Delft University of Technology OvS 97/10-r1
Lips B.V Drunen, proj.no. X52000, CONFIDENTIAL
Electric Thruster
Overview of components and solutions
Preface
This literature study explores electric ship drives and the global features ofa new type of
electric thruster, in the market sector known as `Azipod', a trademark of ABB Azipod. The work is the first part of a design study for such a thruster, carried out for Lips B.V., in Drunen, the Netherlands, and part of an assignment needed for graduation in Marine Technology. This study is supervised by H.T. Grimmelius for the section Marine Engineering of the faculty of Mechanical Engineering and Marine Technology at Delft University of Technology and T. van Beek and J.Ju for Lips.
A short description of electric thruster propulsion is given in chapter 2, with an overview of
operational electric thrusters in paragraph 2.4.2. Information on electric drives is found in the following chapters, and the conclusions in paragraph 3.5 and 5.6. The electric thrusteritself and its features are described in chapter 6.
Joost van Eijnatten
Student Marine Engineering Faculty of Mechanical Engineering and Marine Technology
Delft University of Technology
Contents
Preface
Summary
Introduction
1Ship propulsion
2 2.1 Conventional 2 2.2 Diesel-Electric 3 2.3 Thruster 4 2.4 E-thruster 52.4.1 Benefits and drawbacks of E-Thruster propulsion 6
2.4.2 Current development of E-thrusters 9
Electric propulsion: Motor
103.1 DC Motor 10
3.2 Synchronous (AC) motor 11
1.1.1 Permanent magnet (PM) synchronousmotor 13
1.3 Asynchronous (AC) motor 16
1.4 Parameters 19
1.5 Summary and conclusion 27
Electric propulsion: Converter
28
4.1 Electronic components of converters: Semiconductors 28
4.2 Rectifiers 29
4.3 Inverters 30
4.4 Pulse Width Modulation (PWM) 31
4.5 Synchro Converter (LCI) 33
4.6 Cycloconverter (CCV) 34
4.7 General aspects of converters 35
4.7.1 Harmonic distortion and torque fluctuation 35
4.7.2 Efficiency and cooling of converters 37
4.7.3 Four quadrant operation 37
4.8 Summary of converter data
38
Converter-motor combination
39
... . ...v
,... 4.. . 5.5.1 Possibilities and summary 39
5.2 Power factor cosy 40
1.3 Torque-speed characteristic 42
1.4 Stopping, reversing and braking 44
1.5 Controlling 44 1.6 Conclusion 46
6. E-thruster Concepts
48
6.1 Introduction 48 6.2 E-thruster variations 48 6.2.1 E-motors 48 6.2.2 Propeller 516.2.3 Orientation and layout 53
6.3 Auxiliary E-thruster components 56
6.3.1 Azimuth System 57
6.3.2 Sealings 57
6.3.3 Data and power transmission. 58
6.3.4 Cooling 58
6.3.5 Shaftline and bearings 59
6.3.6 Shaft brake 60
6.4 User Requirements, Classification Demands and Technical Limitations 61
6.5 Field of application 63
6.6 Dimensioning of Propeller, E-motor and pod diameter 66
6.7 Dimensions, hydrodynamic properties and manoeuvrability of the E-thruster 67
6.8 Conclusion 68
Literature
69
... ...
Summary
Lips is currently investigating a new type of steerable thruster that will overcome the power limit of present-day thrusters. The Lips thruster range extends to 6600 kW and the largest thruster ever built is the 7500 kW ARC 1 of Aquamaster. In the new thruster, the bevel gear that normally drives the propeller and that is the limiting factor, is no longer used. The fixed
pitch propeller is directly driven by a large AC motor with the speed (frequency) being controlled by a converter. The first thrusters of this kind are already being sold under the name ABB Azipod, which is the abbreviation of azimuthing podded propulsor.
Lips is interested in the possibilities and technical specifications of such a thruster, to decide if
it should be added to the Lips product range. The electric drive is a major part of the thruster and the Lips knowledge of such electric systems is not up to date. Therefore, the main part of this report deals with the principles of electric motors and frequency controllers, or converters, that control the speed of the motors. Making a good choice for these two components of an electric drive is difficult, because the available information is limited. The E-motors for this application combine a small diameter with a low speed. This means that the motor
construction cannot be based on a standard design, but a totally new design has to be made. There are few manufacturers of large E-motors that are able to do the redesign. An attempt was made to discover parameters that describe the main dimensions of the electric motor, so
that a preliminary design can be made without the complete design ofa real motor. The
parameters also can be used to provide a quick indication of the size of the motor for various power levels. This approach proved to be wrong and a single motor had to be selected as a reference source. Because the possibility exists that the desired electric motor has to be built
within two years, which can be more or less considered as immediate, no future technologies, such as the use of permanent magnets, were selected.
An inventory of the various components of the E-thruster together with the electric motor
provides an impression of the possiblespace and technical problems of such a thruster.
Apart from the size of the motor, the cooling has shown to be a problem. This is the case for
almost any conventional E-motor, whether it's a synchronous or an asynchronous motor. The large flows of cooling medium have to be transported from and to the ship. This holds
especially for air cooled systems, which is the current choice ofmost of the manufacturers.
Another problem is the supply of power to the thruster pod. High voltage, high current and a
low rotational speed make the design of a troublefree slipring device for application in the azimuth mechanism difficult. A short overview of the different requirements for such a thruster indicates that the dimensioning of the azimuth mechanism, the selection of the bearings and the optimisation of the hydrodynamic properties all have to be done with great care.
From a small range of possible future thrusters, one option is chosen for further development. This option is a 7 MW thruster, for pulling mode application on a cruise ship and for
application in an offshore DP system. The motor that is foreseen in the thruster is slow speed, 170 rpm, conventional synchronous motor with very small dimensions. A feasible diameter was estimated at 1700 mm combined with a length of not more than 4000 mm. A first indication by different E-motor manufacturers proves that 2000 is a more realistic minimum at this moment, with the length being estimated correctly at approximately 4000 mm. The direct connection between motor and propeller is responsible for the low speed diameter problems.
I. Introduction
This report is a feasibility study for a new type of steerable thruster, a large power electric thruster. Steerable thrusters are normally equipped witha double or single bevel gear, of
which the size and thus the power capabilities are limited. The intended solution is to design a thruster in which the propeller is driven by an electric motor that is placed in the underwater part of the thruster. The electric motor is speed controlled in both directions and connectedto
a fixed pitch propeller. This kind of large thruster, already introduced into the market by ABB Azipod, makes it possible to apply the benefits of thrusters to a larger number of ship types. This study, carried out for Lips B.V. in Drunen, the Netherlands, has two purposes. The first is to supply Lips with more knowledge on the components of electric drives, so that a better design can be made when considering an electric propulsion system.
The second purpose is to list all the components and to retrieve the dimensions of the main component of the thruster, the electric motor. This should lead to a preliminary design, a thruster designed to fit in a 2.7MW propulsion system for a relatively small cruise liner. In the second chapter, a small explanation of the concepts of diesel-electric (DE) ship propulsion is given. The ship types that currently use the new type of electric thruster are described and a short list of companies that are developing or building the thruster is given. The third, fourth and fifth chapters give an overview of the electric components of a DE system. The emphasis is put on the two main components, the E-motor and the accompanying converter. Because only the relevant installations for ship propulsion are considered, this is not a complete overview of all the available technologies. An effort is made to derive design parameters for electric motors and to indicate the most feasible converter technology for the short-term future.
In chapter six of this report, different options for the number and placing of thrusters on the ship are discussed. Subsequently, the auxiliary components, like the seals, bearings and hydraulic azimuth system, are listed and described. A small overview of future possibilities for an E-thruster range is given. For two main options, propeller and electric motor
dimensions are determined. The chapter concludes with the recommendations for further development.
