A typical application for a transmission of the type mentioned above is a wind power generation plant, and the below description will therefore be made concrete using a wind power plant as an example.

BACKGROUND Wind power plants, today one of the world's most rapidly expanding trades, require transmissions with a high gear reduction. A wind turbine typically works at a speed of about 15-20 rpm, whereas a generator, in order to become of standard design and thus be reasonably economical in manufacture, should run with at least 1500 rpm. This means a transmission gear ratio from about 75: 1 up to 100: 1. A transmission having such a gear ratio is commonly embodied as a planetary gear transmission having several stages, but will still be very large and heavy. In order to adapt the turbine speed somewhat to various wind speeds, generators with two fixed speeds are today commonly used, making the transmission still larger and heavier.

As the transmission is located at the top of the tower of the wind power plant, this size and weight will create problems during assembly as well as maintenance work.

Another alternative would be to run the generator at the slow speed of the wind turbine. In order then to reach the network frequency of 50 Hz, the generator must have a multitude of poles and/or a large diameter in order to reach a sufficient peripheral speed. As the generator is located at the top of the tower of the wind power plant, behind the actual wind turbine, there will be physical limitations to its size, for the same reasons as regarding the transmission. Furthermore, in this case the generator would have to be of special manufacture, making it expensive. In many cases, therefore, frequency converters are used to achieve the appropriate frequency. The frequency converter may in this case further be used to allow the speed of the wind turbine to vary with the wind speed, whilst still delivering, via the frequency converter, 50 Hz to the network.

There is thus a market demand for a transmission, and for a method of controlling said transmission, allowing for example a wind turbine to rotate at a low and continuously variable speed, so as to be able to adapt itself momentarily to the wind speed, thereby making optimum use of the energy contents of the wind, whilst simultaneously allowing the driven generator to run at the highest possible speed, without making the transmission large and heavy in the process. Through momentary adaptation of the turbine speed to the wind speed, there is a theoretical possibility of increasing the efficiency of the wind turbine by about 7 %.

Furthermore, the possibility arises to influence the sound level of the wind turbine through the speed variation.

As wind power plants are always located at the top of a high tower, and often in inaccessible places, for example at sea, there is also a need for minimising the weight and the space requirement of the power plant components, and to reduce the need for, and to simplify the work during, service and maintenance.

The above objects are met by the method and the device for rotational speed control and for achieving a high gear reduction in transmissions, set forth in the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWING Fig. 1 shows a very schematical principle drawing of how known components could be interconnected, in accordance with the method of the invention, to form a device according to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT In the following, a preferred embodiment of the invention will be described, with reference to the figure. The description will by no means be limiting as regards the scope of the invention, which is solely determined by the appended claims.

Fig. 1 shows a planetary gear transmission having an input shaft 1, fixedly connected to the transmission ring gear 2. The planetary gear transmission has, in a known manner, a number of planetary gears 3, mounted on a planet carrier 5 and rotating in constant mesh with the ring gear 2 and a sun gear 4.

The sun gear is, via a shaft 8, connected to an electrical primary generator 9.

The planet carrier 5 is, via a hollow shaft 6, connected to an electrical secondary generator 7.

The primary generator 9 is, as regards frequency, directly connected to a frequency-stable, three-phase electrical network 12, and may be of the novel high- tension type developing 10.000 Volts, allowing direct connection to the network as illustrated in Fig. 1. The generator 9 may also be of the conventional type,

developing a lower tension, in that case connected to the network via a transformer (not shown). The secondary generator 7 is connected to the frequency-stable electrical network 12 via a frequency converter 10 and a transformer 11. However, the secondary generator may also be of the high-tension type, eliminating the need for the transformer 11.

If the prime mover in this embodiment example would be a wind turbine, said turbine would, in a known manner, be connected to the input shaft 1 of the planetary gear transmission, which, in this embodiment example, is connected to the ring gear 2 of the transmission. Between the wind turbine and the input shaft 1 of the planetary gear transmission there is also, in a known manner, a brake arrangement for braking and locking the wind turbine an immobile position. The wind turbine and the braking arrangement are not shown.

In order to facilitate understanding of the function of the device, certain parameters for the comprised components are hereby assumed. These parameter values may, as the person skilled in the art will understand, be modified in order to optimise the device, as long as the modifications are co-ordinated so as to achieve the equivalence in the relationship between the speed ratio of the generators and the gear ratio of the planetary gear transmission, as described below.

The planetary gear transmission is assumed to have a gear ratio of 1: 3 between the rotational speed of the planet carrier 5 and that of the sun gear 4, if the ring gear 2 is immobile. The primary generator 9 is assumed to have only one pair of poles, thus rotating at 3000 rpm when phased into a 50 Hz network. The secondary generator 7 is assumed to have three pairs of poles and would then, if phased directly into a 50 Hz network, rotate at 1000 rpm. However, as the secondary generator is connected to the 50 Hz electrical network via a variable frequency converter, the rotational speed of the secondary generator, in the synchronised or phased-in state, can be made to vary within a range around these 1000 rpm.

Assume that the sun gear 1, being connected to the electrical primary generator 9, is forced by the stable network 12 and the synchronised primary generator 9 to rotate clockwise at 3000 rpm. At the same time, assume that the frequency converter 10 is adjusted to a conversion factor of 1, i. e. no conversion, and that the planet carrier 5, being connected to the secondary generator 7, is forced by the stable network 12 and the synchronised secondary generator 7 to rotate clockwise at 1000 rpm. The ring gear 2 will then, as the gear ratio between the planet carrier 5 and the sun gear 4 is 1: 3, and the ratio between their respective speeds are also 1: 3, be standing completely still.

