THE ELECTRIC WARSHIP V

by

C G Hodge and

D J Mattick OBE

A paper for the Institute of Marine Engineers

To be read at 1730 on Tuesday 14 December 1999

The views expressed are those of the authors alone.

BIOGRAPHIES

DAVID MATTICK David Mattick’s early career resulted from specialisation as a nuclear submarine weapon engineer. After service as the Assistant Weapon Electrical Officer of HMS WARSPITE and in the MoD, he rejoined WARSPITE as the Weapon Electrical Officer of HMS WARSPITE. In 1982 he was appointed as the Marine Engineer Officer of HMS SWIFTSURE. After promotion to Commander in 1984 he headed the electrical power systems specialist group within the MoD, subsequently serving with the VANGUARD Class submarine project and then as a Project Manager with Director General Ship Refitting. In 1994 he served as the Surface Ship Marine Engineering Desk Officer in Director Future Projects (Naval), tasked with concept design of future naval vessels and was appointed as the MoD Electric Ship Programme Manager in February 1996. Since retiring from the Royal Navy in 1999 he has been employed in the Integrated Propulsion Systems Division of Rolls-Royce Marine Power as the Electric Ship Manager. He was awarded the OBE in the 1999 New Year’s Honours List.

CHRISTOPHER HODGE After initial training as a mechanical engineer, and service as an Assistant Marine Engineer in HMS WARSPITE, Christopher Hodge joined HMS ORPHEUS as the Marine Engineer Officer in 1982. He subsequently took an MSc in Electrical Marine Engineering and served in the MoD as the project officer for electrical ship propulsion. After promotion to Commander in 1989 he served as the Marine Engineer Officer of HMS CONQUEROR before returning to MoD as the head of the Nuclear Steam Raising Plant electrical design authority. He was the head of the electrical power system specialist group within the MoD until April 1998 after which time he joined Rolls Royce Marine Power in their Integrated Propulsion Systems Division.

THE ELECTRIC WARSHIP V C G Hodge BSc MSc CEng FIMarE

D J Mattick OBE BSc CEng FIMarE MIEE MINucE

ABSTRACT This paper which follows on from four earlier IMarE papers by the same authors, reviews the progress of the Electric Ship Programme over the last year and focuses on some of the technological developments in equipment and architectures associated with the Electric Warship

INTRODUCTION This is the fifth paper in the Electric Warship series presented at the IMarE and as before it aims to re-state and update the Electric Warship vision and inform readers of progress towards realising the concept in a warship and to record significant developments in its associated technology. In this paper the authors will also compare some of the predictions made in the first two papers to the to actual outcome.

BACKGROUND The following is a very brief summary of the first four Electric Warship papers, References 1,2,3 and 4:

The benefits of employing a common power system for both propulsion and ship’s services can, in an optimised merchant ship installation, allow fuel savings of up to 25%. In a warship, where the constraint of reducing Unit Purchase Cost as well as Running Costs does not allow a system to be fully optimised for fuel economy, the savings may nevertheless be sufficient to allow purchase of one extra ship in a class of around thirty vessels. However as was explained in the first paper, Reference 1, and reaffirmed in the subsequent three papers the size of motors and converters needs to be reduced if they are to be installed in frigate sized warships.

The Electric Ship (ES) concept was developed from Integrated Full Electric Propulsion (IFEP) and it was aimed to reduce Unit Purchase Cost (UPC) and yet retain as much as is practical of the IFEP reduced Running Cost (RC). In the Electric Ship this is achieved with two main features above those to be found in a traditional IFEP vessel.

Minimum Generator Operation: The UPC and space constraints require fewer but more highly rated prime movers than would be found in a merchant IFEP vessel. In order to restore the fuel savings conceded by fitting fewer prime movers the ES runs under a regime of minimum generator operation, often with only one prime mover operational. This brings significant gains in both fuel consumption and maintenance costs due to the minimised engine running hours.

Electrification of Auxiliaries: Additional maintenance and manpower reductions can be achieved by using electric auxiliaries wherever possible. In addition there will be

benefits to be gained from this policy in terms of overall weight of equipment fitted (central energy storage and high reliability ship wide electric power systems).