44.
pint mover (de emire t1d1isscn (tfrtstticdc-st-efts-geertax) ficceller2. Ship propulsion
In this chapter, a global description is given for conventional, diesel-electric, thruster and E-thruster ship propulsion.
2.1 Conventional
A conventional (mechanical) propulsion system consists ofan internal combustion engine
directly connected to the propeller or by means of a reduction gear in case of a medium or
high speed motor. In figure 2.1 the layout ofa diesel-mechanical (DM) propulsion system is
given.
It is possible to use gas turbines as prime movers, but for reasons of simplicity, only diesel engines are considered in this report, where necessary.
figure 2.1 Layout of a diesel-mechanical (DM) ship propulsionsystem
2.2 Diesel-Electric
Recently, DE systems are used more frequently in stead of DM systems for the propulsion of ships. The propellers are driven by E-motors and the AC power for these motors is delivered by diesel generator-sets. In figure 2.2 an example of a layout of a diesel-electric (DE)
propulsion system is given.
figure 2.2 Layout of a diesel-electric (DE) ship propulsion system [/ME 1995-9]
For a number of ship types, such a DE propulsion system can be a good alternative. For vessels with Dynamic Positioning (DP) systems, icebreakers and research vessels, the use of DE propulsion and manoeuvring is well known. The market for DE propulsion has expanded in the past few years to cruise vessels and is supposed to expand to other ship types as well. The current developments in the DE propulsion field are connected with the growing
availability of affordable power electronics that can control large AC E-motors. These motors require less maintenance and have a higher efficiency than their DC equivalents. The power limitation of DC motors is also a key factor in this development, because the current power levels cannot be reached with DC motors. From the side of the owner of the vessel, more demands are made for lower noise and vibration levels, to satisfy growing passenger demands. A DE propulsion system is able to achieve these lower levels, due to the separation of the power generation and the propulsion.
2.3 Thruster
Definition of a thruster:
'A propulsion unit consisting of an underwater part that can turn around a vertical axis, to which a propeller is fixed, that is rotating around a horizontal axis'
translation of [Waalewijn 19931
The way thrusters are referred to vary. Each manufacturer uses a different name, but the main
concept is the same for 'rudder propeller', 'azimuth thruster' or `steerable thruster'. Bow tunnel thrusters and stern tunnel thrusters are also called 'thrusters', which is confusing, but in this report 'thruster' will be a device according to the definition above.
The thruster consists of a pod connected to the ship by means of a strut, see figure 2.3. The propeller can be placed in front of or after the pod, which is respectively called tractor
propeller and pushing propeller (the term 'tractor' propeller is also used for a propelleror thruster mounted at the front of the ship, but this is not meant here).
The propeller is driven with a Z-drive by a diesel engine or electric-motor. In case of an electro-motor driven thruster, the motor can also be mounted vertically and this is called an L-drive. The thruster can be fitted with or without a nozzle, the latterone is called 'open
thruster'. Standard propellers can be used, but also specially designed propellers and complex configurations of propellers, like propellers on both sides of the pod or contra-rotating
propellers.
2.4 E-thruster
The L- or Z-drive transmission in a conventional thruster effects in an extra mechanical efficiency loss when compared to a propeller driven by a standard engine-gearbox combination.
Secondly, a power limitation exists. The largest L-drive to date, an Aquamastere ARC 1 has
an electrical power of 7500 kW.
When the propeller is driven by an E-motor in the thruster pod, this limitation is avoided. Theoretically, the installed E-power can be 20 MW or even higher, although the first systems of that size have to be delivered yet.
Definition of an E-thruster
An E-thruster is a steerable thruster with an E-motor installed in the underwater housing, the thruster pod. This motor drives the propeller directly or by means of a gearbox.
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figure 2.4 E-thruster propulsion: 11.4MW ABB Azipod on MT Uikku
[IA1E1995-9]
5 0 5 10 IS 25 35 40 .5 50
FRAME DISTANCE RIO
-2.4.1 Benefits and drawbacks of E-Thruster propulsion
The E-thruster basically is a conventional thruster without the drawback ofa mechanical bevel gear. The thruster benefits can be directly copied and combined with those of electric
propulsion. In table 2.2 on the next page, the existing and some additional pros and cons are summarised. The improved manoeuvrability shown in figure 2.5 below is one of the examples of what is to gain with E-thruster propulsion for large vessels.
A major drawback for the E-thruster will initially be the high costs per unit. Since the technology is relatively new and unknown, a lot of research has to be done by the shipyard, thruster manufacturer and the supplier of electric installation.
The E-thruster is a propulsion unit in which the components, propeller, E-motor, azimuthing and (thrust)bearing system are highly integrated. Since the unit is placed outside the ships hull, there is a direct gain in consumed space. The concept of integrated propulsion might be
very attractive for both the E-thruster manufacturer and shipbuilder, as it reduces costs and complexity that both increase considerably for high power level conventional thrusters.
figure 2,5 Manoeuvrability footprint comparing triple Azipods with a
traditional installation with flap rudders [NavalArchitect
: see next page
,
Electric drive
Thruster
Manoeuvrability 1
±
full torque available at anyspeed in both directions'
+
I'3600thrustforce especially at low speedsNoise
and
Vibration
-I- resilient mounting of gensets
,
2
,
4-1-noise of converter and
E-motor at different speeds can
vary Space 1 ' 1 -I-,
I components have a high power density 1
,
-I-long propeller shafts with
(thrust)bearings, stem tubes and sealings become
redundant
+
,components can be freely
placed I
-F no hydraulics for CPP are
needed
I
more components take up more space
Weight
-1- better distribution of
components
rudders, shafts, bossings
become redundant 1
-
Imore components weigh
more ,
I
I
Efficiency
±
less fuel consumption at part load
reduced resistance due to absence of rudders, shafts, brackets and stem thrusters° I fat loss of 10 to 12 % by two
extra conversions
±
I simplified stern shapeFlexibility the only form of energy,electricity, can be directed to, multiple users'
Reliability
and
Redundancy
±
E-motors and converters arevery reliable I 1
±
due to sensibly interconnected components, the system is never fully!down 1 1 -, complex system
Maintenance
'1-I- ' E-motors and converters
require minimal maintenance
_
I
±
I
gensets can be maintained without interrupting operation
I
1
1
-I-diesels operating at fixed speed have less engine wear and engine pollution
11
-
genset contains larger number of pistonsCosts
,
FPP propeller'
±
no rudders, shafts, bossings'I and stern thrusters
I
-
more and expensivecomponents simplified stem shape
1
I, -I- 'less building time
table 2.1 E-thruster benefits and drawbacks
+
+
Information in addition to the information in table 2.1 on the previous
page
The combination of E-motor and converter makes it possible to control the motor speed from zero to nominal speed (100%), with full torque available. For ships like icebreakers or vessels with ice-breaking capability, that frequently have to operate at low speeds, this is very useful.
The placing of the E-thruster and the simplified stern shape, allow for the use of a large tip clearance, which reduces propeller induced noise and vibration. Using the thruster in pulling or tractor mode offers an even larger reduction, since the wake field in which the
propeller has to operate, is more uniform.
The total weight of the DE system is, for low power levels, higher than the weight ofa
DM installation and this is mainly due to the E-motor. For higher power levels replacing
a low speed DM system, the DE system is in favour because of the high power density of
the components.
The bevel reduction gears in conventional L or Z-drive thrusters are responsible for transmission losses, in the order of 1.5-3% for each gear. The higher values are for gears that are fully filled with oil and under cold conditions. Although one would expect to find these losses in the table, they are not applicable for the E-thruster, that does not have bevel gear(s), and are therefore omitted.
