Multiport Driving Topology for a Photovoltaic Aircraft Light Transmission System Driven by Switched Reluctance Motors

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Article Multiport Driving Topology for a Photovoltaic Aircraft Light Transmission System Driven by Switched Reluctance Motors

Xiaoshu Zan 1, Wenyuan Zhang 1, Kai Ni 2,* , Zhikai Jiang 1 and Yi Gong 1 1 School of Electrical and Power Engineering, China University of Mining and Technology, Xuzhou 221116, China; [email protected] (Z.X.); [email protected] (W.Z.); [email protected] (Z.J.); [email protected] (Y.G.) 2 School of Electrical and Electronic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China * Correspondence: [email protected]; Tel.: +86-1306-387-2074

Received: 10 April 2020; Accepted: 15 July 2020; Published: 17 July 2020

Abstract: In order to meet the working requirements of high performance and low cost for a photovoltaic (PV) aircraft driven by switched reluctance motors (SRMs), a multiport driving topology (MDT) is proposed. The converter is composed of an asymmetric half-bridge and a multiport power source circuit. Three driving and two charging modes can be realized through simple control of the switches. The output torque and the efficiency of the system are improved, because the excitation and demagnetization processes are accelerated by increasing the commutation voltage. The battery pack can be self-charged when the system is running, and PV panels can be used to charge the battery pack to reduce energy consumption when the system is stationary. The simulation analysis and the experimental verification on an 8/6 SRM confirm the effectiveness of the MFT proposed in this paper.

Keywords: photovoltaic aircraft; switched reluctance motors; multiport power source circuit; output torque; eﬃciency; excitation and demagnetization processes

1. Introduction The survival and development of human beings have a close relationship with energy. With the development of the world, human beings are faced with the contradiction between the increase of energy demand and the decrease of existing fossil energy. With the purpose of reducing fossil energy consumption and its operating cost, together with mitigating pollution of the environment, the concept of electric aircraft emerges and substantial eﬀorts have been made in this area [1]. As shown in Table1, a comparison is made between a traditional aircraft and three types of electric aircraft. The engine of a traditional aircraft is an internal combustion engine, while the engine of an electric aircraft is an electromotor. The weight of an internal combustion engine is about seven times that of an electromotor, thus the electric aircraft can more effectively control the flight attitude due to the reduction of weight. In addition, because the electromotor of an electric aircraft does not need a throttle valve to control the intake pressure and the fuel intake, the high torque of the electromotor can be directly transmitted to the propeller without energy consumption, which is beneficial to reducing losses and improving the efficiency [2]. The three types of electric aircraft are fuel cell aircraft, supercapacitor aircraft, and photovoltaic (PV) aircraft. Compared with natural gas and gasoline, hydrogen is the best fuel for fuel cells, which causes little pollution to the environment, but the production cost of hydrogen is much higher than that of traditional fossil fuel [3]. Supercapacitors have both the characteristics of fast charge and discharge of capacitors and the energy storage characteristics of batteries, but their efficiencies are greatly affected by the temperature and operating voltage [4]. PV aircraft uses PV panels to convert solar energy

Energies 2020, 13, 3687; doi:10.3390/en13143687 www.mdpi.com/journal/energies Energies 2020, 13, 3687 2 of 16 into electricity. A PV panel is a type of power generation device made of semiconductor materials that can generate direct current when exposed to sunlight. Using PV panels to generate electricity is simple, convenient and effective. Compared with other types of aircraft, PV aircraft gets rid of dependence on traditional fossil energy and does not pollute the environment. There is no doubt that the concept of low-carbon environmental protection triggered by PV aircraft is the future developing trend [5].

Table 1. Comparison among the conventional aircraft with three types of electric aircraft.

Energy/Power External Environmental Impact Environmental Engine Cost Source Light Oxygen Temperature Pollution Traditional Aircraft Internal combustion engine Gasoline/Kerosene No Yes No High High Hydrogen/Natural Fuel Cell Aircraft No Yes No High Medium gas/Gasoline Supercapacitor AircraftElectromotor Supercapacitor No No Yes Low Medium PV Aircraft Solar energy Yes No No No Low

Considering the safety of aircraft, three types of electrical machines applied on aircraft have developed rapidly in recent years, which are the induction motors (IMs), permanent magnet synchronous motors (PMSMs), and switched reluctance motors (SRMs) [6]. An IM is also called an asynchronous motor. The rotor of IMs obtains a rotating torque and starts to rotate under the action of a rotating magnetic field. IMs have low moment of inertia and simple rotor structure, which can run for a long time under high temperature and high speed conditions. However, the power factor and efficiency of IMs are low, and the loss of an IM rotor is large [7]. A PMSM is a synchronous motor that generates a synchronous rotating magnetic field by the excitation of a permanent magnet. The permanent magnet acts as a rotor to generate a rotating magnetic field. PMSMs are widely used in electric aircraft due to their high power density, high efficiency, fast dynamic response, and high torque. However, the tensile strength of PMSMs is small, the rotor of PMSMs is prone to encounter irreversible demagnetization at high temperature, and the manufacturing cost of PMSMs is high [8]. The speed control system of SRMs is considered to be the latest generation of speed control system after variable frequency speed control system and brushless direct current motor speed control system, because SRMs have the advantages of high efficiency, robustness, low manufacturing cost, high speed performance, and inherent fault tolerance. The high torque and small current during the starting process make SRMs suitable for the operation of PV aircraft [9,10]. In an SRM drive, the most widely used power converter is the asymmetric half-bridge topology (AHT). However, SRMs need some more practical power converter structures in different fields and actual requirements. In [11–15], the topology was improved with many new topologies proposed in order to adapt to various situations. The design ideas and research methods of these topologies proposed in [11–15] play an important role in promoting the research of this paper. The driving system designed in [11] can improve the performance of the motor, but PV panels and the generator control unit are connected in parallel; the voltage is forced to change, which is not conducive to improving the stability of the system. Paper [12] proposed a converter that uses additional inductance to increase the output torque and demagnetization voltage, but the efficiency and the power density are reduced. Improving the commutation voltage and torque capacity becomes the research direction of this paper. Paper [13] proposed a modular multilevel converter-based SRM drive with a decentralized battery-energy-storage system for hybrid electric vehicle applications. In the proposed drive, a battery pack and a half-bridge converter are connected as a submodule, and multiple submodules are connected. However, the bus voltage is reduced, and the excitation and demagnetization processes are slowed, which are not conducive to improving system efficiency. Paper [14] developed a three-level converter to reduce the number of current rises and falls, but twice as many power transistors as those in an AHT are needed. Paper [15] proposed an integrated multilevel converter of SRMs fed by a modular front-end circuit for plug-in hybrid electric vehicle applications. Several operating modes can be achieved by changing the on–off states of the switches in the front-end circuit. This paper designs Energies 2020, 13, 3687 3 of 16 the multiport power source module, which accelerates the excitation and demagnetization process, and improves the torque capacity and efficiency by generating multilevel voltage. This paper proposes a multiport driving topology (MDT) drive for SRM that is connected to PV panels and a battery pack in order to achieve multiport source operation, adapt to complex weather conditions, and improve the motoring performance. The PV panels and battery pack can achieve three driving and two charging modes according to different conditions. PV panels use sustainable solar energy for the power supply, which is clean and pollution-free, thus the flexible combination of battery pack and PV panels can also overcome the environmental limitations and extend the mileage. The SRM system powered by PV panels and battery packs can achieve multilevel commutation voltages, which can accelerate the excitation and demagnetization process, and improve the torque capacity. The battery pack can be directly charged by the motor windings or can absorb demagnetization current during operation to reduce losses. The battery pack, PV panels, and the asymmetric half-bridge circuit form a compact and integrated converter topology. In this integrated driving mode, this new MDT reduces the complexity of the circuit board and power loss, and it improves the system efficiency and the output torque. However, the conversion rate of PV panels and the energy density of the battery pack are relatively low, and this is where improvement is needed.

