GB2570704A - An electric generator for use with a jet engine - Google Patents

An electric generator for use with a jet engine Download PDF

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Publication number
GB2570704A
GB2570704A GB1801816.8A GB201801816A GB2570704A GB 2570704 A GB2570704 A GB 2570704A GB 201801816 A GB201801816 A GB 201801816A GB 2570704 A GB2570704 A GB 2570704A
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GB
United Kingdom
Prior art keywords
rotor
electric generator
state
windings
magnetic poles
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Application number
GB1801816.8A
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GB201801816D0 (en
Inventor
David Flower Paul
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Safran Electrical and Power SAS
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Safran Electrical and Power SAS
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Publication date
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Priority to GB1801816.8A priority Critical patent/GB2570704A/en
Publication of GB201801816D0 publication Critical patent/GB201801816D0/en
Publication of GB2570704A publication Critical patent/GB2570704A/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
    • H02P9/48—Arrangements for obtaining a constant output value at varying speed of the generator, e.g. on vehicle
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02K—DYNAMO-ELECTRIC MACHINES
    • H02K7/00—Arrangements for handling mechanical energy structurally associated with dynamo-electric machines, e.g. structural association with mechanical driving motors or auxiliary dynamo-electric machines
    • H02K7/18—Structural association of electric generators with mechanical driving motors, e.g. with turbines
    • H02K7/1807—Rotary generators
    • H02K7/1823—Rotary generators structurally associated with turbines or similar engines
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02K—DYNAMO-ELECTRIC MACHINES
    • H02K19/00—Synchronous motors or generators
    • H02K19/16—Synchronous generators
    • H02K19/26—Synchronous generators characterised by the arrangement of exciting windings
    • H02K19/32—Synchronous generators characterised by the arrangement of exciting windings for pole-changing
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P25/00—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details
    • H02P25/16—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details characterised by the circuit arrangement or by the kind of wiring
    • H02P25/18—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details characterised by the circuit arrangement or by the kind of wiring with arrangements for switching the windings, e.g. with mechanical switches or relays
    • H02P25/20—Arrangements or methods for the control of AC motors characterised by the kind of AC motor or by structural details characterised by the circuit arrangement or by the kind of wiring with arrangements for switching the windings, e.g. with mechanical switches or relays for pole-changing
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02K—DYNAMO-ELECTRIC MACHINES
    • H02K2213/00—Specific aspects, not otherwise provided for and not covered by codes H02K2201/00 - H02K2211/00
    • H02K2213/09—Machines characterised by the presence of elements which are subject to variation, e.g. adjustable bearings, reconfigurable windings, variable pitch ventilators
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P2101/00—Special adaptation of control arrangements for generators
    • H02P2101/30—Special adaptation of control arrangements for generators for aircraft

Abstract

An electric generator for generating AC electrical power having a frequency within an operational frequency range of an aircraft electrical system. A rotor of the generator is mechanically coupled to a low pressure (LP) shaft of an aircraft jet engine. The generator operates: in a first state, with a first number of magnetic poles, when the speed of the LP shaft is within a first speed range; and in a second state, with a second number of magnetic poles, when the speed of the LP shaft is within a second speed range. The first number of poles is different to the second number of poles. A switching module may be coupled to the rotor windings to switch the generator between the first and second states. The rotor windings may comprise first and second sets of windings 410, 420. A rotor winding current passing through the first set of windings 410 establishes the first number of poles, and the current passing through the second set of windings 420 establishes the second number of poles. Alternatively, a single set of rotor windings may be used wherein, a first current path through the windings establishes the first number of poles, while a second path through the windings establishes the second number of poles.

Description

The present disclosure relates to an electric generator and method for generating AC electrical power with a frequency within an operational frequency range of an electrical system of an aircraft.
Background
Electric generators on aircraft often take their input mechanical power from the aircraft jet engines. Typically, the rotor of the electric generator is mechanically driven by the High Pressure (HP) shaft of a jet engine, which is the shaft that turns the highpressure compressor and the high pressure turbine of the jet engine.
However, drawing mechanical power from the HP shaft may have a number of undesirable consequences. In particular, the turbomachinery associated with the HP shaft must be redesigned in a manner that makes it substantially less efficient than it would otherwise be. For example, the redesign tends to lower the bypass ratio of a turbofan jet engine (the ratio between the mass flow rate of air through the bypass stream of the turbofan jet engine to the mass flow rate of the air entering the core of the turbofan jet engine), which reduces the propulsive efficiency of the turbofan jet engine.
Summary
In a first aspect of the present disclosure, there is provided an electric generator for generating AC electrical power with a frequency within an operational frequency range of an electrical system of an aircraft, the electric generator comprising: a rotor, wherein the electric generator is configured for the rotor to be mechanically coupled to a low pressure shaft of a jet engine of the aircraft, wherein the electric generator is configured to operate in a first state when the speed of the low pressure shaft is within a first speed range and to operate in a second state when the speed of the low pressure shaft is within a second speed range, and wherein in the first state the electric generator operates with a first number of magnetic poles and in the second state the electric generator operates with a second number of magnetic poles, wherein the first number of magnetic poles is different to the second number of magnetic poles.
Preferably, the rotor comprises rotor windings for establishing a rotor magnetic field.