5
For some types of vessels, like cruise and offshore vessels, the propulsion power is only part of the total energy consumption. The electric load for hotel and other functions, like driving pumps, is already large. Electrifying the propulsion system can be a good solution for such vessels, because the main and auxiliary powerplant then can be combined into one, that is used in the most optimal way by all the electrical consumers. This is also heavily depending on the operational pattern of the vessel.
The generation of the required power is divided over two or more generator sets, that can be individually switched on and off. The generator sets are optimised, for instance for fuel efficiency, and usually running at 80% load. The margin created that way allows the system to cope with peaks in the power demand. When the load for a certain generator set is above 80 percent for a prolonged time, another set is started.
6,I
A controllable pitch propeller (CPP) is no longer obligatory. A fixed pitch propeller (FPP) is sufficient and this makes the propeller and accompanying shaft line simple and thus cost efficient. However, for some applications, a CPP still is an (expensive) option
2.4.2 Current development of E-thrusters
Several manufacturers of propulsion systems are currently developing an E-thrusteror 'a
similarly named device.
Electric drive supplier Asea Brown Boveri (ABB) together with shipyard Kvaemer Masa Yards (1CMY), are the furthest with their Azipod In this report, the name Azipod will in this report only be used to refer to azimuthing electric propulsion units of ABB/KMY/Fincantieri. A small number of Azipod units is already in use. The DAS (Double Acting Ship) tankers build and Uilcku with ice-breaking capability are fitted with single 11.4 MW units. The river service vessel Seili is equiped with one 1.5 MW units and the river icebreaker Rothelstein with two 560 kW units. A number of Azipod driven ships is now on order at ICMY.
The last two ships of the Fantasy Class of ships for Carnival Cruise Lines (CCL), the Elation and Paradise, will be fitted with twin 14 MW Azipod systems and are scheduled for delivery in 1998. For the same year the delivery of a 2*5 MW icebreaker/offshore construction vessel,
build by Finrwards Oy and equipped with Azipods, is scheduled.
ICMY is currently designing the EAGLE class for Royal Caribbean Cruise Lines (RCCL), which will be fitted with two steerable Azipod and one fixed Azipod, each unit having an installed power of 14 MW. When built, the Eagle class ships will be the largest cruise ships ever. As of October 1997, two ships are on order, to first one to be delivered in 1999 and the second one in 2000. Options for two sister ships are pending.
Siemens and Schottel developed the Siemens Schottel Propulsor (SSP), which is similar to the KMY/ABB Azipod but is based on a permanent magnet (PM) synchronous motor and has propellers on both sides of the pod, to lower the propeller load. The SSP is considerably
smaller than an Azipod of the same power. No SSP has been built and installed on a ship yet. The KaMeWa/Cegelec combination claims to have sold an 8 MW podded propulsor for offshore purposes. GEC/Alsthom is said to be developinga similar propulsor of 7MW for application in an offshore DP system.
table 2.2 Ships with E-thruster (Az:pod) propulsion' (November 1997)
Ship and Type Power E-thruster
manufacturer
'Owner Yard Year
Seili, waterway service vessel
1500 kW ASS Azipod Finnish Board
of Navigation
190
Uikku, arctic tankerLunni, arctic tanker
11.4 MW ABS Azipod NEM.ARC
Thsterreichische Donau 1Craftwerke AG 19931 1995 1995
Rothelstein, river lice
breaker
2.560 kW
- ABS Azipod
Elation, cruise ship Paradise, cruise ship
2.14 MW l' ABS Azipod , 1 Carnival Cruise Line KMY to be delivered February and October 1998 Icebreaker/Offshore construction vessel
2.5 MW ABS Azipod Finnish Board
of Navigation
to be delivered in 1998
Project Eagle: "the next generation of cruise ships"
3.14 MW (one fixed unit)
ABS Azipod Royal
Caribbean Cruise Lines 1 Fincantieri I L 2 on order, to be delivered in 1999 and 2000 1 option (2001)
3. Electric propulsion: Motor
The main components of a DE propulsion system are the electric motor (E-motor) and the converter. In this chapter, the different types of E-motors are listed and their principles and characteristics are briefly explained. The goal of this chapter is to give a short overview of the terminology and a search for parameters that will be used to make preliminary designs, rather than to give all the ins and outs of electric propulsion motors. The information is coming from
several sources [Hamels 1992][van den Hut 1995][Sen 1989].
3.1 DC Motor
The number of ship propulsion systems based on direct current (DC) motors is decreasing. DC motors require more maintenance and are, especially for large outputs, large and heavy. A physical limits exists for the motor speed (kW*rpm < 1.6E6) for DC motors, due to the
strength of the commutation mechanism that is used to invert the current in the rotor. For a 7 MW motor, the maximum speed would be approximately 230 rpm. Normally, DC motors are
to 2 MW in size, but 10 MW is possible.
By varying the supplied voltage, the motor speed can be controlled easily and accurately. For a long time, this aspect has made the DC motor superior to the AC motor. Due to the
commutation mechanism, the DC motor requires more maintenance and is (slightly) less reliable than an AC motor.
Nowadays, the accurate speed control characteristics of a DC propulsion system are matched or even outperformed by AC motors combined with modern static converter technology. Therefore, and because of the lowering prices and dimensions for AC motor speed control systems, the choice is made not to consider DC ship propulsion systems motors in this report.
3.2 Synchronous (AC) motor
Motors of the synchronous type are commonly found in ship propulsions. This is not surprising, as synchronous motors are reliable, have a high efficiency and their speed can accurately be controlled with a static converter.
Principles
Generally, a large synchronous motor is consisting of stator windings (armature winding), connected to a 3-phase power supply, which is nowadays normally a static converter. The rotor carries excitation windings (field winding), which are fed through sliprings and brushes by an external DC supply or by an auxiliary winding and a diode rectifier on the rotor. The field winding is divided into 2 or more pole-pairs and generates a constant flux. The rotor can be built cylinder shaped with the windings in grooves or with the windings as separate
'blocks' bolted onto the rotor body. The latter one is called a salient pole rotor. The size of the
gap between the rotor and stator, the `airgap', has a great impact on the properties of the motor. Normally, a smaller airgap means better properties, but the airgap is limited by
mechanical properties like the rotor stifness. A synchronous motor driven by a (gas)turbine or a diesel engine acts as a (three phase) generator.
When the motor is running, the rotor will follow the field in the stator with the same,
'synchronous' speed, given by the following equation:
f
ns =
( 3.1)
in which n, is the synchronous speed in 1/s, f is the frequency of the ACpower supply in Hz and p is the number of pole pairs (half the number of poles).
examples:
p = 1 (2 poles), fne, = 60 Hz nit = 3600 rpm
p = 3 (6 poles), fact = 50 Hz >ri, = 1000 rpm
As can be seen, adding more pole pairs to the rotor decreases the nominal speed of the synchronous motor. Construction of a motor with a larger number of poles also allows for a higher nominal torque. The maximum magnetic force in the airgap is more or less a constant,
1.2-1.4 Tesla, but can be achieved better by the larger number of smaller copper windings and stator core material that is less affected by saturation effects.
The motor can be 'double-wound' which means that the stator carries two 3-phase windings of which the physical orientation is shifted over an angle of 30 degrees.
Because the total current is divided over two windings, these windings are less thick and more
efficiently placed on the stator's contour. Although the total resistance(R) of the double
wound design increases, the heat production (I2R) is lower due to the bisection of the current (I). Furthermore, the double wound design is more redundant because of the possibility to run the motor with one active winding developing half the rated power. Drawback of having two
3-phase windings is the doubling of the converter's number of semiconductors. This will be explained later in chapter 5.
Synchronous motors or generators, sometimes also called 'wound-field' machines, are
available in a number of makes and range from small to very large. The largest synchronous machines are over 500 MW in size and can be found in powerplants such as high-speed generators driven by steam or gasturbines.
