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A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Application J. Y. Chen and C. V. Nayar Centre for Renewable Energy Systems Technology Australia Curtin University of Technology GPO Box U 1987 Perth 6001 Western Australia Telephone: +61 (0)8 9266 3369 Facsimile: +61 (0)8 9266 3107 E-mail: pchenj@alpha2.curtin.edu.au Abstract There is a growing market for battery charging wind generators for remote area power systems in Australia and overseas. The technology based on high speed, gear driven induction generators has not been cost effective and reliable in the low power range. Mechanical simplicity and high efficiency can be achieved by employing direct coupled, high torque, low speed generators using high strength permanent magnets. This paper presents the design details of a 20 kW permanent magnet generator using Neodymium-Iron-Boron magnets for direct coupled wind turbine applications. A prototype has been built and tested. The agreement between experiment and computation shows the feasibility of the design method. 1 INTRODUCTION Wind energy is regarded as an important resource of the renewable energy used for electrical generation. The amount of electricity produced by wind energy can even be comparable with that produced by diesel generating power station in many parts of the world. Most of the wind turbine generators in the world are connected with the power grid, rather than to serve for a local load. The grid readily absorbs the electricity produced by wind energy. Thus, such machines have not been cost effective and reliable in the low power range. There is a large potential market for isolated small wind generator systems. These wind generators are in small size, usually less than 5 kW, and are located in remote areas which are not connected to any electrical grid. There is a demand for higher rating wind generators used for battery charging in small hybrid power system in developing areas. Mechanical simplicity and high efficiency can be achieved by employing direct coupled, high torque, low speed generators using high strength permanent magnets. There have been a variety of high-performance permanent magnet generators [1-4] developed for significant applications. The paper describes the design of a 20 kW low-speed, permanent magnet generator for direct-coupled wind turbine application. This type of generator has an outer-rotor rotating around its centre stator, instead of the conventional machines with their rotors inside of the stator. The Fig.1 shows the outer-rotor, and Fig.2 the prototype 20 kW permanent magnet generator. Fig. 1 Outer-rotor The generator uses sintered Neodymium-Iron-Boron for the magnets so that it can stand a high magnetic loading. All of the radially magnetised square magnets are evenly arranged along the inside periphery of the drum. While the machine is running, the centrifugal force of the magnets applies a pressure to the bonding media, thus, increasing the reliability of their glued joint. The blades of wind turbine can be simply bolted to the front face of the outer-rotor. Due to the absence of gearbox, it was estimated that about 4% of input wind energy could be saved[2] for electrical generation. A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications Chen et al This type of generator may be used to charge battery-bank in an isolated WindSolar-Diesel-Battery hybrid power system that incorporates wind turbines, photovoltaic panels, battery storage and diesel engines. Reliable and simple mechanical configuration makes this sort of machine compact and lightweight, and easy to be installed on a tower. 2 WIND ENERGY ASSESSMENT Fig. 2 It is well known that the wind power Pw in an air stream with a density ρ and a velocity ν through a cross-area is: 0.5 0.45 1 ρAν3 2 (1) A wind turbine, with this swept area in the air stream, will develop a power Pt, and Pt/Pw is the conversion efficiency, sometimes called the power coefficient Cp. Mathematically 1 Cp = Pt/Pw = Pt/ ρAν3 2 (2) 0.35 0.3 0.25 0.2 0.15 0.1 Assuming a circular cross sectional area πR2 with a radius R of blade, then 1 Pt = CpρπR2ν3 2 0.4 Coefficient Cp Pw = 20 kW PM generator (3) 0.05 0 0 1 2 3 4 5 6 7 8 9 10 Blade-Tip-Speed Ratio B More detailed analyses of wind turbine performance[5] Fig. 3 Cp-β curve for a 30 kW horizontal axis wind turbine indicated that maximum power from the wind is obtained when the final down wind velocity is one third of the up wind velocity. The value of power coefficient is then 16/27 or 0.593. This is known as the Betz limit, after the German aerodynamicist who defined it in 1927. Practical wind turbines designed for wind generation have Cp value generally below 0.45. The power coefficient is a function of a turbine‘s wind blade tip speed to speed ratio, given as below β = νt/ν= ωR/ν (4) where, νt is the turbine blade tip speed and ω the turbine shaft angular speed. It can be seen, from the Fig. 3, that a single maximum Cpmax occurs when the tip-speed ratio takes a particular value βmax. Obviously, if the turbine is to extract maximum power from the wind, the shaft speed will vary as ω = βmax ν/R (5) When