GB2506111A

GB2506111A - Electromechanical regenerative retarder

Info

Publication number: GB2506111A
Application number: GB1216660.9A
Authority: GB
Inventors: Marcus Allard; Stephen Grosvenor
Original assignee: DESIGN LIMTIED ETS
Current assignee: DESIGN LIMTIED ETS
Priority date: 2012-09-18
Filing date: 2012-09-18
Publication date: 2014-03-26
Also published as: GB201216660D0

Abstract

An electromechanical regenerative retarder device 1 comprises a frame 2, a first retarding portion on one side of the frame and a second retarding portion on the other side of the frame. Each retarding portion comprises a stator/rotor pair 3, 6, where the stator is arranged to generate a movable magnetic field and where the relative speed at which the magnetic field moves relative to the rotor is adjustable. The regenerative retarder may be operated either as an induction motor to drive or assist in driving an appliance (e.g. a vehicle), or as a regenerative braking unit, where the modes of operation are effected by adjusting the relative speed at which the magnetic field moves relative to the rotor. In an embodiment, the regenerative retarder may be mounted in a vehicle in place of a conventional centre bearing for the vehicles prop shaft, with prop shaft sections fixed to the rotors such that the rotors can drive, or be driven by, the prop shaft. The retarder may be connected to a supercapacitor unit. The device may be used as a retarder for a vehicle, fitted to its propeller shaft, and to apply resistive torque in machines such as exercise bicycles, rowing machines, dynamometers, winding gear, elevators, winches, ski lifts or cable car systems.

Description

Electromechanical device

Field of the invention

The present invention relates to an electromechanical device.

Background of the Invention

Commercial and passenger carrying vehicles have for many years been fitted with secondary braking systems know as retarders, and in some applications such devices are mandatory. The main benefits of retarders are enhanced safety as a result of prevention of overheating of the primary braking system, and extended life of the primary braking system components.

One of the principal types of retarder is the electromagnetic or eddy current retarder. An electromagnetic retarder has two main components, a stator and a rotor, and it is usually mounted in the driveline of the vehicle between the output of the gearbox and the input of the axle.

The stator, which can be fixed to either the gearbox, the axle or the chassis of the vehicle, consists of a circular array of coils positioned around a bearing housing. Each coil consists of wire wound around a core of magnetic material such as iron. The rotor consists of a central shaft and a metal disc at each end, usuallyiron. The discs are positioned such that there is a small air gap between each disc and the shoes which are fixed to the ends of the coil cores.

The shoes themselves are provided to aid with the distribution of the magnetic field generated by the coils. The rotor is permanently fixed to the propeller (prop) shaft of the vehicle so that it rotates as the shaft rotates.

To operate the retarder, a DC voltage is applied to the coils, creating an electromagnetic field.

: This field, generates eddy currents in the rotor discs as they rotate within the field. These eddy currents cause a braking torque to act upon the rotor discs which opposes their rotation, thus slowing the vehicle. Additionally, the eddy currents cause heat to be generated within the rotor discs, which is typically dissipated via cooling fins on the discs. In this way, the kinetic energy of the vehicle is converted to heat. The vehicle slows down and the heat is]ost to the air surrounding the retarder. Once the braking operation has been completed, the DC voltage to the coils is simply switched off to allow the rotor discs, and hence the prop shaft, to rotate freely.

The general principle of regenerative braking in vehicles is also known. In regenerative braking, kinetic energy of a moving vehicle is converted into a useful form of energy during braking and stored for subsequent use, rather than simply being dissipated as heat.

One method of regenerative braking is used for vehicles powered, in whole or in part, by electric motors, and involves reversing the operation of the electric motor under braking conditions, such that the motor acts as an electrical generator. Operating the motor as a gcncrator provides a resistive torque which acts to slow the vehicle, and as such the motor provides a retarding function which assists the primary braking system of the vehicle.

Additionally, electricity generated by the reversely-operated motor may be stored in batteries or capacitors, and later supplied to the electric motor to assist in driving the vehicle.

However, the amount of braking torque provided by this type of system is governed by the amount of generated electrical current which the power control electronics of the system, which directs the generated current to the batteries or capacitors, is able to handle i.e. the electrical power rating of the power control electronics. The higher the power rating, the greater the cost of the power control electronics. Hence, in order to achieve high braking torques, power electronics with a high power capability are required, leading to increased costs for the system.