Efficiency
The efficiency of the synchronous motor is depending on the power fed to the DC windings. This power is converted into heat and therefore it is necessary to minimise this loss.
Nevertheless, the efficiency of a synchronous motor is high. For the motors of ship propulsion systems Ti = 94-96%.
Speed-Torque Characteristics
The synchronous motor delivers a torque only at its synchronous speed. Therefore, the torque-speed characteristic is trivial. However, the maximum motor power and thus the torque are limited, depending on the load of the motor. In the loaded motor, the stator voltage and the rotor have the same frequency or speed, but are shifted in phase. This phase shift is called the power angle and the delivered torque varies with the power angle as can be seen in figure 3.1. For a power angle between 45 and 90°, depending on the motor design, the torque reaches a
maximum, the pull-out torque. Any load that exceeds this torque causes the motor to lose synchronisation.
If the frequency or rotational speed of the rotor does not differ much from the frequency of the rotating stator field, the motor is self-synchronising. For larger differences, the motor torque becomes almost zero until equilibrium is established again, so it is necessary to have an
.figure 3.1 Torque versus power angle for synchronous motors
The parameter on the X-axis is the power or torque angle S,
which is the phase shift between the rotor and stator voltage. 8
is depending on the load of the motor.
accurate control system for the speed of the motor to avoid torque instability.
Speed Control
For two or three different motor speeds, a motor with a switchable number of pole pares can be used, but this is not a common solution for ship propulsion. Normally, speed control of the
synchronous AC motor is done with the use of a static frequency converter that controls the motors input frequency. The voltage also has to be varied proportionally with the frequency to
maintain an optimal magnetic flux. Since the excitation of the rotor is done with a separate DC system, there is an extra flux control parameter and therefore the relation between input frequency and voltage does not have to be very strict.
Benefit of the synchronous motor is that the thyristors in the accompanying converter can be commutated by the motor itself, see section 4.5. This allows for a small and cost efficient converter.
Power factor
The power factor of the synchronous motor has a more or less constant value, which is depending only on the current in the field winding. With variation of the excitation current, the stator current phase shift y can be controlled to be positive or negative, with the power factor cosy being ranging from 0.8 to 1.0.
Starting
The synchronous motor cannot start by itself when fed with a constant frequency, because the speed of the rotor has to be the same as the rotational speed of field in the stator. If a static converter controls the motor, the starting is done by increasing the frequency, starting from zero. If there is no converter, it is possible to install an auxiliary squirrel cage on the rotor. This squirrel cage is used to start the motor and is switched off when the synchronous speed is almost reached.
Reversing and braking
This item is the same for synchronous and asynchronous motors and closely related to the converter technology used. It will be discussed later in section 5.4 Stopping, reversing and braking.
Reliability and maintenance
The synchronous motor is very reliable, but due to the brushes that are needed to feed the
excitation windings, the motor isn't fully maintenance free.
3.2.1 Permanent magnet (PM) synchronous motor
At this moment, synchronous motors are developed that are built with permanent magnets as replacement of the excitation windings on the rotor. The motor can be designed very much
like a conventional synchronous motor, i.e. with a radial flux (RF) distribution or totally new, as is the case with axial (AF) or transverse flux (TF) motors (see figure 3.2, 3.3 and 3.4 on the following pages).
The used magnets are of the 'rare-earth' type, Samarium-Cobalt (Sm2Co7) or
Neodymium-Iron-Boron (NeFeB). An important factor for PM motors is the heat production, since the flux density of permanent magnets decreases as temperature rises. Samarium-Cobalt has good characteristics up to a temperature of 150°C and is cheaper than NeFeB, which is stable up to I20°C. Due to the method of excitation, these machines behave as under-excited motors that
have to be controlled by a self-commutating converter (Pulse Width Modulation PWM or Cycloconverter CCV).
figure 3.3 Layout of a (synchronous) permanent magnet motor with axial flux distribution
is done by coupling the stator housing to the pod, using the surrounding seawater. The lower heat production is also one of the reasons for a higher overall efficiency, which Siemens claims to be as high as 98%. [AES97 002]
figure 3.2 Rotor layout of a (synchronous) permanent magnet motor with
radial flux distribution left: [AES97 002]
Siemens offers a 14 MW radial flux PM motor, with Sm2Co7-magnets, in the Siemens-Schottel-Propulsor (SSP) project. The design is very much like a conventional synchronous motor, i.e. a radial flux motor.
Siemens claims that this kind of motor can have a diameter reduction of 40% with a weight reduction of 15%, compared to a conventional synchronous motor, with the same
performance. Since the excitation is not 'active', i.e. with windingson the rotor, the heat production in the motor is confined to the stator. Keeping the statortemperature low thus
Another benefit of any PM motor, i.e. apart from the flux technology that used, is the possibility to use a large number of poles. This enables an increase of the input frequency without changing the speed of the motor. As the input frequency range gets larger, controlling, the motor speed becomes easier.
Permanent magnets are already applied in small motors, but their use in large motors isnew. At this moment it is unsure if these motors will be feasible for ship propulsion in general and specifically for application in an E-thruster in specific. Little is published about the behaviour of the permanent magnets in time, i.e. whether the flux of the magnets will decrease rapidly or slowly due to normal use or exposure to shocks. ABB states that the production of the
magnets and the assembly of the motors will have to be done in an ultra clean environment with specialised machinery ITheMotorShip 7-19971. Because of the large magnetic forces, the installation of the motor at the shipyard will also have to be carried out with great care and in a similar environment..
figure 3 4 Layout of a (synchronous) permanent magnet motor with a transverse flux distribution
3.3 Asynchronous (AC) motor
Not many asynchronous motors are used for ship propulsion. This is mainly because no combinations of asynchronous motors and converters are available for the high power levels that are needed. The major benefits of the asynchronous motor are its robustness and high efficiency.
Principles
The asynchronous motor is almost identical to the synchronous motor except for the rotor. The stator can again be single or double wound, like the synchronous motors stator, with one or two 3-phase windings and can be fed by a static converter when the motor is used for propulsion purposes. In the brushless asynchronous motors, the rotor is a 'squirrel cage',
consisting of solid bars of copper or aluminium, connected at the ends. According to the required torque-speed characteristic, there can be a single or a double cage and the bass of the cage can have different shapes, for instance deep and narrow or cylindrical bars.
Another asynchronous motor type is the slipring induction motor. It has windings on the rotor that can be connected to resistors outside the motor. By adding resistors to the rotor circuit, high starting currents are avoided, which keeps the rotor temperature low, and themotor
characteristics, for instance the torque, can be controlled.
In both types, the necessary magnetic flux that is responsible for the motors torque is generated by induction.
The configuration of the windings in the asynchronous motor is not as efficient as in the synchronous motor, because the power needed for the excitation has to be transferred by the stator windings. This makes the physical and electromagnetic layout of these windings less optimal, since part of the copper winding capacity has to be used for induction purposes. To ensure a proper induction, the airgap (space between rotor and stator) is relatively small. This makes a large asynchronous motor less resistant to shock loads.
An asynchronous motor cannot be used as a standalone generator, due to the way the rotor field is generated. However, the motor is capable of regenerating braking energy.
Slip, efficiency and power factor
As a logical result of torque generation by induction, the shaft speed of the asynchronous motor is slightly lower than the frequency of the rotational field in the stator windings, defined with the following equation:
n n
(3.2)s = 100%
in which s is the slip of the motor,; is the synchronous speed, the speed of the rotating field
cos
Lp 0.60.4f
122
aZ 0.4.
as
al
0 1.2.The slip of the motor is load dependent: the slip increases when the torque differs from the nominal torque. Typical values for the (nominal) slip are 0.01 to 0.05.