The previous papers described our early vision of an Electric Ship Power System, it is shown below at Figure 1.

It remains a robust vision, although the mode of power distribution for the ship service system remains to be decided and the location of the 6 to 8 MW Gas Turbine Alternator may move across to the high voltage propulsion busbar.

THEN AND NOW The first Electric Warship Paper, Reference 1 made some predictions about the progress of technology. This paper will now compare the principal prediction to the actual progress.

Power Electronics

Reference 1 showed the following graphical representation, Figure 2, of the anticipated progress of Power Electronics. It can be seen that the MOS-Controlled-Thyristor (MCT) was clearly anticipated, as were volume and price reductions from the applications of the newer devices. In addition the expectation was that increasing device-operating frequency would be utilised and result in a lower requirement for harmonic filtering.

Although not all these predictions have been realised, there has certainly been a reduction in the overall size of converters. The 19 MW IGBT based PWM Converter supplied by Alstom Drives and Controls to the USA DOD has linear dimensions about 20% longer than the Thyristor based Type 23 Frigate 1.5 MW half controlled DC Rectifier. Although this represents a five-fold increase in power packing density the IPS converter provides twice the functionality of the Type 23 – it inverts as well as rectifies - and the overall increase in power density if like functionality were compared would exceed the ten-fold increase in packing density predicted at Reference 1.

Although the MCT has started to be marketed it still lacks the ability to cope with hard-switched voltage transients that would be needed if it were to start to rival the IGBT. Its current voltage and current ratings are well below that of the IGBT. It now seems likely that the hybrid VLSI devices reported at Reference 3 will overtake the MCT.

Similarly the ability to switch at high frequency has not been utilised, although there are some examples of high switching frequency being employed in resonant converter circuits these have mainly been low power or laboratory demonstrators. At present switching frequency remains limited by the need to avoid switching losses in order that conversion efficiency remains acceptably high. The balance between waveform quality, filter size and losses and switching frequency remains as fine as ever with many converters switching at 3 kHz or less. Nevertheless the first resonant converter designs are beginning to be produced both in the UK and abroad with the result that significant sizes of power converter utilising switching frequencies of 10 kHz or higher should be available within two years.

Fig

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Figure 1. Electric Warship Power System

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ure 2: Original Predictions of Power Electronic Development

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Figure 3: Original Predictions of Motor Size Reduction

Motors Although this subject is covered later in the paper it is again worth reviewing the prediction made four years ago with the outcome of the intervening development. One important distinction with regard to the development of compact propulsion motors is that this was always considered an area where specific development funding would be required. Whereas the power electronics revolution, anticipated four years ago, has indeed borne fruit through industrially funded and driven developments the development of compact high power (around 20 MW) Propulsion Motors has, as predicted, only been through Government funded programmes. In the USA Kaman have developed axial flux motors while Electric Boat has focussed on radial flux machines, France has recently started to develop an axial flux permanent magnet propulsion motor and the transverse flux has been developed by the UK MOD as previously reported at References 1,2,3 and 4. The same transverse flux topology is now being developed by Siemens in Germany on behalf of the German Ministry of Defence. The prediction for available size and mass reductions made four years ago are shown in Figure 3. These predictions were derived by averaging the reductions thought feasible for each of the three main competing topologies: radial, axial and transverse flux. As such there was some pessimism inherent in the prediction and as events transpired this was fully justified. However despite the growth in the size of the Transverse Flux Motor during the design its size is still within the predictions shown in Figure 3 and remains significantly smaller than all currently available competing technologies, including the new radial flux developments by Siemens and ABB that are covered later in this paper.

SIGNIFICANT RECENT ELECTRIC SHIP ACTIVITIES

The Electric Ship concept grew out of a MoD Marine Engineering Development Strategy that was endorsed at senior level and noted by the Navy Board in 1995. Since then the Strategy has underpinned an Alternative Assumption that has enabled the MoD Electric Ship Management Group to seek tenders for a number of Technology Demonstrations.

