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8/12/2019 Analysis of electric power demands of podded propulsors
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3No. A16 2010 Journal of Marine Engineering and Technology
AUTHORS BIOGRAPHIES
Dr J Prousalidis (Electrical Engineer from NTUA/1991, PhD from
NTUA/1997) is an Assistant Professor at the School of Naval
Architecture & Marine Engineering at the National Technical
University of Athens, dealing with ship electric energy systems
and electric propulsion schemes (jprousal@naval.ntua.gr).
PS Mouzakis is a Research Assistant (Naval Architect and Marine
Engineer, MSc/2009), at the National Technical University of
Athens, School of Naval Architecture & Marine Engineering
(panos.mouzakis@gmail.com).
INTRODUCTION
During the last few decades there have been
several significant improvements relating to the
vital parts of a ship hull form, bulbous bow
and rudders and improvements in ship
manoeuvrability via thrusters and pods. Pods, in particular,
which were introduced some 20 years ago, offer greatflexibilities in vessel manoeuvring and machinery
arrangements as well as propulsion efficiency. But this
progress also introduced new challenges for research and
investigation.
A podded propulsor is defined as a propulsion or
manoeuvring device that is external to the ships hull and
houses a propeller powering capability. Some ship configu-
rations have a system of tandem propellers with two
propellers each connected at each end of the propulsor body.
The pod mechanical system comprises a short propulsion
shaft on which the electric motor is mounted and a set ofrotating elements, radial and thrust bearings. In most cases
propulsion is achieved by means of fixed pitch propellers
powered by an ac synchronous motor (conventional, perma-
nent magnet or high temperature superconductive) or
asynchronous electric motor, fitted inside the pod. Electric
power is supplied through cabling connected to the inboard
ship grid via slip rings.
The loadings generated by the pod are extremely
complicated, as a large number of factors and parameters
should be taken into consideration (eg, the flow around the
fin, the strut, as well as the helicoidal propeller slipstream).
In addition, the interaction between the propeller and the podbody(ies) must be also taken into account. The pod body
design is of great importance because of the need to min-
imise the hull boundary layer and the developed vorticity.
As the hydrodynamic characteristics of the pod strictly
Analysis of electric power demands of podded propulsors
Analysis of electric power demands
of podded propulsorsJM Prousalidis and PS Mouzakis, National Technical University of Athens, School of Naval
Architecture & Marine Engineering, Division of Marine Engineering, Athens, Greece
The authors investigate and analyse the electric power operating conditions of poddedelectric motor drives, mainly in an attempt to explain the high failure rates of the electric
components in pod propulsion installations. The analysis is based on simulations, where
the pod torque demands are obtained from experimental results recently published in
the literature. Thus, the same series of loading scenarios for an entire range of turning
azimuth angles of a twin pod configuration are considered. The approach is rather
generic as only nameplate data of the motor drives are required while, as the simulations
focus on the electric motors, the rest of the ship electric system considered, including gen-
erator sets, is represented by a simplified network.Two different sets of simulations are
considered, one comprising a synchronous ac motor drive and one with an asynchronous
motor of similar rating. In all cases, it is shown that during manoeuvring in a certain range
of azimuth angles significant overloading occurs exceeding the apparent power capacity
of the motors.
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depend on its shape, the body must be designed with the
smallest possible diameter along with the greatest possible
length, hence, the ratio of pod radius to pod body length
must be as low as possible.
Furthermore, if the forces and moments developed
during pod operation are not accurately estimated, the
induced loads to the bearings and the shaft will not bewell assessed either, leading to premature ageing and,
eventually, failure of most components.14 These induced
loads strictly depend on the pod azimuth angle.14 At each
azimuth angle, the thrust and the torque of the pod should
be contrasted with the thrust and torque at zero azimuth
angle, corresponding to sea-going operation. It is noted
that these loading conditions have only recently been
thoroughly investigated on an experimental basis, as
described in a series of papers, conducted by LRS.13 In
the same work it is reported that amongst the highest fail-
ure indices are those of main electric motor parts and, in
particular:
Stator windings and core,
Rotor windings,
Slip ring electric connectors.
These high failure rates are attributed mainly to high
temperature development in the windings, leading to
excessive overheating stresses indicating that the motors have
not been properly matched with the stressful conditions they
are requested to operate in.