As with the synchronous motor, the speed of the motor is depending on the supplied
frequency and the number of poles. Combining equation 3.1 for the synchronous speed ris and equation 3.2 yields the equation for the speed of the asynchronous motor:
n = (I - s)
(3.3) Like the synchronous motor, the asynchronous motor is very efficient, with values ranging from 94 to 97 percent at rated speed. The low values are for small motors (between 30 and 750 kW); the motors for ship drives have efficiencies of 96-97 percent. An increase in the number of poles (motor speed) has a negative influence on the efficiency.
The efficiency is not constant and lowers considerably, down to 80%, for speeds less than 50% of the nominal speed.
The power factor cos cp of the asynchronous motor depends on the load of the motor. As illustrated in figure 3.5, cos y increases with the load, which is due to the slip, that increases with loads different from the design load and influences the mutual induction of rotor and stator.
It becomes clear that the torque, slip, power factor and efficiency are very much coupled, but describing their relations requires a more thorough investigation of the electric and magnetic processes, which is outside the scope of this report.
Speed-Torque Characteristics
The speed torque characteristics for the different types of asynchronous E-motors are given in figure 3.6.
.figure 3.6 Torque versus speed for different types of asynchronous motors.
I) double squirrel cage. 2) squirrel cage with deep and narrow bars. 3) squirrel cage with cilindrical bars. 4) slipring induction
motor.
TN = nominal torque nn = nominal speed [Hamels 19921
Speed Control
The normal way to control the speed ofan asynchronous motor used for propulsion purposes
is by varying the input frequency. The voltage is coupled to this frequency and both have to be precisely controlled to maintain an optimal flux, because the asynchronous motor does not have the benefit of an adjustable excitation. As can be seen in equation 3.4 the voltage Us has to be varied proportional with the frequency f when the flux GI) in the stator is to be kept at a constant maximum.
2
Us = Om. (3.4)
ns
in which w, is the effective number of windings in the stator. 7;
It is said that the speed control of an asynchronous motor is less accurate, but this is only true for very low speeds, i.e. 1 percent of the nominal speed, and not very relevant for ship
propulsion [HOLEC 0011.
Starting
The asynchronous motor is able to start by itself, but ina ship propulsion system, the speed of
the motor is to be controlled by the input frequency, in order to limit the power that is directed
to and can be consumed by the accelerating propeller. So, the starting process of the
asynchronous motor can be compared to that of the synchronous motor, with the frequency being increased from zero to nominal.
Reversing and braking
When braking, which eventually means that the motor is used as a generator, the slip of the motor works the other way around: the shaftspeed for an asynchronous generator is higher than the output frequency. Note that an asynchronous generator isnot capable of
independently supplying power to a network, due to the necessary induction. Therefore, asynchronous generators are seldom used. The asynchronous motor can be reversed by swapping the supply of two of the three stator phases. Braking and reversing will be more elaborately discussed in section 5.4.
Reliability and maintenance
The asynchronous induction motor is even more reliable than the synchronous motor. Due to the absence of the slipring mechanism, the brushless (squirrel cage) motor requires very little maintenance. Normally only the bearings have to be monitored.
3.4 Parameters
For E-thruster application, the E-motor is specially designed and built, which means that the motor and its relevant specifications cannot be chosen from a manufacturer catalogue.
Designing an E-thruster will have to be done in close co-operation with specialists on the field of electric drive/propulsion systems. For this application the requirements for the E-motor are, in order of decreasing importance: small (diameter and length), low speed, reliable, low maintenance, lightweight and silent. Except for the requirements for size, these criteria do not differ much from standard design criteria for electric drive motors.
There seems to be no existence of design parameters for power density, length, diameter and weight for the different E-motor types. This was more or less confirmed by Dr. fr. Gravendeel
[Holec ow] and Prof. Dr. ing Deleroi, section Power Electronics and Electrical Machines from
the faculty of Electrical Engineering at Delft University of Technology. This is astonishingly
different from the situation with diesel engines, wheresuch parameters are 'common
knowledge'. However, parameters or coefficients are necessary to create and evaluate a
preliminary design for an E-thruster,so an attempt was made in this paragraph to find a few of those parameters. In table 3.1, technical data is presented for a small number of systems. On page one, the most recent and relevant data is given. The secondpage is added to give an impression of the technical specifications for older designs of E-motors. The data is extracted
from existing motors, specifications ofmotors found in the literature and data provided by
manufacturers on request of Lips. The accuracy is not the same for all the data and this is made clear in the table by printing the concerning data in italics.
Motors meant for or specifically designed for wilication in E-thrusters
Various 20 MW E-motors added for comparison purposes
ABB [EMY/ABB (102]
I
Siemens fAES97 0021
Jeumont Industries (Lips documentation X40200)
Jeumont Industries fAES97]
Rolls Royce ELME 1995-41
MW 11.4 14-14 3 7.4 16.5 20 20 20 20 20 20 rpm 120-160 160 160 200 140 142 180 180 180 180 180 180 kNm 726 836 836 143 505 1110 1061 1061 1061 1061 1061 1061 It 12 . 18 12 20 40 16 16 60 150 Hz 12-16 20 23.3 47,3 24 24 90 225 mm 5 12 6 mm 1900-2150 2150 2100 1570 2200 4820 5600 2500 3680 3680 3360 2600 mm 3640-4250 4900 3415 3520 3520 3520 3600 2500 3500 3600 2900 2600 m3 10,32-15,42 17.79 11.83 6.81 13.38 64.23 88.67 12.27 37.23 38.29 25.71 13.80 t 413 21 35 85 120 52 80 100 50 40 kW/kg 323 143 211 194 167 385 250 200 400 500 [-] 1,69-2,23 2.28 1.63 2.24 1.60 0.73 0.64 1.00 0.95 0.98 0.86 1.00 MW/m 5,30-6,00 6.51 6.67 1.91 3.36 3.42 3.57 8.00 5.43 5.43 5.95 7.69 MW/m3 0,73-1,10 0.79 1.184 0.440 0.553 0.257 0.226 1.630 0.537 0.522 0.778 1.449 IcNin/m3 47,08-70,35 16.97 70.64 21.02 37.72 17.28 11.97 86.46 28.50 27.71 41.26 76.86 26-51 ---60 40 29 40 194 339 47 142 146 98 53 1.00 1.05 1.58 0.59 0.74 0.34 0.30 2.17 0.72 0.70 1.04 1.93 PI 98 94.8 96 97 97.2 97.2 97 98.2 98.6 kV 2*1,45 3.3 1.5-2.5 5.5 5.5 1.41 A 2900 850-1420 1150 2560 8*1335 H 0.85 0.86 0.7 0.7 0.8 0.87 0.95 0,9' 0,62 ABB S Dw 12 pulse CCV ABB S Dili 12 pulse CCV Siemens S PM 12pulse ccV jeumont AS PWivi Jeumont AS Jeumonl AS Jeumont S Jeumont S PM,TF RR Paper AS DW RR Paper S IDW RR Paper s PM,DW RR Paper 8 PM,T-F 8 phases Azipod
DAS Tankers Uikku & Lunni
Afloat Cruiseships Elation &Paradise rotor 23 t stator 20.3 t rotor 10.3 t stator 10.7 t 4 discs 4 discs 4 rims DA=Double Armature
PM=Permanent Magnet Synch&
LC1=Sync_hro Converter
at full load
DW=Double Wound
TF=Transverse Flux
CCV=Cyclo Converter
?only with sinusoidal current
B=Brushless
PWM=Pulse Width Modulation!