Additionally, the batteries or capacitors may become fully charged under a period of sustained or repeated braking, for example when the vehicle travels downhill. In these circumstances, it is no longer possible to channel further electrical energy to the batteries or capacitors, such that the motor/generator no longer provides a retarding function. To overcome this problem, further electrical energy generated during braking can be channelled via the power control *.... 25 electronics to a resistor to be "dumped" or lost as heat. However, this requires additional componentry, and increases the cost of the system.

. : Retarder-like devices are also used in other applications, for example in fitness equipment.

Some types of fitness equipment, in partkular rowing machines, exercise bicycles and cycle *: trainers use a resistive torque applied to a rotating part as a way of absorbing power generated by the user. This resistive torque may be applied in any of the following ways: I. A fan rotating in air 2. A fluid being pumped through a variable orifice 3. A friction brake 4. A permanent magnet eddy current device 5. An electrical generator The iange of equipment runs from very simple machines with a crudely adjustable fan to complex and expensive virtual reality trainers incorporating a motor to simulate downhill sections of cycle races. Some machines will even feed electricity back into the grid.

However, on many machines the adjustment of resistance is basic, for example by adjusting the inlet vanes on a fan, or by physically moving the magnets on an eddy current device.

Further, the machines may be noisy, for example where a fan is employed to provide the resistive torque, and may need to be powered by mains or battery power supplies.

The present invention seeks to address problems associated with the prior art.

Statement of the Invention

According to a first aspect of the present invention, there is provided an electromechanical device according to claim I. According to a second aspect of the present invention, there is provided a system according to claim 25.

According to a third aspect of the present invention, there is provided a machine according to claim 26.

Brief Description of the Drawings

In order that the present invention may be more readily understood, embodiments thereof will * * now be described, by way of example only, with reference to the accompanying drawings, of o... 25 which. * *

*. FIGURE 1 shows a part-exploded view of a regenerative retarder according to a first embodiment of the present invention; * ..

* . 30 FIGURE 2 is a part cut-away view of the assembled regenerative retarder of Fig. 1; **. 0*. * *

FIGURE 3 is a part cut-away view of a rotor of the regenerative retarder of Fig. I; FIGURE 4 is a part cut-away view of the assembled regenerative retarder of Fig. ]; FIGURE 5 is a schematic drawing according to an embodiment of the present invention; FIGURE 6 is a schematic view of a stator according to an embodiment of the present invention; FIGURE 7A is a schematic view of a conventional axial flux electromagnetic retarder; and ifi FIGURE 7B is a schematic view of an arrangement according to an embodiment of the present invention.

Detailed Description of the Embodiments

1 5 First Embodiment Figure 1 shows, in a part exploded perspective view, various components of a regenerative retarder I according to a first embodiment of the present invention. The regenerative retarder I comprises a frame 2 to one side of which an electromagnetic stator 3 is fixedly attached by means of a stator mandrel 4 and a plurality of bolts 5, and an external rotor 6 rotatably mounted to the frame 2 by means of a bearing housing 7. A further rotor/stator pair, corresponding to that shown in the exploded portion of Fig. I, is shown provided in an assembled fashion on the other side of the frame. This further rotor/stator pair is identical to that shown in the exploded portion of Fig. I, and accordingly will not be described further here.

. : The stator 3 is shown in greater detail in Fig. 2, whereby a portion of the rotor 6 is illustrated *r cut-away to show the stator 3 within. * ..

* * 30 The stator 3 comprises a circular array of electromagnets 8, each comprising a coil 9 wound * onto a respective laminated coil core 10, which array surrounds the central bearing housing 7 to which the rotor 6 is mounted. 5.

The rotor 6 itself comprises a cylinder 11 provided with a plurality of cooling fins 12 around its outer circumference, and which is bolted to a disc portion 13 at one of its ends, such that the rotor 6 has the appearance of a hollow cylindrical drum. The disc portion 1 3 of the rotor 6 is secured at its centre to a central axial shaft 14 of the bearing housing 7 by a bolt 15. In the present embodiment, both the cylinder II and the disc portion 13 are formed of steel. In other embodiments, other ferrous materials may for example be used for the cylinder 11 and disc portion 13.