Hence this paper aims to investigate and analyse the
operating conditions of pod motor drives and their effect on
electric power. The analysis is based on simulations, wherethe pod torque demands are obtained from the experimental
results published;3 moreover, the same series of loading
scenarios, covering the entire operating range of twin pod
configurations, are considered. The approach is as generic as
possible considering that no specific motor data are required,
with the exception of the nameplate data.5 Furthermore, as the
simulations focus on the electric motors, the rest of the ships
electric system, including generator sets, is represented by a
simplified network.5 Two different sets of simulations are
considered, the first corresponding to a synchronous ac motor
drive, the second to an asynchronous one but of similar rated
values.
POD BEHAVIOUR DURING TURNINGMANOEUVRESFigs 13 present the twin pod configuration and operating
scenarios considered, as obtained from.12 In this, when
undertaking a full-turn, the resulting forces and moments
produced by the propellers located on the port and on the star-
board side of the hull, are different. This difference strictly
depends on the side boundary layers and on the flow field in
the way of the propellers. For the purposes of this paper, sim-ulations of pod synchronous motor behaviour were organised
into six different study cases, as shown in Table 1. In the first
three cases (A-C) the pod motor is supposed to be a synchro-
nous ac machine with the nameplate data shown in Table 2,
while cases D-F refer to an asynchronous ac motor, the
data of which are tabulated in Table 3. The motors are of
almost equal rating and have been taken from actual pod
configurations.5
Figs 4613 present the torque measured on the pod
propeller for all three pod operation modes considered
(A, B and C or D, E and F) for different pod azimuthangles.
Fig 1: Pod configuration in case studies cases A & D
Fig 2: Pod configuration in case studies cases B & E
Fig 3: Pod configuration in case studies cases C & F
Analysis of electric power demands of podded propulsors
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Table 1:The case studies under consideration
Table 2: Nameplate data of AC synchronous pod motor
Table 3: Nameplate data of AC asynchronous pod motor
Fig 4:Torque vs pod azimuth angle considered (as measured
in13) in case studies A and D
Fig 5:Torque vs pod azimuth angle considered (as measured
in13) in case studies B and E
Fig 6:Torque vs pod azimuth angle considered (as measured
in13) in case studies C and F
In the starboard-pod experiment,13 positive angles refer to
turns of the pod propeller to starboard (open sea), while
negative angles refer to turn to port (hull side). On the other
hand, with port-pod operation, positive angles refer to turns
of the pod propeller to hull side (starboard side), while
negative angles refer to turns to open sea (port side).
Moreover, regarding twin screw configurations, the following
assumption is made:5
The curve representing the torque demand of a
separate pod torque demand for different azimuthangles remains unchanged* in the twin screw
configuration. This means that it is assumed that
there is no interaction between these two propulsive
devices, which is, in general, a plausible assumption
for the ship structures where twin pods are installed.
The electric power grids of the two main configurations are
depicted in Fig 7a & b, respectively, as modelled in the PSCAD
environment of the Manitoba HVDC Center.6 Specifically, con-
sidering that the interest in this work is focused only on the
operation of pod motors, the power sources are ideal. In this
way, the voltage to pod motor terminals remains constantdespite any current fluctuations, so that the pure power demands
of the pod motors are identified.5 Further, taking into account
Analysis of electric power demands of podded propulsors
Nameplate data of
asynchronous AC pod motor
Nominal Output Power 24.64 MW
Nominal Input Active Power 25.20 MW
Nominal Input Reactive Power 15.27 MVAr
Nominal Power Factor 0.855
Speed at full load 120 RPM
Line Voltage 6600V
Efficiency at Full Load 97.5%
Design Life 30 years
Case studies From To Pod Drive
Star Board Pod x P SCase Study A
Port Pod - - -
Star Board Pod x P S 3-phase ACCase Study B Synchronous
Port Pod x P S Motor
Star Board Pod x P S
Case Study CPort Pod x S P
Star Board Pod x P SCase Study D
Port Pod - - -
Star Board Pod X P S 3-phase ACCase Study E Asynchronous
Port Pod X P S Motor
Star Board Pod X P SCase Study F
Port Pod X S P
Nameplate data of
synchronous AC Pod Motor
Nominal Output Power 25 MW
Nominal Input Active Power 25.64 MW
Nominal Input Reactive Power 19.23 MVAr
Nominal Power Factor 0.8
Speed at full load 120 RPM
Line Voltage 6600V
Efficiency at Full Load 97.5%
Design Life 30 years
* Provided that the angular speed remains constant
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Analysis of electric power demands of podded propulsors
Fig 7: Electric power grid of the two configurations considered (in PSCAD simulation environment) (a) the pod motor is an ac
synchronous machine (b) the pod motor is an ac asynchronous one
that the apparent power absorbed by the motor is the product of
the input voltage times the input current, the apparent power
waveform is directly proportional to that of the current. Should
the source not be ideal, and an actual generator is used instead,
with its associated speed governor and Automatic Voltage
Regulator (AVR), the behaviour is expected to change. In a
future work this interaction between generators and pod motorsas well as the rest of the ship grid is intended to be studied.
