Patent Description:
An electrical machine utilises electromagnetic forces to convert electrical energy to mechanical energy (referred to as an electric motor) or to convert mechanical energy to electrical energy (referred to as an electric generator). Electrical machines typically comprise a stationary stator, a rotor configured to rotate relative to the stator, and one or more windings formed by a coil of conductive material, for example copper, wrapped around portions of the stator. During operation of the electrical machine as a motor, electric current is passed through the winding, which generates a magnetic field. A second magnetic field is generated by the rotor, either by permanent magnets or electromagnets/coils mounted thereon. The interaction between these two magnetic fields induces a torque on the rotor, thus causing the rotor to rotate relative to the stator. This operation may also be conducted in reverse, in that the rotor may be mechanically rotated to induce an electrical current in the winding. Depending on the type of electrical machine, the winding may be mounted to the rotor instead of the stator.

One use for electrical machines is to provide some or all of the energy required for the propulsion of a vehicle. In such electrical machines, the windings are typically mounted in slots in the stator, and are used to generate a magnetic field that interacts with magnetic fields associated with the rotor to produce torque on the rotor.

Power requirements for high performance electrical propulsion motors can exceed <NUM> MW. Therefore, important factors for the design of such high performance electric propulsion motors are power density and specific power. A higher power density and/or specific power means that for a given power requirement, the motor can be smaller and lighter, which is particularly advantageous for electric propulsion motors in vehicles such as aircraft, where space and weight are key limiting factors. In order to help realise a higher power density or specific power, the windings of the electrical machine need to be able to carry as much power as possible. Unfortunately, AC losses and conductor overheating can restrict the power carrying capacity of the windings.

AC losses may be caused by the skin effect, which is the tendency of a highfrequency alternating current to flow through only the outer layer of a conductor, caused by opposing eddy currents induced by the changing magnetic field resulting from the alternating current. The skin effect effectively limits the cross-sectional area of the conductor that can be utilized to carry current, and also increases with frequency. The reduction in current carrying cross-section can also contribute to conductor overheating. It is therefore desirable to employ conductor configurations that minimise such eddy current generation to improve the current carrying capacity and operating temperature for a given cross-section of conductor.

It is also desirable to efficiently cool the conductors to prevent overheating to allow the conductor to carry a higher power. This can be done by providing configurations of conductors that permit liquid cooling thereof e.g., with oil or a similar cooling fluid, to keep the operating temperature of the conductors within a safe range.

Although prior arrangements exist and may be adequate for certain applications, improvements thereto are nevertheless desirable. Accordingly, the present disclosure aims to provide a winding with a conductor configuration that minimizes AC losses and allows effective cooling thereof to maximise power carrying capacity. Such a winding can improve the power density and/or specific power of an electrical machine. This may be particularly suited for electric propulsion motors used in aircraft applications, where there is a high power requirement, but weight and size are to be minimised where possible.

<CIT> and <NPL> disclose a prior art winding according to the preamble of claims <NUM> and <NUM>.

From one aspect, the present disclosure provides a winding for an electrical machine according to claim <NUM>.

In an embodiment of the above, each conductor comprises a U-shaped cross-section defining a base and two sides each joined to a respective end of the base by a substantially <NUM> degree angle. In a further embodiment, one or both of the sides is approximately three times the length of the base. In yet a further embodiment, one of the sides is shorter than the other side.

In a further embodiment of the above, the U-shaped conductors in each pair of conductors are interleaved such that a cooling channel is formed between one of the sides of one of the pair of conductors and the base of the other of the pair of conductors.

In yet a further embodiment, the U-shaped conductors in each pair of conductors are interleaved such that a substantially S-shaped gap separates the pair of conductors.

From another aspect, the present disclosure provides a winding for an electrical machine according to claim <NUM>.

In an embodiment of the above, the long side is approximately three times the length of the short side.

From another aspect, the present disclosure provides an electrical machine comprising a stator. The stator comprises a slot and the winding of the above aspect or any of its embodiments, disposed within the slot.

From yet another aspect, the present disclosure provides an aircraft comprising the electrical machine of the above aspect.

In one embodiment, the aircraft further comprises a propeller, and the electrical machine is configured to drive the propeller.

In another embodiment, the aircraft further comprises a gas turbine engine, and the electrical machine is configured to supplement drive for the gas turbine engine.

Although certain advantages may be discussed in relation to certain features above, other advantages of certain features may become apparent to the skilled person following the present disclosure.

Some exemplary embodiments and features of the present disclosure will now be described by way of example only, and with reference to the following drawings in which:.

