WO2016188721A1

WO2016188721A1 - Speed independent generator

Info

Publication number: WO2016188721A1
Application number: PCT/EP2016/060100
Authority: WO
Inventors: Alan Armstrong
Original assignee: Jaguar Land Rover Limited
Priority date: 2015-05-26
Filing date: 2016-05-04
Publication date: 2016-12-01
Also published as: EP3304703A1; GB201508960D0; GB2538951A; GB2538951B

Abstract

A generator (10), comprising: a first stator (12, 14) comprising a set of electromagnets (24) arranged to produce a magnetic field which rotates at a rotational speed around the first stator (12, 14); a second stator (12, 14) comprising a set of generator windings (26); and a rotor (16) disposed between the first and second stators (12, 14), the rotor (16) comprising a set of core members (32) and an input means arranged to receive a drive force to rotate the rotor (16); wherein the core members (32) are arranged to interact with the magnetic field produced by the set of electromagnets (24) to produce a modulated magnetic field that induces an electrical current in the generator windings (26); and wherein the rotational speed of the magnetic field produced by the set of electromagnets (24) at least in part determines the magnitude of the electrical current induced in the generator windings (26).

Description

SPEED INDEPENDENT GENERATOR

FIELD OF THE INVENTION

The present disclosure relates to a speed independent generator. Particularly, but not exclusively, the present disclosure relates to a speed independent generator for a vehicle. Aspects of the invention relate to a generator, to a vehicle, to a method, to a computer program product, and to a non-transitory computer-readable medium.

BACKGROUND

As a result of recent changes in emissions regulations, there is widespread interest in reducing engine speeds in modern vehicles. In particular, in order to comply with environmental legislation it is desirable to reduce the idling speed of vehicle engines as far as possible, and also the engine speed at which automatic transmission systems select a higher gear.

At the same time, the demand for electrical power in vehicles has been rising. This is in part due to the fact that modern vehicles typically include a large range of internal systems such as climate control, seat warming, infotainment systems, and so on, alongside power train related electrical loads such as heated exhaust gas treatment systems. Often, the time when the electrical demand is highest coincides with a period of engine idling or low engine speed. For example, in a short period immediately following engine ignition the internal systems all initiate while the engine idles. Some systems, in particular heating, ventilation and air- conditioning (HVAC) systems, run at high power during this time in order to condition the driving environment.

Although it is possible to use some stored battery power to meet the electrical demand, it is generally preferred to avoid depleting battery charge during normal running of the vehicle, to conserve battery power for start-up operations and avoid premature aging of the battery. It is therefore desirable to match the electrical generating capacity of the vehicle to the electrical load at all times.

Conventionally, electrical power is generated by an engine-driven alternator, and the generating capacity of the alternator is directly related to the speed at which the alternator is driven. Typically, an armature of the alternator is driven directly by the engine using an auxiliary drive belt. The drive belt is coupled to the engine and the armature using respective pulleys. The ratio of the pulleys is typically configured to provide up to a threefold increase in alternator speed relative to the engine speed. Accordingly, in this arrangement the alternator speed is directly proportional to the engine speed. Therefore, if the pulley ratio is selected so as to provide sufficient generating capacity when the engine is idling, the alternator speed may be very high when the engine is running near a maximum speed. For example, if the armature must be rotated at 3000 rpm to serve the maximum electrical requirement of the vehicle, a 1000 rpm engine idling speed would satisfy this requirement with a 3:1 pulley ratio. Therefore, when the engine speed is near a maximum, typically 7000-9000 rpm, the alternator is spun at approximately 21000-27000 rpm. This is much faster than is necessary to satisfy the electrical power demand, which is wasteful. Furthermore, such high speed operation increases wear and potentially reduces the average time to component failure, and also necessitates robust bearings, which are expensive. Furthermore, to achieve a higher pulley ratio the diameter of the pulley at the alternator must be smaller, thus increasing the amount by which the drive belt wraps round the pulley. As a result, the parasitic load on the engine increases, reducing fuel efficiency.

It is also noted that a larger armature imparts a higher inertial load on the vehicle engine. This can be problematic, particularly for smaller engines which are becoming more common due to a trend to downsize engines for improved fuel economy. It is therefore desirable to reduce the maximum speed at which the alternator is driven, whilst ensuring the generating capacity remains sufficient during engine idling. One way to increase the generating capacity of the alternator is to increase the size of the armature, or even to add a second armature. These approaches allow the pulley ratio to be reduced, and therefore reduce the maximum speed of the alternator, while maintaining the required generating capabilities. However, this approach is not favoured, since raising the size of the armature goes against a general objective of reducing vehicle weight, in line with the overriding aim of reducing emissions. Furthermore, in the case of a larger, heavier armature, more robust bearings would be required. For an arrangement including either a larger armature or multiple armatures, parasitic losses would increase due to increased friction. A second armature would also be costly and difficult to package.