PRESENTATION AND DISCUSSION OFRESULTSThe most representative results of all simulated study cases
are presented as follows. Thus, in each case study the follow-
ing quantities are figuratively presented:
Power and current demands vs azimuth angle (P, Q, S, I
vs a-curves) Active vs reactive power demands (P vs Q -curves).
For the sake of tangible comparison, all waveforms are
expressed in percentage (%) using as reference base values the
corresponding synchronous and asynchronous motor rated
values shown respectively in Tables 2 and 3. However, con-
cerning the active and reactive power quantities, it is question-
able whether they should refer to their corresponding rated
values and not the rated apparent power value.5 More specifi-
cally, considering that active and reactive power complement
one another, if their %-values both refer to the (total) apparentrated power, then, the conditions where the electric motor
capacity is superseded, are more figuratively identified. This is
the reason why two pairs of reactive vs active power demand
diagrams are provided in all case studies. The difference
between them is the reference values and consequently the
motor capacity limit curves.
Regarding the time analysis of the simulations, for the first
three simulations (powered by the synchronous machine)
(Cases A, B & C), the motor(s) starts directly when the
starboard motor azimuth angle is equal to -30.0 degrees
(towards the hull). In addition, the angle range [-30.0o
, +30.0o
]
is covered in a linear manner, within 60 seconds. However, dueto the fact that there is no motor starting procedure before it
reaches pod azimuth angle 30.0o
, a large starting-up transient
phenomenon takes place. For that reason, not all recorded sim-
ulation results refer to the draft examined azimuth angle area
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Analysis of electric power demands of podded propulsors
[30.0o
], but to that of [-25.0o
, +30.0o
]. On the other hand, for
the last three simulations, (powered by the squirrel cage asyn-
chronous machine) (Cases D, E & F), it is considered that the
motor(s) starts at azimuth angle 0.0 and then it turns to the
starting azimuth angle (-30.0o
or 30.0o
).13,5 Finally, the angle
range [-30.0o
, +30.0o
] is covered in 60 seconds.5
Study case A remarksAs already mentioned, in case study A the simulation concerns
the operation of the starboard pod only. The starboard pod turns
from the port side of the hull to the open sea (starboard). The
torque demand as measured in13 is shown in Fig 4. From Fig 8
it can be observed that the active power curve strictly depends
on the propeller torque required, as depicted in Fig 4. Moreover,
significant overloading with respect to the active power is
noted, especially in negative angles. Quite the opposite remark
is made for the reactive power. However, by inspecting the
apparent power and the active vs reactive power diagrams
(Figs 9 and 10) it is seen that significant overloading also occurs
relating to the apparent power capacity of the motor.
Fig 8: Pod power (active, reactive and apparent) demands vs
azimuth angle in case study A
In addition, the starboard pod active power demand
reaches its lowest ever rate at 10o
starboard (towards the
open sea), while at the same point, reactive power reaches its
highest ever rate. On the other hand, active power reaches its
highest rate at 20o
port (towards the hull), while at the same
point, reactive power stands at its lowest ever rate.
Furthermore, the pod requirements at azimuth angle 22.6o
are equal to those at zero azimuth angle, pointing to somekind of symmetry with respect to the angle of 12.3
o
.
In the P-Q diagrams, Figs 9 and 10, a hysteresis phenom-
enon is noted, as the trajectory passing from negative to
positive angles is different from that of the opposite direction.
This hysteresis effect is noticed only in the motor reactive
power. Considering that the reactive power is adjusted by the
synchronous motor exciter, this phenomenon can be attrib-
uted to the integrator module of the motor exciter, which
introduces this kind of memory effect.
