Patent ID: 12206305

DETAILED DESCRIPTION

FIG.1illustrates a vertical take-off and landing (VTOL) aircraft1that may be used for Urban Air Mobility (UAM) applications. The VTOL aircraft1includes a fuselage20that incorporates a cabin for occupants, wings30, a rear flight surface40, and a distributed propulsion system10. The distributed propulsion system10includes six electrical propulsion units (EPUs), four of which are front EPUs100fand two of which are rear EPUs100r. Also visible inFIG.1is a retractable undercarriage50in which a landing platform or gear, in this case having wheels, may be stowed during flight.

The size of the fuselage20and the cabin depends on the application requirements. In this example, the cabin is sized for five occupants including a pilot. Some UAM platforms, however, will not require a pilot and will instead be flown under the control of an autopilot system or may be controlled remotely.

Each EPU100f,100rhas a propeller110driven to rotate by an electric motor. The four front EPUs100fare attached to the wings30of the aircraft1, and the two rear EPUs100rare attached to the separate flight control surface40located towards the rear of the aircraft1. The wings30and the rear control surface40are tiltable between a VTOL configuration (shown inFIG.1) in which the axes of the propellers of the EPUs point upward to provide vertical lift for vertical take-off and landing and a horizontal flight configuration in which the axes of the rotors point forward. The horizontal flight configuration, while principally used for horizontal flight, may also be used for taxiing and possibly short take-off and landing (STOL) operation if supported. In other examples, the wings30and/or the rear control surface40may be fixed in a horizontal position, and the EPUs attached thereto may be tiltable in order to selectively switch between a horizontal flight mode and a vertical flight mode.

The electrical systems, including the electric motors that drive the EPUs110f,110rof the aircraft1, receive electrical power from one or more battery packs and/or fuel cell packs located within the aircraft1. The battery packs and fuel cells packs may be located within any suitable part or parts of the aircraft1, including the EPUs100f,100r, the fuselage20, and the wings30.

While the illustrated aircraft1is a VTOL aircraft, UAM platforms may also be of the STOL or conventional take-off and landing (CTOL) type. Further, while an electric VTOL (eVTOL) aircraft is shown, the propulsion system may be a hybrid-electric propulsion system that includes both engines (e.g., one or more gas turbine engines) and batteries and/or fuel cells. Hybrid-electric platforms may utilize similar distributed propulsion system configurations, but the underlying power system may be a series-hybrid, parallel-hybrid, turboelectric, or other type of hybrid power system.

The configuration of the illustrated VTOL aircraft1is merely one example configuration, and other VTOL aircraft configurations are known and will occur to those skilled in the art. For example, a VTOL aircraft may have a different number of EPUs (e.g., eight EPUs, with four front EPUs100fand four rear EPUs100r). Alternatively, the VTOL aircraft may have a multi-copter (e.g., quadcopter) configuration in which the propellers or fans of the EPUs may not be tiltable and may be ducted. Other VTOL aircraft may have features of more than one type (e.g., a mix of open and ducted propulsors and/or a mix of tiltable and fixed propulsors). The present disclosure is not limited to any particular type of VTOL aircraft.

As noted above, each EPU of the six EPUs100f,100rincludes a propeller or fan110driven to rotate by an electric motor that receives electrical power from an onboard power source. In some examples, each motor may receive the electrical power via its own dedicated power channel, possibly from its own dedicated power source (e.g., a battery module). In other examples, some of the EPUs may share a power channel. This is shown inFIG.2, which is a simplified illustration of an electrical power and propulsion system10that may be used in the VTOL aircraft ofFIG.1.

The power and propulsion system10includes three power and propulsion sub-systems10a,10b,10c. Each sub-system of the sub-systems10a-cincludes two of the six EPUs, and the two EPUs of each sub-system are electrically connected to a shared power channel. The first sub-system10aincludes a first of the front EPUs100f-aand a first of the rear EPUs100r-aconnected to first power channel140a. The second sub-system10bincludes a second of the front EPUs100f-band a second of the rear EPUs100r-bconnected to a second power channel140b. The third sub-system10cincludes the remaining two front EPUs100f-c,100f-d, (e.g., one front-left EPU and one front-right EPU), connected to a third power channel140c. Each power channel140creceives DC electrical power from a battery module150a-c. The battery modules150a-cmay be physically separate from one another or may be part of a common battery pack that outputs three separate power channels140a-c.

In the present example, the power channels140a-creceive the DC electrical power directly from the battery modules150a-c. In other examples, the battery modules150a-cmay interface with the power channels140a-cvia DC:DC power electronics converters. This may allow the DC voltage level of the power channels to be kept substantially constant as the state of charge of the battery modules150a-cdecreases. Also, while in the present example the sole power source is in the form of battery energy storage150a-c, alternative examples may include only fuel cells, or a mix of fuel cells and battery energy storage. In a hybrid system, the power source may include one or more engine-driven electric generators interfacing with the DC power channels via DC:AC power electronics converters (e.g., rectifiers).

Each EPU100f-a,100f-b,100f-c,100f-d,100r-a,100r-bincludes a propeller or fan110with rotation that is driven by an electric motor120. The motor120receives AC electrical power from a DC:AC power electronics converter130(e.g., an inverter130). The inverter130receives DC electrical power from its power channel140a-cand converts the DC electrical power to AC electrical power for supply to its motor120.

Aircraft and power and propulsion systems of the aircraft are subject to strict safety and certification requirements. An aircraft and its safety-critical systems may be tolerant to faults; the aircraft and its safety-critical systems may be capable of continued, safe operation after the failure of a component. To this end,FIG.3illustrates the principle of a laned architecture that may be adopted for the power and propulsion system10ofFIGS.1and2to improve fault tolerance.

FIG.3shows a power and propulsion sub-system10nthat has a laned architecture. The sub-system10nmay be any of the sub-systems10a-cofFIG.2. As inFIG.2, the sub-system10nofFIG.3includes two EPUs100, each of which includes a propeller or fan110driven to rotate by an electric motor120. However, unlike the motors ofFIG.2, the motors120ofFIG.3are multi-lane electric motors. In particular, the motors120ofFIG.3are dual-lane motors with two electrically independent AC inputs (e.g., two independent three-phase inputs).

The term “multi-lane electric motor” or “multi-lane electrical machine,” as used herein, refers to an electric motor that has at least two electrically independent sets of stator coils that may be separately excited and may separately interact with a rotor to produce torque. In this way, in the event of a fault in one lane of the motor (e.g., a stator terminal short circuit), the remaining lane(s) may remain functional, and the multi-lane motor120may thus continue to apply torque to rotate the propeller or fan110. Depending on the number of lanes and the extent to which the motor120is overrated, the motor120may be able to supply all (i.e., 100%) or a large proportion (e.g., 70%, 80%, or 90%) of the torque that would be supplied to rotate the propeller or fan110during normal, fault-free operation. In this example, the motors120have two lanes and may be referred to as dual-lane motors, but a higher number of lanes (e.g., three or four lanes) may be used. The combination of an independent set of stator coils and an associated rotor with which the stator coils interact may be referred to as one lane of the multi-lane electric motor, or as a “sub-machine” of the multi-lane electrical machine.

A multi-lane motor may take one of a number of different forms. In one example, the dual-lane motor ofFIG.3takes the form of two axially stacked motors having rotors that are mechanically coupled to the same output shaft. In this case, the motor has two completely separate sets of active parts (e.g., two axially spaced stators and two axially spaced rotors) but has some shared structures and features, (e.g., shared support structures, casing, and cooling features). In another example, each dual-lane motor ofFIG.3has a single stator structure having a circumference that houses two electrically independent sets of coils, one set belonging to the first sub-machine and the second set belonging to the second sub-machine. The first set of coils and the second set of coils may interact with a common rotor. The above described approaches may be extended to a higher number of lanes (e.g., three axially stacked sub-machines) or may be combined to give a higher number of lanes (e.g., four lanes formed from two axially stacked arrangements, each having two sub-machines). Other examples will occur to those skilled in the art.

FIG.3also shows that each lane of the dual-lane electric motor120receives its multi-phase AC input from a separate, independent DC:AC inverter130i,130ii. Likewise, each inverter130i,130iireceives its DC input from a separate, independent DC power channel140i,140iithat itself receives power from its own battery module150i,150ii. Thus, the propulsion sub-system10nis not only tolerant to faults in the multi-lane electric motor120, but is also tolerant to faults in other sub-system components. For example, a failure in a converter circuit130iof the first lane will not prevent the supply of electrical power to the second lane of the associated motor120. Likewise, the loss of one battery module150ior a fault in a power channel140iwill not prevent the operation of the inverters130iiof the second lane or the second lanes of the motors120. Thus, the use of a laned architecture, including multi-lane motors120, increases the redundancy in and fault tolerance of the power and propulsion system10.

Other measures, not shown in the simplified systems10,10nofFIGS.2-3, are also possible. For example, a propulsion system10may provide for a degree of reconfigurability to improve the fault tolerance and power availability within the system. For example, in the event of a failure of one battery module (e.g., battery module150i), the sub-system10nmay be reconfigured (e.g., by the selective opening and closing of switches such as contactors or solid-state power controllers (SSPCs)) to allow electrical power from the second battery module150iito be supplied to the first power channel140i. As another example, following a fault in the first lane of one or both of motors120, the sub-system10nmay be reconfigured to allow electrical power from the first battery module150iito be supplied to the second power channel140ii.

FIG.4is a schematic illustration of a multi-lane EPU100such as may be used in the VTOL aircraft and propulsion systems ofFIGS.1-3. The EPU100includes a propeller or fan110mechanically coupled to, and thereby drivable by, a rotor of a multi-lane electric motor120. In this example, the multi-lane motor120is a dual-lane motor in which each of the two sub-machines has three phases. Thus, the motor120is shown as having two three-phase inputs that receive power from the outputs of two DC:AC converters130i,130ii. The DC:AC converters130i,130iiinterface with respective DC power channels140i,140ii, the positive and negative rails of which are indicated.

The inverters130i,130iiare controlled by controllers135i,135ii. The controllers135i,135iimay, for example, control the switching frequencies, switching timings, and duty cycles of MOSFETs of the inverters130i,130iito adjust the magnitudes and frequency of the output AC voltage and current waveforms of the inverters130i,130ii. In this way, the controllers135i,135iimay control the lanes of the motor120to produce the required torque, for example. In this example, each lane has its own controller; again, this prevents a fault in one controller (e.g., controller135i) from impacting the entire EPU100.

FIG.4also shows an EPU cooling system125. The EPU cooling system125is responsible for managing the temperature of the motor120and the inverters130i,130iiduring use. For example, the cooling system125provides that the temperature of the insulation of the stator coils does not exceed its rated temperature. The cooling system125may take any suitable form, including a liquid cooling system that utilizes a pumped liquid coolant (e.g., an oil) or an air cooling system that directs a flow of air (e.g., ambient air) to cool the components. In some examples, the cooling system125includes substantially separate cooling systems for cooling the electrical machine120and the inverters130i,130ii. In other examples, the cooling system125is shared by the electrical machine120and the inverters130i,130ii. Further, in some examples, the cooling system125may include separate parts for each lane of the EPU100to prevent a cooling system fault from impacting the entire EPU100.

The EPU110may include a gearbox105. The optional gearbox105may be required to step down the speed of the rotor(s) of the motor120to a lower speed of rotation for the propeller or fan110. VTOL aircraft are expected to have relatively large propellers or fans110(e.g., diameters of 2-4 meters) in order to limit disk loading while increasing propulsive efficiency during VTOL and hover. At the same time, there is a desire to keep aerodynamic noise low, wherein the aerodynamic noise is strongly dependent on the propeller tip speed. The combination of a large propeller diameter and a low propeller tip speed necessitates a relatively low propeller rotational speed. Unless the electrical machine120is capable of providing the required torque at a low rotational speed, which is relatively high given the large propeller, a gearbox105is to be provided.

