GENERATOR AND METHOD FOR CONTROLLING A GENERATOR

Abstract

A switched reluctance generator and devices and methods for its control are concerned with generators and controls which can operate in an aerospace environment. The generator may have: a rotor having rotor poles; a stator having stator poles; and a controller. Either the rotor or stator poles each have windings to which current can be supplied to energise the poles and from which current can be drawn to a load; and the controller is arranged to: periodically excite each of the windings in turn to a pre-determined level of current; measure the current generated in each winding; cease the excitation when the current generated in each winding exceeds the excitation current; and direct the generated current in each winding to the load. The generator may thereby avoid the need to determine the position of the rotor poles relative to the stator poles to provide the commutation of the generator.

Claims

1. A method of controlling a switched reluctance generator, the generator including: a rotor having a plurality of rotor poles; and a stator having a plurality of stator poles, wherein: either said plurality of rotor poles or said plurality of stator poles each have windings to which current can be supplied to energise said poles and from which current can be drawn to a load, the method including the steps of: periodically exciting each of the windings in turn to a pre-determined level of current; measuring the current generated in each winding; ceasing the excitation when the current generated in each winding exceeds the excitation current; and directing the generated current in each winding to the load.

2. A method according to claim 1 wherein each of said windings is excited at a time when said windings are not generating current, or are generating current which is below a pre-determined level.

3. A method according to claim 1 wherein each winding is energised independently of the other windings.

4. A method according to claim 1 wherein the generator outputs a multiple phase output to the load and said multiple phases are supplied from different ones of said windings.

5. A method according to claim 1 further including the steps of: storing the timing of the ceasing of excitation of at least one winding in one cycle; and using said stored timing to determine the timing of exciting one or more of said windings in a subsequent cycle.

6. A method according to claim 1 further including the steps of, for each winding: storing the timing of the ceasing of excitation of the winding in one cycle; and using said stored timing to determine the timing of exciting said winding in the subsequent cycle.

7. A method according to claim 5 further including the step of regulating the power output from the generator by adjusting the timing of exciting said windings compared to the stored timings.

8. A method according to claim 1 further including the step of: regulating the output from the generator to the load by modulating the output current from the windings by adjusting either the amplitude or the timing of the excitation of the windings.

9. A method according to claim 8 further including the step of regulating the output such that there is a minimum current flowing through each winding.

10. A method according to claim 1 further including the steps of: storing energy whilst the generator is not operational; and using said stored energy to energise the windings when the generator is started.

11. A switched reluctance generator, the generator including: a rotor having a plurality of rotor poles; a stator having a plurality of stator poles; and a controller, wherein: either said plurality of rotor poles or said plurality of stator poles each have windings to which current can be supplied to energise said poles and from which current can be drawn to a load; and the controller is arranged to: periodically excite each of the windings in turn to a pre-determined level of current; measure the current generated in each winding; cease the excitation when the current generated in each winding exceeds the excitation current; and direct the generated current in each winding to the load.

12. A generator according to claim 11 wherein the controller is arranged to excite each of said windings at a time when said windings are not generating current, or are generating current which is below a pre-determined level.

13. A generator according to claim 11 wherein the controller is arranged to control each winding independently of the other windings.

14. A generator according to claim 11 wherein the generator is arranged to output a multiple phase output to the load and the controller is arranged to supply said multiple phases from different ones of said windings.

15. A generator according to claim 11 further comprising one or more comparators to determine when the current generated in each winding exceeds said predetermined threshold.

16. A generator according to claim 11 wherein the controller is further arranged to: store the timing of the ceasing of the excitation of at least one winding in one cycle; and use said stored timing to determine the timing of exciting one or more of said windings in a subsequent cycle.

17. A generator according to claim 11 wherein the controller is further arranged to, for each winding: store the timing of exciting the winding in one cycle; and use said stored timing to determine the timing of exciting said winding in the subsequent cycle.

18. A generator according to claim 16 wherein the controller is arranged to regulate the power output from the generator by adjusting the timing of exciting said windings compared to the stored timings.