<FIG> shows one prior art arrangement. A winding comprises a conductor <NUM> having at least one turn forming the winding. The winding is mounted within a slot <NUM> in a stator <NUM>. The conductor <NUM> has a rectangular cross-section. Rectangular conductors are commonly used in high power applications, such as those discussed above, because they can carry a large current and are suitable for liquid cooling. However, AC losses can be particularly high in rectangular conductors due to the skin effect, which may also result in overheating.

With reference to the prior art arrangement shown in <FIG>, one known way of reducing AC losses caused by the skin effect is to form the conductor from multiple strands <NUM> having a smaller cross-section. However, one drawback of this configuration is that it reduces the ratio of the cross-sectional area of conductive material within the stator slot relative to the total cross-sectional area available within the slot. This is known as the slot fill factor. A lower slot fill factor reduces the current that can be passed through the winding, thus reducing power density and specific power. In this arrangement, cooling is also compromised, further reducing power density and specific power.

With reference to the prior art arrangement shown in <FIG>, one particularly effective way of reducing AC losses caused by the skin effect is to form the conductor from Litz wire <NUM>, which is specifically designed to reduce losses caused by the skin effect. However, this configuration can compromise the slot fill factor and cooling even further relative to the previous multi-strand conductor <NUM>.

With reference to the prior art arrangement shown in <FIG>, a known way of maintaining a high slot fill factor while reducing losses caused by the skin effect is to form the conductor from a plurality of flat wires <NUM>. The flat wires <NUM> are stacked within the slot to increase the slot fill factor. However, the flat wires <NUM> may not be suitable for applications which require liquid cooling, as making a channel for a cooling medium to flow may be difficult. Such an arrangement may also result in uneven temperature distribution, with much higher temperatures in the centre of the stack of flat wires <NUM>, again reducing power density and specific power.

With reference to the prior art arrangement shown in <FIG>, very large electrical machines which require significant cooling, such as power station generators, commonly utilise hollow conductors <NUM>. Hollow conductors <NUM> provide a central channel within the conductor <NUM> for flowing a cooling medium. This enhances cooling of the conductors <NUM>, which is particularly important in these very large electrical machines. However, again, the slot fill factor is significantly reduced, thus reducing power density and specific power. Moreover, although skin effect generation may be reduced in such configurations, it is still not minimised.

With reference to <FIG>, in accordance with a first aspect of the present disclosure, a winding comprising a conductor <NUM> having at least one turn forming the winding is disposed within a slot <NUM> of a stator <NUM>. The slot <NUM> has a width W.

When the conductor <NUM> is used in an electrical machine, such as a motor, a cooling fluid, for example oil, is used to fill the slot <NUM> to cool the conductor <NUM>. The electrical machine may have a dedicated pump system for the cooling fluid, or if the electrical machine is to be used for example in a vehicle which has its own coolant system, the cooling fluid may be pumped by said coolant system. A sleeve (not shown) may cover the opening <NUM> of the slot <NUM> to retain the cooling fluid within the slot <NUM>. To this end, the sleeve will extend the width W of the slot <NUM> to completely cover the opening <NUM> and provide a fluid tight seal with the stator <NUM>. The sleeve can take any suitable form, and in some examples, may be provided in the form of a membrane or a cover plate.

The conductor <NUM> is electrically insulated from the cooling fluid and further turns of the conductor <NUM>. To this end, the conductor <NUM> may be sealed within a suitable coating (not shown). Any suitable coating for electrically insulating the conductor <NUM> from the cooling fluid and/or further turns of the conductor <NUM> may be used, for example, an enamel coating.

The conductor <NUM> is a flat conductor that is formed to have an L-shaped cross-section. The flat nature and L-shaped cross-section of the conductor prevents the formation of a circular conduction path within the cross-section of the conductor for eddy currents to flow. This reduces AC losses therein caused by the skin effect. This is unlike, for instance, the conduction configurations shown in <FIG> and <FIG>, which allow for such circular conduction paths to form therein.

The L-shaped cross-section of the conductor <NUM> has a long side <NUM> and a short side <NUM> joined by a <NUM> degree angle. In one example, the long side <NUM> is approximately three times the length of the short side <NUM>. In the depicted embodiment, the long side <NUM> may extend across the full width W of the slot <NUM>. A gap <NUM> is left on either side of the conductor <NUM> between the conductor <NUM> and the slot <NUM> to allow cooling fluid to flow.

In the depicted example shown in <FIG>, one or more pairs of opposing flat, L-shaped conductors <NUM> are disposed opposite each other within the slot <NUM> to form a cooling channel <NUM> for cooling fluid to flow there between. The cooling channel <NUM> is substantially rectangular in cross-section (as shown in dotted outline). The pairs of opposing L-shaped conductors <NUM> are arranged such that there is no contact between the pair of conductors <NUM>, to ensure that no circular path is provided for eddy currents to flow. This pairing arrangement improves the current carrying capacity of the conductors <NUM>, whilst improving the cooling available for the conductor <NUM>. This is unlike, for instance, the conduction configurations shown in <FIG> and <FIG> that do not allow for a substantial cooling channel between conductors.

