Page 300 - Introduction to Marine Engineering

274 Electrical equipment

15°C. For an alkaline battery the specific gravity does not alter much
during charge and discharge but gradually falls over a long period:
when a value of 1.160 is reached it should be replaced.
The electrolyte level should be maintained just above the top of the
plates. Any liquid loss due to evaporation or chemical action should be
replaced with distilled water. Only in an emergency should other water
be used. It is not usual to add electrolyte to batteries.
A battery must be kept clean and dry. If dirt deposits build up or spilt
electrolyte remains on the casing, stray currents may flow and discharge
the battery. Corrosion of the casing could also occur. The battery
terminals should be kept clean and smeared with a petroleum jelly. The
small vents in the cell caps should be clear at all times.
Cell voltage readings are useful if taken while the battery is
discharging. All cells should give about the same voltage reading, This
test method is of particular value with alkaline batteries, where specific
gravity readings for the electrolyte do not indicate the state of charge.

Ward—Leonard speed control system

As a very flexible, reliable means of motor speed control the
Ward-Leonard system is unmatched.
The system is made up of a driving motor which runs at almost
constant speed and powers a d.c. generator (Figure 14.14). The
generator output is fed to a d.c. motor. By varying the generator field
current its output voltage will change. The speed of the controlled
motor can thus be varied smoothly from zero to full speed. Since control

D.C. motor
(controlled)

Rectifier
Figure 14.14 Ward-Leonard speed control