WO2005069465A1

WO2005069465A1 - Electric machine assembly

Info

Publication number: WO2005069465A1
Application number: PCT/FI2005/000021
Authority: WO
Inventors: Juha PYRHÖNEN; Jussi Huppunen; Panu Kurronen; Jarmo Saaranen; Kauko Saaranen
Original assignee: Rotatek Finland Oy
Priority date: 2004-01-14
Filing date: 2005-01-14
Publication date: 2005-07-28
Also published as: FI20040041A0

Abstract

An electric machine assembly, containing electric machine modules, especially generator modules (11-18), which are coupled to the shaft of a wind rotor or other actuator by a transmission mechanism comprising one or more gear wheels (B) directly connected to the shaft of the wind rotor or other actuator, and wherein the electric machine modules are arranged at the rim of the gear wheel and coupled to the gear wheel by second, smaller gear wheels (G) comprised in the transmission mechanism to reduce/increase the rotation speed of the electric machine modules. The shafts of the electric machine modules are connected to the second gear wheels (G) directly without an intermediate gear system, and the electric machine modules are axial-flux machines.

Description

ELECTRIC MACHINE ASSEMBLY

The present invention relates to an electric machine assembly, especially a wind power generator assembly containing electric machine modules, especially generator modules, which are connected to the shaft of a wind rotor or an equivalent actuator by means of a coupling arrangement. In prior-art wind power plants, a wind rotor generally drives one or seldom two generators via a gear system. As the wind rotor in larger wind power plants typically rotates at 10 - 20 rpm, the transmission ratio of the gear system is nearly 100 in order to give the generator a rotational speed of e.g. 1500 rpm. In windmills provided with single-stage planetary gears, the transmission ratio is close to 10, and the generator typically rotates at 150 - 250 rpm. Also known are so-called direct-drive generators, which means that no gear system is used at all and the wind rotor is connected directly to the rotor of the electric machine. A direct-drive generator of this type is disclosed in US patent specification US B1 -6,285,090. It describes a wind power generator comprising one or more modules connected to the shaft of a wind rotor, each module comprising a rotor disc and at least one stator disc, e.g. two stator discs fitted on either side of the rotor. As the power of the machine is dependent on its rotational speed and torque (P=ωT), the diameters and weights of direct-drive generators become very large due to the low rotational speed. Patent specification ES 2140301 discloses another direct-drive modular wind power generator, which comprises two successive generator modules connected to the shaft of a wind rotor. Patent specification US 4691119 A discloses an invention in which several electric machines are mounted after a large gearwheel, each after a separate gear system. Patent specification US 2463349 discloses an invention wherein several electric machines are mounted without a gear system after a large gearwheel. Thus, all known patent specifications deal amply with combinations of different torque wheels and/or gear systems and electric machines, but none of them discloses a solution that would function with very low rotational speeds (10...20 rpm) of the primary shaft and at the same time with a high power, in other words, with a very large torque and large and fast torque variations, as is typically the case e.g. in wind power applications. It is thus characteristic of the whole electric machine assembly that it requires control of large torques and their variations. It is therefore essential to reduce the torque and mass inertia of each generator as observed from the primary shaft, such as the shaft of a wind turbine. The object of the present invention is to overcome the drawbacks of prior art and to produce a solution that is smaller and lighter in mechanical de- sign than prior-art apparatus, a solution that is more reliable than prior-art geared solutions. In the present invention, according to the above-described model according to a prior-art method, the torque of e.g. a wind rotor is divided into several parts using a plurality of generators and increasing the rotational speed of each generator by means of gearwheels, thus creating a foundation on which the aforesaid aims of the invention can be implemented. Mounted on the shaft of a wind rotor is one large gearwheel that drives a plurality of generators, each one directly by means of a small gearwheel instead of a separate gear system as in prior-art wind power solutions. The inventive step of the present invention lies in the circumstance that the gear system and the electric machine are replaced by a frequency-converter- controlled axial-flux machine, because, as no gear system is needed and a slow-speed axial-flux machine having a good torque but a low inertia momentum is used, a small, fairly light, reliable and controllable solution is achieved that can be used even in dynamic applications, such as wind power applications. As the speed of the electric machine is low and the diameter and mass of the rotor are small, a relatively low mass inertia momentum is thus achieved. As this low mass inertia momentum is not 'multiplied' by a gear sys- tem, a low mass inertia momentum even as seen from the primary shaft is achieved. On the other hand, the axial-flux machine is a short electric machine and therefore supports very well the objective regarding small size and light weight. This modular solution saves space and weight, is more reliable due to redundancy, produces power even at low wind velocities, is easy to maintain and repair and provides a flexibility for the connection of frequency converters and extension of the system as a whole. The features of the apparatus of the invention for transmitting power e.g. from the shaft of a wind rotor as electricity to be supplied to an elec- trie network or a motor fed from an electric network are presented in detail in the claims below. According to the invention, the generator solution e.g. for a wind power station