Most significant of these as far as the Electric Warship development is concerned, has been a competitive tendering exercise for a Shore Technology Demonstrator (STD). Bids were submitted just over a month ago and contract award is anticipated around Spring 2000, thus technical aspects cannot be discussed in this paper. Suffice it to say that the UK MoD intention is to erect a single shaft line reflecting half of the generic Electric Ship system shown earlier and to test it against ship-like operations and failures. The outline programme is to design and build the facility such that it can be operating by November 2001 and be decommissioned during 2004. The UK MoD intention is to de-risk such a system so that future warship propulsion system contractors can confidently adopt similar systems.

The naval propulsion community, both MoD and Industry, see this as an invaluable exercise that reflects all that is best in Smart Procurement: A generic demonstration is to be conducted focussed on cost reduction yet considering war fighting facets such as signature and survivability and is to be undertaken ahead of the Main Gate for the relevant platforms. It is important to recognise that as a generic demonstrator, the intention is to demonstrate system functionality rather than to prove ship-ready equipment.

Other Technology Demonstrations for which invitations to tender or contracts are anticipated in the next months include competitions for a 1-2MW GTA, a power dense propulsion motor and advanced machinery controls as well as development of a WR-21 Gas Turbine Alternator package.

Meanwhile, in the US, the NAVSEA Full Scale Advanced Development (FSAD) of their Integrated Power System (IPS) proceeds apace, demonstration of a single shaft line and an advanced distribution system is underway at the Land Based Evaluation Site in Philadelphia. There are a number of papers planned for INEC 2000 that will announce some of the achievements but it is understood to be proving most successful. Alstom Drives and Controls of Rugby have supplied all of the propulsion shaft line including their advanced motor and the converter discussed later in the paper.

Within NATO, the Industrial Advisory group have reported on their All Electric Ship study that concludes that such systems are viable and desirable and that they yield military and cost advantage. The study also reviews the breadth of technologies that might be considered and indicates when they are likely to be sufficiently mature for warship use.

Whilst this paper reviews Electric Warship activities, the boundaries between warships and commercial ships are becoming ever more blurred. This year has seen orders for Gas Turbine Alternators for cruise liners and the continuing build of both the AO and LPD with diesel electric propulsion as well as the military activities described earlier.

Returning to MoD Electric Ship activities, it is understood that the predicated savings attributable to an Electric Warship have been reconfirmed, progress has been reviewed by the Navy Board and again endorsed at higher levels. Simultaneously, the attraction of early savings combined with the rapid maturing of technologies being demonstrated at FSAD and STD mean that both UK Industry and MoD are assessing the suitability of such systems for deployment in the near future; perhaps the Type 45 destroyer could utilise an electric propulsion system loosely derived from existing technologies, a Commercial IFEP (CIFEP) system.

If the technologies prove sufficiently de-risked then Smart Procurement methodologies will enable CIFEP procurement to the demanding timescales required. Such a step would comply with the MoD Marine Engineering Development Strategy and provide opportunities to introduce Electric Ship IFEP (ESIFEP) systems in later batches of the Type 45 and future ship programmes to realise the further cost savings.

ELECTRIC WARSHIP TECHNOLOGY

POWER ELECTRONICS Once again developments have continued across the breadth of the Power Electronics Field. The Power Electronic Building Block (PEBB) programme in the United States of America continues and is now producing hardware (The US Army and Nottingham University have jointly developed a Matrix Converter using PEBB MOS Controlled Thyristors (MCT)). SPCO, with the acquisition of the Research and Development arm of Harris Semi-Conductors (led by Dr Victor Temple) are now leading the development of both the VLSI devices (MCT type) and hybrid Monolithic devices (MTO type).

SPCO now form the principal focus for semi-conductor development in the United States of America. The development of converter topologies has also seen progress over the last year. Alstom Drives and Controls have delivered their 19 MW PWM IGBT based converter to the USA Integrated Propulsion Systems Land Based Test Site at Philadelphia where it continues successfully under the test. This paper will now deal with the areas of device and converter development.

