A Flux-Controllable NI HTS Flux-Switching Machine for Electric Vehicle Applications

applied sciences

Article A Flux-Controllable NI HTS Flux-Switching Machine for Electric Vehicle Applications

Young Jin Hwang * , Jae Young Jang and SangGap Lee Center for Scientiﬁc Instrumentation, Korea Basic Science Institute, Daejeon 34133, Korea; [email protected] (J.Y.J.); [email protected] (S.L.) * Correspondence: [email protected]; Tel.: +82-43-240-5358

Received: 13 February 2020; Accepted: 24 February 2020; Published: 25 February 2020

Abstract: This paper deals with a flux-controllable NI HTS flux-switching machine (FSM) for electric vehicle (EV) applications. In a variable-speed rotating machine for EVs, such as electric buses, electric aircraft and electric ships, an electric motor capable of regulating the flux offers the advantage of constant output operation. In general, conventional HTS rotating machines have excellent flux-regulation performance, because they excite an HTS field coil. However, it is difficult to ensure any flux-regulation capabilities in HTS rotating machines using HTS field coils that apply the no-insulation (NI) winding technique, due to the inherent charge and discharge delays in these machines. Nevertheless, the NI winding technique is being actively researched as a key technology for the successful development of HTS rotating machines, because it can dramatically improve the operational stability of HTS field coils. Therefore, research to implement an HTS rotating machine with flux-regulation capabilities, while improving the operating stability of the HTS field coil using the NI winding technique, is required for EV applications. In this paper, we propose an HTS rotating machine with a flux switching structure, a type of topology of a rotating machine that is being actively studied for application to the electric motors used in EVs. The proposed HTS flux-switching machine (FSM) uses NI field coils, but additional field windings are applied for flux regulation, which enables flux control. In this study, an NI HTS field coil was also fabricated and tested because the characteristic resistance value should be used for the design and characteristic analyses of machines which utilize an NI coil. The simulation model used to analyze the flux-regulation performance capabilities of the NI HTS FSM were devised based on the characteristic resistance values obtained from a charging test of the fabricated NI HTS field coil. This study can provide a good reference for further research, including work on the manufacturing of a prototype NI HTS FSM for EV applications, and it can be used as a reference for the development of other HTS rotating machines, such as those used in large-scale wind power generation, where flux-regulation capabilities are required.

Keywords: electric vehicle; ﬂux regulation; ﬂux-switching machine; HTS; no-insulation

1. Introduction Currently, electric vehicles (EVs) are regarded as good candidates for decreasing environmental pollution by reducing the use of fossil fuels [1–7]. Accordingly, there has been rapid progress in the creation of electric vehicles. An EV, also known as an electric vehicle, is a type of transportation which uses electric motors for propulsion in place of a conventional propulsion system, such as an internal combustion engine (ICE). EVs can include electric cars, electric buses, electric trucks, electric trains, electric aircraft, electric scooters and even electric ships. Motion in EVs can be provided by wheels or propellers driven by traction motors, or in the case of railway vehicles, by linear motors. Two types of electric motors are commonly used for vehicle applications: The induction motor (IM), and the permanent magnet synchronous motor (PMSM), and it is generally known that IMs have

Appl. Sci. 2020, 10, 1564; doi:10.3390/app10051564 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 1564 2 of 20 lower rated torque and efficiency levels compared to PMSMs. In general, PMSMs are more efficient, and for this reason electric vehicles commonly use these types of electric motors. However, because a PMSM uses constant magnetic flux generated by PMs, the flux can only be regulated by means of an armature reaction flux control by a converter. This decreases the machine efficiency, because it significantly increases the winding losses. While PMSMs have employed innovative technologies, it is not easy to increase the electric motor output power any further. As an advanced technology for electric motors, researchers have made progress on the development of high-temperature superconducting rotating machines. Because superconducting wires can handle high-density current flows well, compared to copper wire, it is possible to manufacture a coil capable of generating a very high magnetic field, even with a low number of windings using superconducting wires. Therefore, by applying superconducting coils to electrical machines such as a motor and generator, it is possible to develop electrical machines with high efficiency and high output power levels. Accordingly, many researchers have attempted to apply superconducting motors to EVs, and the applicability of superconducting motors has been largely verified [8–15]. However, for the practical application of superconducting motors to EVs, the operating stability and reliability must be improved. Superconductors can exhibit superconductivity only under critical environmental conditions, such as those with a critical current, critical temperature and critical magnetic field, and superconductivity is lost when this condition deviates for any reason. If this phenomenon, called quenching, occurs, superconductors gain electrical resistance, which can lead to Joule heating and damage. In particular, such damage can be a serious problem in high-temperature superconducting (HTS) wires, given their relatively slow normal zone propagation velocities. Therefore, in order to apply superconducting coils wound with HTS wiring to electrical machines, it is very important to protect the HTS coils from the quenching phenomenon. The approach known as the no-insulation (NI) winding technique is a viable quench-protection method for HTS coils [16–25]. The NI winding technique involves the fabrication of coils without insulation materials between the turns. Figure1 shows the quench-protection mechanism of the NI winding technique. In this figure, Iop is the operating current flowing through the coil. For an HTS coil with turn-to-turn insulation, even if a hot spot occurs due to a partial quench, the current flows continuously through the hot spot, meaning that Joule heat is continuously generated. On the other hand, an NI coil without turn-to-turn insulation has a self-protection mechanism, in that it bypasses the current to the adjacent turns automatically when the HTS coil is partially quenched. Given that the electrical resistance of the superconducting layer under the normal operation condition is nearly zero, the current flows only to the superconducting layer. However, when a hot spot caused by local quenching occurs, electrical resistance occurs at the local portion. Therefore, the current flows through the stabilizer layer constituting the superconducting wire, and it is thus bypassed. As a result, NI coils can be protected from quenching without complicated quench detection schemes and/or protection devices, simplifying these HTS coil systems [16–25]. However, the NI HTS coil is associated with an intrinsic charging/discharging delay phenomenon. The charging/discharging delay phenomenon increases as the coil inductance increases, and the contact resistance between winding turns decreases. Figure2 shows a circuit model of the NI HTS coil. In this figure, the portion indicated by the light blue background represents the simplified equivalent circuit model of the NI HTS coil. As indicated by this circuit model, when the current is charged or discharged, the current in the NI HTS coil is divided into a spiral direction (winding direction) and a radial direction (contact direction between winding turns). The division ratio of the current is determined by the ratio of the azimuthal impedance value, which is in turn determined by the coil inductance and the current charge/discharge rate to the radial impedance value due to the characteristic resistance. The magnetic field generated by the coil stems from the current flowing in the spiral direction. Therefore, if the coil inductance is large, the current flowing through the contact becomes large, such that the delay phenomenon during the charge and discharge of the magnetic field becomes very large. Therefore, in order to develop a robust electric motor using an NI coil, the effects by these charge and discharge delays should be considered. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 21 Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 21

Therefore, in order to develop a robust electric motor using an NI coil, the effects by these charge and Therefore, in order to develop a robust electric motor using an NI coil, the effects by these charge and Appl.discharge Sci. 2020 delays, 10, 1564 should be considered. 3 of 20 discharge delays should be considered.