2. Multiport Driving Topology for a Photovoltaic Aircraft

2.1. Switched Reluctance Motor In order to avoid the short-circuit faults of the bridge arm, the phase winding of the SRM is directly connected in series between the two switching devices. This paper chooses the Pulse Width Modulation (PWM) control method and the nonenergy feedback chopper method [16,17]. The phase current continues to flow in the zero-voltage loop during the chopper freewheeling period to avoid the exchange of reactive energy between the SRM and the power source, which is beneficial to increase the torque and the utilization capacity of power converter, reduce the number of chopping and torque ripples, and suppress the power supply voltage fluctuation [18,19]. Without considering the iron loss and mutual inductance between phase windings, the electromechanical energy conversion of the SRM system is analyzed in detail [20,21]. The voltage of phase K is expressed as follows:

dψ U =R + k k kik dt ∂ψk dik ∂ψk dθk =Rkik + + ∂ik dt ∂θk dt (1) dik ∂Lk(θk, ik) =Rkik + Lk(θk, ik) + ωrik dt ∂θk dik ∂Lk(θk, ik) =Rkik + Lk(θk, ik) ωr + ωrik dθk ∂θk where Uk is the phase K supply voltage, Rk is the phase K resistance, ik is the phase K current, Lk is the phase K inductance, Ψk is the phase K magnetic flux, and θk is the rotor position angle. The magnitude of the current is directly proportional to the supply voltage and inversely proportional to the motor speed when the turn-on and turn-off angles are fixed [22]. Therefore, increasing the excitation voltage at the beginning can accelerate the formation of the excitation current to achieve rapid excitation [23]. The braking torque is generated when the freewheeling current flows through the area where the inductance drops. Therefore, the demagnetization process can be accelerated by increasing the demagnetization voltage [24].

2.2. Multiport Driving Topology For the traditional SRM power converter, stable speed regulation can be achieved in a wide range by applying a ﬁxed voltage. However, a PV aircraft faces complex and changeable weather conditions, Energies 2020, 13, x FOR PEER REVIEW 4 of 16 Energies 2020, 13, x FOR PEER REVIEW 4 of 16 2.2. Multiport Driving Topology 2.2. Multiport Driving Topology For the traditional SRM power converter, stable speed regulation can be achieved in a wide EnergiesrangeFor 2020by the applying, 13 ,traditional 3687 a fixed SRM voltage. power However, converter, a stablePV aircraft speed faces regulation complex can and be changeableachieved in weather a 4wide of 16 rangeconditions, by applying and the amultiport fixed voltage. power However, source control a PV of aircraft PV panels faces and complex the battery and changeablepack is complicated. weather conditions,Therefore, anand MDT the multiport is designed power in this source paper, control which of PVuses panels PV panels and th ande battery a battery pack pack is complicated. to achieve and the multiport power source control of PV panels and the battery pack is complicated. Therefore, Therefore,multistage anspeed MDT regulation is designed in three in this driving paper, modes, which as uses shown PV inpanels Figure and 1. a battery pack to achieve anmultistage MDT is designedspeed regulation in this paper,in three which driving uses modes, PV panels as shown and in a batteryFigure 1. pack to achieve multistage speed regulation in three driving modes, as shown in Figure1.

S9 S9 S1 S3 S5 S7 Battery S1 S3 S5 S7 Ut C1 BatteryBank D9 Ut C1 D1 D3 D5 D7

Bank 9

B B B D B

D1 D3 D5 D7

3 7 1 5

B B B B

3 7 1 5

B B B B

2 4 6 8

B B B B

2 6 8 D11 D2 D4 4 D6 D8 D10 D11 D2 D4 D6 D8 Upv C2 D10 Upv C2 S2 S4 S6 S8 PV S2 S4 S6 S8 PanelsPV S10 Panels S10

FigureFigure 1.1. TheThe multiportmultiport drivingdriving topologytopology ofof thethe switchedswitched reluctancereluctance motormotor (SRM).(SRM). Figure 1. The multiport driving topology of the switched reluctance motor (SRM). AsAs shownshown inin FigureFigure2 2,, a a PV PV aircraft aircraft has has three three driving driving modes modes and and two two charging charging modes. modes. The The three three drivingdrivingAs modes shownmodes are inare theFigure the SRM SRM 2, systema systemPV aircraft powered powered has by three by the the PVdriving PV panels, panels, modes battery battery and pack, two pack, andcharging and both both together.modes. together. Th Thee th twoTheree chargingdrivingtwo chargin modes modesg modes are are the self-charging are SRM self system-charging mode powered mode and static and by the static charging PV charging panels, mode. battery mode. pack, and both together. The two charging modes are self-charging mode and static charging mode.