A plurality of copper bars 16 (numbering 44 in total in this embodiment, although other numbers of bars may be used), linked at each of their ends by two circular copper bands 17, are embedded within the cylinder 11. This may best be seen from Figs. 3 and 4, which show the cylinder 11 in partial cut-away to reveal the copper bars 16 within. In Fig. 4, the further rotor 6 on the other side of the frame 2 is also shown in partial cut-away, to reveal the copper bars 16 within.

According to the embodiment shown in Figure 5, the regenerative retarder I is mounted to the frame 18 of a vehicle in place of a conventional centre bearing for the vehicle's prop shaft 19, and is electrically connected to a supercapacitor unit 20 and a control system and power electronics module 21. Tn such an arrangement, prop shaft sections 1 9A, 19 B are fixed to the central axis of the rotors 6 at either side of the regenerative retarder 1, such that the rotor 6 rotates with the prop shaft 19, and vice-versa.

In such an arrangement, the supercapacitor unit 20 is electrically connected to the stator coils 9 via the control system and power electronics module 21, the latter comprising a variable J 25 frequency inverter 22 and a control module 23. The control system and çiower electronics module 21 is further connected to a rotor speed detector 24 operable to detect the rotational speed of the rotor 6. The rotor speed detector 24 may for example comprise a component of the vehicle's own control system, such as an ABS system, which produces data from which the rotational speed of the rotor 6 may be assessed, or a rotation speed detection device such 30 as an encoder attached to the rotor 6, or a detector which is operable to analyse the current flow in the coils 9 so as to determine the rotational speed of the rotor 6.

The variable frequency inverter 22 converts a DC supply from the supercapacitor unit 20 to a three-phase AC supply which is used to power the array of coils 9 in such a manner that the array of coils 9 produces a rotating magnetic field, directed radially outwardly from the circular array of coils 9.

The three-phase winding of the coils 9 is schematically illustrated in Fig. 6. For simplicity, only nine coils 9 are illustrated, in cross-section, in Fig. 6. In practice, the number of coils may be selected based upon operational requirements and is not limited to the number of coils shown.

Thc coils 9 are split into three groups (indicated as A, B and C in the Figure) with the coils 9 of each group being located alternately around the stator 3 and with each group being connected to a respective one of the three phases of the power supply. The current of each phase varies sinusoidally, and is out of phase by 120 degrees from the following phase, such that, taking one phase as a reference, the other two phases are delayed in time by one-third and two-thirds of a cycle, respectively. Thus the current in each of the three groups A, B, C of coils 9, and therefore the magnetic fields generated thereby, peaks at different times for each of the three groups A, B, C of coils 9. The sum of the magnetic field vectors produced by the coils 9 is a magnetic field vector which rotates around the stator 3 (i.e. clockwise in Fig. 6, as shown by arrow "A", or counterclockwise), and the coils 9 are wound around the coil cores such that the overall magnetic field vector is directed radially of the circular array of coils 9, as shown in Fig. 6, so as to permeate the rotor 6 located radially outwardly of the stator 3.

The control module 23 of the control system and power electronics module 21 is operable to 25 control the frequency of the AC power supply output by the variable frequency inverter 22, and hence to control the speed of rotation of the magnetic field generated by the stator 3. t4

: Additionally, the control system and power electronics module 21 is able to control the amount of current supplied to the stator coils 9. This current control is achieved via pulse * * width modulation in the present embodiment. The control system and power electronics module 21 is funher operable to direct electrical current generated within the stator coils 9, * when the regenerative retarder I is operated as a generator as discussed hereinafter, to the supercapacitor unit 20 for storage.

The regenerative retarder I according to the present embodiment may be operated as an electrical induction motor to drive or assist in driving the vehicle, and as a regenerative retarder which retains full retarding capability regardless of the state of charge of the supercapacitor unit 20. These modes of operation are effected by adjusting the slip speed of the regenerative retarder I i.e. the difference in rotational speed between the rotating radial magnetic field generated by the coils 9 and the speed at which the rotor 6 rotates around the stator 3. At low slip speeds the regenerative retarder I will efficiently convert kinetic energy of the vehicle into electrical energy, and vice versa At high slip speeds the regenerative retarder I is very inefficient and kinetic energy of the vehicle is converted to heat in the steel cylinder ii. This enables the regcnerative retarder 1 to retain its full retarding functionality, even when the supercapacitor unit 20 is fully charged.

The torque acting on the copper bars 16 rapidly increases from zero at zero slip speed to a peak at a relatively low slip speed, and thereafter rapidly decreases at increasing slip speeds.

