GB2536006A

GB2536006A - Eddy current brake assembly, particularly for a verticle axis wind turbine

Info

Publication number: GB2536006A
Application number: GB1503509.0A
Authority: GB
Inventors: Taylor Chris; Blaize Michael; Goater Paul; Crosher Stephen; Jakeman Nigel; Caffall Finch Steven
Original assignee: X-Wind Power Ltd
Current assignee: X-Wind Power Ltd
Priority date: 2015-03-02
Filing date: 2015-03-02
Publication date: 2016-09-07
Anticipated expiration: 2035-03-02
Also published as: GB2536006B; GB201503509D0

Abstract

An eddy current brake assembly, for wind turbines or vehicle braking, comprising a support structure, a shaft 4 supported by the support structure, an electrically conductive target 40, a braking field winding 14, energisation means 30 to supply a current to energise the braking field winding, and a control means 16 for controlling the energisation means, where the braking field winding is mounted on the shaft to rotate with the shaft relative to the to the target with the target being fixed against rotation. Preferably the braking field winding is energised via a pair of inductively coupled coils 33, 34 by a small permanent magnet exciter arrangement 31, 32 and is mounted together with the rotating component of the exciter arrangement on the braked shaft. A control system may control the energisation of the rotor winding responsive to shaft speed, target temperature or other parameters to provide independent endurance braking or over speed control in a wind generator or on the axle of a vehicle.

Description

I

Eddy current brake assembly, particularly for a vertical axis wind turbine This invention relates to eddy current brakes, particularly for use in vertical axis wind turbines (VAWTs) but also for horizontal axis wind turbines (HAWTs) and other applications.

An eddy current device comprises an electrically conductive element, referred to as a target, and a magnetic field which passes through the target. Shaft power is converted into heat by the relative rotation of the target and the field which induces eddy currents in the target.

Eddy current devices are used in some wind turbines as a braking means to prevent the rotor from overspeeding in high wind conditions. In this specification, a wind generator or wind turbine means a machine for generating power from a wind driven rotor.

A particular problem in VAWTs is that it is difficult to adjust the pitch or angle of attack of the blades. This is particularly the case in the so-called Gorlov configuration which is typified by a generally helical arrangement of the blades of the wind-driven rotor about its axis. In this type of rotor the length axis of each blade extends along a curved path and so the blade cannot be angled out of the wind.

Where an eddy current brake is used as the primary means to prevent overspeed of the wind driven rotor, for example in a Gorlov type wind turbine, it is important that it can handle sufficient power to brake the shaft in the worst case overspeed design condition, and moreover that it should be as reliable as possible.

Many eddy current brakes employ a winding to generate the field which acts upon the target.

For example, DE19634464 teaches a wind generator in which one or more retarder assemblies are provided, each comprising a target disc which rotates with the shaft in a magnetic field generated by a fixed coil.

US2009074581 discloses a VAWT with an electromagnetic braking device having windings which generate a magnetic field acting on a rotating disc.

US2012038157 discloses a shrouded HAWT mounted for rotation on a track and having a metallic rotor braked by electromagnets.

An important factor in the reliability of an eddy current braking system is how the field winding with generates the magnetic field is energised. One disadvantage of energising the field winding from grid power is that, of course, braking is lost if the grid power fails.

If the generator which supplies the output power from the wind turbine to the grid is adapted to provide the braking function or to supply power to the field winding of an eddy current brake, then braking may also be lost in the event of a mechanical failure in the drivetrain from shaft to generator. This risk of course can be avoided by arranging the generator on the braked shaft. However, if the generator is controlled by feedback from an inverter supplying the grid output then the operation of the brake may be prejudiced by failures in the generator output control system.

Since the generator retards the shaft in normal service, a separate braking device which acts independently of the generator will provide an additive retarding effect as well as operational redundancy, resulting in a more reliable system.

In order to ensure more reliable operation, many eddy current braking systems employ permanent magnets instead of an inductive winding to provide the magnetic field which interacts with the target.

Permanent magnet eddy current brakes are known for example from W02013155164 which teaches a braking arrangement in a VAWT in which a permanent magnet rotor generates eddy currents in an axially moveable target.

CN201221440 teaches a small wind generator having an eddy current brake comprising a permanent magnet rotor which is biased away from a housing by a spring, the housing acting as the target. The rotor is urged into the braking position by wind pressure acting on the wind driven rotor.

A disadvantage of using permanent magnets to provide the field is that where the brake is designed as the primary means to control overspeed in a commercial wind generator, a permanent magnet rotor capable of provide sufficient magnetic flux to meet the design overspeed condition will be very expensive, while the large size of the mechanical components makes it difficult to implement reliable control of the braking force. It is also not possible to implement purely electronic control of the braking force which in some applications may be preferred over mechanical control.

Electronic control is more easily implemented where the magnetic field which interacts with the target is provided by energising a stator winding.

For example, EP2680433 teaches an inductive unit for generating the magnetic field to interact with a disc which rotates with the shaft, and a control system for controlling the inductive unit via an IGBT switching system.

EP2657516 discloses a HAWT having an eddy current brake on the gearbox output shaft, controlled by signals from a tachometer measuring the rotational speed of the shaft and an anemometer measuring the windspeed.

US2011140425 and RU2339144 teach systems for controlling the operation of an electromagnetic brake in a wind turbine to avoid damaging vibrations.

US4198572 discloses a HAWT in which the wind driven rotor blades are mounted on an eddy current target which rotates in the field from a stator coil energised by secondary brushes on a generator mounted on the same shaft, and a control system which receives an input signal from an anemometer.

DE102014104287 teaches an electro-dynamic brake having a winding which is energised from the DC intermediate circuit of a frequency converter.

One way of providing more reliable overspeed protection is to combine an eddy current brake with another braking or overspeed protection system. For example, many HAWTs are configured to allow adjustment of the pitch or angle of attack of the rotor blades so as to reduce their efficiency in high wind conditions, so that braking devices are not required to dissipate the full shaft power. It is also well known to provide friction brakes as an emergency braking means.

For example, US1894357 discloses a HAWT with twin contra-rotating rotors mounted on a crank so as to pivot away from the wind in overspeed conditions, and an additional electromagnetic braking arrangement which retards the relative motion of the rotors.

W02013166531 teaches a wind generator in which an eddy current device may be used as a service brake in combination with a separate emergency brake.