the turbine is running and keeping the tip speed ratio at βmax, the output power from the turbine is, Pt = ( 1 ρπ Cpmax R2) ν3 2 (6) The expression in the bracket is constant, and hence the output power varies as the cube of the wind speed. It is assumed that wind speed follows a Weibull distribution[6]. In designing a generator, rotating speed is one of key input data for electromagnetic calculation. Substituting Equation (5) for ν, Pt can be alternatively expressed in terms of ω as Pt = ( 1 ρπ Cpmax R5/βmax3) ω3 2 (7) A predicted annual energy output versus mean wind speed for a 30 kW wind turbine was used for wind energy assessment. The mean annual wind speed of 7.1 m/s is derived form the previous statistic distribution of wind speed in a detailed area (Western Australia), and the rotational speed of 170 rpm is initially determined as normal speed of the wind turbine. So, the 68 Hz is determined in the design of the 20 kW permanent magnet generator with 48 poles. 2 Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society Paper 146 A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications 3 DESIGN CONSTRAINTS 3.1 Temperature Rise Chen et al Since the system is to be work in remote location, it was specified that the generator should be completely enclosed as protection against possible environmental damage, such as storm, sun radiation, dust, insects and chemical pollution, etc. With this mechanical construction, there is almost no direct ventilation between inside and outside of the machine. The generator can only emit the extra heat energy into environment through the appearance of the drum. A maximum ambient temperature of 50°C is supposed in user’s area and the glue material fixing the magnets is able to afford 120°C at its up-limitation. Thus, a maximum temperature rise is no more than 70°C for the operation. All of the thermal sources must be catered for the assumed condition and restricted at the bearable level. 3.2 Demagnetisation The magnets can be partially demagnetised by over current or excessive temperature, or a combination of both. Generator is specified to suffer no more than a certain percentage demagnetisation at a possible maximum current and temperature. The procedures should ensure that the demagnetisation current is applied at all orientations so that the worst case is definitely covered. The partial magnetisation effects may be roughly calculated by manual calculation methods and more accurate computation involves finite-element or boundary-element methods. 4 ELECTROMAGNETIC DESIGN Due to so many important aspects of design, it is not easy to derive an optimum design in some simple forms. But, it is possible to obtain an optimum choice of machine construction just keeping in mind the requirement to achieve sufficient output power from a given dimensions, the availability to be able to predict performance at variable load and a prediction of voltage regulation due to winding resistance and inductance. Considering the possible non-linearity, an iterative numerical method is adopted to determine the operating point in magnets. The approaches involve discretisation of magnetic circuit and integration of loop flux path. Repeated calculation and optimised device aim at to obtain possible large output, good performances and low cost of product. The leakage flux resulting from the polarisation of the magnets affects the density in critical parts of the magnetic circuit and main flux. It is necessary to set up a simultaneous equation related to the balance between m.m.f. and reluctance drops throughout the entire path. ∫ H dl − ∫ H dl − ∫ H dl − ∫ H dl − ∫ J dΩ = 0 m s r g (8) Ω Where, subscript m, s, r, and g express the integral paths across magnet, stator, rotor, and air gap respectively. Ω is the integral area surrounded by the entire integral path. The first and last items in the equation (8) are refereed to m.m.f. of magnet and armature reaction. 4.1 Open-circuit operating point Referred to the case of open-circuit of this permanent magnet generator, the flux in the air gap, linking the stator winding, is solely produced by the magnets and induce an open-circuit voltage Eo in the armature, which corresponds to the open-circuit operating point of the magnets. To plot this point on the B/H characteristic, it is assumed that the demagnetising curve is a stabilised one with a single valued relationship. Namely, the demagnetisation within the range of operation is reversible. Normally, Nd-Fe-B material has a linear curve in the second quadrant on B/H characteristic. Although field strength H is negative in the second and third quadrants, it is preferred to regard H as a positive number for more convenient calculation. The B/H line can be defined as No-load operating Λ ex point with nil current Φr Λ∑ Λσ Φm O’ Φ∑ Φσ α’ex Fc Fm Fig. 4 Static operating point Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society Paper 146 3 A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications B = -µ‘H + Br Chen et al (9) So far as a magnet with the given geometry, its Φ/F characteristics can be found from the relations: Φ = BAm and F = Hhm where Am and hm are the magnetic