(a) (b) (a) (b) Figure 1. Illustration of the self-protection mechanism of the no-insulation (NI) superconducting coil FigureFigure 1.1. IllustrationIllustration ofof thethe self-protectionself-protection mechanismmechanism ofof thethe no-insulationno-insulation (NI)(NI) superconductingsuperconducting coilcoil in a comparison: (a) high-temperature superconducting (HTS) coil using the turn-to-turn winding inin aa comparison:comparison: ((aa)) high-temperaturehigh-temperature superconductingsuperconducting (HTS)(HTS) coilcoil usingusing thethe turn-to-turnturn-to-turn windingwinding technique, and (b) HTS coil using the NI winding technique. technique,technique, andand ((bb)) HTS HTS coil coil using using the the NI NI winding winding technique. technique.

FigureFigure 2.2. SimpliﬁedSimplified equivalentequivalent circuitcircuit modelmodel ofof thethe NINI HTSHTS coil.coil. Figure 2. Simplified equivalent circuit model of the NI HTS coil.

VariousVarious researchersresearchers havehave attemptedattempted toto applyapply HTSHTS coilscoils toto electricalelectrical machines,machines, suchsuch asas motorsmotors Various researchers have attempted to apply HTS coils to electrical machines, such as motors andand generators [26–33]. [26–33]. Specifically, Specifically, these these studies studies are more are moreactively actively carried carried out in relation out in to relation electrical to and generators [26–33]. Specifically, these studies are more actively carried out in relation to electrical electricalrotating machines, rotating machines, which require which high require output high power output levels, power such levels, as suchthose as used those for used wind for power wind rotating machines, which require high output power levels, such as those used for wind power powergeneration generation and in and electric in electric vehicle vehicle applications. applications. Thus Thus far, far,most most studies studies on on the the application ofof generation and in electric vehicle applications. Thus far, most studies on the application of superconductingsuperconducting coilscoils toto electricalelectrical rotatingrotating machinesmachines havehave mainlymainly usedused anan air-core typetype ofof structure,structure, superconducting coils to electrical rotating machines have mainly used an air-core type of structure, whichwhich doesdoes notnot useuse ironiron cores.cores. In electrical rotatingrotating machines with anan air-coreair-core structure,structure, thethe which does not use iron cores. In electrical rotating machines with an air-core structure, the superconductingsuperconducting fieldfield coils are are positioned positioned in in the the roto rotorr to to create create a arotating rotating field. field. Therefore, Therefore, in the in superconducting field coils are positioned in the rotor to create a rotating field. Therefore, in the therotating rotating field field coil coil structure, structure, the the cooling cooling system system and and the the excitation excitation system system are are inevitably inevitably complicated, complicated, rotating field coil structure, the cooling system and the excitation system are inevitably complicated, andand requirerequire aa highlyhighly reliablereliable designdesign [[31–34].31–34]. InIn thethe rotorrotor excitationexcitation machine,machine, thethe fieldfield currentcurrent mustmust bebe and require a highly reliable design [31–34]. In the rotor excitation machine, the field current must be fedfed throughthrough thethe slipslip ringring andand brushes,brushes, andand thethe coolingcooling systemsystem requiresrequires dynamicdynamic sealsseals forfor cryogeniccryogenic fed through the slip ring and brushes, and the cooling system requires dynamic seals for cryogenic transfertransfer coupling.coupling. OnOn thethe otherother hand,hand, electricalelectrical rotatingrotating machines,machines, suchsuch asas aa flux-switchingflux-switching machinemachine transfer coupling. On the other hand, electrical rotating machines, such as a flux-switching machine (FSM),(FSM), aa homopolar homopolar machine machine and and a Verniera Vernier machine, machine, use ironuse iron cores, cores, but it but is possible it is possible to realize to realize a robust a (FSM), a homopolar machine and a Vernier machine, use iron cores, but it is possible to realize a machine,robust machine, because because they have they stationary have stationary field coil fiel structures.d coil structures. The cooling The cooling system andsystem excitation and excitation system robust machine, because they have stationary field coil structures. The cooling system and excitation forsystem the field for the coils field in machines coils in machines with a stationary with a stationary field coil structurefield coil canstructure be made can simpler be made than simpler those than in a system for the field coils in machines with a stationary field coil structure can be made simpler than machinethose in a with machine a rotating with fielda rotating coil structure. field coil Forstructure. this reason, For this research reason, on research the application on the application of HTS field of those in a machine with a rotating field coil structure. For this reason, research on the application of coilsHTS tofield electrical coils to rotating electrical machines rotating withmachines a stationary with a fieldstationary coil structure field coil has structure been active. has been In particular, active. In HTS field coils to electrical rotating machines with a stationary field coil structure has been active. In theparticular, FSM with the stationary FSM with HTS stationary field coils HTS is field becoming coils is more becoming attractive, more given attractive, its advantages given its advantages of a robust particular, the FSM with stationary HTS field coils is becoming more attractive, given its advantages rotorof a robust and a reliablerotor and machine a reliabl structuree machine [32 structure–38]. [32–38]. of a However,robust rotor while and thea reliabl FSMe has machine a robust structure rotor structure, [32–38]. the protection issue related to HTS field coils should be considered, as in other HTS rotating machines. The NI winding technique can serve Appl. Sci. 2020, 10, x FOR PEER REVIEW 4 of 21