An alternative approach has been proposed in which a mechanical gearbox is provided between the alternator and its respective pulley. In this arrangement, the gear has two selectable ratios, with an appropriate ratio being selected with respect to the instantaneous engine speed. A higher ratio can be selected at lower engine speeds to maximise generating capacity, and a lower ratio can be selected at higher engine speeds to reduce the drive speed of the alternator. Since two distinct gear ratios are provided, a drawback with this arrangement is that there is a changeover time whilst moving between the gears during which the alternator is not driven. Thus, during this time, generation drops significantly. A further consideration is that the mechanical gearbox introduces extra cost, weight, vibration/noise, complexity and failure potential, and there is currently a desire within the automotive industry to exchange mechanical systems for non-mechanical systems where possible.

Another alternative approach has been proposed in which a variable speed magnetic gear, such as is described in US 201 1 /0037333 A1 , is used in the place of a mechanical gearbox. A known variable speed magnetic gear includes three concentric and coaxial rotors. The innermost and outermost rotors each have a respective set of permanent magnets disposed evenly around their circumference. An intermediate rotor, which is disposed between the inner and outer rotors, comprises a set of core members. The core members are arranged to interact with the magnetic field between the two sets of magnets, such that movement of the intermediate member induces movement of the inner rotor. When the outer rotor is stationary, a fixed gear ratio is defined between the inner rotor and the intermediate rotor. Accordingly, an input shaft may be coupled to the intermediate rotor, and an output shaft may be coupled to the inner rotor, so as to provide a fixed speed increase between the input and the output shafts. In order to vary the gear ratio as required, so as to provide a variable speed increase, the outer rotor is physically rotated. This modulates or influences the coupling between the intermediate and inner rotors. An external means, for example a motor, must be provided for driving the outer rotor.

The combination of the magnetic gear with the external means for driving the outer rotor is complex, large and costly to implement. Therefore, this approach is unlikely to be preferable to simply increasing the size of the armature of the alternator.

Against this background, it would be desirable to provide an improved vehicle generating system, which overcomes or at least substantially alleviates the disadvantages known in the prior art.

SUMMARY OF THE INVENTION

Aspects and embodiments of the invention provide a generator, a vehicle, a method, a computer program product, and a non-transitory computer-readable medium as claimed in the appended claims. According to an aspect of the invention there is provided a generator. The generator may comprise a first stator comprising a set of electromagnets arranged to produce a magnetic field which rotates at a rotational speed around the first stator, a second stator comprising a set of generator windings, and a rotor disposed between the first and second stators. The rotor may comprise a set of core members and an input means arranged to receive a drive force to rotate the rotor. The core members may be arranged to interact with the magnetic field produced by the set of electromagnets to produce a modulated magnetic field that induces an electrical current in the generator windings, and the rotational speed of the magnetic field produced by the set of electromagnets at least in part determines the magnitude of the electrical current induced in the generator windings.

The modulated field is created by the rotation of the core members within the magnetic field, and so the modulated field rotates relative to the outer stator and relative to the outer magnetic field. This relative motion between the modulated field and the second stator provides the varying flux across the generator windings that is required to induce the electrical current. The magnitude of the induced current is proportional to the speed of rotation of the modulated field, which is in turn governed by the rotational speed and sense of rotation of the rotor relative to the rotational speed of the magnetic field produced by the electromagnets. Therefore, by varying the rotational speed, and, optionally, sense of rotation, of the magnetic field, the magnitude of the induced current in the generator windings can be controlled to a desired level. In this way, the generator beneficially decouples the generating output from the input rotor speed, and by extension the engine speed. This increases the ability of the generator to meet electrical demand at all times for any given rotor speed, and also enables the generator to operate in the most efficient regime possible. It is noted that the term 'speed' as used throughout this specification can also denote the sense of rotation, in that a negative speed entails rotation in the opposite sense to a positive speed.

In the generator described above, the input means may comprise an input, for example a pulley which is integrated with the rotor.

The rotor and the first and second stators may be arranged concentrically, in which case the rotor may be generally tubular. In such arrangements, the first stator may be disposed radially outward of the rotor. Alternatively, the second stator may be disposed radially outward of the rotor. The set of electromagnets may comprise a plurality of electromagnets evenly distributed around the first stator in a circular configuration. This arrangement enables creation of a consistent magnetic field around the stator. Similarly, the set of generator windings optionally comprises a plurality of generator windings evenly distributed around the second stator in a circular configuration, and the core members may be spaced from one another and evenly distributed around the rotor in a circular configuration.

The core members may be formed integrally with the rotor in a castellated arrangement, thereby minimising the number of separate parts for ease of assembly.

In some embodiments, the set of electromagnets is arranged such that, when energised, each electromagnet of the set has an opposite polarity to an adjacent electromagnet of the set. Alternatively, the set of electromagnets may be arranged to produce a multi-phase magnetic field.