consists of a plurality of small generators. Thus, it is possible to produce wind power stations for different size classes either by combining a desired number of the same modular components or by combining a desired number of a few different basic components (e.g. of different power). The construction according to the invention allows such high power levels to be achieved that it becomes impossible to implement the traditional combination of a gear and one generator due to the enormous torques, torque impacts (gushes of wind) and speeds. In addition, the mechanics of the generators is significantly easier to manage throughout their service life during manufacture, assembly, transport, installation, maintenance, repair and operation than in the case of a single large generator. This is due to the modular construction, the smaller size and weight of the components. During operation, one or more modules can be controlled differently from each other so that fast variations of torque can be compensated e.g. by taking into account the natural frequencies of the entire wind power station, and thus it is possible to extend the mechanical service life of the entire wind power station. When only one electric machine is used, such compensation is considerably more difficult to implement. In comparison to a direct-drive generator and prior-art solutions based on multiple electric machines, the construction becomes significantly simpler and cheaper. The aim here is to utilize the electromagnetic stresses allowed for electric machines so as to make it possible to produce a considerably more compact solution. Since the torque of the electric machine is a decisive quantity in respect of price, it is advantageous to divide the torque among several modular machines. It is obvious to the person skilled in the art that different embodiments of the invention are not limited to the example described above, but that they may be varied within the scope of the claims presented below. The improvement in space utilization, especially because the length of the entire system is reduced significantly, and the reduction of the weight of the system as a whole are achieved according to the invention because no separate gear system is needed and the power is distributed among several small generators via a gearwheel and a pinion. In this way, the speed of an individual generator can be increased sufficiently to reduce the size. The improvement in reliability (MTTF, Mean Time To Failure) results from the fact that a failure of one generator in the apparatus of the invention will not interrupt the production of power but may only reduce it, whereas prior-art systems have only one gear system and/or one generator and therefore a failure of the gear system or generator will cause an interruption of power production. The probability of failure of a combination of a gearwheel and a pin- ion is lower than that of a gear system, because the number of components is smaller and the stress is lower due to the power distribution principle. The improvement regarding maintenance and repair (MTTR, Mean Time To Repair) results from the fact that in prior-art solutions, when a compo- nent has been damaged, often a crane has to be brought to the site, and especially in difficult areas with poor communication and in marine power plants, bringing hoisting equipment to the site is very expensive and time-consuming, in the apparatus of the invention, all the main components except for the frame can be handled using the wind power station's own hoisting equipment, be- cause the gearwheel is divided into segments that can be changed separately and each generator unit is small and replaceable. This modular construction of the apparatus makes it possible to keep core components in store at a low cost. The improvement in flexibility of the system with the apparatus of the invention is achieved because it allows a desired number of generators or motors of a desired size from a few (2 - 3) alternatives of basic power to be mounted in a single frame. However, considering different wind conditions or similar reasons, each wind power station can be designed to more specific dimensions as the design is not restricted by a given large gear system or generator. For example, an increase of power can be accomplished by adding gen- erators or replacing some of them with larger ones. This also applies to the frequency converters used with the generators, because in the construction according to the invention frequency converters can be connected on the modular principle to an individual generator or to generators operated in parallel, which is more difficult to implement in prior-art methods due to parallel windings and a large number of cable connections. In the following, the invention will be described in detail with reference to an example and the attached drawings, wherein_. Fig. 1 a - 1e present a wind power generator system comprising a 6-part generator assembly, in the axial direction, in cross-sectional side view, a rotor and a stator in side view and a stator frame as seen in the axial direction, Fig. 2 presents a 4-part generator assembly corresponding to figures 1 a - 1e as seen in the axial direction, Fig. 3 and 3b present an axial-flux rotor as presented in Fig. la ic, seen in the axial direction and from the side, Fig. 4 and 5 present other embodiments of the invention, Fig. 6 presents a gearwheel, Fig. 7 illustrates the performance of the electric machine as a function of power, and Fig. 8 illustrates the behavior of the performance of the generator as a function of rotational speed. Figures 1 a - 1e present a generator system for a wind power station, wherein one large gearwheel or gear rim mounted on the shaft of a wind rotor (not shown in the figure) drives a plurality (six in Fig. 1a) of generator modules 11 - 17 arranged symmetrically on its circumference by means of a small toothed wheel or belt at a speed higher than the speed of the rotor shaft. The force is transmitted to each generator either between the two gearwheels directly or by means of a toothed belt. ^■ The rotor of the wind power station is fastened to the shaft A (Fig.