Also shown inFIG.4are a propeller bearing unit111and an EPU lubrication system115. The presence of and designs of the propeller bearing unit111and the EPU lubrication system115will depend on the design of the EPU110and are beyond the scope of the present disclosure and will not described any further.

The VTOL aircraft1, the propulsion system10, and the EPU100described with reference toFIGS.1-4are only intended to be examples, and many other arrangements are possible and within the scope of the present disclosure. As noted previously, the aircraft1may additionally or alternatively include fuel cells or engines. The aircraft1may also be of a different VTOL design (e.g., multi-copter or different number and arrangement of EPUs) or utilize a different electrical power system layout. However, the above description explains certain principles of a VTOL aircraft.

The general design of VTOL aircraft, such as the one described above, results in a number of design challenges. Some of these are discussed below.

In one example, a need for redundancy and propulsive efficiency calls for a distributed propulsion system with a relatively large number of EPUs. In the above example, there are six EPUs, and most proposed VTOL aircraft designs include at least four EPUs. This increases the mass and reduces the power density of the VTOL aircraft because each EPU includes not only lift- and thrust-producing parts but support and attachment structures, cabling, etc.

In a second example, high torque and low speed requirements of the propeller or fan of the EPU, discussed above, may call for a gearbox to step down the output rotor speed of the electrical machine. A gearbox is a relatively heavy component and also introduces additional complexity as well as lubrication and maintenance requirements. Further, each EPU would require its own gearbox, multiplying the gearbox mass by, for example, six times. A direct drive arrangement would eliminate this mass and complexity and be of great advantage. However, designing a high torque, low speed electrical machine that is lightweight and efficient, yet does not have onerous cooling requirements, is a challenge.

Table 1 provides exemplary values of a hover parameter, Ψ, that may be achieved by motors described herein. The hover parameter Ψ is defined (see Equation (26)) as the continuous torque produced by the motor while the VTOL aircraft is hovering (τhover) divided by the continuous angular speed of rotation of the rotor of the motor while the VTOL aircraft is hovering (ωhover), measured in radians per second. By “hovering,” it is provided that the EPUs of the VTOL aircraft are producing sufficient thrust to lift the aircraft and maintain a constant aircraft altitude, without requiring the assistance of airframe lift (e.g., wing-borne lift).

TABLE 1Ψ(Nmsrad−1)Example 1Example 2Example 37.213.116.3

Motors described herein may have values of Ψ in the range 5 to 20 Nmsrad−1, which may allow for omission of the gearbox105, resulting in reduced EPU and aircraft mass.

In another example, strict safety and certification requirements of aircraft call for a fault-tolerant electrical power system (e.g., the laned architecture described above with reference toFIGS.3and4). This results in further multiplication (e.g., duplication) of components in an EPU: electrical machines with multiple sets of active parts; multiple power channels; multiple inverters; and multiple cooling systems. This results in a further increase in EPU mass. It would be desirable to reduce the mass of the active parts and cooling system associated with the electrical machine, to the extent that this is possible, while meeting the platform design requirements.

Table 2 provides examples of values of a take-off parameter, χ, that may be achieved by a VTOL aircraft with one or more EPUs incorporating an electric motor described herein. The take-off parameter, χ, is defined (see Equation (24)) as the maximum tip speed (vtip), measured in ms−1, of the propeller or fan of the EPU to occur during a vertical take-off operation of the VTOL aircraft divided by the active parts torque density (ρact—see Equation (1)).

TABLE 2χ (sm−1)Example 1Example 2Example 31.22.14.7

Motors described herein may have take-off parameters in the range of 0.5 to 7.5 sm−1, which may be associated with reduced system mass and noise.

In another example, multiplication of electrical components such as inverters results in a stacking of losses in the system. For example, a propulsion system with six EPUs and a dual lane architecture would include at least twelve inverters, each having its own losses. One mitigation is to design inverters with high efficiencies, for example, by using state-of-the-art semiconductor materials. Even then, however, it would be desirable to be able to operate the inverters in an operating regime close to their peak efficiency, which occurs when the electrical frequency of the inverter output waveform is relatively high. This is a design challenge, especially for direct drive, because it requires the use of a high inverter output frequency with a low rotor speed of rotation.

Electrical machine designs that are optimized for aircraft propulsion systems, particularly for VTOL aircraft, and may address one or more of the above problems and/or other problems are now described with reference toFIGS.5A-23.

FIG.5Aillustrates a radial flux permanent magnet motor200(e.g., a motor) that may be used for EPUs of electric aircraft, including VTOL aircraft. As noted previously, permanent magnet motors may be provided for VTOL applications because their power density is relatively high compared with most other designs.

For clarity,FIG.5Aonly shows the active parts of the motor200. “Active parts” refers to the components of the motor200that contribute to the production of torque. Thus, “active parts” include magnetic flux generating components such as coils and magnets, and magnetic flux guiding components such iron, but “active parts” do not include support structures, cooling system features, etc., which do not contribute to the production of torque. The active parts of the motor200have a cumulated mass of mact, referred to herein as the active parts mass. For the avoidance of doubt, the active parts mass includes the mass of the end windings and coil insulation because these are integral features of the coils without which a motor cannot produce any torque.

The radial flux motor200includes a stator210and a rotor220arranged to rotate about an axis of rotation230.

The stator210includes an annular stator back iron211, which may also be referred to as a yoke, and a plurality of circumferentially arranged stator teeth212(e.g., stator teeth212) that project radially inwardly from the back iron211. The stator teeth212define stator slots213, which may also be referred to as stator winding space, between the stator teeth212. In the present example, there are twenty-four stator teeth212defining twenty-four stator slots213therebetween. The stator teeth212and/or the stator iron211may, for example, be formed of laminations of a ferromagnetic material to improve their flux guiding performance while reducing the induction of eddy currents in the stator teeth212and/or the stator iron211. In another example, the flux guiding material includes a soft magnetic composite (SMC) such as, for example, a non-iron material with embedded iron particles. The active parts mass, mact, of the motor200includes any carrier material of the flux guiding iron (e.g., non-iron material included in laminations) or the non-iron material in an SMC.

The stator210further includes a plurality of electrically conductive stator coils214(e.g., stator coils) wound around the stator teeth212. The stator coils214may be formed of any suitable material, such as copper or aluminum. Strands of the conductor that form the stator coils214have (e.g., are coated in) an electrically insulating material to prevent short circuits. In the present example, there are twelve stator coils214, and each coil occupies two of the slots213.

In this example, the motor200is a three-phase motor, and thus there are 12/3=4 stator coils214per phase. The three phases are designated U, V, WinFIG.5A, and the stator coils214are labelled 1-4. Stator coils214of the same phase (e.g., U1, U2, U3, U4) are evenly distributed about the circumference of the stator210, while circumferentially adjacent stator coils214(e.g., U1 and V1, or V1 and W1) belong to different phases. Many different stator winding arrangements are known, but, in this example, a distributed winding arrangement in which each coil is wound around two teeth that are located 2π/8=π/4 radians (45 degrees) apart is used. Stator coils214of the same phase are electrically connected (e.g., in series or in parallel). The terminals of each set of phase coils may be connected in, for example, a star or delta configuration, and the input terminals may be connected to an inverter (e.g., a two-level, three phase bridge circuit) from which the stator coils214receive current. In an alternative example, for increased fault tolerance, the stator coils214of each phase may be connected to its own inverter circuit (e.g., an H-bridge circuit).

The rotor220includes an annular rotor back iron221and a plurality of permanent magnets222(e.g., permanent magnets) arranged around a circumference of the rotor220forming a plurality of permanent magnet rotor poles (e.g., permanent magnet poles). Circumferentially adjacent permanent magnet poles are of opposite polarity. The permanent magnet poles are distributed evenly about the rotor circumference and define a pole pitch, Pθ, equal to 2π divided by the number of permanent magnet poles (NP):

Pθ=2⁢πNP(27)

In this example, there are eight permanent magnet poles, so Pθis equal to 2π/8=π/4 radians (45 degrees).

In addition to the pole pitch, Pθ, a pole arc length, PL, of the motor is also defined. Herein, the pole arc length is equal to pole pitch, Pθ, measured in radians, multiplied by the active parts radius, the active parts radius being half the active parts diameter, DAct, of the motor:

PL=Pθ×DAct2(28)

The active parts diameter, DAct, is a diameter corresponding to a radially outermost active part of the motor200. In this example, in which the rotor220is radially inward of the stator210, a radially outer circumference of the stator iron211defines the active parts diameter. When the rotor220is instead radially outward of the stator210, a radially outermost active part of the rotor defines the active parts diameter. In the present example, if the motor200is sized for an EPU of a VTOL aircraft, the active parts diameter may be about 0.45 meters, giving a pole arc length, PL, of about 17.7 cm.

FIG.5Aalso labels an air gap215that separates the rotor220from the stator210. The air gap215has a width, measured in the radial direction for the radial flux motor200, equal to GAir.

In use, the stator coils214of the stator210are excited with AC power to generate a rotating magnetic field that interacts with the magnetic field of the permanent magnets222to produce torque. The torque causes the rotor220to rotate relative to the stator210about the axis of rotation230. The motor200is configured to produce a maximum continuous rated torque of τmax,contand a peak rated torque of τpeak. As used herein, τmax,contis the highest torque the motor can produce and sustain for an extended period (e.g., at least three minutes) at ISA sea level conditions. τmax,contdepends to some extent on the capabilities of the cooling system of the motor, which is configured to remove heat at a rate sufficient to maintain the temperature of the stator coil insulation below its rated temperature while operating at τmax,cont. As used herein, τpeakis a highest torque the motor can produce for a short period (e.g., a three second transient period) at ISA sea level conditions without damaging the motor due to, for example, breakdown of the coil insulation due to excessive voltage or excessive heat generation.

FIG.5Billustrates main magnetic circuits produced by the radial flux motor200during use. The current flowing through each stator coil214produces magnetic flux that flows in a radial direction. For a given stator slot213, the flux produced by the stator coil214flows radially outward along one tooth212and radially inward along another tooth212on a circumferentially opposite side of the slot213. At the radially outer side of the stator210, the magnetic flux flows circumferentially along the stator back iron211between stator teeth212. At the radially inner side of the stator210, the magnetic flux crosses the air gap215to flow to/from a stator tooth212from/to a permanent magnet222of the rotor220. In the rotor210, flux flows from a permanent magnet222to the rotor iron221and then flows circumferentially along the rotor iron221to another permanent magnet222.

FIG.6is a plot1000illustrating how the design of a radial flux motor200may be optimized for use in a EPU of VTOL aircraft. A three-dimensional surface1000is shown, with shading of the surface1000indicating the temperature of the insulation of the stator coils214.

The vertical axis represents the active parts torque density, ρact, of the motor, defined in Equation (1) as the peak rated torque divided by the cumulated mass of the active parts (i.e., the components which contribute to the production of torque) measured in Nmkg−1. In the case of VTOL aircraft, it desirable for the active parts torque density, ρact, to be high because this implies the platform's torque production requirements are met at a low motor mass, which is a significant benefit in VTOL aircraft due in part to the multiplication of components (e.g., multiple EPUs). The remaining two axes show the slot current density, Jslot,peak, (e.g., the slot current density when producing the peak rated torque), measured in A(mm)−2, and the linear RMS current loading, Arms, measured in kA/m. In certain examples, the higher the current loading and the slot current density, the higher the torque production. However, if the current density is high, the stator coil temperature will be higher for a given nominal cooling rate because resistive losses (i.e., I2R losses) will also be higher.

On the surface1000, an isotherm1001(i.e., a line of constant temperature) is shown. The isotherm1001represents operation at the rated temperature of the insulation, assuming operation of a liquid cooling system that cools the stator coils at a nominal rate. In other words, the isotherm1001divides the surface1000into two design space regions: a lower region1002below the isotherm1001in which operation is sustainable at the nominal cooling rate; and an upper region1003above the isotherm1001in which operation is not sustainable at the nominal cooling rate. Thus, the isotherm1001may be regarded as the optimal design.FIG.6further shows three possible stator tooth and slot designs, labelled (i), (ii) and (iii), and operating points thereof that lie on the isotherm1001.

