Electric machine assembly

Abstract

An electric machine assembly 300 includes first 301 and second 302 axially connected radial flux electric machines mechanically phase shifted relative to each other. For any rotational position of the

Description

TECHNICAL FIELD The present disclosure relates to an electric machine assembly having first and second axially connected radial flux electric machines 5 mechanically phase shifted relative to each other. BACKGROUND As aircraft propulsion systems gradually move away from purely hydrocarbon-fuelled gas turbine engines, there is a growing need to provide more electric power to support and progress concepts such as the More 10 Electric Aircraft (MEA) and More Electric Engine (MEE). Electric power for such aircraft may be derived from on-board energy storage such as batteries or from onboard generated sources such as hydrogen fuel cells or C\j the rotating spools of a gas turbine engine. Hybrid electric aircraft include both batteries and onboard generation, with onboard generation either i— 15 combined with propulsion or purely for providing electric power. ¢ / } Extraction of electric power from rotating spools of a gas turbine CM engine has traditionally been achieved using an electric machine such as a wound field synchronous generator. Such a generator can also be used to start the engine by using the generator as a motor. With the increasing need 20 for electric power, permanent magnet (PM) generators are becoming more popular because of their high-power density, which offers improvements in weight and size of the generation plant. Such generators can also provide a starting function. Another known type of electric machine is a reluctance machine, 25 which may be implemented as a switched reluctance or synchronous reluctance machine. Reluctance machines offer some advantages over permanent magnet designs, such as a reduced requirement for rare earth metals that are needed for PM machines. However, acceptance of reluctance machines has been restricted in many applications primarily because of the relatively high torque ripple on the driving shaft by virtue of its operation. Traditional wound field synchronous machines, which have been used in aerospace for AC distribution for many years, do not suffer from a 5 large torque ripple. More modern and power dense permanent magnet machines require a power electronic converter to operate with a variable speed driving shaft. This converter is required to operate with high frequency PVWI type switching to control the quality of the machine phase current waveforms to make them close to sinusoidal, which reduces torque 10 ripple to an acceptable level. High switching frequencies, however, impact on the efficiency of the power electronic converter through increased semiconductor switching losses and require complex control systems to implement. Reluctance machines, due to their high torque ripple, are generally 15 not considered for aerospace electric power generation but may otherwise offer some advantages, including the potential for operating at low temperatures. A general problem to be addressed relates to reducing torque ripple in electric machines. SUMMARY 20 According to a first aspect, there is provided an aircraft electrical power system comprising: a first DC bus; a second DC bus; and an electric machine assembly comprising: 25 a first radial flux reluctance electric machine having a first stator and a first rotor and operable as a generator; a second radial flux reluctance electric machine having a second stator and a second rotor and operable as a generator, the second stator being in a fixed position relative to the first stator; a shaft connecting the first and second rotors to maintain a fixed position of the first rotor relative to the second rotor, the shaft connected to a spool of a gas turbine engine; a first power electronics converter connected to a first plurality of 5 windings of the first electric machine and to the first DC bus; and a second power electronics converter connected to a second plurality of windings of the second electric machine and the second DC bus, wherein, for any first rotational position of the first rotor relative to the first stator, a second rotational position of the second rotor relative to the 10 second stator is shifted relative to the first rotational position by a shift angle. An advantage of radial flux electrical machines, for example over axial flux electrical machines, is that the stator in a radial flux machine acts as containment in the event of a rotor mechanical failure. A further advantage is that the disc structures of axial flux machines are more prone 15 to vibration modes, requiring significant axial stiffening for integrity, which adds weight and complexity to an axial flux machine. Radial flux electric machines are therefore preferred for safety and weight critical applications such as in aerospace applications. The shift angle may be between around 15 degrees and 30 degrees. 