However, while the FSM has a robust rotor structure, the protection issue related to HTS field coils should be considered, as in other HTS rotating machines. The NI winding technique can serve Appl. Sci. 2020, 10, 1564 4 of 20 as a solution in this case. However, the FSM uses a ferromagnetic core to create a switched type of flux, which makes the inductance of the HTS field coil very large. Therefore, the intrinsic charging andas a discharging solution in thisdelay case. effect However, of the NI the coil FSM becomes uses a very ferromagnetic large. This corecan also to create develop a switched into a technical type of problemflux, which during makes the the application inductance of ofan theelectrical HTS field rotating coil verymachine large. requiring Therefore, flux-control the intrinsic capabilities. charging Therefore,and discharging in order delay to apply effect ofan the NI NIHTS coil field becomes coil to very a flux-controllable large. This can alsoFSM, develop a separate into winding a technical is problemrequired duringfor flux theregulation. application of an electrical rotating machine requiring flux-control capabilities. Therefore,In this in paper, order the to applystructure an NI of HTSan NI field HTS coil FSM to acapable flux-controllable of flux control FSM, is a separateproposed. winding An FSM is applyingrequired forthe fluxproposed regulation. structure was designed, and its flux regulation characteristics were analyzed throughIn this a transient paper, themotion structure analysis. of an To NI do HTS this, FSMan NI capable HTS field of fluxcoil was control fabricated is proposed. and tested, AnFSM and theapplying characteristic the proposed resistance structure of the was fabricated designed, NI HTS and itsfield flux coil regulation was calculated characteristics from the were charging analyzed time constantthrough athrough transient a current motion charging analysis. test To doof the this, coil. an NIIt was HTS confirmed field coil wasthat fabricatedthe flux-control and tested, capability and cannotthe characteristic be achieved resistance by applying of the on fabricatedly the NI NI HTS HTS fields field for coil field was calculatedexcitation. fromTherefore, the charging in order time to realizeconstant the through flux-control a current capabilities charging of test the of theNI coil.HTS ItFSM, was confirmeda separate thatwinding the flux-control is required capability for flux regulation.cannot be achieved by applying only the NI HTS fields for field excitation. Therefore, in order to realize the flux-control capabilities of the NI HTS FSM, a separate winding is required for flux regulation. 2. Machine Structure Operating Principle and Design of the Flux-Controllable NI HTS FSM 2. Machine Structure Operating Principle and Design of the Flux-Controllable NI HTS FSM 2.1. Machine Structure of the Flux-Controllable NI HTS FSM 2.1. Machine Structure of the Flux-Controllable NI HTS FSM Figure 3 shows the topology of the flux-controllable FSM proposed in this paper. It is composed of a rotorFigure core,3 shows armature the topology windings, of theNI HTS flux-controllable field coils, and FSM additional proposed field in this windings. paper. It All is composedwindings, includingof a rotor core,the field armature windings, windings, are located NI HTS in fieldthe stator coils, andcore. additional Accordingly, field the windings. machine All has windings, a robust rotorincluding structure. the field A windings,ferromagnetic are located material in the is statorused core.for the Accordingly, stator and the the machine rotor core. has aThree-phase robust rotor armaturestructure. windings A ferromagnetic are considered, material and is used the forarmature the stator windings and the are rotor placed core. in Three-phase each stator slot, armature with windingseach winding are considered,connected in and series. the armature The NI HTS windings field co areils placedgenerate in eachthe main stator magnetic slot, with flux each during winding the steady-stateconnected in series.operation, The NIand HTS additional field coils field generate windings the main are magnetic used for flux flux during regulation. the steady-state Unlike conventionaloperation, and rotor additional excitation field machines, windings the are FSM used has for a doubly flux regulation. salient structure, Unlike conventionalwith the field rotorcoils placedexcitation in the machines, U-shaped the iron FSM core has aof doubly the stator. salient In this structure, machine, with the the rotor field core coils has placed a simple in the laminated U-shaped iron core ofwhich the stator. has salient In this poles. machine, The flux the rotordirection core hasof each a simple field laminatedcoil is opposite iron corebetween which the has adjacent salient poles.field coils. The flux direction of each field coil is opposite between the adjacent field coils.

(a) (b)

Figure 3. Topology of the flux-controllableflux-controllable flux-switching flux-switching machine (FSM): ( (aa)) cross cross section, section, and and ( (b)) three-dimensional configuration.configuration.

2.2. Operating Operating Principle of th thee Flux-Controllable NI HTS FSM Figure4 4 shows shows the the principle principle of fluxof flux switching switching in the in NI the HTS NI FSM. HTS With FSM. the With doubly the salient doubly topology, salient topology,the magnetic the fieldmagnetic generated field generated by the NI HTSby the field NI coilsHTS alwaysfield coils traverses always the traverses path with the thepath minimum with the minimummagnetic reluctance magnetic atreluctance positions at where positions the rotor where poles the and rotor the poles stator and teeth the are stator aligned teeth to are each aligned other. Atto the rotor position shown in Figure4a, the rotor pole aligns with one of two stator teeth, around which an armature winding is wound, and the magnetic flux that is linked in the armature winding, that Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 21

each other. At the rotor position shown in Figure 4a, the rotor pole aligns with one of two stator teeth, around which an armature winding is wound, and the magnetic flux that is linked in the armature winding, that exits the rotor pole and enters the armature winding. When the rotor core rotates until another rotor pole aligns with the right part of the doubly salient pole of the stator, the magnetic flux goes out of the armature coil and into the rotor pole, maintaining the same amount of flux linkage, while reversing the polarity, thus ‘switching’ the flux. Clearly, as the rotor core rotates, the flux linkage in the armature windings will change periodically, inducing back-EMF. Figure 5 represents the flux regulation principle of the flux-controllable FSM. In this machine, the flux-regulation capability is provided by additional field coils wound around the U-shaped doubly salient pole. Figure 5a shows the path of the main magnetic flux generated by the NI HTS field coil. Because the rotor tooth is aligned to the left of the doubly salient pole of the stator, the main magnetic flux forms a clockwise loop with the rotor tooth and the stator. Figure 5b presents the path of the magnetic flux generated by the additional field coil for flux regulation. The flux is generated Appl.by the Sci. additional2020, 10, 1564 field coil marked in yellow in the figure, and the flux also flows in the opposite5 of 20 direction when the direction of the field current changes. As the magneto-motive force (MMF) exitsgenerated the rotor from pole the and additional enters the field armature coil increases, winding. Whenthe flux-regulating the rotor core effect rotates is until enhanced. another If rotor the poledirection aligns of with the magnetic the right partflux ofgenerated the doubly by salientthe additi poleonal of thefield stator, coil is the identical magnetic to fluxthe direction goes out of thethe armaturemain magnetic coil and flux, into the the total rotor magnitude pole, maintaining of the flux the linkage same amount in the ofarmature flux linkage, windings while increases, reversing and the polarity,vice versa. thus Figure ‘switching’ 5c shows the an flux. example Clearly, of asnegative the rotor regulation, core rotates, during the fluxwhich linkage the amplitude in the armature of the windingsflux linkage will in change the armature periodically, windings inducing decreases. back-EMF.

(a)

(b)

FigureFigure 4.4. Operating principle: ( (aa)) starting starting point, point, and and ( (b)) after moving the rotor positionposition byby 180180 electricalelectrical degrees.degrees.

Figure5 represents the flux regulation principle of the flux-controllable FSM. In this machine, the flux-regulation capability is provided by additional field coils wound around the U-shaped doubly salient pole. Figure5a shows the path of the main magnetic flux generated by the NI HTS field coil. Because the rotor tooth is aligned to the left of the doubly salient pole of the stator, the main magnetic flux forms a clockwise loop with the rotor tooth and the stator. Figure5b presents the path of the magnetic flux generated by the additional field coil for flux regulation. The flux is generated by the additional field coil marked in yellow in the figure, and the flux also flows in the opposite direction when the direction of the field current changes. As the magneto-motive force (MMF) generated from the additional field coil increases, the flux-regulating effect is enhanced. If the direction of the magnetic flux generated by the additional field coil is identical to the direction of the main magnetic flux, the total magnitude of the flux linkage in the armature windings increases, and vice versa. Figure5c shows an example of negative regulation, during which the amplitude of the flux linkage in the armature windings decreases. Appl. Sci. 2020, 10, 1564 6 of 20 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 21

(a) (b)

(c)

FigureFigure 5. 5. FluxFlux regulation regulation principle: principle: ( (aa)) main main magnetic magnetic flux flux by by the the NI NI HTS HTS field field coil, coil, ( (bb)) magnetic magnetic flux flux byby the the additional additional field field coil, coil, and and ( (cc)) regulated regulated magnetic magnetic flux. flux.