First referring to design (i), this shows a stator tooth212that is relatively long in the radial direction and includes a large volume of conductor in the slots213defined circumferentially adjacent to the tooth212. The large volume of active parts (e.g., the iron stator teeth and the conductor) results in high torque for a given slot current density. However, the large volume of active parts also results in a high active parts mass, which limits the active parts torque density ρact. Further, the use of radially long stator teeth212provides that, for a motor of a given diameter (noting the diameter will be constrained by the EPU integration requirement), there is relatively little space to flow coolant around the active parts. Thus, while the slot current density is low for a given current value, the extent to which the slot current density may be increased without departing from the isotherm1001into the region1003is limited.

Design (ii) is a more optimized design for VTOL aircraft in that design (ii) better balances torque production, slot current density, and active parts mass. As shown, compared with design (i), design (ii) has radially shorter teeth212with a smaller volume of conductor in the slots. While this reduces torque production at a given value of the slot current density, radially shorter teeth212with a smaller volume of conductor in the slots reduces the active parts mass. This also provides additional room for coolant, which improves cooling and therefore allows for an increase in the slot current density without departing from isotherm1001into the upper region1003. Further, the radially shorter teeth have a lower aspect ratio, which may improve flux guiding, and allow for the use of a larger radius rotor. The use of a larger radius rotor may produce a higher torque. Overall, as shown, tooth design (ii) has the peak value of ρacton the isotherm1001.

Design (iii) illustrates the impact of further reducing the radial length of the stator tooth212and decreasing the volume of conductor in the slot. As before, the reduction in active parts volume results in lower torque production at a given slot current density but also a reduction in active parts mass. The additional free volume for coolant allows the slot current density to be increased, thus increasing active parts torque density ρact, without departing from the isotherm1001. However, resistive losses increase with the square of the current density whereas the torque increases in a linear fashion. There is therefore a point on the isotherm1001after which the increase in torque that results from the increase in slot current density, and which is made possible by the increase in cooling volume, does not compensate the reduction in torque that results from the reduced volume of conductor. Therefore, design (iii) is associated with a lower value of ρactthan design (ii).

While optimized radial flux designs may be used in the EPUs of VTOL aircraft, further performance improvements and optimizations may be provided. To this end,FIG.7A, illustrates a transverse flux electric motor300that may be particularly suitable for use in VTOL aircraft.FIG.7Bis a circumferential cross-section of the motor300ofFIG.7A. As before, for clarity and ease of explanation, only the active parts of the motor300are shown. The illustrated motor300has only a single phase. This is for clarity of explanation; in practice, a motor may have a higher number of phases, and such a motor will be described below.

The transverse flux motor300has a stator310and a rotor320a,320barranged to rotate about an axis of rotation330.

The stator310includes flux guiding stator iron311that defines a circumferentially extending stator slot313(e.g., generally annular stator slot; annular winding space). In this example, the stator iron311includes a plurality of circumferentially arranged flux guiding stator elements312(e.g., stator elements312) that together define and surround the annular winding space313. In the present example, there are eight stator elements312, alternately facing axially up and axially down, defining a single stator slot313. The stator elements312may be formed of laminations of a ferromagnetic material or an SMC to improve their flux guiding performance while reducing the induction of eddy currents. In other examples, the winding space313may be defined by a continuous (e.g., a single piece) stator iron structure as is the case in the radial flux machine ofFIG.5A.

The stator slot313houses a circumferentially extending stator coil314. As in the radial flux motor200, the stator coil314may be formed of any suitable material such as copper or aluminum. The conductor that forms the stator coil314has (e.g., is coated in) an electrically insulating material to prevent short circuits. InFIGS.7A and7B, the stator coil314is a solid piece of conductor (e.g., the coil has a single turn). In practice, the stator coil314may have a plurality of turns; this is discussed in more detail below.

FIG.7Bis a circumferential cross-section through the active parts of the transverse flux motor300, and more clearly shows how the flux guiding stator elements312may define the open slot313. InFIG.7B, two circumferentially adjacent stator elements312i,312iidefining a single stator pole are shown. Stator element312iis in the foreground, and stator element312iiis in the background. Each of the stator elements312i,312iihas a generally claw-shaped form factor and includes a body portion3121and two projections3122,3123that project from the body portion3121. In this example, the projections3122,3123extend axially away from the body portion3121, and the projections3122,3123of circumferentially adjacent stator elements312i,312iiface axially opposite directions. InFIG.7B, the projections3122,3123of the first stator element312iproject axially downwards, whereas the projections3122,3123of the second stator element312iiproject axially upwards. In this way, the two circumferentially adjacent stator elements312i,312iicooperate to define the cross-section of a winding space (e.g., a slot313) therebetween, which in this case has a hexagonal shape, though other shapes may be formed by modifying the shape and curvature of the stator elements312i,312ii. Collectively, the eight stator elements312of the stator310(seeFIG.7A) define an annular winding space, and the stator coil314is housed in the annular winding space. Other stator element form factors are possible and in accordance with the present disclosure.

The rotor320a,320b, is a dual rotor and has two rotor portions: an inner rotor portion320athat is radially inside the stator310; and an outer rotor portion320bthat is radially outside the stator310. In this example, the inner rotor portion320aand the outer rotor portion320bare mechanically connected so that the inner rotor portion320and the outer rotor portion320brotate together. Each of the inner rotor portion320aand the outer rotor portion320bincludes a plurality of circumferentially arranged permanent magnets322a,322bdefining evenly spaced permanent magnet poles (e.g., poles). Circumferentially adjacent poles are of opposite polarity. In this example, the inner rotor portion320aincludes eight permanent magnet poles, and the outer rotor portion320bincludes eight permanent magnet poles. Thus, in the present example, the pole pitch, Pθ, of the motor300is 2π/8=π/4 radians (45 degrees).

The permanent magnets322aof the inner rotor portion320aare affixed to an outer surface of an inner rotor structure321a. Similarly, the permanent magnets322bof the inner rotor portion320bare affixed to an inner surface of an outer rotor structure321b. The inner rotor structure321aand the outer rotor structure321bmay include flux guiding stator iron (e.g., laminations of a ferromagnetic material). However, the use of a dual rotor design, with permanent magnets322a,322bon both radial sides of the stator310, may allow for the omission of iron material from the rotor320a,320bbecause the permanent magnets322a,322bmay define closed magnetic circuits. This is described in more detail below. Other transverse flux motors300in accordance with the present disclosure may not feature a dual rotor, and, in this case, rotor iron or additional stator iron may be provided to define closed magnetic circuits.

As stated above, the pole arc length, PL, of the motor is defined as the pole pitch, Pθ, measured in radians, multiplied by half the active parts diameter, DAct, of the motor. In this example, which features an ironless dual rotor320, the active parts diameter is defined by the outer diameter of the permanent magnets322bof the outer rotor portion. Assuming the motor300is sized for an EPU of a VTOL aircraft, the active parts diameter may be about 0.45 meters, giving a pole arc length, PL, of about 17.7 cm.

FIGS.7A-7Balso label air gaps315a,315bthat separate the inner rotor portion320aand the outer rotor portion320bfrom the stator310. The inner air gap315ahas a width, for example, measured in the radial direction, equal to GAir,1. The outer air gap315bhas a width, for example, measured in the radial direction, equal to GAir,2. The air gap widths GAir,1, GAir,2may be the same to equalize the motor loading.

The transverse flux motor300of the present example has radial air gaps315a,315b. In other words, the two rotor portions320a,320bare on radially opposite sides of the stator310. In other examples, a transverse flux motor has axial air gaps. In other words, the two rotor portions would be on axially opposite sides of the stator. Such an example will be described with reference toFIGS.9-11.

The transverse flux motor300of the present example has only a small number of pole pairs and relatively large values for the pole pitch, Pθ, and pole arc length, PL. This is for ease of explanation. As will be described in detail below, the present disclosure provides the selection of a larger number of pole pairs to improve the characteristics of the motor.

FIG.8shows the magnetic flux paths of the main magnetic circuits formed in the transverse flux motor300ofFIGS.7A-B. The left-hand diagram ofFIG.8is the axial end view ofFIG.7A, with the main magnetic circuits overlaid and labelled by the magnetic flux density vector {right arrow over (B)}. The dotted lines indicate where the flux lines are below the plane of the page. The right-hand diagram, taken through plane A-A, is a zoomed-in perspective view of two circumferentially adjacent stator elements312, forming a single stator pole, and illustrates the three-dimensional shape of the flux path {right arrow over (B)}. The current and force vectors {right arrow over (I)} and {right arrow over (F)} are also shown.

The current flows through the stator coils314in the circumferential direction. This is illustrated inFIG.8by the current vector {right arrow over (I)}. At each circumferential position, the current flow produces a loop of magnetic flux in a plane perpendicular to the direction of current flow. Considering a circumferential position corresponding to a first stator element312i, the loop of flux is guided along the radially extending body portion3121iof the stator element312ibefore entering an axial projection of the stator element312i. From there, the flux crosses the inner air gap315aand enters a permanent magnet322aof the inner rotor portion320a. The flux then flows circumferentially through the inner rotor magnets322ato reach a circumferential position corresponding to the adjacent second stator element312ii. From there, the flux again crosses the inner air gap315a, this time into the second stator element312ii. The flux is then guided along the radially extending body portion3121iiof the second stator element312iibefore entering an axial projection of the second stator element312ii. From there, the flux crosses the outer air gap315band enters a permanent magnet322bof the outer rotor portion320b. The flux then flows circumferentially through the outer rotor magnets322bto reach another circumferential position (e.g., in this case, back to the circumferential position corresponding to the first stator element312i). From here, the flux again crosses the outer air gap315bto the first stator element312i, thus completing the magnetic circuit. The left-hand drawing ofFIG.8shows a similar magnetic circuit for each adjacent pair of stator elements312.

Thus,FIG.8shows that the magnetic circuits of the transverse flux motor300are three-dimensional (e.g., the magnetic circuits have components in the radial, axial, and circumferential directions) and spiral around the annular stator winding region313that houses the stator coil314.

As mentioned previously, a practical motor will include more phases than the single phase shown inFIG.7A. As also mentioned previously, a transverse flux electrical machine may alternatively utilize axial air gaps instead of radial air gaps. To this end, a three-phase axial air gap transverse flux motor60will be described with reference toFIGS.9-11.

FIG.9Ain axial end view of the active parts of a three-phase stator61of a transverse flux motor60that has axial air gaps.

In this example, the three-phase stator61includes six circumferentially arranged phase modules610-1to610-6, distributed evenly about the stator circumference. Radially opposite phase modules (e.g., phase modules610-1and610-4) are associated with the same phase (e.g., phase U) of the motor60to provide mechanical balance. Each phase module610-1to610-6includes flux guiding stator iron611defining a circumferentially extending and open winding space613(e.g., a slot), and a coil614(e.g., a stator coil) housed within the slot613.

FIG.9Bshows one of the phase modules610fixed to a support structure640. In this example, the slot613and the coil614housed within the slot613include first and second radially spaced portions. Specifically, the coil614includes a first, radially inner and circumferentially extending, coil portion614ahoused within a first, radially inner and circumferentially extending, slot portion613a. The coil614further includes a second, radially outer and circumferentially extending, coil portion614bhoused within a second, radially outer and circumferentially extending, slot portion613b. The first coil portion614aand the second coil portion614bare connected at respective circumferential ends by end windings617. In this way, the current flowing through a coil614changes direction in the end windings617, and the current flows through the first coil portion614ain a circumferential direction opposite to (e.g., generally antiparallel to) the current that flows through the second coil portion614b.