DEVICE DEVELOPMENT

One interesting outcome of the merger between the development teams of Harris Semi-Conductors and SPCO is cross-fertilisation between the previously quite distinct areas of VLSI development (Harris and MCTs, FTOs etc) and Monolithic development (SPCO and the MTO). The MTO, being formed by the conjunction of a monolithic GTO and a ring of VLSI style MOSFETs, is in some aspects already a hybrid device, however the new concepts now being developed by SPCO take this a stage further and the advantages of using other VLSI devices, such as IGBTs, MCTs or FTOs, as the gate turn off switch in place of the MOSFETs is being considered.1 These developments appear extremely promising to the authors who relish the opportunity to assess their applicability to an Electric Warship over the next year.

CONVERTER TOPOLGIES

Matrix Converters

The Matrix Converter topology was covered in References 3 and 4 and they continue to show promise in terms of their compactness, efficiency and flexibility. The fact that they have no intermediate energy storage capacitor also makes them a natural choice for future converters if these use Silicon Carbide power electronic devices. Such converters would run at high temperatures, perhaps as high as 300o C, and this would cause significant difficulties in implementing a standard PWM inverter with an electrolytic capacitor as the energy store. Over the past year the application of the Matrix converter to the Electric Warship concept has received significant attention through work conducted by Dr Jon Clare and Dr Pat Wheeler of Nottingham University. They are able to be confident that the Matrix Converter would suit all of the perceived applications within an IFEP installation. The authors gratefully acknowledge the permission of Clare and Wheeler to use their work in this paper. Reference 5, as yet unpublished, records this work performed by Nottingham University for the UK MoD Electric warship Programme.

Matrix Converter Operating Principle

The Authors, at References 3 and 4, have already covered the operating principles of the Matrix Converter, however in view of the potential significance to an Electric Warship (and the sections that follow) the operating principle will be briefly reviewed here.

1 It had been hoped to be able to report on these developments in this paper in detail, however SPCO have to date been unable to give the authors permission to publish the full information which, understandably, is highly commercially sensitive. If this situation changes before 14 December 99 then the additional information will be incorporated, either through a revision of this paper or in its associated presentation.

VcVbVa

VA

VC

VB

Figure 4: Matrix Converter Arrangement

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Figure 5: Matrix Converter Waveforms

The Matrix Converter gains it name from the appearance of the converter’s electrical connections when drawn as shown in Figure 4, as can be seen the nine bi-directional switches appear as a matrix of connections. The bridge produces any desired output waveform by connecting each of the input phases (VA, VB, and VC) to each output phase (Va, Vb, and Vc) in turn within a switching interval of fixed duration. Within the fixed switching interval the periods of output-connection for the three input phases is varied in a manner similar to Pulse width Modulation (PWM) such that the desired output is produced in terms of the average of the input voltages.

The Output Voltage Problem

In order that any desired output voltage can always be produced without regard to the instantaneous phase of the three input voltages the output voltage must be limited to a level below the input voltage envelope. As can be seen from Figure 5 the input voltage envelope restricts the output voltage to 50% of the input. The minimum of the maximum (positive) is half of the peak value, occurring when the two positive voltages are equal. Similarly the maximum of the minimum (negative) is half of the peak value, occurring when the two negative voltages are equal. This is a serious limitation for the Matrix Converter when it is intended for use in a standard power system with standard motors. However it may be overcome by using one of two differing techniques.

Use of Third Harmonic Voltages

The object of this method is to distort the desired output phase voltages by use of a combination of third harmonics of both the input voltage frequency and the desired output frequency. It is shown at Reference 5 that the required additional harmonics are:

Input Voltage Third Harmonic: -1/6 Output Voltage Third Harmonic: +√3/6

When this is done the phase voltages are as shown in Figure 6 and the peak desired output voltage may be raised to 0.866 of the input. Thus the maximum output voltage can be increased from 50% to 86.6% an increase of 36.6 percentage points or 73.2%. Two important points are worth noting with regard to this technique. First, although the phase voltages are distorted this does not appear in the output line voltages because of the natural third harmonic cancellation that occurs in a balanced three-phase system when moving from phase to line voltages. Second, this additional voltage increase has not been achieved at any other performance aspect of the converter, its overall efficiency remains unaltered as does the quality of the output voltage and the level of distortion imposed on the input system.

