US20220037979A1 - Electric machine - Google Patents
Electric machine Download PDFInfo
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- US20220037979A1 US20220037979A1 US17/504,151 US202117504151A US2022037979A1 US 20220037979 A1 US20220037979 A1 US 20220037979A1 US 202117504151 A US202117504151 A US 202117504151A US 2022037979 A1 US2022037979 A1 US 2022037979A1
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- electric machine
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Images
Classifications
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K55/00—Dynamo-electric machines having windings operating at cryogenic temperatures
- H02K55/02—Dynamo-electric machines having windings operating at cryogenic temperatures of the synchronous type
- H02K55/04—Dynamo-electric machines having windings operating at cryogenic temperatures of the synchronous type with rotating field windings
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K55/00—Dynamo-electric machines having windings operating at cryogenic temperatures
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64D—EQUIPMENT FOR FITTING IN OR TO AIRCRAFT; FLIGHT SUITS; PARACHUTES; ARRANGEMENT OR MOUNTING OF POWER PLANTS OR PROPULSION TRANSMISSIONS IN AIRCRAFT
- B64D27/00—Arrangement or mounting of power plants in aircraft; Aircraft characterised by the type or position of power plants
- B64D27/02—Aircraft characterised by the type or position of power plants
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64D—EQUIPMENT FOR FITTING IN OR TO AIRCRAFT; FLIGHT SUITS; PARACHUTES; ARRANGEMENT OR MOUNTING OF POWER PLANTS OR PROPULSION TRANSMISSIONS IN AIRCRAFT
- B64D27/00—Arrangement or mounting of power plants in aircraft; Aircraft characterised by the type or position of power plants
- B64D27/02—Aircraft characterised by the type or position of power plants
- B64D27/24—Aircraft characterised by the type or position of power plants using steam or spring force
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K3/00—Details of windings
- H02K3/02—Windings characterised by the conductor material
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K3/00—Details of windings
- H02K3/04—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors
- H02K3/12—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors arranged in slots
- H02K3/14—Windings characterised by the conductor shape, form or construction, e.g. with bar conductors arranged in slots with transposed conductors, e.g. twisted conductors
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K3/00—Details of windings
- H02K3/42—Means for preventing or reducing eddy-current losses in the winding heads, e.g. by shielding
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K7/00—Arrangements for handling mechanical energy structurally associated with dynamo-electric machines, e.g. structural association with mechanical driving motors or auxiliary dynamo-electric machines
- H02K7/18—Structural association of electric generators with mechanical driving motors, e.g. with turbines
- H02K7/1807—Rotary generators
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K7/00—Arrangements for handling mechanical energy structurally associated with dynamo-electric machines, e.g. structural association with mechanical driving motors or auxiliary dynamo-electric machines
- H02K7/18—Structural association of electric generators with mechanical driving motors, e.g. with turbines
- H02K7/1807—Rotary generators
- H02K7/1815—Rotary generators structurally associated with reciprocating piston engines
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K7/00—Arrangements for handling mechanical energy structurally associated with dynamo-electric machines, e.g. structural association with mechanical driving motors or auxiliary dynamo-electric machines
- H02K7/18—Structural association of electric generators with mechanical driving motors, e.g. with turbines
- H02K7/1807—Rotary generators
- H02K7/1823—Rotary generators structurally associated with turbines or similar engines
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K9/00—Arrangements for cooling or ventilating
- H02K9/19—Arrangements for cooling or ventilating for machines with closed casing and closed-circuit cooling using a liquid cooling medium, e.g. oil
-
- B64D2027/026—
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64D—EQUIPMENT FOR FITTING IN OR TO AIRCRAFT; FLIGHT SUITS; PARACHUTES; ARRANGEMENT OR MOUNTING OF POWER PLANTS OR PROPULSION TRANSMISSIONS IN AIRCRAFT
- B64D2221/00—Electric power distribution systems onboard aircraft
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B64—AIRCRAFT; AVIATION; COSMONAUTICS
- B64D—EQUIPMENT FOR FITTING IN OR TO AIRCRAFT; FLIGHT SUITS; PARACHUTES; ARRANGEMENT OR MOUNTING OF POWER PLANTS OR PROPULSION TRANSMISSIONS IN AIRCRAFT
- B64D27/00—Arrangement or mounting of power plants in aircraft; Aircraft characterised by the type or position of power plants
- B64D27/02—Aircraft characterised by the type or position of power plants
- B64D27/026—Aircraft characterised by the type or position of power plants comprising different types of power plants, e.g. combination of a piston engine and a gas-turbine
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02K—DYNAMO-ELECTRIC MACHINES
- H02K2213/00—Specific aspects, not otherwise provided for and not covered by codes H02K2201/00 - H02K2211/00
- H02K2213/03—Machines characterised by numerical values, ranges, mathematical expressions or similar information
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
- Y02E40/60—Superconducting electric elements or equipment; Power systems integrating superconducting elements or equipment
Definitions
- This disclosure relates to electric machines.
