1. A simple and efficient parametric design approach for marine electrical machines

A simple and efficient parametric design approach for marine electrical machines C. Patsios 1,a, M. Beniakar 1,b, A. Kladas 1,c and J. Prousalidis2,d 1

National Technical University of Athens

School of Electrical and Computer Engineering 9 Iroon Polytechniou st., 15773, Athens, Greece 2

National Technical University of Athens

School of Naval Architecture & Marine Engineering a

hpatsios@central.ntua.gr, bbeniakar.minos@gmail.com, ckladasel@central.ntua.gr, d jprousal@naval.ntua.gr Keywords: Marine electric propulsion, induction machine, synchronous PM motor, POD, finite elements, coupled model.

Abstract. In this paper a parametric design procedure of electrical machines used in naval propulsion systems is developed. The algorithm uses a series of design characteristics i.e. the type of the machine, the winding configuration and key geometrical properties, as parameters and is implemented on MATLAB速 script allowing for a straightforward incorporation with other development tools. Using the proposed algorithm, two of the most common machine configurations involved in marine electrical propulsion systems i.e. the Induction Motor and the Synchronous Permanent Magnet Motor, are designed and 2D finite element modeling and analysis is performed.

MATLAB速 is used to interact with the FEMM software package through ActiveX framework, allowing for a detailed calculation of the electromagnetic properties of the machines examined.

Introduction Nowadays, ship electric propulsion is gaining increased interest owing to a number of presented advantages over its diesel counterpart. There are several cases of cruise ships, shuttle tankers, product carriers, ferrys, icebreakers being built with integrated electric power drive systems [1]-[4] supported by major manufacturers like Converteam, DRS, ABB, Daihastu, LDW and others.

Ship electric propulsion offers a series of advantages over conventional drive systems saving precious cargo space, reducing fuel consumption and emissions and offering an overall greater performance and control flexibility [1]-[8]. Some of the major advantages of electric propulsion include: - Reduction of fuel consumption by improved power flow control. The diesel generator(s) are properly sized in order to provide power to all the ship's electrical loads which means that no idling is involved while multiple combinations of engines and generators of varying sizes can be used in order to produce power in increments. Thus, the engines are kept at appropriate loading and operate at their highest efficiencies.

- A higher redundancy is assured since the loss of a diesel engine does not necessarily mean a respective loss of propulsion, since there is no direct coupling of the diesel engine and the propeller.

- Electric propulsion provides naval architects with many opportunities to optimize ship design since the need for gearboxes and shafts between the propeller & the prime movers is eliminated, allowing the ship designer virtually unlimited flexibility in arranging the ship. This typically means more internal volume is available for cargo or customers for a given ship size, further enhancing the design's cost effectiveness [4].

- Electric motors have almost zero maintenance costs involved, the need for a gearbox is eliminated since they can operate on a wide speed range and can provide higher torque at low speed versus diesel motors.

- The machinery can be vibration-isolated since there is no shaft line hard mounted to the ship, thus lowering acoustic noise and vibrations.

On the other hand, electric propulsion involves a higher acquisition cost, the total efficiency of the energy conversion cycle is lower while the weight and volume of the machines is generally higher.

Machine topologies There are several topologies of electrical machines used for marine purposes having in common that they must be reliable and compact with a high level of mechanical shock tolerance [1]. In the past, DC motors were mainly used in electric propulsion systems due to their wide speed range and easy regulation, large starting and brake torque, and good reversion ability [3]. However, nowadays they have become obsolete, mainly because of the complexity of their rotor receiving power via the commutator and the brushes, thus limiting current and voltage levels. Today, for power levels ranging from tens of kW to several MW, there are three types of motors that seem to dominate the marine business: the wound rotor synchronous motor, the squirrel cage induction motor (IM) and the permanent magnet synchronous motor (PMSM).

Wound rotor synchronous motors, are one the most common topologies in ship propulsion [1].