2.3.2.3. Design Design of of the the Flux-Controllable Flux-Controllable FSM FSM InIn this this study, aa three-phasethree-phase FSM FSM with with a rateda rated output output power power of 20of kW,20 kW, and and a rotation a rotation speed speed of 1000 of 1000rpm, rpm, was considered.was considered. The designThe design was carriedwas carrie outd through out through the following the following process process [32,33 [32,33].].

1.1. TheThe design design specifications speciﬁcations of of the the NI NI HTS HTS FSM, FSM, in in this this case case the the output output power, power, flux ﬂux regulation regulation ratio ratio andand phase, phase, are are determined. determined. r,max max 2.2. TheThe maximum maximum radius radius rrr,max andand length length llmax ofof the the rotor rotor are are calculated calculated considering considering rotor rotor material’s material’s mechanicalmechanical properties, properties, using using Equations Equations (1) (1) and and (2), (2), respectively. respectively. = σ + ρ Ω2 rr,max 4qyield /((3 v) ) (1) 2 rr,max = 4σyield/((3 + v)ρΩ ) (1) = π 2 Ω ρ (2) lmax q/ k EI / S 2 p lmax = π /kΩ EI/ρS (2) 3. The slot pitch is calculated based on the pole pitch and the numbers of slots. The number of 3. conductorsThe slot pitch also is are calculated calculated based using on Equation the pole (3). pitch and the numbers of slots. The number of conductors also are calculatedS = S using⋅ n / Equationβ = I ⋅ n (3)./(J ⋅ β) = d ⋅ w slot Cu 0 0 0 Cu slot slot (3)

4. The dimensions of the armatureS = S conductorCu n0/β = anI0d then0/ slot(JCu areβ) determined = d w by Equation (4). (3) slot · · · slot · slot d = 2⋅d ⋅ β / n ,w = w ⋅ β / 2 (4) 4. The dimensions of the armatureCu conductorslot and0 theCu slotslot are determined by Equation (4). 5. The tangential stress in the rotor is calculatp ed using the rotorp dimension and the torque via d = 2 d β/n , w = w β/2 (4) Equation (5). Cu · slot · 0 Cu slot · 5. The tangential stress in the rotor isσ calculated= T /(2π usingr 2l) the rotor dimension and the torque via F tan (5) Equation (5). 6. The required air-gap field density, B0, is then calculated2 by Equation (6). σF tan = T/(2πr l) (5) = πσ ϕ B0 2 F tan (2r) /(I S n0 q cos ) (6)

7. Based on the required flux regulation ratio and B0, the NI HTS field coil and the additional field winding are designed by means of FEM under a no-load condition. Appl. Sci. 2020, 10, 1564 7 of 20

6. The required air-gap ﬁeld density, B0, is then calculated by Equation (6).

B0 = √2πσF tan(2r)/(ISn0q cos ϕ) (6)

7. Based on the required flux regulation ratio and B0, the NI HTS field coil and the additional field winding are designed by means of FEM under a no-load condition. 8. The cryostat for the NI HTS field coil is designed based on the HTS field coil and other thermal loads. The thermal loads of the cryostat are calculated by the equations below [39–41].

p q 2 q 2 Q = ( P l I /A k A (TH TL)/L ) + 2I ρ k (TH TL) (7) CL CL CL CL − CL CL − CL CL CL −

QK = NLALkL(TH TL)/LL (8) − 4 4 QR = σ(T T )/((1 εH)/εHAH + (1/εL + 2N/εN N)/AL) (9) H − L − −

Here, QCL, QK, and QR are the thermal loads from the conduction heat transfers by a pair of current leads, the leading-in tubes for the signal lines, and the radiation heat transfer. 9. Characteristic analyses of the deigned machine are carried out in the form of a transient motion analysis, using a commercialized FEM software package.

In this design, an NI HTS field coil was considered for field excitation. Therefore, the characteristic resistance, RC, of the NI HTS field coil is required as a design parameter. For this reason, an NI HTS field coil was fabricated to obtain an RC value. The RC value can be calculated from a charging time constant (τ) by a charging test. Figure6 shows the NI HTS field coil fabricated in this study. The HTS coil was fabricated using 2G HTS wire with a 12 mm width. It has a double-layered structure with 40 winding turns. Both the effective length and width of the G10 bobbin for the NI HTS coil are 600 mm and 150 mm, respectively. Table1 shows the specifications of the NI HTS field coil. To determine the characteristic resistance of the fabricated NI HTS field coil, the current charging test was carried out in liquid nitrogen. Figure7 shows the normalized central magnetic field after current charging at 100 A, with a cryogenic Hall sensor used to measure the magnetic field. After the power supply current reached the target current of 100 A, the magnetic field relaxes over time while holding the current. The field increases logarithmically to approximately 25 mT at 600 s. This outcome indicates that the fabricated NI HTS field coil is affected by the charging delay phenomenon. The typical method for calculating RC in NI HTS coils is to use a decay time constant after a sudden discharge of the power supply current. However, RC for the straight section in a racetrack coil depends on the Lorentz force induced by the excitation current. Because the Lorentz force dissipates as the coil current is instantly decreased to zero, RC is not at all affected by the Lorentz force during a sudden charge [32,42]. Therefore, it is more appropriate to calculate the RC of the NI HTS racetrack coil using the charge time constant. The charging time constant, τ, can be defined as the time interval between B1 and B2 in Figure7, where B 1 is the magnetic field density at the moment when the power supply current is fully charged, and B2 is the sum of B1 and B, which is 0.63 between B1 and a fully charged level. In Figure7, the charging time constant ( τ) is 89.34 s. The RC value of the coil calculated from the charging time constant is 145 µΩ. Here, L and τ are the coil inductance and the time constant, respectively. Figure8 shows the measured results of the magnetic field distribution in the NI HTS field coil measured at an excitation current of 100 A. For this measurement, a field mapper with a three-axis motor for sensor movements was used. Considering the charging delay phenomenon of the NI HTS field coil, the magnetic field distribution measurement was performed after 800 s. As indicated in the graph, the central magnetic field generated from the NI HTS field coil at an excitation current of 100 A is 25 mT, and the magnetic field density increases toward the coil winding part. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 21

Insulation type No-insulation

Appl. Sci. 2020, 10, x FORInductance PEER REVIEW 1.3 mH 8 of 21 Operating temperature 77 K Nominal operatingInsulation typecurrent No-insulation 100 A Appl. Sci. 2020, 10, x FOR PEER InductanceREVIEW 1.3 mH 8 of 21 Appl. Sci. 2020, 10, 1564Operating temperature 77 K 8 of 20 NominalInsulation operating type current No-insulation 100 A Inductance 1.3 mH Operating temperature 77 K Nominal operating current 100 A

(a) (b) (a) (b) Figure 6. (a) Pictures of the fabricated NI HTS field coil, and (b) test set-up. Figure 6. ((aa)) Pictures Pictures of of the the fabricated fabricated NI NI HTS HTS field ﬁeld coil, and ( b) test set-up.

(a) (b)

Figure 6. (a) Pictures of the fabricated NI HTS field coil, and (b) test set-up.

Figure 7. Charging test result of the fabricated NI HTS field coil.

FigureFigure 7. Charging 7. Charging test test result result of of the fabricatedfabricated NI NI HTS HTS ﬁeld field coil. coil. Figure 7. Charging test result of the fabricated NI HTS field coil.

