Hybrid Excited Synchronous Machine with Wireless Supply Control System

energies

Article Hybrid Excited Synchronous Machine with Wireless Supply Control System

Marcin Wardach * , Michal Bonislawski , Ryszard Palka, Piotr Paplicki and Pawel Prajzendanc Faculty of Electrical Engineering, West Pomeranian University of Technology, Szczecin, Sikorskiego 37, 70-313 Szczecin, Poland * Correspondence: [email protected]; Tel.: +48-91-449-42-17

Received: 27 June 2019; Accepted: 13 August 2019; Published: 16 August 2019

Abstract: The paper presents an unconventional approach to control the hybrid excited synchronous machine (HESM), which can be used for drives of pure electric vehicles. The hybrid excitation of the additional control coil placed on the rotor of the machine has been realized by the wireless energy transfer system connected with the rotor shaft. Experimental results of back-EMF characteristics obtained on a prototype of HESM were compared with 3-dimentional ﬁnite elements analysis (3D-FEA) predictions. This design, despite some additional complications in the power supply system of the machine, simpliﬁes the mechanical construction and reduces the control coil’s losses compared to the construction with the coil placed on the stator.

Keywords: permanent magnet machines; wireless power transmission; electric vehicles; ﬁnite element methods; variable speed drives

1. Introduction Generally, the air gap flux controlling in a permanent magnet machine can be realized in two ways: By the appropriate control system or by the relevant changes of the machine design. Conventional permanent magnet (PM) machines have a constant, non-adjustable excitation flux, which limits the power, thus becoming an important limitation. In the case of U/f = const. control strategy implementation, due to the limited power of the supply voltage and limited dielectric strength of winding insulation, to achieve higher speeds it is required to weaken the excitation flux, in order to reduce the back-EMF value. A very popular method of field weakening is realized by adjusting the current in the d-axis of the machine. This strategy, however, generates increased losses in the supply system and creates the risk of permanent magnets demagnetization, and consequently decreases the resultant torque of the machine. Conventional permanent magnet excited synchronous machines (PMSM), used as a drive in modern electric vehicles, suffer from the limited battery voltage in high-speed regions because of high back-EMF values. The field weakening (which is obligatory) requires design of machines with lower power conversion characteristics. This caused the development of hybrid excited synchronous machines [1–16]. These machines have PM excitation and an additional toroidal excitation coil fixed on the stator or mounted in the rotor in the machine axial center. By the proper powering of this coil, the amplitude of the induced voltage can be effectively controlled in the range from zero to values above those of classical permanent magnet machines. The hybrid excited synchronous machines have already been designed, optimized and measured very intensively [6–9]. For the reason mentioned above, in this paper an unconventional approach to control the hybrid excited synchronous machine (HESM), which can be used for the drives of electric vehicles, has been proposed. This solution consists of the hybrid excitation, which is characterized by the fact that the

Energies 2019, 12, 3153; doi:10.3390/en12163153 www.mdpi.com/journal/energies Energies 2019,, 12,, xx FORFOR PEERPEER REVIEWREVIEW 2 of 12

2. Review of PM Machines with Adjustable Flux Excitation To realize the controlling of the excitation field (back-EMF) of the machine with PMs, different methods that require structural changes have been used. Regarding to reluctance machines the Energiesmethods2019 that, 12, 3153require structural changes have been used. Regarding to reluctance machines2 ofthe 12 construction with two parts of salient permanent magnets (double salient permanent magnet— DSPM) was proposed. DSPM machines are realized with internal (Figure 1) and external rotor additionalstructures control[10,11], coil wherein is placed the onsolu thetiontion machine ofof thethe rotorouterouter and rotor,rotor, the asas power shownshown supply inin FigureFigure is implemented 2,2, hashas nono additionaladditional using the wirelesscontrol coil. energy The transfer advantage system of these connected machines with is the a lower rotor production shaft. cost caused by small amounts of permanent magnets. 2. Review of PM Machines with Adjustable Flux Excitation The other design of cylindrical machines with permanent magnets is one with a movable rotor, which—atTo realize higher the speeds—moves controlling of the in the excitation axial direction, ﬁeld (back-EMF) leaving its of part the outside machine of with the stator PMs, [12]. diﬀerent Such methodsa solution that leads require very effectively structural to changes the reduction have been of back-EMF, used. Regarding but its disadvantage to reluctance is machinesa complicated the constructionstructure and with the two lack parts of possibility of salient permanentto increase magnets the excitation (double flux, salient useful permanent atat lowlow magnet—DSPM)speedspeed operationoperation wasrange. proposed. DSPM machines are realized with internal (Figure1) and external rotor structures [ 10,11], whereinInIn thethe the literatureliterature solution cancan of alsoalso the bebe outer foundfound rotor, designsdesigns asshown withwith aa dodo inuble Figure rotor.2, hasIn this no solution additional there control is one shaft, coil. Thewhich advantage is common of theseto two machines different isrotor a lower topologies: production A conventional cost caused PM by rotor small and amounts laminated of permanentreluctance rotor magnets. without magnets. This conception allows to obtain a machine which has a high ratio Ld/Lq of inductances Ld/Lq [13].[13].

