WO2005012763A1

WO2005012763A1 - Drive train for a renewable-energy generating machine

Info

Publication number: WO2005012763A1
Application number: PCT/NZ2004/000175
Authority: WO
Inventors: Andrew Stephen Horsfall
Original assignee: Andrew Stephen Horsfall
Priority date: 2003-08-05
Filing date: 2004-08-05
Publication date: 2005-02-10

Abstract

Planetary gear technology protects synchronous generators of windmills using a torque limiter in which one gear (103) is servo-rotated by a worm drive (201) whenever excess wind gust torque is detected at the worm (201). The transfer ratio is immediately altered and excess power is diverted into the rotating windmill blades. Similarly a two-speed gearbox is constructed by reversibly braking the sun gear (303 fig 3). If not braked, a one-way clutch (312) locks the input (301) and input (302) shafts together; if braked the planetary gear set (314, 308, 309) applies a rotational speed increase about the clutch (312) which over-runs. Both functions may be combined.

Description

TITLE DRIVE TRAIN FOR A RENEWABLE-ENERGY GENERATING MACHINE

FIELD This invention relates to power generation from renewable resources such as wind energy, to devices that couple a generator to a windmill, and in particular this invention relates to coupling devices including an assembly of gears for adapting the generator to changing wind speeds.

BACKGROUND Sustainable power generation uses natural phenomena (winds, tides, solar energy and the like) rather than fossil fuels including nuclear fuels as a power source. This is of increasing current interest. The UK Government target is to source 10% power from sustainable sources by year 2010, involving about 6 GW of generating capacity from large windmills. Typical windmills are mounted on typically 60-80 m high towers, have two or three rotor blades (dia 30-50 m) that turn at a relatively slow rate between 20 and 50 revolutions per minute, a gearbox for raising the rate of angular motion of a shaft to drive an AC generator running at typically 1500 rpm coupled to an AC power grid operating at a constant frequency and typical windmills of this class are rated at from about 400 up to 1500 kilowatts. They are "computer-controlled". Usually the blades are pitch controlled in order to match the variable speed of the wind to a fixed ratio of the constant speed of the rotor of the synchronous generator. The gearbox is required to reliably handle large torques and powers.

One way to maximise the power available from the wind would be by using a fully variable speed transmission which would allow the blades to operate at their most efficient angles across a wide range of wind velocities. A fully variable, efficient gearbox has not been developed for use at this power level and there is still a need for better matching of the windmill rotation speed to wind speed in order that the maximum amount of energy can be extracted from the wind. Some multi-speed gearboxes that allow more than one rotor speed are known, but those developed so far are expensive and complex. A DC generator or variable-speed AC generator not restricted in rotation speed is known, but inversion into constant-frequency AC at the power levels involved is costly. Generators of the two-speed pole-changing type may be used, but they are also costly.

The invention to be described in this specification is generally suited to a wind turbine having a single speed gearbox coupled to a synchronous generator connected to a power utility grid. The invention may have other applications. A particular aspect of wind generation is coping with gusts, gales or hurricanes. Uncontrolled increases in wind speed beyond a design limit would result in stressing mechanical and electrical parts beyond their design limits, and would interfere with the stability of the AC grid fed from the generator. Therefore a torque limiting device is a preferred component of a windmill, at least as a short-term response to a gust of wind until either blade angle or entire windmill direction have been adjusted in order to cope with excessive wind speed. Unexpected gusts may be likely for windmills operated downwind from mountain ranges where rotors may cause gusts, or may occur during turbulent weather conditions such as cyclones, the passage of fronts, or in or near thunderstorms.

PRIOR ART

The present invention is relevant to high-power windmills that have an about constant-speed generator turning in synchrony with the frequency of an AC power utility grid that is being supplied. Windmills not having synchronous generators are electrically more complex and hence more expensive.

Henderson (US 5140170) describes means to dissipate energy in a gust of wind and prevent it from reaching the generator. The means comprises the allowance of slip in a sun gear of a gear train in a secondary gear box in the event of a high torque. The sun gear is coupled to a spur gear to which is coupled a hydraulic pump. Unless a pressure-responsive valve on the outlet of the pump is forced open by a torque apphed through the spur gear, the spur gear and hence the sun gear is stationary. When slip occurs, the hydraulic pump absorbs some of the excess torque by forcing compressed fluid through a restricted, closed circuit and through cooling means to return to the pump. Meanwhile the sun gear turns and the transfer ratio of the secondary gear box drops. At the same time, the blades tend to turn faster. Some of the excess torque is returned to the blades.