- the magnetic core is formed from ferromagnetic material and thus typically constitutes from around 50 percent to in some instances 70 percent of the active weight of the machine.
- Such a machine comprises a stator having a fully non-magnetic core and stator windings formed of a non-superconducting transposed conductor to reduce eddy current losses.
- a cryogenic cooling system is arranged to cool the stator windings to reduce conduction losses in the stator windings.
- stator magnetic core removal of the stator magnetic core will expose the stator windings to this high field. This would be the case either with conventional, non-superconducting stator windings, or superconducting stator windings. This is because superconducting stator windings only exhibit ideally zero loss when exposed to time-invariant fields—in an electric machine the stator windings are clearly exposed to time-varying fields from the rotor. These time-varying fields create eddy current losses which may cause a superconducting stator winding to quench and thus require shut-down of the machine.
- the approach taken is to use a non-superconducting transposed conductor.
- the transposed conductor may be a litz conductor.
- a cooling system is provided to cool the stator windings. The inventors have found a surprising increase in performance of the machine by using a cryogenic cooling system to maximise the reduction in conduction losses.
- FIG. 1 shows an aircraft of an embodiment
- FIG. 2 shows an aircraft of another embodiment
- FIG. 3 is a block diagram of the propulsion systems in the aircraft of FIG. 1 and the aircraft of FIG. 2 ;
- FIG. 4 is a diagram of an electric machine that may be used in the system of FIG. 3 ;
- FIG. 5 sets out a method for establishing an operational temperature for the stator in the electric machine of FIG. 4 ;
- FIG. 6 shows a plot of efficiency versus stator temperature for four configurations of electric machine.
- FIG. 1 An aircraft 101 is illustrated in FIG. 1 , and in this example comprises two electric propulsion units 102 .
- the electric propulsion units 102 are connected via a power distribution bus 103 to a source of electrical power, which in the present example is an internal combustion engine configured to drive an electric machine.
- a source of electrical power is turboelectric generator 104 , in which the internal combustion engine is a gas turbine engine.
- the internal combustion engine may be a piston engine or similar.
- the source of electrical power could alternatively by a fuel cell, battery, etc.
- Each electric propulsion unit 102 includes an electric machine configured to drive a propulsive fan.
- the electric propulsion units 102 are ducted fans, although it will be appreciated that 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.
- FIG. 2 An alternative aircraft 201 is illustrated in FIG. 2 , and in this example comprises two turbofan engines 202 which include respective electric machines.
- the electric machines act as a source of electrical power, which is provided, via a power distribution bus 203 , to a boundary layer ingestion system 204 .
- the boundary layer ingestion system 204 comprises an electric machine configured to drive the fan, which improves the propulsive efficiency of the aircraft 201 .
- a gas turbine engine 301 drives an electric machine 302 operating as a generator via an interconnecting shaft, thus providing a source of electrical power.
- This power is distributed on the bus 303 .
- Power is drawn from the bus 303 by another electric machine 302 , which in this instance operates as a motor.
- This drives a propulsive fan 304 via an interconnecting shaft, thereby providing propulsion.
- the generator and motor are provided by the same configuration of electric machine 302 .
- the motor and generator may be of different configurations. It will be appreciated that depending upon the configuration of the aircraft (for example aircraft 101 ) further propulsive fans and electric machines to drive them may be provided.
- additional sources of power in the form of additional turboelectric generators, or fuel cells, batteries, etc. or any combination thereof, may be provided (for example as with aircraft 201 ).
- the electric machine 302 is of the type claimed herein. It is shown in cross-section along its meridional plane in FIG. 4 , along with supporting hardware.
- the electric machine 302 comprises a stator 401 having a fully non-magnetic core 402 (also known as “air-cored”) and non-superconducting windings 403 .
- the non-magnetic core 402 in the present embodiment comprises a resin in which the non-superconducting windings 403 are embedded.
- the resin is a polymeric resin such as an epoxy resin, polyurethane resin, etc.
- Alternative non-magnetic core materials and/or support frames may be used to support the non-superconducting windings 403 to deal with the reaction torque during operation
- the non-superconducting windings 403 are formed from a transposed conductor.
- Transposed conductors are multi-strand conductors in which each strand is insulated, and is transposed in order to occupy each possible position along a specific length.
- the transposition of the strands may be continuous, discrete, or random. In this way, when the conductor is exposed to a magnetic field, each strand will on average link with the same number of flux lines as every other strand, thus dividing current equally among the strands.
- the strands are of small enough diameter that little skin effect can occur, thereby reducing losses due to induced eddy currents caused by the rotating rotor field.
- the non-superconducting windings 403 in the stator 401 are formed from litz conductor.
- Litz conductors are a particular type of transposed conductor in which strands of round cross-section are transposed continuously along the cable length.
- Other transposed conductors may be contemplated, such as Roebel conductors which use rectangular strands transposed at discrete intervals.
