Abstract:
A variable ratio transmission having an infinitely variable ratio, for enabling a tidal turbine ( 10 ) or the like running at relatively low but continuously varying speeds, to drive smoothly a constant speed machine, such as a synchronous generator or the like, without excessive transient torques. A preferred transmission includes a primary differential gear train ( 120 ), having a driven input ( 50 ), and a driving output, wherein the ratio of the input speed to output speed is variable, and wherein said ratio is governed by the reaction torque provided by speed controlling gearing ( 150 ) within the primary differential ( 120 ), the transmission including a secondary differential gear train ( 220 ), which is in rotational communication with the input ( 50 ) via a first route ( 115 ), and is in rotational communication with the speed controlling gearing ( 150 ) via a second different route ( 125 ), wherein the secondary differential gear train includes two parallel power paths.

Description:
FIELD OF THE INVENTION 
     The present invention relates to variable ratio transmission having an infinitely variable ratio, for example enabling a prime mover, such as a wind or tidal turbine running at relatively low but continuously varying speeds to drive smoothly a constant speed machine, such as a synchronous generator, without excessive transient torques. 
     BACKGROUND OF THE INVENTION 
     In particular, the requirement for speed increasing transmissions for wind turbines stems from their low rotational speed compared with the preferred generator speed, typically 1500 rpm. The low turbine speed is dictated by the fact that wind energy generation is a function of the turbine-swept area and the blade tip speed limits. Thus, the higher the power, the lower the rotor speed. In effect, power is directly proportional to the rotor diameter squared, while rotor speed is inversely proportional to tip diameter and/or the square root of rotor power; e.g. a 3000 kW turbine would run at 16 rpm compared with 44 rpm for a 400 kW machine with the same tip speed. Since rotor weight and torque are directly proportional to the rotor diameter cubed; bigger turbines not only have bigger transmission step up ratios but even bigger input torques and therefore a lower power to weight ratio. For example, whereas a 3000 kW turbine generates 7.5 times the power of a 400 kw machine, its torque and weight are increased by a factor of 20.54 (i.e. 7.5 raised to the power of 1.5), while its ratio is increased by a factor of 2.74 because its speed is reduced by the square root of 7.5. 
     Since the volume, weight and price of a gearbox is governed by its torque and overall ratio, there is an incentive to reduce weight by reducing the parasitic transient overload torques, which typically occur (and for which provision is made) in all fixed ratio wind turbine transmissions. These are created by the stochastic variations in wind speed, air density and unit aerodynamic energy over the large swept area of the turbine. Such variations lead to speed fluctuations of the turbine rotor hub at its input to the gearbox, which may occur many times per revolution. This is further complicated, firstly by steady changes in wind speed and secondly by the sudden changes which occur during gusts. Since wind energy is directly related to air velocity cubed, a 50% transient speed increase will increase aerodynamic power by a factor of 3. While some of this power will be dissipated by lower efficiency and some by the increased speed and kinetic energy in the turbine, it follows that with a fixed ratio transmission additional torques will be generated in trying to accelerate the generator. This stems from the fact that the polar moment of inertia of a generator about its own axis when referred to the axis of the turbine rotor is multiplied by the square of the step up ratio. Thus a generator requiring a step up ratio of 80/1 would have a referred inertia 6400 times that about its own axis. A 1-degree angular deviation of the turbine rotor hub from its mean speed of rotation would therefore suggest an 80 degree fluctuation of the generator over the same time scale. 
     An asynchronous generator can employ power conditioning to produce a smooth electrical output, but this masks the problem because mechanical acceleration torques are still required to change its speed. Such transient acceleration torques can only be mitigated by strain energy in the mechanical transmission path and so more rigid, transmissions will have higher torques. 
     The variable ratio transmission obviates this problem by changing its ratio in a complementary way, at the same rate as the transient change in the turbine speed. By so doing, generator torque, speed and phase angle are kept constant by allowing the turbine to accelerate and absorb the transient excess of power in the form of kinetic energy. 