Although friction brakes may be used to hold stationary the wind driven rotor of a wind generator, it is also known to use an eddy current brake for this purpose, for example, as taught by EP0486765 and US 2011187107.

The reliability of an eddy current braking system also depends on the manner in which the target is cooled.

US 4486638 discloses a VAWT having an eddy current brake arrangement comprising a permanent magnet rotor and an axially moveable target with cooling water passages. The target is moved along the shaft axis by a motor controlled by a closed loop control system to vary the braking force responsive to the shaft speed.

WO 2014063708 teaches a HAWT having an eddy current brake in which the target is cooled by a flow of cooling fluid induced by a rotating blower.

It is also known to use eddy current devices as a means for converting shaft power to heat a working fluid.

VAWTs comprising an eddy current device for heating a working fluid are known, for example from DE2620236, FR2328931, DE202008010970U, DE4429386, FR2912786, DE3009027, and GB2207739 which teaches a cylindrical target having an integral cooling passage for the heated fluid.

Similarly, an eddy current device may be used in a HAWT for heating a working fluid, for example, as taught by 1P5545436, JP2011210656, JP5435357, US4421967, and W02011029445 and W02011029446 in which the output from the eddy current device can be controlled by adjusting the relative axial position of the rotor and target along the shaft axis.

Eddy current brakes are also well known in other applications such as in road and rail vehicles where they are useful as retarders for endurance braking.

Vehicular eddy current braking systems are known for example from U55485901, US2058024, US2003117012, EP0416413, U52920220 which discloses an eddy current brake on a rail vehicle, and U52017665 which discloses a rail vehicle bogey with an eddy current brake mounted on the drive motor of each axle.

EP1751838 discloses an eddy current brake arrangement comprising a motorised control system for moving respective arrays of permanent magnets, one relative to the other so as to control the braking force.

US2767367 discloses a generator driven by a turbine and having an eddy current brake on the turbine shaft which is controlled by the output from an AC generator on the turbine shaft to maintain a constant shaft speed.

US 2014060980 discloses a motor in which the rotor may be used as an eddy current 10 target.

In view of the above mentioned problems it is a general object of the present invention to provide an eddy current braking system which is reliable and cost effective. In certain embodiments of the invention it is a more particular objective to provide such a system in a VAWT.

In accordance with the various aspects of the present invention there are provided an eddy current brake assembly, a wind turbine comprising an eddy current brake assembly, and a road or rail vehicle comprising an eddy current brake assembly, as defined in the claims.

A preferred embodiment of the novel eddy current brake assembly comprises a support structure supported against rotation about a shaft axis, and a shaft supported by the support structure for rotation about the shaft axis relative to the support structure. The assembly further comprises an electrically conductive target and a braking field winding together with an energisation means which is arranged to supply a current to energise the braking field winding, and a control means for controlling the energisation means. The braking field winding is arranged to generate a magnetic field passing through the target when energised by the energisation means.

In a first aspect, the braking field winding is mounted on the shaft to rotate with the shaft relative to the target, and the target is fixed against rotation with the shaft about the shaft axis relative to the support structure.

In a second aspect, the energisation means includes an exciter winding and a permanent magnet exciter assembly which is arranged to produce a magnetic field which passes through the exciter winding. A respective one of the permanent magnet exciter assembly and the exciter winding is mounted on the shaft to rotate with the shaft relative to the other respective one of the permanent magnet exciter assembly and the exciter winding so as to generate a current in the exciter winding.

Advantageously, the above mentioned features can be combined. The novel eddy current brake assembly provides a highly reliable overspeed protection or endurance or emergency braking system which can be arranged to operate without ancillary cooling systems or external power.

Further features and advantages of the invention will be evident from the various illustrative embodiments of the invention which will now be described, purely by way of example and without limitation to the scope of the claims, and with reference to the accompanying drawings, in which: Fig. 1 shows a VAWT incorporating an eddy current brake assembly in accordance with a first embodiment; Fig. 2 shows a longitudinal section through the enlarged upper portion of the tower of Fig. 1; Fig. 3 shows the exciter assembly according to a second embodiment; Fig. 4 shows the exciter assembly according to a third embodiment; Fig. 5 shows the rotor and stator assembly according to a fourth embodiment; Fig. 6 shows a fifth embodiment in which an eddy current brake assembly is arranged within an axle casing of a vehicle; Fig. 7 shows how braking power can be increased proportionately to shaft power; and Fig. 8 shows a TSR curve for the VAWT of Figs. 1 and 2.

Corresponding reference numerals indicate corresponding features in each of the figures.

Referring to Figs. 1 and 2, a vertical axis wind turbine 1 comprises a wind driven rotor 2 which drives a generator 3 via a vertical shaft 4. The rotor is mounted on the shaft on top of a support structure comprising a vertical tower 5 which is supported on the ground against rotation about the shaft axis 6. The tower supports the shaft on bearings 7 for rotation about the shaft axis relative to the support structure.

The tower comprises cylindrical upper portion 9 and a longer, cylindrical lower portion 10, and an eddy current brake assembly 11 is arranged within the upper portion 9 which has an enlarged diameter relative to the lower portion and also houses the generator 3 which is axially spaced from the brake assembly.

The wind driven rotor 2 may be of a helical or Gorlov type as shown, having a blade or blades configured to rotate about the shaft axis, wherein the or each blade has a length and is curved along its length around the shaft axis, so that when considered in a static position of the wind driven rotor, the blade or blades together extend through at least most of a full revolution around the shaft axis. In the illustrated embodiment there are two such blades 12 which describe a cylindrical swept envelope as the wind driven rotor rotates.

The brake assembly comprises an electrically conductive target 40 and a braking field winding 14 together with an energisation means 30 which is arranged to supply a current to energise the braking field winding. The brake assembly and the generator are controlled together by a control system 16, which includes a controller 16" for controlling the generator and a control means 16' for controlling the energisation means. Advantageously, the energisation means 30 is powered solely by rotation of the shaft, so that the novel brake assembly can operate in the event of failure of the grid or other external systems.