FigureFigure 1.1. DSPMDSPM machinemachine withwith internalinternal rotor.rotor.

FigureFigure 2.2. DSPMDSPM machinemachine withwith externalexternal rotor.rotor.

TheMany other designs design are of described cylindrical in paper machines [14], with where permanent thethe authorsauthors magnets presentpresent is solutionssolutions one with ofof a movable hybridhybrid excitedexcited rotor, which—atmachines higherfor both speeds—moves cylindrical and in disk-type the axial direction, machines. leaving Similar its characteristics part outside of can the statorbe obtained [12]. Such with a solutionsuitably leadslocated very magnetic effectively barriers to the [15]. reduction In the literature of back-EMF, are also but known its disadvantage designs of isclaw a complicated pole [16,17] structure(Figure(Figure 3)3) and andand the disc-typedisc-type lack of possibility machinesmachines withwith to increase adjustableadjustable the excitation excitationexcitation flux, fluxflux useful [18].[18]. at low speed operation range. In the literature can also be found designs with a double rotor. In this solution there is one shaft, which is common to two different rotor topologies: A conventional PM rotor and laminated reluctance rotor without magnets. This conception allows to obtain a machine which has a high ratio of inductances Ld/Lq [13]. Many designs are described in paper [14], where the authors present solutions of hybrid excited machines for both cylindrical and disk-type machines. Similar characteristics can be obtained with suitably located magnetic barriers [15]. In the literature are also known designs of claw pole [16,17] (Figure3) and disc-type machines with adjustable excitation flux [18]. Hybrid excited Vernier machines are a large group of machines with adjustable excitation flux. In a previous paper [19], structures with magnets mounted on the surface and using magnetic concentrators were presented. Whereas, [20] presents a Vernier permanent magnet machine that uses homopolar topology. The authors of [21] presented research of the machine, whose rotor consists of two parts: One has permanent magnets, and the other electromagnets analogous to that of the wound field synchronous machines. Energies 2019, 12, x FOR PEER REVIEW 2 of 12

2. Review of PM Machines with Adjustable Flux Excitation To realize the controlling of the excitation field (back-EMF) of the machine with PMs, different methods that require structural changes have been used. Regarding to reluctance machines the construction with two parts of salient permanent magnets (double salient permanent magnet— DSPM) was proposed. DSPM machines are realized with internal (Figure 1) and external rotor structures [10,11], wherein the solution of the outer rotor, as shown in Figure 2, has no additional control coil. The advantage of these machines is a lower production cost caused by small amounts of permanent magnets. The other design of cylindrical machines with permanent magnets is one with a movable rotor, which—at higher speeds—moves in the axial direction, leaving its part outside of the stator [12]. Such a solution leads very effectively to the reduction of back-EMF, but its disadvantage is a complicated structure and the lack of possibility to increase the excitation flux, useful at low speed operation range. In the literature can also be found designs with a double rotor. In this solution there is one shaft, which is common to two different rotor topologies: A conventional PM rotor and laminated reluctance rotor without magnets. This conception allows to obtain a machine which has a high ratio of inductances Ld/Lq [13].

Figure 1. DSPM machine with internal rotor.

Figure 2. DSPM machine with external rotor.

Many designs are described in paper [14], where the authors present solutions of hybrid excited machines for both cylindrical and disk-type machines. Similar characteristics can be obtained with Energies 2019, 12, 3153 3 of 12 suitably located magnetic barriers [15]. In the literature are also known designs of claw pole [16,17] (Figure 3) and disc-type machines with adjustable excitation flux [18].