     Prior variable ratio gearboxes for example as described in the present applicant&#39;s prior patent application WO 2004/109157, have been employed for applications including wind power turbines. However, their complication, for example, the need to use consumable components such as clutches and alternative power routes, increases their size, weight and manufacturing costs. The inventor has realised that a simple variable ratio gearbox is required which, in embodiments, provides generator speed control and a simple design. 
     SUMMARY OF THE INVENTION 
     According to one aspect the present invention consists in a variable ratio transmission for wind or tidal power generation applications, suitable for providing a substantially constant speed output from a prime mover input which varies in speed at least within an input speed range where all power generating takes place, the transmission including a primary differential gear train, having a prime mover input, and a driving output, wherein the ratio of the input speed to output speed is variable, and wherein said ratio is governed by the reaction torque provided by speed controlling gearing within the primary differential, the transmission including also a secondary differential gear train, which is in rotational communication with the input via a first route is in rotational communication with the speed controlling gearing via a second different route, wherein the secondary differential gear train includes two parallel power paths, one path including a hydraulic bypass having two hydraulic units in a hydraulic circuit; the power transmitted and direction of power transmitted in the hydraulic circuit of the bypass path being selectively variable in use to vary the speed in the other path, thereby to alter the power in the second route and to alter the reaction torque of the speed controlling gearing and thereby to govern said ratio, the transmission being characterized in that the first and second routes are utilized for the whole input speed range. 
     Preferably, the first and second routes are clutchless. 
     In an embodiment the units have power transmitted in one direction between them, when the input speed is below a predetermined value in the range, and said power transmission direction is reversed when said speed is reached or exceeded. 
     Preferably the hydraulic units are each driven at a speed which is proportional to the input speed. 
     More preferably each hydraulic unit is a positive displacement device having a variable stroke to alter hydraulic volumetric flow rate or pressure. Preferably the stroke of the one or both units is adjusted to alter the power transmitted in the bypass path. 
     In one arrangement, the primary and/or secondary differential gear trains include epicyclic gear trains. If so, the primary and secondary gear trains may both be epicyclic gear trains and any one or more of the following epicyclic arrangements may exist: 
     the first input to the primary differential is the planet carrier of the primary differential; 
     the output of the primary differential is the annulus of the primary differential; 
     the speed controlling gear of the primary differential is the sun wheel of the primary differential; 
     the two power paths of the secondary differential include the planet carrier and the sun wheel of the secondary differential. 
     The invention extends to a transmission system including: (a) a low speed fixed ratio step up stage; and (b) a variable ratio stage, having an input shaft, for receiving a steplessly variable input speed from the fixed ratio stage and providing a constant output speed on its output shaft wherein the variable ratio stage includes or comprises gearing as claimed in any one preceding claim. 
     Preferably, the fixed ratio stage includes two epicyclic stages in series. The invention extends to a fluid driven prime mover drivingly connected to a generator via a variable ratio transmission as described above. 
     The invention extends to any novel feature described herein and to any novel combination of features described herein, whether or not those features are described in combination herein. For example, the invention may consist in a variable ratio transmission for wind or tidal power generation applications, suitable for providing a substantially constant speed output from a prime mover input which varies in speed at least within an input speed range where all power generating takes place, the transmission including a primary differential gear train, having a prime mover input, and a driving output, wherein the ratio of the input speed to output speed is variable, and wherein said ratio is governed by the reaction torque provided by speed controlling gearing within the primary differential, the transmission including also a secondary differential gear train, which is in rotational communication with the input via a first route and is in rotational communication with the speed controlling gearing via a second different route, wherein the secondary differential gear train includes two parallel power paths, one path including a hydraulic bypass having two hydraulic units in a hydraulic circuit, the power transmitted and direction of power transmitted in the hydraulic circuit of the bypass path being selectively variable in use to vary the speed in the other path, thereby to alter the power in the second route and to alter the reaction torque of the speed controlling gearing and thereby to govern said ratio, the transmission being characterized in that the first and second routes are clutchless. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention may be performed in various ways and a specific embodiment will now be described with reference to the accompanying drawings in which: 
         FIG. 1   a  is a sectional schematic view of step-up gearing for use with the gearing showing in  FIG. 1   b;    
         FIG. 1   b  is a part cross sectional schematic view of an assembly of gearing and a generator. 