SRM SRM

Battery BPackattery Multiport Fault-tolerant Control Pack MultiportPower F Caultonverter-tolerant CSontrolystem PV Power Converter System PanelsPV Panels

SRM SRM

FigureFigure 2.2. PhotovoltaicPhotovoltaic electricelectric aircraftaircraft drivedrive system.system. Figure 2. Photovoltaic electric aircraft drive system. FigureFigure3 3presents presents the the relationship relationship between between the the phase phase voltage voltage and and phase phase current. current. In In the the three three drivingdrivingFigure modes, modes, 3 presents the the process process the of relationship excitation of excitation and between demagnetization and demagnetizationthe phase isvoltage accelerated is and accelerated becausephase current. the because initial In theexcitationthe initialthree voltagedrivingexcitation and modes, voltage demagnetization the and process demagnetization of voltage excitation are voltage increased, and demagnetizationare andincreased, the output and is t torqueacceleratedhe output is improved. torque because is improved. When the initial the SRMexcitationWhen system the voltageSRM is poweredsystem and demagnetization is by powered the PV panelsby the voltage andPV panels batteryare increased, and pack battery together, and pack the the outputtogether, commutation torque the commutationis improved. voltage is improvedWhenvoltage the is comparedimprovedSRM system tocompared that is powered in the to traditionalthat by in the the PV SRMtraditional panels system and SRM driven battery system by thepack driven AHT, together, by as the shown AHT,the incommutation as Figure shown3a,c. in AlthoughvoltageFigure 3 isa, PVcimproved. Although panels arecompared PV not panels directly to are that involvedn otin directlythe traditional in theinvolved power SRM in supply, thesystem power the driven PV supply, panels by the the help AHT, PV to panels increaseas shown help the into excitationFigure 3a, andc. Although demagnetization PV panels voltage are not when directly the involved SRM system in the is poweredpower supply, by a battery the PV pack, panels as shownhelp to in Figure3b. The battery pack can also increase initial excitation and demagnetization voltage when the SRM system is powered by PV panels, as shown in Figure3d. Energies 2020, 13, x FOR PEER REVIEW 5 of 16 increase the excitation and demagnetization voltage when the SRM system is powered by a battery packEnergies, as2020 shown, 13, 3687 in Figure 3b. The battery pack can also increase initial excitation and demagnetization5 of 16 voltage when the SRM system is powered by PV panels, as shown in Figure 3d.

(a) (b)

Energies 2020, 13, x FOR PEER REVIEW 6 of 16 (c) (d)

whereFigureFigure Ut is 3.the Relationship battery pack between between voltage, phase phase U currentpv current is the and PV and phase panel phase voltage: voltage, voltage (a): drivingU(aa) isd rivingthe by phase asymmetric by asymmetric A voltage, half-bridge half Ud -is the phasebridgetopology D voltage, topology (AHT), Ra ( isAHT (b the) powering), phase(b) powering A by resistance, the by battery the battery i pack,a is the pack (c )phase powering, (c) pAowering current, by the by PV L thea panelsis PV the panels phase and battery and A inductance,battery bank ϴa is bankthetogether, rotor together and position, ( dand) powering ( dangle) powering of by phase the by PV the A, panels. PVand panels. ωr is the rotor angular velocity. 2. When the phase D freewheeling current is smaller than the phase A current, or phase D has 2.3.2.3. Analysis Analysis of of MDT MDT in in Three Three Driving Driving Modes Modes finished the freewheeling stage, it cannot help phase A to form the exciting current, as shown in Mode 1: When the aircraft is powered by the battery pack, switch S is turned on and switch S ModeFigure 1: 4b. When The voltagethe aircra offt phase is powered A is expressed by the battery as follows: pack, switch S9 is turned on and switch S10 isis turned turned off. off. PV PV panels panels can can help help increase increase the the commutation commutation voltage, voltage, as as shown shown in in Figure Figure 33b.b. When the solarsolar power power is insufficient, insufficient, thethe PVPV panelpanel voltagevoltage푈푎 isis= zero, zero,푈푡 so so the the working working condition condition of of SRM SRM is is similar similar(4 to) the traditional mode. to3. theFigure traditional 4c shows mode. the chopper freewheeling stage. The voltage of phase A is expressed as follows: WhenWhen ϴθonon << ϴθ << ϴθoffo,ff the, the exc excitationitation process process of of the the SRM SRM system system powered powered by by the the battery battery pack pack is is shownshown in in Figure Figure 44.. The phase currentcurrent changes are푈푎 mainlymainly= 0 presentedpresented byby threethree stages.stages. (5) 1. When the phase D freewheeling current is larger than the phase A current, the phase D freewheeling current not only flows through the battery pack and the energy storage capacitor

C2, but also flows through phase A to help it form the exciting current, as shown in Figure 4a. The phase A and phase D voltages are expressed as follows:

푈푑 = −푈푡 − 푈푝푣 (2)

UUad

UUt pv (3) dia Lia a, a  Ra i a  L a a, i a  r   r i a (a) d  aa (b)  (c)

Figure 4. WhenWhen ϴθon << ϴθ < <ϴoffθ,o ﬀthe, the excitation excitation process process of of the the SRM SRM system system is is powered powered by by the the battery battery pack. pack. (a) Stage 1. ( (b)) Stage Stage 2. ( (cc)) Stage Stage 3. 3.

When ϴoff < ϴ < 2 ϴoff − ϴon, the demagnetization process of the SRM system powered by the battery pack is shown in Figure 5. The current changes are mainly presented by four stages. 1. When phase A starts the demagnetization process while phase B has not yet started the excitation process, the phase A freewheeling current is not related to phase B, as shown in Figure 5a. The voltage of phase A is expressed as follows:

UUUa  t  pv

dia Lia a, a  (6) Ra i a  L a a, i a  r   r i a daa 2. When phase B starts the excitation process and the phase A freewheeling current is larger than the phase B current, the phase A freewheeling current helps phase B to establish the excitation current, and the demagnetization voltage of phase A does not change, as shown in Figure 5b. The voltage of phase A is expressed as follows: 푈푎 = −푈푡 − 푈푝푣 (7) 3. When phase A continues to be in the freewheeling stage and the phase A freewheeling current is smaller than the phase B current, the phase A freewheeling current is not large enough to help phase B to establish the exciting current. In this case, the battery pack helps phase B to establish the exciting current, and the phase A voltage starts to change, as shown in Figure 5c. The phase A and phase B voltages are expressed as follows: 푈푏 = 푈푡 (8) 푈푎 = −푈푏 = −푈푡 (9) 4. when the phase A freewheeling current flows through the phase B PWM chopper freewheeling stage, because phase B is in a zero-voltage loop, the phase A freewheeling current is larger than

Energies 2020, 13, 3687 6 of 16

1. When the phase D freewheeling current is larger than the phase A current, the phase D freewheeling current not only ﬂows through the battery pack and the energy storage capacitor C2, but also ﬂows through phase A to help it form the exciting current, as shown in Figure4a. The phase A and phase D voltages are expressed as follows:

U = Ut Upv (2) d − −

Ua = U − d =U + U t pv (3) dia ∂La(θa, ia) =Raia + La(θa, ia) ωr + ωria dθa ∂θa

where Ut is the battery pack voltage, Upv is the PV panel voltage, Ua is the phase A voltage, Ud is the phase D voltage, Ra is the phase A resistance, ia is the phase A current, La is the phase A inductance, θa is the rotor position angle of phase A, and ωr is the rotor angular velocity. 2. When the phase D freewheeling current is smaller than the phase A current, or phase D has ﬁnished the freewheeling stage, it cannot help phase A to form the exciting current, as shown in Figure4b. The voltage of phase A is expressed as follows:

Ua = Ut (4)

3. Figure4c shows the chopper freewheeling stage. The voltage of phase A is expressed as follows:

Ua = 0 (5)

When θ < θ < 2 θ θon, the demagnetization process of the SRM system powered by the oﬀ oﬀ − battery pack is shown in Figure5. The current changes are mainly presented by four stages.