Fig 11 presents the reactive power demands of the exam-
ined asynchronous motor of Case A vs its active power
demands referred to their rated values. In detail, this graph isthe product of four continuous operations with return to its
initial position (starboard pod azimuth angle -30.0o
). As
already mentioned, these loops are attributed to memory
effects of the exciter integrator.
Fig 9: Pod reactive power vs active power in case study A.
Active and reactive power quantities are expressed in % with
respect to their corresponding rated values
Fig 10: Pod reactive power vs active power in case study A.
Active and reactive power quantities are expressed in % with
respect to the rated apparent power value so that the
operation limits are identified
Fig 11: Explanatory figure on how the hysteresis loops of
Fig 9 are created
Study case BIn study B, the simulation concerns the operation of both
pods. The starboard pod turns from the hull side to the open
sea, while the port pod turns from the open sea to the hull
side. During this operation, significantly different loading
torque is noted between the two motors (Fig 5). More specif-
ically, by inspecting the results depicted in Figs 1214, thefollowing remarks are made:
By comparing Figs 8 and 11, the loading pattern in case
B is more symmetrical with respect to 0 angle than in
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case A. The symmetry of this case is due to the operation
of both pods. However, especially in large overload
conditions [-30.0o
, -17.0o
] and [+30.0o
, +17.0o
] small
asymmetry has been observed in reactive power require-
ments and thus in apparent power and current curves.
In this case, no overloading in reactive power demand is
noted in the entire range of angles.
Fig 12: Pod power (active, reactive and apparent) demands vs
azimuth angle in case study B
Fig 13: Pod reactive power vs active power in case study B.
Active and reactive power quantities are expressed in % with
respect to their corresponding rated values
Fig 14: Pod reactive power vs active power in case study B.
Active and reactive power quantities are expressed in % with
respect to the rated apparent power value so that the
operation limits are identified
On the contrary, overloading is noticed in active and
apparent power, as well as the current waveforms.
At zero azimuth angle, the total active power demands of
both pods stand at the lowest ever rate, while the correspon-
ding demands for reactive power stand at the highest rate.
The total pod requirements in active power depend on
the azimuth angle regardless of the angle direction.However, the total pod requirements in reactive power,
total current and apparent power depend on angle
direction possibly due to memory effects as well as the
collaboration effects of the two exciters.
Study case CIn study C, the simulation corresponds to the operation of the
starboard and port pod, both turning from the hull side to
the open sea. In Figs 1517 the total power demands are
depicted. In this case, which resembles closely case A, the
following remarks are made:
The azimuth angle for each pod where the maximum or
minimum total demand for active and reactive power
occur, are the same with that of study case A.
Like in case A, the pod power demands at 22.6o
are equal
to those at 0o
.
Fig 15: Pod power (active, reactive and apparent) demands vs
azimuth angle in case study C
Fig 16: Pod reactive power vs active power in case study C.
Active and reactive power quantities are expressed in % with
respect to their corresponding rated values
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Fig 17: Pod reactive power vs active power in case study C.
Active and reactive power quantities are expressed in % with
respect to the rated apparent power value so that the
operation limits are identified
As in case A, a minor overloading in terms of reactive
power is noted in the range between 0o and 22.6o
. On the
contrary, in this region no overloading is noted in terms
of active/apparent power and current; however, signifi-
cant overloading occurs in the complementary regions of
[-30, 0o
] and [+22.6o
, +30o
].
In overloading conditions, there are differences noted
between motor reactive power demands in Cases A and
C, of up to 7% of their rated values. On the other hand,
no differences between these two study cases are noted
regarding active power requirements.
Fig 18: Pod power (active, reactive and apparent) demands vs
azimuth angle in case study D
Study case DLike case A, this case involves only the use of starboard pod,
which turns from the port side of the hull to the open sea
(starboard). The corresponding results are presented in Figs
1820, from which the following remarks are made:
Unlike the previous cases, both active and reactive power
quantities have identical variation. This is due to theasynchronous type of the motor drive and its lack of inde-
pendent excitation circuit regulating reactive power.
Evidently, apparent power and consequently current follow
similar track. Hence, all the examined magnitudes strictly
depend on the applied torque (Fig 4), with the minimum
values in 10o
towards the open sea (+10o
), while the maxi-
mum values 20o
towards the hull of the ship (ie, -20o
).