page: 1 of 2
t-t
All E-motors on this page are meant fi - ff.jspecificaly
designed for application in Farusters
GEC Alsthom (Lips documentation X40200)
Ansalqo Lips documentation X40200)
Mw ..::::i. 7 7.4 74 16.5 16.5 16.5 16.5 16.5 2x4260 2)(2)(2137 2x9450 2x2x4750 2x9400 2x9400 2x9400 rpm 1000 1000 170 170 140 140 140 140 160 160 160 IcNm 48 67 281 393 505 505 1125 1125 985 985 985 16 16 16 16 12 12 12 Hz 18.7 18.7 18.7 18.7 16 16 16 nun >9 29 mm 1900 1900 3540 3600 3200 2400 4600 3200 2580 2580 2310 mm 2650 3000 4000 4900 7800 15000 7900 15500 9900 4340 3500 rn3 7.51 831 39.37 49.88 62.73 67.86 131.29 124.66 51.76 22.69 14.67 t 24 - 29 46.00 65 110 112 172 224 154 155 kW/kg ' 20833 2417311 108.70 107.69 67.27 66.07 95.93 73.66 107.14 106.45
ii
1.39'1 8
1.13 1.36 2.44 6.25 1.72 4.84 3.84 1.68 1.52 MW/tri 2.63 68 1.41 1.94 2.31 3.08 3.59 5.16 6.40 6.40 7.14 MW/m3 0,665 0.823 0.127 0.140 0.118 0.109 0.126 0.132 0.319 0.727 1.125 kNin/m3 6.35 7.86. 7.13 7.88 8.05 7.44 8.57 9.03 19.03 4140 67.14 186 202 390 3' 70 176 77 50 089 1,10 0.17 0.19 0.16 0.15 0.17 0.18 0.43 0.97 1.50 Fl 96.5 96.2 97 96.5 97.5 97 97 kV 2x2.6 2)(2si2.6 2x2.8 2x2x2.8 2.6 2.6 1.6 A 2x946 2)(2x474 2x2098 2x1055 2x1550 2x1550 [-] 0.9 0.9 0.9 0.9 0.9 0.9 0.9 GEC Alsthom S LCI : M incL GIX medium speedGEC Alsthom
S
LC1I
Mind. 013X medium speed
GEC Alsthom S WI GEC Alsthom S LC1 Ansaldo S B,DW 12 pulse WI Feb-95 Ansaldo S B,DA,DW 24 pulse LCI Feb-95 Ansaldo S B,DW 12 pulse ClL Feb-95 Ansaldo S 13, DA,DW 24 pulse LCI Feb-95 Ansaldo S
B,DW 12 pulse LCI Jun-95
Ansaldo
S
DW
24 pulse LCI rotor 67t Oct-95
Ansaldo
S
OW
24 pulse LC1 Oct-97
DA=Double Armature
PM=Permanent Magnet Synchr.
LCI=Synchro Converter
DW=Double Wound
TF=Transverse Flux
CCV=Cyclo Converter
B=Brushless
PWM=Pulse Width Modulation
page 2 of 2
roximation,s_or taken from sketch dra
-.
ti
gad: (4/01/98
The (dimensionless) coefficients normally used by designers of E-motors are primarily for the calculation of electric and magnetic properties and cannot be used directly.
The basic equations for an electric motor show that the effective torque Te is depending on (rotor)diameter, length and the magnetic force in the airgap. A general equation can be given as:
=Km D2 .1.13. -Js
(3.5)Km is a form factor depending on the behaviour of the magnetic field and is constant, but different for each motor type. B. and J, are factors that indicate the magnetic and electric load of the motor and can be more or less considered as fixed values. D is the rotor outer diameter and 1 the active length of the motor.
Equation 3.5 can be simplified, which results in equations 3.6 and 3.8 for torque Te and internal mechanical power Pmech:
=C1 D2 -1 Pined = 2 . 71" T - n Pmech = C2 D 2 1 n (3.6) (3.7) (3.8)
CI and C2 are motor constants.
From these equations it follows that the active volume of the motor, 1/4-7E-D2-1, has the largest influence on the maximum torque that can be reached. For thisreason, the 'active' volume of the motor was added as the motor parameter 'Volume V'to table 3.1. This parameter is based
on the outer diameter of the stator as this is the only value that is available for most of the motors. The number of poles also has an influence on the size of a motor. The same motor (speed and power) with a greater number of poles has a larger diameter.
For the parameter generation it was decided to keep the number of poles out of focus, since this number does not differ much for the conventional motors that are considered here and it would make things unnecessarily complicated.
As can been seen in figure 3.7, the expected linear relation between the torque and the volume of the motor is not found. This result is the. same when the synchronous motors are !considered as a separate group. 140y00 120.00 100,00 80,00 60,00 40,00 20100 0100 0 Ansaldo S ICI 1-Jeumont AS PM A Siemens S PM CCV ABB S CCV RR Paper Jeumont $ Jeumont S RATE A 0:
r
figure 3.7 Volume (m3) Torque (Wm)
The same remark can be made for the data in figure 3.8 below, where 1321n power is displayed, and again the expected or desired linear relation does not appear.
figure 3.8 D2117(m3/s) Power OM149
0 0 5 10 151 20, .25 Power '(MW) 500 400 x 200 100 0 Ansaldo S LCI Jeumont AS PWM A Siemens S PM CCV ABB S CCV RR Paper Jeumont S Jeumont S PAAJF 1' _ 1 A 0 I 0 El o JI o o 9
SI
Ii -- r---O' nE 300 - -I 0, .200 400 600 800 10001 1200 Torque (kNm) -0 0 E 0 060001 5000 - 40001-,E ir2 3000 1E !al
a
2000'-11000 OArtsaldo S WI*GEC Alsthom $ LCI medium speed
Jeumont AS PVVM
Z GEC Alsthom S LCI A Siemens S PM CCV ABB S CCV RR Paper Jounce $ !turnout S PM,TF 4 0 _ 1 I.
Although it would be an unexpected result, diameter and torque or diameter and power
this is not the case. The figures show that a
diameter when compared to a synchronous
figure 3:10 Diameter (m) Torque (kNm)
it would be interesting if a relation between
was found. As can be seen in figures 3.9 and 3.10
PM motor does not necessarily have a smaller motor of the same speed.
6000 5000 4000 11-1 3000 a)
P
1000 0Ansaklo $ ICI*GEC AlPhom $ WI medium speed
+Jeumont AS PWM X GEC Ahlhom S LCI A Siemens S PM CCV ABS S CCV RR Paper Jeumont S Jeumont S PM,TF _/14 0 200 400 600 Torque (kNm) 0 -
----LI
- _ - L-1 800 1000 1200figure 3.9 Diameter (in), Power (MW)
5 10 15 Power (MW); 2000 a) 0 0 I E 0 I L L 0 20 25
Because sensible parameters for these motors could not or not easily be calculated, a very practical approach was chosen. In figure 3.11 the volume-power relation is displayed. The specific volume (MW/m3) will be used as the main parameter in the remainder of this report. As can be seen, the values for the ABB Azipod motors are relatively low.
This non-dimensionless coefficient for the motor volume can also be found in table 3.1. It cannot be used for the design of an E-motor, but it will be sufficient for this study.
Another reason to choose a simple parameter is that the detailed information neededto design
an electric motor is unknown during the preliminary thruster design stage.
0
0 Ansaldo S LC!
*GEC Alsthom S LCI medium speed +Jeumont AS PVVM
*GEC Alsthom S LCI A Siemens S PM CCV ABI3 S CCV ...RR Paper Jeumont S Jeumont S PM,TF
80
---40 11
1 , 1 ',-- 4 20--+ i + - , , + 4.1
, 0 5 TO 15 20 25 Power (MW) -0 4.figure 3.11 Volume (m3) Power (MW)
140
120
100
-9 60
figure 3.12 Efficiencies, based on a normal propulsion characteristic, of
different AC motors [IME 1995-4]
In the table below, a few representative values for the different E-motor parameters are collected. When no source is given, the data is extracted from table 3.1 and printed in italics.