The control means 16' may comprise an automatic voltage regulator (AVR) or any other suitable circuit components as known in the art for controlling energisation of the braking field winding. It will be understood that the energisation of the braking field winding may be controlled by controlling current, voltage, frequency, or any other related parameter. The control means may receive external signals from a manual control interface, sensors such as an array of anemometers 80, a temperature sensor or sensors 81 for sensing the temperature of the target, a shaft speed sensor means 82 or any other desired input, and may generate for example an infinitely variable 0-10V output signal to the AVR or other device which controls (directly or indirectly) the energisation of the braking field winding. Analogue rather than digital control of the shaft torque may be preferred in order to avoid inducing vibration in the wind driven rotor.

The braking field winding 14 is mounted on the shaft to rotate with the shaft relative to the target 40, and the target is supported against rotation with the shaft about the shaft axis relative to the support structure. The braking field winding 14 is arranged to generate a magnetic field passing through the target 40 when energised by the energisation means 30 so that whenever the braking field winding is energised, rotation of the shaft will cause the magnetic field from the braking field winding to generate eddy currents in the target as it rotates through the target. The target may be fixed to prevent any rotation about the shaft axis relative to the support structure. It is possible also to envisage alternative arrangements to permit some very limited rotation of the target about the shaft axis (for example, by mounting the target on a helical thread so that its axial position along the shaft axis is adjustable by rotating it on the thread), but in any such arrangement the target will of course be fixed so that it is restrained relative to the support structure (for example, by abutment with a locknut or a collar at an axial end of the thread) so that it cannot rotate freely with the shaft.

The target 40 is arranged radially outwardly of the braking field winding and is coaxial with the shaft. It may form part of the support structure which mechanically supports the shaft, e.g. by forming a cylindrical portion of the tower, in which case it may advantageously form an exterior surface of the support structure. Alternatively the target may be thermally coupled to the exterior surface 17' of the support structure.

In the illustrated embodiment, the eddy current target 40 comprises a plurality of aluminium segments 41 bolted together to form a ring having a continuous radially inward portion 42 and radially outwardly extending fins 43 which are thermally coupled to the steel outer wall 17 of the tower. Air passages 44 defined between the fins provide for convective cooling to supplement the cooling which occurs by conduction via the tower wall. The target is cooled at least in part by an air current 90 flowing upwardly through the tower between an inlet or inlets 19 in a lower region of the tower and an outlet or outlets 20 in an upper region of the tower.

Any closed loop fluid cooling system is subject to failure in the event of loss of the working fluid, and for this reason, it is advantageous in some embodiments to cool the target solely or mainly by means of a passive cooling system using ambient air.

Moreover, by arranging the target as a stator, it is possible (without adding inertial mass to the shaft) to provide the target with sufficient thermal capacity to absorb transient peaks in shaft energy, allowing it to maintain shaft speed within safe limits without relying on an active cooling system. By arranging the target outwardly of the braking field winding it is further possible to provide for passive cooling by radiation, conduction and convection. This provides a passively cooled, failsafe braking arrangement which is capable of maintaining shaft speed within safe limits during the most severe conditions of sustained high winds envisaged by a practical design scenario, even in the event of failure of the generator. This is particularly useful where the wind driven rotor is of the helical type as shown in Fig.1, where the pitch or angle of attack of the blades is not adjustable.

A further advantage of a passively cooled system is that the efficiency of the wind turbine in normal operation is not reduced by the parasitic load from a cooling fan or pump.

The passive cooling effect is particularly effective where the target forms or is thermally coupled to an exterior surface of the support structure, as shown in the illustrated embodiments. This configuration of the brake is further advantageous, particularly where the brake is arranged in an enlarged diameter upper portion of the tower of a VAWT, because the enlarged diameter of the tower increases the surface area available for cooling without increasing the wind resistance of the remainder of the tower, and also increases brake diameter relative to length, and braking torque is proportional to the square of the diameter of the braking field winding but increases only linearly with its axial length.

The exterior surface of the tower may be provided with passive cooling fins 21, which may extend for example vertically (for passive cooling by convection, perhaps with a shroud arranged outwardly of the fins to carry the convective air current) or horizontally around the circumference of the support tower as shown in the illustrated embodiment. The circumferential fins are always aligned with the wind direction so that passive cooling is proportional to windspeed, and hence also to the excess shaft power to be dissipated.

In applications other than wind turbines, cooling fins can similarly be arranged for example on an axle casing in which the shaft is mounted, and may be aligned with the direction of travel of the vehicle so that the airflow over the axle casing similarly varies with vehicle speed and shaft power.

A VAWT is particularly suited to the passive cooling of a fixed target by ambient airflow because the shaft is coaxial with the tower, allowing the fixed target to be embodied as a surface of the tower which lies outside the airstream passing through the wind driven rotor. (In a HAWT the blades extend in front of the tower for most of its height, so any adaptation of the tower surface will have some effect on the airstream exiting the blades.) The height of the tower provides a convective airflow which increases the capacity of the passive cooling system. The air current may be driven at least in part by convection, and/or at least in part by a fan. In addition, the braking field winding and other rotor windings may include cooling passages 91 extending in parallel with the shaft, so that the air current passes through the cooling passages to cool the windings inside the target.

The convective airflow is enhanced by the ambient external pressure differential reflecting variations in windspeed over the height of the tower. In alternative embodiments the shaft may be elongated so that the eddy current target can be positioned to heat the air in the base of the tower to further enhance convection. The generator 3 can also be positioned at the base of the tower so that the load on the tower is minimised while still benefiting from convective cooling.

Of course, if required, the target may be cooled by an active cooling system. This may be a forced air system in which air is blown through ductwork by a fan driven directly from the shaft or by a separate motor. The generator may comprise a fan which cools the rotor and/or target of the eddy current brake. Alternatively a liquid heat transfer medium can be used to transfer heat from the target to a radiator or other heat sink or heat rejection device where it is cooled and then returned to the target by a pump. Similarly, a passive cooling system may be arranged to transfer heat via a liquid medium in a heat pipe or thermosyphon. In each case the target may be arranged to incorporate a heat transfer interface such as liquid coolant passageways. The braking field winding and other rotor components may also comprise flowpaths 91 through which cooling air can flow.

The energisation means 30 includes an exciter winding (i.e. a set of coils or windings) 31 and a permanent magnet exciter assembly 32, comprising an array of permanent magnets defining multiple magnetic poles, which is arranged to produce a magnetic field which passes through the exciter winding. A respective one of the permanent magnet exciter assembly and the exciter winding is mounted on the shaft to rotate with the shaft relative to the other respective one of the permanent magnet exciter assembly and the exciter winding so as to generate a (preferably polyphase) current in the exciter winding. Conveniently, the permanent magnet exciter assembly 32 is mounted on the shaft and the exciter winding 31 comprises a stator winding, as shown.

