ment: dcdc, acac, acdc, and acFFCac. An ac
setup. Its advantages are the availability of full
system is preferred because of its light weight and
torque over the entire range of speed, no loss of
higher efficiency. The problems of commutation
synchronization, operation of the prime mover at
in dc systems are not present in ac systems.
its optimum speed, and the availability of power
The advantages of an electrical transmission
for auxiliary systems from the main generators.
over a mechanical one are that the characteristic
Its main disadvantages are the high capital cost,
of the drive can be exactly matched with the mis-
high volume and weight, and relatively poor over-
sion profile of a ship, and that the total power for
all transmission efficiency of 9092% (estimated),
the ship can be divided among a number of en-
although the transmission efficiency of acFFC
gines. There is flexibility in the placement of gen-
ac systems is higher than that for acdc and dcdc
erators in a ship. An electrical system also isolates
systems.
the prime mover from the overload torque caused
by ice loads on the propellers. The disadvantages
Azimuth propulsion drive
of an electrical transmission system are the higher
Azimuth propulsion drives have been installed
costs, greater weight and larger space require-
on different types of vessels, such as icebreakers,
ments.
cargo ships, ferries, cruise ships, etc. One of the
With medium-speed diesel engines as prime
Lunni series tankers, Uikku, was converted in 1993
movers, the dcdc system is most commonly used
to accommodate 11.4-MW azimuth propulsion
in icebreakers. The maximum speed of a dc gen-
drives (one of the world's most powerful units),
erator must be less than 100 rpm owing to the lim-
replacing the original medium-speed diesel, gear-
ited capacity of the commutator brushes to trans-
ing, shafting and controllable-pitch propellers. In-
mit current. The advantages of a dc system are its
stallation of these units on the multipurpose ice-
simplicity, ease of control, good torque character-
breakers Fennica and Nordica has produced excel-
istics (especially at low speed) and lower cost than
lent icebreaking and maneuvering capabilities.
other electrical systems. In comparison to mechani-
With their advanced hulls (designed to give excel-
cal transmission systems, the disadvantages of this
lent seakeeping in open waters [Fig. 9]), these ves-
system are its higher cost, greater weight and vol-
sels can make continuous progress through 1.8-
ume, lower transmission efficiency (about 85%)
m-thick ice. Their icebreaking capabilities are also
and a relatively high requirement for manpower.
very good when they are moving astern. The azi-
The acdc system combines the advantages of
muth thruster units allow these ships to turn on
ac generators with the precise speed control of dc
the spot in ice conditions. Lohi et al. (1994) give
motors. The generated power, in three-phase
the results of full-scale ice tests with Fennica dur-
alternating current, is converted with low losses
ing her trials in the Baltic.
to direct current by the use of thyristors, which
There are two commercial azimuth propulsion
were developed in the 1960s.
systems available--Aquamaster and Azipod. In an
The acac propulsion system is based on syn-
Azipod unit, an ac electrical motor is located in-
chronous motors. The speed is changed by chang-
side the pod, whereas the motor is located above
ing the speed of the prime mover. It is the simplest
the azimuth thruster units in Aquamaster drives.
and least expensive. This system, while perhaps
The motor, controlled by a frequency converter,
being the economical choice for open water ships,
directly drives a fixed-pitch propeller, which is ei-
is not suitable for icebreaking ships. The genera-
ther open or placed in a nozzle. These drives azi-
muthally move 360 and supply full power in all
tor and the motor may fall out of synchronization
when the propellers are subjected to large ice loads.
directions.
Other disadvantages of this system are the low
Figure 17 shows the difference between conven-
torque at start up and the excitation of resonant
tional dieselmechanical and azimuth propulsion
vibrations.
systems on an arctic tanker. The azimuth system
The acac system with Full Frequency Control
has the following advantages:
(FFC), or a cyclo-converter, is the most suitable but
1. Gives excellent dynamic performance and
also the most expensive acac system. It has been
maneuvering characteristics.
used in the Finnish icebreakers Otso, Finnica and
2. Eliminates the need for long shaft lines, trans-
Nordica, in the Russian Taymyr-class icebreakers
verse stern thrusters, controllable-pitch propellers
and in Canadian light icebreakers. By employing
and reduction gears.
cyclo-converters, the motors can be precisely and
3. Allows new ways for designing machinery
steplessly controlled by a highly reliable control
and cargo spaces.
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