20 The second stator may be rotationally shifted relative to the first stator by the shift angle. The first and second rotors may form a common rotor extending across the first and second stators. The second rotor may be rotationally shifted relative to the first rotor 25 by the shift angle. Each electric machine may also be operable as a motor. The electric machine assembly may further comprise: a third radial flux electric machine having a third stator and a third rotor, the third stator being in a fixed position relative to the first and second 30 stators, the shaft connecting the first, second and third rotors, wherein, for any first rotational position of the third rotor relative to the third stator, a third rotational position of the third rotor relative to the third stator is shifted relative to the second rotational position by the shift angle. Each rotor may have four or more rotor teeth and each stator may 5 have six or more stator teeth. The aircraft electrical power system may further comprise a switching controller configured to control switching of a plurality of power transistors in the first and second power electronics converters. The skilled person will appreciate that except where mutually 10 exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore, except where mutually exclusive any feature described herein may be applied to C\j any aspect and / or combined with any other feature described herein. BRIEF DESCRIPTION OF THE DRAWINGS 1— qq 15 Embodiments will now be described by way of example only with CM reference to the accompanying drawings, which are purely schematic and not to scale, and in which: Figure 1 is a schematic cross-sectional diagram of an example reluctance machine; 20 Figure 2 is a plot of torque as a function of rotation angle for a reluctance machine; Figure 3 is schematic diagram of an example pair of electric machines having a common rotor with associated power electronics converter circuits connected to each electric machine; 25 Figure 4 is a schematic diagram of an example pair of reluctance electric machines connected to respective power electronics converter circuits; Figure 5 is a plot of torque as a function of rotation angle for a pair of axially connected reluctance machines; Figure 6 is a schematic diagram illustrating a general arrangement of a turbofan engine for an aircraft, including a rotary electric machine assembly on each spool; and Figure 7 is a schematic diagram illustrating a fluid pumping assembly 5 including an electric machine assembly. DETAILED DESCRIPTION Reluctance machines may be used as motors or generators and take the general form illustrated in Figure 1. An outer stator 101 surrounds an inner rotor 102 separated by a small air gap. The stator 101 comprises a 10 plurality of stator teeth 103a-f, around which are provided a corresponding set of windings 104a-f. The windings 103a-f are driven in pairs by an electric CO current to provide pole-pairs, forming an electromagnetic field between C\l opposing pole-pairs. The rotor 102 is constructed from a ferromagnetic metal and is shaped with a plurality of rotor teeth 105a-d that interact with 1 15 the electromagnetic field generated by current passing through the stator CO windings 103a-f. In the position shown in Figure 1, with stator windings 103b, 103e energised the rotor 102 experiences no torque due to the reluctance being minimised as rotor teeth 105a, 105c are in the fully aligned position relative to stator teeth 103b, 103e. Rotor teeth 105d, 105b, 20 however, are in a fully unaligned position between stator teeth 103a-103f and 103c-103d. With windings 104a, 104d energised, the rotor 102 will experience a clockwise torque such that rotor teeth 105d, 105b are forced towards alignment with stator teeth 103a, 103d. By sequencing when current is passed through the pairs of windings around the stator teeth 103a-25 f, the rotor 102 can be made to continuously rotate. It can be seen from the basic principle of operation that the reluctance torque imposed on the rotor 102 will change from a maximum value in the fully unaligned position to zero in the fully aligned position. This means a reluctance machine will develop a significant torque ripple as it 30 rotates. This is illustrated in Figure 2, which shows a plot of torque 201 as a function of rotation angle for an example reluctance machine with 6 stator teeth and 4 rotor teeth (commonly termed a 6 / 4 reluctance machine). The torque fluctuates with maxima and minima occurring every 60 degrees. In practice, reluctance machines are designed with a number of 5 stator and rotor teeth such that some are in an unaligned position at the same time that others are aligned to produce a more continuous torque. The number of stator teeth is therefore generally different, and typically greater than, the number of rotor teeth. A typical reluctance machine will have 6 or more stator teeth and 4 or more rotor teeth. Increasing the number of teeth 10 on both the rotor and stator will tend to reduce this ripple, but adds to the complexity and cost of the machine. CO Whilst reluctance machines suffer from a torque ripple problem, an C\J advantage of such machines is that the rotor does not require permanent magnets as are used today in the most power dense starter-generator 15 designs. PM machines have a further disadvantage in that they continue to develop a voltage across the stator windings whenever they are rotating and CM may continue to deliver significant current to the electric system or power electronic converter during a fault. Additional hardware in the form of complex circuit breakers and / or mechanical disconnect mechanisms, which 20 decouple the rotor from the rotating engine spools, are being considered to deal with such fault conditions where PM machine generation is used. In the case of a reluctance machine-based starter-generator system for a gas turbine engine, an advantage is that the rotating ferromagnetic based rotor can be allowed to continue to rotate without contributing fault 25 current if the power electronic converter is prevented from passing current to the stator windings, i.e. if the electromagnets are de-energised. Given the potential advantages of reluctance machines, particularly for use with gas turbine engines, reducing the torque ripple effect would provide a further justification for using such reluctance machines over other 30 types of machines. Figures 3 and 4 illustrate schematically an example electric machine assembly 