Page 301 - Introduction to Marine Engineering

Electrical equipment 275
is achieved through the generator shunt field current, the control
equipment required is only for small current values. A potentiometer or
rheostat in the generator field circuit enables the variation of output
voltage from zero to the full value and also in either direction. The
controlled motor has a constant excitation: its speed and direction are
thus determined by the generator output.
Depending upon the particular duties of the controlled motor, series
windings may be incorporated in the field of the motor and also the
generator. This may result in additional switching to reverse the
controlled motor depending upon the compounding arrangements.
The driving motor or prime motor for the Ward—Leonard system can
be a d.c. motor, an a.c. motor, a diesel engine, etc. Any form of constant
or almost constant speed drive can be used, since its function is only to
drive the generator.

In the event of a main generating system failure an emergency supply of
electricity is required for essential services. This can be supplied by
batteries, but most merchant ships have an emergency generator. The
unit is diesel driven and located outside of the machinery space (see
Chapter 10, Emergency equipment).
The emergency generator must be rated to provide power for the
driving motors of the emergency bilge pump, fire pumps, steering gear,
watertight doors and possibly fire fighting equipment. Emergency
lighting for occupied areas, navigation lights, communications systems
and alarm systems must also be supplied. Where electrical control
devices are used in the operation of main machinery, these too may
require a supply from the emergency generator.
A switchboard in the emergency generator room supplies these
various loads (Figure 14.8). It is not usual for an emergency generator to
require paralleling, so no equipment is provided for this purpose.
Automatic start up of the emergency generator at a low voltage value is
usual on modern installations.

Navigation lights

The supply to the navigation lights circuit must be maintained under all
circumstances and special provisions are therefore made.
To avoid any possibility of accidental open circuits the distribution
board for the navigation lights supplies no other circuit. A changeover
switch provides an alternative source of supply should the main supply