The rotor windings may comprise a first set of rotor windings and a second set of rotor windings, wherein the rotor is configured such that: in the first state, a rotor winding current passes through the first set of rotor windings to establish the first number of magnetic poles, and in the second state, the rotor winding current passes through the second set of rotor windings to establish the second number of magnetic poles.
Alternatively, the rotor windings may comprise a single set of rotor windings, wherein the rotor is configured such that: in the first state a rotor winding current follows a first current path through the single set of rotor windings to establish the first number of magnetic poles, and in the second state the rotor winding current follows a second current path through the single set of rotor windings to establish the second number of magnetic poles.
The electric generator may further comprise first exciter windings for establishing a first exciter magnetic field; and second exciter windings for establishing a second exciter magnetic field; wherein the rotor further comprises: a first exciter armature corresponding to the first exciter windings, wherein the first exciter armature is coupled to the rotor windings such that the electric generator operates in the first state when the first exciter magnetic field induces a rotor current in the first exciter armature; and a second exciter armature corresponding to the second exciter windings, wherein the second exciter armature is coupled to the rotor windings such that the electric generator operates in the second state when the second exciter magnetic field induces a rotor current in the second exciter armature.
The rotor may further comprise a switching module coupled to the rotor windings to switch the electric generator between the first state and second state. The switching module may be controllable wirelessly from an off-rotor location.
The electric generator may further comprise a controller configured to switch the electric generator between the first state and the second state to maintain the frequency of generated AC electrical power within the operational frequency range of the electrical system of the aircraft. The controller may be configured to switch the electric generator between the first state and the second state based at least in part on at least one of the following: a measured rotational speed of the low pressure shaft of the jet engine; a measured rotational speed of the rotor and/or a measured frequency of generated electrical power.
A lower limit of the first speed range may be less than a lower limit of the second speed range and/or an upper limit of the second speed range may be greater than an upper limit of the second speed range, wherein the first number of magnetic poles is greater than the second number of magnetic poles.
The electric generator may be configured for the rotor to be mechanically coupled to the low pressure shaft of the jet engine with a fixed speed ratio.
The electric generator may be configured for the rotor to be directly mechanically coupled to the low pressure shaft of the jet engine.
The electric generator may further comprise a fixed ratio speed converter for mechanically coupling the rotor to the low pressure shaft of the jet engine. The fixed ratio speed converter may be a gearbox.
The first speed range may partially overlap the second speed range. Alternatively, the first speed range may be non-contiguous with the second speed range.
In a second aspect of the present disclosure, there is provided a jet engine comprising a low pressure shaft; and the electric generator of the first aspect, wherein the rotor of the electric generator is mechanically coupled to the low pressure shaft such that the rotor is mechanically driven by the low pressure shaft.
In a third aspect of the present disclosure, there is provided an aircraft comprising the jet engine of the second aspect.
In a third aspect of the present disclosure, there is provided a method for generating AC electrical power with a frequency within an operational frequency range of an electrical system of an aircraft using an electric generator, the method comprising mechanically driving a rotor of the electric generator using a low pressure shaft of a jet engine of the aircraft; operating the electric generator in a first state when the speed of the low pressure shaft is within a first speed range; and operating the electric generator in a second state when the speed of the low pressure shaft is within a second speed range, wherein in the first state, the electric generator operates with a first number of magnetic poles, and wherein in the second state, the electric generator operates with a second number of magnetic poles, and wherein the first number of magnetic poles is different to the second number of magnetic poles.
Drawings
Aspects of the present disclosure are described, by way of example only, with reference to the following drawings, in which:
Figure 1 shows an example representation of a turbofan jet engine and electric generator in accordance with an aspect of the present disclosure;
Figure 2 shows a schematic representation of a first mechanical coupling of the electric generator to the turbofan jet engine of Figure 1;
Figure 3 shows a schematic representation of a second example of the electric generator to the turbofan jet engine of Figure 1;
Figures 4A and 4B show schematic representations of a first example rotor winding of the electric generator of Figure 1;
Figures 5A and 5B show schematic representations of a second example rotor winding of the electric generator of Figure 1;
Figure 6 shows a schematic representation of a first example of the electric generator of Figure 1; and
Figure 7 shows a schematic representation of a second example of the electric generator of Figure 1.
The drawings are not drawn to scale. Like reference numerals are used throughout the drawings to denote the same features.
Detailed Description
The Low Pressure (LP) shaft of a jet engine is the shaft that drives the low pressure compressor and the low pressure turbine of the jet engine. The inventors have recognised that the LP shaft is designed to provide substantial amounts of mechanical power (for example, in a turbofan jet engine, the LP shaft also drives the fan of the jet engine and provides the source of propulsive power to the aircraft, which requires substantial amounts of mechanical power). Consequently, if an electric generator were to take its mechanical power from the LP shaft, the design of the jet engine would not be compromised to the extent that it is compromised when the electric generator mechanical power is drawn from the HP shaft. As a result, the aircraft propulsion system as a whole could be made substantially more efficient.
However, there are problems associated with driving an electric generator from the LP shaft, which have resulted in a technical prejudice against using the LP shaft. The most severe of these problems is the mismatch between electrical frequency requirements of aircraft electrical systems and the rotational speed range of the LP and HP shafts. Historically, aircraft have required a fixed electrical frequency, usually
400Hz. More recent aircraft allow for a range of electrical frequencies, for example between about 380Hz - 850Hz (although the particular operational frequency range of aircraft electrical systems will vary between different aircraft models and manufacturers). However, the wide speed range of LP shafts can make it very difficult to operate within even the more flexible electrical frequency ranges of modern aircraft, which is the main reason why the LP shaft has not been used for electrical power generation.