They are supplied by current source inverters (synchroconverters), which are very reliable up to very high powers at the cost of increased weight and volume due to the inductors involved. Volume is a major issue especially in case Propulsors with Outboard Drives (PODs) are used where space is very limited.

The IM topology is favorable in terms of its simplicity, lacking several parts such as rotor windings, an exciter, rotating diodes or permanent magnets while on the same time are easier to repair on-board. IM machines are conveniently operated through reliable voltage source inverters (VSI) allowing for safe operation under several hazardous conditions such as stator short circuits, that would prove a liability as in the case of PMSMs where there is no control over the excitation and the magnets could be demagnetized. Furthermore, manufacturing techniques such as slot skewing can lead to reduced acoustic noise and vibrations. The simplicity of IM machines render them highly attractive in terms of cost of acquisition.

PMSMs are gaining increased interest as main propulsion motors. Their main benefits include high efficiency, since rotor losses are eliminated and larger power density. Radial field Surface Mounted Permanent Magnet (SMPM) motors are the most common topology though more complex topologies such as axial field PM motors and transverse flux PM motors are also employed [1].

There are several issues to be addressed regarding PMSM operation the most notable of which being the high currents involved under short circuit operation that combined with the lack of control of the excitation could lead to demagnetization. Furthermore there are operating restrictions associated with the permanent magnets regarding operating temperature and mechanical stresses.

Finally, the cost of acquisition for PMSMs is still higher than that of IM machines of the same power levels.

Machine design and modelling For the design process the geometrical parameters can be separated in two categories [1]: - The parameters affected by the manufacturing constraints limiting the machine size i.e. the stator and rotor diameter, bar and slot sizes, winding sizes.

- The parameters specified by the designer i.e. number of poles, number of slots and pole pitch winding.

In order for electric ship propulsion systems to be attractive, the achievement of the rated performances has to be satisfied for a minimum volume and weight. Thus, it is crucial to optimize machine geometry in order to achieve a high power and torque density without violating the manufacturing or operating constrains of both the machine and drive systems.

As a result, a sensitivity analysis for the machines developed must be performed, requiring multiple simulations for different topologies and geometrical variants. The design optimization and simulations can be both computationally demanding and time consuming, thus a parametric approach can substantially reduce the resources required. In this paper, a convenient parametric design algorithm for FEM analysis has been developed.

Machine Parametric design The design algorithm for FEM analysis has been developed, with the type of the machine, the number and type of stator slots per pole, the excitation system, the magnet position and formation, the width and length of stator and rotor slots and the airgap length, as integrated parameters. The algorithm facilitates both the design process, saving precious time and effort, as well as the sensitivity analysis allowing for multiple designs to be simulated through iterative program calls.

Figure 1 shows examples of various machines modelled. The algorithm is implemented on MATLAB® script [9] allowing for seamless integration with various software applications.

(a) (b) (c) (d) Figure 1. 2D FEM pole models (a) PMSM (b) Salient pole synchronous machine (c) IM with axial rotor slots (d) IM with slotted and cage-wound rotor Machine Model A first estimation of the machine structure and determining of important parameters is achieved by considering classical machine design techniques. After determining the basic structure of the machines under consideration according to this procedure, 2D finite element models have been introduced for detailed parameter calculation.

In this paper the FEMM software package [10] is used for the FEM analysis, which consists of a set of programs for solving low frequency electromagnetic problems on two-dimensional planar and axisymmetric domains. FEMM utilizes the Lua programming language. Lua is an extension language designed to support general procedural programming with data description facilities [11].

FEMM also allows for interprocess communication via ActiveX. MATLAB® can connect to FEMM as a client via Active X. From MATLAB®, one can send commands to the FEMM Lua interpreter and receive the results of the command. To aid in the use of FEMM from MATLAB®, a toolbox, called OctaveFEMM, implements MATLAB® commands that subsume the functionality of Lua using equivalent MATLAB® commands. Using the toolbox, all details of the ActiveX interface are taken care of, in a way that is completely transparent to the user [11].