- a strand diameter of less than 1 millimetre may be selected.
- strand diameters of 0.3, 0.2, 0.1 and 0.08 millimetres may be used.
- the transposed conductor may, in an embodiment, be formed of copper. In another embodiment, it may be formed of aluminium. Alternatively, any other non-superconducting conductor may be selected.
- the electric machine 302 further comprises a rotor 404 which also has a fully non-magnetic core, along with superconducting windings 405 .
- the rotor 404 may comprise superconducting magnets instead.
- the superconducting windings 405 are mounted to a shaft 406 that is supported by bearings 407 .
- a cryogenic cooling system is provided to maintain the superconducting windings 405 in a superconducting state.
- the cryogenic cooling system comprises a cryogenic tank 408 for storing a cryogenic cooling fluid, along with a pump 409 for circulating the fluid, which, in the present embodiment, is liquid nitrogen.
- a cooler 410 is included in the return path to remove heat.
- Alternative fluids may be used, such as liquid helium etc. Such arrangements will be familiar to those skilled in the art.
- cryogenic cooling system is also used to cool the non-superconducting stator windings 403 .
- two cryogenic cooling systems may be provided, for dedicated cooling of each of the stator 401 and the rotor 404 .
- the cryogenic cooling system is a high temperature cryogenic cooling system. It will be appreciated by those skilled in the art that the term “high temperature cryogenic” has, in terms of temperature, a generally-defined upper limit in the art of 223 kelvin. In an embodiment, the cryogenic cooling system is configured to maintain the non-superconducting windings 403 at a cryogenic temperature, i.e. at 223 kelvin or below.
- one advantage of cooling the non-superconducting windings 403 with the same cryogenic cooling system as the superconducting windings 405 is that it reduces the temperature differential between it and the superconducting windings 405 in the rotor 404 . This facilitates a reduction in air gap as less thermal insulation is required.
- the airgap length between the rotor 404 and stator 401 comprises 10 millimetres of mechanical standoff plus 2 millimetres of thermal insulation to facilitate sufficient thermal isolation therebetween.
- This allows higher cryogenic operational temperatures in the stator compared to in the rotor.
- bringing the operational temperature of the non-superconducting windings 403 closer to that of the superconducting windings 405 will facilitate reduction of this airgap to smaller lengths, for example, 5 millimetres.
- the airgap length will be no longer dictated by the required thermal insulation, but instead by rotordynamic constraints which do not require such large airgap lengths.
- the stator 401 further comprises a magnetic screen 411 to contain the high field generated by the superconducting windings 405 .
- the magnetic screen may be a superconducting magnetic screen, with cooling thereof provided by the cryogenic cooling system.
- the electric machine 302 is configured to have a power output of 1 megawatt at a speed of 12000 revolutions per minute. In the present example, it is an 8-pole machine, however a 16-pole configuration may be contemplated. Further, the current density in the stator windings 403 may be 8 amps per square millimetre, or may alternatively be 16 amps per square millimetre depending upon desired power output. It is also contemplated that the stator windings 403 will be supplied with alternating current at a frequency of 800 hertz.
- the electric machine 302 combines non-superconducting stator windings 403 formed of a transposed conductor in a non-magnetically cored stator 401 , with cooling thereof by a cryogenic cooling system.
- an initial set of parameters are set:
- the mean diameter D s of the non-superconducting windings 403 in the stator 401 may be evaluated by using the following relation:
- Equations 1, 2, and 3 stem from a two-dimensional analytical solution of the magnetic field distribution in free space for a current sheet placed at the mean diameter of the superconducting winding 405 , D r , which is related to D s by h s , h s , and g. This enables the magnetic field at D s to be expressed in terms of D r , J r , and n.
- Equation 1 may be solved for D s using, for example, Newton's method.
- the magnetic field at that location may be evaluated at step 503 .
- the eddy current losses, P eddy in one strand of the non-superconducting windings 403 may be expressed as:
- B is the magnetic field strength
- w is the frequency which is dependent upon rotor speed and pole-pair number n
- r strand is the radius of a strand in the transposed conductor
- ⁇ (T) is the resistivity of the transposed conductor at a given absolute temperature T.
- the conduction losses, P conduction , in one strand of the non-superconducting windings 403 may be expressed as:
- V is the total volume of the non-superconducting windings 403 .
- the optimum temperature to which to cool the non-superconducting windings 403 may therefore be determined at step 505 by differentiating Equation 6 with respect to p to determine the value of p that gives the lowest losses. This produces the following relation for p:
- this value of p may be converted into a temperature by use of established conductivity versus temperature data for the particular material selected for the non-superconducting windings 403 .
- FIG. 6 An exemplary plot of efficiency of the electric machine 302 against operational temperature of the non-superconducting windings 403 in the stator 401 is shown in FIG. 6 . It can be seen that high efficiencies are obtained over a range of cryogenic temperatures.
