GB2574827A

GB2574827A - Generating electric power

Info

Publication number: GB2574827A
Application number: GB201810005A
Authority: GB
Inventors: Herbert Douglas
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2018-06-19
Filing date: 2018-06-19
Publication date: 2019-12-25
Also published as: GB201810005D0

Abstract

Electrical power generation apparatus for an aircraft (fig 1, 101), comprising: an internal combustion engine; an electric machine (fig 2, 201) for generating power, coupled to be driven by the engine, the machine comprising a stator 301 with windings 303, 304 immersed in fluid or encapsulant having a dielectric strength of between 5 and 100 megavolts per metre, and wherein each one of the windings has a minimum separation distance DS from any other winding of from 0.01-10mm. The windings may be single-layer concentrated windings, and DS is the separation distance between circumferentially adjacent windings, and is the smaller of: the separation distance between windings on circumferentially adjacent windings or the separation distance between radially adjacent windings. DS may be from 0.2-0.5mm. The dielectric strength may be between 15-75 megavolts/metre. The stator may comprise teeth defining slots, where slots per pole per phase is less than 1. The encapsulant may be a potting compound. The liquid may be a coolant circulated by a pump system (fig 2, 211). The engine may be an Otto cycle, Atkinson cycle, Diesel cycle or Brayton cycle engine. The engine may be a two shaft gas turbine having a high and low pressure spool, in which the generator is connected with a low pressure spool.

Description

Generating Electrical Power

TECHNICAL FIELD

This disclosure relates to electrical power generation apparatus for an aircraft.

BACKGROUND

At sea level, the dielectric strength of air is typically in the region of megavolts per metre. However, in regions of low air pressure, such as experienced by aircraft at cruise altitude, the dielectric strength may drop to the order of only hundreds of volts per metre.

Electric generators are typically subjected to voltage derating as altitude increases to prevent the likelihood of electrical breakdown and the attendant phenomena such as arcing and corona, which may cause catastrophic failure modes.

Hybrid propulsion systems for aircraft are currently being developed, in which an electric machine is connected with an internal combustion engine, such as a gas turbine for generating electricity. Said electricity is then supplied to motors to drive a propulsive fan, for example. Due to the thrust demand placed upon the overall propulsion system and the necessarily high voltage requirement of the generators in such applications, it may not be possible to circumvent the issue by simply moving to a voltage regime which will not exceed the breakdown voltage.

Other measures to mitigate the risk of electrical breakdown in electric machines are therefore required.

SUMMARY

The invention is directed towards electrical power generation apparatus for an aircraft.

In an aspect, such apparatus comprises an internal combustion engine, and an electric machine for generating electric power, which is coupled to and configured to be driven by the internal combustion engine. The electric machine comprises a stator having a plurality of windings immersed in a fluid or an encapsulant having a dielectric strength of between 5 and 100 megavolts per metre, and wherein each one of said windings have a minimum separation distance D_s from any other one of said windings of from 0.01 to 10 millimetres..

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which:

Figure 1 shows an aircraft of an embodiment, including an electric generation unit;

Figure 2 shows the electric generation unit of Figure 1 which includes an electric machine;

Figures 3A and 3B show, respectively, axial and transverse cross sections of the electric machine of Figure 2;

Figure 4 is a magnified version of the view of Figure 3B; and

Figure 5 shows an alternative embodiment of the stator of the electric machine of Figure 2.

DETAILED DESCRIPTION

Figure 1 illustrates an aircraft 101, which in the present embodiment is powered by two electric propulsion units 102 in combination with an electrical generation unit 103.

In the present embodiment, the electric propulsion units 102 are ducted fans, and generate thrust for the aircraft by utilising electric motors to drive said fans. The pressure rise results in the generation of a net thrust upon the aircraft.

In alternative embodiments the electric propulsion units 102 may be configured as open propellers (a type of propulsive fan), or any other configuration able to produce thrust by causing a pressure rise in the incident airflow.

Electrical power is distributed by a bus 104 from the electrical generation unit 103 to the electric propulsion units 102. It will be appreciated by those skilled in the art that the bus 104 may include conductors, power electronics, and may possibly include energy storage systems such as batteries or flywheels to provide extra capacity.

The electrical generation unit 103 is illustrated in schematic form in Figure 2.