Use of a Fictitious DC Link

Again this technique is described briefly at Reference 5, a more complete description is at References 6 and 7. The technique divides the modulation function into two stages, the first deals with input voltages, the second with the generation of the output voltages. It is termed a fictitious DC link because in order to maximise the output voltage the first modulation stage chooses to work between the most positive and most negative input voltages, as would a standard uncontrolled DC rectifier. The overall maximum voltage output that can be achieved with this technique is shown at

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Figure 6: Matrix Converter Third Harmonic Waveforms

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Figure 7: Two Level PWM Waveforms

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Figure 9: Three Level PWM Converter Limb Configuration

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Figure 8: Two Level PWM Converter Limb Configuration

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Figure 10: Three Level PWM Waveforms

Reference 6 to be 6√3/π2 or 105.3%. Although the modulation function is developed by considering two distinct phases of time-continuous operation this cannot be the case when the modulation function is applied to the converter – there is only one switch between the input and the output and no intermediate stage. In practice the two separate switching strategies are combined into one unified requirement before the modulation function is applied. Techniques to achieve this are covered at References 6 and 7. There are however significant disadvantages:

Although this technique overcomes the voltage limitation of the Matrix Converter completely this gain is bought at the expense of the overall harmonic performance of the converter: the harmonics present in the input voltage or the output current (or indeed both) must increase. It is perhaps not surprising - since the modulation function is in part derived through consideration of DC rectification - that the largest increase in harmonics are those associated with this process itself (5th, 7th, 11th etc). An additional source of harmonic distortion arises when the voltage is forced above 86.6%, when this is done it is no longer possible for the average output voltage in each switching interval to equal the target voltage and additional low frequency harmonics result. It is interesting to note that a Matrix Converter operating at minimum harmonic distortion under a fictitious DC link scheme will be subject to the same voltage limit as a converter operating under the third harmonic strategy.

There are practical difficulties in developing the overall modulation function through two stages in real time so that implementation of a converter producing continuously varying frequency and voltage (as would be required in many of the IFEP applications) would be difficult.

Multi–Level Converters Multi-level converters apply a PWM strategy to differing voltage levels that are selected, across the desired output waveform’s period, to suit its generation. Figure 7 shows the simplest application of PWM and represents the waveforms that would arise from a converter employing two levels – full positive DC volts and full negative DC volts. The arrangement of a single limb of this type of converter is shown in Figure 8.

It is self-evident from Figure 7 that applying full negative and positive volts across to the output phase is an inelegant method of generating the desired output waveform. At its simplest, when the desired output voltage is close to the maximum it is advantageous to balance the full positive volts with a lower level closer to the voltage than the full minimum. This requires a converter with additional voltage levels and is normally termed a Multi-Level Converter. The simplest multi-level converter employs three voltage levels: full positive DC volts, zero DC volts, and full negative DC volts. A single limb of such a converter is illustrated at Figure 9. The waveforms that would arise from a converter employing three levels is shown at Figure 10.

Clearly, the same principle of using multiple voltage levels can be readily extended beyond three to five or higher. As well as providing a better quality of output waveform operating a PWM converter at multiple voltage levels confers another, more important, advantage: the switching losses are reduced. The switching losses are directly related to the Volt-Amp product of the circuit being interrupted. Inspection of Figures 7 and 10 shows clearly that the voltage level being switched is halved between a two level and three level converter. For the same current level this implies that the switching losses

will also be halved although other practical considerations such as the power loss created by the circuit components that create the intermediate voltage will reduce this advantage. Nevertheless reduction of switching loss is a powerful argument to move to multi-level topologies. Whereas the incremental gains in waveform quality become less significant when moving to higher numbers of multiple levels (an example of diminishing returns) the efficiency gains do not suffer from the same disadvantage as early in the progression.

The SPCO Multi Level Converter As has been previously reported SPCO are very active in the development of new topology power electronic devices, among others; the MCT and MTO. They are also active in developing new types of converter and have recently been involved in conceptual work investigating the feasibility of producing a five level semi-soft switched PWM converter. Although far from complete SPCO have given permission for this concept to be covered by this paper. The authors are extremely grateful for this permission and would like to record their gratitude to Kevin Donegan, SPCO’s Vice-President of Strategic Marketing.