Makino et al (US 6720670 and US 6608397)) describe in very broad but not enabling terms a variable-ratio gear assembly (Figs lb to 3) for a wind machine in which there is a sensor responsive to output attached to the generator itself, and control means controlling the gear assembly to maintain the generator rotation speed relatively constant. One detailed example employs toothed pinions and no control of gear ratio. Friction cones, which are not compatible with gear boxes for usefully high power levels, provide variable ratio. Apart from that incompleteness, this invention has the disadvantages (a) that the fate of any excess energy is unspecified, and (b) the generator must have experienced the excess load at least in part before it is sensed and action to compensate can begin. OBJECT

It is an object of this invention to provide a multi-speed transmission for use in a wind generator, or at least to provide the public with a useful choice.

STATEMENT OF INVENTION

In a first broad aspect this invention provides a variable ratio drive train for a renewable- energy generating machine for coupling an electrical generator to a rotating source of power; the drive train including a gearbox of the planetary type having a frame or housing and three gear sets: a sun gear, a plurality of planet gears, and an annular gear all held together in meshed alignment, and including an input drive shaft having when in use a first rate of rotation and an output drive shaft having when in use a second rate of rotation related to the first rate by a transfer ratio; the input shaft being connected to a selected one of the gear sets; the output shaft being connected to a selected either of the remaining gear sets, wherein the intermediate gear set, being that set not connected to either drive shaft, is at least partially externalised from the frame or housing and is brought into contact with externally controllable restraining means capable of affecting the rotation rate of the intermediate gear set with respect to the frame or housing so that the actual transfer ratio of the drive train may thereby be controlled during use.

Preferably the drive train or gear box is adapted for use within a sustainable energy plant such as a wind mill, wave, or tide plant, and is capable of transmitting the power typically collected by wind vanes or rotors to an electrical generator. In a second broad aspect, the drive train may operate in either of two modes of operation thereby serving as a dual ratio gearbox and the housing is capable of rotation with respect to an external support; the gearbox includes as externally controllable restraining means a braking means mounted on the external support in functional apposition to an externalised surface attached to the intermediate gear set; wherein during a first mode of operation the braking means is inactive and the intermediate gear is allowed to rotate freely, thereby causing the input shaft to tend to spin faster than the output shaft so causing the one-way clutch to become engaged and thereby transmit power from the input shaft to the output shaft at a 1:1 transfer ratio without factional loss in the drive train; or wherein during a second mode of operation the brake is actuated thereby preventing movement of the intermediate gear set so that the planetary gearbox becomes activated causing the output shaft to rotate faster than the input shaft and thereby causing the one-way clutch to become disengaged hence permitting power at a transfer ratio greater than 1:1 from the input shaft to be transmitted to the generator; the change in ratio being effected by application of the brake. Preferably the ratios provided are 1:1 and 1:1.5 (that is, a step-up from the input shaft to the output 100 shaft). In a subsidiary aspect the sun gear serves as the intermediate gear set and bears an externalised concentric brake disk; the input shaft is coupled through an aperture in the sun gear directly to a mount for the planetary gears, and the output shaft is coupled to the annular gear, so that the drive train is adapted to serve as a dual transfer ratio 105 gearbox dependent on the status of the brake, and so that a power generating device may be more easily matched to a fluctuating supply of power. In a third broad aspect, the intermediate gear of the drive train is attached to an exteriorised concentric worm wheel; the framework of the drive train supporting a worm screw meshed with the worm wheel and mounted on a worm shaft rotatably mounted

110 on an external support; the external support being provided with transducing means capable of sensing an axial load placed on the worm shaft by a torque from the concentric disk; the transducing means being connected to motor control means connected to a variable speed motor capable of causing the worm shaft to rotate hence causing the intermediate gear to rotate thereby causing a change of the ratio of the drive train, and so that

115 in response to a sensed axial load the drive train ratio may be adjusted so that the axial load is held substantially constant thereby preventing the electrical generator attached to the output shaft of the drive train from experiencing an excessive torque. Preferred transducing means include load cells, strain gauges (suitably mounted), and displacement transducers in combination with spring mounts.

120 Preferably the sensed axial load is used as an input to a negative feedback control loop in which the output comprises power to the variable speed motor drive and the sense of feedback is such that the control loop seeks to hold the axial load as sensed at no more than a predetermined threshold level. Preferably, if the sensed axial load is less than a predetermined threshold level, the 125 variable speed motor drive is not energised, while if the sensed axial load is greater than the predetermined threshold level, the variable speed motor drive is energised in proportion. In a first related aspect the invention provides a windmill having an electrical generator and equipped with a drive train as described previously in this section, wherein the 130 drive train protects the generator from excess torque; the excess torque being returned to the blades as a result of an instantaneous change in the ratio of the drive train. In a second related aspect the invention provides a windmill having an electrical generator and equipped with a drive train as described previously in this section, wherein the drive train is capable of being quickly changed from a first ratio to a 135 second ratio so that the windmill can be operated in a manner matched to the available wind. In a fourth broad aspect, the intermediate gear of the drive train is a modified, slideable sun gear capable of being forced, in a first mode, to a first position towards one end of a shaft; in which position the sun gear is locked to an externally rotatable wheel thereby