The frequency of electrical power generated by an AC electric generator is proportional to the rotational frequency of the rotor of the electric generator, which is mechanically driven by the jet engine. The HP shaft of jet engines typically operate in a rotational speed range of about 2:1 between the fastest and slowest speeds (for example between about 10,000 to 20,000 revolutions per minute (rpm), although the particular rotational speeds will vary between different jet engine models and manufactures). Since the operational frequency range of electrical systems in modern aircraft is typically slightly wider than the speed range of the HP shaft, an electric generator that is mechanically driven by the HP shaft can generate electrical power within the operational frequency range, across the entire speed range of the HP shaft.
In contrast, the LP shaft of jet engines typically operates in a rotational speed range of about 5:1 between the fastest and slowest speeds (for example, 800 to 4,000 rpm, or 685 to 3540 rpm, etc although the particular rotational speeds will vary between different jet engine models and manufactures). Since the rotational speed range of the LP shaft is larger than the operational frequency range, if the electric generator is to be mechanically driven by the LP shaft, it has traditionally been required to mechanically reduce the rotational speed range using a variable speed ratio gearbox (for example, a fixed speed output gearbox), or modify the frequency of the generated electrical power to be compatible with the aircraft electrical systems.
US6,467,725 describes an arrangement where an electric generator is driven by an LP shaft of a jet engine. In order to address the LP shaft speed/electrical frequency issues described above, the generated AC electrical power is rectified to DC electrical power, which is then converted to AC electrical power using power electronics (such as an inverter) to produce AC electrical power at the frequency required by the aircraft electrical system. This is required because the rotational speed range of the LP shaft is too wide for the AC output of the generator to be fed directly to the aircraft electrical systems.
However, power electronics such as rectifiers and inverters tend to be financially costly to purchase and maintain, and also relatively heavy (often more expensive and heavier than the electrical generator itself), resulting in a weight (and therefore, fuel efficiency) penalty for the aircraft. Furthermore, there are electrical efficiency loses for each conversion (from AC to DC and then from DC to AC), which adversely affect the fuel efficiency of the aircraft.
In a further alternative, a variable speed ratio gearbox, driven by the LP shaft, may be used to rotate the rotor of an AC electric generator at a constant speed regardless of the LP shaft rotational speed, in order to generate AC electrical power at a frequency within the operational frequency range of the aircraft electrical system. However, such mechanical gearboxes are expensive to purchase and maintain. They are also heavy, resulting in a weight (and, therefore, fuel efficiency) penalty for the aircraft. Furthermore, they tend to be quite large, which may require the size of the jet engine nacelle to be increased, thereby increasing the aerodynamic drag of the engine and decreasing fuel efficiency.
Consequently, because of the cost and weight drawbacks associated with driving electric generators from the LP shaft, there has previously been a technical prejudice against using the LP shaft. Instead, manufacturers have typically driven electric generators using the HP shaft, on the understanding that the required jet engine design compromises are outweighed by the cost and weight benefits for the electric generator.
However, the inventors have realised that the benefits to the design of jet engines that can be achieved by driving the electric generator using the LP shaft rather than the HP shaft may be greater than previously appreciated due to changes in jet engine and aircraft design. For example, as the electric power needs of aircraft have increased, the size of electric generators relative to the size of the jet engines has increased, which means that the benefits of using the LP shaft to drive the electric generator may be greater than previously appreciated. This, coupled with the recognition that aircraft weight minimisation is gradually becoming a less significant factor in aircraft design as aircrafts become ever more efficient, has led them to reconsider the use of the LP shaft in driving electric generators, contrary to the prevalent technical prejudice against the use of the LP shaft.
Upon reconsidering this problem, the inventors unexpectedly recognised that since a number of modern aircraft electrical systems now accept a range of different electrical frequencies, rather than the fixed frequency that was historically required, there may now be increased flexibility for a generator that is driven by the LP shaft. By recognising this increase in flexibility, the inventors have identified that by changing the number of rotor magnetic poles in the generator during operation of the generator, the frequency of generated electrical power may be kept within the operational frequency range (for example, 380-850Hz) of the electrical system of an aircraft. This is because the frequency of generated electrical power is proportional to the rotational speed of the rotor and the number of magnetic poles of the electric generator. For example, in a synchronous generator, the relationship between speed and frequency is:
NP ~ 120 where f is the frequency of generated electrical power in Hz, N is the rotational speed of the rotor in RPM and P is the number of magnetic poles. Therefore, it can be seen that if N remains constant, reducing the number of magnetic poles from 4 to 2 (for example), will halve the frequency of the generated electrical power. Likewise, if the rotational speed of the rotor N were to reduce by half, the frequency of generated electrical power may be kept constant by doubling the number of magnetic poles.
However, there may be some drawbacks to varying the number of magnetic poles in the electric generator. For example, the weight of the electrical generator may be greater to accommodate a variable number of magnetic poles. Consequently, it would go against the natural intuition of a person skilled in the technical field of electrical power generation to vary the number of magnetic rotor poles.