- Curve 601 was produced by the method of FIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles, copper litz windings 403 having a strand diameter of 0.1 millimetres, and a current density J s equal to 8 amps per square millimetre. Following determination of sizing of such a configured electric machine 302 , an estimated power density of 20.1 kilowatts per kilogram was derived. The optimal operational temperature for the non-superconducting windings 403 was found to be 104.8 kelvin which gave an estimated efficiency of 99.87 percent.
- Curve 602 was produced by the method of FIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles, copper litz windings 403 having a strand diameter of 0.1 millimetres, and a current density J s equal to 16 amps per square millimetre. Following determination of sizing of such a configured electric machine 302 , an estimated power density of 29.4 kilowatts per kilogram was derived. The optimal operational temperature for the non-superconducting windings 403 was found to be 68.4 kelvin which gave an estimated efficiency of 99.85 percent.
- Curve 603 was produced by the method of FIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles, aluminium litz windings 403 having a strand diameter of 0.1 millimetres, and a current density J s equal to 8 amps per square millimetre. Following determination of sizing of such a configured electric machine 302 , an estimated power density of 26.3 kilowatts per kilogram was derived. The optimal operational temperature for the non-superconducting windings 403 was found to be 87.7 kelvin which gave an estimated efficiency of 99.86 percent.
- Curve 604 was produced by the method of FIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles, aluminium litz windings 403 having a strand diameter of 0.1 millimetres, and a current density J s equal to 16 amps per square millimetre. Following determination of sizing of such a configured electric machine 302 , an estimated power density of 36.6 kilowatts per kilogram was derived. The optimal operational temperature for the non-superconducting windings 403 was found to be 70.3 kelvin which gave an estimated efficiency of 99.85 percent.
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Abstract
An electric machine including a stator having a fully non-magnetic core and stator windings formed of a non-superconducting transposed conductor to reduce eddy current losses. It further includes a rotor having a fully non-magnetic core and superconducting windings or superconducting magnets which produce a magnetic field for interaction with the stator windings. A cryogenic cooling system is arranged to cool the stator windings to reduce conduction losses in the stator windings.
Description
- This application is a continuation of U.S. application Ser. No. 16/548,032, filed Aug. 22, 2019, the contents of which are incorporated herein by reference.
- This disclosure relates to electric machines.
- Whilst the use of electric machines for aerospace propulsion is known, conventional electric machines generally have insufficient specific torque and power densities (whether measured in terms of weight or volume) for use in large airliners.
- A large fraction of the weight of conventional electric machines may be found in its magnetic core (often referred to as the “iron”). The magnetic core is formed from ferromagnetic material and thus typically constitutes from around 50 percent to in some instances 70 percent of the active weight of the machine.
- There are no known practical magnetic materials which are not ferromagnetic, and thus the only approach available to reduce the weight of the stator magnetic core is to remove it entirely (often referred to as an “air-cored” configuration). However, this has a drastic impact upon the magnetic field strength in the stator due to the increase in reluctance, and in turn therefore the torque developed. Indeed, the impact is such that the power to weight ratio may be worse than if the magnetic core was retained.
- The invention is directed towards electric machines. In one aspect, such a machine comprises a stator having a fully non-magnetic core and stator windings formed of a non-superconducting transposed conductor to reduce eddy current losses.
- The machine further comprises a rotor having a fully non-magnetic core and superconducting windings or superconducting magnets which produce a magnetic field for interaction with the stator windings
- A cryogenic cooling system is arranged to cool the stator windings to reduce conduction losses in the stator windings.
- As set out previously, removal of the magnetic core in conventional machines results in a large reduction in field strength. The use however of superconducting windings or superconducting magnets enables the production of extremely high fields, which, despite the fully non-magnetic core, enables the production of high torque and thus a high power-to-weight ratio.
- It will be appreciated however that removal of the stator magnetic core will expose the stator windings to this high field. This would be the case either with conventional, non-superconducting stator windings, or superconducting stator windings. This is because superconducting stator windings only exhibit ideally zero loss when exposed to time-invariant fields—in an electric machine the stator windings are clearly exposed to time-varying fields from the rotor. These time-varying fields create eddy current losses which may cause a superconducting stator winding to quench and thus require shut-down of the machine.
- Thus, in the present invention the approach taken is to use a non-superconducting transposed conductor. In an embodiment, the transposed conductor may be a litz conductor. In this way, eddy current losses are minimised. Clearly, however, conduction losses still exist in a conventional non-superconducting conductor and so a cooling system is provided to cool the stator windings. The inventors have found a surprising increase in performance of the machine by using a cryogenic cooling system to maximise the reduction in conduction losses.