area and height of the magnet. This Φ/F characteristic is linear with intercepts equal Φr = BrAm and Fc = Hchm, as shown in Fig. 4. Integral paths are selected both for the main flux and for leakage fluxes, and a balance is applied between the reluctance drop and the m.m.f. The total flux Φ, supplied by permanent magnets, equals the sum of main flux Φ∑ and leakage flux Φσ distributed along the flux path. It is expressed as: Φ = Φ∑ + Φ σ = Fm/(Rg+Rs+Rr) + FmΛσ = FmΛΣ + FmΛσ (10) The main flux path within a pair of poles consists of rotor yoke, two magnets, two sections of air gap, stator teeth and yoke. Because of that the flux density in the teeth is unsaturated and the permeability µr in steel is far greater than unit, the reluctance drops in the steel of stator and rotor are only a small part comparing with the magnetic potential difference across the air gap. The resultant permeance of the entire magnetic circuit can be expressed as: Λex = Φ/Fm = ΛΣ + Λσ (11) The Λex is the slope of no-load line and represents the external magnetic affection to the magnet. The operating point corresponding to the open-circuit load is plotted at the intersection of no-load line and demagnetising curve, as shown in Fig. 4. The corresponding coordinate on Φ-axis represents the total flux coming out from the magnet. The angle of the no-load line is αex = arctan pex. 4.2 Dynamic operating point With the current flowing through the winding, the magnets of the generator is significantly affected by an external magnetomotive force which is caused by armature reaction, and its operating point vary with the exerteded external field strength. The corresponding position of the dynamic operating point depends on both the direction and amplitude of the armature reaction. The air gap field, set up by the armature current, possesses a direct-axis component and a quadrature-axis component, which bear a close relation to the direct-axis and quadrature-axis inductances. Open-circuit operating point with nil current Br Pex load operating point with armature reaction O Bm L Load line αex Hc αex Had Fig. 5 Dynamic operating point The direct-axis flux passes across a magnet face and either add to or subtract from the magnet flux, depending on the direction of the direct-axis current which is defined by the load-angle θ between Eo and load current I. A positive sign for Id means a magnetising action and a negative sign a demagnetising action with respect to the magnets. In the case of this generator, more attention is paid to latter. The demagnetising field strength caused by the direct-axis armature current can be deduced from a set of equations. It is known that the amplitude of m.m.f. produced by the three-phase D-axis armature current under one pole one phase is Fad = W k 3 2 2WΦ k w I cos θ = 2.7 Φ w I cos θ π p p (12) Before being used for plotting the load operating point directly on the B/H characteristic of the magnets, equation (12) is needed to be divided by hm: Had = Fad / hm = 2.7 WΦ k w I cos θ phm (13) To determine the magnetic operating point on the B/H characteristic, the field strength Had is caused by armature reaction and can be directly iterated to the open-circuit characteristic. The no-load line pex is shifted along the H-axis for a Had and intersects the magnetic demagnetisation curve at point L shown in Fig.5. The L is the wanted load operating point of the magnet. 4 Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society Paper 146 A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications 5 Chen et al INVESTIGATION OF MACHINE PERFORMANCE 5.1 Open-circuit characteristics The open-circuit test was made under various rotational speed or frequencies. Fig.6 shows output RMS line-line voltages from computation and measurement. It is seen that the computed results from the method described in section 4.1 has a very good agreement with measured data. The test results shows that the output terminal EMF was linearly proportional to the rotational speed, and so the machine operated at unsaturated region which is identical with the flux density in the design. Output line-line voltage, V To investigate the performance of this type of generator, a prototype machine was built with an intention to deliver an output power of 20 kW at a supposed rotational speed of 170 rpm. A comparison of the performance is made between the results of the computation and measurement. 