In contrast, the torque on the steel cylinder II increases steadily with increasing slip speed to a maximum at a much higher slip speed than that which occurs in the copper bars 16.

In more detail, when a driving demand is made of the regenerative retarder I (which may for example be signalled by a driver of the vehicle depressing an acceleration pedal), the control system and power electronics module 21 provides a three-phase AC supply to the stator coils 9 by drawing a current from the supercapacitor unit 20. In particular, the control module 23 of the control system and power electronics module 21 notes the rotational speed of the rotor 6 as detected by the rotor speed detector 24 and controls the variable frequency inverter 22 to output the AC supply at a frequency corresponding to a low slip speed between the rotational * ...: 25 speed of the magnetic field and the detected rotational speed of the rotor 6.

Specifically, the magnetic field is controlled to rotate at a similar, but slightly higher, -rotational speed than the rotor 6. This relative rotational speed differential results in the * ** magnetic field experienced by the copper bars 16 of the rotor 6 varying with time, resulting in a current being induced in the copper bars 16. This induced current in turn creates a magnetic field which interacts with the magnetic field originating from the stator coils 9, with the result that a driving torque is applied, acting in the same direction of rotation, to the rotor 6. The control system and power electronics module 21 is able to control the amount of driving

S

torque produced in this way by adjusting the amount of current flowing to the coils 9, using pulse width modulation The driving torque is transmitted by the rotor 6 to the prop shaft 19, thus providing a driving force to move the vehicle. Hence, the regenerative retarder I acts as a induction motor in this mode.

When a braking demand is made of the regenerative retarder 1 (which may for example be signalled by a driver of the vehicle depressing a brake pedal), the regenerative retarder I is operated in an inductive generator mode. In this mode, the control module 23 of the control system and power electronics module 21 again notes the rotational speed of thc rotor 6 as detected by the rotor speed detector 24 and draws a current from the supercapacitor unit 20, and controls the variable frequency inverter 22 to supply an AC current to the coils 9 to create a magnetic field which rotates at a similar but slightly slower speed than the rotor 6. As each rotor 6 is therefore again moving relative to the magnetic field, currents are induced in the copper bars 16 which in turn establishes a magnetic field around the bars 16. This induced magnetic field then induces currents in the stator coils 9, which currents are transmitted by the control system and power electronics module 21 to the supercapacitor unit 20 for storage.

Hence, the regenerative retarder I acts as an induction generator in this mode, and the charge stored in the supercapacitor unit 20 can subsequently be used to power the regenerative retarder 1 when it is operated in its induction motor mode.

Additionally, when the regenerative retarder 1 is operated in its induction generator mode, a retarding torque results on the rotor 6, hence acting to slow the rotation of the prop shaft 19 and accordingly to assist in slowing the vehicle itself The control system and power electronics module 21 is able to control the amount of retarding torque produced in this way by adjusting the amount of current flowing to the coils 9, using pulse width modulation. * . . * .

However, as noted previously, once the supercapacitor unit 20 is fully charged it will accept no further charge, such that the retarding force resultant from the regenerative retarder I being *::: operated as a generator diminishes.

The present regenerative retarder I can however retain its full retarding capability in these circumstances, as once the supercapacitor unit 20 is fully charged, the control system and power electronics module 21 then operates to increase the slip speed between the magnetic Field and the rotor 6. The efficiency of the regenerative retarder 1 as a generator reduces significantly as the slip speed increases, such that the kinetic energy of the vehicle is no longer efficiently converted into electrical energy in the coils 9. However, eddy currents generated in the steel cylinder Il increase significantly as the slip speed increases, resulting in a significant retarding torque being applied thereto, as well as heat being generated in the rotor 6. That is, as the slip speed increases, the regenerative retarder I acts decreasingly as an induction generator, and increasingly as a conventional electromagnetic retarder.

Consequentially, the retarding capability of the regenerative retarder I may be retained, even though the supercapacitor unit 20 is fully charged.