140 forming an extemahsed intermediate gear for creation of a variable ratio planetary gearbox, and the slideable sun gear is capable in a second mode of being forced into a second position away from one end of the shaft; in which second position teeth included about the periphery of the sun gear cause the sun gear to become locked to the planet gears and hence the drive train becomes locked in a transfer ratio of 1:1, so

145 that the drive train can operate in more than one mode. More particularly, the slideable sun gear when in the first position towards one end of a shaft is locked to a separately rotatable worm wheel thereby forming a composite sun gear and drivable worm capable of being rotated by an external, torque-responsive motive source, conferring a torque limiting property on the drive train and when in the 150 second position the sun gear is locked by means of a second annular set of gear teeth included about the periphery of the sun gear about both sides of the planet gears hence locking the drive train in a transfer ratio of 1:1, so that the same drive train can operate as either a torque-limited gearbox having a selected transfer ratio or as a direct transfer gearbox.

155 Preferably the drive train or gear box may be provided as an accessory capable of being used within an existing windmill design. Alternatively the drive train or gear box may be provided as part of or in conjunction with an AC generator assembly suitable for use with a sustainable energy plant so that the generator simulates a two-speed generator even though, in use, the rotor of the generator itself turns at a

160 single rate (determined by the frequency of an AC power grid to which the generator is coupled) so that the generator can be used with a wider variety of input rotation speeds. Alternatively the dual-speed drive train or gear box may be constructed within or upon an existing gearbox suitable for use with a sustainable energy plant so that the existing gearbox assumes a dual-speed function hence rendering any associated generator more effective.

165 PREFERRED EMBODIMENT The description of the invention to be provided herein is given purely by way of several examples and is not to be taken in any way as limiting the scope or extent of the invention. DRAWINGS Fig 1 : is a longitudinal sectional view of the gearbox portion of example 1, a torque-limiting 170 device. Fig 2: is a diagram showing a front elevation view of example 1. Fig 3: is a diagram showing a longitudinal section of example 2. Fig 4: is a diagram showing a perspective view of example 2. Fig 5: is a diagram showing a perspective view of example 3 in a first mode.

175 Fig 6: is a diagram showing example 3 in a second mode. Fig 7: shows the transfer functions of a prior art gearbox and several Examples.

The Examples relate to a planetary gear type coupling for use in power-generating windmills at power levels of typically up to 0.5 to 1.5 MW. Wind power as available is to be collected

180 by a propellor and matched, with overload protection, to the rotation rate preferred by a usually synchronous AC electrical generator. Fig 7 shows example transfer functions. The Y axis 701 shows generator power output and a design maximum level at the dotted line 703. Axis 702 shows several wind power inputs. Curve 704/705 is a prior-art curve in which the windmill control system reacts slowly to overload by yawing the windmill or changing the pitch

185 of the blades, so that overload (as at 705) may occur. The Examples provide a set of mechanisms that manipulate the sun gear while in use in order to alter the transfer function of the propellor-gearbox-generator coupling. Example 1: Torque limiter as in Fig 7, lines 709-710. Operation is along line 709, but if a sudden gust of wind takes the input power over a threshold the torque limiter has immediate

190 effect by means of a variable ratio gearing, whereupon operation occurs along line 710, protecting the generator. Excess input power is diverted into the turning windmill blades, for return shortly afterwards, or for dissipation. Example 2: Speed changer as in Fig 7, two slopes of lines 706, 707, 708/A. The windmill is operative over a wider range of wind speeds. If the sun gear is held stationary the transfer 195 function follows the slope of line 707/708 ; otherwise the function is on the slope of 706 corresponding to a 1:1 transfer ratio coupling. Example 3 (Combination): Ratios as for example 2; alternative slopes of lines 706 or the mode suited to a higher wind speed 707, and entering the solid horizontal portion 708A if the torque limiter comes into effect. In the low speed mode the device is a 1:1 coupler with low losses and 200 in the example described the torque limiter is not active in that mode.

EXAMPLE 1 This type of torque-limiting gearbox is adapted for use in conjunction with a windmill for generating electric power for distribution by an AC power utility grid. The gearbox is a stand-alone regulating device by which we mean that excess torque or power is

205 sensed within the invention rather than elsewhere within the generator and is dealt with locally, by conversion into blade rotational inertia, by actions within the gearbox and is not passed on to the generator. See Fig 7, lines 709 and 710. This Example is intended to be placed between an existing step-up gearbox and the generator itself, where it extends the capability of the windmill to operated effectively in a range of wind speeds.