Optionally, each electromagnet of the set may be operable independently.

The generator may comprise a control means arranged to control operation of the electromagnets to control the rotational speed of the magnetic field, thereby to control the magnitude of the electrical current induced in the generator windings in a self-contained arrangement. The control means may be arranged to provide an alternating current to the set of electromagnets such that the polarity of each electromagnet alternates, thereby to rotate the magnetic field relative to the first stator at a speed determined by the frequency of the alternating current. The control means may comprise a power inverter for providing the alternating current, in which case the power inverter is operable to control the frequency of the alternating current.

The control means may comprise a controller, for example a controller comprising a processor, a memory module, one or more inputs arranged to receive signals indicative of rotor speed and electrical demand, and one or more outputs arranged to transmit control signals for controlling the electromagnets.

The generator may comprise sensing means arranged to detect a signal indicative of a rotational speed of the input means. The signal indicative of the rotational speed of the input means may comprise a measurement of the frequency and/or the amplitude of the electrical current induced in the generator windings, for example for closed loop feedback control. The sensing means may comprise a sensor, for example a speed sensor for direct measurement of the speed of the rotor, or alternatively a voltmeter or ammeter for measuring the current and voltage of the current induced in the generator windings. In another aspect, there is provided a generator system comprising a generator according to the above aspect in combination with energising means arranged to energise the set of electromagnets to produce the rotating magnetic field around the first stator. The energising means may comprise an energising circuit arranged to produce an electrical waveform with which to energise the electromagnets.

The invention also extends to a vehicle comprising the generator or the generator system of the above aspects.

Another aspect of the invention provides a method of operating a generator. The generator may comprise a first stator including a set of electromagnets, a second stator including a set of generator windings, and a rotor disposed between the first and second stators. The rotor may include a set of core members. The method may comprise sensing a rotational speed of the rotor, and energising the electromagnets so as to produce a magnetic field that rotates relative to the first stator at a speed determined according to the rotational speed of the rotor, such that a modulated field produced through interaction of the core members with the magnetic field induces a desired electrical current in the generator windings.

The method may comprise energising the set of electromagnets into a first state, so as to define an initial position for the magnetic field, and subsequently energising the set of electromagnets into a second state, so as to displace the magnetic field from its initial position, thereby rotating the magnetic field. The first state may define a respective initial polarity for each electromagnet, in which case in the second state the polarity of each electromagnet is opposite to its respective initial polarity. The method may comprise energising the electromagnets such that each electromagnet alternates between the first and second states to define an alternation frequency, in which case the method may further comprise adjusting the alternation frequency so as to control the speed of rotation of the magnetic field around the electromagnets. Alternatively or in addition, the method may comprise applying an alternating current to the electromagnets to alternate between the first and second states.

The above method relates to scenarios in which, for example, the rotor of the generator is driven by an engine of a vehicle, so that the rotational speed of the rotor is proportional to the speed of the engine. In such scenarios, the method optionally comprises sensing an electrical demand of the vehicle, and setting the desired electrical current in dependence on the electrical demand. In other aspects, the invention extends to a computer program product arranged to implement the method of the above aspect, to a controller arranged to implement the method of the above aspect or the computer program product, and to a non-transitory computer-readable medium loaded with the computer program product. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF DRAWINGS

One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which like components are assigned like numerals, and in which:- Figure 1 is a schematic drawing of a generator arrangement according to an embodiment of the invention;

Figure 2 corresponds to Figure 1 but shows the generator in radial cross-section; Figure 3 is a graph showing typical generator performance characteristics;

Figure 4 is a graphical representation of operating regimes for the generator arrangement of Figure 1 ; Figure 5 corresponds to Figure 3 but indicates the impact of a generator according to an embodiment of the invention on performance; and Figure 6 is a schematic illustration of a vehicle including the generator arrangement of Figure 1 .

DETAILED DESCRIPTION

Figures 1 and 2 illustrate a generator 10 according to an embodiment of the invention. The generator 10 includes three generally tubular members disposed in concentric relation and separated by air gaps: an inner stationary member, hereafter referred to as an inner stator 12; an outer stationary member, hereafter referred to as an outer stator 14; and an intermediate rotatable member disposed between the inner and outer stators 12, 14, hereafter referred to as a rotor 16. The rotor 16 is supported by bearings 18, and is arranged for rotation about a common central axis 20 of the generator 10.

In this embodiment, the inner and outer stators 12, 14 are formed integrally with one another, in that the stators 12, 14 are defined by respective concentric tubular protrusions extending axially from a common circular side wall 22. An annular groove is defined between the protrusions, in which the rotor 16 is received.

The outer stator 14 has a set of electromagnets 24 disposed around its circumference. The electromagnets 24 are of equal size to one another, and are equidistantly spaced around the outer stator 14. Each electromagnet comprises a coil of wire wound around a soft iron magnetic core, such that magnetic poles are formed when the wire is energised. The orientation of the magnetic poles, i.e. the polarity of the electromagnet, is dependent on the direction of the current flowing through the coil. To energise the electromagnets 24, a voltage may be applied, for example from an energy storage device such as a battery.