1. When phase A starts the demagnetization process while phase B has not yet started the excitation process, the phase A freewheeling current is not related to phase B, as shown in Figure5a. The voltage of phase A is expressed as follows:

Ua = Ut Upv − − dia ∂La(θa, ia) (6) =Raia + La(θa, ia) ωr + ωria dθa ∂θa

2. When phase B starts the excitation process and the phase A freewheeling current is larger than the phase B current, the phase A freewheeling current helps phase B to establish the excitation current, and the demagnetization voltage of phase A does not change, as shown in Figure5b. The voltage of phase A is expressed as follows:

Ua = Ut Upv (7) − − 3. When phase A continues to be in the freewheeling stage and the phase A freewheeling current is smaller than the phase B current, the phase A freewheeling current is not large enough to help phase B to establish the exciting current. In this case, the battery pack helps phase B to establish the exciting current, and the phase A voltage starts to change, as shown in Figure5c. The phase A and phase B voltages are expressed as follows:

Ub = Ut (8)

Ua = U = Ut (9) − b − Energies 2020, 13, 3687 7 of 16

4. when the phase A freewheeling current ﬂows through the phase B PWM chopper freewheeling stage, because phase B is in a zero-voltage loop, the phase A freewheeling current is larger than Energiesthe 2020 phase, 13, x BFOR current, PEER REVIEW and the phase A demagnetization voltage returns to the original voltage,7 of as16 shown in Figure5d. The phase A and phase B voltages are expressed as follows: the phase B current, and the phase A demagnetization voltage returns to the original voltage, as

shown in Figure 5d. The phase A and phaseUa = B voltagesUt Upv are expressed as follows: (10) − − 푈푎 = −푈푡 − 푈푝푣 (10) Ub = 0 (11) 푈푏 = 0 (11)

(a) (b)

FigureFigure 5.5. When θϴoff < ϴθ << 22 ϴθoff − ϴonθ,on the, the demagnetization demagnetization process process of of the the SRM SRM system system powered by the oﬀ oﬀ − batterybattery pack.pack. (a) Stage 1. ( b) Stage 2. ( (cc)) Stage Stage 3. 3. ( (dd)) Stage Stage 4. 4.

WhenWhen θϴ >> 2θϴoff−ϴθonon, ,phase phase B B or or phase phase C C is is in in the the freewheeling freewheeling stage, stage, because because the voltage of each oﬀ− phasephase freewheelingfreewheeling isis aa pulsedpulsed alternating voltage, which is didifferentﬀerent fromfrom thethe single-levelsingle-level voltagevoltage ofof thethe SRMSRM systemsystem driven driven by by the the AHT. AHT. The The phase phase A windingsA windings will will generate generate a small a small induced induced current, current, and theand phase the phase A voltage A voltage will startwill start to change. to change. The voltageThe voltage of phase of phase A is expressedA is expressed as follows: as follows: 푑푖푎 푈푎 = −퐿푎 (12) dia푑푡 Ua = La (12) − dt Mode 2: When the aircraft is powered by the PV panels and battery pack together, switches S9 and SMode10 are 2:turnedWhen on. the The aircraft operation is powered mode byis similar the PV to panels the traditional and battery mode, pack but together, the excitation switches andS9 anddemagnetizationS10 are turned voltages on. The are operation always modehigh, as is shown similar in to Figure the traditional 3c and 6. mode, but the excitation and demagnetizationWhen phase voltagesA is in the are excitation always high, stage, as the shown voltage in Figures of phase3c andA is6 expressed. as follows: 푈푏 = 0 (13) When phase A is in the chopper freewheeling stage, the voltage of phase A is expressed as follows:

Energies 2020, 13, x FOR PEER REVIEW 8 of 16

UUUa t pv

dia Lia a, a  (14) Ra i a  L a a, i a  r   r i a daa When phase A is in the freewheeling stage, the voltage of phase A is expressed as follows: Energies 2020, 13, 3687 8 of 16 푈푎 = −푈푡 − 푈푝푣 (15)

(a) (b) (c)

FigureFigure 6.6. TheThe workingworking stage of the SRM system powered by the PV panels and battery pack together. ((aa)) ExcitationExcitation stage.stage. ((bb)) ChopperChopper freewheelingfreewheeling stage.stage. ((cc)) FreewheelingFreewheeling stage.stage.

WhenMode phase3: When A isthe in aircraft the excitation is powered stage, by the the voltage PV panels, of phase switch A is S expressed9 is turned asoff follows: and switch S10 is turned on. The working stages are similar to the SRM system powered by the battery pack, and the difference is that PV panels act as a power sourceUb = instead0 of the battery pack, and the magnitude (13) of the phase voltage is different, as shown in Figure 3d. When phase A is in the chopper freewheeling stage, the voltage of phase A is expressed as follows: After the above analysis, the initial excitation and demagnetization voltages are increased during the operation process of the aircraft. Excitation and demagnetization processes are accelerated Ua =Ut + Upv and the efficiency is improved. The multilevel commutation voltage is achieved in mode 2 and mode dia ∂La(θa, ia) (14) 3. =Raia + La(θa, ia) ωr + ωria dθa ∂θa

