Page 296 - Introduction to Marine Engineering

270 Electrical equipment

starting current period. An undervoltage or 'no volts' protective device
ensures that the motor is properly started after a supply failure,

Maintenance

With all types of electrical equipment cleanliness is essential for good
operation. Electrical connections must be sound and any signs of
sparking should be investigated. Parts subject to wear must be examined
and replaced when necessary. The danger from a.c. equipment in terms
of electric shocks is far greater than for similar d.c. voltages. Also a.c,
equipment often operates at very high voltages. Care must therefore be
taken to ensure isolation of equipment before any inspections or
maintenance is undertaken.
The accumulation of dirt on electrical equipment will result in
insulation breakdown and leakage currents, possibly even an earth fault.
Moisture or oil deposits will likewise affect insulation resistance. Regular
insulation resistance measurement and the compiling of records will
indicate the equipment requiring attention. Ventilation passages or
ducts may become blocked, with resultant lack of cooling and
overheating. Oil deposits from a direct-coupled diesel engine driving an
open generator (usually d.c.) can damage windings and should therefore
be removed if found. Totally enclosed machines should be periodically
opened for inspection and cleaning since carbon dust will remain inside
the machine and deposit on the surfaces.
Brushgear should be inspected to ensure adequate brush pressure
and the springs adjusted if necessary. New brushes should be 'bedded in'
to the commutator or slipring shape with fine glass paper. Sparking at
the commutator will indicate poor commutation. This may require
polishing of a roughened commutator surface. The mica insulation
between commutator segments may require undercutting if it prot-
rudes, or simply cleaning if deposits have built up.
Control equipment, such as starters, will require attention to contacts
which may be worn or pitted as a result of arcing. Contactors usually
have a moving or wiping action as they come together. This helps clean
the surfaces to provide good electrical contact, and also the arc produced
during closing and opening is not at the finally closed position. The
contactor contact surfaces of frequently used equipment should
therefore be subject to regular inspections.

Batteries

The battery is a convenient means of storing electricity. It is used on
many ships as an instantly available emergency supply. It may also be