3
' PM: Permanent Magnet; RF: Radial Flux; IF: TransverseFlux;2[AES 97002]; [IME 1995-4];
4 5
[Holec 001]; - [van den Hul 95]; 6 [SEN 1989]; 7 See figure 3.12;8 9
[Jeumont 001]; [IME 1995-7] '9 ABB motors; II Other motors
En ic lency A ,co
..
So 70 GO 50 40 30 10 i 0 (X) IDA PM Sync In duc con Wound-field syncnronous30 efs SO/ SO 70 so, so Ion
Asynchronous Synchronous Synchronous
brushless Synchronous PM,RF" Synchronous PM,TFI Size relative 1 I 1.1 0.99 0.719 Volume/Power MW/m3 0.75-1.109 0.75-1.109 0.7 1.182 1.5 Parameter 0.10-0.7310 0.10-0.7310 Weight relative l '' 12' 1.3 0.852 0.719 0.469 Efficiency' [A] 954 94-9455 almost 982 98.23 97.2 97' identical to synchronous motors
Power Factor relative Variable with 0.85 0.85 0.852 0.78
motorspeed 0.8-1.0' 0.7-0.85'
Reliability relative ++ +-+ ++? ±4-?
Costs relative j2 1,152 ? ? ?
table 3.2 E-motor parameters
+
3.5 Summary and conclusion
Summarised comments on size and type
In general, it can be said that the motors that have to be used in an E-thruster are slender (thin and long) and have a less optimal electrical design, mainly because of a low speed combined with a small diameter. For the E-thruster, this diameter is the most important factor. The size of any E-motor is a trade off between length and diameter, with the length increasing with 20% if a 10% smaller diameter is chosen. The length itself is limited by the stiffness of the rotor and the desired airgap that has to be maintained for both electrical (torque) and
mechanical (shock resistance) reasons. For asynchronous motors, the airgap is small to ensure proper functioning of the induction mechanism. The synchronous motor is less critical on this point.
The number of motors that is actually built for application in an E-thruster or designed to fit in a comparable space is very small. The electric installations in the current and near future E-thrusters are all from one manufacturer, ABB, and they do not lavishly publish their technical specifications. The motors in these ABB installations are operational, so they can be more or
less considered as 'proven technology'.
This is supported by the trend towards smaller dimensions that becomes visible when looking at the sizes for Ansaldo motors in table 3.1. The values between brackets in the last column are be the preliminary dimensions of a motor built at this moment.
From the same table 3.1, it can also be concluded that a straightforward choice for a certain type of E-motor is not possible, since the motor type is depending on the manufacturer and acceptation by the client.
For ship propulsion systems, the synchronous motor seems to be the most common option (when applying an AC drive). Although induction motors have the benefit of being slightly smaller and very reliable, not many of them are found in ship propulsion, which is contrary to other industries, where induction (asynchronous) motors are preferred. The efficiency of both the synchronous and asynchronous motor is more or less equal when considering these motor sizes. Asynchronous motors will be a slightly smaller and lighter, but not so much, so no large influence on the thruster design is to be expected.
The PM motor technology looks very promising, but no large motors are commercially operational yet and this will probably be the case for the next three to five years. The PM motor is an interesting future development that is rising and is to be watched.
As for the influence on the E-thruster design, PM motors will fit relatively easy in an existing E-Thruster concept. The motor is smaller, weighs less, is demanding less cooling capacity and applying a PM motor will certainly improve the electrical characteristics of the E-thruster. Conclusion
The synchronous motor that ABB has installed in the Azipods for the DAS tankers Lunni and Uikku is chosen as the reference for the size of large synchronous motors. The specific
volume, value 0.75 MW/m3, obtained from the ABB design for these ships, will be used in the remainder of this report. A more detailed motor design may lead to an improved value, but this is beyond the scope of this report.
4. Electric propulsion: Converter
The second main part of a DE propulsion system is the converter or frequency controller. The speed of the (AC) motor in the propusion system is varied by the converter and the increasing availability of large converters is the main reason why electric drives are becoming popular. The information for this next part can be found in [Hamels 1992], l[vanden Flu! 1995], [TheMotorShip 2-1997[, [SEN 19891 and [Siemens 0021. First, the basic components, the semiconductors, are briefly described. Then, attention is given to rectifiers and inverters, which are consisting of semiconductor bridges and form the main components of pulse-width-modulation, cyclo and synchro converters. The characteristics of each of these converter technologies are explained. Because an unlimited number of converter technologies seem to exist, only the ones that are common for application in an electric ship propulsion are described. Finally, a comparison between the converters is made.
The field of the converter technology is considerably larger than indicated by the explanation and the examples given in this chapter. However, the information should be sufficient to understand the basics of converters for ship propulsions.
4.1 Electronic components of converters: Semiconductors
The main component for every converter is a semiconductor, re. a diode, thyristor or a transistor.
A diode is capable of allowing current in only one direction. Other aspects of the current cannot be controlled. The main application for diodes is the use in uncontrolled rectifiers.. Normal thyristors are similar to diodes but can be made conductive by applying a voltage
from an external electronic circuit. They are switched off when a zero crossing of the current occurs.
Thyristors can thus be used as a switch and are applied in converters for higher powers due to their high voltage and current capabilities.
Both on and off switching is possible with Gate Turn Off thyristors (GTO) at frequencies up to 600 Hz. They are used in converters with very accurate control characteristics. "To be switched on or off, they need high current pulses, up to several hundred amps (A). Even
though it is needed for a very short time, these current pulses make thegate drive circuitry complex and costly."[IME 1995-6] The current needed to shut the thyristor off is about 20% of
the nominal current and has to be very precisely controlled.
It is also possible to use transistors as the switching part in converters. The Insulated Gate Bipolar Transistor (TORT) is a transistor type that allows fast switching, frequencies of 2 kHz
are possible, and is controlled by an electronic circuit that operates with low currents. "IGBT
are voltage controlled, and require little power and low voltage, both can be implemented simply."[ImE 1995-6]
Transistors are not yet suitable for high power installations, but efforts are made to overcome
these limits. In the near future, it will be possibleto build converters for high power levels based on transistors.
The table below shows the present and near future capacities of the different semiconductors. Table 4.1 Semiconductors (source: [AES97 003])
42 Rectifiers
Rectifiers are used as a power supply for DC motors, the excitation windings of a
synchronous AC motor or as the first stage of converters with a DC link. The use with DC motors is not further described here, since DC propulsion is not considered in this report.
Uncontrolled rectifier
A normal rectifier for three phase AC consists of six diodes in a bridge. The output of the rectifier is a fixed DC voltage with a superimposed distortion. When the rectifier is fed with a net frequency of 60 Hz, this distortion has a frequency of 300 Hz. The voltage and current cannot be reversed, so a normal rectifier is not capable of power regeneration.
Controlled rectifier
The diodes in the rectifier bridge can be exchanged for thyristors, see figure 4.1 on the next page. The moment of conduction of a thyristor can be controlled by ark-electronic circuit. If
the thyristors in the rectifier circuitare switched on at the moment of the highest voltage over
the thyristor, the rectifier works the same as a diode rectifier. When the switching or 'firing' of the thyristors in the circuit is delayed, the output voltage of the rectifier decreases, as can be derived from equation 4.1:
V0 Vpcos a
It-with the firing angle cc being between 0 and 180° and VP the AC input voltage.