Figure 8. Measured magnetic field distribution of the fabricated NI HTS field coil.

Based on the aforementioned design process, an NI HTS FSM with a 12-tooth stator was initially chosen. Table 2 presents the design parameters of the NI HTS FSM. A liquid nitrogen cryostat, a vacuum-layeredFigure cryostat 8. Measured with a magneticpair of current ﬁeld distribution leads, a liquid of the fabricatednitrogen inlet, NI HTS a liquid ﬁeld coil. nitrogen outlet, Figure 8. Measured magnetic field distribution of the fabricated NI HTS field coil. BasedFigure on the8. Measured aforementioned magnetic design field process, distribution an NI HTS of the FSM fabricated with a 12-tooth NI HTS stator field was coil. initially chosen. Table 2 presents the design parameters of the NI HTS FSM. A liquid nitrogen cryostat, a Basedvacuum-layered on the aforementioned cryostat with a design pair of current process, leads, an NIa liquid HTS nitrogen FSM with inlet, a a12-tooth liquid nitrogen stator outlet,was initially chosen. Table 2 presents the design parameters of the NI HTS FSM. A liquid nitrogen cryostat, a vacuum-layered cryostat with a pair of current leads, a liquid nitrogen inlet, a liquid nitrogen outlet, Appl. Sci. 2020, 10, 1564 9 of 20

Table 1. Key Parameters of the NI HTS Field Coil.

Item Value 2G HTS wire 2G HTS wire manufactured by AMSC Wire width 12 mm Winding structure Racetrack and single pancake Length of the straight coil part 600 mm Inner radius of the coil end part 65 mm Total winding turns 40 Insulation type No-insulation Inductance 1.3 mH Operating temperature 77 K Nominal operating current 100 A

Based on the aforementioned design process, an NI HTS FSM with a 12-tooth stator was initially chosen. Table2 presents the design parameters of the NI HTS FSM. A liquid nitrogen cryostat, a vacuum-layered cryostat with a pair of current leads, a liquid nitrogen inlet, a liquid nitrogen outlet, and leading-in tubes, were designed. In order to account for the radiation heat loss, multi-layer insulation (MLI) in the vacuum layer was considered. According to Equations (7)–(9), the total heat Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 21 loss for each cryostat was calculated and found to be approximately 15 W when three layers of MLI were inserted into the vacuumTable space. 2. FigureKey Design9 shows Parameters the designed of the FSM. LN 2 cryostat.

Table 2. KeyItem Design Parameters of the FSM. Value Maximum output power 36 kW ItemPhases Value3 Maximum outputRotating power speed 36 1000 kW rpm PhasesStator inner diameter 563 3 mm Rotating speed 1000 rpm Stator inner diameterAir-gap distance 563 mm3 mm Air-gap distanceRotor inner diameter 3 175mm mm Rotor inner diameterActive stack length 175 600 mm mm Active stack lengthNo. of stator slots 600 mm 12 No. of statorNo. slots of rotor poles 12 10 No. of rotor poles 10 Stator tooth arc 7.5° Stator tooth arc 7.5◦ Slot arcSlot arc 8.25 8.25°◦ Winding turnsWinding per armature turns coilper armature coil 12 12 No. of field coilsNo. of for field flux regulationcoils for flux regulation 12 12 Winding turns per field coil for flux regulation 20 Winding turns per field coil for flux regulation 20

Figure 9. Liquid nitrogen (LN ) cryostat for the NI HTS ﬁeld coil. Figure 9. Liquid nitrogen (LN2) cryostat for the NI HTS field coil.

3. Characteristic Analysis of the Flux-Controllable NI HTS FSM

3.1. Field Distribution of the Proposed FSM In this study, the operating characteristics of the designed NI HTS FSM were analyzed by a combination of the finite element method (FEM) and an electrical circuit analysis. The electrical circuit model was devised based on the simplified circuit model shown in Figure 2. The RC value in the electrical circuit model was calculated from the current charging test of the NI HTS coil. Simulations were performed using FEM software, and the electrical circuit was modeled using a circuit modeler, and was simulated by interworking with the electromagnetic analysis, as shown in Figure 10. In this model, the azimuthal resistance of the HTS coil is assumed to be zero. Figure 11 presents the simulation results of the current charging characteristics of the NI HTS field coil under a 100 A field current with a ramping rate of 1 A s−1. The charging time constant, τ, can be calculated from the time interval between I1 and I2 in Figure 11, as in NI HTS field coil test, because the magnetic field density is proportional to the coil current. Here, I1 is the coil current at the moment when the power supply current reaches the target, and I2 (66.48 A) represents the sum of I1 and I, which is 63% (57.08 A) between I1 (9.40 A) and a fully charged coil current (100 A). The value of τ is 608 s. The coil current is fully charged approximately 9500 s after the excitation current reaches the target. Therefore, the initial time for the simulations of the designed NI HTS FSM in the steady-state condition was set to 10,000 s, considering a full charge of the field current. Figure 12 shows the magnetic flux vector at the rotor position of 0 degrees and 15.75 degrees 10,000 s after the field current reaches the target of 100 A. Appl. Sci. 2020, 10, 1564 10 of 20

3. Characteristic Analysis of the Flux-Controllable NI HTS FSM

3.1. Field Distribution of the Proposed FSM In this study, the operating characteristics of the designed NI HTS FSM were analyzed by a combination of the finite element method (FEM) and an electrical circuit analysis. The electrical circuit model was devised based on the simplified circuit model shown in Figure2. The RC value in the electrical circuit model was calculated from the current charging test of the NI HTS coil. Simulations were performed using FEM software, and the electrical circuit was modeled using a circuit modeler, and was simulated by interworking with the electromagnetic analysis, as shown in Figure 10. In this model, the azimuthal resistance of the HTS coil is assumed to be zero. Figure 11 presents the simulation results of the current charging characteristics of the NI HTS field coil under a 100 A field 1 current with a ramping rate of 1 A s− . The charging time constant, τ, can be calculated from the time interval between I1 and I2 in Figure 11, as in NI HTS field coil test, because the magnetic field density is proportional to the coil current. Here, I1 is the coil current at the moment when the power supply current reaches the target, and I2 (66.48 A) represents the sum of I1 and I, which is 63% (57.08 A) between I1 (9.40 A) and a fully charged coil current (100 A). The value of τ is 608 s. The coil current is fully charged approximately 9500 s after the excitation current reaches the target. Therefore, the Appl. Sci.initial 2020, 10 time, x FOR for thePEER simulations REVIEW of the designed NI HTS FSM in the steady-state condition was set to11 of 21 10,000 s, considering a full charge of the field current. Figure 12 shows the magnetic flux vector at the Therotor simulation position of 0results degrees show and 15.75 that degrees the polarity 10,000 s afterof the the fieldmagnetic current flux reaches applied the target to ofthe 100 armature A. The simulation results show that the polarity of the magnetic flux applied to the armature winding winding changes as the rotor moves. Figure 13 shows the air-gap flux density distributions due only changes as the rotor moves. Figure 13 shows the air-gap flux density distributions due only to the to the excitation of of the the NI HTSNI HTS field coils.field Thecoils. local The maximum local maximum value of the value air-gap of flux the density air-gap appears flux atdensity appearsthe at position the position where the where stator the tooth stator is fully tooth overlapped is fully with overlapped the rotor pole. with The the local rotor maximum pole. value The local maximumdirectly value results directly in the maximumresults in fluxthe ofmaximum the armature flux winding of the A armature1, and thus winding is more representative A1, and thus than is more representativethe absolute than maximum the absolute value [ 43maximum]. Therefore, value flux regulation [43]. Therefore, can be achieved flux regulation effectively bycan adjusting be achieved effectivelythe local by adjusting maximum the value. local maximum value.