Nevertheless, by virtue of an extensive and holistic understanding of both jet engine design and optimisation, and electric generator design and optimisation, the inventors have unexpectedly realised that by driving an electric generator with a variable number of magnetic poles using the LP shaft of a jet engine, the mechanical and propulsive gains in the turbofan jet design may outweigh any weight increase that results from using variable pole numbers. This is achieved whilst still keeping the frequency of the generated power within the operational frequency range of the electrical system of the aircraft and avoiding the cost and weight penalties associated with power electronic rectifiers and inventers, or variable speed ratio gearboxes.
As such, the inventors have developed an electric generator configured for the rotor to be mechanically coupled to the LP shaft of a turbofan jet engine, wherein the number of magnetic poles of the electric generator is variable such that the electric generator can operate with a first number of magnetic poles when the LP shaft has a rotational speed within a first speed range and a second number of magnetic poles when the LP shaft has a rotational speed within a second speed range. Consequently, the jet engine design may be better optimised (for example, in a turbofan jet engine, the bypass ratio may be increased, thereby increasing the efficiency of the engine) and the generated electrical power kept within the operational frequency range of the aircraft electrical systems without incurring the cost and weight penalties associated with power electronic rectifiers and inventers, or constant speed output gearboxes. Furthermore, reliability may also be improved, as a variable pole electric generator should be considerably more reliable than power electronics or constant speed output gearboxes. As a result, an overall improvement in aircraft efficiency and reliability may be achieved.
In a further benefit, in the event of an engine failure such as a flame out, the fan of a turbofan jet engine will continue to rotate under a wind milling effect, meaning that the electric generator may continue to generate electricity in an emergency situation. Consequently, it may be possible to omit auxiliary emergency generators, such as a ram air turbine, from the aircraft design, thereby even further improving cost and weight benefits for the overall aircraft design.
Figure 1 shows an example representation of a turbofan jet engine 100. The turbofan jet engine 100 comprises an LP shaft 110, on which is mounted a fan 112, a low pressure compressor 114 and a low pressure turbine 116. The turbofan jet engine 100 further comprises an HP shaft 120, on which is mounted a high pressure compressor 122 and a high pressure turbine 124. The configuration and operation of these components of the turbofan jet engine 100 are well understood and documented in the art and, consequently, shall not be explained in detail in the present disclosure.
Mechanically coupled to the LP shaft 110 is an electric generator 130 in accordance with an aspect of the present disclosure. The mechanical coupling is such that as the LP shaft 110 rotates, a rotor of the electric generator 130 rotates, thereby causing electrical power to be generated, which can then be supplied to the aircraft electrical systems. The generated power is represented in Figure 1 by the generated voltage Vg.
The mechanical coupling between the LP shaft 110 and the electric generator 130 may be any form of fixed speed ratio coupling.
Figure 2 shows a schematic representation of one example fixed speed ratio coupling, whereby a rotor 132 of the electric generator 130 is directly mechanically coupled to the LP shaft 110, such that the rotor 132 has the same rotational speed as the LP shaft 110. Also represented in Figure 2 is a stator winding 134, where the generated electrical power may be induced by virtue of the rotation of the rotor magnetic field within the stator winding 134.
Figure 3 shows a schematic representation of an alternative example fixed speed ratio coupling, where the electric generator 130 comprises a fixed speed ratio converter 300, such as a gearbox or transmission, coupling the LP shaft 110 to the rotor 134. The fixed ratio speed converter 300 is configured such that the ratio between the LP shaft 110 rotational speed and the rotor 132 rotational speed is fixed, so a change in the LP shaft 110 rotational speed will cause a change in the rotational speed of the rotor 132. It will be appreciated that whilst the fixed ratio speed converter 300 may come at some cost and weight penalty, the cost and weight penalty is likely to be considerably less than for a more complex constant speed output gearbox (for example, a fixed ratio speed converter typically weighs in the region of 5kg, whereas constant speed output gearboxes typically weigh in the region of 80kg).
It will be appreciated that the type of mechanical coupling used may be chosen based on the typical operating speeds of the LP shaft 110, the number of magnetic poles that the electric generator 130 is configured to have and the desired range of frequencies of the generated electrical power.
The rotor 132 of the electric generator 130 according with the present disclosure may have rotor windings that establish a rotor magnetic field when a rotor winding current passes through them. The rotor windings may be configured in any suitable way to be able to establish a rotor magnetic field with a number of magnetic poles that may be varied during operation of the electric generator 130. For example, the rotor windings may comprise a plurality of different sets of rotor windings, each different set being configured to establish a different number of magnetic poles when a rotor winding current passes through them.
Figures 4A and 4B show an example cross-sectional schematic representation of the rotor 132 comprising a rotor shaft 405, a first set of rotor windings 410 and a second set of rotor windings 420. In Figure 4A, the first set of rotor windings are highlighted, with magnetic poles of a first type (for example, north) identified by single line shading and magnetic poles of a second type (for example, south) identified by cross-hatched shading. It can be seen that the first set of rotor windings 410 in this example comprises four magnetic poles (two pole pairs). In Figure 4B, the second set of rotor windings are highlighted, again with the magnetic poles of the first type (for example, north) identified by single line shading and magnetic poles of the second type (for example, south) identified by cross-hatched shading. It can be seen that the second set of rotor windings 420 in this example comprises eight magnetic poles (four pole pairs). Therefore, by passing a current through either the first set of rotor windings 410 or the second set of rotor windings 420, the number of magnetic poles of the electric generator may be changed. Different examples of how the rotor winding current may be controlled to pass through the first set of rotor windings 410 or the second set of rotor windings 420 are explained later.