- 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:
-
FIG. 1 shows an aircraft of an embodiment; -
FIG. 2 shows an aircraft of another embodiment; -
FIG. 3 is a block diagram of the propulsion systems in the aircraft ofFIG. 1 and the aircraft ofFIG. 2 ; -
FIG. 4 is a diagram of an electric machine that may be used in the system ofFIG. 3 ; -
FIG. 5 sets out a method for establishing an operational temperature for the stator in the electric machine ofFIG. 4 ; and -
FIG. 6 shows a plot of efficiency versus stator temperature for four configurations of electric machine. - An
aircraft 101 is illustrated inFIG. 1 , and in this example comprises twoelectric propulsion units 102. - The
electric propulsion units 102 are connected via apower distribution bus 103 to a source of electrical power, which in the present example is an internal combustion engine configured to drive an electric machine. In the specific embodiment ofFIG. 1 , the source of electrical power isturboelectric generator 104, in which the internal combustion engine is a gas turbine engine. In an alternative embodiment, the internal combustion engine may be a piston engine or similar. The source of electrical power could alternatively by a fuel cell, battery, etc. - Each
electric propulsion unit 102 includes an electric machine configured to drive a propulsive fan. In the present example, theelectric propulsion units 102 are ducted fans, although it will be appreciated that in alternative embodiments theelectric 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. - An
alternative aircraft 201 is illustrated inFIG. 2 , and in this example comprises twoturbofan engines 202 which include respective electric machines. The electric machines act as a source of electrical power, which is provided, via apower distribution bus 203, to a boundarylayer ingestion system 204. The boundarylayer ingestion system 204 comprises an electric machine configured to drive the fan, which improves the propulsive efficiency of theaircraft 201. - Common features of the propulsion systems employed by both
aircraft 101 andaircraft 201 are illustrated in block diagram form inFIG. 3 . In the present example, therefore, agas turbine engine 301 drives anelectric machine 302 operating as a generator via an interconnecting shaft, thus providing a source of electrical power. This power is distributed on thebus 303. Power is drawn from thebus 303 by anotherelectric machine 302, which in this instance operates as a motor. This in turn drives apropulsive fan 304 via an interconnecting shaft, thereby providing propulsion. It will be appreciated that in this embodiment, the generator and motor are provided by the same configuration ofelectric machine 302. However, in alternative embodiments, the motor and generator may be of different configurations. It will be appreciated that depending upon the configuration of the aircraft (for example aircraft 101) further propulsive fans and electric machines to drive them may be provided. - It will also be appreciated that additional sources of power, in the form of additional turboelectric generators, or fuel cells, batteries, etc. or any combination thereof, may be provided (for example as with aircraft 201).
- In the present embodiment, the
electric machine 302 is of the type claimed herein. It is shown in cross-section along its meridional plane inFIG. 4 , along with supporting hardware. - As set out previously, the
electric machine 302 comprises astator 401 having a fully non-magnetic core 402 (also known as “air-cored”) andnon-superconducting windings 403. Thenon-magnetic core 402 in the present embodiment comprises a resin in which thenon-superconducting windings 403 are embedded. In a specific embodiment, the resin is a polymeric resin such as an epoxy resin, polyurethane resin, etc. Alternative non-magnetic core materials and/or support frames may be used to support thenon-superconducting windings 403 to deal with the reaction torque during operation - The
non-superconducting windings 403 are formed from a transposed conductor. Transposed conductors are multi-strand conductors in which each strand is insulated, and is transposed in order to occupy each possible position along a specific length. The transposition of the strands may be continuous, discrete, or random. In this way, when the conductor is exposed to a magnetic field, each strand will on average link with the same number of flux lines as every other strand, thus dividing current equally among the strands. The strands are of small enough diameter that little skin effect can occur, thereby reducing losses due to induced eddy currents caused by the rotating rotor field. - In the present embodiment, the
non-superconducting windings 403 in thestator 401 are formed from litz conductor. Litz conductors are a particular type of transposed conductor in which strands of round cross-section are transposed continuously along the cable length. Other transposed conductors may be contemplated, such as Roebel conductors which use rectangular strands transposed at discrete intervals. - In an embodiment, a strand diameter of less than 1 millimetre may be selected. For example, strand diameters of 0.3, 0.2, 0.1 and 0.08 millimetres may be used.
- The transposed conductor may, in an embodiment, be formed of copper. In another embodiment, it may be formed of aluminium. Alternatively, any other non-superconducting conductor may be selected.