Further, by controlling the slip speed it is also possible to convert a desired proportion of the vehicle's kinetic energy into electrical energy for diversion to the supercapacitor unit 20, and to "dwnp" a further proportion as heat in the rotor 6, by selecting a suitable slip speed and hence a corresponding effective efficiency of the induction generator function of the regenerative retarder I -It should also be noted that the amount of retardation which may be provided by the regenerative retarder 1 is not limited by the amount of current which may be channelled from the coils 9 to the supercapacitor unit 20 by the control system and power electronics modulc 21, unlike a conventional electric regenerative braking system, as by adjusting the slip speed the regenerative retarder I may convert significant amounts of the kinetic energy of the vehicle into heat in thc rotor 6, in addition to or in place of converting kinetic energy of the * vehicle into electrical energy. Indeed, if the frequency of the AC supply to the retarder coils 9 is reduced to zero by the control module 23, then it becomes a DC supply and the regenerative retarder will function in exactly the same way as a conventional electromagnetic retarder. In *.

* the extreme, therefore, the regenerative retarder I may in fact provide a braking torque *. without having to channel any current to the supercapacitor unit 20 (or to a resistive dump * :: unit to be lost as heat, as in a conventional electric regenerative braking system).

" 30 Heat generated in the rotor 6 is dissipated therefrom, and in particular from the cooling fins 12, as in a conventional retarder. Advantageously, increased rotation speed of the rotor 6 results in a greater air cooling effect, thus helping to dissipate higher levels of eddy current heating which may be encountered such as when braking is initiated.

An additional advantage of the present embodiment is that a conventional electromagnetic retarder typically requires a vehicle to have an upgraded alternator to supply the necessary excitation current (i.e. the current required to generate the magnetic fields which result in a braking torque being applied to the rotor). This may however potentially be avoided according to the present embodiment, as currents generated by the regenerative retarder 1, when it is operated as an induction generator, may be used as the excitation current for the slator coils 9. Hence, upgrades to the main vehicle electrical system may potentially be avoided.

Alternative applications The first embodiment of a regenerative retarder I is described above in use in a vehicle braking system. However, the present invention is not limited to this application, and embodiments of the present invention may be employed in other applications. For example, regenerative retarders I according to embodiments of the invention may be employed to provide a braking and/or driving torque in applications such as dynamometers, winding gear, elevator, ski lifts and cable car systems.

In a further example, regenerative retarders I according to embodiments of the invention may be employed as the primary source of resistive torque in fitness equipment machines, such as rowing machincs, exercise bicycles, cycle trainers and the like, with a number of potential advantages, in particular in terms of improvements in control and noise reduction.

Furthermore, by operating the regenerative retarder I in its induction generator mode, power * 25 may be generated as the user exercises, which power may be uscd to power the coils 9 of the stator 3 to produce the rotating magnetic field discussed above, as well as to run control and display functions of the fitness equipment machine. As such, machines provided with a regenerative retarder I according to the present invention would not require an external power source such as a mains supply. Yet further, by operating the regenerative retarder I in its induction motor mode, downhill simulation on cycle trainers may for example be provided, whereby the actions of the user in turning the pedals of the machine are aided by a driving torque being applied to the rotor 6 of the regenerative retarder I, in the manner described above.

In more detail, a regenerative retarder I according to an embodiment of the present invention may be employed in place of a fan rotating in air in a conventional rowing machine, potentially to provide a more compact, much quieter unit, with the possibility of programmable resistance settings which could be stored in memory to suit a particular user. It would also be possible for users of different weights to compete against each other by selecting the resistance according to weight. The different resistance settings may be achieved by the control system and power electronics module 21 providing a greater or lesser current to the coils 9. Alternatively, the control system and power electronics module 21 can adjust the slip speed, and hence the overall torque experienced by the rotor 6.

Yet further, a regenerative retarder 1 according to an embodiment of the present invention may be employed in place of a permanent magnet eddy current brake, such as may typically be found in a mid-priced cycle trainer. By being run in its induction generator mode, the regenerative retarder I could self-power the electromagnetic coils 9 of the stator 3, as well as providing stepless resistance control to a user -by adjusting the slip speed and/or the amount of current directed to the coils, the power electronics and control module 21 can finely adjust the resistive torque experienced by the rotor 6. The onboard control electronics for the cycle trainer may also be powered by the electricity generated by the regenerative retarder, such that mains power for the trainer would not be required.

In other examples, the regenerative retarder I could replace the friction pad typically used for resistance in club type cycle trainers, to allow for self-generated power to be produced. This : power may for example be used to power the control system and power electronics module 21 of the regenerative retarder I. Furthermore, as the regenerative retarder is controlled : electronically (namely, by the control system and power electronics module 21), more accurate resistance control may potentially be provided than by a mechanically-operated * .. friction pad. Furthermore, the machine would typically require lower maintenance, as unlike a friction pad, no wear is caused on the parts of the regenerative retarder I during its operation.