Figure 3. Design of the claw pole machine.

Another solution is presented in [22]. The paper shows simulation and experimental research of hybrid excited flux-switching PM machines with iron flux bridges. Research on a similar construction is in [23]. Furthermore, in [24] an influence of stator and rotor pole combinations on machine parameters were theoretically and experimentally tested. A very wide review of hybrid-excited machines together with theoretical and experimental research was presented in [25]. The authors searched for the optimal construction of an electric machine for use in an electric vehicle.

3. Hybrid Excited Synchronous Machine Design Control systems of hybrid excited synchronous machines differ from those of conventional permanent magnet excited synchronous machines (PMSM). They have the ability to adjust the excitation flux by using an additional control coil [6–8]. This feature gives the possibility of the dynamic flux control that provides an additional degree of freedom of the machine control. The hybrid excitation source increases the efficiency of the drive system in a wide range of load torques and speeds. All advantages of using the hybrid excitation can be obtained by the proper design, both of the machine and the control system. In previous designs of hybrid-excited synchronous machines the additional excitation coil was placed on the stator. In this paper, a new design of the coil location on the machine rotor is presented. Usually, in order to supply the windings placed on the rotor, it is necessary to use brushes and slip rings. Alternatively to this, contactless energy transfer (CET) systems can be used [8]. Figure4 shows construction details of the hybrid excited synchronous machine and in Table1 the main data are listed. Supply coils used for the wireless power transfer have been designed and initially optimized. They are connected with the housing of the hybrid excited synchronous machine.

Table 1. The main data of the machine.

Name Value Unit Stator inner diameter 164 mm Stator axial length 40, 50, 40 mm Number of slots 36 - Number of turns in slot 2 6 - × Rotor outer diameter 163 mm Rotor axial length 40, 50, 40 mm Number of poles 12 - PM type NdFeB - PM Br 1.2 T PM µr 1.05 - Air gap 0.5 mm Energies 2019, 12, x FOR PEER REVIEW 3 of 12

Figure 3. Design of the claw pole machine.

Hybrid excited Vernier machines are a large group of machines with adjustable excitation flux. In a previous paper [19], structures with magnets mounted on the surface and using magnetic concentrators were presented. Whereas, [20] presents a Vernier permanent magnet machine that uses homopolar topology. The authors of [21] presented research of the machine, whose rotor consists of two parts: One has permanent magnets, and the other electromagnets analogous to that of the wound field synchronous machines. Another solution is presented in [22]. The paper shows simulation and experimental research of hybrid excited flux-switching PM machines with iron flux bridges. Research on a similar construction is in [23]. Furthermore, in [24] an influence of stator and rotor pole combinations on machine parameters were theoretically and experimentally tested. A very wide review of hybrid-excited machines together with theoretical and experimental research was presented in [25]. The authors searched for the optimal construction of an electric machine for use in an electric vehicle.

3. Hybrid Excited Synchronous Machine Design Control systems of hybrid excited synchronous machines differ from those of conventional permanent magnet excited synchronous machines (PMSM). They have the ability to adjust the excitation flux by using an additional control coil [6–8]. This feature gives the possibility of the dynamic flux control that provides an additional degree of freedom of the machine control. The hybrid excitation source increases the efficiency of the drive system in a wide range of load torques Energiesand speeds.2019, 12 All, 3153 advantages of using the hybrid excitation can be obtained by the proper design,4 both of 12 of the machine and the control system.

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Table 1. The main data of the machine.