For similar reasons, ie, the absence of excitation circuit
with integrator module, no hysteresis effect is noted in
the active vs reactive power diagrams, Figs 19, 20.
While apparent and active power overloading is noted inthe range of angles [-30
o
, -10o
] and [+25o
, +30o
], the
corresponding range of angles of reactive power over-
loading is [-30o
, 0o
] and [+20o
, +30o
].
Fig 19: Pod reactive power vs active power in case study D.
Active and reactive power quantities are expressed in % with
respect to their corresponding rated values
Fig 20: Pod reactive power vs active power in case study D.
Active and reactive power quantities are expressed in % with
respect to the rated apparent power value so that the
operation limits are identified
Study case ESimilar to case B, case E involves two pods (port and
starboard), but are both driven by asynchronous pods. The
starboard pod turns from the hull side to the open sea, while
the port pod turns from the open sea to the hull side. From the
simulation results presented in Figs 2123, the followingremarks are made:
As in case D, all quantities examined follow the track of
the pod torque demands, see Fig 5. Thus, a symmetry
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with respect to 0o
, the point with minimum power
demands, is noticeable.
While overloading situation with respect to active and
apparent power is noted in the range of angles [-30o
, -10o
]
and [+10o
, +30o
], reactive power overloading is apparent
in the entire range [-30o
, +30o
].
Fig 21: Pod power (active, reactive and apparent) demands vs
azimuth angle in case study E
Fig 22: Pod reactive power vs active power in case study E.
Active and reactive power quantities are expressed in % with
respect to their corresponding rated values
Fig 23: Pod reactive power vs active power in case study E.
Active and reactive power quantities are expressed in % with
respect to the rated apparent power value so that the
operation limits are identified
Study case FLike case C, case F simulation concerns the operation of the
starboard and port pod with both of them turning from the
hull side to the open sea. From the simulation results present-
ed in Figs 2426, the following remarks are made:
The resemblance between cases A and C is repeatedbetween cases D and F.
Fig 24: Pod power (active, reactive and apparent) demands vs
azimuth angle in case study F
Fig 25: Pod reactive power vs active power in case study F.
Active and reactive power quantities are expressed in % with
respect to their corresponding rated value
Fig 26: Pod reactive power vs active power in case study F.
Active and reactive power quantities are expressed in % with
respect to the rated apparent power value so that the
operation limits are identified
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All the examined quantities (active and reactive power,
as well as three phase current) follow the pattern of the
propeller torque demand (Fig 6). Hence, minimum
values appear 10o
towards the open sea, while maxi-
mum values are noted 20o
towards the hull of the ship
referring to pod 0o
.
In addition, the pod requirements in azimuth angle 22.6
o
are equal to those in zero azimuth angle.
While apparent and active power overloading is noted
in the range of angles [-30o
, -10o
] and [+25o
, +30o
], the
corresponding range of angles of reactive power over-
loading is [-30o
, 0o
] and [+20o
, +30o
].
RESULTS/DISCUSSION
In this section, the results of all case studies are compared
to each other and discussed further. For comparison
purposes, certain characteristic values concerning over-
loading are tabulated in Table 4, eg, range of azimuth
angles where overloading occurs, as well as maximum and
minimum values. Hence, the following conclusive remarks
are made:
In general, results of case A resemble to a great extent
those of case C. This is due to the fact that the hydro-
dynamic loading in case A is supposed to be the same
as case C. Thus, at each simulation instant, the wave-
forms of active and reactive power as well as motor
currents are approximately the same. For the same
reason, regarding asynchronous motor configurations,
the simulation results of case D are the same as thosein case F.
Cases A, C, D and F seem to be significantly worse than
cases B and E from the electric power overloading point
of view, ie, in terms of both:
maximum values of all power quantities P,Q,S (and I)
range of azimuth angle with overloading in terms of
P,Q,S (and I).
This overloading can be stressful for the generator
sets, too, and has to been taken into account in the
electric balance analysis and the generator selection
study. It is noted that this situation is not a fast tran-
sient, eg, in terms of inrush current during motorstarting-up with a duration of 100500ms, but could
last for several minutes and hence could cause several
adverse phenomena.
Furthermore, it seems that the worst overloading operat-
ing condition of the electric motor is in the vicinity of
-20o
, as in all cases this is where the maximum apparent
power demands occur reaching occasionally, an over-
loading level of 160% the rated one.
