditions of the engine will inevitably result in
variations of the alternator driving speed. A hydraulic pump and
gearbox arrangement may be used to provide a constant-speed drive, or
the alternator output may be fed to a static frequency converter. In the
static frequency converter the a.c. output is first rectified into a variable
d.c. voltage and then inverted back into a three-phase a.c. voltage. A
feedback system in the oscillator inverter produces a constant-output a.c,
voltage and frequency.

Distribution system
An a.c. distribution system is provided from the main switchboard which
is itself supplied by the alternators (Figure 14.8). The voltage at the
switchboard is usually 440 volts, but on some large installations it may be
as high as 3300 volts. Power is supplied through circuit breakers to
larger auxiliaries at the high voltage. Smaller equipment may be
supplied via fuses or miniature circuit breakers. Lower voltage supplies
used, for instance, for lighting at 220 volts, are supplied by step down
transformers in the distribution network.
The distribution system will be three-wire with insulated or earthed
neutral. The insulated neutral has largely been favoured, but earthed