* * 30 Alternative embodiments In the above-described embodiment, the bars 16 and bands I 7 are formed from copper, but other materials, for example non-ferrous materials such as aluminium or brass may be used.

Further, although the bars 16 are connected by inner and outer hands 17, either or both of these rings may be omitted.

In the above-described embodiment, the coil cores ID are laminated to reduce the effect of eddy current heating during operation. However, other means for reducing heating may be employed. For example, the cores 10 may be made from soft magnetic composites.

A supercapacitor unit 20 is used to store electrical energy in the above embodiment. However, electrical energy may instead be stored in one or more batteries, or in a combination of batteries and supercapacitors.

The regenerative retarder I of Fig. I is fitted in place of a conventional centre-bearing unit on the prop shaft 19. However, a vehicle may be provided with one or more of the regenerative retarders 1, fitted in other ways. For example, a regenerative retarder I may be fitted at the wheel hub of one or more wheels of the vehicle.

The first embodiment has a number of potential advantages. For example: 1. Low cost. The design of the regenerative retarder I makes it potentially significantly cheaper to manufacture than a permanent magnet machine of similar power.

2. Full retarder capability may be maintained even when energy store (i.e. the supercapacitor unit 20) is full, as if the energy store is full, excess energy can be dissipated in the rotor 6 as * heat.

* 25 3. Maximum retarder power can be much higher than the maximum power which may be handled by the control system and power electronics module 21, by dissipating some or all of the kinetic energy of the vehicle into heat in the rotor 6 rather than simply relying on the * regenerative retarder I being operated in its induction generator mode to provide a resistive torque to the rotor 6. As such, the power handling capability of the control system and power r": 30 electronics module 21 can be optimised for cost and fuel saving at a fairly low power, but the system retains the capability of providing extremely high braking torque even at high vehicle speeds.

4. Retarding torque on the rotor may potentially be increased as compared to a conventional axial flux electromagnetic retarder. Specifically, a detail from a conventional axial flux electromagnetic retarder 25 is shown in the attached Fig. 7a, whereby the top half of a rotor 26 is shown located to one side of an electromagnet 27, and is permeated by a magnetic field emanating in an axial direction (i.e. from left to right in Fig. 7a) from the electromagnet 27. In comparison, Fig. 7b illustrates an embodiment according to the present invention, whereby the electromagnet 8 is located in the same plane as the plane of rotation of the illustrated top half of the rotor 6, which is permeated by the radial magnetic field (i.e. magnetic field in the vertical direction in Fig. 7B) produced by the electromagnet 27. The torque experienced by the rotor in each case is equal to the force produced in the gap between the electromagnet and the rotor multiplied by the effective radius. Thus, torquc will be greater in the radial flux embodiment of Fig. 7b as compared to the conventional axial flux embodiment of Fig. 7a for a given rotor diameter due to the larger effective radius in the former, as shown in the Figures.

5. The present embodiment may be used to provide an axially compact arrangement as the stator occupies the same plane as the plane of rotation of the rotor 6, rather than the rotor rotating in a plane to one side of the stator, as in a conventional axial flux electromagnetic retarder.

SECOND EMBODIMENT

According to a second embodiment (not illustrated), the rotor is located radially inwardly of the stator, rather than radially outwardly as in the First EnThodiment. * . S * *S

*r"i As in the first embodiment, the second embodiment allows for an axially compact form of regenerative retarder as the rotor and stator are contained within the same plane. S.., * . S

S

S * S. * S S * *5 * S

Claims

CLAIMSI. An electromechanical device, comprising: a first member and a second member comprising first and second retardation portions each arranged to rotate within a plane occupied by the first member; wherein the first member is arranged to generate a movable magnetic field and wherein the relative speed at which the magnetic field moves relative to the second member isadjustable,wherein at a first relative speed the interaction of the magnetic field with the second member results in the first retardation portion generating an electrical current, and a retardation torque opposing the movement of the second member is applied to the first retardation portion; and wherein at a second relative speed, different from the first relative speed, the amount of current generated by the first retardation portion is less than that generated by the first retardation portion at the first relative speed; and the interaction of the magnetic field with the second member results in an amount of retardation torque opposing the movement of the second member being applied to the second retardation portion, which amount is greater than the amount of retardation torque opposing the movement of the second member applied to the second retardation portion at the first relative speed.
2. The electromechanical device of claim I, wherein the first and second retardation portions are arranged to rotate radially outwardly of the first member. S. * S S

*
3. The electromechanical device of claim 1, wherein the first and second retardation portions are arranged to rotate radially inwardly of the first member. 5.. * * * ** *
4. The electomechanical device of any one of the previous claims, wherein the first member *::: extends axially.