300, with Figure 3 showing a schematic mechanical arrangement for a pair of general electric machines and Figure 4 illustrating electric connections for a pair of reluctance machines. The assembly 300 comprises 5 first and second radial flux electric machines 301, 302. The first electric machine 301 has a first stator 303 and a first rotor 304. The second electric machine 302 has a second stator 305 and a second rotor 306. The second stator 305 is in a fixed position relative to the first stator 303, for example by both stators 301, 302 being mounted to a common frame (not shown). A 10 shaft 307 connects the first and second rotors 304, 306 to maintain a fixed position of the first rotor 304 relative to the second rotor 306 so that both rotors 304, 306 rotate together. As shown in Figure 4, the rotational position CO of the second rotor 306 relative to the second stator 305 is shifted by a shift angle a relative to the rotational position of the first rotor 304 relative to the 15 first stator 303. In the illustrated example, the rotational shift is 30 degrees because the number of stator teeth in each machine 301, 302 is six and the CO number of rotor teeth in each machine 301, 302 is four. This maximises torque ripple cancellation by setting the rotational shift to half that of the relative rotation between torque maximum points for each machine, which 20 in this case is every 60 degrees. In alternative examples, the rotors 304, 306 may be rotationally shifted relative to each other by the shift angle a, which will have the same effect as the stators being shifted. The rotors 304, 306 may be separate rotors fixed to a common shaft 307, as shown in Figure 3, or in some cases 25 may be provided as a common rotor extending across first and second rotationally shifted stators. Each electric machine 301, 302 is connected in parallel to a respective power electronics converter 308, 309 to supply power to, or draw power from, a DC power bus 401. The power electronics converters 308, 30 309 operate to convert between a DC voltage on the DC bus 401 and a plurality of windings on the electric machines 301, 302. In alternative arrangements, each converter 308, 309 may be connected to a separate DC bus so that the electric machines 301,302 can supply power to, or draw power from, different loads or sources as well as provide galvanic isolation between the converters 308, 309 that may be beneficial in some 5 applications. The converters 308, 309 may be controlled to provide equal amounts of power to, or draw equal amounts of power from, the respective motors 301,302 to maximise the effect of torque ripple cancellation. In some cases, however, the amount of power drawn from or provided to each machine 301, 302 may differ while still providing some torque cancellation 10 during operation. To operate each electric machine 301, 302 as a motor, the power electronics converters 308, 309 convert the DC supply on the bus 401 to an AC supply provided to the windings on each electric machine 301, 302. Conversely, to operate each electric machine 301, 302 as a generator, the 15 power electronics converters 308, 309 convert the AC supply provided by each electric machine 301, 302. Operation of the converters 308, 309 is controlled by a switching controller 310, which controls switching of a plurality of power transistors 3111-6, 312i-e in each converter 308, 309. For operation of the reluctance machines 301, 302 illustrated in Figure 4, the 20 power transistors 3111-6, 312i-6 are switched to control current through a plurality of windings 31 3i-3, 314i-3 in the first and second electric machines 301,302. To operate the electric machines 301,302 as either generators or motors, the switching sequences provided by the switching controller 310 to each converter 308, 309 with an electric phase shift corresponding to the 25 mechanical shift angle a between the electric machines 301, 302. The transistors of converters 308 and 309 need to be switched at different times. In practice, the electromagnets at the stator poles each need to be energised to pull the rotor pole into alignment with an electrical phase shift to match the mechanical phase shift for a 6 / 4 machine. The relationship between electrical degree and mechanical degree for an electrical machine with P poles is given by: Electrical degree = P / 2 * Mechanical degree 5 For a 6 / 4 switched reluctance machine, there are 6 stator poles. For a 3-phase system, the 3 stator poles driven by windings 313i-3 are combined. Therefore, there are only 2 stator poles in a 3-phase 6 / 4 arrangement, resulting in the electrical degree being equal to the 10 mechanical degree. The assembly 300 in Figure 3 relates to a switched reluctance motor-generator operated with asymmetric power electronics converters 308, 309 and may in general be considered as a type of stepper-motor. Synchronous reluctance machines operate in a similar manner and also generate 15 significant torque ripple for the reasons explained above. Synchronous reluctance machines are normally 3-phase and operate at a “synchronous” speed set by the frequency of the power electronics converter or AC supply. Synchronous reluctance machines use a more traditional 6-switch converter, which is widely used for variable speed drives. The arrangements 20 described herein relate to the reduction of torque ripple using a tandem machine arrangement with suitable phase shift, which applies equally to both synchronous reluctance machines and switched reluctance machines. As mentioned above, PM machines use high frequency PWM control of the converter to draw high quality sinusoidal currents from stator 25 windings, which tends to largely eliminate torque ripple. Using a lower PWM switching frequency