The flux guiding stator iron611includes two sets of flux guiding stator elements: a radially inner first set of flux guiding stator elements612aand a radially outer second set of flux guiding stator elements612b. The radially inner first set of stator elements612adefine the radially outer first slot portion613athat houses the first coil portion614a. The radially outer second set of stator elements612bdefines the radially outer second slot portion613bthat houses the second coil portion614b. Each of the two sets of stator elements612a,612bincludes a plurality of circumferentially arranged, evenly distributed stator elements612. In the present example ofFIG.9B, the first, radially inner set of stator elements612ahas twenty-five stator elements612, whereas the second, radially outer set of stator elements612bhas twenty-seven stator elements612. Other numbers of stator elements are possible.

Each stator element612is substantially as described above with reference toFIGS.7-8(e.g., each element612includes a body portion3121and two projections3122,3123). However, in the example ofFIGS.9-11, the stator elements612are oriented so that the body portions3121extend axially and the projections3122,3123extend radially. The projections3122,3123of circumferentially adjacent stator elements612i,612iiof each set612a,612balternately project radially inwardly and radially outwardly so as to define an open slot cross-section, similar to that shown inFIG.7Bbut with the radial direction and the axial direction swapped. Collectively, all of the stator elements612within a set (e.g., set612a) define a slot portion (e.g., radially inner slot portion613a).

FIG.10Ais a circuit diagram of the stator61illustrating how the coils614of the six phase modules610-1to610-6may be connected together.

The stator61has three phase terminals U, V, W by which the stator61receives AC electrical power from an inverter arrangement. For example, each phase terminal may be connected to a two-level, one-phase H-bridge inverter circuit, or each phase terminal may be connected to one of the phase legs of a two-level, three-phase DC:AC inverter circuit.

The coil614of each phase module610-1to610-6has two terminals. Respective first terminals of the coils614of the radially opposite first phase module610-1and fourth phase module610-4are connected in parallel to the first phase terminal U. Respective first terminals of the coils614of the radially opposite second phase module610-2and fifth phase module610-5are connected in parallel to the second phase terminal V. Respective first terminals of the coils614of the radially opposite third phase module610-3and sixth phase module610-6are connected in parallel to the third phase terminal W. Respective second terminals of the first phase module610-1, second phase module610-2, and third phase module610-3are connected at a first star point616-1. Respective second terminals of the fourth phase module610-4, fifth phase module610-5, and sixth phase module610-6are connected at a second star point616-2. Thus, in this example, the phases are connected in a star configuration. In other motors in accordance with the present disclosure, the phases may be connected in a delta configuration.

FIG.10Aalso shows the measurement of the voltage between the two star points616-1,616-2. A difference in the voltage between the star points may be used to diagnose a fault in the stator coils614.

FIG.10Bis a hybrid diagram combining the axial end view of the stator61(FIG.9A) and the circuit diagram of the stator61(FIG.10A). As well as showing the connection of the coils614together, and to the phase connections U,V, W and the star points616-1,616-2,FIG.10Bshows the coils614in schematic form.FIG.10Bshows that the coils614have end windings617at circumferential ends, respectively, which allows the current to reverse direction between the inner coil portion614aand the outer coil portion614b.FIG.10Balso shows that each coil614has multiple winding turns6140. Although only two turns are illustrated, in practice, each coil may have more than two turns.

FIG.11shows the motor60in cross-section. While the previous figures have only illustrated certain active parts of their respective motors,FIG.11also shows various other features, including features of an EPU in which the motor may be integrated.

The motor60includes a main motor housing601that includes, amongst other things, the stator61. Various components of the stator61are visible and labelled inFIG.11. This includes, for the two of the six phase modules610-1to610-6that are visible in the cross-section ofFIG.11, the first, radially inner set of flux guiding stator elements612a, the second, radially outer set of flux guiding stator elements612b, the first, radially inner slot portion613adefined by the first set of stator elements612a, and the second, radially outer slot portion613bdefined by the second set of stator elements612b. The coil portions614a,614bare omitted fromFIG.11to more clearly show the slots613(e.g., the winding space).

The rotor62includes a rotor housing625mechanically coupled to the EPU drive shaft630via a coupling structure626that, for example, may be disk-shaped. Thus, in this example, the rotor housing625rotates with the drive shaft630about an axis of rotation63. The active parts of the rotor (e.g., the permanent magnets that interact with the active parts of the stator61) are located within the rotor housing625and also rotate together with the housing625.

The permanent magnets include four groups of permanent magnets622a-1,622a-2,622b-1,622b-2, each of which are circumferentially distributed around the rotor62. In one example, each group of permanent magnets622a-1,622a-2,622b-1,622b-2is arranged as a Halbach array. The first group of permanent magnets622a-1and the second group of permanent magnets622a-2form a first set of magnets622athat interact with the magnetic field associated with the first, radially inner coil portion614aand the first set of flux guiding stator elements612a. The third group of permanent magnets622b-1and the fourth group of permanent magnets622b-2form a second set of magnets622bthat interact with the magnetic field associated with the second, radially outer coil portion614band the second set of flux guiding stator elements612b.

The first group of magnets622a-1is located axially adjacent to (e.g., axially above) and facing a first axial end of the radially inner portions612a,613a,614aof the active parts of the stator61. The first group of magnets622a-1is separated from the first axial end of the active parts of the stator61by a first axial air gap, schematically indicated inFIG.11but too small to see, having a width GAir,1in the axial direction. The second group of magnets622a-2is located axially adjacent to (e.g., axially below) and facing a second axial end of the radially inner portions612a,613a,614aof the active part of the stator61. The second group of magnets622a-2is separated from the second axial end of the active parts of the stator61by a second axial air gap, schematically indicated inFIG.11but too small to see, having a width GAir,2in the axial direction. The values of GAir,1and GAir,2may be the same to balance loading.

The third group of magnets622b-1is located axially adjacent to (e.g., axially above) and facing a first axial side of the radially outer portions612b,613b,614bof the active parts of the stator61. The third group of magnets622b-1is separated from the first axial end of the active parts of the stator61by a third axial air gap, schematically indicated inFIG.11but too small to see, having a width GAir,3in the axial direction. The fourth group of magnets622b-2is located axially adjacent to (e.g., axially below) and facing a second axial end of the radially outer portions612b,613b,614bof the active parts of the stator61. The fourth group of magnets622b-2is separated from the second axial end of the active parts of the stator61by a fourth axial air gap, schematically indicated inFIG.11but too small to see, having a width GAir,4in the axial direction. The values of GAir,3and GAir,4may be the same to balance loading, and may be the same as GAir,1and GAir,2.

In use, the stator coils614of the phase modules610-1to610-6are excited with current from inverter circuits. The current flows in a circumferential direction through the inner coil portion614aand the outer coil portion614bof the phase modules610-1to610-6, changing direction in the end windings617. The magnetic flux generated by the current is guided in magnetic circuits axially through the body portions of the stator elements612, radially through the projections of the stator elements612, axially across the axial air gaps, and circumferentially between rotor magnets622. The magnetic field produced by the rotor magnets622interacts with the stator field to produce torque, which drives rotation of the rotor62and, via the coupling structure626, rotation of EPU drive shaft630. Rotation of the drive shaft630drives rotation of a propeller or fan, and a propeller interface65is shown inFIG.11.

Thus, a three-phase transverse flux motor60with axial air gaps and two coils per phase has been described. For completeness,FIGS.12A and12Billustrate an equivalent transverse flux motor70with radial air gaps. It will be appreciated that a radial air gap transverse flux motor300was described with reference toFIGS.7-8, but for a single phase having a single coil.

FIG.12Ais an axial end view of a transverse flux motor70having a stator71and a rotor72. The rotor72rotates about an axis of rotation73. Once again, the stator71includes six circumferentially arranged phase modules710-1to710-6. Radially opposite phase modules (e.g., phase modules710-1and710-4) are associated with the same phase (e.g., phase U) and are connected, for example, as shown inFIG.10A. The rotor includes a radially inner rotor portion72aand a radially outer rotor portion72b, with the stator71positioned radially therebetween. The stator coils74are omitted fromFIG.12Afor clarity, but a single stator coil74of one phase module710is illustrated in and will be described with reference toFIG.12B.

As in the motor60ofFIGS.9A-9B,10A-10B, and11, each phase module710-1to710-6of the motor70ofFIGS.12A-12Bincludes two sets of stator elements712a,712bdefining two circumferentially extending slot portions713a,713bhousing two circumferentially extending coil portions714a,714bof the coil714. However, while the two sets of stator elements612a,612bof the previously described motor60are radially spaced, the two sets of stator elements712a,712bof the motor70of the present example are axially spaced. This can be most easily appreciated fromFIG.12B, which shows one phase module710.FIG.12Bshows the two sets of axially spaced stator elements712a,712band the coil portions714a,714b, which are connected at circumferential ends of the phase module710by end windings717. The direction of current flow is indicated by the circumferential arrows labelled “{right arrow over (I)}”.FIG.12Bshows that in this example, each coil714is formed of a plurality of winding turns.

In the present example, each set of stator elements712a,712bincludes a plurality of circumferentially arranged, evenly distributed flux guiding stator elements712(e.g., stator elements712). Each of the stator element712is substantially as described above with reference toFIGS.7-8(e.g., each stator element712includes a body portion7121and two projections7122,7123). The stator elements712are oriented so that the body portions7121extend radially and the projections7122,7123extend axially from the body portion7121. The projections7122,7123of circumferentially adjacent stator elements712i,712iiof each set712a,712balternately project axially inwardly and axially outwardly so as to define an open slot cross-section, similar to that shown inFIG.7B. Collectively, all of the stator elements712within a set (e.g., set712a) define a slot portion (e.g., first axial slot portion713a).

ComparingFIG.12AandFIG.12B, the stator phase modules710-1to710-6extend axially into the plane of the page such that only one axial portion (e.g., portion712a) of each phase module710is visible. Similarly, the two rotor portions72a,72bextend axially into the plane of the page. Each rotor portion72a,72bincludes first and second axially spaced groups of permanent magnets such that, in total, the rotor72has four groups of magnets: a radially inner and axially inner first group722a-a, a radially inner and axially outer second group722a-b, a radially outer and axially inner third group722b-a, and a radially outer and axially outer fourth group722b-b. The dashed lines used for labels722a-band722b-bindicate that the permanent magnets of the second group722a-band the fourth group722b-bare axially behind those of the first group722a-aand the third group722b-a. Each group of permanent magnets722a-a,722a-b,722b-a,722b-bmay be arranged as a Halbach array.

The magnets of the first group722a-aface a radially inner side of the stator71at a first axial height and are separated from the radially inner side by a first radial air gap of width GAir,1. The magnets of the second group722a-bface the radially inner side of the stator71at a second axial height and are separated from the radially inner side by a second radial air gap of width GAir,2. The magnets of the third group722b-aface a radially outer side of the stator71at the first axial height and are separated from the radially outer side by a third radial air gap of width GAir,3. The magnets of the fourth group722b-bface the radially outer side of the stator71at the second axial height and are separated from the radially outer side by a second radial air gap of width GAir,4. The first radial air gap width GAir,1and the second radial air gap width GAir,2may be the same to balance loading. Likewise, the third radial air gap width GAir,3and the fourth radial air gap width GAir,4may be the same to balance loading. In some examples, all four radial air gaps widths are the same. The radial air gaps are only schematically indicated inFIGS.12A-12Bbecause the air gaps are too small to resolve.

In use, the stator coils714of the phase modules710-1to710-6are excited with current from inverter circuits. The current flows in a circumferential direction through the axially inner portion714aand the axially outer coil portion714bof the phase modules710-1to710-6, changing direction in the end windings717. The magnetic flux generated by the current is guided in magnetic circuits radially through the body portions of the stator elements712, axially through the projections of the stator elements712, radially across the radial air gaps, and circumferentially between rotor magnets. The magnetic field produced by the rotor magnets interacts with the stator field to produce torque that drives rotation of the rotor72a,72b.

Thus, a three-phase transverse flux motor70with radial air gaps and two coils per phase has been described.

As described above with reference toFIGS.3and4, for VTOL applications, it may be desirable to utilize multi-lane (e.g., dual-lane) electric motors.FIG.13andFIG.14illustrate how a multi-lane architecture may be implemented in a transverse flux electric motor.