210 The gearbox includes a sun, planets and annulus (orbital) type of gearing but the sun is modified so that it may be forcibly rotated, hence making the gearbox variable. With reference to Fig 1, a longitudinal section 100 shows the layout of a preferred embodiment. This gearbox is rated for about 0.5 to 1.5 MW operation with adequate safety margins. Dimensions of a realistic working prototype provide that the disk 103

215 has a diameter of 320 mm and the distance from the disk to the end of shaft 101 is 400 mm. Rotational energy from the blades passes through the windmill gearbox and drives the input shaft 102 to the gearbox, and will be passed on to the generator by rotation of output shaft 101. The input shaft 102 is rigidly connected with a planet carrier comprising disk 106 carrying a number (preferably 3, 4 or 5) of planet pinion

220 gears 105. These pinions may be mounted on Hicks-style cantilevered axles 112 for improved load sharing, as is known in the art. The input shaft is supported by bearings 111 within the output shaft. More bearings Ilia are provided between the input shaft 102 and the sun gear 113, according to standard practice. The pinions 105 are always engaged with both the annulus gear 108 and the sun gear 113. The sun gear 225 is also rigidly connected with a externalised radial disk 103, provided at its periphery with a worm thread (see Figs 5 or 6) engaged with a worm screw 201 (see Fig 2) mounted from a supporting frame. Oil seals 109 and 110 are shown, but other covers, lubrication means, etc (according to standard practice) are not shown in this illustrative example.

230 Fig 2 shows some special aspects of this invention not included in Fig 1 from one end. Fig 2 is a face view of the input shaft 102 surrounded by the disk 103 carrying a peripheral worm thread 104. Rotation (if any) of the disk will be in a clockwise direction. A worm screw 201 rigidly supported on shaft 201A is constantly engaged with the thread 104. The worm screw on its shaft is firmly held by means of thrust

235 (212, 213) and rotational support bearings 202, 203, though one skilled in the art will be able to substitute appropriate alternatives such as deep-groove bearings or the like. Sensing means capable of locally detecting the torque apphed by the edge of disk 103 against the worm screw is provided. One version of sensing means comprises allowing and sensing a small amount of movement in an axial direction in relation to a housing

240 (part shown as 210). Bearing 213 is supported by a pre-loaded (biased) compression spring 209 in compression (optionally to a controllable amount by adjustment of confinement) against a frame or housing 210. If a large torque is apphed to the input shaft 102, the worm tends to be moved (to the right) against the compression spring 209, the strength and loading of which is set so that applied torques of less than the

245 limit amount permitted do not cause any movement of the worm shaft.. Excessive torque will overcome the push of the spring and cause the shaft to move to the right. Linear displacement transducer 208 detects that movement. A worm gear is preferred because of its non-reversibility by which we mean that it is difficult to force a worm screw to turn in a step-up mode by turning the worm itself. Other forms of gear can

250 exhibit this property and frictional loading on conventional gearing is another way to achieve an effect of this type. Locking may also be caused to occur within the motor 206 itself, such as by application of a steady magnetic field in an AC motor. In an alternative sensor arrangement, a load cell is used to support the thrust bearing 213 and is placed between bearing 213 and frame or housing 210. Typical load cells 255 employ strain gauges to measure bending of internal beams and exhibit a quite small displacement when maximally loaded. The load cell output signal is passed to a control box implementing a proportional/integral servo drive that causes the motor 206 (supported on frame 207) to turn proportionally to torque, once the torque signal exceeds a predetermined threshold.

260 For both sensors the motor causes the worm to be driven so that the disk 103 is made to rotate in the direction that will cause the effective torque as sensed to be reduced. The geared coupling 204 and 205 permits a positive rotational drive to be maintained to the worm even if worm screw displacement occurs. Rotation of the worm causes the sun gear 113 to rotate, causing the effective ratio of the planetary gear box to change so

265 that the device immediately acts as a variable speed gear, lowering the transfer ratio during a period between the "initial attack" of the excessive wind gust and application of other well-known procedures built into the windmill operating system that can only respond more slowly to excessive wind (such as yawing the entire windmill about a vertical axis and blade pitch control. Hence the excessive energy from the wind makes

270 the blades spin faster although the generator input power is held constant (Fig 7, 710). Torque limiting can be combined with a pre-existing variable speed mode of operation. The worm can rotate at a set rate and then speed up if excess torque is detected. Increased angular momentum in the turbine blades a result of the extra energy collected from the gust may be recovered by returning the ratio of the gearbox towards

275 its sub-threshold value when the wind dies down to a sub-limiting speed. As the torque applied to the invention drops, the motor driving the worm slows to a stop, the gearbox ratio increases and the potential energy (the extra angular momentum) within the faster-rotating blades is returned to the generator while the blades slow. If the excess wind speed should be sustained, the turbine blades are used to dissipate the