2.4. AnalysisWhen phase of Efficiency A is in theand freewheelingPower Density stage, the voltage of phase A is expressed as follows: The SRM input power transfer function is as follows: Ua = Ut Upv (15) 푃푖푛−= 푈푠퐼푠− (16) Mode 3: When the aircraft is powered by the PV panels, switch S is turned off and switch S is The SRM output power transfer function is as follows: 9 10 turned on. The working stages are similar to the SRM system powered by the battery pack, and the difference is that PV panels act as a power source푃표푢푡 instead= 휔푟푇푎푣 of the battery pack, and the magnitude of(17 the) phaseThe voltage SRM is efficiency different, transfer as shown function in Figure is as3d. follows: After the above analysis, the initial excitation and demagnetization voltages are increased during 푃표푢푡 휔푟푇푎푣 휂 = = the operation process of the aircraft. Excitation and푃푖푛 demagnetization푈푠퐼푠 processes are accelerated and(18 the) efficiency is improved. The multilevel commutation voltage is achieved in mode 2 and mode 3. where η is the SRM efficiency, Pin is the SRM input power, Pout is the SRM output power, Nr is the 2.4.number Analysis of rotor of Effi poles,ciency U ands is the Power excitation Density voltage, Is is the bus current, Tav is the SRM average torque. The SRM power density transfer function is as follows: The SRM input power transfer function is as follows: 푃표푢푡 푆 = 푉푀 (19) Pin = UsIs (16) where S is the SRM power density and VM is the SRM characteristic volume. TheThe SRMinput output power power of the transfer SRM is functionconverted is asinto follows: the magnetic field storage of the winding and mechanical power output. The battery pack or PV panels can increase the excitation voltage at the

beginning of the excitation stage, but it doesP notout = powerωrTav the system, so the input power of the system(17) does not change significantly. When ωr, ϴon, and ϴoff are fixed, the output power can be increased by The SRM eﬃciency transfer function is as follows:

P ω T η = out = r av (18) Pin UsIs where η is the SRM eﬃciency, Pin is the SRM input power, Pout is the SRM output power, Nr is the number of rotor poles, Us is the excitation voltage, Is is the bus current, Tav is the SRM average torque. Energies 2020, 13, 3687 9 of 16

The SRM power density transfer function is as follows:

P S = out (19) VM where S is the SRM power density and VM is the SRM characteristic volume. The input power of the SRM is converted into the magnetic field storage of the winding and mechanical power output. The battery pack or PV panels can increase the excitation voltage at the beginning of the excitation stage, but it does not power the system, so the input power of the system does not change significantly. When ωr, θon, and θoff are fixed, the output power can be increased by increasing the output torque, and the increase of output power can help improve the system efficiency and the power density. k=1 k=1 X3 X3 1 2 ∂Lk(ik, θ) Te = Tek = i (20) 2 k dθ When the phase winding is energized in the rising region of the phase inductance, the rotating electromotive force is positive and the SRM generates positive electric torque. Otherwise, when the phase winding is energized in the area where the phase inductance drops, the rotating electromotive force is negative and the SRM generates braking torque.

dω J + µω = Te T (21) dt − l where Te is the total electromagnetic torque, Tl is the load torque, J is the total inertia moment of the motor and the load, and µ is the combined friction coeﬃcient of the motor and the load. Positive and negative torques can be obtained by controlling the phase current in the rising or falling region of the inductance, respectively. When the excitation voltage is low, the current rises slowly and a negative torque is easily generated due to the excitation current existing in the area where the inductance decreases, which is not conducive to improving the performance of the motor. However, this negative torque can be eliminated by increasing the excitation and demagnetization voltages. A constant DC power source cannot achieve a multilevel voltage, so the output torque is limited during high-speed operation. Compared with the AHT, the excitation voltage of the MDT system increases at the beginning of excitation, and the rise time of phase current in the minimum inductance region is prolonged, which increases the current entering the eﬀective working area. The output torque and demagnetization voltage are increased, the demagnetization stage is accelerated, the braking torque is avoided or reduced, and the output torque of the system is improved.

2.5. Charging Modes of the Battery Pack There are two battery charging modes: self-charging mode and static charging mode. The self-charging mode is mainly in the freewheeling stage. Because the freewheeling current is small, its main role is to reduce the power consumption. The PV panels charge the battery pack when the static charging mode is applied, which can get rid of the dependence on fossil fuels and traditional power grids, reducing the cost and environmental pollution. Self-charging mode: When the SRM is in operation, taking phase A as an example, and S1 and S2 are turned off, the phase current will be fed back to the battery pack through the freewheeling diode. This regenerative effect results in a higher efficiency for the SRM. When the SRM system is powered by the PV panels, the battery pack does not participate in the excitation process, but receives the current feedback during the freewheeling stage. When the battery pack participates in the excitation process, it also can receive the current feedback during the freewheeling stage. Static charging mode: When the SRM is in standstill, the battery pack is charged by the PV panels. Because the power level of the PV panels is relatively low, only one phase winding is required to be Energies 2020, 13, 3687 10 of 16

charged. Take phase A as an example; switch S9 is turned off, switch S10 is turned on, switches S1, S2 are turned on, and the switches in the other phases are turned off. Phase A windings are equivalent to inductors, and the PV panels provide energy to them, as shown in Figure7a. Then switches S1 and S2 Energies 2020, 13, x FOR PEER REVIEW 10 of 16 are turned off to feed the current in the phase winding back to the battery pack, as shown in Figure7b.

The charging current can be easily controlled by controlling푖푎푚−푖푎0 the duty cycle of switches S1 and S2, along 푖푐(푡) = 푖푎0 − (푡 − 퐷푇) with the switch PWM frequency of phase A. (1−퐷)푇 (23)

(a) (b)

FigureFigure 7. The 7. The PV panelsPV panels provide provide energy energy to to phase phase ((aa)) windings. (b (b) The) The winding winding current current of phase of phase A is A is fed backfed back to the to batterythe battery pack. pack.

The3. Simulation winding currentResults of phase A is expressed as follows: In order to verify the feasibility of MDT, an SRM system model was built in simulation. The iam ia0 parameters of the model are shown inia Table(t) = 2.ia 0 + − t (22) DT Table 2. The system parameters of the SRM. where ia0 is the initial phase current, iam is the maximum phase current, D is the duty cycle of switch S1 and switch S2, and T is the switching period.Parameter Value The battery pack charging currentPhase of phase number A is expressed as4 follows: Rotor pole number 6 Stator pole numberiam ia0 8 ic(t) = ia0 − (t DT) (23) Opening angle− (1 D)T − 5° Turn-off angle − 20° 3. Simulation Results Battery pack voltage 36 V PV panels voltage 60 V In order to verify the feasibilityLoad of MDT,torque an SRM system0.1 N·m model was built in simulation. The parameters of the model are shown in Table2. Figure 8 presents the simulation waveforms of the proposed motor drive operating at 600 rpm, where ia, ib, ic, and id are the phase currents of phase A, B, C and D, respectively, Ua is the phase A Table 2. The system parameters of the SRM. winding voltage, ibus is the bus current, it is the battery pack current, and ich is the battery pack charging current. Parameter Value The excitation and demagnetization voltages are both single-level voltages when the SRM is Phase number 4 powered by PV panels and drivenRotor by the pole traditional number AHT, as shown 6 in Figure 8a. The PV panels do not participate in the excitation Stator process pole when number the SRM is powered 8 by the battery pack, but the excitation and demagnetization voltages are both increased, which accelerates the excitation and Opening angle 5◦ demagnetization processes. The efficiencyTurn-oﬀ isangle improved due to 20 the◦ presence of multilevel voltage during the commutation process,Battery and the pack freewheeling voltage current 36 flows V through the battery pack to charge the battery, as shown in FigurePV panels 8b. The voltage commutation voltage 60 V is increased when the SRM is powered by the PV panels, as shownLoad in Figure torque 8d, where the working 0.1 N m modes are similar to those in · Figure 8b. The working modes of the SRM powered by the PV panels and battery pack together are similar to the SRM powered by the traditional AHT, and the difference is that the excitation and Figure8 presents the simulation waveforms of the proposed motor drive operating at 600 rpm, demagnetization processes maintain a high voltage level, which can accelerate the excitation and where i , i , i , and i are the phase currents of phase A, B, C and D, respectively, U is the phase demagnetizationa b c dprocesses, avoid braking torque, and increase the electromagnetic torque,a as shown