- The
electric machine 302 further comprises arotor 404 which also has a fully non-magnetic core, along withsuperconducting windings 405. Alternatively, therotor 404 may comprise superconducting magnets instead. In the present example thesuperconducting windings 405 are mounted to ashaft 406 that is supported bybearings 407. - A cryogenic cooling system is provided to maintain the
superconducting windings 405 in a superconducting state. In the present example, the cryogenic cooling system comprises acryogenic tank 408 for storing a cryogenic cooling fluid, along with apump 409 for circulating the fluid, which, in the present embodiment, is liquid nitrogen. A cooler 410 is included in the return path to remove heat. Alternative fluids may be used, such as liquid helium etc. Such arrangements will be familiar to those skilled in the art. - In the present embodiment, the cryogenic cooling system is also used to cool the non-superconducting stator windings 403. In an alternative arrangement, two cryogenic cooling systems may be provided, for dedicated cooling of each of the
stator 401 and therotor 404. - In the present embodiment, the cryogenic cooling system is a high temperature cryogenic cooling system. It will be appreciated by those skilled in the art that the term “high temperature cryogenic” has, in terms of temperature, a generally-defined upper limit in the art of 223 kelvin. In an embodiment, the cryogenic cooling system is configured to maintain the
non-superconducting windings 403 at a cryogenic temperature, i.e. at 223 kelvin or below. - It should be noted that one advantage of cooling the
non-superconducting windings 403 with the same cryogenic cooling system as thesuperconducting windings 405 is that it reduces the temperature differential between it and thesuperconducting windings 405 in therotor 404. This facilitates a reduction in air gap as less thermal insulation is required. - For example, in the present embodiment the airgap length between the
rotor 404 andstator 401 comprises 10 millimetres of mechanical standoff plus 2 millimetres of thermal insulation to facilitate sufficient thermal isolation therebetween. This allows higher cryogenic operational temperatures in the stator compared to in the rotor. However, it is contemplated that bringing the operational temperature of thenon-superconducting windings 403 closer to that of thesuperconducting windings 405 will facilitate reduction of this airgap to smaller lengths, for example, 5 millimetres. Thus the airgap length will be no longer dictated by the required thermal insulation, but instead by rotordynamic constraints which do not require such large airgap lengths. - In the specific embodiment of
FIG. 4 , thestator 401 further comprises amagnetic screen 411 to contain the high field generated by thesuperconducting windings 405. In an example, the magnetic screen may be a superconducting magnetic screen, with cooling thereof provided by the cryogenic cooling system. - In the present embodiment, the
electric machine 302 is configured to have a power output of 1 megawatt at a speed of 12000 revolutions per minute. In the present example, it is an 8-pole machine, however a 16-pole configuration may be contemplated. Further, the current density in thestator windings 403 may be 8 amps per square millimetre, or may alternatively be 16 amps per square millimetre depending upon desired power output. It is also contemplated that thestator windings 403 will be supplied with alternating current at a frequency of 800 hertz. - It will be appreciated however that selection of pole number, current density, and frequency is very much dependent upon the intended application of the machine, particularly in terms of power output and supply characteristics, and thus different values of each parameter may be adopted.
- As described previously, the
electric machine 302 combinesnon-superconducting stator windings 403 formed of a transposed conductor in a non-magnetically coredstator 401, with cooling thereof by a cryogenic cooling system. - It will be appreciated that as temperature decreases, the magnitude of the eddy currents in a loop, and the losses associated therewith, increases due to the attendant rise in conductivity. Eddy currents are also proportional to the square of the conductor strand radius. Conduction loss is proportional to resistivity, but is dependent only upon total conductor volume, not strand radius. Its magnitude is therefore not affected by use of transposed conductor—only the temperature changes it.
- Thus, as the temperature drops, there is a sharper drop in conduction loss than there is increase in eddy current loss. Eventually, the temperature will drop to the point of the residual resistivity of the windings and the conduction loss will be substantially constant with eddy current losses dominating. The inventors have identified that the use of a cryogenic cooling system arranged to cool the
non-superconducting stator windings 403 places themachine 302 into an operating regime which provides a surprisingly effective balance between these loss mechanisms. - Whilst it is contemplated that operation of the
non-superconducting stator windings 403 at any cryogenic temperature as herein defined will result in this effect, a method of identifying the optimum cryogenic temperature for thestator windings 403 is set out inFIG. 5 . - Conventional machine design equations typically relate the rotor torque to the rotor volume and the assumed electric and magnetic loadings. In the case of the
electric machine 302, however, thenon-magnetic core 402 means that the magnetic field produced by thesuperconducting windings 405 is not constant, but is instead a complex function of radial position. - Assuming, however, that there are no harmonic fields and the power factor of the
electric machine 302 is close to unity, it is possible to instead relate the torque developed by the machine to the rotor volume and only the electric loading of the stator and the rotor. - Thus, at
step 501, an initial set of parameters are set: -
- τ: torque to be developed by the
electric machine 302; - g: magnetic airgap length between the
stator 401 and therotor 404; - n: pole-pair number;
- Js: current density in the
non-superconducting windings 403 of thestator 401; - Jr: current density in the
superconducting windings 405 of therotor 404; - α: aspect ratio of the
electric machine 302; - hs: radial height of the
non-superconducting windings 403 of thestator 401; - hr: radial height of the
superconducting windings 405 of therotor 404.