1000 900 800 700 600 500 400 300 measured 200 predicted 100 0 0 100 200 Fig. 6 5.2 300 400 500 600 Rotational speed, rpm Open circuit characteristics Performance of load test The prototype generator was tested with a symmetrical 3-phase resistive load. Varying with the different load resistance in the test, the armature reaction has significant influence on the output characteristics of the machine. At this resistive load test, it was found that although the no-load output voltage waveform was not a satisfied sinusoid the generator could produce a better sine wave output when the load current was increased beyond 13 A. The stronger the load current was, the more standard sine wave the output waveform tended to be. It is a very interesting behaviour and the expected performance that the generator is capable of supplying power system with a good sine wave over wide loading range. This trend was promoted when load current was raised as a result of that either load resistance dropped at certain working frequency, or frequency increased at the constant load resistance. Under either of the cases the inductive component of load current was enhanced. This varying tendency could be easy to be detected through the tests. The Fig.7 shows the obvious difference of waveforms between the cases at no-load test and at the resistive load test of 11Ω per phase. Both pictures of the Fig.7 (a) and (b) were taken at the operating frequency of 68 Hz. The Fig. 7(b) obviously has a more standard sinusoidal shape. phase voltage Vp-p=452 V (a) V p-p =379 V Phase voltage at no-load phase current Ip-p=147A (b) Voltage and current at resistive load Fig. 7 Output waveform A comparison of the load performances is made between the results of the computation and measurement. The designed parameters, methods and a proper phasor diagram have been used to predict the output characteristics. The prediction and measurement of output phase voltage varying with load current at 6 different speeds are shown in Fig. 8. Comparing with the Xd and Xq at low operating frequency, stator armature resistance ra has a significant influence to the performances of the permanent magnet machine. It is seen in Fig. 8 that the prediction of regulation at relative lower rotational rang reaches a satisfied agreement with the measurement by taking account of ra. A positive voltage regulation can been seen through the measurement. The computed results include stator resistance ra and armature reaction with influence of Xq. A good agreement between computations and measurements is obtained within normal Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society Paper 146 5 A Low Speed, High Torque, Direct Coupled Permanent Magnet Generator for Wind Turbine Applications Chen et al load current range. With higher value of current, difference between the two voltages is gradually increased. The possible reasons are: 200 Output voltage per phase, V (1) Variation of stator armature resistance due to temperature rises. In the computation, a constant resistance ra at constant temperature was only used. But, increased current yielded higher temperature and caused the ra changed a lot in the test. 180 160 140 120 100 80 60 40 20 0 0 10 20 30 40 50 60 70 (2) When frequency Load current, A increased, load angle and power factor angle varied Fig. 8 Out put voltage at resistive load in a more complicated function, which is related to the features of magnetic material and symmetry of the machine. 75 rpm computed 75 rpm measured 100 rpm computed 100 rpm measured 125 rpm computed 125 rpm measured 150 rpm computed 150 rpm measured 170 rpm computed 170 rpm measured 200 rpm computed 200 rpm measured The synchronous performance of this prototype machine was tested under varying load condition at 6 different rotational speeds. The experimental results show that when the output power of the generator reached 20 kW at the rotational speed of 170 rpm, the efficiency was 86%, load current 52 A and output phase voltage 134 V. After one hour’s full load test, the measured temperature rise of the stator armature was 57°C at the ambient temperature of 31°C. This exhibits that the machine is able to work at the supposed environmental condition with a reliable performance. 6 CONCLUSION The paper has described the design of a direct coupled, outer-rotor, permanent magnet generator for wind turbine application. A prototype machine was built and tested. The discussed essential parameters of this machine model was determined by using computational and experimental methods. There is a reasonable agreement between computed and measured results, which exhibits the feasibility of those approaches developed in the design. 7 ACKNOWLEDGMENT The study was founded by MERIWA as Project No. E233. The authors appreciate the cooperation of Wester Wind and the supply of experimental equipment from the Curtin University of Technology. Also, the authors thank the advice from Dr W. W. Keerithpala. 8 REFERENCES 1 Chalmers, B.J. (1994), “Performance of interior-type permanent-magnet alternator”, IEE Proc.-Electr. Power Appl, Vol. 141, (4) 1994, pp.186-190. 2 Spooner, E. and Williamson, A.C. (1996), “Direct coupled, permanent magnet generators for wind turbine application”, IEE Proc.-Electr. Power Appl, Vol. 143, (1), and pp.1-8. 3 Nayar, C.V. (1991), “Investigation of capacitor-excited induction generators permanent magnet alternators for small scale wind power generation”, Renewable energy, Vol. 1, No.3/4, pp381-388. 4 Binns, K.J. and Low, T.S. (1979), “Performance and application of multistacked imbricated permanent-magnet generator”, IEE Proc.-Electr. Power Appl, Vol.130, (6), pp. 407-414. 5 Golding, E.W., (1976), “The Generation of Electricity from Wind Power”, E&F Spon Ltd., London. 6 Johnson, G.L. (1985), “Wind Energy Systems”, Prentice-Hall, U.S.A. 6 Proceedings of Solar ’97 - Australian and New Zealand Solar Energy Society Paper 146