In this embodiment, the coils of the electromagnets 24 are all connected together in series, and are wound in opposite senses to create an alternating polarity from each electromagnet to the next. A single length of wire may be used to form the coils of all of the electromagnets 24. Due to the alternating polarity, the application of an alternating current to the coils creates a rotating magnetic field around the outer stator 14 in a similar manner to the stator of a conventional electric motor. This is because the polarity of each electromagnet switches each time the electrical current changes direction. As the polarity of each electromagnet alternates, the overall magnetic field created collectively by the set of electromagnets 24 effectively rotates in the same way as if the stator were to be physically rotated. The rotational speed of the magnetic field is determined by the frequency of the alternating current, and so this speed can be controlled by manipulating the alternation frequency. The electromagnets 24 would typically be energised using a control strategy such as pulse- width modulation (PWM), in which case it is noted that the magnitude of the magnetic field produced by the electromagnets 24 can be varied by controlling the duty cycle of the pulse- width modulated signal.

A set of generator windings 26 is disposed around the circumference of the inner stator 12. The generator windings 26 are all equally sized, and are evenly distributed around the inner stator 12. In this embodiment the generator windings 26 are generally identical to the electromagnets 24, although the generator windings 26 may include a different number of turns than the electromagnets 24 according to the electrical voltage that each is intended to handle. Both the generator windings 26 and the electromagnets 24 are oriented such that the sources and sinks of magnetic flux are aligned perpendicular to the common axis 20, and with alternating polarity. The rotor 16 is arranged to be driven by a drive force, which in this embodiment is provided by a vehicle engine. The rotor 16 includes an input means in the form of a pulley section 28 of enlarged radius relative to the rest of the rotor 16, the pulley section 28 including an annular recess 30 arranged to receive a drive belt, the drive belt being wrapped at its other end around a pulley coupled to an engine output in an arrangement that mirrors that for a conventional vehicle alternator. Therefore, when the engine drives the drive belt, the rotor 16 is turned about the common axis 20 at a speed proportional to the engine speed. In contrast with the conventional alternator arrangement, in which a pulley ratio of around 3:1 is typically used, with the generator 10 of this embodiment a lower pulley ratio can be used, for example 2:1 or lower. The reason for this is explained later.

The rotor 16 includes a set of core members 32 which are equally sized and evenly distributed around the circumference of the rotor 16, separated by air gaps or non-magnetic material. The core members 32 are made from a ferromagnetic material, and in this embodiment are formed as teeth that are integral with the rotor 16 in a castellated structure.

In use, the generator 10 induces a current in the generator windings 26 of the inner stator 12 using a similar principle to that used in magnetic gearboxes. The rotating magnetic field produced by the electromagnets 24 extends radially inwards across the core members 32 of the rotor 16. As the rotor 16 is driven by the drive belt, the core members 32 pass through the magnetic field. The presence of the core members 32 modulates the magnetic field produced by the electromagnets 24 as they pass through it, such that a modulated field is created in the air gap between the rotor 16 and the inner stator 12. Since both the core members 32 and the magnetic field rotate, the spatial distribution of the modulated field is not fixed; the modulated field rotates at a speed which is governed by the relative speed of the core members 32 and the speed of the magnetic field produced by the electromagnets 24, noting again that the polarity of the speed of rotation is indicative of the sense of rotation. The number of core members 32 and number of poles of the magnetic field produced by the electromagnets 24 is not arbitrary, but determined by known principles applied to achieve magnetic coupling used in magnetic gear systems. The rotating modulated field passes over the generator windings 26 of the inner stator 12, such that each of the windings 26 is subjected to a varying magnetic flux. This causes an electric current to be induced in the generator windings 26, the magnitude of which is proportional to the magnitude and rate of change of the magnetic flux cutting the windings, which is in turn dictated by the magnitude and rotational speed of the modulated field. The induced electrical current is of an alternating nature, and so additional circuitry is used to rectify the current into a DC output, in this embodiment in the range of 12V-16V, that is suitable for charging a battery.

As the rotational speed of the modulated field is dependent on the various factors noted above, the modulated field can have a different rotational speed to that of the rotor 16. As the rotor speed is directly related to the engine speed, the use of the modulated field to induce a current in the generator windings 26 allows the electrical output of the generator 10 to be decoupled from the instantaneous engine speed. In particular, for any given rotor speed, the rotational speed of the magnetic field produced by the electromagnets 24 can be varied so as to control the rotational speed of the modulated field at any desired level within its operational range. Therefore, the modulated field produced by the electromagnets 24 together with the core members 32 replaces the magnetic field of an armature of a conventional generator, such that the speed of the modulated field can be thought of as an effective alternator speed. In contrast with a conventional generator, with the generator 10 of this embodiment the effective alternator speed can be varied to provide a desired electrical output, thereby emulating an armature that rotates at a desired speed for any engine speed input.

