CIMAC Congress - Schiff & Hafen

SHIPBUILDING & EQUIPMENT | CRUISE & FERRIES
The Norwegian Epic offers accommodation for 4,200 passengers
Norwegian Epic powered by
innovative propulsion system
INDUCTION MOTORS | Built
at STX Europe Saint-Nazaire
(France), Norwegian Cruise
Line’s (NCL) new 150,000 gt
cruise vessel Norwegian Epic implements
an innovative high
power propulsion system with
induction motors fed by PWM
converters. France-based Converteam
has supplied the electric
propulsion solution, which
is driven by six diesel engines
from MaK-Caterpillar (12M43C)
providing 80.4 MW in total. The
system is based on high-torque
density induction motors and
PWM MV7000 converters. The
ship is powered by two shaftlines,
directly driven by two 24MW
slow-speed induction motors,
at 130 rpm. The order also includes
six generators with a total
power of 80 MW, to be supplied
by Converteam Ltd (UK), and
six electric motors for thrusters
with a total power of 15 MW, to
be manufactured at Converteam
Motors’ Nancy plant (France).
The induction motor rotor has no
components, such as insulated
windings, exciter, rotating diodes
or permanent magnets but has
copper bars short-circuited by
rings instead. Furthermore, the
induction motor has a reduced
16 Ship & Offshore | 2010 | N o 3
acoustic noise and vibration
level and its simplicity decreases
maintenance time and cost. Converteam
has designed the dedicated
marine HTD (High Torque
Density) induction motor with
an optimised cooling, a large air
gap and a low resistive rotor cage
to decrease rotor losses and a low
frequency machine with dedicated
number of poles.
Converter and controller
The PWM MV7000 converter
allows electrical energy to be
converted from a given level of
voltage/current/frequency to
another one by using the presspack
Insulated Gate Bipolar
Transistors (IGBT) as high level
switching components.
The ability to adjust, automatically
as well as during operation,
both PWM patterns and
frequency allows the control
to keep the drive output active
power constant regardless of the
motor power factor variation.
Thus, no converter over-sizing is
required, linked to motor power
factor design. The press-pack
IGBT technology enhances the
converter effi ciency and availability.
It features safest components
(due to their intrinsic ca-
pacity to limit any over-currents)
and a high switching frequency
capability (allowing low output
harmonic currents).
Efficiency
The main benefi t of such an inverter
arrangement is the low
level of current harmonics,
which results in the following
advantages:
� Reduced motor losses
� AC supply harmonic level
reduction
�
Power factor optimisation,
close to 1 over the whole
speed range (> 0.96).
Consequently, the propulsion
plant effi ciency is high (around
94%) and harmonic fi lters do
not need to be compliant with
the maximum Total Harmonic
Distortion (THD) allowed by
the classifi cation societies. In addition,
the size of the generators
is reduced (power factor up to
0.9).
Redundancy
The propulsion motor supply
chain is fully redundant with a
master/slave selection. In case
of failure, the PWM MV7000
converter arrangement offers
the possibility to automatically
operate with “half converter” at
reduced torque. Due to the harmonic
free PWM power supply,
which avoids torque pulsation,
the marine HTD induction motor
is designed with a single stator
winding.
This arrangement provides the
same level of redundancy as a
conventional dual winding synchronous
motor, but increases
its availability when operating
in half converter mode (higher
torque is available due to higher
current capability).
The slow-speed induction motor
by Converteam