ABSTRACT

The electromechanical system of a wind power plant usually consists of three main parts: turbine, gearbox and generator. However, the wind power plant can be simplified by taking off the gear and by using a directly driven low-speed generator.

A directly driven, grid connected permanent-magnet wind generator was designed. The rated power of the generator was 500 kW and the synchronous rotational speed 40 rpm. The excitation of the generator was made by NdFeB permanent magnets mounted onto the surface of the rotor yoke. The construction of the machine was designed so that the losses were small, air-gap torque high and the phase voltages also had an almost sinusoidal waveform. The electrical performance of the generator was calculated by the finite element method.

The aim of this study is to design a machine, which can be connected directly to the grid.
 
 

INTRODUCTION

Wind power technology has been developed much during the last decade. Nowadays more than 3500 MW wind power capacity has been installed in Europe. The machines now entering the market generate 300 - 800 kW per turbine rather than the 100 kW average of the late eighties' models. Many manufacturers are presently engaged in the development of 1 MW and larger machines and some of them have already come to the market.

The electromechanical system of a wind power plant usually consists of three main parts: turbine, gearbox and generator. The rotor of a typical wind turbine rotates at a speed of 20 - 100 rpm. In conventional wind power plants the generator is coupled to the turbine via a gear so that the generator can rotate at a speed of 1000 or 1500 rpm. However, the gearbox adds weight, generates noise, demands regular maintenance and increases losses. The wind power plant can be simplified by taking off the gear and by using a directly driven low-speed generator, Fig. 1.

There are different alternatives for the design of a directly driven generator. The machine type can be, for example, an asynchronous machine, a permanent-magnet synchronous machine or a synchronous machine excited by a traditional field winding. Furthermore, the machine can be a radial-, axial- or transverse-flux machine. The path of the magnetic flux is perpendicular to the direction of the rotor shaft in a radial-flux machine and perpendicular to the radial direction of the rotor shaft in an axial-flux machine. The transverse-flux principle means that the path of the magnetic flux is perpendicular to the direction of the rotor rotation. The stator core can be slotted or slotless, and there can, for example, be a toroidal stator winding in an axial-flux machine. The first commercial directly driven generator in the power range of some hundred kilowatts was a synchronous machine excited by a traditional field winding. Nowadays, the greatest interest is in permanent-magnet generators for directly driven wind power plants, because the characteristics of permanent-magnet materials are improving and their prices are decreasing. Permanent-magnet excitation allows us to use a smaller pole pitch than in conventional generators. The efficiency can also be higher in the permanent-magnet machine than in the conventional machines. Different types of directly driven wind generators were presented, for instance in ref. [1-4].

The study focuses on the electromagnetic design of a multipole, low-speed permanent-magnet wind generator. The rated power of the machine is 500 kW and the rotational speed 40 rpm. The nominal frequency of the generator is 50 Hz. The electrical performance of the machine was calculated by the finite element method. The aim of this study is to design a machine, which can be connected directly to the grid.

 
 

Fig. 1. Typical and directly driven wind
power plants.

METHOD OF ANALYSIS

The calculation of the operating characteristics of the generator is based on a time-stepping, finite element analysis of the magnetic field. The field is assumed to be two-dimensional. The rotor is rotated at each time-step by changing the finite element mesh in the air gap, and the time-dependence is modelled by the Crank-Nicholson method. The laminated stator core was modelled as a non-conducting, magnetically non-linear medium. The rotor core, which was made of solid steel, was modelled as a conducting, magnetically non-linear medium. The permanent magnets were modelled as conducting material. The eddy current losses were neglected in the stator coil. The program was developed in the Laboratory of Electromechanics at Helsinki University of Technology by Arkkio [5] and Väänänen [6].

PERMANENT-MAGNET GENERATOR

Construction of the generator

A directly driven, low-speed permanent-magnet generator was designed. The rated power of the machine was 500 kW and the synchronous rotational speed 40 rpm. The low-speed generator should have a great number of poles and therefore the diameter of the generator may be rather large. The pole pitch and slot pitch should be large enough for the machine to be rigid and feasible to manufacture. Permanent-magnet excitation allows us to use a smaller pole pitch than in conventional generators. The efficiency can also be higher in the permanent-magnet machine than in the conventional machines.

The simple way to construct a rotor with a great number of poles is mounting the magnets onto the surface of the rotor yoke. In this study it was necessary to use high-energy magnets such as NdFeB magnets to provide an acceptable flux density in the air gap. The remanence of the NdFeB permanent magnets used was Br = 1.14 T, the coercivity Hc = 850 kA/m and the conductivity s = 461 kS/m.

The machine designed was a radial-flux synchronous generator. The stator was designed as a conventional variant. The stator core was slotted and the core segments were made of 0.5 mm laminations (3.6 W/kg, 50 Hz, 1 T). The voltage was chosen to be on the low-voltage level (<1000V). The winding was a three-phase, two-layer round-wire winding. The rotor yoke was a cylinder made of solid steel. The generators could be used for electrodynamics braking by connecting resistors and capacitors to the generator terminal.

The open stator slot and the surface mounted magnets can cause cogging torque and torque ripple in the machine [7]. A fractional slot winding was chosen to make the torque ripple and cogging torque smaller than with integral slot winding. The diameter of the copper wire was 1.9 mm and the width of the slot opening was chosen as small as possible, 3 mm. The cogging torque could also be decreased, if the magnets were displaced asymmetrically on the rotor surface. The symmetric magnet location has been chosen for the machine designed, because the cogging torque is not especially high and the machine is then also easier to manufacture. The number of slots per pole and phase was chosen to be 1.5 and the width of the magnets 2/3 times the pole pitch, because the losses were then small, air-gap torque high and the phase voltages also had an almost sinusoidal waveform.

