(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 04 Issue: 01, September 2011

Optimal Design of Axial Flux Permanent Magnet Generator for Hybrid Electric Vehicle Prof. B.S. Dani, Prof. M.D.Khardenvis, Prof. V.M.Jape Dept of Electrical Engineering, Govt College Of Engineering, Amravati, Maharashtra India Abstract—The axial flux permanent magnet machine is an attractive alternative to radial flux machines in wind turbine applications. The axial flux configuration is amenable to the low-speed, high-torque operation of a direct drive wind energy system. Axial flux permanent magnet (PM) machines are being developed for many applications due to their attractive features. An extensive literature exists concerning the design of a variety of types of axial flux PM machines. The objective of this paper is to optimize the design since design formulae involves various parameters and constraints. Modern day approach of Artificial Intelligent technique is also introduced as an effective design optimization method. Cost and qualitative parameters can be segmented easily with such approach. Case study design is discussed and effect of varying design parameters on performance and economy is concluded in the paper. Efforts are proposed and also discussed to minimize losses, improved efficiency and torque during design process. Key Words : Permanent Magnet (PM), Axial Flux Machine AFM.

Introduction Axial-flux machines appeared a few years ago, with the evolution of the needs (and especially for the applications with strong volume constraints). The axial flux motors become interesting when obtaining large surfaces of air-gap with the traditional machines is no longer possible. This motor can be used for direct-drive, low speed and high torque. They are usually employed in land transport applications, aeronautical field or for wind power generation system. Hybrid electric vehicles, as their name implies, draw their operating power from two or more sources of energy. Typically, these sources are an electric drive train, consisting of an electric motor and a battery and an internal combustion engine. HEVs are currently under development by auto manufacturers throughout the world and lots of research is devoted to further improvement of them. This persistent activity is also directly related with the concerns about global warming. HEVs are without any doubt only short-term or even mid-term solution to reduce the worldwide carbon dioxide emissions by an acceptable level. The paper aims to develop an optimal electrical machine that would satisfy the technical specifications demanded by this particular design. AFPM machines are becoming quite acceptable in electric vehicle applications. For instance, Zhang et. al. investigated several possible structures of AFPM wheel machines for electric vehicles. Permanent magnet (PM) motor drives have come of age. The invention of high performance magnets, like samarium cobalt and neodymium-boron-iron, have made it possible to achieve motor performances that can surpass

the conventional DC or induction motors (IM). Particular areas where permanent magnet drives can be superior include hifh power density, high torque to inertia ratio, and high efficiency. Hence, depending on the application, there are many instances where permanent magnet motor drives are preferable. AFPM machine has following advantages compared to RFPM machines . • simple winding • low cogging torque and noise (in slotless machine) • short axial length of the machine • higher torque/volume ratio However, there are certain the limitations of AFPM machines compared to RFPM machines as follows. • lower torque/mass ratio • larger outer diameter, large amount of PM, and structural instability (in case of slotless machine) • difficult to maintain air gap in large diameter (in slotted machine) • difficult production of stator core (in slotted machine) According to the survey on AFPM machines, the followings conclusions can be drawn. • slotless machines need a large outer diameter. • mass of AFPM machine is heavier than RFPM machine. • to maintain air gap, the construction must be strong or even complicated. • stator core production is difficult in slotted machines. Therefore to apply AFPM machines in direct-drive application for large scale wind turbine, these disadvantages must be solved or even improved significantly, since those cause cost increase and difficult manufacture. I.

Structural Aspects of permanent synchronous machines (PMSM)

magnet

The AFPM machine is a machine producing magnetic flux in the axial direction with permanent magnets. Fig. 1 depicts different AFPM machines such as slotless, toroidal-stator, slotted, coreless machines. A slotless, toroidal-stator PM machines have been discussed with several advantages such as the compactness, the short axial length, the suitable integration with the engine and others in reference designs [1][2] [3]. An axial flux PM generator named Torus for a directdrive variable speed wind turbine application. The Torus machine is a slotless, toroidal-stator, double-sided, axial, disc-type, permanent magnet, brushless machine.

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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 04 Issue: 01, September 2011 The direct-drive, low-speed, axial flux PM synchronous machine for 100 kW wind turbine has been proposed considering both mechanical and electromagnetic designs in [C33][C34].

between the windings are filled with epoxy resin to increase robustness and provide better conductor heat transfer. The rotor structure is formed by surface mounted NdFeB magnets, rotor core and shaft. It should be noted that only the windings facing the rotor PMs are used for torque production in RFMs. The portions of the windings on the outside surface of the stator and the portions on both sides are considered to be end windings in this topology. Therefore, this topology has long end windings when the aspect ratio D/L (diameter over axial length) is small. In that case, small aspect ratio could result in high copper loss. Besides, the flux density is reduced due to the large airgap. However, one important advantage of this machine is that the structure transfers the heat from the stator frame very easily. Therefore, machine electrical loading can be relatively high.

II. Need for optimized Design

Fig. 1. Different AFPM machines

The stator is toroidal with the iron sheet core to avoid eddy currents, and the winding is wound directly on the toothless stator core. The design and development of an axial-flux PM generator for a direct-drive generator with small-scale wind turbine has been described .