19. A generator according to claim 11 wherein the controller is further arranged to: regulate the output from the generator to the load by modulating the output current from the windings by adjusting either the amplitude or the timing of the excitation of the windings.

20. A generator according to claim 19 wherein the controller is arranged to regulate the output such that there is a minimum current flowing through each winding.

21. A generator according to claim 11 further including an energy storage device which is arranged to store energy which is used to energise the windings when the generator is started.

22. An engine having a dedicated generator which is a switched reluctance generator, the generator including: a rotor having a plurality of rotor poles; a stator having a plurality of stator poles; and a controller, wherein: either said plurality of rotor poles or said plurality of stator poles each have windings to which current can be supplied to energise said poles and from which current can be drawn to a load; and the controller is arranged to: periodically excite each of the windings in turn to a pre-determined level of current; measure the current generated in each winding; cease the excitation when the current generated in each winding exceeds the excitation current; and direct the generated current in each winding to the load.

23-24. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0092] Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

[0093] FIG. 1 shows an overview of the functional elements of a dedicated power generator in an aerospace implementation and has already been described;

[0094] FIG. 2 shows a known regulator scheme for a PMA generator and has already been described;

[0095] FIG. 3 shows a switched reluctance arrangement with three sets of stator windings and two rotor flux paths and has already been described;

[0096] FIG. 4 shows the electrical equivalent circuits for the switched reluctance machine of FIG. 3 and has already been described;

[0097] FIG. 5 shows a ducted fan gas turbine engine to which a generator according to an embodiment of the invention may be attached;

[0098] FIG. 6 is a super-position of the movement of the rotor with the various electrical characteristics of the winding of coil n shown;

[0099] FIG. 7 is a functional block diagram for a drive controller for a single phase of a generator according to an embodiment of the present invention;

[0100] FIG. 8 is a schematic electrical circuit diagram of the drive controller of FIG. 7;

[0101] FIG. 9 shows the results of a simulation of the drive controller of FIGS. 7 and 8 with and without the threshold comparator operating;

[0102] FIG. 10 is a functional block diagram for a drive controller for a single phase of a generator according to a further embodiment of the present invention;

[0103] FIG. 11 is a schematic electrical circuit diagram of the drive controller of FIG. 10;

[0104] FIG. 12 shows the results of stimulation of the drive controller of FIGS. 10 and 11 with the predictive timing regulation in operation;

[0105] FIG. 13 shows the results of a simulation of the operation of a four phase generator according to a further embodiment of the present invention; and

[0106] FIG. 14 is a schematic circuit diagram of a multi-phase drive controller including average last cycle timing control.

DETAILED DESCRIPTION AND FURTHER OPTIONAL FEATURES OF THE INVENTION

[0107] With reference to FIG. 5, a ducted fan gas turbine engine according to an embodiment of the present invention is generally indicated at 10 and has a principal and rotational axis X-X, The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.

[0108] During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.

[0109] The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.

[0110] Attached to one of those shafts is an off-take for a gearbox (not shown). The gearbox drives various ancillary components, including a dedicated alternator, which may be a generator according to a further embodiment of the present invention, for example as described further below.

[0111] Whilst the operation of the SRM is relatively easy to grasp at a simplistic level the operation of the SRG can at first seem more difficult. References in published works often relate to the complexity of driving the coils to achieve the desired output and like the SRM, sensor-based DSP solutions are often mandated.

[0112] What often appears to be missing from the discussion of the SRG is a simple practical model on which to build understanding. The paper “Equivalent Circuit of Switched Reluctance Generator Based on DC Series Generator” published in the Journal of Electrical Engineering Vol 59 No 1 2008 23-28 by Martin Liptàk, Valéria Hrabovcovâ and Pavol Rafajdus of University of {hacek over (Z)}ilina, whilst still founded on the use of DSPs, provides a significant insight into the basic operation of the machine type will be summarised below.

