Page 195 - Introduction to Marine Engineering

Deck machinery and hull equipment 181

Hydraulic systems

The open-loop circuit takes oil from the tank and pumps it into the
hydraulic motor. A control valve is positioned in parallel with the motor.
When it is open the motor is stationary; when it is throttled or closed the
motor will operate. The exhaust oil returns to the tank. This method can
provide stepless control, i.e. smooth changes in motor speed.
The live-line circuit, on the contrary, maintains a high pressure from
which the control valve draws pressurised oil to the hydraulic motor (in
series with it), as and when required.
In the closed-loop circuit the exhaust oil is returned direct to the
pump suction. Since the oil does not enter an open tank, the system is
considered closed.
Low-pressure systems use the open-loop circuit and are simple in
design as well as reliable. The equipment is, however, large, inefficient
in operation and overheats after prolonged use.
Medium-pressure systems are favoured for marine applications, using
either the open or closed circuit. Smaller installations are of the
open-loop type. Where considerable amounts of hydraulic machinery
are fitted the live-circuit, supplied by a centralised hydraulic power
system, would be most economical.

Electrical operation

Early installations used d.c. supply with resistances in series to provide
speed control (see Chapter 14). This inefficient power-wasting method
was one possibility with d.c., but a better method was the use of Ward
Leonard control. The high cost of all the equipment involved in Ward
Leonard control and its maintenance is, however, a considerable
disadvantage.
Machines operated on an a.c. supply require a means of speed control
with either pole-changing or slip-ring motors being used. Slip-ring
motors require low starting currents but waste power at less than full
speed and require regular maintenance. Pole-changing motors are of
squirrel cage construction, providing for perhaps three different speeds.
They require large starting currents, although maintenance is negligible
(see Chapter 14).
Apart from the advantages and disadvantages for each of the drive
and control methods, all electric drives have difficulty with heavy
continuous overloads. Each system has its advocates and careful design
and choice of associated equipment can provide a satisfactory
installation.