280 energy held therein by "beating the air" while at least partially feathered and perhaps the windmill is yawed so that they slow down. There is no additional device used for energy dissipation. In relation to wear on the worm and bronze screw (which are to be lubricated in accordance with standard practice), and in relation to power required to turn the screw, note 285 that in normal use, below the torque reduction threshold, the worm and screw are stationary. When active, the worm is serving to change the ratio of the gear box and it is not itself acting as a site where the excessive energy (perhaps of the order of 0.5 MW) is to be dumped. Instead, this invention converts the excess energy of a wind gust into angular momentum. The variable-speed, unidirectional motor 206 is likely to be

290 rated in the range of 500W to 1 KW and its combination with a controller (211) is a device well known in the engineering arts. The combination of transducer 208, controller 200, worm drive (motor 206, coupling 204/205) and spring 209 comprises a feedback controlled regulator of torque applied to the worm screw by the shaft, always endeavouring to maintain the torque at no more than a certain value and so that axial

295 displacement of the worm screw is held at no more than a small value. In applying regulation at that point the device limits any torque applied through the gearbox to the generator without sampling parameters of the generator itself. It is convenient to consider the application of this invention as having two modes. In a first mode of operation, where the applied torque is less than a predetermined limiting

300 amount, the output shaft 101 is driven from the input shaft 102 through the orbital gears. The sun gear is stationary, being locked in place by friction at the sliding interface between the stationary worm 201 and the worm thread 103. hi this mode it may also be convenient to include a change in rate of rotation between the input and output shafts, but in order to render this invention a convenient add-on accessory for

305 field installation, it may be preferable that it has a 1:1 ratio. The example prototype gearbox has a 1:1 ratio. This mode corresponds to line 709 in Fig 7. In the second mode of operation, the sun gear 103 is caused to rum as a result of the worm screw 201 being driven by the motor 206 in response to an excessive apphed torque, so altering the transfer ratio of the gear box and having the effect of holding 310 excess kinetic energy from a gust of wind as a blade speed increase rather than causing overload damage to any part of the windmill. The resulting increase in efficiency depends on the variability of the wind itself. This mode corresponds to line 710 in Fig 7. In relation to fitting to existing windmills, this invention may have its own control 315 means although a power source for the motor is required. Some or total control by the windmill computer control device is used. Windmills with synchronous generators are constant-speed machines and the computer in control may not have means to measure the blade rotation rate. Existing windmill control systems usually include means to either change the pitch of the propellor blades or yaw the windmill when the excess 320 speed has risen to an undesirable amount, having regard to blade strength for example, and these may be retained.

EXAMPLE 2 This invention relates to a type of dual-speed gearbox intended to extend the operating range of a windmill used for generating power of the order of 0.5 MW for connection to a power

325 utility grid. This Example is intended to be placed between the step-up gearbox and the generator itself, where it extends the capability of the windmill to operate effectively in a range of wind speeds. One ratio is 1:1. The other ratio is typically a step-up of 1:1.5. The gearbox includes a sun planets and annulus (orbital) type of gearing. With reference to Fig 3, a longitudinal section 300 shows a preferred embodiment. The entire assembly may rotate during use,

330 unless the externalised disk 303 is being held stationary with respect to a windmill frame during one mode of use. Dimensions are similar to those of Example 1. Rotational energy from the existing windmill gearbox (not shown) drives the input shaft 302 and is coupled from the gearbox to the generator by output shaft 301. At the termination of the input shaft there is a rigid connection 311 with the planet gear carrier 313. Also, the invention includes a one-way

335 clutch 312 optionally as part of a bearing assembly so that the input shaft 302 can be directly coupled though the clutch to the output shaft 301 in one mode (1:1 ratio; brake off) when the speed of the input shaft already matches the desired speed of the generator rotor. The planet carrier has a number (preferably 3, 4 or 5) of pinion gears assembled on to it, such as pinion 314 (a void is shown on the lower side where a planet gear may be placed). Pinions 314 may

340 be mounted on Hicks-style cantilevered axles for improved load sharing - this gearbox being expected to transfer power of typically up to 0.5 to 1.5 MW. Cantilevering and other forms of acoustic noise reduction are desirable. These pinions 314 are always engaged with an inner sun gear 308 which is rigidly connected with a radial brake disk 304 and being directly connected to neither the input nor the output shaft is called an intermediate gear. The sun gear may rotate,

345 during use, in relation to the input shaft 302. A brake mechanism 315 for either holding or releasing the disk 303 follows conventional disk brake design as is well known in the art. Bearings 310 and 307, and an oil seal 306 are provided in order to allow relative rotation. These pinions 314 are also always engaged with an outer annular ring gear 309, which is rigidly connected to the output shaft 301. As is usual engineering practice, another oil seal 305 closes