Energies 2020, 13, 3687 11 of 16

Energies 2020, 13, x FOR PEER REVIEW 11 of 16

A winding voltage, ibus is the bus current, it is the battery pack current, and ich is the battery pack in Figure 8c. The self-charging mode can reduce the power consumption when the SRM system is charging current. running.

1 1

id ia ib ic id ia ib ic

)

) A

A 0.5 0.5

(

( Phase Current Phase Phase Current Phase 0 0 100 100 Ua Ua

50 50 )

) 0 0

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-50 -50

Phase Voltage Phase Phase Voltage Phase -100 -100 2 2

1 1

ibus it

) )

urrent 0.5 0.5

(

( C

0 0 Bus Bus

BatteryCurrent ich -0.5 -0.5 0.530 0.532 0.534 0.536 0.538 0.540 0.542 0.544 0.546 0.548 0.550 0.530 0.532 0.534 0.536 0.538 0.540 0.542 0.544 0.546 0.548 0.550 Time (s) Time (s)

(a) (b)

1.5 1.5

1 1

id ia ib ic id ia ib ic

)

) A A 0.5

0.5 (

( Phase Current Phase Phase Current Phase 0 0 100 100 Ua Ua

50 50 )

) 0 0

( (

-50 -50

Phase Voltage Phase Phase Voltage Phase -100 -100 1.5 1.5

1 1

it )

0.5 ) 0.5 A

A it

( (

0 0

BatteryCurrent BatteryCurrent -0.5 ich -0.5 ich 0.530 0.532 0.534 0.536 0.538 0.540 0.542 0.544 0.546 0.548 0.550 0.530 0.532 0.534 0.536 0.538 0.540 0.542 0.544 0.546 0.548 0.550 Time (s) Time (s)

FigureFigure 8. The 8. simulationThe simulation waveforms waveforms of of the the proposed proposed motor drive drive operating operating at at600 600 rpm. rpm. (a) Driving (a) Driving by AHT. (b) Powering by the battery pack. (c) Powering by the PV panels and battery bank together. by AHT. (b) Powering by the battery pack. (c) Powering by the PV panels and battery bank together. (d) Powering by the PV panels. (d) Powering by the PV panels. Figure 9 presents the static charging mode with the duty cycles of 30% and 70%. The charging The excitation and demagnetization voltages are both single-level voltages when the SRM is current can be changed by controlling the duty cycles of the phase switches, and the charging current poweredcan be by increase PV panelsd by and increasing driven the by duty the traditional cycle. AHT, as shown in Figure8a. The PV panels do not participate in the excitation process when the SRM is powered by the battery pack, but the excitation and demagnetization voltages are both increased, which accelerates the excitation and demagnetization processes. The efficiency is improved due to the presence of multilevel voltage during the commutation process, and the freewheeling current flows through the battery pack to charge the battery, as shown in Figure8b. The commutation voltage is increased when the SRM is powered by the PV panels, as shown in Figure8d, where the working modes are similar to those in Figure8b. The working modes of the SRM powered by the PV panels and battery pack together are similar to the SRM powered by the traditional AHT, and the difference is that the excitation and demagnetization processes maintain a high voltage level, which can accelerate the excitation and demagnetization processes, avoid braking torque, and increase the electromagnetic torque, as shown in Figure8c. The self-charging mode can reduce the power consumption when the SRM system is running. Energies 2020, 13, 3687 12 of 16

Figure9 presents the static charging mode with the duty cycles of 30% and 70%. The charging Energies 2020, 13, x FOR PEER REVIEW 12 of 16 current can be changed by controlling the duty cycles of the phase switches, and the charging current can be increased by increasing the duty cycle. Energies 2020, 13, x FOR PEER REVIEW 12 of 16

1 2 ia )

) ia A

A 0.5 1

( (

1 2 ia

Phase Current Phase Current Phase )

0 ) ia 0 A

A 0.5 1 ( 100 ( 100

Ua Ua Phase Current Phase 50 Current Phase 0 500 100 100

Ua )

) 0 Ua 0

V V (

( 50 50 )

-50 ) 0 -500

V V

Phase Voltage Phase Voltage Phase

( (

-100 -50 -100-50 Phase Voltage Phase 1 Voltage Phase 2 -100 -100

0.5 1 ich 21 ich )

) 0 0.5 ich 10 ich

A A

( ( )

) 0 0

A A (

-0.5 ( -1 BatteryCurrent BatteryCurrent -0.5 -1

-1 -2

BatteryCurrent BatteryCurrent 0.6400 0.-64021 0.6404 0.6406 0.6408 0.6410 0.6412 0.6414 0.6416 0.6418 0.6420 0-.26400 0.6402 0.6404 0.6406 6.0408 0.6410 0.6412 0.6414 0.6416 0.6418 0.6420 0.6400 0.6402 0.6404 0.6406 0Time.6408 (s0).6410 0.6412 0.6414 0.6416 0.6418 0.6420 0.6400 0.6402 0.6404 0.6406 6.0408 0.6410Time0 .(6412s) 0.6414 0.6416 0.6418 0.6420 Time (s) Time (s)

(a) ( a) ( b )( b )

FigureFigure 9. The 9. Thesimulation simulation waveforms waveforms of of of the the static static charging charging mode.mode. ( a() (a aAt)) At At 30% 30% 30% duty duty duty cycle. cycle. cycle. (b) At( (bb )70%) At At 70% 70% duty dutycycle. cycle.