The exciter winding 31 is arranged to energise the braking field winding 14 as follows.

Preferably, the energisation means further comprises a stator 33 and a rotor winding 34 mounted on the shaft to rotate with the shaft relative to the stator as shown. The stator 33 is arranged to produce a magnetic field passing through the rotor winding 34 so that the rotor winding generates the current to energise the braking field winding 14 when energised by rotation through the magnetic field produced by the stator.

Preferably as shown the stator 33 is a stator winding and the exciter winding 31 is arranged to energise the stator winding. Preferably a full wave rectifier unit 35 is mounted on the shaft to rotate together with the shaft and arranged to convert the (preferably polyphase) AC output from the rotor winding 34 to a DC input to the braking field winding 14.

The control means 16' is arranged to control the braking force by controlling the strength of the magnetic field passing through the rotor winding 34 responsive to a rotational speed of the shaft and/or to another control parameter or external control signal as further described below. In the illustrated embodiment the exciter winding 31 is connected to the stator winding 33 via the control means 16' which controls the energisation of the stator winding from the exciter winding, and preferably includes an exciter rectifier unit for rectifying AC input from the exciter winding so as to supply a DC output to the stator winding 33.

Advantageously, the permanent magnet exciter assembly 32 is small relative to the size of the braking field winding and the target. This minimises the cost of the permanent magnets and also makes it possible to control the braking force by a control means 16' acting upon the relatively small current from the exciter winding, so that expensive high power control components are not required.

In alternative embodiments the stator 33 could be replaced by a permanent magnet assembly for energising the rotor winding 34 which in turn energises the braking field winding, in which case no separate exciter assembly and exciter winding is needed. An example of such an arrangement is shown in Fig. 5. The size of the permanent magnet assembly required to energise the braking field winding is smaller and so substantially less expensive than a permanent magnet assembly arranged to interact directly with the target as known in the art.

Returning to the embodiment of Figs. 1 and 2, it has been found that the helical type of rotor is capable of self starting even in relatively light winds, which makes a VAWT with this rotor type more efficient. However, to enable self starting, and also to avoid braking the rotor in normal service, it is desirable that the braking field winding should produce substantially no magnetic field when braking is not required. Preferably therefore the control means 16' is operable to define a zero braking condition in which substantially no magnetic field passes through the rotor winding 34. Advantageously, the use of a small permanent magnet exciter assembly 32 makes it possible to confine the permanent magnetic field in such a way that it does not interact at all with the rotor winding 34, so that in the zero braking condition the rotor winding is not excited by the exciter assembly.

Preferably the permanent magnet exciter assembly 32 is mounted on the shaft to rotate with the shaft relative to the exciter winding, while the exciter winding 31 is fixed against rotation relative to the support structure as shown. This means that even if the weak extremity of the permanent field from the permanent magnet exciter assembly passes through the rotor winding 34, there is no relative rotation and so no excitation of the rotor winding.

The target may be made from a non-magnetic metal, for example, aluminium, which is a good flux concentrator, which ensures that no residual magnetic field is present. Copper is also suitable. Of course, it could be made from a magnetic metal, e.g. steel, if desired.

In alternative embodiments, the stator winding 33 may be switchable so as to be powered alternatively from the mains grid 100 or other power source and from the exciter winding 31 which is energised by rotation of the braked shaft 4. This may enable the novel braking assembly to be used to hold the rotor stationary. A mechanical rotor lock may be provided to maintain the rotor in a predetermined rotational position for maintenance, and the generator may be operable as known in the art to rotate the rotor to the position in which the rotor lock can be engaged.

In less preferred embodiments the stator winding 33 may be powered from the mains grid or other power source instead of from an exciter winding energised by rotation of the braked shaft.

In alternative embodiments it is possible to supply AC rather than DC current from the rotor winding 34 to the braking field winding 14, in which case the rectifier unit 35 is not required. This however is less preferred due to the additional heating effect of the collapsing field on the winding. Similarly the stator winding 33 could if desired by energised by an AC supply.

Advantageously, the generator 3 is a permanent magnet generator, i.e. a generator having a permanent magnet rotor 50 for energising the generator stator winding 51.

This is more efficient in use because there is no rotor winding to generate back [ME. However, the relatively high cost of the permanent magnet rotor makes it uneconomic to oversize the generator to act as a brake in infrequent overspeed conditions. By combining a permanent magnet rotor with an eddy current brake using a braking field winding energised by a rotor winding, the eddy current brake may be powered entirely from the driven shaft, preferably by a relatively small permanent magnet exciter arrangement 31, 32, to provide a failsafe and cost effective braking system while delivering high efficiency in normal service.

In alternative embodiments, the main generator could be for example a brushless alternator or any other suitable type as known in the art. It is also possible to use the novel braking assembly in a wind turbine which produces output energy as heat rather than electrical power, for example, as a working fluid to power a steam turbine at ground level.

Referring to Fig. 5, in a yet further embodiment, where the braking field winding is energised by a rotor winding 34, the stator which energises the rotor winding may comprise a permanent magnet stator assembly 101 which is fixed against rotation with the shaft 4 relative to the support structure. The control means may be arranged to move the permanent magnet stator assembly axially relative to the rotor winding as shown, or alternatively to move the rotor winding relative to the permanent magnet stator assembly, e.g. by arranging the stator winding to rotate in axially fixed relation to a ring which is axially moveable by the control means. The moveable part may be moved by the control means via an electromechanical mechanism 102 as shown (which in the illustrated embodiment acts upon the permanent magnet stator assembly 101) or purely mechanically by changes in a rotational speed of the shaft 4, e.g. by arranging a governor mechanism similar to that shown in Fig. 3 to act upon the permanent magnet stator assembly or the stator winding.