would reduce switching losses and improve efficiency of the converter, but at the expense of introducing some torque ripple at the driving shaft, along with additional heating of the stator windings due to harmonic current flow. In some applications it may be beneficial to allow 30 individual machines to operate in this manner with the associated torque ripple reduced using the tandem machine arrangement with a phase shift as described herein. The same principle, may also be applied to other variable speed applications using converter control, including induction machines. Figure 5 illustrates the effect of rotationally shifting a pair of electric 5 machines of the type described above. The torque 501 generated by the first electric machine is shifted relative to the torque 502 generated by the second electric machine. The effect is that the torque ripple is partially cancelled by aligning the minimum torque for the second machine with the maximum torque for the first machine. Adding further machines may reduce 10 the torque ripple further. The arrangements described herein include two electric machines with corresponding power electronics converter. In other arrangements more than two electric machines with a corresponding number of power electronics converters may be used. The torque ripple may thereby be 15 reduced further, although at the expense of additional complexity and size of the assembly. In the case of 6 / 4 reluctance machines, the shift angle between each successive electric machine may be selected to be 60 / n, where n is the number of electric machines. Therefore, for 2 machines the shift angle is 30 degrees, for 3 machines the shift angle is 20 degrees, for 4 20 machines the shift angle is 15 degrees and so on. Advantages of the above described electric machine assembly include one or more of the following: Torque ripple, particularly relating to reluctance type machines, can be significantly reduced by virtue of the tandem arrangement with suitable 25 phase shifting. This can make the assembly more suitable for use in an aerospace electric starter-generator application, which torque ripple is known to introduce vibration, acoustic noise and mechanical stress / wear. The electric machine assembly may provide a dual power supply with galvanic isolation between channels. Unequal loading of the power 30 channels may still lead to some torque ripple cancellation. Reluctance machines can be prevented from supplying current simply by converter control, which is important during fault conditions, i.e. the generator can be turned off. This may remove the need for additional hardware such as a mechanical disconnect mechanism or circuit breakers, 5 which may be required with permanent magnet generation. The overall system, including any required protection components, could thereby be made smaller, lighter and cheaper. Reluctance based tandem generation may offer a fault tolerant generation system in case of a fault in one machine, albeit that 10 disconnection of one machine would prevent torque ripple cancellation. Nevertheless, increased torque ripple may be acceptable under fault conditions until the fault can be repaired. CO The electric machine assembly may use either switched reluctance CM or synchronous reluctance machines. 15 The electric machine assembly may be extended to three or more machines, offering improved torque ripple cancellation and more isolated CO power channels if desired. CM Reluctance machines may be more robust and suitable for very harsh environments due to the lack of magnets on the rotor. Examples of 20 such applications may include operation at cryogenic temperatures, for example for use as a liquid hydrogen pump. Figure 7 schematically illustrates a fluid pumping assembly 700 including the described electric machine assembly 300. The shaft 307 of the electric machine assembly 300 is coupled to a fluid pump impellor 710. 25 The electric machine assembly may use permanent magnet and induction machine drives operating with a lower PWM switching frequency such that torque ripple generated by individual electric machines may be cancelled by virtue of the phase shifted tandem arrangement. A hybrid design of assembly may be envisaged, in which different 30 types of electric machines are connected in a tandem arrangement, for example with a PM electric machine connected to a switched reluctance machine, to increase the torque or power of the machine assembly. Figure 6 illustrates a general arrangement of a gas turbine engine 600 for an aircraft. The engine 600 is of a turbofan configuration, comprising 5 a ducted fan 602 that receives intake air A and generates two pressurised airflows: a bypass flow B which passes axially through a bypass duct 603 and a core flow which enters a core gas turbine. The core gas turbine comprises, in axial flow series, a low-pressure compressor 604, a high-pressure compressor 605, a combustor 606, a high-10 pressure turbine 607, and a low-pressure turbine 608. In operation, the core flow C is compressed by the low-pressure compressor 604 and is then directed into the high-pressure compressor 605 C\J where further compression takes place. The compressed air exhausted from the high-pressure compressor 605 is directed into the combustor 606 15 where it is mixed with fuel and the mixture is combusted. The resultant hot combustion products then expand through, and thereby drive, the high-pressure turbine 607 and in turn the low-pressure turbine 608 before being exhausted to provide a small proportion of the overall thrust. The high-pressure turbine 607 drives the high-pressure compressor 20 605 via an interconnecting shaft. The low-pressure turbine 608 drives the low-pressure compressor 604 via another interconnecting shaft. Together, the high-pressure compressor 605, high-pressure turbine 607, and associated interconnecting shaft form part of a high-pressure spool of the engine 600. 