- τ: torque to be developed by the
- At
step 502, the mean diameter Ds of thenon-superconducting windings 403 in thestator 401 may be evaluated by using the following relation: -
τ=k τ J s J r f(D s) [Equation 1] -
where: -
k τ=μ0π/8αh s h r [Equation 2] - and:
-
- Equations 1, 2, and 3 stem from a two-dimensional analytical solution of the magnetic field distribution in free space for a current sheet placed at the mean diameter of the superconducting winding 405, Dr, which is related to Ds by hs, hs, and g. This enables the magnetic field at Ds to be expressed in terms of Dr, Jr, and n.
- Equation 1 may be solved for Ds using, for example, Newton's method.
- Once Ds has been evaluated, the magnetic field at that location may be evaluated at
step 503. - At
step 504, the eddy current losses, Peddy, in one strand of thenon-superconducting windings 403 may be expressed as: -
- where B is the magnetic field strength, w is the frequency which is dependent upon rotor speed and pole-pair number n, rstrand is the radius of a strand in the transposed conductor, and ρ(T) is the resistivity of the transposed conductor at a given absolute temperature T.
- The conduction losses, Pconduction, in one strand of the
non-superconducting windings 403 may be expressed as: -
P conduction =J s 2ρ(T) [Equation 5] - Defined in this way, both Peddy and Pconduction are in terms of watts per cubic metre.
- An expression for the total losses Ptot due to these mechanisms at a particular absolute temperature T may therefore be constructed:
-
P tot =V(P eddy +P conduction) [Equation 6] - where V is the total volume of the
non-superconducting windings 403. - The optimum temperature to which to cool the
non-superconducting windings 403 may therefore be determined atstep 505 by differentiating Equation 6 with respect to p to determine the value of p that gives the lowest losses. This produces the following relation for p: -
- Then, this value of p may be converted into a temperature by use of established conductivity versus temperature data for the particular material selected for the
non-superconducting windings 403. - It will of course be appreciated by those skilled in the art that other methods of establishing an optimal operational temperature may be valid and may be used in place of the method set out in
FIG. 5 . - An exemplary plot of efficiency of the
electric machine 302 against operational temperature of thenon-superconducting windings 403 in thestator 401 is shown inFIG. 6 . It can be seen that high efficiencies are obtained over a range of cryogenic temperatures. -
Curve 601 was produced by the method ofFIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles,copper litz windings 403 having a strand diameter of 0.1 millimetres, and a current density Js equal to 8 amps per square millimetre. Following determination of sizing of such a configuredelectric machine 302, an estimated power density of 20.1 kilowatts per kilogram was derived. The optimal operational temperature for thenon-superconducting windings 403 was found to be 104.8 kelvin which gave an estimated efficiency of 99.87 percent. -
Curve 602 was produced by the method ofFIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles,copper litz windings 403 having a strand diameter of 0.1 millimetres, and a current density Js equal to 16 amps per square millimetre. Following determination of sizing of such a configuredelectric machine 302, an estimated power density of 29.4 kilowatts per kilogram was derived. The optimal operational temperature for thenon-superconducting windings 403 was found to be 68.4 kelvin which gave an estimated efficiency of 99.85 percent. -
Curve 603 was produced by the method ofFIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles,aluminium litz windings 403 having a strand diameter of 0.1 millimetres, and a current density Js equal to 8 amps per square millimetre. Following determination of sizing of such a configuredelectric machine 302, an estimated power density of 26.3 kilowatts per kilogram was derived. The optimal operational temperature for thenon-superconducting windings 403 was found to be 87.7 kelvin which gave an estimated efficiency of 99.86 percent. -
Curve 604 was produced by the method ofFIG. 5 , for a machine with a 1 megawatt target power, a speed of 12000 revolutions per minute, 8 poles,aluminium litz windings 403 having a strand diameter of 0.1 millimetres, and a current density Js equal to 16 amps per square millimetre. Following determination of sizing of such a configuredelectric machine 302, an estimated power density of 36.6 kilowatts per kilogram was derived. The optimal operational temperature for thenon-superconducting windings 403 was found to be 70.3 kelvin which gave an estimated efficiency of 99.85 percent. - 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 may be employed separately or in combination with any other features and the invention extends to and includes all combinations and sub-combinations of one or more features described herein.
Claims (21)
1-15. (canceled)
16. A method of operating an electric machine,
the electric machine comprising: a stator having a fully non-magnetic core and stator windings formed of a non-superconducting transposed conductor; a rotor having a fully non-magnetic core and superconducting windings or superconducting magnets which produce a magnetic field for interaction with the stator windings; and a cryogenic cooling system arranged to cool the stator windings,
the method comprising: using the cryogenic cooling system, cooling the stator windings to a predetermined temperature of 223 Kelvin or below, the predetermined temperature minimizing a sum of eddy current losses and conduction losses in the stator windings.
17. The method of claim 16 , in which the predetermined temperature minimizes:
P tot =V(P eddy +P conduction)
P tot =V(P eddy +P conduction)
wherein Peddy and Pconduction are the eddy current losses and the conduction losses in the stator winding measured in Watts per cubic metre, V is a total volume of the stator windings and Ptot is the total losses due to eddy current losses and conduction losses in the stator windings measured in Watts.
