STUDY OF UNCONVENTIONAL WINDING CONFIGURATION OF MULTIPHASE PERMANENT MAGNET SYNCHRONOUS MACHINE TO IMPROVE RELIABILITY AND TORQUE QUALITY FOR POD PROPULSION APPLICATION

F. Scuiller, J.F. Charpentier, Institut de Recherche de l’Ecole Navale, French Naval Acadamy, BP 600, 29240 BREST-ARMEES, FRANCE E. Semail, S. Clénet, Laboratoire d’Electrotechnique et d’Electronique de Puissance, Ecole Nationale Supérieure d’Arts et Métiers, 8, bd Louis XIV, 59046 LILLE, FRANCE P. Letellier, Jeumont SA, France

In this paper, a design methodology of a 10-phase surface mounted Permanent Magnet (PM) machine is proposed. The winding is made up of two 5-phase windings each one being star-connected. This design methodology is based on the use of a multi-machine representation of multi-phase machine. This strategy is applied using, as a basis, a set of specifications which corresponds to a realistic 2100 kW/105 rpm POD propeller. The presented original machine, so called 2x5pSM (double 5-phase Starconnected surface mounted PM Machine), is compared with a conventional 3-phase machine corresponding to the same specifications. This original multiphase structure is characterized by a sinusoidal current supply and a fractional slot concentrated winding. This design allows to separate magnetically and physically the two-star windings. This independence between the two star windings allows for a very simple control of the two-star supply and a straightforward fault operating mode. Moreover, the new design minimizes the torque ripples which underlie the acoustic behaviour of the system. The end turns are also drastically reduced, which improves the compactness of the machine in comparison to the conventional 3-phase design. For all these reasons, this structure appears particularly suitable for podded propulsion application since it increases the machine performances in terms of compactness, reliability and quality of torque. 1. Introduction Electrical marine propulsion widely uses multiphase motors for reasons such as reliability, smooth torque and partition of power. Usually supplied by Pulse Amplitude Modulation Current Source Inverter (PAM CSI), these motors can nowadays be controlled by Pulse Width Modulation Voltage Source Inverter (PWM VSI) thanks to advances in high power semiconductors (IGBT, IGCT) and Digital Signal Processor (DSP)[1-2]. This kind of supply increases the flexibility of control. In POD propulsion application, the main challenges are to increase the torque density of the electrical machine for minimization of mass and volume, and to improve the reliability of the system. To achieve this goal, the use of a Multiphase Permanent Magnet Synchronous Machine fed by a Pulse Width Modulation (PMW) VSI is probably one of the more advantageous solutions [3]. With this solution, a multiphase motor in POD propulsion applications can also be very interesting to improve the quality of torque. With a high number of phases, it is possible to find a machine design and control strategies which minimize the pulsating torque and consequently improve the acoustic behaviour of the system [4-5].

The multiple-star winding also leads itself to the reduction of the pulsating torques. Let us consider windings divided into two N-phase stars shifted by a π/2N electrical angle. If we suppose a sinusoidal current supply, the first harmonic of pulsating torque created by the first N-phase star is the opposite of the pulsating torque first harmonic created by the second N-phase star. That means a sharp reduction of the pulsating torque in comparison with a N-phase machine. Besides, multiple-star configurations are also of high interest as their control under normal and fault modes is straightforward. Double 3-phase star-connected machines supplied by PAM CSI are widely used because the corresponding control is very simple. Two independent 3-phase PAM CSI can be used, each one supplying one 3-phase star-connected winding. The two CSI have not to be precisely synchronized. Moreover the control in fault mode is very simple: when one phase is opened, its corresponding 3-phase star is no longer supplied. So, there remains only a 3-phase machine with a conventional control. When VSI supply is selected in order to improve the dynamic of control, it is generally not advised to use two independent 3-leg VSIs to supply the two star-connected windings because of the inherent magnetic coupling between the two stars inducing high parasitic currents. To retain the same straightforward control as in the PAM CSI one, it is consequently necessary to design a multiple-star machine with an extremely low magnetic coupling between the star-connected winding. It is one constraint in the design of the proposed machine. This analysis can be extended to multiple-star windings with N phases per star. A way to take advantage of the benefits of the two systems (multiphase and multiple-star) is to design a multiple-star arrangement where each star is made up of a multiphase winding. This solution allows to minimize the pulsating torques and to benefit from efficient and simple fault operating modes. In this paper, a methodology for the design of a 10-phase surface mounted PM machine is proposed. The two 5-phase star-connected windings that make up the ten phases are shifted by 18 electrical degrees. In the first section, a multi-machine modelling of surface mounted PM motors is presented. With this methodology, a 5-phase machine is proved to be equivalent to a set of one 1-phase and a set of two 2phase fictitious machines, each one being characterized by a specific harmonic family. A simple control strategy consists in supplying the first 2-phase fictitious machine by a current which contains only one harmonic. With this strategy, each 5-phase star-connected winding is controlled in order to have only sinusoidal currents. Furthermore, it is necessary to impose zero average currents in the other fictitious machines. To limit the amplitude of the parasitic currents due to the PWM supply, the time constant associated with each fictitious machine will be large enough. In the second section, the methodology allows us to determine design criteria to really take advantage of this control strategy. One first criterion consists in determining an optimal back-EMF waveform and winding coefficient which allows increasing the 1st fictitious machine torque. Another main design criterion is to control the inductance value associated with each fictitious machine in order to get sufficiently large time constant. A third design constraint is to minimize the torque ripples. The last constraint is to aim at an extremely weak magnetic coupling between the two star-connected windings. These properties allow to control independently the two stars and to minimize the influences of one star on the other in case of fault.