As many pairs of opposing L-shaped conductors <NUM> are placed into the slot <NUM> as space will allow to maximise the slot fill factor and the power carrying capacity/density for the electrical machine. In the depicted example there are three pairs of opposing L-shaped conductors <NUM> disposed within the slot <NUM>. However, more or fewer pairs may be present depending on the slot shape, size and space available.

It is to be understood that although L-shaped conductors <NUM> are depicted having a long side <NUM> and a short side <NUM> joined by a <NUM> degree angle; within the scope of this disclosure, the conductors <NUM> may be only substantially L -shaped (i.e., generally L-shaped) and/or the long side <NUM> and short side <NUM> may not be joined at exactly a <NUM> degree angle, but rather substantially at <NUM> degrees (e.g., ±<NUM> degrees).

With reference to <FIG>, another winding in accordance with another aspect of the present disclosure is shown. The winding is substantially the same as the previous aspect, but instead comprises a flat conductor <NUM> having a U-shaped cross section. Much like the L-shaped cross section, the flat nature and U-shaped cross-section prevents the formation of a circular conduction path in the cross-section of the conductor <NUM> for eddy currents to flow, thereby reducing AC losses caused by the skin effect. It also further improves the slot fill factor and thermal performance of the winding, as discussed more below.

In the depicted example shown in <FIG>, the U-shaped conductor <NUM> has a base <NUM> and two sides <NUM>, <NUM> each joined to a respective end of the base <NUM> by a <NUM> degree angle. One or both of the sides <NUM>, <NUM> is approximately three times the length of the base <NUM>. The sides <NUM>, <NUM> may be equal in length; however, in the depicted example, one of the sides <NUM> is shorter in length than the other side <NUM>. This provides a gap for the base <NUM> of an adjacent U-shaped conductor <NUM> when disposed opposite the conductor <NUM>, as explained below. The difference between the lengths of the sides <NUM>, <NUM> may be approximately equal to the thickness of the base <NUM>. The longer side <NUM> may extend across substantially the full width W of the slot. In the depicted example, a gap <NUM> is left between either side of the longer side <NUM> and the slot <NUM> to allow cooling fluid to flow.

In a similar manner to the L-shaped conductors <NUM>, in the depicted example shown in <FIG>, one or more pairs of opposing U-shaped conductors <NUM> are disposed opposite each other and interleaved together within the slot <NUM> to increase the slot fill factor. The conductors <NUM> are interleaved such that one of the sides <NUM>, <NUM> of each conductor <NUM> is arranged within the U-shape of the opposite conductor <NUM> (i.e., one of the sides <NUM>, <NUM> of one conductor <NUM> is positioned adjacent the base <NUM> of the opposite conductor <NUM>).

The pair of opposing U-shaped conductors <NUM> are arranged such that a cooling channel <NUM> is formed therebetween. This can further enhance cooling of the conductors <NUM> during operation. The cooling channel <NUM> is substantially rectangular in cross-section (as shown in dotted outline). The cooling channel <NUM> is defined between the base <NUM> and two sides <NUM>, <NUM> of one of the pair of opposing U-shaped conductors <NUM>, and an end of the interleaved side <NUM>, <NUM> of the opposing conductor <NUM>. In the depicted embodiment, the interleaved side is the shorter side <NUM>, therefore the shorter side <NUM> is arranged within the U-shape of the opposite conductor <NUM> and the cooling channel <NUM> is formed between the end of the shorter side <NUM> of one conductor <NUM> and the base <NUM> of the opposite conductor <NUM>. The interleaved side being shorter may allow for the cooling channel <NUM> to be provided whilst reducing the separation needed between the opposing conductors <NUM>. This can further improve the slot fill factor, without negatively effecting cooling.

As discussed above, there is no contact between the pair of opposing U-shaped conductors <NUM> to ensure that no circular path is provided for eddy currents to flow. The arrangement of the pair of opposing U-shaped conductors <NUM> is such that a gap is left between the sides <NUM>, <NUM> of the opposing conductors <NUM> to allow cooling fluid to flow there between. Owing to the interleaved U-shaped cross-sections of the conductors <NUM>, the gap and the cooling channel <NUM> together define a substantially S-shaped cross-sectional area between the pair of opposing U-shaped conductors <NUM> for fluid to flow (as shown in dotted outline).