[0113] The basic concept is that the machine can be seen as an analogue of the series wound brushed DC generator, elements of which have been in use since the inception of electrical generation.

[0114] The key feature of the series wound brushed DC generator for the purpose of understanding the SRG is that the output current flow occurs in the same direction and in the same magnetic elements needed to increase the magnetic field of the machine in a positive direction. Thus as the load current on the machine increase so does the ability of the machine to deliver more power, so a positive feedback mechanism is achieved.

[0115] Therefore in the SRG application, if we can establish a current flow in the coils, the current flow in the coils will increase in those same coils, providing a load is present. The presence and understanding of what the load represents also need to be expanded on since it is assumed that once a current flow has been established in the necessary coils the voltage generated across the coils will be a function of the flux created and the rate of change of that flux.

[0116] The greater the reluctance, the lower the flux for a given current flow in the coils; so as the rotor 105 moves the reluctance reduces and the voltage rises to a peak. If this peak is higher than the voltage across the output, a current will flow and a load current can be drawn. Clearly it is important to maintain the correct relationship between load or output voltage and coil current. Too much current delivered to the coils and the output voltage could collapse to very little and not enough voltage will be generated to power the control electronics and switches.

[0117] One practical point is that all of the above relies on a current flow in the coils to generate flux to create voltage. However, without a flux to start the machine there is no way to create the current flow so the machine will never start, in practice it is nearly impossible to de-magnetise a machine completely so there will almost certainly be some residual flux to create a voltage. However, to ensure this is the case it is common to introduce small permanent magnets in the flux path to give enough voltage to begin the process, even if this small amount of voltage needs to be boosted and stored to provide enough energy to establish the necessary flux to start the generation process.

[0118] The basic principle that can be extracted from the discussion of SRGs above is that if all the windings in the machine are excited at a low level of current there will come a point as the rotor pole moves over the stator pole where the EMF in the stator winding will be of the correct polarity to deliver current to the load. As the current is delivered to the load it will naturally exceed the value set by the PWM controller for that phase and this will cause the phase drive to switch off leaving the steering diodes to steer the winding output to the load.

[0119] FIG. 6 shows schematically the linkage between the rotor 105 spinning counter-clockwise and the inductance in the coil of winding pole n. As the rotor pole leaves pole n−1 the inductance of winding n begins to rise and an EMF is generated in winding n that opposes the flow of current in the desired direction. At this point in the cycle the PWM controller would be actively maintaining the desired current flow. This current is maintained as the pole and rotor cross each other and the inductance begins to fall at which point the voltage across the winding reverses polarity and, if a load is present, current begins to flow that causes an increase in flux and power generation as the inductance of pole n again returns to its minimum condition.

[0120] For the purposes of discussion, of the detailed embodiments below only a single phase of the SRG will be detailed but in practice the SRG can have any number of phases and a single or multiple analogue control loops would be used to achieve final closed loop voltage control. The skilled person would readily appreciate how the embodiments detailed below could be extended in this manner.

[0121] The embodiment shown in FIG. 14 moves towards this final system although final voltage loop closure is not completed. Also the excitation currents in the embodiment shown in FIG. 14 are generated using power FETs operating in linear mode, whereas in practice it may be preferable that the FETs are driven in PWM switch mode (the simpler linear solution appears to be viable, in which case use of the more complex PWM solution may not be necessary).

[0122] The basic idea behind the generators of the embodiments described below is to base a switched reluctance generator regulator on a little used feature of the SRG that it behaves like a series wound DC generator. This means that once a phase is generating the field current is naturally increased during the rotor pole transit over the stator pole winding. This produces positive feedback making the average output current greater than that established by the field excitation and the commencement of current flow can be used as a robust timing indicator for the rotor pole transition.

[0123] This means that the SRG can be used as a generator without complex timing functions and that each stator winding can be controlled completely independently resulting in the possibility of very high integrity by redundancy as well as providing greater efficiency under normal running conditions than is possible with a PMA

[0124] Embodiments of the invention can vary but will generally consist of a uni-polar switch mode current drive per phase of the machine. Such a drive might feature one or two active switches but would follow the typical arrangement for an SRM motor drive.