The electrical generation unit 103 includes an electric machine 201 which is driven by an internal combustion engine. In the present example, the electrical generation unit 103 is configured as a turboelectric generator, in which the internal combustion engine is a gas turbine engine 202, i.e. a Brayton cycle engine. Alternatively, the internal combustion engine could be a piston engine, such as a Diesel cycle engine, or any other type of internal combustion engine, such as those operating in accordance with an Otto or Atkinson cycle.

In the present example, the gas turbine engine 202 is configured as a two shaft engine, having a low pressure (LP) and high pressure (HP) spool. Accordingly, the LP spool includes an LP compressor 203 coupled via an LP shaft 204 to an LP turbine 205. The HP spool includes an HP compressor 206 coupled via an HP shaft 207 to an HP turbine 208. A combustor 209 is located between the HP compressor 206 and the HP turbine 208.

The gas turbine engine 202 operates in the conventional manner such that inlet air A is initially compressed by the LP compressor 203, and then subjected to further compression by the HP compressor 206. The compressed air exhausted from the HP compressor 206 is directed into the combustor 209 where it is mixed with a supply of fuel F and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the HP turbine 208 and the LP turbine 205, before exiting as exhaust E. The HP turbine 208 drives the HP compressor 206 via the HP shaft 207, whist the LP turbine 205 drives both the LP compressor 203 and the electric machine 201.

The electric machine 201 thus produces electrical power for distribution via the bus 104. In the present example, the electric machine

201 is an interior rotor, permanent magnet radial flux AC machine. As will be appreciated by those skilled in the art, though, other machine types, such as those which use one or more of exterior rotor, induction, axial flux, DC, etc. configurations may be used.

In the present example the electric machine 201 is liquid-cooled by a coolant loop 210. The coolant loop comprises a pump 211 which pumps a liquid coolant through the electric machine 201, removing heat therefrom, and carrying it to a heat exchanger 212 which rejects heat from the liquid into a supply of inlet air A.

The electric machine 201 is shown in Figure 3A in cross section along B-B of Figure 3B, which in turn is a cross section of the electric machine 201 at C-C of Figure 3A.

Referring first to Figure 3A, the electric machine 201 in the present example includes a stator 301 surround a rotor 302 which are both concentric around the axis D-D of the electric machine.

In the present example, the stator 301 comprises a lamination stack of the known type. In the present example, the electric machine 201 is a permanent magnet machine and thus the rotor 302 comprises permanent magnets, which interact with the magnetic field generated by windings in the stator 302 to generate torque. As, a radial flux machine, end windings 303 emerge axially from the lamination stack of the stator 301.

Referring to Figure 3B, the stator 301 includes a plurality of circumferentially adjacent windings 304 mounted upon teeth 305 having slots 306 therebetween, the manner of which will be familiar to those skilled in the art.

In the present example, the stator 301 uses a concentrated winding arrangement, which will be familiar to those skilled in the art, in which there is a one layer of windings in the slot. In another embodiment, the stator 301 may use a concentrated winding arrangement, but in which there is two layers of windings in the slot, which will again be familiar to those skilled in the art. It will be apparent to those skilled in the art that distributed windings could be used in other alternative embodiments.

The number of slots-per-pole-per-phase of the electric machine 201 may, in an example, be less than unity. In alternative embodiments, the number of slots-per-pole-per-phase may be equal to or greater than unity.

It will be appreciated though that the electric machine 201 could employ a slotless design in alternative embodiments.

The slots 306 in the electric machine are filled with either a dielectric fluid coolant or with an encapsulant. In order to prevent breakdown, the fluidor encapsulant has a dielectric strength of between 5 and 100 megavolts per metre.

In a specific embodiment, both the stator 301 and rotor 302 of the electric machine 201 are cooled with a fluid having a dielectric strength of at least 5 and 100 megavolts per metre. In this configuration, a dielectric coolant is passed through the slots 306 to cool the windings 304, and around the end windings 303. The rotor may have a liquid coolant passed through its centre-bore.

In a specific example, the coolant may be Midel (RTM) 7131 available from M&l Materials Ltd. of Manchester, England, which is a synthetic ester-based coolant and has a dielectric strength of 29.5 megavolts per meter.

Alternatively, in another specific example, the coolant may be FC3283, available from 3M Specialty Materials of St. Paul, Minnesota, USA, which is a fully-fluorinated coolant and has a dielectric strength of 17 megavolts per meter.

Alternatively, in another specific example, the coolant may be Paratherm LR, available from Paratherm Corp, of King of Prussia, Pennsylvania, USA, which is an aliphatic-hydrocarbon-based coolant and has a dielectric strength of 8.7 megavolts per meter. Any other dielectric liquid with good heat transfer properties may be used, for example a deionised water-glycol mix, etc.