In a more complex and hence less preferred embodiment, where the rotor winding 34 which energises the braking field winding 14 is arranged to rotate through the magnetic field produced by a stator winding 33, an exciter winding may be mounted on the shaft and a permanent magnet assembly fixed against rotation relative to the support structure so that its magnetic field passes through the exciter winding. In this case the exciter winding is arranged to energise a second rotor winding, preferably via a first rectifier unit which rotates with the shaft, and a second stator winding is fixed against rotation relative to the support structure so that the magnetic field of the second rotor winding passes through it. The second stator winding in turn supplies current to energise the first stator winding, preferably via a rectifier unit. The control system may be arranged to move any of the respective windings or the magnet assembly relative to the other respective winding or magnet assembly with which it is inductively coupled to regulate the strength of the magnetic field passing through the rotor winding 34, or alternatively may be arranged to control the power (e.g. by controlling current, voltage, frequency, or any other parameter) supplied by any of the windings to the respective winding which it energises. Each respective winding is most simply energised by connecting it directly or via a rectifier and/or control unit to the winding which supplies it with current, but it could of course be energised indirectly, such as via another pair of inductively coupled coils.

In less preferred embodiments, rather than energising the braking field winding 14 from a rotor winding 34 inductively coupled to a stator winding 33 or rotating through the magnetic field of a permanent magnet stator 101, the braking field winding 14 may be energised from the energisation means via a current transfer means such as slip rings or even a commutator through which the braking field winding is electrically connected to a current source. This arrangement of course incurs the risk of failure of the moving current transfer means.

Referring again to the embodiment of Figs. land 2, the rotor is preferably a lift type rotor, i.e. of the type having aerofoil blades which generate lift as the airstream passes over them, as distinct from a reaction type rotor in which the shaft is rotated by the force imparted by the wind pushing against the blades. The efficiency of the rotor is defined as the power coefficient Cp representing the proportion of power contained in the windstream passing through the swept area of the rotor which is transformed into shaft power.

Referring also to Fig. 8, efficiency will be maximised at a given tip speed ratio (TSR), i.e. the ratio of the speed of the tip of the rotor blades (that point on the rotor blades which is radially at the greatest distance from the rotation axis) to the speed of the windstream. The change in Cp with TSR can be expressed graphically as a characteristic curve for the rotor, with the optimal TSR at the highest point A of the curve. The rotational speed of the tip of the rotor blades is a function of the rotational speed R of the shaft and the diameter of the rotor.

The generator 3 is preferably controlled by a suitable controller 16" as known in the art to manage its output power and the torque reaction it produces on the shaft 4 so as to allow the rotor to accelerate to a rotational speed at which it begins to generate power and thereafter to manage the rotational speed of the rotor, preferably maintaining it at its optimal tip speed ratio. The rotor may advantageously be self starting, and/or the controller may be arranged to operate the generator as a motor to power the rotor through some or all of the speed range up to the point at which it begins to generate power from the wind. The wind turbine control system including the generator controller and eddy current brake control means can include a PID or any other known control arrangement and can control the energisation means to manage the speed of the rotor based on an input signal from an anemometer 80 (which provides near-instantaneous windspeed information and so enables the controller to predict fluctuations in shaft speed) and/or based on the sensed rotational shaft speed (e.g. from a separate rotation sensor 82), generator power output, torque and/or other indicators of rotor performance which may be derived from the generator output or from separate sensors as known in the art. An advantage of controlling the operation of the generator and/or the eddy current brake based on shaft rotation and not anemometer input is that control is not influenced by erroneous anemometer signals arising for example from shadowing of the anemometer by the tower. Multiple anemometers 80 may be used to provide redundancy with the signals being compared to remove tower shadow effects.

The generator 3 is designed to produce maximum continuously rated output power (GPmax) at an optimal rotational shaft speed Rmax corresponding to an optimal rated windspeed Wmax. Fig. 7 illustrates the power curves for VAWT in which GPmax is 100kW at a shaft speed Rmax of 70RPM at a maximum rated windspeed Wmax of about 10m/s. The generator will typically be capable of producing a maximum peak output power GPpeak substantially in excess of GPmax.

The shaft speed R may be varied by the controller to maintain the TSR at an optimal value TSRopt indicated by point A on the TSR curve of Fig. 8 up to the maximum rated windspeed.

Preferably the control means forms part of a control system 16 which is arranged to bring the shaft (and hence the rotor) to a stop when shaft power SP transmitted by the shaft exceeds a threshold shaft power limit value SPstop.

Advantageously, the control system 16 may be arranged to vary the value SPstop to manage an operating temperature of the target.

The limit value SPstop represents the maximum shaft power at which the energy transmitted by the shaft can be sustainably transformed or dissipated by the generator and the brake assembly. The limit value can be expressed as a maximum windspeed, shaft torque, rotational shaft speed, combined output power of the brake and generator, and/or any other parameter or combination of parameters which determines or represents the power transmitted by the shaft. For a wind turbine in which the shaft does not drive a generator, SPstop can be based on the combined output power of the brake assembly and any other shaft driven means for transforming output power, e.g. into heat or mechanical action.

A control system 16 arranged in this way is particularly useful where the eddy current target 40 is arranged to dissipate excess shaft energy at least in part by passive cooling, and so can advantageously be employed in a wind turbine, e.g. where the brake assembly is arranged in the tower of a VAWT so as to be passively cooled by the airstream passing the tower or by convection via the tower.

The control system arranged in this way makes it possible to provide a failsafe endurance braking system using a passively cooled target and to select its thermal capacity to correspond to the braking requirements of an excess windspeed scenario of given probability or frequency, while actively managing the system to prevent failure by overheating of the target, for example, during an exceptionally long period of sustained high windspeed. The worst case scenario can include failure of the generator or grid connection.

It can also be employed in other applications of the eddy current brake assembly, for example, where it is used to retard the motion of a road or rail vehicle having service brakes in addition to the novel eddy current brake assembly, both braking means being controlled by a control system. In such applications, it can be used to provide an endurance braking system which ensures that the vehicle is brought to a halt, either by means of its service brakes or by means of the eddy current brake assembly, before the target reaches a temperature beyond which it could no longer safely or sustainably absorb the energy required to bring the vehicle to a halt. In vehicular applications, this means that if desired, the eddy current brake assembly could be used as an emergency brake or as an endurance brake for very occasional use, and passively rather than actively cooled so as to avoid the need for a fan or other circulatory system which would increase the size and complexity of the assembly and cause a parasitic drain on the motive power of the vehicle.

SPstop can be adjusted as a function of at least one operating parameter or as a function of more than one operating parameter.