In the present analysis, the FEM model is time stepping, inputting motor currents from a MATLAB® script and outputting EMF and instantaneous torque per pole for different loading conditions and rotor positions. One pole of the each machine has been modelled, by using appropriate anti-periodic lateral boundary conditions. The mesh for the one pole 2D finite element model can consist of several thousand nodes. Non-oriented silicon steel is used for the machine stator and rotor core while in the PMSM case the permanent magnet is NdFeB. Figure 2a shows a detailed FEM model for one pole of an IM depicting the boundary conditions employed.

The stator currents shift depending on the rotor displacement in order to model a periodic timespace phenomenon. FEM iterative calls are generated through a MATLAB® script that modifies motor geometry and stator slot current densities. Regarding data acquisition, in order to achieve sufficient resolution of the computed parameters to be used as model outputs, a convenient rotor – input current space–time shift has been adopted. Half degree rotor angular displacement is implemented which corresponds to 0.22 msec time shift of the input currents for a supply frequency

of 50Hz. The above time step is a suitable trade-off between computational cost and precision of calculation needed for the phenomena under study.

(a) (b) Figure 2. a) 2D FEM pole models for an IM and boundary conditions (b) Distinctive snapshots of the FEM iterative calls Figure 2b shows three distinctive snapshots of the FEM iterative calls for a PMSM machine, assuming full load conditions, where motor geometry and stator slot current densities are properly modified. The model outputs the electromagnetic torque for both sinusoidal and non-sinusoidal input currents. Maxwell stress tensor is used for the calculation of the instantaneous torque.

The proposed model can be coupled with a external circuit model as described in [1] for a more extensive calculation of machine characteristics.

Results and discussion For the purposes of performance evaluation a 3.5 MW machine is considered with a nominal voltage of 3000 V at 235 RPM. Table I summarizes the properties of the PM machine while Table II summarizes the properties for the IM machine.

Table I PM machine properties.

Table II IM machine properties Rotor radius 600 mm Rotor radius 600 mm inner radius 606 mm inner radius 606 mm Stator Stator outer radius 850 mm outer radius 850 mm length 230 mm number 168 Magnet width 40 mm Rotor Slots length 100 mm depth 2400 mm depth 2400 mm Machine depth 2400 mm Machine depth 2400 mm Output Power 3.7 MW Output Power 3.7 MW Voltage 3000 V Voltage 3000 V Current 712 A Current 712 A Pole/Phase/Stator Slots 16/3/144 Pole/Phase/Stator slots 16/3/144 PMSM analysis Figure 3a shows the phase-to-phase back EMF waveforms for the PMSM machine modelled.

Through iterative calls corresponding to a half pole rotor step, considering sinusoidal current inputs, the model outputs the machine’s electromagnetic torque. Instantaneous torque under full load conditions is shown in Fig. 3b.

Figure 4a summarizes torque results for three different power angles d. Results clearly demonstrate that the torque ripple reflects basic machine design characteristics e.g. number of stator slots per pole, magnet position and formation.

In order to reduce the amplitude of the machine’s torque ripple a fractional slot number per phase and per pole can be used resulting in the reduction of the space harmonics of the stator magnetomotive force (MMF). Figure 4b shows the respective instantaneous torque waveforms and their FFT analysis for different loading conditions using double layer 8/9 slots per phase and pole.

Figure 4c shows the FFT analysis of the electromagnetic torque for the two winding types. It can be observed that the use of the fractional winding has a definite impact on the 6th and 12th order

harmonics, however, the calculation of the optimum topology for the type of fractional winding to be employed requires an in-depth analysis as well as the consideration of several operating parameters and exceeds the scope of this work.