25 Similarly, the low-pressure compressor 604, low-pressure turbine 608, and associated interconnecting shaft form part of a low-pressure spool of the engine 600. Such nomenclature will be familiar to those skilled in the art. The fan 602 is driven by the low-pressure turbine 608 via a reduction 30 gearbox in the form of a planetary-configuration epicyclic gearbox 609. Thus in this configuration, the low-pressure turbine 608 is connected with a sun gear of the gearbox 609. The sun gear is meshed with a plurality of planet gears located in a rotating carrier, which planet gears are in turn are meshed with a static ring gear. The rotating carrier drives the fan 602 via a fan shaft 5 610. In alternative arrangements a star-configuration epicyclic gearbox may be used, in which the planet carrier is static and the ring gear rotates and provides the output. To facilitate electric generation by the engine 600, a first rotary electric machine assembly 611 capable of operating both as a motor and 10 generator is mechanically coupled with the high-pressure spool. The first electric machine assembly 611 may comprise first and second electric machines of the type described herein. The first electric machine assembly 611 is coupled to the high-pressure spool via a high-pressure spool driven, core-mounted accessory gearbox 612 of conventional drive configuration. 15 Thus, as well as operation as a generator to supply an aircraft on which the engine 600 is installed with electric power, the first electric machine assembly 611 may drive the high-pressure spool to facilitate starting of the engine 600 in place of an air turbine starter, and may also drive it in certain flight phases to improve operability, fuel consumption, etc. 20 The first electric machine assembly 611 may alternatively be mounted coaxially with the turbomachinery in the engine 600. For example, the first electric machine assembly 611 may be mounted axially in line with the duct between the low- and high-pressure compressors 604 and 605. Similarly, a second rotary electric machine assembly 613 capable of 25 operating both as a motor and generator is mechanically coupled with the low-pressure spool. The second rotary electric machine assembly 613 may also comprise first and second electric machines of the type described herein. In the arrangement in Figure 6, the second electric machine assembly 613 is mounted in the tail cone 614 of the engine 600 coaxially 30 with the turbomachinery and is coupled to the low-pressure turbine 608. In alternative arrangements, the second rotary electric machine assembly 613 may be located axially in line with low-pressure compressor 604, which may adopt a bladed disc or bladed drum configuration to provide space for the second rotary electric machine assembly 613. It will of course be appreciated by those skilled in the art that any 5 other suitable location for the first and second electric machine assemblies may be adopted. The first and second electric machine assemblies are connected with power electronics, for example as described above. Extraction of power from, or application of power to the electric machines is performed by a 10 power electronics module (PEM) 615. In the present embodiment, the PEM 615 is mounted on the fan case 616 of the engine 600, but it will be appreciated that it may be mounted elsewhere such as on the core gas C\j turbine, or in the vehicle to which the engine 600 is attached, for example. Control of the PEM 615 and of the first and second electric machine 15 assemblies 611 and 613 is in the present example performed by an engine electronic controller (EEC) 617. In the arrangement of Figure 6, the EEC CM 617 is a full-authority digital engine controller (FADEC), the configuration of which will be known and understood by those skilled in the art. It therefore controls all aspects of the engine 600, i.e. both of the core gas turbine and 20 the first and second electric machine assemblies 611 and 613. In this way, the EEC 617 may holistically respond to both thrust demand and electric power demand. The internal configuration of PEM 615 guarantees fault-tolerant transfer of electric power between the first electric machine assembly 611 25 and second electric machine assembly 613. In this way, the turbomachinery may be designed to exploit the attendant advantages conferred by transfer of power between the high-pressure spool and the low-pressure spool. For example, transfer of power from the low-pressure spool to the high-pressure spool during the approach phase reduces the effective thrust of the engine 600 whilst maintaining sufficient high-pressure spool rotational speed to safely initiate a go-around manoeuvre. Further, in engine 600, transfer of power from the high-pressure spool to the low-pressure spool during a deceleration manoeuvre reduces 5 the risk of weak extinction, therefore enabling a more optimal combustor design. In an implementation contemplated herein, the PEM 615 is configured such that it may output to or receive electric power from two de busses - a configuration contemplated for future more electric aircraft 10 platforms. Other embodiments are intentionally within the scope of the invention, which is defined by the appended claims. CO Various examples have been described, each of which feature CM various combinations of features. It will be appreciated by those skilled in 15 the art that, except where clearly mutually exclusive, any of the features may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub- CM combinations of one or more features described herein.