For example, the temperature of the target 40 may be an operating parameter, and the control system may be arranged to sense a temperature of the target and to vary the value SPstop responsive to variations in the temperature of the target.

Where the control system is software based it can be arranged to vary SPstop responsive to an input signal from a temperature sensor 81 which senses the temperature of the target. The control system can be arranged to vary SPstop responsive to the signal so as to maintain the operating temperature of the target 40 below a maximum temperature limit Tmax or to limit the rate of increase of the target temperature.

Another possible operating parameter is the rate at which current is supplied to the braking field winding 14 or energy is supplied to the target 40. This may be calculated as a function of current and time, e.g. as an average of current over a rolling time period, and including a dissipation factor which represents the relationship between the temperature of the target 40 and the rate at which its thermal energy is dissipated by passive or active cooling.

Where the eddy current brake assembly is incorporated into a wind turbine in which the shaft drives a generator or other means for converting shaft power into heat or mechanical work or other useful output, the wind turbine control system 16 will include the control means 16' of the eddy current brake and the controller 16" of the generator or other output means, which may be separate or may be combined into a single control unit. Often the control system will also control the operation of a friction brake for use in bringing the rotor to a stop and holding it static, either in an emergency or for servicing. In this and other applications, the control system may be a computer system including software running on a processor, and/or may include mechanical control means.

The novel control system can be used both in electronically controlled embodiments where there are no moving parts, and in embodiments including a governor type mechanism or other mechanical control means for regulating the current in the braking field winding 14. For example, in a mechanically controlled embodiment such as that shown in Fig. 3 or Fig. 4 in which a permanent magnet exciter assembly arranged as a rotating governor is inductively coupled with an exciter stator winding, the novel control means can be arranged to move the position of the exciter stator winding relative to the permanent magnet exciter assembly responsive to the temperature of the target or any other selected operating parameter or parameters. This could be accomplished by an electromechanical system or even by a purely mechanical system, e.g. using an assembly of thermally responsive bimetallic strips arranged to sense the temperature of the target (i.e. thermally coupled to the target) and to amplify the movement generated by thermal expansion. Alternatively the exciter stator winding could be mounted in a fixed position and its output current controlled directly by a circuit based on a temperature dependent resistor or other suitable components.

Advantageously, the wind turbine control system may be arranged to control the rotor in four phases as windspeed rises above Wmax. Of course, the phases could be combined or only some of the phases implemented.

In a first phase, the generator controller is arranged to control the reaction torque applied to the shaft so as to maintain the shaft speed R at a value which reduces TSR to a point below TSRopt (e.g. to point B on the TSR curve of Fig. 8), preferably progressively as windspeed increases. This reduces Cp so that progressively less of the power contained in the moving airstream is converted into shaft power. During the first phase the generator controller may maintain output power GP at GPmax. This may be desirable if there is a tariff threshold or other commercial restriction on the power supplied to the grid. Alternatively, the controller may allow generator output power GP to rise above GPmax, in which case the additional output power may be supplied to the grid or dissipated, for example via a dump resistor or other energy or heat sink.

If windspeed increases the power SP transmitted by the shaft to a level which would raise R above Rmax despite the reduced efficiency of the rotor, then in a second phase the generator controller further increases the reaction torque applied to the shaft by increasing generator output power GP. Again, excess power can be supplied to the grid or dumped.

The maximum windspeed at which phase two control may be applied will depend inter alia on the size of the generator and how much power can be dissipated by the resistive dumping arrangement.

During both phase land phase 2 the control system may maintain the shaft speed Rat Rmax. Alternatively shaft speed may be allowed to increase within a defined envelope with the reaction torque of the generator being increased proportionately to shaft speed. Phase 1 and phase 2 may be combined.

If windspeed rises to a level at which the shaft power SP exceeds a threshold shaft power limit SPbrake, which may be at or below GPpeak, then in phase 3 the control system begins to actuate the brake, preferably by progressively increasing the braking power (conveniently, by increasing the current supplied to the braking field winding 14) so that the brake transforms the excess energy transmitted by the shaft into heat in the target. During phase 3 the control system may continue to maintain the shaft speed at Rmax, with the braking power being increased to the level required to maintain shaft speed at Rmax. Alternatively the shaft speed may be allowed to increase within a defined envelope with the braking power being increased proportionately to shaft speed.

In phase 4, if the windspeed rises to a level at which the shaft power SP exceeds the threshold shaft power limit SPstop at which the energy transmitted by the shaft can be sustainably transformed or dissipated by the generator and the eddy current brake, then the control system brings the rotor to a stop and holds it static. This can be accomplished by increasing the generator output power GP and/or the current supplied to the braking field winding 14 to a higher value for a sufficient time to bring the rotor to a stop, and/or by actuating a separate mechanical brake, e.g. a disc brake, which can afterwards be used to hold the rotor static until the windspeed and the temperature of the target 40 have both dropped to a level at which normal operation can resume.

The control system may be arranged to vary the value SPstop by progressively reducing SPstop, for example, as the temperature of the target increases, or as a function of another operating parameter or a combination of operating parameters. SPstop can be adjusted continuously or step-wise. Alternatively the control system may be arranged to reduce SPstop as a single event, i.e. in a single step from a normal value to a reduced value. For example, SPstop could be reduced to a value equal to SPbrake when the temperature of the target exceeds a threshold value, and then returned to its normal value when the temperature of the target falls to another predetermined threshold value.

The thermal capacity of the target 40 may be selected so as to absorb the excess shaft 30 energy up to a windspeed corresponding to SPstop in the worst case overspeed design scenario, taking into account the predicted rate of energy input to the target and rate of passive heat dissipation from the target with changing target temperature. Preferably the brake assembly is arranged to dissipate most, for example, about 95% of the excess shaft energy in the target and only a small proportion in the braking field winding. To give just one example, if Wmax is 13m/s, SPstop may be expressed as a rotational shaft speed corresponding to a windspeed of 20m/s, and the target may be designed to dissipate up to about 300kW of heat compared with only about 15 -20kW in the braking field winding at SPstop.

The power rating Pw of the braking field winding may be substantially greater than GPmax and may readily be calculated by those skilled in the art based on the well-known formula: Pw = Cp x 1/2 x rho x S x V3 wherein Cp = power coefficient of the wind driven rotor; rho = air density = projected swept area of the wind driven rotor normal to the wind direction; V = wind speed.