FIG.13illustrates a transverse flux motor70′ (e.g., a motor) with radial air gaps and two independent power lanes (e.g., two sub-machines). The motor70′, which is similar in its construction to the motor70ofFIGS.12A-12B, includes six phase modules710-1′ to710-6′. While the motor70ofFIGS.12A-12Bhas two coils per phase with radially opposite coils connected and belonging to the same phase, the motor70′ has two three-phase sub-machines70a′,70b′ with one coil per phase, and radially opposite coils correspond to different sub-machines70a′,70b′. Each sub-machine70a′,70b′ receives its power from a different inverter so that if an inverter fails, one sub-machine70a′,70b′ is not affected by the failure.

In more detail, the circumference of a stator of the motor70′ is circumferentially divided into two sectors each spanning IT radians (180 degrees): a first sector70a′ and a second sector70b′. The first sector70a′ corresponds to a first three-phase sub-machine70a′ and has three phase modules710-1′ to710-3′, each corresponding to one phase of the first sub-machine70a′. The second sector70b′ corresponds to a second three-phase sub-machine70b′ and has three phase modules710-4′ to710-6′, each corresponding to one phase of the second sub-machine70b′. The stators of the sub-machines70a′,70b′ share and interact with a common rotor, which is configured in a same or similar way to the dual rotor of the motor70ofFIG.12A.

By increasing the number of sectors into which the circumference is divided, the number of sub-machines may be increased. For a number of sub-machines equal to NL, there may be NLsectors each spanning 2π/NLradians (360/NLdegrees). The number of coils per phase may be increased by increasing the number of phase modules per sector.

FIG.14illustrates a transverse flux motor60′ with axial air gaps and two independent power lanes (e.g., two sub-machines). The two sub-machines60a′ and60b′ are implemented by axially stacking two sets of active parts. Specifically, the motor60′ has two axially stacked sub-machines60a′,60b′, each of which is of similar construction to and operates in much the same way as the axial air gap motor60ofFIGS.9A-9B,10A-10B and11. Rotors of the two sub-machines60a′,60b′, each of which is of dual-rotor construction, are mechanically coupled so that the rotors rotate together, though it will be appreciated the two rotors may instead be independent and be separately connected to an output drive shaft.

An advantage of the axial stacking approach is that, as well as implementing multiple sub-machines for increased fault tolerance, the torque developed by the motor60′ is increased without requiring an increase in the motor diameter or the slot current density. Although the active parts mass does increase, the use of some common features (e.g., non-active features such as cooling and support structures) limits the overall increase in the mass of the motor60′. The number of power lanes may be increased beyond two, if this is desired, by axially stacking more than two sub-machines and/or dividing the circumference of each stator into multiple sub-machines as shown inFIG.13.

Motors in accordance with the present disclosure may be configured to have particularly high active part torque densities, defined in Equation (1). For example, motors may have a value of ρactof at least 50 Nmkg−1. Table 3 illustrates the calculation of ρactfor three motors in accordance with the present disclosure, each of which is a transverse flux motor.

TABLE 3mactτpeakρact(kg)(Nm)(Nmkg−1)10.287085.213.4130097.017.8145081.5

As shown, each of the example transverse flux motors has a particularly high value of ρact, in excess of 80 Nmkg−1. Noting that a VTOL aircraft may include at least four EPUs, such a high value of ρactresults in a significant mass saving when compared even to VTOL aircraft utilizing optimized radial flux motors.

The increased active parts torque density may be understood by comparing the two-dimensional magnetic circuits of the radial flux motor (FIG.5B) and the three-dimensional magnetic circuits of the transverse flux motor (FIG.8). Referring first toFIG.5B, the magnetic circuits are substantially two-dimensional (e.g., the magnetic circuits lie in planes perpendicular to the axis of rotation230) and pass through the annular region of the stator210in which the coils214are housed. There is, therefore, competition for space in this annular region between the conductor, which carries the current that generates the stator field, and the stator teeth that guide the flux in the radial direction. Thus, for a motor of a given active parts diameter, any gain in performance that may be realized by increasing the conductor volume may be offset by the effects of a corresponding reduction in a stator iron volume, and vice versa. For example, a higher conductor volume may allow more torque to be produced, or the same torque to be produced at lower current density, the latter reducing the cooling burden. However, this would require more slender stator teeth, which are less efficient flux guides, and/or fewer stator teeth, which may result in higher torque ripple (e.g., especially at the low rotor speeds for VTOL aircraft). Thus, an improvement in one performance metric (e.g., peak torque) will likely require either a reduction in another performance metric (e.g., efficiency and torque ripple) and/or an increase in the mass of the active parts of the motor. Referring now toFIG.8, in contrast withFIG.5B, the magnetic circuits are three-dimensional and spiral around the annular winding space313that houses the conductor. There is therefore no competition or much more limited competition for space in the stator between the conductor and the flux guiding stator iron. Consequently, both the stator pole design and number may be optimized (e.g., without reducing the conductor volume), resulting in a higher active parts torque density.

In designing for a high value of ρact, it is useful to introduce a dimensionless machine parameter Γ, defined in Equation (5) as the cumulated volume of the conductor (e.g., the stator coils), Vconductor, included in the motor divided by cumulated volume of the flux guiding iron material, Viron, included in the motor. In accordance with the present disclosure, a notably high value of Γ, greater than or equal to 0.25, may be selected to promote the production of high torque with a low active parts mass. Table 4 shows values of Γ for three motors in accordance with the present disclosure and sized for an EPU of a VTOL aircraft:

TABLE 4VconductorViron(cm3)(cm3)ΓExample 139.262.20.63Example 234.192.20.37Example 352.249.71.02

The iron material may be present in both the stator and the rotor. However, in the transverse flux motors of the examples described herein, only the stator includes iron material. This reduces the iron volume and promotes a higher value of Γ.

Another characteristic parameter for the purposes of an EPU of a VTOL aircraft is Λ, defined in Equation (3) as the ratio of the active parts torque density and the slot current density at the peak rated current. Λ may be a useful parameter for optimizing a motor for a VTOL EPU because the parameter rewards torque production but penalizes the addition of active parts mass, which increases the EPU weight, and at the same time penalizes the use of a high slot current density, which creates onerous cooling requirements and increases the likelihood of failures. In accordance with the present disclosure, a particularly high value of Λ (e.g., greater than or equal to 5 μN3mkg−1A−1) may be selected. Table 5 shows values of Λ for three transverse flux motors in accordance with the present disclosure:

TABLE 5ρactJslot,peakΛ(Nmkg−1)(Amm−2)(μN3mkg−1A−1)846141087.512.576116.9

As noted above, the radial flux motor200has magnetic circuits that are two-dimensional and pass radially through the annular region of the stator210in which the slots213are defined. This creates competition for space in the annular region of stator210between the flux guiding material (e.g., the stator teeth212) and the slot213that houses the conductor (e.g., the coils214). This provides that increasing the number of stator pole pairs requires a decrease in the volume of conductor. Equivalently, increasing the volume of conductor requires a decrease in the number of stator pole pairs. In contrast, in a transverse flux motor, there is no, or much more limited, competition for space in the annular stator region. Thus, the number of stator pole pairs, formed by the stator elements312in this example, may be increased with no impact on the volume of conductor. The impact of this may be appreciated fromFIG.15.

FIG.15is a plot1100illustrating how, for a motor of a given diameter, the tangential force (y-axis) developed by the motor varies with the pole pitch, Pθ(x-axis), and the air gap width, GAir. Two plots1101,1102corresponding to two air gap widths are shown: a 3 mm air gap (plot1101) and a smaller 1.4 mm air gap (plot1102). These values are shown purely for the purpose of explanation.

As can be seen from both plots1101,1102, at small values of the pole pitch, Pθ, the tangential force increases as the pole pitch increases. However, the tangential force eventually reaches a maximum at a particular value of the pole pitch Pθ,max. Increasing the pole pitch beyond Pθ,maxdecreases the tangential force and thus reduces the torque developed by the motor. By comparing the two plots1101,1102, it is also shown that: (i) the tangential force increases as the air gap decreases; and (ii) the value of Pθ,maxdecreases as the air gap decreases.

FromFIG.15, it may be appreciated that to increase the torque density of a motor of a given diameter, it is desirable to decrease the air gap width. However, for a given air gap width, torque production may only be maximized if the pole pitch may be decreased to Pθ,max. In a radial flux motor, the extent to which Pθmay be decreased (e.g., by increasing the number of pole pairs) is limited by competition for space in the stator. In a transverse flux motor, however, the extent to which Pθmay be decreased (e.g., by increasing the number of pole pairs) may instead only be limited by manufacturing constraints and the flux guiding efficiency of the stator iron. Thus, a transverse flux motor may access an upper-left region of the plot1100corresponding to high torque density, whereas a radial flux motor may only access a relatively lower-right region of the plot.

In accordance with the present disclosure, to further optimize a motor for use in an EPU of a VTOL aircraft, the value of a motor parameter γ, defined in Equation (9) as the product of the pole pitch, Pθ, and the air gap width, GAir, may be selected to be in the range 5 to 100 micro radian-meters. Table 6 shows examples of values of γ for three example motors in accordance with the present disclosure. Values are provided in micro radian-meters.

TABLE 6Υ (10−6radian-meters)Example 1Example 2Example 311.026.768.1

For motors sized for VTOL aircraft, the selection of a value ofin this range may optimize the torque-producing tangential force and thus increase the active parts torque density, ρact. Small values of Pθ(e.g., less than or equal to 10 degrees, or less than 5 degrees) and high values of the pole pair number (e.g., at least 15, or greater than or equal to 50) may be provided, along with small values of the air gap width (e.g., less than or equal to 1.5 mm). Motors in accordance with the present disclosure may have more than one air gap because of the use a dual rotor design and/or the use of axial stacking of active parts to implement multiple lanes. All air gaps of a given motor may be approximately the same size, such that the value ofwill be approximately the same for all air gaps of a motor. Where different air gaps are used, however, the largest air gap may be used to calculateas the largest air gap may limit the torque density.

Motor-inverter combinations in accordance with the present disclosure may also have optimized values of a parameter Π, defined in Equation (11). Π is the ratio of the pole arc length, PL(see Equation (28)) and the maximum value of the electrical frequency, fmax, of the current output by the inverter and received by the stator coils of the motor during use. In accordance with the present disclosure, the value of Π may be between 1 and 30 μms, which is unusually low. Table 7 shows examples of values in accordance with the present disclosure.

TABLE 7PL(mm)fmax(kHz)Π (μms)Example 17.01.54.7Example 24.20.67.0Example 317.51.214.6

As described above with reference toFIG.4, particularly in point d), the propulsion system of a VTOL aircraft has a large number of inverters as a result of its distributed propulsion system and fault tolerant electrical architecture. This results in the stacking of inverter losses and provides that inverter efficiency may have a significant impact on performance (e.g., aircraft mission range). Selecting an unusually low value for Π (e.g., in a range of 1 to 30 μms or in a range of 3 to 15 μms) has been found to reduce inverter losses when operating at a relatively low rotor speed. The use of a low value of Π may therefore not only reduce inverter losses, but also allow for the omission of a speed-reducing gearbox in the EPU without sacrificing low aerodynamic noise or motor efficiency.

An important consideration in the context of aerospace electrical machines is fault tolerance. In accordance with the present disclosure in which the electrical machines may be the permanent magnet type, the tolerance to a stator terminal short circuit fault may be particularly important.

In the event of a stator terminal short circuit fault (e.g., a short circuit fault condition in the electrical network connected to the stator terminals), the rotation of the rotor will drive a fault current into the network for as long as the rotor excites the stator windings. In motor designs that feature rotor windings, it is possible to stop excitation of the rotor windings to prevent the excitation of a voltage in stator windings and thus stop the fault current. However, in a permanent magnet motor, the rotor is permanently excited and will, unless the permanent magnets are demagnetized or the rotor is moved away from the stator, continue to excite a voltage in the stator windings that will drive the fault current. With zero or little impedance in the short-circuited electrical network, this fault current may be very large. The heat dissipated by the stator windings, which causes heating of the coil insulation, increases with the square of the current (I2R losses).