It will be appreciated that this is one non-limiting example of a plurality of sets of rotor windings and that each set of rotor windings may be configured to establish any number of magnetic poles. Furthermore, in an alternative, the rotor windings may comprise more than two sets of rotor windings, for example three sets of rotor windings or four sets of rotor windings, each set being configured to establish a different number of magnetic poles. Thus, the plurality of sets of rotor windings may be configured to enable the electric generator 130 to operate in two or more different states, where in each state a different number of magnetic poles are established by the rotor windings
Figures 5A and 5B show an alternative example cross-sectional schematic representation of the rotor 132 comprising a rotor shaft 505 and a single set of rotor windings 510. In Figure 5A, a rotor winding current is directed to follow a first current path through the rotor windings 510 in order to establish a first number of magnetic poles (in this example, four magnetic poles, or two magnetic pole pairs). In Figure 5B, the rotor winding current is directed to follow a second, different, current path through the rotor windings 510 in order to establish a second number of magnetic poles (in this example, two magnetic poles, or a single magnetic pole pair). Different examples of how the rotor winding current may be controlled to take different paths through the rotor windings 510 are explained later.
It will be appreciated that this is one non-limiting example of a single set of rotor windings that are configured to be able to establish a variable number of magnetic poles. The rotor windings 510 may be configured to establish any number of magnetic poles in each state and/or may be configured to be controllable to operate in more than two different states (for example, generate a first number of magnetic poles, or a second number of magnetic poles, or a third number of magnetic poles, depending on the path of the rotor current through the rotor windings 510).
Thus, it will be appreciated that the rotor windings 510 may be configured in a number of different ways to enable the electric generator 130 to operate in two or more different states, where in each state a different number of magnetic poles are established by the rotor windings.
Figure 6 shows an example schematic cross-sectional representation of an electric generator 600 in accordance with an aspect of the present disclosure. The electric generator 600 comprises a rotor shaft 610, on which is mounted rotor windings 612, a first exciter armature 614 and a second exciter armature 616. The first exciter armature 614 and the second exciter armature 616 are each electrically coupled to the rotor windings 612 via electrical couplings 615 and 617 respectively. The mechanical fixings holding the rotor windings 612 and the armatures 614 and 616 to the rotor shaft 610 are not represented for the sake of simplicity.
Stator generator windings 622 correspond with the rotor generator windings 612. First exciter windings 624 for establishing a first exciter magnetic field correspond with the first exciter armature 614. Second exciter windings 626 for establishing a second exciter magnetic field correspond with the second exciter armature 616. A controller 630 may be configured to control which of the stator exciter windings 624 or 626 is turned on.
As the rotor shaft 610 is driven by the LP shaft to rotate about its longitudinal axis, the armatures 614 and 616 will rotate within the exciter windings 624 and 626. Therefore, when the first exciter winding 624 is turned on by the controller 630 to establish a first exciter magnetic field, a rotor current is induced in the first armature 614. When the second exciter winding 626 is turned on by the controller 630 to establish a second exciter magnetic field, a rotor current is induced in the second armature 616. The first armature 614 may be coupled to the rotor windings 612 in such a way that when a rotor current is induced in the first armature 614, the rotor current causes the rotor windings 612 to establish a rotor magnetic field with a first number of magnetic poles. Likewise, the second armature 616 may be coupled to the rotor windings 612 in such a way that when a rotor current is induced in the second armature 616, the rotor current causes the rotor windings 612 to establish a rotor magnetic field with a second number of magnetic poles. For example, when the rotor windings 612 comprise two sets of rotor windings, as described above with reference to Figures 4A and 4B, the first armature 614 may be coupled to the first set of rotor windings and the second armature 616 may be coupled to the second set of rotor windings. In an alternative, when the rotor windings 612 comprise only a single set of rotor windings, such as that described above with reference to Figures 5A and 5B, the first armature 614 may be coupled to the rotor windings 612 in such a way that the rotor current follows a first path through the rotor windings 612 to establish a rotor magnetic field having a first number of magnetic poles and the second armature 616 may be coupled to the rotor windings 612 in such a way that the rotor current follows a second path through the rotor windings 612 to establish a rotor magnetic field having a second number of magnetic poles.
In some implementations, the controller 630 may be configured to turn on only one of the exciter windings 624 or 626 at a time. In an alternative, the rotor windings 612 may be configured such that the controller 630 can turn on one or more of the exciter windings 624, 626 at any one time. In this case, the rotor magnetic field may have a first number of magnetic poles when only the first exciter windings 624 are turned on, a second number of magnetic poles when only the second exciter windings 626 are turned on and a third number of magnetic poles when both the first and second exciter windings 624 and 626 are turned on.
The controller 630 is configured to operate the electric generator 600 in a first state where the electric generator has a first number of poles when the speed of the LP shaft is within a first speed range and to operate the electric generator 600 in a second state where the electric generator has a second number of poles when the speed of the LP shaft is within a second speed range. For example, the first speed range could be the lower half of the expected operational speeds of the LP shaft (such as 800-2650 rpm, for example) and the second speed range could be the upper half of the expected operational speed of the LP shaft (such as 2650-4500rpm, for example). In this case, the first number of poles will be greater than the second number of poles, in order to reduce the frequency of the generated electrical power when the LP shaft is rotating at higher speeds. The controller 600 may be configured to switch between the different states based on a parameter indicative of the rotational speed of the LP shaft, such as at least one of a measurement of the rotational speed of the LP shaft and/or a measurement of the rotational speed of the rotor (which is indicative of the rotational speed of the LP shaft) and/or a measurement of the frequency of the generated electrical power (which is indicative of the rotational speed of the LP shaft).