In this context, an effective gear ratio is defined in the generator 10 between the rotor speed and the effective alternator speed. This gear ratio is variable by virtue of the ability to vary the rotational speed of the magnetic field produced by the electromagnets 24. As the magnetic field can rotate in either sense, the gear ratio can be both positive and negative. This is advantageous because it allows the generator 10 to supply sufficient electrical power when demand is high and engine speed is low and also to maintain optimal efficiency of generation during other circumstances, such as high engine speed and low/medium power demand. In contrast, in a conventional alternator arrangement, the minimum generating capacity of the alternator, namely the electrical power output when the vehicle engine is idling, has to correspond to the maximum possible vehicle demand to ensure that demand can always be met. As noted above, this approach adds cost and weight to a vehicle with little benefit in return, and so is wasteful and puts unnecessary strain on the components of the alternator.

Decoupling the generator 10 output from the engine speed is a significant benefit for the reasons already described; namely, generating output can be increased at low engine speeds to ensure electrical demand is met, while the speed at which the rotor 16 must rotate in order to generate the required electrical power can be reduced. This avoids the need to use an alternator with a larger and therefore heavier armature in order to ensure that the electrical demand can be met at lower engine speeds. This also enables the use of a lower pulley ratio, thereby extending the life of the bearings 18. It is noted that the increase in the pulley ratio is limited by belt wrap considerations, in that parasitic losses can become unacceptable if the pulley ratio is too high. For this reason, a pulley ratio of around 2:1 is used in this embodiment. The use of a smaller armature in combination with a lower pulley ratio is also favourable in terms of the inertial load applied to the engine, which as already noted has a significant impact on lower capacity engines.

The number and size of the core members 32 are configured to produce the modulated field with at least one pole pair to interact with the generator windings 26. Said another way, the core members 32 modulate the magnetic field produced by the electromagnets 24 to form a magnetic field with a different number of pole pairs and rotational speed which results in flux cutting across the generator windings 26 at a different rate and/or magnitude than would result from direct interaction with the magnetic field produced by the electromagnets 24. This ability to vary the speed of the modulated magnetic field independently of the speed of the rotor 16 by varying the speed of the magnetic field produced by the electromagnets 24 increases the capability of the generator 10 to convert mechanical energy from the rotor 16 into electrical energy as a current induced in the generator windings 26. It is noted that the magnetic field produced by the electromagnets 24 can be rotated in either sense, and is optionally stationary. This maximises the flexibility of the generator 10 to provide a desired electrical power output from a range of engine speed inputs. For example, noting that the speed of the modulated field is dictated in part by the speed of the core members 32 relative to the magnetic field, the speed of the modulated field can be maximised by rotating the magnetic field in the opposite sense to the rotation of the rotor 16, thereby maximising the induced current in the generator windings 26.

It is through the action of the core members 32 to modulate the magnetic field produced by the electromagnets 24 that kinetic energy supplied to the rotor 16 from the engine is converted into electrical energy in the generator windings 26. While some electrical energy is required by the electromagnets 24 to produce the magnetic field, this field can be relatively weak compared with the field induced in the generator windings 26. As the core members 32 pass through the magnetic field at relatively high speed, the kinetic energy of those members allows the creation of a modulated field that is stronger and faster-moving than the magnetic field produced by the electromagnets 24. Therefore, the electrical energy recovered in the form of the electrical current induced in the generator windings 26 is significantly larger than the energy required to power the electromagnets 24.

Figure 3 shows a plot 34 of typical vehicle generator performance characteristics such as will be familiar to the skilled person. Since the generator 10 of the above described embodiment of the invention emulates the behaviour of a conventional generator, the graph 34 shown in Figure 3 is applicable. However, the ability to decouple the effective alternator speed from the engine input speed means that it is possible to operate in any region of the graph irrespective of the engine speed.

For example, as shown in Figure 3, a typical alternator operates with peak efficiency at an alternator speed of approximately 1500rpm. Beyond this, back torque acting to resist movement of the rotor increases the load on the engine, and so begins to reduce efficiency. Therefore, to operate the above described generator 10 at peak efficiency, the rotational speed of the magnetic field produced by the electromagnets 24 is set such that, for a given rotor speed, the modulated field that induces a current in the generator windings 26 rotates at around 1500rpm. The effect of this is to maximise the generator output relative to the machine input power.

Alternatively, if the priority is to maximise the output in response to high electrical demand from the vehicle, the effective alternator speed, namely the rotational speed of the modulated field, can be increased such that the generator windings 26 are producing the maximum amount of electrical power for which they are designed. As noted above, this typically entails rotating the magnetic field produced by the electromagnets 24 in the opposite sense to the rotation of the rotor 16 so as to maximise the speed of the magnetic field relative to the rotor 16.