As many pairs of opposing interleaved U-shaped conductors <NUM> are placed into the slot <NUM> as space will allow to maximise the slot fill factor and the power carrying capacity/density for the electrical machine. In the depicted example there are three pairs of opposing U-shaped conductors <NUM> disposed with the slot <NUM>. However, more or fewer pairs may be present depending on the slot shape, size and space available.

It is to be appreciated that the flat conductor configuration shown in <FIG> results in smaller cooling channels <NUM> between conductors <NUM> than the cooling channels <NUM> between the flat conductors <NUM> in <FIG>, but it improves the slot fill factor. Nonetheless, the configurations of the conductors <NUM>, <NUM> shown in <FIG> and according to this disclosure permit improved power density and specific power for a winding for an electrical machine. These configurations achieve this by allowing conductor AC losses to be reduced and conductor cooling to be improved.

It is to be understood that although U-shaped conductors <NUM> are depicted having a base <NUM> and two sides <NUM>, <NUM> each joined to a respective end of the base <NUM> by a <NUM> degree angle; within the scope of this disclosure, the conductors <NUM> may be only substantially U-shaped (i.e., generally U-shaped) and/or the two sides <NUM>, <NUM> may not each be joined to a respective end of the base <NUM> by exactly a <NUM> degree angle, but rather at substantially a <NUM> degree angle (e.g., ±<NUM> degrees).

Furthermore, although not depicted, in another embodiment within the scope of this disclosure, the conductors may instead be substantially V-shaped in cross-section. In other words, when comparing to the U-shaped conductors <NUM>, the base <NUM> is dispensed with and the two sides <NUM>, <NUM> are joined together at an acute angle to form a general V-shape. Opposing V-shaped conductors could then either be interleaved as with the U-shaped conductors <NUM> or opposed as in the L-shaped conductors <NUM> to provide a substantially diamond-shaped cooling channel there between. If V-shaped conductors are interleaved, then one side of the V-shape may be shorter than the other side, in order to define a cooling channel when interleaved within the opposing V-shaped conductor.

In the depicted examples, the slot width W is between <NUM> to <NUM>, and in one example is about <NUM>. Nonetheless, within the scope of this disclosure, the slot <NUM> can have any suitable width W, and will depend on the size of the electrical machine and application. The conductors <NUM>, <NUM> may have a substantially uniform thickness T. The thickness T may be between <NUM> to <NUM>, and in one example is about <NUM>. Nonetheless, as will be appreciated, the thickness T can be readily varied according to a particular application e.g., according to the skin depth required for a particular AC frequency.

The conductors <NUM>, <NUM> may be manufactured by pressing a flat conductor into the desired L- or U-shape. The conductors <NUM>, <NUM> may alternatively be formed by additive manufacturing, extrusion, or any other suitable method. The conductors <NUM>, <NUM> may be interconnected through end windings which may be formed by any suitable means, e.g., by hairpin, busbar, or other methods. The conductors <NUM>, <NUM> may be made of any suitable conductive material. In general, however, the conductors <NUM>, <NUM> are made of a high conductivity metal, such as copper or aluminium.

Utilising the conductors <NUM>, <NUM> of the present disclosure in an electrical machine can result in a more compact and lightweight machine for a given power requirement. This means the electrical machine will have a higher power density and/or specific power. Such an electrical machine is suitable for use as an electrical propulsion motor for electric or hybrid vehicles in which power density and specific power are important factors. Such an electrical machine is particularly suitable for use in an aircraft, where these factors are critical due to weight and size concerns. Such aircraft applications may include utilising the electric propulsion motor to drive or provide supplemental power to a propeller or gas turbine engine. In such an application, the electric propulsion motor can be said to be used as part of a hybrid electric powertrain for an aircraft.

The conductors <NUM>, <NUM> may also be utilised in other types of electrical machines other than electric propulsion motors. For example, they may also be used for windings in a transformer or other suitable electrical machinery where high power carrying capacity windings are required.

Claim 1:
A winding for an electrical machine, the winding comprising at least one flat conductor (<NUM>, <NUM>) having at least one turn to form the winding, wherein the at least one flat conductor (<NUM>, <NUM>) comprises a substantially L-shaped, U-shaped or V-shaped cross-section, wherein the at least one flat conductor (<NUM>, <NUM>) comprises one or more pairs of conductors (<NUM>, <NUM>), wherein the conductors (<NUM>, <NUM>) in each pair of conductors (<NUM>, <NUM>) are disposed opposite each other and form a cooling channel (<NUM>, <NUM>) there between, wherein there is no contact between the conductors (<NUM>, <NUM>);
characterised in that:
the conductors (<NUM>, <NUM>) of each pair of conductors (<NUM>, <NUM>) are interleaved with each other.