350 off the interior of the gearbox at the site where relative rotation between the annulus and the sun gear occurs. In the first mode of operation, the output shaft is driven directly at a 1:1 ratio through the oneway clutch 312 by the input shaft 302. The brake 304/315 is off, so that the effect of rotation of the planet carrier is to cause the sun gear and the attached disk 303 to spin freely. The gearbox 355 acts as a 1:1 coupling with no losses. In the second mode of operation, the sun gear 308 is locked by means of the brake 315 clamping upon the braking surface 304 of disk 303. The planet carrier causes the annular ring 309 to rotate in the same direction as the input shaft but at a faster rate (1.5 times the input shaft rate, in this Example), so that the one-way clutch 312 placed between the input and

360 output shafts is allowed to freewheel or overrun and is decoupled. In this Example, the speed change is effected by locking or unlocking the brake on disk 303. This action can be carried out by an actuator (not shown) working the brake mechanism in response to a command from a controlling digital processor (not shown). One design has the gear box stepping up the rotation of the output shaft by preferably 1.5 times for use when winds are light. Another

365 design conserves as much energy as possible when winds are light and provides a step-down ratio for heavy winds. (Of course other ratios may be provided as will be apparent to a person skilled in the art). During transitions from one mode to the other, the generator attached to the output shaft will drop off line from the grid and stop the windmill rotor, either by braking or by feathering the

370 propellors. Although modes of operation lie largely outside the the scope of this invention, it is probable that this dual speed gearbox will not be used to change speeds while the windmill rotor is turning, on account of rotational inertia, brake wear, and excessive torques that may occur. One preferred mode of use for synchronous generators is that the generator is disconnected from the grid, and the rotor is brought to a standstill, either by feathering the blades or

375 using the main brake (already part of a usual windmill) or both, then either clamping or releasing the brake disk 303 of the invention, then starting the windmill and allowing the generator speed to stabilise at about 1500 rpm before the generator is reconnected to the grid at a suitable moment, having regard to instantaneous phase (as is known in the art). Alternatively, speed change may be effected during use by a procedure involving braking or 380 release of the sun gear (through disk 303) so that either the braking accelerates the input shaft to the required generator speed, or the release causes the input shaft to accelerate up to generator speed. In both cases, the generator speed is allowed to stabilise before the generator is reconnected to the grid. For example the invention allows rotor rotation speeds during power generation of either 36 rpm or 48 rpm.

385 A suitable one-way clutch should be optimised for long life at the typical power levels used. Because the speed changes are effected while the blades are not turning, the one-way clutch does not need to absorb a great deal of impact energy at the time of becoming engaged. In the absence of torque limiting means, the power transfer relationship (Fig 7) lies along line 706, or line 707 including the dashed portion.

390 EXAMPLE 3 This may be regarded as a variant of Example 1. The planetary gearbox of this invention may also be used so as provide a useful constant or alterable change of rotational speed, if that was required for any other reason. It has the advantage that the torque limiting feature may stay in effect even if the primary mode of operation is as a

395 variable drive. (Refer to Figs 1 or 2). In this Example, the hardware is modified by including means to determine the rate of rotation of the motor driving the worm screw and to report that rate to the windmill operating computer. A suitable rotation transducer (not shown, but well known to workers in the relevant art) is mounted on the motor shaft (or internal 400 motor properties may be used, or the disk 103 may include a device for monitoring its rate of rotation. Alternatively the controller 211 may have a capability to inherently determine the motor speed to a sufficient accuracy. It might be a form of synchronous or stepping motor. In use, if the operating computer determines that a different rate of blade rotation to 405 that provided when the sun gear is stationary is desired, the motor 206 is driven through controller 211 at a speed that has the effect of causing the disk attached to the sun gear to rotate at a desired rate, to give this gearbox the desired ratio. Again, the power requirements of motor 216 are low in comparison with the power being transmitted through the drive train (including 102 to 101).

410 Preferably any indication of displacement of shaft 201 A through transducer 208 is permitted to add to the motor rotation speed, so that even if the invention is running in this third mode it retains the torque limiting function described above.

EXAMPLE 4 This is a combination of Example 1 and Example 2. Fig 5 shows this Example in a first mode; 415 the sun gear coupling 507 is engaged with the worm gear 502. In Fig 5, the input shaft 503 at left is rigidly coupled to the annulus gear 505 which is always meshed with the planet spur gears 506, 506' and these gears are also always meshed with the sun gear 507 which is the intermediate gear. The planet gears are on a carrier which is part of the output shaft 504. The rotational velocity of the sun gear may be altered (in this mode) by rotation of the externally 420 fixed worm screw 501 working against the worm gear 502. For this mode, the sun gear 507 must have been moved axially along the output shaft 504 so that dog teeth 509 on the side face of the sun gear have meshed with complementary dog teeth 508 on the side face of the worm gear. In this mode the device acts as a 1:X transfer ratio with active torque control (as previously described for Example 1). The value of X depends on gear diameters and typically