* 30
5. The electromechanical device of claim 4, wherein the axis of the first member corresponds to the axes of rotation of the first and second retardation portions.
6. The electromechanical device of any one of the preceding claims, wherein the magnetic field generated by the first member is directed radially.
7. The electromechanical device of any one of the preceding claims, wherein at the second relative speed the amount of retardation torque opposing the movement of the second member applied to the first retardation portion is less than that applied thereto at the first relative speed.
8. The electromechanical device of any one of the preceding claims, wherein the first retardation portion comprises at least one elongate, electrically conductive element.
9. The electromechanical device of claim 8, wherein said at least one elongate, electrically conductive element comprises a non-ferrous material.
10. The electromechanical device of claim 8 or claim 9 as dependent upon claim 4, wherein the at least one elongate, electrically conductive element has its axis parallel to the axis of the first member.
II. The electromechanical device of any one of claims 8 to 10, wherein the first retardation portion comprises a plurality of said at least one elongate, electrically conductive elements, respective first andlor second ends of which are electrically connected to each other.
12. The electromechanical device of any one of claims 8 to II, wherein the secondretardation portion comprises a cylindrical portion.

.. :
13. The electromechanical device of claim 12, wherein the cylindrical portion comprises a different material from a material of the at least one elongate, electrically conductive element. * *.
14. The electromechanical device of claim 12 or 13, wherein the cylindrical portion comprises a ferrous material.
15. The electromechanical device of any one of claims 12 to 14, wherein the axis of the at least one elongate element is parallel to the axis of the cylindrical portion.
16. The electromechanical device of any one of the preceding claims, wherein the first retardation portion is embedded within the second retardation portion.
17. The electromechanical device of any one of the preceding claims, wherein the second retardation portion is provided with at least one cooling fin.
18. The electromechanical device of any one of the preceding claims, wherein the second retardation portion is connected to a rotatable shaft.
19. The electromechanical device of any one of the preceding claims, wherein the first member comprises a stator, the second member comprises a rotor arranged to rotate relative to the stator,. and the magnetic field comprises a rotatable magnetic field.
20. The electromechanical device of claim 1 9, wherein the stator comprises a plurality of coils which produce a rotating niagnetic field when powered by a three-phase AC power supply.
21. The electromechanical device of any one of the preceding claims, wherein the first * retardation portion generates a current at the first relative speed by induction in at least one coil.
22. The electromechanical device of claim 21, wherein the first member comprises at least one coil to generate the movable magnetic field, and wherein the first retardation portion induces a current in the at least one coil of the first member. * 25
23. The elcctromechanical device of any one of the preceding claims, wherein the second relative speed is higher than the first relative speed.
24. The electromechanical device of any one of the preceding claims, wherein the movement 30 of the magnetic field relative to the second member may be adjusted to a relative speed at which the interaction of the magnetic field with the second member results in a driving torque acting to accelerate the second member being applied to the first retardation portion.
25. A system comprising at least one electromechanical device of any one of the preceding claims, a controller for controlling the relative speed at which the magnetic field moves relative to the second member, and an electrical power store for storing the current generated by the first retardation member at the first relative speed.
26. A machine provided with at least one electromechanical device according to any one of claims I to 24or a system according to claim 25.
27. The machine of claim 26, wherein kinetic energy of the machine or a change in potential energy of the machine is converted into heat in the second retardation portion at the second relative speed.
28. The machine of claim 27, wherein the heat in the second retardation portion is generated by eddy currents which are induced in the second retardation portion as a result of thc IS interaction of the magnetic field with the second member.
29. The machine of any one of claims 26 to 28, wherein the machine comprises one of a vehicle, a fitness equipment machine, a dynaniometer, winding gear, elevator, a winch, a ski lift or cable car system.
30. An electromechanical device substantially as herein described with reference to Figs. 1 to 6 and 7B. a. * * . * **S * a S5* S S * S* SSS S 5* S * * S 55 aS.....S