         FIGS. 2 to 4  show the gearing of  FIG. 1   b , illustrating the power transmission during various modes of operation. 
         FIG. 5  is a graph indicting relationship of the stroke rates of the hydraulic units used and the speed of one of those hydraulic units. 
         FIG. 6  is a plot of rotor speed against rotor power and torque, and the speed of the hydraulic unit mentioned immediately above; 
         FIG. 7  is a plot of hydraulic unit speed and bypass power; and 
         FIG. 8  is a table of variables. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1   a , step-up gearing is shown having two epicyclic gear trains  20  and  30 , in series which converts the relatively slow rotation of a wind turbine input shaft  10  into a faster rotation at pinion  40 . Since the gearing in  FIG. 1   a  produces a large increase in rotation speed, it follows that small variations in turbine speed at input shaft  10  are amplified into large changes in the speed of the pinion  40 . It is desirable to have a constant speed to drive a synchronous generator and so pinion  40  is connected to the input gear  50  of variable ratio gearing shown in detail in  FIG. 1   b.    
     Referring to  FIG. 1   b , the input gear  50  is shown which is driven at varying speeds by the gearing shown in  FIG. 1   a . Input gear  50  drives a primary pinion  100 . Primary pinion  100  is in turn connected to a planet carrier  110  of a primary epicylic differential  120  and also drives a secondary pinion  200  along a first power transmission route  115 . The planet carrier  110  drives generator  300 , via an annulus  130  of the primary epicyclic differential  120 . 
     In use, the reaction torque of the sun gear  150  of the primary differential can control the speed of the generator  300 . In use, when the input is slow, the sun gear  150  needs to be turned in the same direction as the input (the planet carrier  110 ) to increase the effective speed of the annulus  130  and hence the generator  300 , and when the input is faster, the sun gear  150  needs to stop, or slip in the opposite direction to the input to slow the annulus  130  down. The reaction torque on the sun gear  150  can be used to control its speed, either driving the annulus  130  faster when higher torque is applied or being driven (slipping) in the opposite direction, when lower torque is applied. 
     The reaction torque of the sun gear is controlled by a secondary differential epicyclic gear train  220 . The secondary differential has an annulus  230  connected to the secondary pinion  200 . Two power paths P′ and P″ are defined by the planet carrier  210  and the sun gear  250  of the secondary differential  220 . The planet carrier  210  is in rotary connection with one of two hydraulic units  180 , the sun gear  250  being in rotary connection with the other unit  500 . In this instance the sun gear  250  is connected also a pair of pinions  260 ,  265  which receive torque from or provide torque to the sun gear  150  of the primary differential  120  and define a second power route  125 . 
     The power transmitted through the hydraulic bypass and the direction in which it flows are controlled by altering the stroke of the two hydraulic units. This in turn alters the reaction torque at sun gear  150  and consequential alters the output speed of the primary differential. A controller  600  is used to monitor, input speed and pressure in the hydraulic bypass, as well as controlling the stroke rates in the respective hydraulic units  500  and  180 . 
     In use the gearing has a number of modes of operation as are described below. 
     Firstly, with reference to  FIG. 2 , when the input shaft is at or below 11.5 r.p.m then the gearing will operate as a fixed ratio. Hydraulic unit  180  will be set to a near maximum stoke but it will be rotating relatively slowly and hydraulic unit  500  will be set to a near minimum stroke but rotates relatively quickly, so that it is being driven by unit  180 . This causes power to be transmitted through the hydraulic bypass and through the gearing in the direction of the arrows P and sun gear  150  provides as much torque as possible in the direction of rotation of the planet carrier  110 , to increase the speed of the generator  300 . However, at this input speed the generator does not reach the desired 1500 r.p.m. and so the output speed (generator speed) is allowed to vary with input speed and the generator is not connected to the grid system because it is not at rotating at the correct speed. 