Figure 4 shows four distinct operating regimes 40, 42, 44, 46 for the above described generator 10, each of which dictates the effective alternator speed that is required so as to achieve a balance between electrical output and generating efficiency. Each regime 40, 42, 44, 46 corresponds to a quarter of a graph of input rotor speed against vehicle electrical demand. The bottom left regime 40 of the graph corresponds to low rotor speed and low electrical demand. This may relate to a scenario in which a vehicle cruises at a steady, low speed, for example. As the electrical demand is low, the effective gear ratio of the generator 10 is set to a medium level so as to raise the effective alternator speed slightly relative to the input rotor speed, to allow the generator 10 to operate in the region of maximum efficiency.

In the bottom right regime 42 of the graph, the rotor speed is high while the energy demand remains low. In this situation, a low effective gear ratio is used so as to drop the effective alternator speed relative to the rotor speed to enable the generator 10 to operate in the most efficient region of its range while meeting the instantaneous electrical demand.

The upper left portion of the graph contains a regime 44 corresponding to low rotor speed and high electrical demand. This may, for example, correspond to the period immediately following engine ignition during which many electrical systems of the vehicle initialise. To meet the high demand, the effective gearing is increased to raise the effective alternator speed and so raise the output current from the generator windings 26. As a consequence, the efficiency of operation is necessarily lower for this operating region.

Finally, the regime 46 in the upper right portion of the graph corresponds to high rotor speed and high electrical demand. Since the rotor speed is high, a low effective gear ratio can be used to ensure that the effective alternator speed is sufficient to provide for the energy demand. Again, efficiency has to be sacrificed to some extent in this operating region in order to meet the vehicle demand.

Figure 5 illustrates the impact that the above described generator 10 is able to make on generating performance compared with a conventional alternator. As illustrated by the arrows on the line representing efficiency, the generator 10 is able to control operation so as to move towards peak efficiency, regardless of the input rotor speed. On the output current line, as indicated by the arrow, the generator 10 can adapt operation so as to produce a higher output than an equivalent conventional alternator for any given input rotor speed.

Figure 6 illustrates the generator 10 of Figures 1 and 2 in the context of use in a vehicle 50, in which the generator 10 is arranged to be driven by an engine 52 of the vehicle 50. The vehicle 50 is further provided with a sensor 54 configured to detect the speed of the engine 52, and an engine control unit (ECU) 56 arranged to process incoming data from the sensor 54 and to control the effective gear ratio of the generator 10 according to the detected engine speed and the instantaneous electrical demand. Additionally, a vehicle battery 58 is provided to supply power for all electrical systems of the vehicle 50, including the electromagnets 24 of the generator 10. A power inverter 60 is included for the purpose of converting the DC supplied by the battery 58 into AC for driving the electromagnets 24 to create a rotating magnetic field. In other embodiments, the generator 10 is configured as a self-contained unit with integral control circuitry. In these arrangements, the generator 10 includes a sensor arranged to detect the speed of the rotor 16 either directly or indirectly, and a dedicated processor arranged to process data output from the sensor and to control the effective gear ratio of the generator 10 according to the detected rotor speed and the instantaneous electrical demand. The generator also includes a power inverter to convert battery supplied DC into AC for driving the electromagnets 24, and a rectifier to convert AC induced in the windings 26 into DC to charge the battery 58.

Whether the generator 10 is self-contained or part of a distributed system within a vehicle, as an alternative to measuring rotor speed input as a control variable, the generator 10 can alternatively be controlled by implementing closed loop feedback control on the output voltage with reference to a target voltage. In any event, the effect is to modify the rotational speed of the magnetic field produced by the electromagnets 24 so as to meet the instantaneous electrical demand whilst operating in the most efficient regime possible.

The electromagnets 24 are energised using electrical power supplied by the vehicle 12V electrical power system including the battery 58, which creates the magnetic field.

The pulley section 28 of the rotor 16 is driven by a drive belt 62 attached to the vehicle engine 52. This causes the core members 32 of the rotor 16 to rotate relative to the inner and outer stators 12, 14 and so modulate the magnetic field from the electromagnets 24 to produce the modulated magnetic field. As described above, the modulated field induces a current in the generator windings 26 that can be used to charge the vehicle battery 58 and to supply power to vehicle electrical loads and systems.

The power inverter 60 is controllable by the ECU 56 such that the AC frequency can be adjusted so as to control the rotational speed of the magnetic field produced by the electromagnets 24.