Name Value Unit Stator inner diameter 164 mm Stator axial length 40, 50, 40 mm Number of slots 36 - Number of turns in slot 2 × 6 - Rotor outer diameter 163 mm Rotor axial length 40, 50, 40 mm Number of poles 12 - PM type NdFeB - PM Br 1.2 T PM µr 1.05 - Air gap 0.5 mm Figure 4. Design of the hybrid-excited synchronous machine. 4. FEA Results of Field Control Predictions of HESM 4. FEAIn Resultsprevious of designs Field Control of hybrid-excited Predictions synchronous of HESM machines the additional excitation coil was placedInIn order on order the to tostator. predict predict In the this the field fieldpaper, control control a new (FC) (FC) design characte characteristics, of theristics, coil locationthree three dimensional dimensional on the machine finite finite rotorelement element is presented. analysis analysis (3D-FEA)(3D-FEA)Usually, studiesin studies order have haveto supply been been carried carriedthe windings out out under under placed no-load no-load on the armature armature rotor, itwindings windingsis necessary conditions. conditions. to use brushesFigure Figure 5 5and shows shows slip thetherings. magnetic magnetic Alternatively flux flux distribution distribution to this, contactless within within a a3D-FEA ener 3D-FEAgy transfermo modeldel of of(CET) an an HES HES systems machine machine can obtainedbe obtained used [8]. at at no-load no-load of of the the rotorrotor DCFigure DC control control 4 shows coil. coil. Toconstruction To analyze analyze a afielddetails field control control of the rang rangehybride (FCR) (FCR) excited factor, factor, synchronous as as a aratio ratio of machine of a afield field strengthening and strengthening in Table 1 I (FS)(FS)the operationmain operation data gained gainedare listed. by by positive positiveSupply value coils value usedof of the the for rotor rotor the DC DCwireless control control power coil coil current current transfer ( (IDC DChave),), to to beena afield field designed weakening weakening and I (FW)(FW)initially operation operation optimized. obtained obtained They at are negativenegative connected valuevalue with ofof I theDCDC, ,ho threethreeusing didifferentff oferent the hybrid magnetomotive magnetomotive excited synchronous forces forces of of the the machine. rotor rotor DC DCcontrol control coil coil in the in the range range from from1000 −1000 to 1000 to 1000 Ampere-turns, Ampere-turns, it means it means at I at= I0DC and = 0 Iand= IDC1.0 = ±1.0 A have A − DC DC ± havebeen been analyzed. analyzed. The resistanceThe resistance of the of rotor the DCrotor control DC control coil is coil 19 Ω isand 19 Ω the and inductance the inductance is about is 400 about mH. 400 mH.

Figure 5. Three-dimensional finite element analysis (3D FEA) results of no-load magnetic flux density Figure 5. Three-dimensional finite element analysis (3D FEA) results of no-load magnetic flux density distribution on HESM. distribution on HESM. Figure6 shows the phase back-EMF waveforms simulated at the rotor constant speed of 1000 rpm Figure 6 shows the phase back-EMF waveforms simulated at the rotor constant speed of 1000 by three operating conditions. These results show that FCR up to 4:1 can be effectively obtained with a rpm by three operating conditions. These results show that FCR up to 4:1 can be effectively obtained small rotor DC control coil having power losses of max. 20 W. with a small rotor DC control coil having power losses of max. 20 W.

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Figure 6. TheThe 3D-FEA 3D-FEA predictions predictions of ofback back-EMF-EMF waveforms waveforms at three at three differe diﬀnterent DC DCfield ﬁeld excitations excitations of HESM.of HESM.