FigureFigure 10 presentspresents 10 presents thethe the comparisonscomparisons comparisons of of of the the the average averageaverage torque, torque,torque, eefficiencyffi efficiencyciency,, andand, and output output torque torque torque when when when the thespeed speedthe is speed changed is changed is changed from from 600 from to600 900600 to to r900/min 900 r/min r/min between between between the traditional ththee traditionaltraditional drive drive drive mode mode mode by by employing employingby employing the the AHT the AHT and the proposed drive mode by employing the proposed MFT, where Tav is the average torque, AHTand the and proposed the proposed drive drive mode mode by employing by employing the proposed the proposed MFT, MFT, where whereTav is T theav is averagethe average torque, torque,η is ղ is the system efficiency, and n is the system speed. The traditional drive mode is powered by a 60 ղthe is systemthe system effi ciency,efficiency, and andn is n the is the system system speed. speed. The The traditional traditional drive drive mode mode is poweredis powered by by a 60a 60 V V DC source, the proposed drive mode is powered by the 60 V PV panels, switch S9 is turned off and VDC DC source, source, the the proposed proposed drive drive mode mode is is powered powered by by the the 60 60 V V PV PV panels,panels, switchswitch SS99 is turned off off and switch S10 is turned on. As shown in Figure 10, the average torque and output torque are increased switchdue S10 to is isthe turned turned acceleration on. on. As As of shown shownthe excitation in in Figure Figure and 10,demagnetization10, the the average average torqueprocesses, torque and and an output outputd the efficiency torque torque isare are slightly increased due to improved the acceleration due to the of increase the excitation of output and and torque. demagnetization demagnetization The proposed processes, processes,drive mode an andcand the themake efficiency effi theciency speed is and slightly improvedimprovedtorque due due stabilize to to the more increase quickly of when output the torque.speed is The Thechanged proposed proposed from 600 drive drive to 900 mode mode r/min. can can make make the the speed speed and and torquetorque stabilize more quickly when the speed is changed from 600600 toto 900900 rr/min./min. 0.8 Traditional Drive Mode Proposed Drive Mode Tav Proposed Drive Mode 0.6 0.8 0.8 Traditional Drive Mode

orque Proposed Drive Mode ) Tav T Proposed Drive Mode m 0.4 0.6 0.6 .

N 0.8

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0.2 . 0.4

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Average 0.05 N

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Average 0.05 0 90 87.4% 0.2 0.02 86.7% 0.04 100 82.6% Output 80 Traditional Drive Mode 82.1% ղ

(%) Proposed Drive Mode 0

0.01 0.03 ficiency 90 f 87.4% Traditional Drive Mode E 70 0.02 82.6% 86.7% 0.8 80 8260.1% -0.01

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f n Traditional Drive Mode Traditional Drive Mode

) E 70 Proposed Drive Mode orque

T -0.03 m

900 900r/min . 0..48

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Speed n

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300r/min Proposed Drive Mode orque 300 0

T -0.03 m

900 900r/min . 0.4

N ) 0 (

600 0.4 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 00..240 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80

min

/ Output Output

r Time (s) Time (s) Speed ( 300r/min 300 0

0 0.4 0.45 0.50 0.55 0 .60 (a)0 .65 0. 70 0 .75 0 .80 0 .40 0 .45 0. 50 0 .55 (b0.)60 0 .65 0. 70 0 .75 0. 80 Time (s) Time (s) Figure 10. The dynamic performance of the SRM system when the speed is changed from 600 to 900 r/min. (a) Average torque and efficiency. (b) Output torque. (a) (b) 4. Experimental Verification Figure 10. 10. TheThe dynamic dynamic performance performance of ofthe the SRM SRM system system when when the thespeed speed is changed is changed from from 600 600to 900 to r/min.900 r/Amin. ( a500) Average( aW) AverageSRM torque converter torque and andefficiency.is experimentally eﬃciency. (b) Output (b) Outputverified torque. torque. with the same parameters as those used in the simulation, and the experimental platform is shown in Figure 11. The SRM is driven by MDT, 4. Experimental Verification A 500 W SRM converter is experimentally verified with the same parameters as those used in the simulation, and the experimental platform is shown in Figure 11. The SRM is driven by MDT,

Energies 2020, 13, 3687 13 of 16

4. Experimental Verification A 500 W SRM converter is experimentally verified with the same parameters as those used in the Energies 2020, 13, x FOR PEER REVIEW 13 of 16 simulation,Energies 2020, 1 and3, x FOR the experimentalPEER REVIEW platform is shown in Figure 11. The SRM is driven by MDT, which13 of is16 established with the insulated gate bipolar transistor (IGBT) connected with a fast recovery antiparallel which is established with the iinsulated gate bipolaripolar ttransistorransistor ((IGBTIGBT)) connectedconnected withwith aa fastfast recoveryrecovery diode to drive the motor. PV panels are simulated by using an adjustable 46 V DC power source, and antiparallelel diodediode toto drivedrive thethe motor.motor. PVPV panelspanels areare simulatedsimulated byby usingusing anan adjusadjusTable 46 V DC power three batteries are connected in series to form a 36 V battery pack. The rotor position is determined source,source, and three batteries are connected in series to form a 36 V battery pack. The rotor position is by a 1000-line incremental encoder. The main control system is implemented by the TMS320F28335 determined by a 1000--lineline incremental incremental encoder. encoder. The The main main control control systemsystem is is implemented implemented by by the the digital signal processor produced by Texas Instruments. A Hall effect current sensor is used to collect TMS320F28335 digital signal processor produced by Texas Instruments. A Hall effecteffect currentcurrent sensorsensor the phase current signal. isis usedused toto collectcollect thethe phasephase currentcurrent signal.signal.

Battery Battery pack pack

Power Current Controller Power Current SRM Encoder Controller inverter sensor SRM Encoder inverter sensor

Figure 11. Experimental platform. FigureFigure 11.11. Experimental platform.