In this example, the first and second speed ranges are non-overlapping. However, in an alternative, they may partially overlap (for example the first speed range may be 800-3000 rpm and the second speed range may be 2400 - 4500 rpm). In this way, a hysteresis effect may be achieved whereby the electric generator 600 is controlled to stay in its current state until the rotational speed of the LP shaft exits a speed range and enters the other speed range, thereby avoiding the state being switched regularly when the LP shaft speed is fluctuating around the switch over point between the first state and the second state.
In a further example, the two speed ranges may be non-contiguous. For example, the first speed range may be between 685 - 1050 rpm for typical ground settings (such as aircraft taxiing) and the second speed range may be between 1660 - 3340 rpm for flight settings. As such, the electrical generator may operate in the first state when the aircraft is on the ground and the second state during flight.
Additionally or alternatively, the speed ranges may be of unequal size (for example, the second speed range may be larger than the first speed range). In this way, the optimal number of magnetic poles may be associated with the larger speed range such that the electric generator should operate in the most efficient state for more of the time. Thus, the first and second speed ranges may be set in order to optimise the generation efficiency of the electric generator 600.
Figure 7 shows a further example schematic cross-sectional representation of an electric generator 700 in accordance with an aspect of the present disclosure. The electric generator 700 comprises a rotor shaft 710, on which is mounted rotor windings 712 and an exciter armature 714. The exciter armature 714 is electrically coupled to the rotor windings 712 via an electrical coupling 715 and a switching module 740. The mechanical fixings holding the rotor windings 712 and the exciter armature to the rotor shaft 710 are not represented for the sake of simplicity.
Stator generator windings 722 correspond with the rotor generator windings 712. The exciter windings 724 for establishing an exciter magnetic field correspond with the exciter armature 714. The operation of the exciter windings 724 and the exciter armature 714 for generating a rotor current to pass through the rotor windings 712 will be well understood by the skilled person.
In this arrangement, the switching module 740 is configured to switch the electric generator 700 between a first state and a second state (or alternatively between any of three or more states), wherein the rotor magnetic field has a different number of magnetic poles in each state. The way in which the switching circuit 740 may do this depends on the configuration of the rotor windings 712. For example, when the rotor windings 712 comprise two sets of rotor windings, as described above with reference to Figures 4A and 4B, the switching module 740 may straightforwardly switch the coupling of the exciter armature 714 between the first set of rotor windings and the second set of rotor windings. In an alternative, when the rotor windings 712 comprise only a single set of rotor windings, such as that described above with reference to Figures 5A and 5B, the switching module 740 may be configured to alter the way in which the exciter armature 714 is coupled to the rotor windings 712 so as to change the path of the rotor current through the rotor windings 712 to change the number of poles in the rotor magnetic field. The switching module 740 may comprise any suitable number of switches and the switches may be of any suitable type, for example mechanical switches or solid-state switches such as transistor or thyristor switches.
The switching module 740 may be wirelessly controlled by the controller 730 which is in an off-rotor location, for example using any suitable wireless communications protocol/standard/architecture, such as an RF communications link, WiFi, Bluetooth, etc. In an alternative, the switching module 740 may be configured to be controlled by the controller 730 in any other suitable way, for example the controller 730 may be located on the rotor shaft 710 and therefore be electrically coupled to the switching module 740, or the controller 730 and switching module 740 may be coupled using a brushes and slip ring arrangement, etc. The controller 730 may be configured to control the switching between states in the same way as described above with reference to controller 630.
Whilst the electric machine 700 comprises an exciter winding 724 and an armature 714, it will be appreciated that the rotor current may be supplied to the rotor winding 712 in any other suitable way, for example using an off-rotor power source and brushes and sliprings, etc.
By way of non-limiting example, some LP shaft speed ranges and magnetic pole numbers are given below:
LP shaft speed range (rpm) Rotor frequency (Hz) Electrical output (Hz) No. of magnetic poles
First state 685-1660 72-175 360-872 10
Second state 2901-3540 305-372 610-744 4
In this example, the electric generator comprises a fixed speed ratio converter with a ratio of 6.31:1. The first speed range of the LP shaft corresponds to modes such as taxiing, high idle and hold, whereas the second speed range of the LP shaft corresponds to modes such as cruise, climb and takeoff.
It will be appreciated that various other LP shaft speed range, rotor frequencies and magnetic pole numbers may be used, depending on the design and operation of the jet engine and aircraft electrical system.
The skilled person will readily appreciate that various alterations or modifications may be made to the above described aspects of the disclosure without departing from the scope of the disclosure.
For example, a switching module may be used with the electric machine 600 represented in Figure 6. In this example, the outputs of the first and second armatures 614 and 616 may control the switching module to change the number of poles in the rotor magnetic field.
Whilst some specific LP shaft operational speed ranges have been identified above, it will be appreciated that the electric generator of the present disclosure may be driven by an LP shaft of a jet engine that is designed to operate across other speed ranges. Likewise, whilst some specific operational frequency ranges of the electrical system of the aircraft have been identified above, it will be appreciated that the electric generator according to the present disclosure may be configured to generate electrical power for any other operational frequency ranges.