5. Mathematical Mathematical Model of HESM DefiningDefining thethe adequateadequate mathematicalmathematical machinemachine model model is is necessary necessary to to develop develop a controla control strategy strategy in inboth both steady steady state state and inand transient in transient modes. modes. This model This ismodel also indispensable is also indispensable to realize ato new realize proposition a new propositionof the machine of the power machine supply power system supply control system in control a fault in mode a fault of mode operation, of operation, and also and in also the in field the weakeningfield weakening and strengthening and strengthening ranges. Thisranges. model This has model been developedhas been underdeveloped typical under assumptions: typical assumptions:armature windings armature are windings symmetric, are windings symmetric, inductance windings andinductance resistance and are resistance constant, are permanent constant, permanentmagnets flux magnets is also constant,flux is also and constant, magnetic and flux magnet densityic higherflux density harmonics higher are harmonics neglected. are Finally, neglected. the d- Finally,and q-axis the voltages d- and q and-axis the voltages electromagnetic and the electromagnetic torque of hybrid-excited torque of hybrid-excited machines can be machines written as:can be written as: dψd U = RI + pΩmψq, (1) d d dt − URId  p , (1) dddt mq dψq U = RI + + pΩ ψ , (2) q q dt m d d q URI  p ,  (2) qqT md Trel  syncdt TDC   z }| { 3 3 z}|{ z }| {  T = p I I = p I + M I I + L L I I  e qψd dψq ψpm q DC DC q d Tq d q (3)  Tsync TDC rel  2 − 2     −   33  TpIIeqddqpmqDCDCqdqdq pIMIILLII   (3) 22 where p—number of pair poles; Ψ d, Ψ q—d- and q-axis magnetic fluxes; Ψ pm—flux generated by PMs; Id, Iq—d- and q-axis stator currents; Ld, Lq—d- and q-axis inductances; MDC—mutual inductance of the where p—number of pair poles; Ψd, Ψq—d- and q-axis magnetic fluxes; Ψpm—flux generated by PMs; additional coil; IDC—current in the additional coil. I I L L M d, q—Equationd- and q (3)-axis shows stator the currents; possibility d, ofq— formingd- and aq-axis specified inductances; electromagnetic DC—mutual torque atinductance different valuesof the additional coil; IDC—current in the additional coil. of the individual components of the stator current (Id, Iq) and additional coil current IDC. EquationDue to the (3) additional shows the degree possibility of freedom of forming in the form a specified of resultant electromagneti flux control byc thetorque coil current,at different it is I I I possiblevalues of to the generate individual a specific components electromagnetic of the stator torque current at di ff( erentd, q) and values additional of stator coil current current components DC. Due to the additional degree of freedom in the form of resultant flux control by the coil current, Id and Iq. If high torque is desired, it is preferred to increase the excitation flux of the machine, thus it itis possibleis possible to reduceto generate the stator a specific current. electromagnetic In the high rotational torque speeds, at different due to thevalues induced of stator high voltage,current I I componentsthe proper power d and supply q. If high of the torque machine is desired, requires it the is preferre weakeningd to of increase the excitation the excitation flux. Reducing flux of the machine,flux can also thus aff itect is thepossible reduction to reduce of losses the in stator the machine current. magnetic In the high circuit. rotational Appropriately speeds, controllingdue to the inducedthe auxiliary high winding voltage, of the the proper hybrid machinepower supply can finally of the increase machine its effirequiresciency [the7]. weakening of the excitation flux. Reducing the flux can also affect the reduction of losses in the machine magnetic circuit.6. Supply Appropriately and Control controlling System of the HESM auxiliary winding of the hybrid machine can finally increase its efficiency [7]. The power supply of the control coil on the rotor use the classical system of slip rings and 6.brushes Supply or and a rotary Control transformer. System of Due HESM to the reliability, quiet operation and low maintenance, the use of

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The power supply of the control coil on the rotor use the classical system of slip rings and brushes or a rotary transformer. Due to the reliability, quiet operation and low maintenance, the use of contactless energy transfer is best. Many design variants of this solution have already been analyzed in [26–27]. The analyzed machine uses a transformer whose winding is formed by a double-sided printed circuit board (PCB) with 70 µm copper thickness. This allowed the simplification of the structure and reduces the width. On both parts of the secondary and primary plates, ferrite sheets were used as the Energiespath of2019 the, 12magnetic, 3153 flux (Wurth Electronic® WE-FSFS flexible ferrite sheet, number 344003) [28].6 ofAir 12 gap between TX and RX-coil is about 1.2 mm. Constructional details are shown in Figure 7 and PCBs contactlesscoils are shown energy in transferFigure 8. is The best. control Many algorithm design variants takes into of this account solution the have mathematical already been model analyzed of the inHESM [26,27 (1)–(3).]. TheIn order analyzed to enable machine the change uses a transformerof direction of whose the coil winding current is in formed the rotor, by a a double-sided DC/DC controller printed of circuitIDC and board the control (PCB) circuit with 70 wereµm located copper on thickness. the secondar This allowedy side of thethe simplificationrotary transformer of the (rotating structure part). and reducesThe required the width. values On of boththe current parts of are the transmitte secondaryd andfrom primary the master plates, control ferrite system sheets by were using used radio as thetransmitter path of theand magnetic receiver flux(2.4 GHz (Wurth integrated Electronic radio®WE-FSFS module). flexible The transmitting ferrite sheet, coil number is supplied 344003) with [28 a]. Airvoltage gap inverter between having TX and an RX-coil operating is about frequency 1.2 mm. of 100 Constructional kHz. Wireless details energy are transfer shown to in the Figure secondary7 and PCBscoil was coils investigated are shown in for Figure series-series8. The control resonant algorithm operation takes using into account two resonant the mathematical capacitors model without of theadditional HESM (1)–(3).compensation circuits [29]. Operating frequency of coil’s current controller was 10 kHz. A block diagram of the control coils power system is shown in Figure 9.