One potential mitigation to this problem is for the EPU to include a mechanism or device to physically disconnect the permanent magnet rotor from the propeller fan so that the inertia of the propeller does not continue to force rotation of the rotor. For example, a freewheel transmission may be included in the EPU. However, this solution may add mass, complexity, and maintenance requirements to the EPU. Another potential mitigation would be to provide additional overrating to the cooling system of the motor, so that the cooling system may maintain the temperature of the insulation at or below its rated temperature even in the presence of a terminal short circuit fault. However, this also adds mass to the EPU and may make air cooling (described in more detail below) unfeasible, adding even more mass to the EPU due to the requirement to adopt liquid cooling.

In accordance with the present disclosure, an electrical machine may have a short-circuit insulation temperature parameter, ζ, defined in Equation (17) that satisfies the inequality:

ζ=θins,cont(ISC)θins,cont(Ic⁢o⁢n⁢t)≤1.1.

In the above equation, ISCis the steady-state short circuit current, and Icontis the continuous rated current (e.g., the highest current the stator coils are rated to carry for a sustained period; this is associated with production of the maximum continuation rated torque, τmax,cont). θins, cont(ISC) is the temperature of the insulation when carrying the steady-state short circuit current, and θins, cont(Icont) is the temperature of the insulation when carrying the continuous rated current. Designing a motor to have a short-circuit insulation temperature parameter, ζ, less than or equal to 1.1 may allow the stator coils and their insulation to be sufficiently cooled following a terminal short circuit fault without additional overrating the cooling system. A value of ζ in the range of 0.7 to 1.0 may be provided and may, for example, allow for the use of air cooling in a transverse flux motor without additional overrating of the cooling system or a reduction in the performance of the motor during normal operation.

Additionally or alternatively, a short circuit current ratio, ξ, defined in Equation (16), may satisfy the inequality:

0.5≤ξ=ISCIpeak≤1.2

In the above equation, Ipeakis the peak rated current (e.g., the current associated with production of the peak rated torque, τpeak). A value of this ratio in a range of 0.6 to 0.9 (e.g., in a transverse flux motor) may strike a good balance between fault tolerance and good electrical and mechanical performance.

A further motor design optimization in accordance with the present disclosure is to select a design with a value of a characteristic motor parameter Δ, defined in Equation (6), greater than or equal to 65 Nmkg−1. Table 8 shows the calculation of Δ for three exemplary transverse flux motors, sized for use in the EPU of a VTOL aircraft.

TABLE 8ρact(Nmkg−1)cos(Ø)Δ (Nmkg−1)Example 1850.65131Example 2950.75127Example 3740.8587

The selection of a value of greater than or equal to 65 Nmkg−1, particularly a value in a range of 80 to 190 Nmkg−1, may provide a surprising combination of low EPU mass and fault tolerance. For example, such a selection may correspond to a sweet spot in the combined mass of an EPU's motor, inverter, and cooling system while offering good tolerance against stator terminal short circuit faults. This may be understood in terms of the effect of the power factor and its relationship with the torque density of the motor. A motor with a low power factor may require oversized power electronics but will also have a lower steady state terminal short circuit current. Thus, the selection of the power factor affects the inverter mass and also the required cooling system mass, as the cooling system may be sized to cool the motor under short circuit conditions. At the same time, the value of the power factor is mediated by the inductance of the motor, which depends on the quantity and distribution of active parts. This affects the active parts mass and the peak rated torque. A value of Δ in a range of 80 to 190 Nmkg−1may strike an effective balance between these competing requirements.

It is also useful to introduce a motor parameter Z, defined in Equation (14) as the product of the power factor of the motor and active parts mass divided by the efficiency, n, of the motor. The efficiency is defined as the efficiency when the motor is producing the maximum continuous rated torque, τmax,cont, at ISA sea level conditions. Table 9 illustrates values of Z in accordance with the present disclosure. The values of Z are notably low and may be associated with a strong balance between efficiency and fault tolerance in a motor sized for VTOL aircraft.

TABLE 9Z (kg)Example 1Example 2Example 3117.214.5

In accordance with the present disclosure, the value of Z may be less than or equal to 30 kg or in a range of 5 to 15 kg. This may be achieved most effectively in a transverse flux motor, where the inductance may be tuned to achieve a desirable power factor, (e.g., in a range of 0.6 to 0.9), without a significant negative impact on the efficiency of the motor. In a radial flux motor, the length of the magnetic circuits (illustrated inFIG.5B) may be shorter than those in a similarly-sized transverse flux motor (illustrated inFIG.8). The tuning of the inductance of the radial flux motor may therefore require the addition of significant active parts mass, or the selection of a sub-optimal design (e.g., the selection of long and narrow stator teeth; seeFIG.6, design (i)), which may increase the inductance but at the same time reduce the efficiency of the motor.

In a similar manner, a value of a motor parameter λ, defined in Equation (20) as the product of the efficiency and the inductance of the machine divided by its active parts mass, may be tuned to improve balance between efficiency and fault tolerance. Table 10 illustrates values of λ in accordance with the present disclosure. The values of λ are notably high.

TABLE 10λ (μHkg−1)Example 1Example 2Example 36.81.83.0

The value of λ may be selected to be greater than or equal to 1.4 μHkg−1, while values in the range of 2.1 to 5.5 μHkg−1may provide a particularly good balance between the competing constraints. The machine inductance itself, Lmachine, may be relatively high, especially relative to the mass of the active parts (i.e., Lmachinedivided by mactmay be particularly high).

Another important consideration in the design of a motor for an EPU of an aircraft is the capability of the cooling system of the motor. The cooling system may be capable, at all relevant operating conditions, of removing heat from the motor at a rate sufficient to keep the motor below a rated temperature. Herein, the rated temperature may be a maximum rated temperature of the coil insulation, θins,max. The cooling system may significantly add to the mass of the EPU, and this additional cooling system mass (mcool) is multiplied by the number of EPUs on the aircraft. Thus, rather than designing an EPU with a high torque production capability and an aggressive cooling system, which may have a high mass, it may be desirable to consider a parameter ∇, defined in Equation (12):

∇=τmax,contma⁢c⁢t×Cmax,cont(12)

In this equation, τmax,contis the maximum continuous rated torque, and Cmax,contis the heat capacity cooling rate required to maintain the coil insulation at or below its rated temperature, θins,cont, assuming operation at ISA sea level conditions. Cmax,contmay be defined as the product of the specific heat capacity of a coolant of the cooling system (at ISA sea level conditions) multiplied by the mass flow rate of the coolant required to maintain the coil insulation at or below θins,max. In accordance with the present disclosure, a value of ∇ may be selected so that the combined mass of the active parts and the cooling system may be optimized relative to the torque producing capability of the motor. Table 11 illustrates values of 7 for a motor sized for an EPU of an aircraft in accordance with the present disclosure. The values of V, which are notably high, are quoted in units of Kskg−1(Kelvin-seconds-per-kg):

TABLE 11∇ (Kskg−1)Example 1Example 2Example 30.190.270.55

The value of ∇ may be selected to be greater than or equal to 0.1 Kskg−1, greater than or equal to 0.18 Kskg−1, or greater than or equal to 0.21 Kskg−1.

It is also useful to introduce a dimensionless figure of merit, F, for a motor of a VTOL aircraft. F is defined in Equation (22):

F=τmax,contma⁢c⁢t⁢pair,0Cp⁢m.max,cont⁢(θins,max-θa⁢i⁢r,0)⁢2⁢π×Drefωmech,cont⁢(DrefDact)2(22)

In this equation, τmax,contis the maximum continuous rated torque, mactis the active parts mass, Cpis the specific heat capacity of the coolant at ISA sea level conditions, θins,maxis the maximum rated temperature of the insulation for operation at the maximum continuous rated torque, {dot over (m)}max,contis the mass flow rate of the coolant required to maintain the insulation at or below θins,maxduring ISA sea level operation at τmax,cont, ωmech,contis the angular speed of rotation (in radians per second) of the rotor of the motor while producing the maximum continuous rated torque, and Dactis the active parts diameter. The remaining values are fixed, nominal operational, values: pair,0is a nominal ambient air pressure equal to 100 kPa, pair,0is a nominal ambient air temperature of 318 Kelvin, and Drefis a nominal motor diameter set equal to 0.5 meters.

For the purposes of comparing two motors, any value may be selected for Drefas long as the same value of Drefis used for both calculations. The value of F is decreased by using an active parts diameter, Dactgreater than Drefbut increased by using an active parts diameter, Dact, less than Dref. In other words, the equation for F penalizes the use of an arbitrarily large active parts diameter to meet the torque and speed requirements of the motor, as the use of an arbitrarily large diameter would create installation and aerodynamic drag issues. The selection of 0.5 meters for Drefreflects that 0.5 meters is a reasonable value for certain EPU designs. If calculating and comparing values of F for a smaller platform (e.g., an unmanned aerial vehicle (UAV) or drone), a smaller value of Drefmay be selected (e.g., 0.1 meters). If calculating and comparing values of F for a larger platform (e.g., a larger aircraft), a higher value of Drefmay be selected (e.g., 1.0 meters). Accordingly, in Equation (22), it is the value of Dact, and not Dref, that characterizes the motor.

In order to provide a particularly good balance between the competing requirements of physical size (e.g., active parts diameter), mass, torque production, and cooling, electrical machines in accordance with the present disclosure may have a particularly high value of F. The first two rows of Table 12 illustrate values of F for motors in accordance with the present disclosure. For comparison, the third line of Table 12 illustrates the value of F for an exemplary radial flux motor designed for use in a CTOL aircraft having a more conventional value of F.

TABLE 12FAir-cooled transverse flux motor (VTOL)5.2Liquid-cooled radial flux motor (VTOL)2.3Liquid-cooled radial flux motor (CTOL)0.3

According to the present disclosure, the value of F may be greater than or equal to 1.9. In some examples of electric motors for EPUs of VTOL aircraft, particularly those utilizing a transverse flux arrangement, the value of F may be greater than or equal to 2.5.

A motor and EPU for these applications may use an air cooling system. This is partly due to a reduction in the complexity and maintenance requirements associated with a liquid cooling system. However, a potentially more significant benefit is the reduction in the cumulated mass of the components of the cooling system, which may otherwise make a substantial contribution to the EPU mass and platform mass. For example, a liquid cooling system will include not only the mass of the liquid coolant, but may also include: the mass of the coolant tank; conduits (e.g., piping) through which the coolant flows; the mass of pumps, valves, and other fluid flow modulating components; the mass of filters; and the mass of heat exchangers. In one example, the mass of a liquid cooling system sized for a motor of a VTOL aircraft EPU is about 14 kg, representing approximately 20-25% of the overall mass of the motor. If each one of the six EPUs of the exemplary VTOL aircraft1ofFIG.1had such a cooling system, the total motor cooling system mass for the entire aircraft would be about 84 kg, which is on the order of the mass of a passenger. In contrast, an example of an air-cooling system having a mass generally limited to the mass of filter components and flow directing components may only be about 3-5 kg per EPU.

While the advantages of selecting an air-cooling system may be clear, implementing an air-cooling system in a motor for an EPU of a VTOL aircraft requires more consideration. Air has a relatively low specific heat capacity compared with certain liquid coolants (e.g., 1006 Jkg−1K−1for air, compared with 1745 Jkg−1K−1for one oil-based coolant), and the available mass flow rate may be limited in VTOL applications due to both the low density of air compared to liquid and the relatively slow movement of the aircraft at some operating points. This may limit the rate at which heat may be removed from the motor. If the slot current density, Jslot, is high, there will be high resistive losses (I2R losses) and/or a lack of free space in the slot to effectively cool the coils, which may make air-cooling impractical. If the slot current density, Jslot, is too low, the motor may not be able to meet its torque production requirements.