Active power P follows the pattern of pod torque
demands as it is directly related with it. Thus, due to the
significant overloading occasionally occurred, pod
motors could eventually encounter adverse conse-quences, such as premature ageing or failure of the
propeller, the shaft and the bearings. In the asynchronous
motor cases (D, E, F) the overloading noticed in torque
demand is accompanied by a decrease in speed; this
explains why the resultant active power demand is less
than the corresponding torque demand.
Reactive power Q has a more complicated behaviour. In
cases A-C with the synchronous motor drive, reactive
power is adjusted and controlled by the independent
AVR of the motor and, hence, appears to follow a pat-
tern complementary to that of the active power P andtorque T; thus minimum Q values coincide with maxi-
mum P (and T) values and vice versa, due to the effort
of the AVR to keep the total apparent power demand to
low level.
According to starboard simulation results, the starboard
pod for the azimuthing angle range [-25.0 -14.75]
requires from the system a negative amount of reactive
power. This means that the motor produces rather than
consumes reactive power. Moreover, the presence of
the AVR in these cases (A-C), results in an intriguing
hysteresis effect in the P-Q diagrams, according to
which tracking from positive to negative angles is dif-
ferent from the other way round. On the other hand, in
cases D-F, with the asynchronous motor drive, where
no reactive power regulation device is present, reactive
power Q follows an identical pattern to that of the
active power. Therefore, in cases D-F, reactive power
pattern resembles that of the pod torque demand
leading in certain cases to even more significant over-
loading as the motor operation cannot be adjusted at all.
This remark points why the asynchronous motor could
be completely inappropriate for pod applications.
Finally, in these last three cases where no reactive
power regulation can be done, no hysteresis effect in
the P-Q diagrams is noticed.
Moreover, apparent power S is yielded from the combi-
nation of both active and reactive power according to the
well known expression:
(1)
In general, apparent power demands resemble more the
active power, which can be explained considering that this
power portion is always predominant. The angle regions,
where rated apparent power is exceeded are most significant
as the apparent power shows the motor overall capacity towithstand overloading or not. This is why the P-Q diagrams
where both active and reactive power refer to the rated
apparent power rather than their own rated values are
most useful.
Finally, current changes in a manner identical to the one
of the apparent power. This is explained as the current is
directly proportional to the apparent power, considering that
they are related via the expression:
(2)
As already mentioned, in this work the terminal voltage V of
the motor is considered to be constant and provided by an
ideal source. In a future work, the pod motor will be
IS
V
=
3
S P Q= +2 2
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considered connected to the actual ship grid, and the terminal
voltage will be subjected to fluctuations by several grid
operation factors including the pod motor itself.
Apparently, considering that overloading takes place in an
entire range of values, the appropriate selection of the pod
motor drive should be the result of a customized optimisation
methodology. More specifically, besides any other designconstraints, two contradictory concepts have to be taken into
account:
1. If the motor rated power is selected according to the
mean power demand then significant overloading would
occur leading to accumulated insulation stressing,
premature ageing and failure of components as already
statistically recorded.1
2. On the other hand, if the motor sizing is based on the
overloading operating conditions noticed in certain
pod turning angles, then the motor will normally (ie, in
sea-going conditions) operate in values significantly
lower than the rated ones, resulting in lower efficiency
and power factor which consequently means higher
fuel consumption of the generators and higher total
operation cost.
In any case, the actual overloading limitation is the apparent
power as already discussed on the occasion of the P vs Q
diagrams. Furthermore, in synchronous motor drives, where
an independent excitation circuit exists, reactive power Q
follows a complementary pattern to that of active power P
resulting in comparatively lower power demands. This is a
major advantage of this motor type, despite even the hystere-
sis effects, ie, the fact that different overloading is notedwhen turning from positive angles to negative ones than the
opposite.
In the case of asynchronous motor drive where no
independent excitation is present, reactive power has a
similar behaviour to that of active power, resulting in more
significant overloading in terms of apparent power. This is
the major reason why this motor type is to be exempted
from pod applications. Nevertheless, novel alternative
motor configurations, especially those regarding theirpower demand controllability, have to be sought and inves-
tigated in depth a direction that future work by the authors
is focused on.