Economy and performance parameter such as losses and efficiency of machine is prime objective of design. Design depends on several parameters with various mechanical constraints. The parameters for axial flux machine are torque, efficiency, speed and outer and inner diameter. Varying one parameter at a time, all design parameters are calculated and then analysed accordingly. Following design formulas [7] were dominant during design. The fundamental torque for one stator face of the AFPM machine is found by

The generator consists of two rotor discs with PM located around its periphery. The stator is made of non- Where Ta is axial torque which needs to be optimized for magnetic non-conducting material and has a number of Kr. bobbin-wound armature coils located around its periphery. A. Radial flux surface mounted pm machines Conventional radial flux PM machines have now been used extensively for decades. Many papers exist in the literature concerning the RFM machine, the most common type of PM machine used in industry. These machines are well known to have higher torque capability than the more common induction machine (IM). The efficiency is also higher than an IM due to the lack of rotor windings have higher power density and higher torque per ampere ratio. However, an important manufacturing disadvantage of the RFM is that magnet maintenance must be carefully implemented so that the rotor does not fly apart. The non-slotted version of the conventional radial flux PM machine has also been analysed in the literature. The two major differences between the slotted and non-slotted versions of the radial flux PM machine are the existence of slots and the type of polyphase winding. The stator structure is non-slotted and consists of a stack of laminated steel. Back-to-back connected polyphase windings are wrapped around the stator in a toroidal fashion and termed air gap windings since the windings are not placed into slots. The places in

III. Design Parameters and Results The existing design of the total drive system for a hybrid electric vehicle puts specific requirements on the electrical machine, which can be summarized as follows: 1. Space allocated to the electrical machine: From the prototype technical drawings it may be seen that the electrical machine must be integrated with the flywheel: it is proposed to be placed inside the flywheel. The machine (including its frame) should be designed small enough to fit in a cylindrical volume of 150 mm height and 240 mm diameter. 2. Torque requirement: The machine is supposed to supply a mechanical torque of 18 Nm in the motoring mode under rated conditions. Short-time overloading can be necessary, e.g. if the vehicle is starting on a hill or in the case of a coupled trailer. In generating mode (the flywheel energy is recuperated) the machine should be able to supply a power of 30 kW.

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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 04 Issue: 01, September 2011 3. Rotational speed: Since the electrical machine is integrated with the flywheel, the rotational speed of the machine is the same as that of the flywheel which corresponds to 7000 rpm in city driving and a maximum of 16000 rpm while recuperating the brake energy. Table1 Specification of motor used for design optimisation. Sr. no 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Quantity

Value

Mechanical torque Maximum Speed Power Stator current(r ms) Max. inverter frequency Line to line e mf (r m s) Terminal voltage Phase Synchronous inductance Per unit Synchronous reactance Air gap flux density (at 600C),Bg0 Stator Outside diameter (Dg) Stator inside diameter (Di) Stator yoke length (Lst) Total stator axial length (Ly) Number of poles (2p) Number of slots/pole/phase(nspp) Number of turns/phase/stator(Nph) Air gap length(g) Total(x2)magnet axial length(Lm) Slot bottom width(wsb) Slot top width(ws) Slot depth(db) Slot top depth 1(dt1) Slot top depth 2(dt2) Total slot depth(ds)

18 Nm 1600r p m 30.16kw 53A 533Hz 330V 345V 0.115mH 0.203 0.735 T

Fig. 2 : Effect of Torque on Losses (Speed=16000 rpm)

190 mm 110 mm 30mm 45mm 4 2 16 1.5mm 6mm 6mm 1.5mm 11mm 2mm 2mm 15mm

The motor design procedure was subjected to variables such as torque, outer diameter, speed and corresponding effect on performance parameters such as efficiency, losses was obtained for different constrained parameters. Efficiency and losses passes through a point of turn in characteristics which correspond to optimized design. Maxima or minima of such parameters yielded selection of specific optimized value. were recorded which gave optimized result Data such as optimized outer diameter and optimized speed ensure low loss, high efficiency motor. Designers need to go through such procedure so as to come out with best possible design. This logic can be applied to weight also. Weight can also pass through a minimum value for certain set of input parameters such as speed, outer diameter, torque etc. Such set of data will help is designing light weight machine.

Fig. 3 : Effect of Torque on Efficiency (Speed=16000 rpm)

Fig. 4 : Effect of Speed on Different Losses (Torque = 18 N-m)

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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 04 Issue: 01, September 2011

Fig. 5 : Effect of speed on Different Losses (Torque = 18 N-m)

Fig. 8 : Effect of Outer Diameter on Efficiency (at speed 16000 rpm)

Fig. 6 : Effect of Outer Diameter on Different Losses (at speed 16000 rpm)

Fig. 9 : Effect of Kr (inside to outside diameter ratio) on Axial Torque

Fig. 7 : Effect of Outer Diameter on weight (at speed 16000 rpm)

CONCLUSIONS As seen from extensive design case study that there exist optimized point where performance criteria goes through a transition. This point of turn itself is a optimization criteria. Once a particular parameter say Kr (inside to outside diameter ratio) is fixed then other parameter under the shadow of constant optimized Kr will ensure optimized performance for other parameters such as outer diameter. Weight is also another parameter which leads to economical design. In this way designer should take multiple trials of design variation for satisfying multi-parameter optimization technique.

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(IJEECS) International Journal of Electrical, Electronics and Computer Systems. Vol: 04 Issue: 01, September 2011 Although many areas have been covered by the papers, much is yet to be done in the areas of machine design, control algorithms and drive design. As the technology for manufacturing the motors becomes more established and higher performance magnets become available, it can be expected that due to economies of scale, the cost of the machines and their drives will reduce, encouraging many more applications for the machines and drive systems.

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