Referring to Fig. 3 and Fig. 4, in alternative embodiments the control means may include or consist of a mechanical assembly 70 which is arranged to move the permanent magnet exciter assembly 71 or the exciter winding 72, one relative to the other. In the examples shown the control means is actuated mechanically by changes in a rotational speed of the shaft, comprising a mechanical linkage having weights which are moved from a rest position radially outwardly from the shaft axis by increasing shaft speed. Advantageously the weights may be permanent magnets 73 of the permanent magnet exciter assembly. Fig. 3 illustrates an arrangement in which the weights comprising permanent magnets 73 are arranged in the configuration corresponding to a classic Watt governor, pivotably mounted on twin parallel arms 74 which raise them into a vertical orientation to induce a current in the exciter winding 72 when shaft speed increases to a value corresponding to SPbrake. Fig. 4 shows an alternative arrangement with the vertical shaft in cross section, in which the weights comprising permanent magnets 73 move out in a horizontal plane with increasing shaft speed. The weights may be biased to the rest position by elastic bias means such as springs (not shown). The remainder of the brake assembly may correspond mutatis mutandis to that of the foregoing embodiments. If desired, the exciter winding 72 may be directly connected (preferably via a rectifier unit) to the stator winding 33 which energises the rotor winding 34 as the shaft 4 rotates, the rotor winding energising the braking field winding 14 as previously described. This provides a very simple self regulating braking system which can regulate shaft speed, optionally without any electronic controls.

Referring to Fig. 6, the novel eddy current brake assembly may be arranged to provide an endurance braking system in a road or rail vehicle 60 wherein the shaft 4' is driven by a wheel 61 of the vehicle which supports the vehicle on a road or rail. In the embodiment shown, the wheel is mounted on the shaft 4', and the support structure is an axle casing 5' of the vehicle which forms the eddy current target. The remaining components of the assembly are generally as described above and so are indicated by corresponding reference numerals, and the control means 16' may be arranged mutatis mutandis to operate generally as previously described, including by sensing a temperature of the target via a thermal sensor 81, or alternatively in accordance with the vehicle braking control methodology as known in the art.

In summary, a preferred embodiment provides an eddy current brake assembly comprising a braking field winding 14 which induces eddy currents in a preferably static target 40. Preferably the braking field winding is energised via a pair of inductively coupled coils 33, 34 by a small permanent magnet exciter arrangement 31, 32 and is mounted together with the rotating component of the exciter arrangement on the braked shaft 4. The novel brake assembly thus preferably comprises a small permanent magnet generator arrangement, referred to herein for ease of reference as an exciter arrangement, which is preferably independent of the main output generator 3; a pair of inductively coupled windings 33, 34 which are energised by the exciter arrangement and also use the rotational power of the shaft to generate a higher power supply to the braking field winding; and a rotating rectifier assembly for rectifying the higher power supply from the rotor winding 34 to supply a DC current to the braking field winding which produces a magnetic field which rotates with the shaft. The target may be passively cooled. A control system 16 may control the energisation of the rotor winding responsive to shaft speed, target temperature or other parameters to provide independent endurance braking or overspeed control in a wind generator or on the axle of a vehicle. Braking force is proportional to the rotational speed of the shaft (although it may be non-linearly proportional) and is preferably controlled by varying the power supplied to the stator winding 33.

In alternative embodiments the various features of the novel eddy current brake assembly need not be combined together. For example, the novel control means can be applied also to an eddy current brake where the target rotates with the shaft and the braking field is generated by a stator winding energised by an exciter assembly powered by rotation of the shaft. Alternatively the eddy current target can be fixed against rotation and energised by a rotating braking field winding as shown, with the braking field winding being energised by an external power supply rather than from the rotating shaft. If preferred, the wind driven rotor, wheel or other prime mover may be mounted on a separate shaft and drivingly connected to the shaft on which the brake assembly is arranged.

Many further adaptations will be evident to those skilled in the art within the scope of the claims.