Energies 2019, 12Figure,Figure x FOR 7.PEER7. Design REVIEW view of the proposed CETCET (left:(left: Receiver coil, right: Excitation coil). 7 of 12

(a) (b)

FigureFigure 8. 8. PCBPCB coils coils of of the the proposed proposed CET: CET: (a ()a )Receiver Receiver coil, coil, (b (b) )Excitation Excitation coil. coil.

In order to enable the change of direction of the coil current in the rotor, a DC/DC controller of IDC and the control circuit were located on the secondary side of the rotary transformer (rotating part). The required values of the current are transmitted from the master control system by using radio transmitter and receiver (2.4 GHz integrated radio module). The transmitting coil is supplied

Figure 9. Power supply system.

For the research, simulation models were made using the PLECS® software (Figure 10) in order to examine properties of novel contactless energy transfer systems. In order to test the transient and averaged voltages and currents, time domain analyses have been conducted. These analyses were also led in order to examine the thermal stress of the semiconductor. This numerical model was divided into four subsystems: • High frequency inverter for transformer supply • Rotary transformer model • Secondary part: Rectifier and digital PI regulator based current controller • Rotor control coil

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Energies 2019, 12, 3153 7 of 12 with a voltage inverter having an operating frequency of 100 kHz. Wireless energy transfer to the secondary coil was investigated for series-series resonant operation using two resonant capacitors without additional compensation(a) circuits [29]. Operating frequency of coil’s(b) current controller was 10 kHz. A blockFigure diagram 8. PCB of thecoils control of the proposed coils power CET: system (a) Receiver is shown coil, ( inb) FigureExcitation9. coil.

FigureFigure 9.9. Power supply system.

For the research, simulation models were made using the PLECSPLECS®® software (Figure 1010)) in order toto examineexamine propertiesproperties ofof novelnovel contactlesscontactless energyenergy transfertransfer systems.systems. In order to test the transienttransient andand averaged voltagesvoltages and and currents, currents, time time domain domain analyses analyses have have been been conducted. conducted. These These analyses analyses were were also ledalso in led order in order to examine to examine the thermal the thermal stress ofstress the semiconductor.of the semiconductor. This numerical This numerical model was model divided was intodivided four into subsystems: four subsystems: High• High frequency frequency inverter inverter for transformer for transformer supply supply • Rotary• Rotary transformer transformer model model • Secondary• Secondary part: part: Rectiﬁer Rectifier and digitaland digital PI regulator PI regulator based based current current controller controller • • Rotor control coil Rotor control coil Energies• 2019, 12, x FOR PEER REVIEW 8 of 12

Figure 10. Time domain simulation model of the contactless energy transfer system. Figure 10. Time domain simulation model of the contactless energy transfer system. 7. Experimental Setup and Validation 7. Experimental Setup and Validation To verify the presented control strategy and the FCR of the prototype, an experimental setup for the proposedTo verify the HESM presented as a generator control strategy was established. and the FCR It was of the mechanically prototype, an connected experimental to a high-speedsetup for theinduction proposed machine HESM (Figure as a generator 11). was established. It was mechanically connected to a high-speed inductionSecondly, machine the (Figure rotating 11). part (rectifier, control system, radio receiver and IDC current controller) of the wireless energy transfer system was implemented as a flexible PCB (Figure 12) fixed to a plastic support.

Figure 11. Test stand.

Secondly, the rotating part (rectifier, control system, radio receiver and IDC current controller) of the wireless energy transfer system was implemented as a flexible PCB (Figure 12) fixed to a plastic support. The main construction elements of the secondary supply system include IPD082N10N3 Mosfet power transistors for the IDC current controller and a IDD04SG60C SiC Schottky diode for the rectifier. As a control and central processing unit the TMS320F28027 (Piccolo family) by Texas Instruments has been used. The excitation system was a full bridge converter (Mosfets CSD88599Q5DC), also controlled by the TMS320F28027 processor. In order to verify the basic properties of proposed wireless power transfer system configuration, the experimental test stand was built. Basic waveforms obtained from simulations and measurements (output voltage and current from excitation inverter) confirm correct resonant operation of the supply system (Figure 13). Additionally, in Figure 14, measured efficiency and power losses of the wireless rotor power supply system are shown.