In accordance with the present disclosure, the selection of a transverse flux motor with one or more of the optimizations described above (e.g., an optimized value ofto access the peak of the torque curve illustrated inFIG.15) may allow for use of air cooling in an EPU of a VTOL aircraft, particularly where the flow of cooling air is supplied to directly contact the coils.FIGS.16to22illustrate transverse flux electrical machines with air cooling systems and, more specifically, transverse flux motors with directly cooled conductors.

FIG.16AandFIG.16Bshow a motor60in perspective view. The motor60includes a rotor62that is coupled to a drive shaft630via a coupling structure626and rotates about an axis of rotation63. The motor60further includes a bearing unit64. Reinforcement ribs670are arranged circumferentially around the bearing unit64and the axis of rotation63and are fixed to the bearing unit64and to a base plate672of the bearing unit64. The coupling structure626is visible in the perspective bottom view ofFIG.16B. At a radially outer region674of the coupling structure626, the coupling structure626is connected to the rotor62. At a radially inner region676of the coupling structure626, the coupling structure626is connected to the EPU drive shaft630.

FIG.17shows the motor60ofFIGS.16A and16Bin cross-section.FIG.17shows the same motor60asFIG.11, but further shows cooling channels602in the stator61and omits the active parts for clarity. An empty volume662that would accommodate the active parts is labelled, as are the locations of axial air gaps615formed between the active parts of the stator61and the rotor62. In operation, an external flow of ambient air that impinges on the EPU due to, for example, movement of the aircraft and/or wind enters the motor housing and is guided by the cooling channels602in a radially outward direction into the volume662where the active parts are located. Thus, the ambient air directly contacts and cools the active parts, including the stator coils.

For effective direct cooling, the volume662may not be completely filled and leaves space through which the cooling air may pass. For example, the stator coils614may define an effective cooling surface area that is directly exposed to air.FIGS.18-20illustrate an example of a stator phase module structure610by which the stator coils define an effective cooling surface area for direct cooling.

FIG.18AandFIG.18Bshow a stator phase module610mounted to its support structure640. The phase module610is comparable to the one shown inFIG.9B, which is referred to for a detailed explanation.FIG.18AandFIG.18Bshow the same embodiment; however,FIG.18Ais cut in a radial plane, enabling a view of the cross section of the first coil portion614aand the second coil portion614bof the coil614, and the first slot portion613aand the second slot portion613bof the slot613.FIG.18Bshows the end windings617.

The stator according toFIG.18Aincludes an assembly680that may be a modular (e.g., prefabricated) component. The assembly680extends in the radial direction (r) and in the circumferential direction (@), and includes two axially spaced, non-magnetic and non-magnetizable support structures640i,640ii. These have radially inner fastening areas690,692, at which the support structures640i,640iimay be connected to a plurality of ribs of the stator. This is done, for example, via retaining projections of the ribs, as will be described with reference toFIG.21.

Flux guiding stator elements612extend between the support structures640i,640ii, and collectively provide the flux guiding stator iron611of the stator. The stator elements612define the slots613(i.e., the winding space) extending in the circumferential direction, in which the coil614extending in the circumferential direction is arranged.

According to the present example, a flow of air (e.g., the flow of external ambient air that enters the EPU and is directed by the stator cooling channels602ofFIG.17) flows radially through the assemblies680in the region between the two support structures640i,640iiand flows across the stator elements612and the coil614. This flow of air cools the stator elements612and the coil614. The stator elements612are each aligned radially. The stator elements612each have two radially aligned side surfaces694,696spaced apart in the circumferential direction, both of which are cooled by a cooling air flow.

The coil614includes multiple individual winding turns6140(seeFIG.18A) that are formed from a continuous winding wire. Further, each coil portion614a,614bof the coil614includes two axially spaced winding packages: the first coil614aportion has a first axially spaced winding package614a-iand a second axially spaced winding package614a-iiin the first slot portion613a, and the second coil portion614bhas a third axially spaced winding package614b-iand a fourth axially spaced winding package614b-iiin the second slot portion613b. The winding packages614a-i,614a-ii,614b-i,614b-iieach have sections extending longitudinally in the circumferential direction of the coil614. As shown inFIG.18B, through the deflected coil section that forms the end winding617, the winding packages614a-i,614a-ii,614b-i,614b-iiform a coil614. A corresponding end winding617is found at the other end of the coil614.

The winding packages of each pair of winding packages614a-i,614a-ii, and614b-i,614b-iiare spaced apart in the axial direction from one another and from the support structures640i,640ii. In this way, cooling air may flow around the winding packages on their upper side and on their lower side. This is illustrated inFIG.19A. The assembly680defines three radially extending and axially spaced cooling air flow passages A1, A2, A3 for cooling the winding packages614a-i,614a-ii,614b-i,614b-ii. A first cooling air flow passage A1 runs adjacent to the upper support structure640i, a second cooling air flow passage A2 runs in an area between the winding packages614a-i,614b-iand the winding packages614a-ii,614b-ii, and a third cooling air flow passage A3 runs adjacent to the lower support structure640ii. The division of the coil614into axially spaced winding packages614a-i,614a-ii,614b-i,614b-iiincreases the surface area of the winding that is available for direct cooling. While two axially spaced windings packets per winding portion are shown in this example, more than two axially spaced winding packets may also be provided. Alternatively, it is also possible that only one winding package is arranged in each slot portion613a,613b.

According toFIG.18AandFIG.19A, two winding packets614a-i,614a-iiand614b-i,614b-ii, respectively, (or, e.g., one coil portion614a,614b) may each be fixed by a fixing material6130in the respective slot portion613a,613b. In the present example, the fixing material6130only slightly extends in the circumferential direction (e.g., being in the shape of a disk or plate) so as not to impair cooling by obstructing the cooling air flow. The fixing material may be arranged radially in front of or behind a stator element612, so as to limit a reduction in the cross-section facing and exposed to a radial air flow.

To avoid physical contact of the coil614with the stator elements612, a mechanical protective layer may also be applied to the stator elements612on the side facing the slot613(e.g., the slot portions613a,613b). For example, an aramid paper may be used, analogous to the use of slot papers in the slots of radial flux machines.

Referring again toFIG.18A, the arrangement of the stator elements612for defining the slot portions613a,613bis shown. The stator elements612are arranged in four circumferential rows: two rows612a-i,612a-iibeing of the first set612aof stator elements612and defining the first slot portion613a, and two rows612b-i,612b-ii, being of the second set6122bof stator elements612and defining the second slot portion613b.

The stator elements612are, like those shown in previous examples, curved and/or bent. For example, the stator elements612may be claw-shaped and/or curved in a C-shape. The stator elements612of the respective radially inner rows612a-i,612b-iiare concave, viewed from the radially outer side, and the stator elements of the respective radially outer rows612a-ii,612b-iiare convex, viewed from the radially outer side, so that their mutually facing sections together define the slot portions613a,613b. The stator elements612of each of the two rows delimit the slot portions613a,613btransversely to the circumferential direction. For this purpose, each stator element612of a given row (e.g., row612a-i) forms a pair of stator elements with a circumferentially adjacent stator element belonging to a radially adjacent row (e.g.612a-ii) of the same set of stator elements (e.g.,612a), and stator elements of a pair are oriented such that the stator elements of the pair oppose each other.

End portions (e.g., projections) of the stator elements612form pole heads (e.g., upper pole heads and lower pole heads; see, e.g., the pole heads3122,3123inFIG.7B). The end portions are positioned adjacent the permanent magnets of the rotor62and are separated from the permanent magnets of the rotor62only by an air gap (e.g., corresponding to the air gap615ofFIG.17). For this purpose, it is provided that the end portions or pole heads are each arranged in one of the support structures640i,640iiand terminate flush with their outer sides641i,641ii. Accordingly, the upper end portions of the stator elements612lie in the outer plane of the upper outer side641iof the upper support structure640i, as shown inFIG.18A.

A motor includes a plurality of the assemblies680, adjoining one another in the circumferential direction. For example, six assemblies680may be provided for the motor described with reference toFIGS.9-11, with two per phase.

FIG.19Bis a schematic sectional view of an embodiment of a coil614. The coil614has a continuous winding wire that is wound in a number of winding turns6140, with each winding turn6140extending over an angle of 360°. A total of fourteen winding turns6140are provided in the exemplary embodiment considered. The coil614is configured such that the total of fourteen winding turns6140are arranged in four levels or coil layers L1, L2, L3, L4, with three winding turns6140-1to6140-3arranged in the first coil layer L1, four winding turns6140-4to6140-7arranged in the second coil layer L2, four winding turns6140-8to6140-11arranged in the third coil layer L3, and three coil turns6140-12to6140-14arranged in the fourth coil layer L4. In the embodiment shown, each coil layer L1, L2, L3, L4 is arranged in an axial plane, parallel to each other.

The winding order is indicated by the arrows6142. From the winding sequence, it follows that in the case of the winding turns6140of the first coil layer L1, a turn diameter DTurnof the winding turns6140decreases as the number of winding turns increases. In other words, the continuous winding wire or conductor is moving inwards with every winding turn6140in the first coil layer L1. Thus, winding turn6140-1has a larger turn diameter than winding turn6140-2, which has a larger turn diameter than winding turn6140-3. “Turn diameter,” in this context, refers to the average diameter of a 360° loop of one winding turn6140around winding turn axis “W”, and not to the diameter of the wire or conductor. An example of the turn diameter DTurnIS shown inFIG.19Bfor the twelfth winding turn6140-12. A winding turn with a smaller turn diameter lies radially (e.g., with respect to the winding turn axis) within an adjacent winding turn with a larger diameter. For example, winding turn6140-2lies within winding turn6140-1.

In contrast, in the coil layer L2, the turn diameter of the winding turns6140increases with an increasing number of winding turns. For example, the winding turn6140-5has a larger turn diameter than the winding turn6140-4. In the third coil level L3, the turn diameter of winding turns6140decreases again as the number of windings increases, and in the fourth coil level L4 the turn diameter increases again.

The described coil614forms winding packages614a-i,614a-ii,614b-i,614b-iicorresponding to the winding packages614a-i,614a-ii,614b-i,614b-iiofFIGS.18A and19A.

FIG.20Ashows an embodiment of a coil614with a structure according toFIGS.19A-Bin a view from above.FIG.20Ashows that the coil614may have a curved shape similar to that of a banana. Accordingly, the coil614includes longitudinally extending sections614a,614bthat may be concavely bent. The longitudinally extending sections614a,614bare bent over and form deflected sections at the end windings617. The top view ofFIG.20Ashows the coil turns6140-1,6140-2, and6140-3of the first coil layer L1 ofFIG.19.

FIG.20Bschematically shows a coil614that corresponds toFIGS.19A and20Ain terms of structure. The side view ofFIG.20Bshows the individual coil layers L1, L2, L3, and L4 in which a plurality of winding turns6140are formed. Further, the coil614ofFIG.20Bis additionally shown with fixing material6130in the form of fixing disks6132that correspond to the fixing material6130ofFIGS.19A and20A, and serve to arrange and position the coil614in the slot613.

The shape of the coil614inFIGS.19A-19B and20A-20Bis a non-limiting example. In principle, the coil614(and/or the winding packages614a-i,614a-ii,614b-i,614b-ii) in the winding diagram and structure shown inFIG.19Bmay have other shapes, (e.g., circular, elliptical, or with a plurality of concave and convex areas). The use of fixing disks6132according toFIGS.20A,20Bis also optional.

As explained above with reference toFIG.17, ambient air may enter a motor and be directed radially outwardly by channels602towards the active parts of the motor. As explained with reference toFIGS.18A-B,19A-B, and20A-B, the stator phase modules may be arranged so that there are gaps (e.g., radial gaps between stator elements612and passages A1, A2, A3) for air flow for effective direct air cooling.FIGS.21and22illustrate this in more detail for a dual-lane transverse flux motor80.