Claims

1. An aircraft electric power system comprising: a first DC bus;5 a second DC bus; andan electric machine assembly (300) comprising:a first radial flux reluctance electric machine (301) having a first stator (303) and a first rotor (304) and being operable as a generator;10 a second radial flux reluctance electric machine (302) havinga second stator (305) and a second rotor (306) and being operable CO as a generator, the second stator (305) being in a fixed positionCM relative to the first stator (303);a shaft (307) connecting the first and second rotors (304, 306) 1 15 to maintain a fixed position of the first rotor (304) relative to theCO second rotor (306), the shaft connected to a spool of a gas turbineCMengine (600);a first power electronics converter (308) connected to a first plurality of windings (313i-s) of the first electric machine (301) and to 20 the first DC bus; anda second power electronics converter (309) connected to a second plurality of windings (314i-s) in the second electric machine (302) and the second DC bus,wherein, for any first rotational position of the first rotor (304) 25 relative to the first stator (303), a second rotational position of thesecond rotor (306) relative to the second stator (305) is shifted relative to the first rotational position by a shift angle (a).

2. The aircraft electrical power system (300) of claim 1, wherein the shift 30 angle (a) is between around 15 degrees and 30 degrees.

3. The aircraft electrical power system (300) of claim 1 or claim 2, wherein the second stator (305) is rotationally shifted relative to the first stator (303) by the shift angle (a).

54. The aircraft electrical power system (300) of claim 3, wherein the first and second rotors form a common rotor extending across the first and second stators (303, 305).10 5. The aircraft electrical power system (300) of claim 1 or claim 2,wherein the second rotor (306) is rotationally shifted relative to the first rotor (304) by the shift angle (a).coc\j6. The aircraft electrical power system machine assembly (300) of any15 one of the preceding claims, further comprising:i—a third radial flux electric machine having a third stator and a thirdCOrotor, the third stator being in a fixed position relative to the first and second stators (301, 302), the shaft (307) connecting the first, second and third rotors,20 wherein, for any first rotational position of the third rotor relative tothe third stator, a third rotational position of the third rotor relative to the third stator is shifted relative to the second rotational position by the shift angle (a).25 7. The aircraft electrical power system of any one of the precedingclaims, wherein each rotor (304, 306) has four or more rotor teeth and each stator (303, 305) has six or more stator teeth.

8. The aircraft electrical power system of claim 6, further comprising a 30 switching controller (310) configured to control switching of a plurality ofpower transistors (3111-6, 312i-e) in the first and second power electronics converters (308, 309).COCM

Images