Fig. 4 – Torque obtained with the new design (perfect sinusoidal current) Moreover, this figure 4 does not consider the PWM VSI supply which generates high frequency harmonic currents and therefore additional pulsating torques in the higher frequency range (typically above 2 kHz). Notwithstanding, these torque ripples can be easily filtered (sine wave filtering) or even ignored in most marine applications due to the strong attenuation from the source to the surfaces that are emitting the noise into the water. However, these promising results must be moderated by the issue of the regularity of the stator magnetomotive force [7] (due to MMF sub harmonics). In fact, this force is less correctly distributed in the 2x5pSM than in the 3-phase one. Consequently, the potential resulting mechanical stresses will have to be estimated in order to determine if they are acceptable. 5. Conclusion In this paper, an original design of a 2x5pSM fed by a PWM VSI is proposed. This new design is based on a multi-machine design strategy. This machine is characterized by an original concentrated winding fractional slot configuration which allows a magnetic and physical separation of the two stars windings. The objective of this design is twofold: to enhance the fault tolerance capability of the system and to optimize the quality of the torque by reducing the electromagnetic torque ripples which underlie the acoustic behaviour of the motor. This new design is compared with a more conventional 3-phase PM machine design for the same set of specifications corresponding to 2100 kW/105rpm podded propeller motor. Numerical studies show that the use of this original design allows reducing drastically the electromagnetic torque ripples for the same specifications. The end windings of this original design also feature reduced dimensions. Therefore, this structure seems particularly promising for podded propulsion applications where the main goals for the design of the electrical machine are compactness, fault tolerance and acoustic behaviour. Further studies are envisaged to fully assess the influence of the PWM supply in regard to the weak magnetic coupling between phases and the impact of the actual MMF distribution. 6. References

1. 2.

3. 4.

5.

6

7

D. Gondouin, F. Menneron, “New diesel-electric ship propulsion system topologies”, All Electric Ship AES 2000, 26-27 oct 2000, pp 66-71 S. Siala S., E. Guette, J. L. Pouliquen, “Multi-inverter PWM control: a new generation drives for cruise ship electric propulsion”, European Power Electronics Conference (EPE’2003), September 2003, Toulouse (France), CD-ROM P. Letellier, “High Power Permanent magnet machines for electric propulsion drives”, All Electrical Ship AES 2000, 26-27 Oct. 2000 Paris, pp 126-132 F. Scuiller, E. Semail, J.F. Charpentier, S. Clénet, “Comparison of conventional and unconventional 5-phase PM motor structures for naval application system”, IASME Transactions, Issue 2, vol.1, April 2004, pp.365-370 F. Scuiller, J.F. Charpentier, E. Semail, S. Clénet, “A multi-phase surface mounted permanent magnet design to reduce torque ripples and Joule losses for Naval applications”, in the proceedings of the All Electrical Ship symposium (AES 2005), 13-14 October 2005, Versailles, France, CD-ROM M. Lajoie-Mazenc, H. Hector, R. Carlson, “Procédé d’analyse des champs électrostatiques et magnétostatiques dans les structures planes et de révolution : programme DIFIMEDI”, Compumag’78, Grenoble France 4-6 sept. 1978 F. Magnussen, D. Svechkarenko, P. Thelin, C. Sadarangani, “Analysis of a PM machine with Concentrated fractional pitch windings”, in the proceedings of the Nordic Worshop on Power and Industrial Electronics (NORPIE), Trondheim, Norway, 14-16 June 2004