Figure Figure10. Field-circuit 10. Field-circuit coupled coupled finite ﬁnite element element method method (FEM)(FEM) model model of theof the designed designed NI HTS NI FSM.HTS FSM.

Figure 11. Current charging characteristics of the NI HTS field coil installed in the designed FSM. Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 21

The simulation results show that the polarity of the magnetic flux applied to the armature winding changes as the rotor moves. Figure 13 shows the air-gap flux density distributions due only to the excitation of the NI HTS field coils. The local maximum value of the air-gap flux density appears at the position where the stator tooth is fully overlapped with the rotor pole. The local maximum value directly results in the maximum flux of the armature winding A1, and thus is more representative than the absolute maximum value [43]. Therefore, flux regulation can be achieved effectively by adjusting the local maximum value.

Appl. Sci. 2020, 10, 1564 11 of 20 Figure 10. Field-circuit coupled finite element method (FEM) model of the designed NI HTS FSM.

Appl. Sci.Figure 2020, 10 11., x CurrentFOR PEER charging REVIEW characteristics of the NI HTSHTS ﬁeldfield coil installed in the designed FSM.12 of 21 Appl. Sci. 2020, 10, x FOR PEER REVIEW 12 of 21

(a) (b) (a) (b) FigureFigure 12. MagneticMagnetic flux ﬂux density density vector vector plots plots according according to the rotor position: ( (aa)) 0°, 0◦, and and ( (bb)) 15.75°. 15.75◦. Figure 12. Magnetic flux density vector plots according to the rotor position: (a) 0°, and (b) 15.75°.

Figure 13. Air-gap flux density distribution due only to the excitation of the NI HTS field coils when Figure 13. theFigure rotor 13. position Air-gap is flux ﬂuxon the density d-axis. distribution due only to the excitation of the NI HTS ﬁeldfield coils when the rotor position is on the d-axis.d-axis. 3.2. Operating Characteristics of the Proposed FSM Due Only to NI HTS Field Coil Excitation 3.2. Operating Characteristics of the Proposed FSM Due Only to NI HTS Field Coil Excitation In this study, the operating characteristics of the NI HTS FSM were analyzed when excited only with Inthe this HTS study, field the coil. operating Figure 14 characteristics presents the offlux the linkage NI HTS waveform FSM were of analyzed the three-phase when excited armature only windings.with the HTS The fieldexcitation coil. Figure current 14 of presents the NI HTSthe flfieldux linkage coil in waveformthis analysis of isthe 100 three-phase A, and the armature rotation speedwindings. of the The rotor excitation is 1000 rpm.current In thisof the graph, NI HTS the xfield-axis coil represents in this analysisthe elaps ised 100 time A, 10,000and the s afterrotation the coilspeed current of the reaches rotor is 100 1000 A. rpm. As shown In this in graph, Figure the 14, x -axisthe flux represents linkage thewaveforms elapsed aretime nearly 10,000 sinusoidal, s after the andcoil currentthe peak reaches value 100of theA. As flux shown linkage in Figure is close 14, to the 0.6 flux Wb. linkage Figure waveforms 15 shows arethe nearly no-load sinusoidal, voltage waveformsand the peak of valuethree-phase of the fluxarmature linkage windings. is close to The0.6 Wb.peak Figure value 15 of shows the no-loadthe no-load voltage voltage is approximatelywaveforms of 770three-phase V. The no-load armature voltage windings. wavefo rmsThe exhibitpeak harmonicsvalue of thecomponents no-load duevoltage to the is lowerapproximately saturation 770 degree V. The and no-load the rough voltage comparison wavefo torms conventional exhibit harmonics machines. components The roughness due isto due the tolower the saturationshield effect degree of the and NI the coils. rough Figure comparison 16 shows to the conventional waveforms machines. of the phase The roughnessvoltage and is linedue currentto the shield in the effectunit-load of the condition. NI coils. The Figure current 16 shows and voltage the waveforms waveforms of arethe nearly phase sinusoidal.voltage and From line thecurrent data inshown the unit-load in Figure condition. 16, it is confirmed The current that andthe outputvoltage power waveforms is 20 kW. are nearly sinusoidal. From the data shown in Figure 16, it is confirmed that the output power is 20 kW. Appl. Sci. 2020, 10, 1564 12 of 20

3.2. Operating Characteristics of the Proposed FSM Due Only to NI HTS Field Coil Excitation In this study, the operating characteristics of the NI HTS FSM were analyzed when excited only with the HTS field coil. Figure 14 presents the flux linkage waveform of the three-phase armature windings. The excitation current of the NI HTS field coil in this analysis is 100 A, and the rotation speed of the rotor is 1000 rpm. In this graph, the x-axis represents the elapsed time 10,000 s after the coil current reaches 100 A. As shown in Figure 14, the flux linkage waveforms are nearly sinusoidal, and the peak value of the flux linkage is close to 0.6 Wb. Figure 15 shows the no-load voltage waveforms of three-phase armature windings. The peak value of the no-load voltage is approximately 770 V. The no-load voltage waveforms exhibit harmonics components due to the lower saturation degree and the rough comparison to conventional machines. The roughness is due to the shield effect of the NI coils. Figure 16 shows the waveforms of the phase voltage and line current in the unit-load condition. The current and voltage waveforms are nearly sinusoidal. From the data shown in Figure 16, it is confirmed that the output power is 20 kW. Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 21 Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 21

Figure 14. Flux linkage due only to the NI HTS field coil. FigureFigure 14. Flux 14. Flux linkage linkage due due onlyonly to to the the NINI HTS HTS field ﬁeldcoil. coil.