Claims

CLAIMS1. An eddy current brake assembly for braking a rotating shaft, comprising: a support structure, the support structure being supported against rotation about a shaft axis; a shaft supported by the support structure for rotation about the shaft axis relative to the support structure; an electrically conductive target;a braking field winding;an energisation means arranged to supply a current to energise the braking field winding; and a control means for controlling the energisation means; the braking field winding being arranged to generate a magnetic field passing through the target when energised by the energisation means; wherein the braking field winding is mounted on the shaft to rotate with the shaft relative to the target, and the target is fixed against rotation with the shaft about the shaft axis relative to the support structure.
2. An eddy current brake assembly according to claim 1, wherein the energisation means is powered by rotation of the shaft.
3. An eddy current brake assembly according to claim 1, wherein the energisation means comprises: a stator; and a rotor winding mounted on the shaft to rotate with the shaft relative to the stator, the stator being arranged to produce a magnetic field passing through the rotor winding; and the control means is operable to control a strength of the magnetic field passing through the rotor winding; the rotor winding being arranged to generate the current to energise the braking field winding when energised by rotation through the magnetic field produced by the stator, the braking field winding being arranged to generate the magnetic field passing through the target to generate eddy currents in the target when energised by the rotor winding.
4. An eddy current brake assembly according to any of claims 1-3, wherein the target is arranged radially outwardly of the braking field winding and is coaxial with the shaft.
5. An eddy current brake assembly according to any of claims 1-3, wherein the target forms part of the support structure.
6. An eddy current brake assembly according to any of claims 1-3, wherein the target forms or is thermally coupled to an exterior surface of the support structure.
7. An eddy current brake assembly according to claim 6, wherein the exterior surface of the support structure is provided with passive cooling fins.
8. An eddy current brake assembly according to claim 3, wherein the control means is operable to define a zero braking condition in which substantially no magnetic field passes through the rotor winding.
9. An eddy current brake assembly according to claim 3, wherein the control means is arranged to control the strength of the magnetic field passing through the rotor winding responsive to a rotational speed of the shaft.
10. An eddy current brake assembly according to claim 3, wherein the control means is arranged to control the strength of the magnetic field passing through the rotor winding responsive to an external control signal.
11. An eddy current brake assembly according to claim 3, wherein a rotor winding rectifier unit is arranged to convert an AC output from the rotor winding to a DC input to the braking field winding, and the rotor winding rectifier unit is mounted on the shaft to rotate together with the shaft.
12. An eddy current brake assembly according to claim 2, wherein an exciter winding is provided; and a permanent magnet exciter assembly is arranged to produce a magnetic field which passes through the exciter winding, a respective one of the permanent magnet exciter assembly and the exciter winding being mounted on the shaft to rotate with the shaft relative to the other respective one of the permanent magnet exciter assembly and the exciter winding so as to generate a current in the exciter winding; and the exciter winding is arranged to energise the braking field winding.
13. An eddy current brake assembly according to claim 3, wherein the stator comprises a stator winding; and an exciter winding is provided; and a permanent magnet exciter assembly is arranged to produce a magnetic field which passes through the exciter winding, a respective one of the permanent magnet exciter assembly and the exciter winding being mounted on the shaft to rotate with the shaft relative to the other respective one of the permanent magnet exciter assembly and the exciter winding so as to generate a current in the exciter winding; and the exciter winding is arranged to energise the stator winding.
14. An eddy current brake assembly according to claim 12 or claim 13, wherein the permanent magnet exciter assembly is mounted on the shaft to rotate with the shaft relative to the exciter winding.
15. An eddy current brake assembly according to claim 13, wherein the control means is arranged to control the energisation of the stator winding from the exciter winding.
16. An eddy current brake assembly according to claim 12 or claim 13, wherein the control means is arranged to move the permanent magnet exciter assembly or the exciter winding, one relative to the other.
17. An eddy current brake assembly according to claim 16, wherein the control means is actuated mechanically by changes in a rotational speed of the shaft.
18. An eddy current brake assembly according to claim 16, wherein the control means comprises a mechanical linkage having weights which are moved from a rest position radially outwardly from the shaft axis by increasing shaft speed.
19. An eddy current brake assembly according to claim 18, wherein the weights are permanent magnets of the permanent magnet exciter assembly.
20. An eddy current brake assembly according to claim 3, wherein the stator comprises a permanent magnet stator assembly, the permanent magnet stator assembly being fixed against rotation with the shaft relative to the support structure, and the control means is arranged to move the permanent magnet stator assembly or the rotor winding, one relative to the other.
21. An eddy current brake assembly according to claim 20, wherein the control means is actuated mechanically by changes in a rotational speed of the shaft.
22. An eddy current brake assembly according to any of claims 1 -21, wherein the control means forms part of a control system which is arranged to bring the shaft to a stop when shaft power SP transmitted by the shaft exceeds a threshold shaft power limit value SPstop, and the control system is arranged to vary the value SPstop to manage an operating temperature of the target.
23. An eddy current brake assembly according to claim 22, wherein the control system is arranged to sense a temperature of the target and to vary the value SPstop responsive to variations in the temperature of the target.
24. A wind turbine comprising an eddy current brake assembly according to any of claims 1-23, and a wind driven rotor which drives the shaft.
25. A wind turbine according to claim 24, wherein an anemometer is provided, and the control means is arranged to control the energisation means responsive to a rotational speed of the shaft and to a signal from the anemometer.
26. A road or rail vehicle comprising an eddy current brake assembly according to any of claims 1 -23, wherein the shaft is driven by a wheel of the vehicle which supports the vehicle on a road or rail.
27. A road or rail vehicle according to claim 26, wherein the wheel is mounted on the shaft, and the support structure is an axle casing of the vehicle.
28. A vertical axis wind turbine comprising an eddy current brake assembly according to any of claims 1 -3, a wind driven rotor which drives the shaft, and a generator driven by the shaft; wherein the support structure is a vertical tower, and the wind driven rotor is mounted on the shaft on top of the tower.
29. A vertical axis wind turbine according to claim 28, wherein the tower comprises cylindrical upper and lower portions, and the eddy current brake assembly is arranged within the upper portion, the upper portion having an enlarged diameter relative to the lower portion.
30. A vertical axis wind turbine according to claim 28, wherein the wind driven rotor has a blade or blades configured to rotate about the shaft axis, and the or each blade has a length and is curved along its length around the shaft axis, so that when considered in a static position of the wind driven rotor, the said blade extends or the said blades together extend through at least most of a full revolution around the shaft axis.
31. A vertical axis wind turbine according to claim 28, wherein the target is cooled at least in part by an air current flowing upwardly through the tower between an inlet in a lower region of the tower and an outlet in an upper region of the tower.
32. An eddy current brake assembly for braking a rotating shaft, comprising: a support structure, the support structure being supported against rotation about a shaft axis; a shaft supported by the support structure for rotation about the shaft axis relative to the support structure; an electrically conductive target;a braking field winding;an energisation means arranged to supply a current to energise the braking field winding; and a control means for controlling the energisation means; the braking field winding being arranged to generate a magnetic field passing through the target when energised by the energisation means; wherein the energisation means includes: an exciter winding, and a permanent magnet exciter assembly arranged to produce a magnetic field which passes through the exciter winding; a respective one of the permanent magnet exciter assembly and the exciter winding being mounted on the shaft to rotate with the shaft relative to the other respective one of the permanent magnet exciter assembly and the exciter winding so as to generate a current in the exciter winding.
33. An eddy current brake assembly according to claim 32, wherein the energisation means further comprises: a stator winding; and a rotor winding mounted on the shaft to rotate with the shaft relative to the stator winding, the stator winding being arranged to produce a magnetic field passing through the rotor winding; and the control means is operable to control a strength of the magnetic field passing through the rotor winding; the rotor winding being arranged to generate the current to energise the braking field winding when energised by rotation through the magnetic field produced by the stator winding, the braking field winding being arranged to generate the magnetic field passing through the target to generate eddy currents in the target when energised by the rotor winding; and the exciter winding is arranged to energise the stator winding.
34. An eddy current brake assembly according to claim 32 or claim 33, wherein the permanent magnet exciter assembly is mounted on the shaft to rotate with the shaft relative to the exciter winding.
35. An eddy current brake assembly according to claim 32 or claim 33, wherein the control means forms part of a control system which is arranged to bring the shaft to a stop when shaft power SP transmitted by the shaft exceeds a threshold shaft power limit value SPstop; and the control system is arranged to vary the value SPstop to manage an operating temperature of the target.
36. An eddy current brake assembly substantially as described with reference to the drawings.