Consequently, if the conversion factor of the frequency converter 10 is now increased or decreased, the synchronised rotational speed of the secondary generator will increase or decrease from 1000 rpm. This at the same time entails that the ring

gear 2 of the planetary gear transmission will begin to rotate at a speed varying with the difference between the speed of the primary generator 9 and the speed of the secondary generator 7, multiplied by the gear ratio between the planet carrier and the ring gear of the transmission. For a planetary gear transmission having a gear ratio of 1: 3 between the planet carrier 5 and the sun gear 4, the gear ratio between the ring gear 2 and the planet carrier 5 will be 2: 3.

If the synchronised speed of the secondary generator 7 is raised, by means of the frequency converter 10, to 1030 rpm clockwise, the ring gear will thus also rotate clockwise, but at a speed of only (1030-1000)/1,5 = 20 rpm. If the synchronised speed of the secondary generator 7 is instead lowered, by means of the frequency converter 10, to 970 rpm clockwise, the ring gear will instead rotate anti- clockwise, at a speed of (1000-970)/1, 5 = 20 rpm.

In the illustrated embodiment example of the method and the device according to the invention, the extremely frequency-stable electrical network 12 is thus used for maintaining one of the three movable main components (sun wheel 4/primary output shaft 8) of the planetary gear transmission at a constant, relatively high speed, rather than, as in the conventional way, maintaining one main component immobile, that is at the constant speed of zero. Another one of the transmission main components (planet carrier 5/secondary output shaft 6) is allowed to rotate at a speed that is also very stable (due to the electrical network 12) but adjustable (thanks to the frequency converter 10) about that nominal rotational speed where the speeds of the two transmission main components (4,5) concerned, with regard to the gear ratios, cancel each other out, making the third main component (ring gear 2/input shaft 1) stand still. Hereby, the rotational speed of the input shaft 1 can be continuously controlled in both directions about zero rpm, through varying the setting of the frequency converter 10. The setting of the frequency converter 10 can easily, and in a known manner, e. g. by a microprocessor-based control system, be controlled in response to an external parameter, in the described embodiment example e. g. in response to the wind speed, hereby allowing the rotational speed of the wind turbine to be continuously adjusted in relation to the wind speed.

In accordance with this inventive method, there is provided on the one hand an extremely high gear ratio between the input shaft 1 and the primary output generator shaft 8, in the demonstrated embodiment example no less than 3000: 20 or 150 : 1, and between the input shaft 1 and the secondary output generator shaft 6 about 1030: 20 or 51,5: 1, on the other an elegant possibility of continuous speed control of the input drive shaft 1, in the clockwise as well as the anti-clockwise direction. Thanks to the stability of the electrical network 12, the control will be independent of changes in the input power on the input shaft 1; if the power increases, the two generators 7 and 9 will simply generate more power onto the

electrical network 12. We assume, as a matter of course, that all components in the device are adapted to their required power, speed, torque, etc.

In the illustrated embodiment of a wind turbine/generator, the device is started in the following manner: a. The shaft 1 of the wind turbine is initially braked and immobile. b. The secondary generator 7 is started unexcited (on idle) by regulating the frequency converter 10 from the frequency zero and upwards, and is phased into the net at its nominal speed, i. e. 1000 rpm. c. As the wind turbine is immobile, the ring gear 2 of the transmission is standing still. This entails that when the secondary generator 7 starts to rotate, the rotation of the planet carrier 5 will also rotate the sun gear 4, and thereby the primary generator 9, which is also started and phased unexcited (on idle) into the network at its 3000 rpm. d. When both generators are phased-in or synchronised, they are excited, at the same time as the wind turbine brake is released and the frequency converter 10 is adjusted to such a frequency conversion factor that a secondary generator 7 speed is created, which deviates from the nominal (1000 rpm) in such a direction and with such an amount that the wind turbine will rotate in its normal operating direction at a rotational speed adapted to the wind speed. e. The rotational speed of the wind turbine can now easily be adapted to the wind speed in an optimum way, by measuring the wind speed in a known manner and controlling the conversion factor of the frequency converter 10 in response thereof, for example via a microcomputer, making the wind turbine adapt its rotational speed continuously, in an optimum manner, to the wind speed.

The method and the device according to the invention could also be applied in a case where no shaft could be maintained at a stable and constant speed. In such a case, the speed of one of the transmission main components (e. g. planet carrier 5/secondary output shaft 6) could be arranged to be controlled by an external parameter (e. g. the wind speed) as well as by the speed of another main component (e. g. sun gear 4/primary output shaft 8), causing the speed of the secondary output shaft 6 to vary about a nominal speed that is a constant factor times the speed of the primary output shaft 8, the constant factor being equal to the transmission gear ratio between the two shafts 6 and 8. If the external parameter remains constant, then a constant speed of the third transmission main component (e. g. ring gear 2/input shaft 1) will still be maintained, even if the speed of the primary output shaft 8 is varied, as the speed of the secondary output shaft 6 is varied simultaneously, by the external

parameter through the control system, with a corresponding amount, considering the transmission gear ratio.

The method and the device according to the invention thus provides among others a continuous speed control of the input shaft 1, as a function of an external parameter, said control being completely independent of the input power applied onto said shaft, whilst simultaneously providing an extremely high gear ratio between the input shaft 1 and the two output shafts 6 and 8, without requiring to that end a larger and heavier transmission. This allows a large generator diameter and a high generator weight to be avoided, and the costs to be kept down.

A person skilled in the art will surely recognise several possibilities for modification and change of the method and the device according to the invention, without departing from the scope of the invention, which is solely limited by the appended claims.