Whilst the above disclosure focuses particularly on synchronous generators, the disclosure is not so limited and the electric generator may be any other type of AC electric generator, such as an asynchronous generator. Furthermore, the electric generator may be configured to generate electrical power with any number of phases, for example single phase, two phase, three phase, etc.
Furthermore, whilst the above disclosure focusses on wound rotor electric generators, the electric generator according to the present disclosure may have other types of rotor design. For example, the rotor may be a squirrel cage rotor where the number of 5 magnetic poles of the electric generator may be changed by changing the number of stator magnetic poles, which in turn changes the number of rotor magnetic poles.
Furthermore, whilst the above disclosure focusses particularly on turbofan jet engines, the electric generator may be coupled to any other type of jet engine that comprises 10 an LP shaft, such as a turboprop jet engines or open-rotor jet engines.
It will be appreciated that the example electric generators represented in Figures 6 and may comprise any number of additional components, such as rotor rectification diodes, etc, which are not included in the Figures for the sake of clarity.

Claims (15)

1. An electric generator for generating AC electrical power with a frequency within an operational frequency range of an electrical system of an aircraft, the electric generator comprising:
a rotor, wherein the electric generator is configured for the rotor to be mechanically coupled to a low pressure shaft of a jet engine of the aircraft, wherein the electric generator is configured to operate in a first state when the speed of the low pressure shaft is within a first speed range and to operate in a second state when the speed of the low pressure shaft is within a second speed range, and wherein in the first state the electric generator operates with a first number of magnetic poles and in the second state the electric generator operates with a second number of magnetic poles, wherein the first number of magnetic poles is different to the second number of magnetic poles.
2. The electric generator of claim 1, wherein the rotor comprises rotor windings for establishing a rotor magnetic field.
3. The electric generator of claim 2, wherein the rotor windings comprise a first set of rotor windings and a second set of rotor windings, and wherein the rotor is configured such that:
in the first state, a rotor winding current passes through the first set of rotor windings to establish the first number of magnetic poles, and in the second state, the rotor winding current passes through the second set of rotor windings to establish the second number of magnetic poles.
4. The electric generator of claim 2, wherein the rotor windings comprise a single set of rotor windings, and wherein the rotor is configured such that:
in the first state a rotor winding current follows a first current path through the single set of rotor windings to establish the first number of magnetic poles, and in the second state the rotor winding current follows a second current path through the single set of rotor windings to establish the second number of magnetic poles.
5. The electric generator of any of claims 2 to 4, further comprising:
first exciter windings for establishing a first exciter magnetic field; and second exciter windings for establishing a second exciter magnetic field; wherein the rotor further comprises:
a first exciter armature corresponding to the first exciter windings, wherein the first exciter armature is coupled to the rotor windings such that the electric generator operates in the first state when the first exciter magnetic field induces a rotor current in the first exciter armature; and a second exciter armature corresponding to the second exciter windings, wherein the second exciter armature is coupled to the rotor windings such that the electric generator operates in the second state when the second exciter magnetic field induces a rotor current in the second exciter armature.
6. The electric generator of any of claims 2 to 4, wherein the rotor further comprises a switching module coupled to the rotor windings to switch the electric generator between the first state and second state.
7. The electric generator of claim 6, wherein the switching module is controllable wirelessly from an off-rotor location.
8. The electric generator of any preceding claim, further comprising:
a controller configured to switch the electric generator between the first state and the second state to maintain the frequency of generated AC electrical power within the operational frequency range of the electrical system of the aircraft.
9. The electric generator of claim 8, wherein the controller is configured to switch the electric generator between the first state and the second state based at least in part on at least one of the following:
a measured rotational speed of the low pressure shaft of the jet engine;
a measured rotational speed of the rotor;
a measured frequency of generated electrical power.
10. The electric generator of any preceding claim, wherein a lower limit of the first speed range is less than a lower limit of the second speed range and/or an upper limit of the second speed range is greater than an upper limit of the second speed range, and wherein the first number of magnetic poles is greater than the second number of magnetic poles.
11. The electric generator of any preceding claim, wherein the electric generator is configured for the rotor to be mechanically coupled to the low pressure shaft of the jet engine with a fixed speed ratio.
12. The electric generator of claim 11, wherein the electric generator is configured for the rotor to be directly mechanically coupled to the low pressure shaft of the jet engine.
13. The electric generator of claim 11, further comprising a fixed ratio speed converter for mechanically coupling the rotor to the low pressure shaft of the jet engine.
14. A jet engine comprising:
a low pressure shaft; and the electric generator of any preceding claim, wherein the rotor of the electric generator is mechanically coupled to the low pressure shaft such that the rotor is mechanically driven by the low pressure shaft.
15. An aircraft comprising the jet engine of claim 14.
16. A method for generating AC electrical power with a frequency within an operational frequency range of an electrical system of an aircraft using an electric generator, the method comprising:
mechanically driving a rotor of the electric generator using a low pressure shaft of a jet engine of the aircraft;
operating the electric generator in a first state when the speed of the low pressure shaft is within a first speed range; and operating the electric generator in a second state when the speed of the low pressure shaft is within a second speed range, wherein in the first state, the electric generator operates with a first number of magnetic poles, and wherein in the second state, the electric generator operates with a second number of magnetic poles, and wherein the first number of magnetic poles is different to the second number of magnetic poles.