425 provides a ratio of 1.5:1. In this mode, the annulus turns the planet gears which use the sun gear as a reference, which might be moved by action of the worm 501. The second mode, shown in Fig 6, is activated by sliding the sun gear away from the worm gear by means of an actuator arm (not shown) ending with a roller miing in a circumferential groove 512. (This groove and actuator were not shown in Fig 5, and other means known in the

430 art for sliding a wheel along a shaft in order to engage or disengage may be substituted). In this mode both the inner margin of the sun gear at 504A and the outer margin at 507 A, also toothed as if it was an annulus gear, mesh with the planetary gears 506, 506' so that the planet gears cannot rotate at all. In this mode, the gear box transfers power with a 1:1 transfer ratio and with minimal loss, since all parts are turning as one. Some of the bearings used in a

435 practical construction are shown in Fig 5; worm gear support bearings 511, carrier support bearing 510. Sliding bearings (not shown) are placed between the slideable sun gear 507 and the output shaft 504.

VARIATIONS It is possible to make gears of a planetary group other than the sun gear serve as the exter- 440 nalised controllable member of the planetary gearbox. In one example a controllable disk is connected to the annulus gear with the drive applied to the sun gear, and the output taken from the planetary gears. (These versions are not illustrated). The worm gear of Figs 1, 2, 5 and 6 may be exposed to too much torque. Solutions to that problem include forced lubrication of the bronze screw, making the diameter of the steel worm 445 as large as is feasible, and inserting further planetary gears that increase the relative rotation rate of the worm perhaps by a 4:1 ratio. Variations of gear tooth configuration (for example, straight cut versus helical gears) may reduce noise and wear, as is known in the art. If the gear box of Example 2 was used in a mode wherein the brake was used to cause a 450 change in rotation rate for a change of ratio during power transmission, the brake acting on the disk 103 would need to be substantial and dissipate a significant amount of heat, and wear would occur. This mode is possible though presently believed to be "not preferable". There is another propellor brake in most windmills. This type of operation may suit some types of windmill control and management.

455 Normally this gearbox can be sold as a retro-fit option or as an accessory, while it may be incorporated within or upon the housing of a generator and sold as part of the generator itself, thereby creating a cheaper and simpler kind of two-speed generator than an electrically implemented version or alternatively sold as a torque-protected generator. Alternatively, this gearbox is incorporated within and sold as part of an existing "main" 460 gearbox, which thereby becomes a more versatile kind of gearbox than was previously available..

INDUSTRIAL APPLICABILITY and ADVANTAGES Incremental improvements of efficiency are keenly sought after in the power windmill industry and these examples conserve power under light-wind conditions in which there is relatively 465 little energy, by minimising the "processing" of the power, while providing means to overcome wind power surges. The windmill or its blades might be optimised for higher permissible wind speed ranges (such as use of a normal and high-speed mode of a two-speed supplementary gearbox, rather than low and normal-speed modes). Incorporation of this gearbox between the output of a gearbox and the generator input within 470 an existing windmill is a reasonably simple operation. Unlike prior-art means for limiting excessive torque, this invention does not dump some excess energy within a heat sink; it returns all of the excess energy to the blades as rotational momentum and may recover the blade energy after a gust or allow it to be dissipated as drag on the blades. The invention uses existing, slower-to-act alterations to operating mode (such as 475 blade feathering (change of pitch) or windmill yawing) to bring the blade rotation speed down again, if the excess wind turns out to be steady. The operating cost to efficiency of the torque limiting invention is small; the motor if in use may use 0.02% of the power developed under load. The insertion cost is also small; at sub- torque limiting speeds the related losses are only those of a standard orbital gear transmission.

480 The invention is inherently quick to respond to an excess torque; responses within 1 second are feasible. The invention is a stand-alone, self-regulating device, needing perhaps 1 KW of electric power for the torque limiter function (when active). Self-regulation is a useful fail-safe property. The technology is like that already used in windmill power trains (gearboxes, electromechanical 485 actuators, and active control. Mechanics and engineers do not need training in new fields. The capital cost is small and lower than alternatives.

The invention may be scaled up or down for use in applications other than propellor-driven wind generation.

Finally, it will be understood that the scope of this invention as described and/or illustrated herein is not limited to the specified embodiments. Those of skill will appreciate that various modifications, additions, known equivalents, and substitutions are possible without departing from the scope and spirit of the invention as set forth in the following claims.