It is noted that the frequency of the induced current in the generator windings 26 is indicative of the rotational speed of the rotor 16. Therefore, closed loop control of the generator 10 is possible by detecting the frequency of the current induced in the generator windings 26, and using that information to determine an appropriate frequency at which to energise the electromagnets 24 in order to adjust the effective gear ratio to provide an optimum effective alternator speed. The above described generator 10 can alternatively be used as a power inverter to provide a low-voltage supply from a high voltage battery when the vehicle engine 52 is not running. This is of particular interest for hybrid vehicles, in which a high voltage battery having an output in the range of 300V to 500V is used to provide drive to the vehicle 50. In order to provide power for the internal low voltage (nominal 12V) electrical systems of the vehicle 50, a DC/DC converter must be provided in order to reduce the high voltage of the traction battery to the low voltage that such systems run on, typically 12V.

When the vehicle engine 52 is not running the rotor 16 is not driven, and so the core members 32 are stationary. A rotating magnetic field produced by the electromagnets 24 still moves relative to the stationary core members 32, and so a modulated field will still be produced. Accordingly, the rotational speed of the magnetic field can be controlled so as to cause the modulated field to rotate at a speed which results in electrical power being supplied to the low voltage system. Therefore, the generator 10 is able to provide for the electrical demands of the internal systems of the vehicle 50, which means that a DC/DC converter and, potentially, a low- voltage battery, are not required. Conventional DC/DC converters are typically expensive and difficult to package/cool, so the use of a generator 10 of this embodiment of the invention in place of a conventional DC/DC converter provides a significant benefit.

It is noted that a generator 10 to be used as a power inverter would have significantly larger electromagnets 24 than would be required in a non-hybrid vehicle implementation. This is because the strength of the magnetic field produced by the electromagnets 24 would need to be much greater to enable transfer of a suitable amount of power to the low voltage system when the rotor 16 is stationary. It is noted that a generator 10 to be used as a power inverter would require electrical power to supply the electromagnets 24 to be sourced from the high voltage electrical system (nominal 300V) as opposed to the low voltage electrical system (nominal 12V).

Many variations and modifications of the embodiments described above are possible. For example, the inner and outer stators 12, 14 may not be tubular in shape. The outer stator 14 could have any shape, provided it has a generally cylindrical bore which is suitable to receive the rotor 16, with the electromagnets 24 being distributed in a circular configuration around the bore. The rotor 16 may carry separate core members 32 instead of the core members 32 being integral with the rotor 16.

The roles of the inner and outer stators 12, 14 could be interchanged, such that the electromagnets 24 are positioned on the inner stator 12 and the generator windings 26 are carried on the outer stator 14. This arrangement may be beneficial as the magnetic flux produced by the electromagnets 24 is relatively low, and so it makes sense to locate the electromagnets on the smaller diameter member, namely the inner stator. In such an arrangement, the modulated field would rotate at a lower speed than the rotor if the magnetic field produced by the electromagnets 24 was stationary. However, the modulated field may include more pole pairs than the magnetic field produced by the electromagnets 24, which would increase the rate of cutting of the windings 26, thereby increasing generated electrical current via electromagnetic induction.

As an alternative to a central rotor 16 that carries core members 32 between two stators, either the inner or the outer member that carries electromagnets 24 or windings 26 could act as the rotating member, with the intermediate core members 32 being stationary. However, the above described arrangement in which the electromagnets 24 and windings 26 are stationary has the benefit that there is no need to transfer an electrical current to or from a rotating member, which requires additional, expensive components such as a commutator, brushes and slip ring. The skilled reader will also appreciate that use of the generator 10 of Figures 1 and 2 is not restricted to use within a vehicle, and could be used in any suitable context, such as in a power station or in a wind turbine, for example. The generator 10 may be arranged such that rather than connecting the electromagnets 24 in series, the electromagnets 24 are powered and controlled independently. In this scenario, the electromagnets 24 may be operated individually, or alternatively the electromagnets 24 may be selectively operated in pairs, triplets and so on. In this way, the ratio of electromagnets 24 to permanent magnets may be varied, so as to vary the gear ratio. In another variant, the electromagnets 24 may be divided into groups, each group being powered independently. In either case, the electromagnets 24 may be powered according to a multi-phase system, for example a three-phase system.

Furthermore, the skilled reader will appreciate that there are several alternative methods for operating the electromagnets 24 so as to produce a rotating magnetic field. For example, in one embodiment the electromagnets 24 are divided into two groups, the first group alternating with the second group. The first group is switched on, and the second group is switched off, and then this is reversed repeatedly, to produce a rotating field in the same manner as in a conventional stepper motor.

The generator windings 26 may be arranged such that they all have the same polarity.

In another example, the generator 10 may be provided in a planar arrangement having coaxial rotating discs, with a central rotor 16 including annular gear teeth arranged to be driven by a corresponding gear wheel that is driven by an engine.

In the illustrated embodiments, the electromagnets 24 of the generator 10 are powered from the vehicle battery 58. In other embodiments, the electromagnets 24 may be powered directly from the output from the generator windings 26. A separate, dedicated controller may be provided between the generator windings 26 and the electromagnets 24 so as to adjust the frequency of the current supplied to the electromagnets 24 as required.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims.

