Page 285 - Introduction to Marine Engineering

Electrical equipment 259

Alternating current generators

A coil of wire rotating in a magnetic field produces a current. The
current can be brought out to two sliprings which are insulated from the
shaft. Carbon bushes rest on these rings as they rotate and collect the
current for use in an external circuit. Current collected in this way will
be alternating, that is, changing in direction and rising and falling in
value. To increase the current produced, additional sets of poles may be
introduced.
The magnetic field is provided by electromagnets so arranged that
adjacent poles have opposite polarity. These 'field coils', as they are
called, are connected in series to an external source or the machine
output.
If separate coils or conductors are used then several outputs can be
obtained. Three outputs are usually arranged with a phase separation of
120°, to produce a three-phase supply. The supply phasing is shown in
Figure 14.5. The three-phase system is more efficient in that for the
same mechanical power a greater total electrical output is obtained. Each
of the three outputs may be used in single-phase supplies or in
conjunction for a three-phase supply. The separate supplies are
connected in either star or delta formation (Figure 14.6). The star
formation is most commonly used and requires four sliprings on the
alternator. The three conductors are joined at a common slipring and
also have their individual siipring. The central or neutral line is common
to each phase. The delta arrangement has two phases joined at each of
the three sliprings on the alternator. A single-phase supply can be taken
from any two sliprings.

Phase 1 Phase 2 Phase 3

Voltage

Armature angle
Figure 14.5 Three-phase alternator output