18. The method of claim 16 , in which the predetermined temperature is determined using predetermined conductivity versus temperature data for a material from which the transposed conductor of the stator windings is made.
19. The method of claim 18 , in which determining the predetermined temperature comprises using the predetermined conductivity versus temperature data and a conductivity value for the transposed conductor of the stator windings.
20. The method of claim 19 , in which the conductivity value for the transposed conductor of the stator windings is given by:
wherein ρ is the conductivity value of the transposed conductor of the stator windings, rstrand is a radius of a strand in the transposed conductor, Js is a current density in the stator windings, B is a magnetic field strength in the transposed conductor of the stator windings, and w is a frequency of the electric machine which is dependent upon a speed of the rotor of the electric and a pole-pair number n for the electric machine.
21. The method of claim 20 , in which B, the magnetic field strength in the transposed conductor of the stator windings, is the magnetic field at a location DS corresponding to a mean diameter of the stator windings.
22. The method of claim 21 , in which, DS satisfies:
wherein:
τ is a torque to be developed by the electric machine;
g is a magnetic airgap length between the stator and the rotor;
Jr is a current density in the windings of the rotor;
α is an aspect ratio of the electric machine;
hs is a radial height of the stator windings; and
hr is a radial height of the windings of the rotor.
23. The method of claim 16 , in which the electric machine is operated at a power of at least 1 MW.
24. The method of claim 16 , in which the current density in the stator windings is, in operation, at least 8 Amps per square millimetre.
25. The method of claim 16 , in which the stator windings are supplied with an alternating current at a frequency of at least 800 Hertz.
26. An electric machine comprising:
a stator having a fully non-magnetic core and stator windings formed of a non-superconducting transposed conductor to reduce eddy current losses;
a rotor having a fully non-magnetic core and superconducting windings or superconducting magnets which produce a magnetic field for interaction with the stator windings; and
a cryogenic cooling system arranged to cool the stator windings to a predetermined cryogenic temperature of 223 kelvin or below, the predetermined temperature cryogenic temperature minimizing a sum of the eddy current losses and the conduction losses in the stator windings.
27. The electric machine of claim 26 , in which the transposed conductor is a litz conductor.
28. The electric machine of claim 26 , in which the stator windings are formed from wire strands having a diameter of less than 1 millimetre.
29. The electric machine of claim 26 , in which the stator windings are formed from one of:
copper;
aluminium.
30. The electric machine of claim 26 , in which the fully non-magnetic core in the stator comprises a resin.
31. The electric machine of claim 26 , comprising one of:
8 poles;
16 poles.
32. A propulsion system for an aircraft, comprising:
an electrical network for distributing electrical power;
a source of electrical power connected with the electrical network; and
one or more electric machines according to claim 26 connected with the electrical network for driving a fan to propel the aircraft.
33. The propulsion system of claim 32 , in which at least one of the one or more electric machines drives a boundary layer ingestion fan.
34. A propulsion system for an aircraft, comprising:
an electrical network for distributing electrical power;
one or more electric machines according to claim 26 connected with the electrical network for generating said electrical power; and
one or more electric propulsion units connected with the electrical network for propelling the aircraft.
35. The propulsion system of claim 34 , in which the or each electric machine is driven by a respective internal combustion engine, and optionally wherein the or each internal combustion engine is one of:
a piston engine;
a turbomachine.
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US17/504,151 US20220037979A1 (en) | 2018-09-18 | 2021-10-18 | Electric machine |
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GR20180100429 | 2018-09-18 | ||
GR20180100429 | 2018-09-18 | ||
US16/548,032 US11152844B2 (en) | 2018-09-18 | 2019-08-22 | Electric machine |
US17/504,151 US20220037979A1 (en) | 2018-09-18 | 2021-10-18 | Electric machine |
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US16/548,032 Continuation US11152844B2 (en) | 2018-09-18 | 2019-08-22 | Electric machine |
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DE102022204761A1 (en) | 2022-05-16 | 2023-11-16 | Rolls-Royce Deutschland Ltd & Co Kg | Propulsion system for an aircraft |
US20240030766A1 (en) * | 2022-07-22 | 2024-01-25 | Raytheon Technologies Corporation | Indirect rotor cooling of electric machine |
EP4376266A1 (en) * | 2022-11-24 | 2024-05-29 | GKN Aerospace Services Limited | Electric machine having cryocooled armature litz windings |
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CN110912376B (en) | 2023-07-04 |
EP3627674A1 (en) | 2020-03-25 |
US20200091810A1 (en) | 2020-03-19 |
US11152844B2 (en) | 2021-10-19 |
GB201817883D0 (en) | 2018-12-19 |
EP3627674B1 (en) | 2022-01-12 |
EP3982524A1 (en) | 2022-04-13 |
CN110912376A (en) | 2020-03-24 |
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