Claims

495 I CLAIM : 1. A variable ratio drive train for a renewable-energy generating machine for coupling an electrical generator to a rotating source of power; the drive train including a gearbox of the planetary type having a frame or housing and three gear sets: a sun gear, a plurality of planet gears, and an annular gear all held together in meshed alignment, and including an

500 input drive shaft having when in use a first rate of rotation and an output drive shaft having when in use a second rate of rotation related to the first rate by a transfer ratio; the input shaft being connected to a selected one of the gear sets; the output shaft being connected to a selected either of the remaining gear sets, characterised in that the intermediate gear set, being that set not connected to either drive shaft, is at least partially externalised from

505 the frame or housing and is brought into contact with externally controllable restraining means capable of affecting the rotation rate of the intermediate gear set with respect to the frame or housing so that the actual transfer ratio of the drive train may thereby be controlled during use. 2. A drive train as claimed in claim 1, characterised in that the drive train housing may rotate 510 with respect to an external support and an externalised surface of the at least partially externalised intermediate gear is in controllable contact with a braking means mounted on the external support; wherein during a first mode of operation the braking means is controlled so that the intermediate gear is allowed to rotate freely, thereby causing the input shaft to tend to spin faster than the output shaft hence causing the one-way clutch to become

515 engaged and transmit power directly from the input shaft to the output shaft; and wherein during a second mode of operation in which the braking means brakes and thereby prevents rotation of the intermediate gear set, consequent action of the planetary gearbox causes the output shaft to rotate faster than the input shaft thereby causing the one-way clutch to become disengaged hence including the planetary gearbox in the power train and enabling

520 power at a selected transfer ratio to be transmitted to the generator. 3. A drive train as claimed in claim 2, characterised in that the sun gear serves as the intermediate gear set and bears an extemahsed concentric brake disk; the input shaft is coupled through an aperture in the sun gear directly to a mount for the planetary gears, and the output shaft is coupled to the annular gear, so that the drive train is adapted to serve as a

525 dual transfer ratio gearbox dependent on the status of the brake, and so that a power generating device may be more easily matched to a fluctuating supply of power. 4. A drive train as claimed in claim 1, characterised in that the intermediate gear of the drive train is attached to an exteriorised concentric worm wheel; the framework of the drive train supporting a worm screw meshed with the worm wheel and mounted on a worm shaft 530 rotatably mounted on an external support; the external support being provided with transducing means capable of sensing an axial load placed on the worm shaft by a torque from the concentric disk; the transducing means being connected to motor control means connected to a variable speed motor capable of causing the worm shaft to rotate hence causing the intermediate gear to rotate thereby causing a change of the ratio of the drive 535 train, and so that in response to a sensed axial load the drive train ratio may be adjusted so that the axial load is held substantially constant thereby preventing the electrical generator attached to the output shaft of the drive train from experiencing an excessive torque. 5. A drive train as claimed in claim 4, characterised in that the sensed axial load is used as an input to a negative feedback control loop in which the output comprises power to the 540 variable speed motor drive and the sense of feedback is such that the control loop seeks to hold the axial load in the worm shaft as sensed at no more than a predetermined threshold level and so that the drive train has the function of protecting the generator from excess torque; the excess torque being diverted to the source of rotational power in the form of increased rotational kinetic energy.

545 6. A drive train as claimed in claim 5, characterised in that if the sensed axial load is less than a predetermined threshold level, the variable speed motor drive is not energised, while if the sensed axial load is greater than the predetermined threshold level, the variable speed motor drive is energised in proportion to the axial load. 7. A windmill having an electrical generator and equipped with a drive train according to claim 550 6, characterised in that the windmill includes a drive train including a gear box having a torque limiting ratio controller, adapted to prevent the transmission of power in excess of a predetermined amount from being passed to the generator. 8. A drive train as claimed in claim 1, characterised in that the intermediate gear of the drive train is a modified, slideable sun gear capable of being forced, in a first mode, to a first

555 position towards one end of a shaft; in which position the sun gear is locked to an externally rotatable wheel thereby forming an extemahsed intermediate gear for creation of a variable ratio planetary gearbox, and the slideable sun gear is capable in a second mode of being forced into a second position away from one end of the shaft; in which second position teeth included about the periphery of the sun gear cause the sun gear to become locked to

560 the planet gears and hence the drive train becomes locked in a transfer ratio of 1:1, so that the drive train can operate in more than one mode. 9. A drive train as claimed in claim 8, characterised in that the slideable sun gear when in the first position towards one end of a shaft is locked to a separately rotatable worm wheel thereby forming a composite sun gear and drivable worm capable of being rotated by an

565 external, torque-responsive motive source, conferring a torque limiting property on the drive train and when in the second position the sun gear is locked by means of a second annular set of gear teeth included about the periphery of the sun gear about both sides of the planet gears hence locking the drive train in a transfer ratio of 1:1, so that the same drive train can operate as either a torque-limited gearbox having a selected transfer ratio or as a direct

570 transfer gearbox. 10. A drive train as claimed in any previous claim, characterised in that the drive train is manufactured as a self-contained unit capable of being used as an accessory for an existing renewable-energy generating machine.