1a, 1b), which again is connected to the gearwheel B according to the present invention. The shaft A is mounted with bearings on the frame F. Parts H, C and E form an axial-flux generator having a rotor C to which are secured permanent magnets D (e.g. sector-shaped permanent magnet pieces as shown in Fig. 3a). The rotor C is secured to a shaft E, which is mounted with bearings on the frame F of the generator module, said frame consisting of two plate-like parts and intermediate parts between them. Mounted on the other end of the shaft E is a pinion G, whose rotational speed is thus determined by the gearwheel B. The permanent magnets D of the rotating rotor C induce a current in the wind- ings H of the stator of the generator, from where the electricity is conducted either directly or via a frequency converter to the electrical power network. Figures 1 c and 1 d additionally show how the rotor C is secured to the shaft E with brackets T, and present the stator S, which is secured to a plate-like stator frame I having the shape of a circular sector as illustrated in Fig. 1e. The stator frame has holes J to allow it to be secured to frame brackets U, and apertures V for ventilation. As can be seen from Fig. 1 b, there are two stators S arranged on either side of the rotor, and the stators and the rotor can be protected with a cover plate W between the frame parts of the stator. The generator wheel, can also be provided with a cover plate Y (Fig. 6). As shown in Fig. 1 b, the generator modules may be arranged either on only one side of the gearwheel according to Fig. 4, which represents a machine with eight generator modules 11-18, or else 2 packets can be arranged in parallel, in which case e.g. max 12 - 24 generator modules (Fig. 5) are ob- tained. The generators may be different or identical to each other. The generators can be connected electrically in series and in parallel according to the situation (e.g. wind force). By selecting a given number of generators and their power classes, it. is possible to combine different technical values for the wind power station as a whole. A separate frequency converter may be provided for each generator or some or all of the generators feed the same frequency converter. When identical generators are operated at the same power angle, exactly the same switching instructions can be applied to each frequency converter of the individual generators. The generators, complete with shafts and gearwheels, can be mounted in the frame and dismounted from it as modules. The large gearwheel can be composed of segments that can be changed at the site of operation. Two generators can be mounted on the same shaft on either side of the gearwheel. Several generator units can be installed one after the other on the same shaft, such as e.g. on the same wind turbine shaft. It is also possible to select an asynchronous motor for use as a generator. An individual generator can also be coupled by prior-art methods via a gear system. The shaft of each generator is provided with a clutch which in a fault situation can disengage the gearwheel from the toothed rim or decouple the generator. A brake that can stop the wind power station or hold it still can be placed on the large gearwheel or as a part of an individual generator or in place of an individual generator. Each generator separately or the shaft of the wind turbine can be provided with a power engine which is used when no sufficient wind or other force is available. In place of an individual generator, it is possible to mount some other actuator, such as e.g. a cooling compressor, a coolant circulating pump, an oil pump, and a hydraulic pump for other functions of either the generator or the motor or the power station. The performance of an electric machine typically rises with the increase of power. Fig. 7 shows the performance of a 300-rpm generator as a function of power. It can be seen from the figure that the performance is not significantly improved after the power reaches about 500 kW. Accordingly, in the case of a modular generator, a good performance can be achieved by selecting individual machines with a power level of about 500 kW. Fig. again illustrates the behavior of the performance of a 3000-kW generator as a function of rotational speed. It can be seen that a direct-drive generator is not advantageous in respect of performance because the performance of the machine is significantly improved as the rotational speed increases. From the point of view of design of the electric machine, rotational speeds below 100 rpm are undesirable. It is obvious to the person skilled in the art that different embodiments of the invention are not limited to the example described above, but that they may be varied within the scope of the claims presented below. Thus, instead of axial-flux generators, it is also possible to use radial-flux generators. In addition, it can also be used as a generator in other power production plants besides wind power stations and also as an electric motor, in which case power supplied from an electric network is used to drive an actuator connected to the shaft of the motor assembly.