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Figure 10. Time domain simulation model of the contactless energy transfer system.

7. Experimental Setup and Validation To verify the presented control strategy and the FCR of the prototype, an experimental setup for Energies 2019, 12, 3153 8 of 12 the proposed HESM as a generator was established. It was mechanically connected to a high-speed induction machine (Figure 11).

Energies 2019, 12, x FOR PEER REVIEW Figure 11. 9 of 12 Figure 11. Test stand.

The main construction elements of the secondary supply system include IPD082N10N3 Mosfet power transistors for the IDC current controller and a IDD04SG60C SiC Schottky diode for the rectifier. As a control and central processing unit the TMS320F28027 (Piccolo family) by Texas Instruments has been used. The excitation system was a full bridge converter (Mosfets CSD88599Q5DC), also controlled by the TMS320F28027 processor. In order to verify the basic properties of proposed wireless power transfer system configuration, the experimental test stand was built. Basic waveforms obtained from simulations and measurements (output voltage and current from excitation inverter) confirm correct resonant operation of the supply system (Figure 13). Additionally, in Figure 14, measured efficiency and power losses of the wireless rotor power supply system are shown. In Table2 and Figure 15 are depicted RMS values of back-EMF at 1000 rpm rotor speed for the three operating conditions obtained experimentally and during 3D-FEA analysis. (a) (b)

FigureTable 13. 2. Comparison Basic waveforms of 3D-FEA for serial predictions resonant and operation experimental (IDC = results1 A): measurements; of no-load back-emf (a) simulation (RMS). results; (b) top waveforms: Output voltage from inverter, bottom: Current from inverter. Operation 3D-FEA Experiment

FS at IDC = 1.0 A 37.5 V 40.0 V No-load 23.8 V 24.0 V FW at I = 1.0 A 10.5 V 10.1 V DC −

Figure 14. Measured efficiency (blue) and power losses (red) of the wireless rotor power supply system.

In Table 2 and Figure 15 are depicted RMS values of back-EMF at 1000 rpm rotor speed for the three operating conditions obtained experimentally and during 3D-FEA analysis.

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Energies 2019, 12, 3153 9 of 12 Figure 12. Coil current controller (on the rotor). Figure 12. Coil current controller (on the rotor).

(a) (b) (a) (b) Figure 13.13. Basic waveforms for serial resonant operation (IIDC = 1 A): measurements; (a) simulation Figure 13. BasicBasic waveformswaveforms forfor serialserial resonantresonant operationoperation ((DCIDC == 1 A):A): measurements;measurements; ((a)) simulationsimulation results; (b) top waveforms: Output voltage from inverter, bottom: Current from inverter. results;results; ((bb)) toptop waveforms:waveforms: OutputOutput voltagevoltage fromfrom inverter,inverter, bottom:bottom: CurrentCurrent fromfrom inverter.inverter.

Figure 14. Measured efficiency (blue) and power losses (red) of the wireless rotor power supply Figure 14. Measured efficiency (blue) and power losses (red) of the wireless rotor power supply EnergiesFiguresystem. 2019 14. , 12Measured, x FOR PEER eﬃ REVIEWciency (blue) and power losses (red) of the wireless rotor power supply system. 10 of 12 system. In Table 2 and Figure 15 are depicted RMS values of back-EMF at 1000 rpm rotor speed for the In Table 2 and Figure 15 are depicted RMS values of back-EMF at 1000 rpm rotor speed for the three operating conditions obtained experimentally and during 3D-FEA analysis. three operating conditions obtained experimentally and during 3D-FEA analysis.

FigureFigure 15. 15.Experimental Experimental and and 3D-FEA 3D-FEA results results of back-EMF of back-EMF characteristics characteristics versus DCversus control DC coil control current. coil current. Waveforms of induced voltages at selected values of IDC currents are shown in Figure 16. Table 2. Comparison of 3D-FEA predictions and experimental results of no-load back-emf (RMS).

Operation 3D-FEA Experiment FS at IDC = 1.0 A 37.5 V 40.0 V No-load 23.8 V 24.0 V FW at IDC = −1.0 A 10.5 V 10.1 V

Waveforms of induced voltages at selected values of IDC currents are shown in Figure 16.

