Page 280 - Introduction to Marine Engineering

254 Electrical equipment

temperature and the heating effect of current in the windings,
Temperature rise is measured above this total temperature. Adequate
ventilation of electrical equipment is therefore essential. Classification
Societies have set requirements for the various classes of insulation. The
usual classes for marine installations are E, B and F where particular
insulation materials are specified and increasing temperature rises
allowed in the order stated.

Enclosures

Depending upon the location, a motor or generator will have one of a
number of possible types of enclosure. 'Drip-proof is most common and
provides protection from falling liquids or liquids being drawn in by
ventilating air. A 'watertight enclosure' provides protection for
immersion under a low head of water for up to one hour.
'Weatherproof, 'hose proof and 'deck watertight' provide immersion
protection for only one minute. 'Totally enclosed' can also be used or an
arrangement providing ducted ventilation from outside the machinery
space. A 'flameproof enclosure is capable of withstanding an explosion
of some particular flammable gas that may occur within it. It must also
stop the transfer of flame, i.e. contain any fire or explosion.

Direct current generators

A current is produced when a single coil of wire is rotated in a magnetic
field. When the current is collected using a ring which is split into two
halves (a commutator), a direct or single direction current is produced.
The current produced may be increased by the use of many turns of
wire and additional magnetic fields.
With many coils connected to the commutator, sparking will occur as
the current collecting brushes move across the insulated segments.
Commutating poles or interpoles are used to reduce this sparking. They
are in fact electromagnets having a polarity the same as the main pole
which follows in the direction of rotation.
The magnetic field between the poles is produced by what are known
as 'field coils'. These coils are excited or energised by the current
produced in the machine. The soft iron core of the field coils retains
some magnetism which enables a preliminary current generation to
build up eventually to the full machine output. The field windings can
be connected to the output current in a number of ways—shunt, series
or compound. The compound wound arrangement is usual since it
provides the best voltage characteristics.