     Secondly, with reference to  FIG. 3 , when the input is in the range of about 11.5 r.p.m to about 17.3 r.p.m, which is the usual input speed range, a second mode of operation is employed. In this mode the generator, will be synchronised with the grid system and providing power to the grid, and therefore it is necessary to keep its speed constant. Consequently, the gearing is operated with a variable ratio because the input shaft speed will vary with fluctuations in wind speed etc. The reaction torque provided by sun gear  150  must be sufficient to increase the generator speed, but it must be gradually reduced as the input speed increases from about 11.5 to about 17.3 r.p.m. To achieve this, the amount of power routed through the hydraulic bypass is altered. The power flowing through the bypass can be controlled by adjusting the stroke of one or both hydraulic units. In this mode, the stroke of unit  180  remains at or near maximum and the stroke of unit  500 , initially at a minimum, is increased in response to an increase in pressure in the bypass, as a function of input speed. As a consequence of the arrangement of the secondary differential, decreased power flowing through the bypass decreases, overall, the amount of reaction torque produced by sun gear  150 , which keeps the output speed to the generator constant as the input speed increases. As the input speed starts to approach about 17.3 r.p.m. the stroke of unit  180  is reduced, but the stroke of unit  500  is maximised. 
     When the input speed is at about 17.3 r.p.m. then no power flows through the bypass because the stroke of unit  180  is zero and the speed of unit  500  is zero. At this speed the reaction torque required of sun gear  150  is just that needed to stop it slipping. This is the predicted average turning speed for the turbine. 
     A third mode of operation is used where the rotor speed exceeds 17.3 r.p.m but does not exceed 19 r.p.m. This mode is illustrated in  FIG. 4 . The generator is still producing power at the desired rotational speed. The sun gear  150  is allowed to slip in the opposite direction to the planet carrier  110  and so it is effectively being driven, to slow the output to the generator  300 . To achieve this the stroke of unit  500  remains maximised but the stroke of unit  180  is reversed so the power flow is in the opposite direction. The rotation of the twp units is reversed also. The arrangement of the secondary differential is such that power flows in the direction of arrows P. As the rotor speed increases from about 17.3 to about 19 r.p.m. the stroke of hydraulic unit  180  is progressively increased in the reverse direction, causing more power to flow in the bypass path P′. This causes even more slip in the sun gear  150 , to slow the output further. 
     So as not to cause too much power to flow through the bypass, brakes  145  and/or  245  are used to slow the system when the rotor speed exceeds 19 r.p.m. the brakes are also used for safety purposes, for example during maintenance or when a failure of a component takes place. 
     In practice, the rotor input speed will be varying constantly and so the stroke rates of the hydraulic units  180  and  500  will be adjusted according to  FIG. 5  to provide the necessary reaction torque at sun gear  150 .  FIG. 6  shows the relationship between rotor power and the speed of hydraulic unit  180 . The speed of unit  180  increases as rotor speed increases.  FIG. 7  shows how the bypass power is controlled as the speed of hydraulic unit  180  (and hence rotor speed) increases.  FIG. 8  is a table showing the variables of the transmission system and how each of them change with increasing rotor speed. The respective strokes of the units  180  and  500  may be changed quickly enough to vary the gear ratio and precisely control the output speed. The very high inertia of the turbine ensures that it cannot change speed quickly and therefore, input torque can be very precisely controlled by varying the ratio. 
     In this application the particular advantage of using a hydraulic transmission to bypass power, is its negligible referred inertia compared with the generator. It is relevant to point out that the collective referred inertia of the gear elements is also negligible for practical purposes. A cross-line controllable relief valve may be employed to respond to any very sudden speed/torque fluctuations which might conceivably outpace the reaction times of the stroke controls of the units. 
     The use of just two different power routes  115  and  125  to the secondary differential, utilized across the whole power generating input regime (the second and third power generating modes) simplifies the transmission construction and reduces costs and weight. The use of the two routes means that clutches etc are not required to for example to alter the path of the power. 
     One specific embodiment has been described above and illustrated, but it will readily apparent to the skilled addressee that many variants, modifications and alternatives will be possible within the ambit of the claims. For example, epicyclic differential gear trains have been described and illustrated, but other differential arrangements can be employed to useful effect.