The converter would develop the five levels of voltage by use of GTO based switches while the final inversion to AC would be by IGBTs as shown in Figure 11. The overall connection is shown schematically at Figure 12, which also shows that the converter will employ soft switching for the IGBT based final stage. In operation the base GTOs switch relatively slowly and control the voltage level being presented to the IGBTs, they do not impose much switching loss due to the low frequency of their operation. Conversely the IGBTs are switching much faster but use a soft switching system thus again they impose low switching losses. In addition the converter produced a good quality output waveform, due to high switching speed of the IGBTs, and thus filtering requirements are reduced with consequential gains in efficiency (reduced passive losses in the filter) and reductions in converter size and mass. The design offers much reduced switching loss and it is hoped to investigate this topology further in order to assess fully the potential gains in efficiency. It was reported at Reference 3 that the size of the PMPM had increased to around 65 tonnes from the 40 tonnes, reported at Reference 2. This had resulted, in part, from a requirement to reduce the air-gap shear stress in order to improve the machine’s power factor such that converter-switching losses did not dominate and reduce the system efficiency unacceptably. Due to its potential low switching loss the SPCO converter offers the possibility of supporting a TFM based PMPM with a low power factor but high overall efficiency. This would mean, given successful development of the converter, that the PMPM could be once again reduced in size and mass. Other factors, such as mechanical strength, may prevent returning to 40 tonnes, nevertheless significant reductions do appear feasible and these will be investigated further once the design of the SPCO converter has been defined. The schematic representations of the SPCO converter shown here in Figures 11 and 12 are the property of SPCO and are used in this paper by permission of Kevin Donegan.

Multi-Port Inversion The novel conversion technique of Sequential Capacitive Discharge (SCD) was reported at References 3 and 4. The circuit schematic diagram me for an MPI converter

(a) EquivalentFive-Level Switch

(b) EquivalentTwo-Level Switches

(c) Implementation withThyristors and IGBTs

Figure 11: Development of Multiple Voltage Levels

7-kVDC5-Level

Super GTO5000-V3000-A

Super GTO2500-V1500-A

IGBT3300-V1200-A

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Figure 12: SPCO Five Level Semi-Soft Switched Converter

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Figure 14: MPI Current waveforms

Output Switches Output Filter Section

Co

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Figure 13: MPI Circuit Schematic Configuration

VOLUME:3.587 cubic meters

Figure 15: Conceptual MPI Converter Arrangement

is shown at Figure 13, together with the current waveforms for one charge and discharge cycle at 14.

As described in Reference 4, and briefly summarised here, the technique utilises the fact that in any balanced multi-phase system the main phase currents at any time sum to zero, simplistically: for any given time interval there will have been as much positive current as negative flowing in the circuit. The charges delivered and received balance precisely. Hence these positive and negative currents can all be supplied from a single capacitor whose positive plate and negative plate charges self evidently sum to zero. When considering a 3 phase to 3 phase MPI converter within the time intervals of its operation; there will always be a phase which delivers, or receives the highest charge, while the remaining two phases are of opposite sign and combine to balance the overall charge transfer. Therefore when operating one input or output phase is connected to one side of the capacitor throughout the charge or discharge cycle while the other two are connected in turn to the plate. The only control requirement during this phase is to determine at which point the two minor phases should be commutated.

Over the last year a limited amount of work has been conducted towards assessing the viability of constructing a Multi-Port Inverter. Figure 15 shows the general arrangement of a conceptual MPI. The general operating principles have also received independent endorsement from third party assessments. The authors are grateful to Rudy Limpaecher and SAIC for permission to use this information.