In an alternative embodiment, the stator 301 may be cooled in a water-jacket arrangement, and include an encapsulant such as a potting compound or equivalent in the slots. In an example, the encapsulant may have high thermal conductivity, so as to carry heat through the stator back iron to the coolant whereupon heat exchange may be effected.

Thus, in a specific example, the encapsulant may be CoolTherm EP-3500 available from Lord Corp, of Cary, North Carolina, USA, which is a thermally conductive epoxy encapsulant and has a dielectric strength of

15.4 megavolts per meter. It will be appreciated that other encapsulant types may be suitable.

In another specific embodiment, only the stator 301 of the electric machine 201 is cooled with a fluid having a dielectric strength of at least 5 and 100 megavolts per metre, with the rotor being air cooled, i.e. air being channelled down the centre-bore of the rotor 302.

In a specific embodiment, a fluid is selected which has a dielectric strength of between 15 and 75 megavolts per metre.

Figure 4 is a magnified view of the electric machine 201 as shown in Figure 3B.

As may be seen in the Figure, the circumferentially adjacent windings 304 have a linear separation distance D_s, which is the minimum linear distance between any two conductors on the adjacent windings. The stator 301 is configured such that this separation distance D_s is from 0.01 to 10 millimetres. In a more specific example, the separation distance D_sis from 0.2 to 0.5 millimetres. In this way, risk of breakdown in the electric machine is minimised.

Those skilled in the art will appreciate that this distance may be controlled by varying the size of the windings, by varying the number of slots, or by varying the tooth thickness, etc., or any combination thereof.

Figure 5 is a magnified view similar to that of Figure 4, however illustrating an alternative embodiment in which the stator 301 concentrated windings 401 but with two layers in each slot. Thus, windings 401 are radially adjacent in addition to being circumferentially adjacent.

In this example, the adjacent windings 401 also have a radial linear minimum separation distance D_s of from 0.01 to 10 millimetres. As with the embodiment of Figure 4, in a more specific example, the separation distance D_s is from 0.2 to 0.5 millimetres. It will be appreciated that although the separation of the windings on adjacent teeth (i.e. those that have circumferential adjacency) may be the same as of windings on each layer (i.e. those that have radial adjacency) on a particular tooth, this is not 5 a requirement. Instead, the minimum separation distance D_s as defined herein is the smallest of the two, thereby allowing them to differ.

Various examples have been described, each of which feature various combinations of features. It will be appreciated by those skilled in the art that, except where clearly mutually exclusive, any of the features 10 may be employed separately or in combination with any other features and the invention extends to and includes all combinations and subcombinations of one or more features described herein.

Claims

1. Electrical power generation apparatus for an aircraft, comprising:

an internal combustion engine;

an electric machine for generating electric power, which is coupled to and configured to be driven by the internal combustion engine, the electric machine comprising a stator having a plurality of windings immersed in a fluid or an encapsulant having a dielectric strength of between 5 and 100 megavolts per metre, and wherein each one of said windings have a minimum separation distance Ds from any other one of said windings of from 0.01 to 10 millimetres.

2. The apparatus of claim 1, in which said plurality of windings are single-layer concentrated windings, and Ds is the separation distance between windings on circumferentially adjacent coils.

3. The apparatus of claim 1, in which said plurality of windings are concentrated windings, and Ds is the smaller of:

the separation distance between windings on circumferentially adjacent coils;

the separation distance between radially adjacent coils of each layer.

4. The apparatus of claim 1, in which Ds is from 0.2 to 0.5 millimetres.

5. The apparatus of claim 1, in which said dielectric strength is between 15 and 75 megavolts per metre.

6. The apparatus of claim 1, in which the stator comprises teeth thereby defining slots, in which the slots per pole per phase of the electric machine is less than 1.

7. The apparatus of claim 1, in which the encapsulant is a potting compound.

8. The apparatus of claim 1, in which the liquid is a coolant.

9. The apparatus of claim 8, in which the coolant is circulated 5 by a pump system.

10. The apparatus of claim 1, in which the internal combustion engine is one of:

an Otto cycle engine;

an Atkinson cycle engine;

10 a Diesel cycle engine;

a Brayton cycle engine.

11. The apparatus of claim 1, in which the internal combustion engine is a two shaft gas turbine engine having a high pressure spool and a low pressure spool, in which the generator is connected with a low

15 pressure spool.