Claims

A generator, comprising:

a first stator comprising a set of electromagnets arranged to produce a magnetic field which rotates at a rotational speed around the first stator;

a second stator comprising a set of generator windings; and

a rotor disposed between the first and second stators, the rotor comprising a set of core members and an input means arranged to receive a drive force to rotate the rotor;

wherein the core members are arranged to interact with the magnetic field produced by the set of electromagnets to produce a modulated magnetic field that induces an electrical current in the generator windings; and

wherein the rotational speed of the magnetic field produced by the set of electromagnets at least in part determines the magnitude of the electrical current induced in the generator windings.

The generator of claim 1 , wherein the rotor and the first and second stators are arranged concentrically.

The generator of claim 2, wherein the rotor is generally tubular.

The generator of claim 2 or claim 3, wherein the first stator is disposed radially outward of the rotor.

The generator of claim 2 or claim 3, wherein the second stator is disposed radially outward of the rotor.

A generator according to any preceding claim, wherein the set of electromagnets comprises a plurality of electromagnets evenly distributed around the first stator in a circular configuration.

A generator according to any preceding claim, wherein the set of generator windings comprises a plurality of generator windings evenly distributed around the second stator in a circular configuration.

8. A generator according to any preceding claim, wherein the core members are spaced from one another and are evenly distributed around the rotor in a circular configuration.

9. A generator according to any preceding claim, wherein the core members are formed integrally with the rotor in a castellated arrangement.

10. A generator according to any preceding claim, wherein the set of electromagnets is arranged such that, when energised, each electromagnet of the set has an opposite polarity to an adjacent electromagnet of the set.

1 1 . A generator according to any of claims 1 to 9, wherein the set of electromagnets is arranged to produce a multi-phase magnetic field.

12. A generator according to any preceding claim, wherein each electromagnet of the set is operable independently.

13. A generator according to any preceding claim, comprising a control means arranged to control operation of the electromagnets to control the rotational speed of the magnetic field, thereby to control the magnitude of the electrical current induced in the generator windings.

14. A generator according to claim 13, wherein the control means is arranged to provide an alternating current to the set of electromagnets such that the polarity of each electromagnet alternates, thereby to rotate the magnetic field relative to the first stator at a speed determined by the frequency of the alternating current.

15. A generator according to claim 14, wherein the control means comprises a power inverter for providing the alternating current, and wherein the power inverter is operable to control the frequency of the alternating current.

16. A generator according to any preceding claim, comprising sensing means arranged to detect a signal indicative of a rotational speed of the input means.

17. A generator according to claim 16, wherein the signal indicative of the rotational speed of the input means comprises a measurement of the frequency and/or the amplitude of the electrical current induced in the generator windings.

18. A generator according to any preceding claim, wherein the input means comprises a pulley which is integrated with the rotor.

19. A generator system comprising a generator according to any preceding claim in combination with energising means arranged to energise the set of electromagnets to produce the rotating magnetic field around the first stator.

20. A vehicle comprising the generator of any of claims 1 to 18, or the generator system of claim 19.

21 . A method of operating a generator, the generator comprising a first stator including a set of electromagnets, a second stator including a set of generator windings, and a rotor disposed between the first and second stators, the rotor including a set of core members, the method comprising:

sensing a rotational speed of the rotor; and

energising the electromagnets so as to produce a magnetic field that rotates relative to the first stator at a speed determined according to the rotational speed of the rotor such that a modulated field produced through interaction of the core members with the magnetic field induces a desired electrical current in the generator windings.

22. The method of claim 21 , comprising energising the set of electromagnets into a first state, so as to define an initial position for the magnetic field, and subsequently energising the set of electromagnets into a second state, so as to displace the magnetic field from its initial position, thereby rotating the magnetic field.

23. A method according to claim 22, wherein the first state defines a respective initial polarity for each electromagnet, and in the second state the polarity of each electromagnet is opposite to its respective initial polarity.

24. A method according to claim 22 or claim 23, comprising energising the electromagnets such that each electromagnet alternates between the first and second states to define an alternation frequency.

25. A method according to claim 24, comprising adjusting the alternation frequency so as to control the speed of rotation of the magnetic field around the electromagnets.

26. A method according to claim 24 or claim 25, comprising applying an alternating current to the electromagnets to alternate between the first and second states.

27. A method according to any of claims 21 to 26, wherein the rotor of the generator is driven by an engine of a vehicle, so that the rotational speed of the rotor is proportional to the speed of the engine.

28. A method according to claim 27, comprising sensing an electrical demand of the vehicle, and setting the desired electrical current in dependence on the electrical demand.

29. A computer program product arranged to implement the method of any of claims 21 to 28.

30. A controller arranged to implement the method of any of claims 21 to 28 or the computer program product of claim 29.

31 . A non-transitory computer-readable medium loaded with the computer program product of claim 29.

32. A generator, a vehicle or a method substantially as herein described, with reference to the accompanying figures.