Figure 16. Experimental results of back-EMF versus DC coil current (1000 rpm).

8. Conclusions By controlling the auxiliary rotor coil current and the stator current, it is possible to use the specific features of hybrid machines in the whole range of resulting torques and speeds. Application of the rotary transformer complicates the supply system, however as shown by simulation studies, the value of power delivered to the additional coil placed on the rotor is big enough in order to influence the main flux of the machine. The paper also presents results of experimental tests that confirmed the efficiency of power supply and control of the excitation current using the novel wireless system. The further improvement of the efficiency of energy transfer requires additional optimization of the whole rotor coil supply system, which will be the subject of future research.

Author Contributions: All authors worked on this manuscript together and analyzed the obtained data. Conceptualization, R.P.; investigation, M.W., M.B., P.Paplicki and P.Prajzendanc; methodology, R.P.; validation, M.W., M.B., P.Paplicki and P.Prajzendanc; writing—original draft, M.W., M.B., P.Paplicki and P.Prajzendanc; writing—review and editing, R.P. All authors revised and approved the final version of the paper.

Funding: This work has been supported with the grant of the National Science Centre, Poland 2018/02/X/ST8/01112.

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

Energies 2019, 12, x FOR PEER REVIEW 10 of 12

Figure 15. Experimental and 3D-FEA results of back-EMF characteristics versus DC control coil current.

Table 2. Comparison of 3D-FEA predictions and experimental results of no-load back-emf (RMS). Waveforms of induced voltages at selected values of IDC currents are shown in Figure 16. Operation 3D-FEA Experiment FS at IDC = 1.0 A 37.5 V 40.0 V Energies 2019, 12, 3153 No-load 23.8 V 24.0 V 10 of 12 FW at IDC = −1.0 A 10.5 V 10.1 V

FigureFigure 16. 16. ExperimentalExperimental results results of of back-EMF back-EMF versus versus DC DC coil coil current current (1000 (1000 rpm).

8.8. Conclusions Conclusions ByBy controlling thethe auxiliary auxiliary rotor rotor coil coil current current and and the statorthe stator current, current, it is possible it is possible to use theto use specific the specificfeatures features of hybrid of machineshybrid machines in the whole in the rangewhole ofrange resulting of resulting torques torques and speeds. and speeds. Application Application of the ofrotary the rotary transformer transformer complicates complicates the supply the system,supply system, however however as shown as by shown simulation by simulation studies, the studies, value theof power value deliveredof power to delivered the additional to the coil additional placed on coil the rotorplaced is bigon enoughthe rotor in is order big toenough influence in order the main to influenceflux of the the machine. main flux The of paper the alsomachine. presents The results paper of also experimental presents results tests that of confirmedexperimental the tests efficiency that confirmedof power supply the efficiency and control of power of the excitationsupply and current control using of the the excitation novel wireless current system. using The thefurther novel wirelessimprovement system. of theThe e ffifurtherciency improvement of energy transfer of the requires efficiency additional of energy optimization transfer requires of the whole additional rotor optimizationcoil supply system, of the whole which rotor will be coil the supply subject system of future, which research. will be the subject of future research.

Author Contributions: AllAll authors authors worked worked on on this this manuscript manuscript to togethergether and and analyzed analyzed the the obtained obtained data. data. Conceptualization,Conceptualization, R.P.; investigation,investigation, M.W.,M.W., M.B., M.B., P.P. P.Pap (Piotrlicki Paplicki) and P.Prajzendanc; and P.P. (Pawel methodology, Prajzendanc); R.P.; methodology, validation, M.W.,R.P.; validation, M.B., P.Paplicki M.W., and M.B., P.Prajzend P.P. (Piotranc; Paplicki) writing—original and P.P. (Pawel draft, Prajzendanc); M.W., M.B., P.Paplicki writing—original and P.Prajzendanc; draft, M.W., M.B., P.P. (Piotr Paplicki) and P.P. (Pawel Prajzendanc); writing—review and editing, R.P. All authors revised and writing—reviewapproved the final and version editing, of theR.P. paper. All authors revised and approved the final version of the paper. Funding:Funding: ThisThis work work has has been supportedbeen supported with the with grant ofthe the gr Nationalant of Sciencethe National Centre, PolandScience 2018 Centre,/02/X/ST8 Poland/01112. 2018/02/X/ST8/01112. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References

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