This type of rectifier can be used to control the speed of DC motors. Power regeneration is
possible with the rectifier in 'inversion mode', i.e. whenthe firing angle is set between 90 and
180°, the voltage reverses, which leads to a negative power flow. If two such controlled rectifiers are combined, both current and voltage can be reversed and true four-quadrant operation is possible. (4.1) Maximum Voltage Maximum Current Symbol (V) (A) Diode 6600 Thyristor 8000 4000 ---ii--- GTO GTO 4500 4500 future GTO 6000 (9000) 6000 Transistor 500 500 1GBT 3300 1200 future IGBT 5000-6000 2400 = I
-figure 4.1 Thyristor (controlled) rectifier layout [Sen 1989]
Semi controlled rectifier
A rectifier bridge with 3 thyristors and 3 diodes has the same driving characteristics as the controlled rectifier, but does not allow voltage reversal. No power regeneration is possible.
4.3 Inverters
An inverter is a rectifier operating in reverse mode and converts a fixed voltage DC to a fixed or variable voltage AC with variable frequency. Two types of inverters exist: current and voltage source inverters, CSI and VSI respectively.
The CSI is fed by a constant current DC source, opposed to the constant voltage source of the VSI, and generates a controllable, variable current with variable frequency. These inverters are very rugged and reliable since the current is controlled and therefore cannot increase unexpectedly, not even with shorted output terminals.
The CSI can be found in synchro (LCI) converters, with the current being delivered by a
controlled rectifier.
A VSI has to be fed by a 'stiff voltage DC source and deliversa controllable, variable voltage
with a variable frequency. It is most suitable for ship propulsion with an asynchronous motor,
of which both frequency and voltage have to be controlled to maintaina constant torque. It is found as the inverter stage in PWM converters, with the 'stiff voltage being supplied by an
4.4 Pulse Width Modulation (PWM)
The main components of a PWM converter are an uncontrolled rectifier and a VSI. The rectifier feeds a DC link, connected to the inverter that is build up with normal transistors, Gate Turn Off Thyristors (GTO) or Insulated Gate Bipolar Transistors (IGBT). See figure 4.2 for the schematic layout of a twelve pulse PWM converter.
Forced (line) commutation of the semiconductors is used to generate pulses with adjustable width and frequency. The output voltage that is the result of this process is approximately the desired sinusoidal AC voltage, see figure 4.2 again. The output frequency of a PWM
converter can be as high as 140 Hz [Holec NIL the distortion increases with the frequency. In the VSI, a conversion from 3 to 2 phases and a conversion back to three phases again takes place. Controlling the converter thus requires more calculations, but this is not a significant problem with current technology. The VSI enables an accurate control of frequency and
voltage [Holec 001]. 3-PHASE POWER SUPPLY E-1AOTOR !CRT PVIII01 TRANSISTOR OR
INSULATED GATE BIPOLAR TRANSISTOR (IG9T)
= [NODE
_figure 4.2 Pulse Width Modulation (PWilip
left: converter layout with a 12 pulse diode rectifier and anti-parallel diodes for idle operation
right: PWM (Voltage source inverter VSI) waveform with a) block
modulation. b) sinus modulation) (Helmets 1992]
a
One of the benefits of PWM is the constant power factor cosy = 1.0.
Since the commutation of the converter is not controlled by the &motor, the PWM converter can be used to control more than one E-motor, which can be of the synchronous or the asynchronous type. The PWM converter needs additional circuitry to control the firing of the
thyristors or IGBT's, which takes up extra space in the converter. By managing both
frequency and amplitude, the PWM converter has good speed controlling capabilities. Siemens GTO Thyristor converter
The Siemens GTO converter is a combination of a rectifier and an inverter based on GTO thyristors. Like the PWM converter, the GTO converter uses the fast switching capabilities of the thyristors to create a sinusoidal voltage. The difference is that the first stage of the
converter is formed by a controlled rectifier. The output current of this converter is smoothed by a capacitor circuit, to feed the motor with a near sinusoidal current. The controlled rectifier allows for reversal of the energy flow in the converter and this makes 4 quadrant operation possible, without additions to the converter. The layout and waveform examples in figure 4.3 hold for the Siemens GTO converter, for the power range of 3 to 10 MW, with
voltages.ranging from 1.5 to 6 kV and frequencies up to 130 Hz.
=NI
1111.111IMMII
1?,
Propeller
drive motor Asynchronous motor
Motor current Motor voltage
1-11,,111111,11IITITT
Capicitor current:
Inverter output current
filirilh11111,11/111,Irlir
figure 4.3 GTO Converter (GTO) converter layout and waveform
[Siemens 002]
This converter is also suitable for synchronous and asynchronous motors.
Since the Siemens GTO is the only one of this type to be found and is basically similar to the PWM converter, the GTO converter is not further described in the remainder of this report, in order to keep the converter overview compact and comprehensible.
4.5 Synchro Converter (LCI)
This type of converter is also called synchronous cycloconverter. The abbreviation for this converter type is LCI, which stands for Load Controlled Inverter.
Unlike the PWM converter, a synchroconverter has a controlled rectifier as the first stage, connected to the DC link. A thyristor based CSI is connected to this link, see figure 4.4, and is providing the variable voltage and frequency that is used to control the motors power and speed. The schematics show the layout of a 12 pulse LCI.
This motor has to be of the synchronous type as the inductive voltage created in the stator windings of the motor commutates the thyristors of the inverter. Each time when a rotor pole passes a certain stator winding, the voltage in this winding momentarily lowers to zero. This causes the (conductive) thyristor that is in connection with the winding to shut off. The next
thyristor is then fired by means of an external circuit. This process, for which the motor is
partly responsible, generates the desired AC voltage. Motor and converter have to be carefully matched to obtain good speed control properties.
figure 4.4 Synchro (LCD converter left: layout for a 12 pulse LC1
right: LCI output waveform all: [Siemens 002]
Increasing the motor's speed is done by increasing thecurrent. The magnetic force and the
torque become higher and the motor speeds up. The thyristors in the inverter commutate faster until the AC frequency, motor speed and current reach an equilibrium. The same method is used to start the motor.
The output frequency of the LCI cannot be directly controlled, the speed of the motor is set by varying the delivered torque. This makes that controlling the motor speed with an LCI is slightly less accurate. Because the synchronous motor is not generating sufficient reactive power at low speeds, the commutation of the converters thyristors at these speeds has to be done by an additional electronic circuit. From zero to ten percent of the speed range of the motor the commutation is controlled by this circuit.
As the power that is needed in this low speed range is also low, this circuitry does not have to be very large, making the LCI a small and robust converter type.
4.6 Cycloconverter (CCV)
A cycloconverter (CCV) is a converter without a DC link and a combination of three pairs of antiparallel switched controlled rectifiers, an example of a 6 pulse CCV can be seen in figure 4.5. With accurate controlling of the firing of the thyristors in each pair, a sinusoidal voltage can be generated. Combining the three 120° phase shifted pairs of rectifiers results in a three phase system that feeds the motor. This makes that a large number of thyristors is used: for a three phase 6 pulse converter there are 3*2*6 = 36 thyristors needed.
IA Rectifier operation Propeller drive motor
WI
II IA II / Dead interval Synchronous Or asynchronous motor Rectifier operatic/re nverter operationstimis Ionia
141111101111sionimie IMMO
111111111 NMI MOM
MIS
RSA Me
ass
figure 4.5 Cyclo (CCTO converter
top: layout for a 12 pulse CCV
bottom: CCV output waveform all: 'Siemens 002]
Because the thyristors are fired via an extensive electronic circuit, i.e. there is forced
commutation, the CCV converter can be used in combination witha synchronous or an
asynchronous motor. Frequency (motor) speed and torqueare both accurately adjustable.
As the CCV 'cuts' chunks out of the original, supplied frequency (see figure 4.5 and 4.6 on the next page), the frequency output of a CCV converter is stable to approximately one third of the input frequency. With a supplied net frequency of 60 Hz a CCV can thus supply any frequency between zero and 20 Hz. Because of these low frequencies the typical use for a