MOTOR DEVELOPMENTS

PROPULSION MOTORS

Development of the Transverse Flux Motor (TFM) based Permanent Magnet Propulsion Motor (PMPM) continues with the 2.5 MW representative machine now being installed at the Defence Evaluation and Research Agency at Pyestock. Other developments continue, the French Ministry of Defence has initiated development of a prototype radial flux machine, while three TFM developments are now under way in Germany Siemens are developing a TFM for the Federal German Navy while both Daimler-Chrysler and Voigt are developing TFM units for transport applications. In addition Radial Flux Machines have received considerable development and three machines deserve particular mention.

UK MoD PMPM

The Transverse Flux Motor development continues and the principal design parameters have not changed since those reported at Reference 3. Over the last year significant effort has been expended on finalising the design and construction technique for the C cores of the motor stator. These crucial elements of the machine are illustrated in Figure 16. The original concept used a wound c core design where the laminations were continued around the C such that they were continuous but always parallel to the flux path. Unfortunately this design suffers, with the current binding technology, from delamination at the C core pole tips. This failure mode has been experienced on the wound cores installed in the no load test rig shown dismantled at Figure 17.

Figure 16: C Core Location

Figure 17: No-Load Rotation Rig

Siemens Radial Flux

Siemens have also developed a new radial flux machine which to date has been demonstrated at up 1 MW, although not yet advertised as available in the power and torque ranges required for a UK MoD warship application the machine is notably compact. A sketch design using the same parameters as the Siemens production motor indicates that a PMPM could be produced against the UK MoD warship requirement in around 30% more space and mass than the current TFM based design. It should be noted however that extension of the design to these torque and power levels is unlikely to be a simple undertaking.

ABB Radial Flux

Similarly to the Siemens machine this utilises a radial flux path but employs a higher pole number and a less standard winding pattern. It has been demonstrated at 4.3 MW. Again, a sketch design against the UK MoD requirement indicates that a motor could be produced in about 25% more space than the current TFM based UK design. It should be noted that this design being less conventional inherently assumes higher development risk when extension of the design to the higher torque and power requirement is contemplated

Comparison of Machine Designs The extrapolated machine Siemens and ABB designs using the same design principals and parameters as the Siemens and ABB machines are compared, together figures for the original TFM design and the latest revised design, below in Table 1.

It should be noted that the figures for the TFM have been adjusted from those reported at Reference 3 in order to reflect a cylindrical design and allow direct comparison with the estimates given for the extrapolated radial flux designs. The original TFM figures reflect those that pertained for an air gap shear stress of 120 kN/m2 and a power factor of 0.4 while those for the revised TFM reflect those for an air gap shear stress of 100 kN/m2 and a power factor of 0.6.

The TFM topology as being developed by the UK MoD clearly still represents the most

compact form for use in a Naval Warship application where space and weight are at a premium.

CONCLUSION The authors remain confident that the Electric warship remains a viable concept that will be able to offer reduced first and through life costs. Developments are now proceeding in many countries associated with the Electric warship and many nations are the concept for their future classes of surface warship. The authors share the same excitement at being able to participate in this programme and look forward with keen anticipation to the next two years which should see the many of the concepts proven in hardware.

TFM (Original)

TFM (Revised)

Radial flux ABB

Radial flux Siemens

Overall diameter 2.6 m 2.75 m 2.8 m 2.8 m Overall length 2.6 m 2.85 m 4.4 m 4.7 m Overall weight 39 t 58 t 70 t 85 t Frequency 195 Hz 195 Hz 300 Hz 36 Hz Number of Poles 130 130 200 24 Number of Phases 8 8 12 12 Power Factor 0.4 0.6 0.75 0.94 %Volume increase 0.00% +22.63% +96.27% +109.65%

Table 1: Comparison of extrapolated Siemens and ABB Radial Flux Machines with the TFM

REFERENCES

1. Cdr C G Hodge and Cdr D J Mattick, ’The Electric Warship’ Trans IMarE, Vol 108, Part 2, The Institute of Marine Engineers (1996).

2. Cdr C G Hodge and Cdr D J Mattick, ’The Electric Warship II’ Trans IMarE, Vol 109, Part 2, The Institute of Marine Engineers (1997).

3. Cdr C G Hodge and Cdr D J Mattick, ’The Electric Warship III’ Trans IMarE, Vol 110, Part 2, The Institute of Marine Engineers (1998).

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