The generator can be directly connected to a power network with frequency of 50 Hz. The number of poles was 152 and the line voltage of the generator was 690 V. The number of stator slots per pole and phase was 1.5 and therefore the machine has 684 stator slots. The air-gap diameter was 3.3 m and length 0.3 m. The machine factor of the generator was 3.85 kVAmin/m3. The cross-sectional geometry of the permanent-magnet generator is shown in Fig. 2. The main parameters of the generator are given in Table 1 and the weights of the active material are shown in Table 2.

Main parameters of the permanent-magnet generator.

Performance of the generator designed

The calculation of the operating characteristic of the machine was made by the finite element method. The field was assumed to be two-dimensional.

Active material weight of the permanent-magnet generator.

The waveforms of the winding voltage, flux linkage and cogging torque at no-load are shown in Fig. 3. The waveform of the induced voltage was calculated so that the stator winding was open, i.e. it was not connected to any external source. That means that the stator current was zero. The no load voltage of the generator was 690 V. The cogging torque was very low, under 1% of the air-gap torque at rated load.

The efficiency and power factor of the permanent-magnet generator are shown in Fig. 4. The power factor was 1.0 at no load and 0.96 at rated load. The efficiency at rated load was over 95% and also high at part load. The efficiency should be high also at part load, because usually the load is smaller than rated output in wind power applications. Then the total efficiency of the generator is high.

The torque ripple can cause a problem of noise or of vibration in the machine and therefore it should be low. The torque ripple was about +/- 2% of the air-gap torque at rated load. The maximum air-gap torque of the generator was 2.7 times the rated air-gap torque.

Fig. 4. Efficiency and power factor of the permanent-magnet generator.

Fig. 3. The waveforms of the winding voltage, flux linkage and cogging torque at no-load.
(The waveform of the induced winding voltage was calculated with open-connected stator winding.)

CONCLUSIONS

The electromechanical system of a wind power plant usually consists of three main parts: turbine, gearbox and generator. The rotor of a typical wind turbine rotates at a speed of 20 - 100 rpm. In conventional wind power plants the generator is coupled to the turbine via a gear so that it can rotate at a speed of 1000 or 1500 rpm. However, the gearbox adds weight, generates noise, demands regular maintenance and increases losses. The wind power plant can be simplified by taking off the gear and by using a directly driven low-speed generator. However, the low-speed generator should have a great number of poles and therefore the diameter of the generator may be rather large. The greatest interest is nowadays in permanent-magnet generators for directly driven wind power plants, because the characteristics of permanent-magnet materials are improving and their prices are decreasing.

A directly driven, grid connected permanent-magnet wind generator was designed. The rated power of the generator was 500 kW and the synchronous rotational speed 40 rpm. The excitation of the generator was made by NdFeB permanent magnets mounted on the surface of the rotor yoke. The electrical performance of the generator was calculated by the finite element method.

The stator slots and the surface mounted magnets can cause cogging torque and torque ripple in the machine. A fractional slot winding was chosen to make the torque ripple and cogging torque smaller than with integral slot winding. The number of slots per pole and phase was chosen to be 1.5 and the width of the magnets 2/3 times the pole pitch, because the losses were then small, air-gap torque high and the phase voltages also had an almost sinusoidal waveform.

A low-speed permanent-magnet synchronous generator would be a good solution for the construction of a directly driven wind generator.
 
 

REFERENCES

1. Caricchi, F., Crescimbini, F., Honorati, O., Santini, E. 1992. "Performance evaluation of an axial-flux pm generator." International Conference on Electrical Machines (ICEM'92), Manchester, U.K., 15-17 September 1992, Proceedings, vol. 2, p.761-765.

2. Deleroi, W. 1992. "Linear induction motor in generation use for windmills." Symposium on Power Electronics, Electrical Drives, Advanced Electrical Motors (Speedam'92), Positano, Italy, 19-21 May 1992, Proceedings, p. 71-76.

3. Spooner, E., Williamson, A.C. 1996. "Direct coupled, permanent magnet generators for wind turbine applications." IEE Proceedings, Electric Power Applications, Vol. 143, No. 1, January 1996. p. 1-8.

4. Weh, H. 1995. "Transverse flux machines in drive and generator application." International Symposium on Electric Power Engineering (Stockholm Power Tech), Stockholm, Sweden, 18-22 June 1995, Proceedings, Part Invited Speakers' session, p. 75-80.

5. Arkkio, A. 1987. "Analysis of induction motors based on the numerical solution of the magnetic field and circuit equations." Helsinki, Finland: Acta Polytechnica Scandinavica, Electrical Engineering Series, Doctoral thesis, No. 59, 97 p.

6. Väänänen, J. 1994. "Construction of a power electronic simulator including two-dimensional finite element modelling of electrical machines", Electrical Engineering - Archiv für Elektrotechnik, 78, 1994, 1, p. 41-50.

7. Lampola, P., Perho, J., Saari, J. 1995. "Electromagnetic and thermal design of a low-speed permanent magnet wind generator." International Symposium on Electric Power Engineering (Stockholm Power Tech), Stockholm, Sweden, 18-22 June 1995, Proceedings, part Electrical Machines and Drives, p. 211-216.