Figure 15. No-load voltage due only to the NI HTS field coil. FigureFigure 15. No-load 15. No-load voltage voltage due due only to to the the NI NIHTS HTS field ﬁeldcoil. coil. Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 21

Appl. Sci. 2020, 10, 1564 13 of 20

Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 21

Figure 16. Phase voltage and line current in resistance-load condition due only to the NI HTS field coil. Figure 16.FigurePhase 16. voltage Phase voltage and line and current line current in resistance-load in resistance-load conditioncondition due due only only to the to theNI HTS NI HTSfield field coil. 3.3. Flux-Regulationcoil. Characteristics of the Proposed FSM 3.3. Flux-Regulation Characteristics of the Proposed FSM The flux-regulation3.3. Flux-Regulation characteristics Characteristics of the were Proposed analyzed FSM by means of a transient motion analysis. With The flux-regulation characteristics were analyzed by means of a transient motion analysis. With the FEM, the Theno-load flux-regulation magnetic characteristics field distributions were analyzed and by flux means contours of a transient of the motion proposed analysis. machineWith with the FEM, the no-load magnetic field distributions and flux contours of the proposed machine with different MMFthe FEM, levels the no-load of the magnetic additional field field distributions windin andgs wereflux contours analyzed. of the Figure proposed 17 machine shows withthe magnetic different MMFdifferent levels MMF oflevels the of additional the additional field field windings windings were were analyzed. analyzed. Figure Figure 17 shows 17 showsthe magnetic the magnetic field distributions according to the MMF of the additional field windings. In this analysis, the field distributionsfield distributions according according to the MMFto the ofMMF the of additional the additional field field windings. windings. In In this this analysis, analysis, the the excitation excitationexcitation current currentof the ofNI the HTS NI HTS field field coil coil was was ma maintained at at100 100 A, whileA, while the MMF the ofMMF the additional of the additional current of the NI HTS field coil was maintained at 100 A, while the MMF of the additional field winding field windingfield windingwas changed. was changed. Figure Figure 17c 17c presents presents thee magneticmagnetic field field distribu distribution andtion flux and contour flux contour was changed. Figure 17c presents the magnetic field distribution and flux contour under only NI HTS under onlyunder NI onlyHTS NI field HTS coilfield excitation.coil excitation. Figures Figures 17a,b show show the the field field distributions distributions with negative with negative field coil excitation.regulation current Figure levels 17a,b of show20 A and the 10 field A, re distributionsspectively. Correspondingly, with negative Figures regulation 17d,e show current the levels of regulation current levels of 20 A and 10 A, respectively. Correspondingly, Figures 17d,e show the 20 A and 10field A, distributions respectively. withCorrespondingly, positive regulation current Figure levels 17d,e of 20 show A and the 10 fieldA. These distributions figures show withthat positive field distributions with positive regulation current levels of 20 A and 10 A. These figures show that regulationas current the regulation levels current of 20 A increases, and 10 A.the Thesemagnitud figurese of the show magnetic that asfield the density regulation in the FSM current also increases, as the regulationincreases. current increases, the magnitude of the magnetic field density in the FSM also the magnitude of the magnetic field density in the FSM also increases. increases.

(a) (b) (c)

(d) (e)

Figure 17. Flux vector contour and magnetic ﬁeld distribution according to the magneto-motive force (MMF) of the additional ﬁeld winding: (a) 4 kA T, (b) 2 kA T, (c) 0 kA T, (d) 2 kA T, and (e) 4 kA T. − · − · · · · Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 21

Figure 17. Flux vector contour and magnetic field distribution according to the magneto-motive force Appl. Sci.(MMF)2020, of10 ,the 1564 additional field winding: (a) −4 kA∙T, (b) −2 kA∙T, (c) 0 kA∙T, (d) 2 kA∙T, and (e) 4 kA∙T.14 of 20

Figure 18 shows the flux linkage of the phase C armature winding under different regulation currentFigure levels. 18 Theshows regulation the flux current linkage was of themaintained phase C at armature zero until winding 14 milliseconds, under di andfferent the regulationregulation currentcurrent levels.was applied The regulation to the currentadditional was maintainedfield winding at zero after until 14 14milliseconds. milliseconds, By and controlling the regulation the currentregulation was current, applied the to theflux additional linkage of field the armature windingafter winding 14 milliseconds. can be regulated. By controlling When the the NI regulation HTS field current,coils and the additional flux linkage field of windings the armature work winding together, can a flux-regulation be regulated. When operation the NI is achieved. HTS field Figure coils and 19 additionalcompares the field no-load windings voltage work waveforms together, a flux-regulationunder different operationregulation is modes achieved. at 1000 Figure rpm. 19 comparesFor solely thethe no-loadNI HTS voltagefield coil waveforms excitation undermode, dithefferent peakregulation value of the modes no-load at 1000 voltage rpm. is Forapproximately solely the NI 770 HTS V. fieldThe peak coil excitationvalues in mode,the maximum the peak negative value of regulati the no-loadon current, voltage and is approximatelythe positive regulation 770 V. The current, peak valuesare 410 in and the 980 maximum V, respectively. negative regulationBy applying current, MMF andof − the20 kA positive∙T, the regulation amplitude current, of the no-load are 410 andvoltage 980 V,can respectively. be reduced By by applying nearly 53%, MMF from of 77020 kA V toT, the410 amplitudeV. By applying of the MMF no-load of voltage20 kA∙t, can the be amplitude reduced byof − · nearlythe no-load 53%, fromvoltage 770 can V to be 410 increased V. By applying by 27%, MMF from of770 20 V kA to t,980 the V. amplitude Figures 20 of and the no-load21 correspondingly voltage can · beshow increased the phase by 27%,voltage from and 770 phase V to 980current V. Figures waveforms 20 and at 21 the correspondingly rated operation show condition. the phase In this voltage case, andthe armature phase current winding waveforms coils were at theconnected rated operation to a resistance condition. load. The In this simulation case, the results armature confirm winding that coilsthe output were connected power of tothe a FSM resistance in the load. negative The simulationregulation resultsmode can confirm be reduced that the to output approximately power of the6.7 FSMkW. inOn the the negative other regulationhand, in the mode positive can be regula reducedtion to mode, approximately the output 6.7 kW.power On theof the other FSM hand, is inapproximately the positive regulation36.2 kW, which mode, is theincreased output by power 180%, of compared the FSM to is that approximately in the solely 36.2 NI HTS kW, whichfield coil is increasedexcitation bymode. 180%, Figure compared 22 shows to that the in thetorque solely curve NI HTS according field coil to excitationthe MMF mode.of the Figureadditional 22 shows field thewinding. torque From curve these according simulation to the results, MMF of it the is confirmed additional that field the winding. torque Fromof the thesemachine simulation in the positive results, itregulation is confirmed mode that is theapproximately torque of the 4700 machine N∙m, inwhich the positive is increased regulation by 210%, mode compared is approximately to that of 4700 the Nsolelym, which NI HTS is increasedfield coil excitation by 210%, comparedmode. to that of the solely NI HTS field coil excitation mode. ·

FigureFigure 18.18. Flux linkage of Phase C according toto thethe MMFMMF ofof thethe additionaladditional ﬁeldfield winding.winding. Appl. Sci. 2020, 10, 1564 15 of 20 Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 21 Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 21

Figure 19. No-load voltage of Phase C according to the MMF of the additional field winding. FigureFigure 19.19. No-load voltage of Phase C accordingaccording toto thethe MMFMMF ofof thethe additionaladditional ﬁeldfield winding.winding.

FigureFigure 20.20. Phase voltage of Phase C in thethe resistance-loadresistance-load condition according to the MMFMMF ofof thethe Figure 20. Phase voltage of Phase C in the resistance-load condition according to the MMF of the additionaladditional ﬁeldfield winding.winding. additional field winding. Appl. Sci. 2020, 10, 1564 16 of 20 Appl.Appl. Sci.Sci. 20202020,, 1010,, xx FORFOR PEERPEER REVIEWREVIEW 1717 ofof 2121

FigureFigureFigure 21.21.21. PhasePhase currentcurrent ofof PhasePhase CC inin thethethe resistance-loadresistance-loadresistance-load conditioncondition accordingaccording toto thethe MMFMMFMMF ofofof thethethe additionaladditionaladditional ﬁeld fieldfield winding. winding.winding.