Claims

1. An electric machine assembly, containing electric machine mod- ules, especially generator modules (11-18), which are coupled to the shaft of a wind rotor or other actuator by a transmission mechanism, said transmission mechanism comprising one or more gear wheels (B) directly connected to the shaft of the wind rotor or other actuator, wherein the electric machine modules are arranged at the rim of the gear wheel and coupled to the gear wheel by second, smaller gear wheels (G) comprised in the transmission mechanism to reduce/increase the rotational speed of the electric machine modules, and wherein the shafts of the electric machine modules (11-18) are connected to the second gear wheels (G) directly without an intermediate gear system, characterized in that the electric machine modules are axial-flux machines. 2. An electric machine assembly according to claim 1 , especially a wind power generator assembly, characterized in that the number of modules is at least three. 3. An electric machine assembly according to claim 1 , characterized in that the modules are identical. 4. An electric machine assembly according to claim 1 , characterized in that at least some of the modules are different. 5. An electric machine assembly according to claim 1 , characterized in that the module contains at least one rotor disc, and a stator is provided on only one side of it. 6. An electric machine assembly according to claim 1 , characterized in that the module contains at least one rotor disc, and stators are provided on either side of it. 7. An electric machine assembly according to claim 1 , characterized in that the transmission ratio of the transmission mechanism and the mounting position of the stators and rotors of the electric machine modules have been so selected that all the electric machines have the same power an- gle. 8. An electric machine assembly according to claim 1 , characterized in that, to optimize the voltage e.g. with respect to the wind-force, the modules can be electrically connected in series when a low find-force prevails and/or in parallel when a fairly high wind-force prevails and one or more modules can be controlled by means of a frequency converter so that the change of torque caused by the electric machine assembly will impose as little stress as possible on the wind power station. 9. An assembly according to claim 1 , characterized in that, by selecting a given number of modules and their power classes, a desired power level of each individual plant, such as each individual wind power station in a wind power park, is achieved. 10. An assembly according to claim 1 , characterized in that a separate frequency converter is provided for each module or a few or all of the modules are connected to one frequency converter. 11. An assembly according to claim 7, characterized in that when identical modules are used with the same power angle, exactly the same switching instructions can be applied to each frequency converter of an individ- ual electric machine. 12. An assembly according to claim 1 , characterized in that it is implemented without a separate gear system by integrating the gear wheels and electric machines into the same unit or they can also be implemented as separate units. 13. An assembly according to claim 1 , characterized in that the electric machines with their shafts and gear wheels are modules, so that they can be mounted in the frame and dismounted from it as modules. 14. An assembly according to claim 1 , characterized in that its gear wheel can be implemented by composing it from segments that can be replaced at the site of operation. 15. An assembly according to claim 1, characterized in that two electric machines can be mounted on the same shaft on either side of the gear wheel. _.16. An assembly according to claim 1, characterized in that sev- eral units can be placed on the same shaft one after the other. 17. An assembly according to claim 1 , characterized in that the shaft of each module is provided with a clutch which, e.g. in a fault situation, can disengage the pinion from the gear rim or decouple the electric machine. 18. An assembly according to claim 1, characterized in that a brake that can stop the wind power station or equivalent or hold it still can be mounted on the large gearwheel or as a part of an individual generator or in place of an individual generator.

19. An assembly according to claim 1 , characterized in that each generator separately or the shaft of the wind turbine can be provided with a power engine, which is used when no sufficient wind or other force is available. 20. An assembly according to claim 1 , characterized in that the ro- tor is secured to the shaft (E) with brackets (T). 21. An assembly according to claims 1 - 20, characterized in that in place of an individual electric machine it is possible to mount some other actuator, such as e.g. a cooling compressor, a coolant circulating pump, an oil pump, and a hydraulic pump for other functions of either the generator or the motor or the power station. 22. An assembly according to claim 1 , characterized in that the stator (S) is preferably secured to a plate-like stator frame (I) having the shape of circular sector and having holes (J) to allow it to be secured to frame brackets (U), and apertures (V) for ventilation.