FIG.21shows an electric drive unit with the motor80. InFIGS.11,16A-B, and21, like reference numerals label like parts. The motor80includes a rotor81, a stator82, an axis of rotation83, and a bearing unit84. The bearing unit84includes an axially arranged, rotatable EPU drive shaft830and a static bearing part822that supports the EPU drive shaft830. The coupling structure described with reference toFIGS.16A and16Bis not shown inFIG.21, but may be included in a corresponding manner.FIG.21shows a stator82as a ring structure with a large number of ribs820that adjoin one another in the circumferential direction and each form a cooling air passage821between the ribs820. The active components of the stator82are held and positioned by the ribs820. For this purpose, the ribs820have retaining projections823.

A difference fromFIGS.11,16A and16Bresults from the fact that the motor unit ofFIG.21includes two rotor-stator assemblies8110,8120, each forming a sub-machine of the dual-lane motor, which are axially stacked (e.g., arranged one behind the other in the axial direction) and are fixed to one another. Accordingly, the rotor81includes three axially spaced outer walls811,812,814, each of which has or integrates permanent magnets85, and two radially outer end walls813,815. The outer walls811,812,814and the end walls813,815form two axially spaced volumes882of the two rotor-stator assemblies8110,8120, each containing the active components of the stator82of the respective assembly.

The permanent magnets85of the rotor are only shown on the right-hand side ofFIG.21for the sake of clarity. The permanent magnets85are arranged on the insides of the outer walls811,812,814. The air gap615shown inFIG.17Aruns between the permanent magnets85and the associated stator poles of the assembly.

FIG.21also shows how a cooling air flow may be provided through the cooling air passages821and the active components of the stator82arranged in the volume882. The transverse flux machine may have a first end8010facing a mechanical load to be driven (e.g., a propeller) and a second end8020facing away from the load to be driven. InFIG.21, the transverse flux machine forms openings801at its first end8010, which enable an air flow860to enter the motor unit in an initially primarily axial orientation. This may be supported by a fan891, which is, however, optional. For example, the airflow may come from a propeller driven by the EPU drive shaft830.

The second end8020facing away from the load to be driven is hermetically sealed to prevent inflowing air from leaving the motor unit again in the axial direction. For this purpose, a cover plate802is provided, which is shown schematically. The cover plate802is connected to the stator82inFIG.21, but may alternatively be connected to the rotor81, or be formed by a coupling structure similar to the coupling structure626ofFIGS.11,16Aor the like, depending on the design.

By the cover plate802, the inflowing air flow860flows radially outwards as an air flow861through the cooling air passages821and the active components of the stator arranged in the volume882. The radial air flow861may also be optionally supported by fans892.

The end walls813,815of the rotor81are provided with radial openings816that enable the cooling air flow861to be directed into the environment. Alternatively, openings may be formed in the motor unit at the second end8020facing away from the load to be driven, while the first end8010facing the load to be driven may be sealed airtight in this case. A further alternative is that a cooling air flow is directed radially inwardly through the stator82. For this purpose, an air flow located at the outer circumference of the rotor, which may originate from a propeller, for example, may be deflected by baffles or other deflecting mechanisms or devices, and guided through openings816in the walls813,815of the rotor81into the stator82. In this way, the radial direction of the air flow may be reversed, with the air flow going from radially outside to radially inside through the active components of the stator (e.g., that are arranged in the volume882) and the cooling air passages821.

FIG.22shows a detail cutaway and perspective view of the example motor80shown inFIG.21. The first rotor-stator assembly8110and the second rotor-stator assembly8120(e.g., sub-machines) are visible. The connection between the radially inner fastening areas690,692of stator assemblies680and retaining projections823of the stator82(e.g., integrally connected to the ribs820, respectively) is also visible. Further, the axial air gaps815, which are arranged between the permanent magnets85of the rotor82and each support structure640i,640ii, are indicated.

As noted above, for the purposes of aircraft (e.g., VTOL aircraft), an air cooling system may be used because of the associated reduction in EPU mass, complexity, and maintenance requirements. However, meeting the platform torque production requirements (e.g., with a high active parts torque density) while using air cooling may necessitate the use of direct air cooling (e.g., a cooling system in which heat is transferred directly from the coils into the cooling air, rather than via a heat exchanger).FIGS.16to22show motors60,80, stator modules680, and coils614that incorporate spaces through which air may flow to directly cool the active parts. For example, the illustrated coils have multiple (e.g., two) winding packages to define an intermediate air passage A2 for direct cooling of axial inner regions of the coil, with additional spaces left in the axial outer regions to define axial outer air channels A1, A3.

Increasing the amount of free space in the active parts region (e.g., the empty volume662shown inFIG.17) may improve the direct air cooling of the active parts, since the effective cooling surface of the coils (e.g., the percentage of the coil surface area directly exposed to the air) may be increased. However, this is balanced against the reduction in the slot current density that results from reducing the amount of conductor that is packed into the slot. Reducing the slot fill factor (e.g., packing factor) too far may necessitate an increase in the current, which may result in additional heat production (I2R) that is greater than the additional heat removal capability of the cooling system gained from the reduced slot packing.

In accordance with the present disclosure, the coil design may be selected to tune the effective cooling surface area to optimize the balance between direct air cooling efficiency and slot current density. For example, the number of winding packages, the number of turns per winding package, the arrangement of turns within a winding package, the turn cross-section, the turn radius, or any combination thereof may be adjusted to optimize the effective cooling surface area. The effective cooling surface area percentage, expressed as a percentage, is defined in Equation (29):

Effective⁢cooling⁢surface⁢area⁢%=Exposed⁢coil⁢surface⁢areaTotal⁢coil⁢surface⁢area×1⁢0⁢0⁢%(29)

In this equation, the total coil surface area is the sum of the surface areas of each winding turn of the coil. The exposed coil surface area is the sum of the areas that are exposed to the cooling flow of air for direct cooling.

FIGS.23A and23Billustrate how, for a given number of winding packages and turns (e.g., in this case, two winding packages each having seven turns arranged in a trapezium shape), the effective cooling surface area may be tuned by adjusting the conductor cross-section and radius.

In the first example, depicted inFIG.23A, each turn6140of the winding package614a-ihas a circular cross-section. In the second example, depicted inFIG.23B, each turn6140of the winding package614a-imay have a rectangular cross-section with rounded corners. In both cases, the turns6140are arranged in two rows: a first row of three turns and a second row of four turns, wherein the turns are packed as closely as possible. The effective cooling surface area of the winding package614a-iis the outer surface area of the winding package614a-l, which is exposed to the flow of air. In both cases, the curvature of the turn cross-section results in empty central regions6145that are not occupied by conductor but are also inaccessible to cooling air, and thus do not form part of the effective cooling surface area.

Comparing the first and second examples, the empty central regions6145are larger in the example inFIG.23Athan in the example inFIG.23Bdue to the greater curvature of the turns in the example inFIG.23A. This reduces the slot current density, which reduces torque production, without offering any improvement in cooling. However, portions of the effective cooling surface areas adjacent regions A1, A2 (e.g., corresponding to the air flow passages A1, A2 inFIG.19A) are larger in the example inFIG.23Athan in the example inFIG.23B. Specifically, the rectangular shape of the turns in the example inFIG.23Bresults in linear, planar effective cooling surfaces adjacent regions A1, A2. The result of this is that the surfaces adjacent to the air flow passages A1, A2 only expose one side of the rectangle (e.g., slightly over 25%) to the cooling air. In contrast, the curvature of the circular turns6140in the example inFIG.23Aresults in cooling surface areas adjacent regions A1, A2 that are non-linear. This exposes more of the turn surface area (e.g., slightly under 50%) to the cooling air. Overall, the example inFIG.23Bhas a slightly higher packing factor and slot current density, resulting in slightly greater torque production. However, the example inFIG.23Ahas superior cooling due to the greater cooling surface area adjacent the air passages A1, A2.

In accordance with the present disclosure, the effective cooling surface area may be at least 25% of the overall surface area of the coil. Values of between 35% and 70% may strike a particularly good balance between cooling and torque production in a transverse flux motor.

For completeness, Table 13 summarizes the configuration and properties of a transverse flux electrical machine that is optimized for the use in the EPU of a VTOL aircraft. This is merely one example and does not limit the present disclosure to an electrical machine of this configuration.

TABLE 13Air-cooled 150 kW Dual-LaneTransverse Flux MotorAir gap configurationDouble rotor, axial air gapLane ConfigurationTwo lanes, axially stacked activepartsCooling system typeDirect air coolingContinuous rated power (Pcont)150 kWPeak rated power (Ppeak)175 kWMaximum continuous rated torque1300 NmPeak rated torque1440 NmHover torque975 NmRotor speed at maximum continuation120 rads−1rated torque (ωmax,cont)Rotor speed at hover (ωhover)96 rads−1EPU tip speed (υtip)171 ms−1Efficiency (η)0.94 (94%)Maximum continuous rated current200 A (RMS)Peak rated current230 A (RMS)Steady-state terminal short circuit174 A (RMS)currentSlot current density (peak)8.1 A/(mm)2Continuous rated voltage900 VActive parts diameter0.46 metersAir gap0.7 mmActive parts mass15.2 kgCooling system mass4.2 kgTotal motor mass56 kgConductor volume38.8 cm3Iron volume61.1 cm3Pole⁢pair⁢number⁢(NP2)80Pole⁢pitch⁢(Pθ=2⁢πNP)0.039 radians (2 degrees)Pole⁢arc⁢length⁢(PL=Pθ×DA⁢c⁢t2)8.97 mmMachine inductance43 μHPower Factor (cos (Ø))0.72Max. electrical frequency (fmax)1.4 kHzInsulation rated temperature (θins,max)475 KCoolant specific heat capacity (Cp)1006 Jkg−1K−1Required mass flow rate at0.27 kgτmax,cont({dot over (m)}max,cont)Cmax,cont= Cp× {dot over (m)}max,cont272 JK−1ρact=τpeakmact94.7 Nmkg−1ρact+cool=τpeakmact+mcool74.2 Nmkg−1Λ=τpeakmact×Jslot,peak11.7 μNm3kg−1A−1Λ*=τpeak(mact+mcool)×Jslot,peak9.3 μNm3kg−1A−1Γ=VconductorViron0.64Δ=ρactcos⁢(∅)132 Nmkg−1Δ*=τpeak(mact+mcool)×cos⁢(∅)103 Nmkg−1= Pθ× GAir27.3 micro radian-meters* = PL× GAir6.3 μm2∏=PLfm⁢ax6.4 μms∇=τm⁢ax,contmact×Cma⁢x,cont0.31 Kskg−1∇*=τma⁢x,cont(mact+mcool)×Cma⁢x,cont0.24 Kskg−1Z=cos⁡(∅)×mactη11.6 kgZ*=cos⁡(∅)×(ma⁢c⁢t+mc⁢o⁢o⁢l)η14.9 kgξ=ISCIp⁢e⁢a⁢k0.76ζ=θins,cont(IS⁢C)θi⁢ns,cont(Ic⁢o⁢n⁢t)0.94β = Lmachine× ρact4.1 mHNkg−1λ=η×Lmachinemact2.7 μHkg−1λ*=η×Lmachine(mact+mcool)2.1 μHkg−1F (see Equation (22))6.2F* (see Equation (23))4.9χ=vtip×mactτpeak1.8 sm−1χ*=vtip×(mact+mcool)τpeak2.3 sm−1Ψ=τhoverωhover10.2 Nmsrad−1Effective cooling surface area42%Slot fill factor28%

Various examples have been described, each of which features various combinations of features. It will be appreciated by those skilled in 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 disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

It is to be understood that the elements and features recited in the appended claims may be combined in different ways to produce new claims that likewise fall within the scope of the present disclosure. Thus, whereas the dependent claims appended below depend on only a single independent or dependent claim, it is to be understood that these dependent claims may, alternatively, be made to depend in the alternative from any preceding or following claim, whether independent or dependent, and that such new combinations are to be understood as forming a part of the present specification.