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(a) (b) Figure 3. a) PMSM machine EMF b) PMSM machine electromagnetic torque under full load conditions for a half-pole rotor displacement

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(a) (b) (c) Figure 4. a) PMSM machine electromagnetic torque for a half-pole rotor displacement b) PMSM machine electromagnetic torque for a half-pole rotor displacement (fractional slot winding case) c) FFT analysis of the electromagnetic torque under full load conditions for the two winding types IM analysis In the case of the IM machine the FEM analysis is somewhat more complicated since it requires the solution of the problem on the frequency plane considering the currents of the rotor are induced rather than enforced. In this case, the FEMM software can adequately solve the problem considering a single frequency matching the slip frequency between the stator and rotor, however, since the stator currents are passed as input to the model, the above analysis can only be performed for a single slip frequency - stator RMS current value combination on every occasion.

Considering the above, on a first step a preliminary analysis of the IM machine geometry characteristics is considered. A single layer winding is selected for the stator and an axial slot rotor.

Overlapping single layer winding has been considered. Using the FEMM software the optimum slip frequency for maximum torque is calculated considering constant current. Figure 5 shows simulations of the instantaneous torque profile versus slip frequency for stator currents I = 330 Arms, I = 504 Arms and I = 670 Arms respectively. For a more detailed analysis of the IM machine a different software suite should be used allowing for the uncoupling of rotor and stator field frequencies.

Conclusions In this paper a parametric design procedure for electrical machines used in naval propulsion systems is developed. A 2D finite element modeling for detailed calculation of electromagnetic

properties has been performed for two of the most common machine types in naval systems, using FEMM software package in conjunction with MATLAB®. This allows modifying of the motor geometry and stator slot current densities in order to reproduce a periodic time-space phenomenon and model important operating parameters such as the electromagnetic torque ripple.

The developed algorithm facilitates both the design process as well as the sensitivity analysis allowing for multiple designs to be simulated through iterative program calls.

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Inominal 66% of Inominal

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Figure 5. IM instantaneous torque profile versus slip frequency ACKNOWLEDGMENT This research has been co-financed by the European Union (European Social Fund – ESF) and Greek national funds through the Operational Program "Education and Lifelong Learning" of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES: Reinforcement of the interdisciplinary and/or inter-institutional research and innovation.

References [1] Lateb R., Takorabet N., Meibody-Tabar F., Mirzaian A., Enon J.,Sarribouette A., 2-6 Oct. 2005, "Performances comparison of induction motors and surface mounted PM motor for POD marine propulsion", 40th IAS Annual Meeting, Vol.2, pp.1342-1349, Hong Kong.

[2] F. Caricchi, F. Crescimbini, Member, O. Honorati, September 1999, "Modular Axial-Flux Permanent-Magnet Motor for Ship Propulsion Drives", IEEE Transactions on Energy Conversion, Vol. 14, No. 3.

[3] Sanggon Lee, Yong-Jae Kim, Yu-seok Jeong, Sung-Chin Hahn, Sang-Yong Jung, "Seasonal Power Characteristic Analysis and Propulsion Motor Comparison for Electric Vessels",10-13 Oct. 2010, 2010 International Conference on Electrical Machines and Systems (ICEMS).

[4] McCoy T.J., 25-25 July 2002, "Trends in Ship Electric Propulsion", Power Engineering Society Summer Meeting, 2002 IEEE. [5] Radan D., 2004, "Power Electronic Convereters for Ship Propulsion Electro Motors, Tech.

Report, Department of Marine Technology", project: Energy-Efficient All Electric Ship, NTNU, Trondheim, Norway.

[6] Vossen, C., 2011, "Diesel electric propulsion on ΣIGMA class corvettes", Dept. for Amphibious Support Ships & Naval Auxiliaries, DAMEN Schelde Naval Shipbuilding (DSNS), Netherlands.

[7] Prousalidis J.M., Hatziargyriou N.D., Papadias B.C., 2001, "On Studying Ship Electric Propulsion Motor Driving Schemes", Proceedings of 4th International Conference on Electromagnetic Transients, IPST '01, Rio de Janeiro (Brazil).

[8] Ådnanes A. K., 2003, "Maritime Electrical Installations And Diesel Electric Propulsion", ABB AS Marine.

[9] http://www.mathworks.com/products/matlab/ [10] http://www.femm.info/wiki/HomePage [11] http://www.lua.org/manual/4.0/