Claims
07 02 19
1. An electric generator for generating AC electrical power with a frequency within an operational frequency range of an electrical system of an aircraft, the electric
5 generator comprising:
a rotor, wherein the electric generator is configured for the rotor to be mechanically coupled to a low pressure shaft of a jet engine of the aircraft, wherein the electric generator is configured to operate in a first state when the speed of the low pressure shaft is within a first speed range and to operate in a second state
10 when the speed of the low pressure shaft is within a second speed range, and wherein in the first state the electric generator operates with a first number of magnetic poles and in the second state the electric generator operates with a second number of magnetic poles, wherein the first number of magnetic poles is greater than the second number of magnetic poles, and wherein
15 a lower limit of the first speed range is less than a lower limit of the second speed range and an upper limit of the first speed range is greater than the lower limit of the second speed range and less than an upper limit of the second speed range.
2. The electric generator of claim 1, wherein the rotor comprises rotor windings for 20 establishing a rotor magnetic field.
3. The electric generator of claim 2, wherein the rotor windings comprise a first set of rotor windings and a second set of rotor windings, and wherein the rotor is configured such that:
25 in the first state, a rotor winding current passes through the first set of rotor windings to establish the first number of magnetic poles, and in the second state, the rotor winding current passes through the second set of rotor windings to establish the second number of magnetic poles.
30 4. The electric generator of claim 2, wherein the rotor windings comprise a single set of rotor windings, and wherein the rotor is configured such that:
in the first state a rotor winding current follows a first current path through the single set of rotor windings to establish the first number of magnetic poles, and in the second state the rotor winding current follows a second current path
35 through the single set of rotor windings to establish the second number of magnetic poles.
07 02 19
5. The electric generator of any of claims 2 to 4, further comprising: first exciter windings for establishing a first exciter magnetic field; and second exciter windings for establishing a second exciter magnetic field; wherein the rotor further comprises:
5 a first exciter armature corresponding to the first exciter windings, wherein the first exciter armature is coupled to the rotor windings such that the electric generator operates in the first state when the first exciter magnetic field induces a rotor current in the first exciter armature; and a second exciter armature corresponding to the second exciter windings,
10 wherein the second exciter armature is coupled to the rotor windings such that the electric generator operates in the second state when the second exciter magnetic field induces a rotor current in the second exciter armature.
6. The electric generator of any of claims 2 to 4, wherein the rotor further
15 comprises a switching module coupled to the rotor windings to switch the electric generator between the first state and second state.
7. The electric generator of claim 6, wherein the switching module is controllable wirelessly from an off-rotor location.
8. The electric generator of any preceding claim, further comprising:
a controller configured to switch the electric generator between the first state and the second state to maintain the frequency of generated AC electrical power within the operational frequency range of the electrical system of the aircraft.
9. The electric generator of claim 8, wherein the controller is configured to switch the electric generator between the first state and the second state based at least in part on at least one of the following:
a measured rotational speed of the low pressure shaft of the jet engine;
30 a measured rotational speed of the rotor;
a measured frequency of generated electrical power.
10. The electric generator of any preceding claim, wherein the electric generator is configured for the rotor to be mechanically coupled to the low pressure shaft of the jet
35 engine with a fixed speed ratio.
07 02 19
11. The electric generator of claim 10, wherein the electric generator is configured for the rotor to be directly mechanically coupled to the low pressure shaft of the jet engine.
5 12. The electric generator of claim 10, further comprising a fixed ratio speed converter for mechanically coupling the rotor to the low pressure shaft of the jet engine.
13. A jet engine comprising:
10 a low pressure shaft; and the electric generator of any preceding claim, wherein the rotor of the electric generator is mechanically coupled to the low pressure shaft such that the rotor is mechanically driven by the low pressure shaft.
15. A method for generating AC electrical power with a frequency within an operational frequency range of an electrical system of an aircraft using an electric generator, the method comprising:
20 mechanically driving a rotor of the electric generator using a low pressure shaft of a jet engine of the aircraft;
operating the electric generator in a first state when the speed of the low pressure shaft is within a first speed range; and operating the electric generator in a second state when the speed of the low
25 pressure shaft is within a second speed range, wherein in the first state, the electric generator operates with a first number of magnetic poles, and wherein in the second state, the electric generator operates with a second number of magnetic poles, and
30 wherein the first number of magnetic poles is greater than the second number of magnetic poles, and wherein a lower limit of the first speed range is less than a lower limit of the second speed range and an upper limit of the first speed range is greater than the lower limit of the second speed range and less than an upper limit of the second speed 35 range.
15 14. An aircraft comprising the jet engine of claim 13.
GB1801816.8A 2018-02-05 2018-02-05 An electric generator for use with a jet engine Withdrawn GB2570704A (en)

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Application Number Priority Date Filing Date Title
GB1801816.8A GB2570704A (en) 2018-02-05 2018-02-05 An electric generator for use with a jet engine

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GB2570704A true GB2570704A (en) 2019-08-07

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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2445281A (en) * 2006-12-26 2008-07-02 Hamilton Sundstrand Corp Pole shifting generator
US20090121688A1 (en) * 2007-11-14 2009-05-14 Lemmers Glenn C Pole count changing generator

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2445281A (en) * 2006-12-26 2008-07-02 Hamilton Sundstrand Corp Pole shifting generator
US20090121688A1 (en) * 2007-11-14 2009-05-14 Lemmers Glenn C Pole count changing generator

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