[0125] The phase drives can be linked together by a common demand that will set the minimum current flow in the phase. This minimum current flow can set the flux as a function of the average air gap whilst the rotor speed sets the resulting EMF. It is important to ensure that the energy consumed in the flux generation process at no point exceeds the energy delivered by the machine at any given speed. This problem might occur if the output voltage rises to high for the combination of flux and speed to accommodate so current does not flow to output or if the losses in the current drive exceed the power available from the machine.

[0126] How this control is managed will depend on implementation; it may be done centrally by monitoring rise and fall of output voltage or it may be more advantageous to average the input and output current flows and reduce the demand to match the available power.

[0127] FIG. 7 shows a functional block diagram of a very basic phase drive according to an embodiment of the present invention which incorporates a threshold comparator 201 that provides the “self commutation” of the generator. Without this threshold comparator 201, the excitation drive is active whilst the SRG is delivering power and losses are higher than is necessary.

[0128] FIG. 8 shows a schematic circuit diagram of the phase drive of FIG. 7. By simulating the design shown in FIG. 8 with and without the threshold comparator 201 active the effect of this component can be seen.

[0129] FIG. 9 shows the results; although the generated phase current is very similar in each case the power lost in the excitation is much higher, peaking at 45 W if the “self commutation” feature is not active.

[0130] The basic self commutating design of this embodiment can be refined by further suppressing the delivery of excess current by timing its application relative to the “self commutation”. This refinement results in a further reduction in loss power from 660 mW to 200 mW.

[0131] The ability to perform predictive timing of excitation current, based on the “self commutation” feature opens up the possibility for further refinement namely the possibility of output current control by excitation timing.

[0132] The essence of the timing control is that it uses the “self commutation” timing from the previous SRG phase output cycle to predict the optimal point at which to apply the excitation voltage for the next phase power output cycle. If it is used in this manner it simply minimizes the power lost in the drive stage. In the present embodiment this timing data is generated by capacitors and stored from cycle to cycle on capacitors but it would be possible for this to be in digital form if the reliability of such digital circuits was considered adequate.

[0133] FIG. 10 shows a functional block diagram for a single phase drive of an SRG according to a further embodiment of the present invention in which a predictive timing control 202 is added which works in parallel with the threshold comparator to control the “self commutation” of the generator. FIG. 11 shows a schematic circuit diagram of the single phase drive with predictive timing control.

[0134] Once the predictive timing function is present it is possible, using the control input “Demand”, shown in FIG. 11 to modulate the output current from the SRG. The result is that the “Self Commutation” feature of the SRG of this embodiment can now be used both to minimize power loss in the driver and to provide first order regulation of the SRG phase output current. This effect is shown in FIG. 12.

[0135] Whilst is it possible for an SRG to only have a single phase, multiple phase outputs are more normal; to configure a phase drive with predictive timing for such an application, one possibility is to simply treat each phase individually with a common demand input based on the state of the output voltage. However, in practice the balance of the machine output currents can be improved by using common “last cycle” timing data to produce an average value for the machine. This configuration is shown in FIG. 14 which shows a SRG according to an embodiment of the present invention having 4 stator poles 306 which have coils attached to separate control devices 307 and 6 rotor poles 305. Each phase drive controller X2 to X5 is an instantiation of the design shown in FIG. 11 but running from a common supply (generator 301) into a single load 302. The SRG is created from a set of independent phases with time shifting to represent the relative position of a six position rotor. The control devices 307 share a common “Demand” input and a common timing reference (“T_Ref”).

[0136] Simulation results for the four phase SRG of FIG. 14 are shown in FIG. 13, where the predictive timing element is modified by the demand voltage over a range that first improves the power lost to excitation and then reduces the peak output current of the phases, resulting in a controlled reduction in the average current flow in the load Id(M1).

[0137] While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.

[0138] All references referred to above are hereby incorporated by reference.