Page 286 - Introduction to Marine Engineering

260 Electrical equipment

Phase 3

Delta connection

Phase3

Star connection
Figure 14.6 Star and delta three-phase connections

So far, alternator construction has considered the armature rotating
and the field coils stationary. The same electricity generating effect is
produced if the reverse occurs, that is, the field coils rotate and the
armature is stationary. This is in fact the arrangement adopted for large,
heavy duty alternators.
The field current supply in older machines comes from a low-voltage
direct current generator or exciter on the same shaft as the alternator.
Modern machines, however, are either statically excited or of the
high-speed brushless type. The exciter is required to operate to counter
the effects of power factor for a given load. The power factor is a
measure of the phase difference between voltage and current and is
expressed as the cosine of the phase angle. With a purely resistance load
the voltage and current are in phase, giving a power factor of one. The
power consumed is therefore the product of voltage and current.
Inductive or capacitive loads, combined with resistance loads, produce