NOMENCLATURE
AVR: Automatic Voltage Regulator
I: motor current
P: active power demand of the motor
Q: reactive power demand of the motor
S: apparent power demand of the motor
T: motor output torque
V: terminal supply voltage of the motor
CONCLUSIONS
In this paper an effort is made to investigate and analyse
the electric power operating conditions of pod electric
motor drives in an attempt to explain the high failure rates
of the electric components in pod propulsion installations.
The analysis is based on simulations for a twin pod
configuration, the torque demands of which were obtained
from experimental results recently published. Two alter-native electric motor types are considered, a synchronous
and an asynchronous one. Significant overloading is
Analysis of electric power demands of podded propulsors
Case Range of angles where Overloading Maximum Power Minimum Power Demand
Study occurs Demand
P Q S, I P Q S, I P Q S, I
[-25o
, 0o
] [-25o
, -14o
] 155% 110% 125% 90% -40% 98%
A and [0, +22.6o
] and at -20o
at 10o
at -20o
at 10o
at -20o
at 10o
[+22.6o
, +30o
] [+22.6o
, +30o
]
[-25o
, -10o
] [-25o
, -18o
] 130% 108% 98% in -35% at 97%
B and - and at +30o
- at +30o
the range -20o
at 15o
[+10o
, +30o
] [+11o
, +30o
] [-10o
, 10o
] and +25o
[-25o
, 0o
] [-25o
, -14o
] 155% 110% 125% 90% -33% 97%
C and [0, +22.6o
] and at -20o
at 10o
at -20o
at 10o
at -18o
at 10o
[+22.6o
, +30o
] [+22.6o
, +30o
]
[-30o
, -10o
] [-30o
, -0o
] [-30o
, -7.5o
] 135% 160% 141% 87% 95% 89%
D and and and at -20o
at -20o
at -20o
at 10o
at 10o
at 10o
[+25o
, +30o
] [+20o
, +30o
] [+23o
, +30o
]
[-30o
, -14o
] [-30o
, -5o
] [-30o
, -13o
] 115% at 130% at 120% 92% in 100% in 95% in
E and and and -30o
and -30o
and at -30o
the range the range the range
[+14o
, +30o
] [+5o
, +30o
] [+13o
, +30o
] +30o
+30o
and +30o
[-10o
, 10o
] [-10o
, 10o
] [-10o
, 10o
]
[-30
o
, -10
o
] [-30
o
, -0
o
] [-30
o
, -7
o
] 135% 160% 142% 87% 95% 89%F and and and at -20
o
at -20o
at -20o
at 10o
at 10o
at 10o
[+25o
, +30o
] [+20o
, +30o
] [+24o
, +30o
]
Table 4: Overview of overloading critical values in all case studies
Journal of Marine Engineering and Technology No. A16 201012
8/12/2019 Analysis of electric power demands of podded propulsors
11/11
noticed in certain operating ranges of pod azimuth turning
angles, eg, in turning-to-port manoeuvring conditions,
exceeding as much as 150% of the motor capacity.
Besides the pod torque defining the active power
demands, the reactive power demands play a significant
role. Thus, it is shown that the situation is milder in the
synchronous motor drive due to the presence of the exci-tation circuit regulating the motor reactive power
demands, and hence also its total apparent power
demands. Nevertheless, extra research work is required in
terms of optimising the motor performance without
exceeding its capacity.
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to Professor
Gerassimos Politis for his valuable advice on pod propeller
hydrodynamic behaviour.
REFERENCES
1. Ball WE and Carlton JS. 2006. Podded propulsor shaft
loads from model experiments for berthing manoeuvres.
International Journal of Maritime Engineering, RINA.
2. Ball WE and Carlton JS. 2006. Free-running model
experiments in calm-water and waves, International Journal
of Maritime Engineering. RINA.3. Carlton JS. 2008. Podded propulsors: Some results of
recent research and full scale experience. IMarEST Journal
of Marine Engineering & Technology (Part A11). April 2008,
pp 316.
4. Islam MF, He M, Veitch B and Liu P. 2007. Cavitation
characteristics of some pushing and pulling podded propellers.
International Journal of Maritime Engineering, RINA.
5. Mouzakis P. 2009. Analyzing and resolving electric
power supply quality problems in ship electric networks due
to large power machine operation. Graduation Diploma thesis.
6. HVDC Center, PSCAD/EMTDC Users Manual,
Manitoba (Canada), 2006.
Analysis of electric power demands of podded propulsors
13No A16 2010 Journal of Marine Engineering and Technology
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