FigureFigure 22.22. TorqueTorque curvecurve accordingaccording tototo thethethe MMF MMFMMF of ofof the thethe additional additionaladditional field fieldfield winding. winding.winding. 4. Discussion 4.4. DiscussionDiscussion This paper deals with a flux-controllable NI HTS flux-switching machine (FSM) for electric ThisThis paperpaper dealsdeals withwith aa flux-controllableflux-controllable NINI HTSHTS flux-switchingflux-switching machinemachine (FSM)(FSM) forfor electricelectric vehicle (EV) applications. In a variable-speed rotating machine for EVs, flux-regulation capabilities are vehiclevehicle (EV)(EV) applications.applications. InIn aa variable-speedvariable-speed rotarotatingting machinemachine forfor EVs,EVs, flflux-regulationux-regulation capabilitiescapabilities very important for constant output operation. Conventional HTS rotating machines have excellent areare veryvery importantimportant forfor constantconstant outputoutput operatiooperation.n. ConventionalConventional HTSHTS rotatingrotating machinesmachines havehave flux-regulation capabilities, but for an HTS rotating machine using no-insulation (NI) HTS field coils, excellentexcellent flux-regulationflux-regulation capabilities,capabilities, butbut forfor anan HTSHTS rotatingrotating machinemachine usingusing no-insulationno-insulation (NI)(NI) HTSHTS ensuring flux-regulation capabilities is difficult, due to their inherent charge and discharge delays. fieldfield coils,coils, ensuringensuring flux-regulationflux-regulation capabilitiescapabilities isis difficult,difficult, duedue toto theirtheir inherentinherent chargecharge andand dischargedischarge However, the NI winding technique can dramatically improve the operational stability of HTS field delays.delays. However,However, thethe NINI windingwinding techniquetechnique cancan dramaticallydramatically improveimprove thethe operationaloperational stabilitystability ofof coils. Therefore, research to implement an HTS rotating machine with flux-regulation capabilities, HTSHTS fieldfield coils.coils. Therefore,Therefore, researchresearch toto implemenimplementt anan HTSHTS rotatingrotating machinemachine withwith flux-regulationflux-regulation capabilities,capabilities, whilewhile improvingimproving thethe operatingoperating stabilitystability ofof thethe HTSHTS fieldfield coilcoil viavia thethe NINI windingwinding technique,technique, isis requiredrequired forfor EVEV applications.applications. Appl. Sci. 2020, 10, 1564 17 of 20 while improving the operating stability of the HTS field coil via the NI winding technique, is required for EV applications. In this paper, we propose an HTS rotating machine with a flux switching structure and an NI HTS field coil. The proposed NI HTS flux-switching machine (FSM) uses additional windings for flux regulation, which enables feasible flux control. In addition, an NI HTS field coil was fabricated and tested, because the characteristic resistance, RC, of the NI HTS field coil is required as a design parameter. Based on the RC value of the fabricated NI HTS field coil, an NI HTS FSM was designed, and the flux-regulation performance outcomes were analyzed. The flux-regulation performance capabilities were analyzed using a combination of the finite element method (FEM) and an electrical circuit analysis. The electrical circuit model was devised based on the equivalent circuit model of the NI HTS coil. The simulation results showed that the charging time constant (τ) of the NI HTS FSM is 608 s, and that the coil current becomes fully charged approximately 9500 s after the excitation current reaches the target. This means that the HTS FSM using NI HTS field coils cannot control the flux by adjusting the field current of the HTS field coil. Therefore, additional windings are required for the flux control of the NI HTS FSM. In a simulation of the model with additional windings for flux regulation, the flux linkage and back-EMF characteristics, the output characteristics under a resistance load, and the torque characteristics of the proposed machine, were investigated. The simulation results demonstrated that the NI HTS FSM is capable of effective flux-regulation performance, enabling power and torque regulation. However, further research on the applicability of the proposed NI HTS FSM is needed, with the fabrication and testing of prototype models.

5. Conclusions In this paper, a novel flux-controllable NI HTS flux-switching machine (FSM) for electric vehicle applications was proposed. This topology makes the NI HTS FSM more attractive, because it improves the flux-regulation capabilities. The proposed machine adopts both NI HTS field coils and additional windings. The proposed HTS flux-switching machine (FSM) uses NI field coils, but additional field windings are applied for flux regulation, which enables good flux control. An NI HTS field coil was also fabricated and tested. The characteristic resistance of the NI HTS field coil was obtained from a current charging test of the coil. The characteristic resistance was used for the design and in a characteristic analysis of the NI HTS FSM. To analyze the flux-regulation performance capabilities of the NI HTS FSM, a simulation model including an electrical circuit was devised based on the characteristic resistance of the fabricated NI HTS field coil. This study shows that flux-regulation capabilities can be realized through the proposed structure, even in the NI HTS FSM. The results here also provide a good reference for those involved in the manufacturing of a prototype NI HTS FSM for EV applications, and they can also be used as a reference for other HTS rotating machines, such as those used in association with large-scale wind power generation.

Author Contributions: All authors contributed to this work by collaboration. Conceptualization, Y.J.H.; Formal analysis, Y.J.H.; Methodology, Y.J.H.; Supervision, Y.J.H.; Supervision, Y.J.H.; Writing-original draft, Y.J.H.; Visualization, J.Y.J.; Validation, J.Y.J.; Writing-review & editing, S.L. All authors have read and agree to the published version of the manuscript. Funding: This work was funded by the Korea Basic Science Institute under Grant D010200. Acknowledgments: This work was supported by the Korea Basic Science Institute under Grant D010200. Conﬂicts of Interest: The authors declare no conﬂict of interest. Appl. Sci. 2020, 10, 1564 18 of 20

Nomenclature

σyield yield strength v Poisson’s ratio ρ density of the rotor material n0 number of armature conductors per slot I0 total current per slot r outer radius of the rotor l eﬀective length of the rotor Ω mechanical angular speed E Young’s modulus k safety factor SCu cross-sectional area of the armature conductor JCu the allowable current density of the armature conductor dslot slot height wslot slot width dCu armature conductor height wCu armature conductor width β packing factor P rated output power σFtan tangential stress of the rotor T torque of the rotor IS RMS value of the phase current q number of the slots cos ϕ power factor ρcl electrical resistivity of the current lead lcl length of the current lead I transport current through the current lead Acl cross-sectional area of the current lead kcl thermal conductivity of the current lead TH temperature at the high-temperature part TL temperature at the low-temperature part Nl no. of signal lines or leading-in tubes Al cross-sectional area of a signal line or a leading-in tube kl thermal conductivity of a signal line or a leading-in tube Ll length of a signal line or a leading-in tube σ Stefan–Boltzmann constant (5.67 10 8 W m 2 K 4) × − · − · − εH emissivity of the vessel εN emissivity of MLI N no. of MLI layers AH surface area of the high-temperature part AL surface area of the low-temperature part

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