analysis of electric power demands of podded propulsors

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 ([email protected]).

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

([email protected]).

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


Journal of Marine Engineering and Technology No. A16 20104


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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


in13) in case studies B and E


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


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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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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[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.


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.


azimuth angle in case study B

Fig 13: Pod reactive power vs active power in case study B.



Fig 14: Pod reactive power vs active power in case study B.




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

.


azimuth angle in case study C

Fig 16: Pod reactive power vs active power in case study C.






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Fig 17: Pod reactive power vs active power in case study C.




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.


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.



Fig 20: Pod reactive power vs active power in case study D.




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

].


azimuth angle in case study E

Fig 22: Pod reactive power vs active power in case study E.



Fig 23: Pod reactive power vs active power in case study E.




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.


azimuth angle in case study F

Fig 25: Pod reactive power vs active power in case study F.


respect to their corresponding rated value

Fig 26: Pod reactive power vs active power in case study F.







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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




10/11

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


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



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.


13No A16 2010 Journal of Marine Engineering and Technology

analysis of electric power demands of podded propulsors

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