FigureFigure 1212 presentspresents the the experimental experimental waveforms waveforms of of the the proposed proposed motor motor drive drive operating operating at 600 at rpm600 andrpm 0.8and N 0.8m, N where· m, whereia is the ia is phase the phase A current, A current,Ua is U thea is phase the phase A winding A winding voltage voltage (the battery (the battery pack rpm and 0.8· N · m, where ia is the phase A current, Ua is the phase A winding voltage (the battery current),pack current and), iandc is theic is the battery battery pack pack charging charging current. current. The The excitation excitation voltage voltage andand demagnetizationdemagnetization pack current), and ic is the battery pack charging current. The excitation voltage and demagnetization voltagevoltage are are both both increased increased to to accelerate accelerate the the excitation excitation and and demagnetization demagnetization processes. processes. Multilevel Multilevel voltagevoltage appearsappears duringduring commutation,commutation, asas shownshown inin FigureFigure 1212aa,c.,c,c.. The The excitation excitation and and demagnetization demagnetization processesprocesses maintainmaintain high voltagevoltage level when the SRM is is poweredpowered byby thethe PVPV panelspanels andand batterybattery packpack together,totogether, asas shownshown inin FigureFigure 1212b.b.

t(2ms/div) t(5ms/div) ia:1A/div i t(5ms/div) ia:1A/div i :1A/div a t(2ms/div) t(5ms/div) i :1A/div ia t(5ms/div) a i ia:1A/div a a ia ia ia:1A/div ia

Ua:50V/div Ua:50V/div Ua:50V/div Ua Ua:50V/div Ua U :50V/div U Ua U a a a Ua:50V/div Ua

it、ic:1A/div ic:1A/div it it、ic:1A/div it it、ic:1A/div ic:1A/div ic it it、ic:1A/div it ic

ic ic i i c c (a) (b) (c) (a) (b) (c) Figure 12. The experimental waveforms of the proposed motor drive operating at 600 rpm. (a) FigureFigure 12. 12.The The experimental experimental waveforms waveforms of the of proposed the proposed motor motor drive operating drive operating at 600 rpm. at 600(a) Powering rpm. (a) Powering by the battery pack. (b) Powering by the PV panels and battery pack together. (c) Powering byPowering the battery by the pack. battery (b) Poweringpack. (b) Powering by the PV by panels the PV and panels battery and packbattery together. pack together. (c) Powering (c) Powering by the by the PV panels. PVby the panels. PV panels.

Figure 13 presents the experimental waveforms of charging in the static charging mode with the duty cycles of 30% and 70%. The charging current can be changed by controlling the duty cycle of the phase A switches, and the charging current becomes larger with the increase of duty cycle.

Energies 2020, 13, 3687 14 of 16

ia:1A/div t(0.25ms/div) ia:1A/div ia t(0.625ms/div)

Ua:50V/div Ua:50V/div Ua Ua

ic:0.5A/div ic:0.5A/div ic ic

(a) (b)

Figure 13. The experimental waveforms of of static charging mode. ( (aa)) At At 30% 30% duty duty cycle. cycle. ( (bb)) At At 70% 70% duty cycle.

5. Conclusions Conclusions In this paper,paper, thethe MDTMDT for for a a PV PV aircraft aircraft driven driven by by SRMs SRMs is proposed,is proposed, which which not not only only improves improves the theperformance performance of the ofaircraft the aircraft system system in the operatingin the operating condition, condition, but also but implements also implements flexible charging flexible chargingfunctions. functions. The multilevel The multilevel voltage is voltage obtained is andobtained the effi andciency the isefficiency improved. is improved. Compared Compared with traditional with traditionalconverters, converters, the MDT can the increase MDT can the increase excitation the voltage excitation to speed voltage upthe to speed establishment up the establis of phasehment current of phaseand increase current the and demagnetization increase the demagnetization voltage to speed voltage up the demagnetization to speed up the process,demagnetization which can process, increase whichthe output can increase torque, the output output power, torque, effi outputciency, power, and power efficiency density., and Compared power density. with Compared existing electric with existingaircraft systems, electric aircraft the MDT systems, can achieve the MDT multilevel can achiev voltagee multilevel and multiple voltage operating and multiple modes with operating fewer modeselectronic with components fewer electronic and simpler components control and strategies. simpler Incontrol addition, strategies. the battery In addition, pack, PV the panels, battery and pack, the PVasymmetric panels, and half-bridge the asymmetric circuit formhalf- anbridge integrated circuit topology,form an integrated which can topology, increase thewhich bus can voltage increase and theabsorb bus energy voltage in andthe demagnetization absorb energy in process. the demagnetization Simulation and process.experimental Simulation results proveand experimental that the new resultsinverter prove is suitable that the for new the useinverter of PV is electric suitable aircraft for the driven use of by PV SRM. electric The aircraft working driven mode by and SRM. control The workingstrategy aremode analyzed and control in detail. strategy The mainare analyzed contributions in detail. of this The paper main are cont asributions follows. of this paper are as follows. 1. Three driving modes are realized. The flexible power supply mode of the PV panels and battery 1. Threepack enablesdriving themodes PV aircraftare realized. to operate The flexible normally power under supply different mode environmental of the PV panels conditions and battery and packwith enables different the speed PV aircraft requirements. to operate The normally operating under cost anddifferent the environmental environmental pollution conditions can and be withsignificantly different reduced speed require comparedments. to otherThe operating types of aircraft. cost and the environmental pollution can be 2. significantlyTwo charging reduced modes arecompared realized. to Withother the types demagnetization of aircraft. current, the battery pack is charged 2. Twoin the charging self-charging modes mode are realized. and the With static the charging demagnetization mode, respectively. current, the The battery flexible pack energy is charged control inis the achieved self-charging through mode the simple and the control static strategies.charging mode, respectively. The flexible energy control 3. isThe achieved excitation through and demagnetization the simple control voltages strategies. are increased, and the multilevel voltage is achieved 3. Theby the excitation PV panels and and demagnetization battery pack in the voltages driving mode. are increased, The output and torque, the multilevel system effi ciency, voltage and is achievedpower density by the can PV be panels improved and duebattery to thepack increase in the ofdrivi commutationng mode. The voltage. output torque, system efficiency, and power density can be improved due to the increase of commutation voltage. Although this article is targeted at PV aircraft applications, the technology developed can also Although this article is targeted at PV aircraft applications, the technology developed can also be applied to other applications suitable for PV panels, such as electric vehicles, traction drives, and be applied to other applications suitable for PV panels, such as electric vehicles, traction drives, and electric ships. electric ships.

Author Contributions: Conceptualization, X.Z. X.Z. and and W.Z.; W.Z.; methodology, methodology, W.Z.; W.Z.; software, software, W.Z.; W.Z.; validation, X.Z., X.Z., W.Z., and K.N.; formal analysis, W.Z.; investigation, Z.J.; resources, Y.G.; data curation, W.Z.; writing—original W.Z., and K.N.; formal analysis, W.Z.; investigation, Z.J.; resources, Y.G.; data curation, W.Z.; writing—original draft preparation, W.Z.; writing—review and editing, K.N.; visualization, W.Z.; supervision, K.N.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Conflicts of Interest: The authors declare no conflict of interest.

Energies 2020, 13, 3687 15 of 16 draft preparation, W.Z.; writing—review and editing, K.N.; visualization, W.Z.; supervision, K.N.; project administration, X.Z.; funding acquisition, X.Z. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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