Article Inductive Power Transfer Subsystem for Integrated Motor Drive

Zbigniew Kaczmarczyk 1, Marcin Kasprzak 1, Adam Ruszczyk 2, Kacper Sowa 2 , Piotr Zimoch 1,* , Krzysztof Przybyła 1 and Kamil Kierepka 1

1 Department of Power Electronics, Electrical Drive and Robotics, Silesian University of Technology, 44100 Gliwice, Poland; [email protected] (Z.K.); [email protected] (M.K.); [email protected] (K.P.); [email protected] (K.K.) 2 ABB Corporate Technology Center, 31038 Cracow, Poland; [email protected] (A.R.); [email protected] (K.S.) * Correspondence: [email protected]; Tel.: +48-32-237-12-47

Abstract: An inductive power transfer subsystem for an integrated motor drive is presented in this paper. First, the concept of an integrated motor drive system is overviewed, and its main components are described. Next, the paper is focused on its inductive power transfer subsystem, which includes a magnetically coupled resonant circuit and two-stage conversion with an appropriate control method. Simplified complex domain analysis of the magnetically coupled resonant circuit is provided and the applied procedure for its component selection is explained. Furthermore, the prototype of the integrated motor drive system with its control is described. Finally, the prototype based on the gallium nitride field effect transistors (GaN FET) inductive power transfer subsystem is experimentally tested, confirming the feasibility of the concept.

 Keywords: motor drives; power transfer (WPT); inductive power transfer (IPT); coupled  circuits; resonant converters Citation: Kaczmarczyk, Z.; Kasprzak, M.; Ruszczyk, A.; Sowa, K.; Zimoch, P.; Przybyła, K.; Kierepka, K. Inductive Power Transfer Subsystem 1. Introduction for Integrated Motor Drive. Energies (WPT) has been an object of increased interest of researchers 2021, 14, 1412. https://doi.org/ in recent years. Contactless energy transmission eliminates cables and connectors, thus 10.3390/en14051412 increasing reliability. The technology is also naturally safe by providing electrical insulation. Inductive power transfer (IPT), which usually employs two magnetically coupled coils Academic Editor: Alon Kuperman (forming a loosely coupled ) and compensation , is one of multiple technologies used for WPT [1–3]. IPT systems allow to achieve relatively long transmission Received: 19 January 2021 distances with relatively high efficiencies. Their efficiencies depend heavily on the Accepted: 1 March 2021 Published: 4 March 2021 coefficient between primary and secondary coils and their quality factors [4,5]. They can also supply multiple receivers [6]. IPT technology is a very popular solution for a wide

Publisher’s Note: MDPI stays neutral range of applications [7–9], e.g., contactless charging of electric vehicles [10,11] and mobile with regard to jurisdictional claims in devices [5], powering biomedical implants [4], flexible [12], or fluorescent published maps and institutional affil- lighting [13]. Recent advances in this regard relate to the structures of IPT systems [14–16], iations. compensation circuits [15,17] and coil designs [5,18,19]. So far, little attention has been paid to the possibility of using IPT technology for motor drive systems. For example, [20] proposes a wireless in-wheel motor drive for electric vehicles to solve the problems with power cables and signal wires exposed to harsh environments. This system applies an additional controller and power converter Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. at the motor side. In turn, in [21] a wireless bidirectional servo motor drive is described This article is an open access article and [22] presents a wireless switched reluctance motor drive. In both cases the principles distributed under the terms and of the so-called “energy encryption” [23] and selective wireless power transfer [24] are conditions of the Creative Commons applied. The reluctance motor drive in [22] is switched by using receiver coils of different Attribution (CC BY) license (https:// resonant , while the servo motor drive in [21] is based on the same method creativecommons.org/licenses/by/ for bidirectional motion. In [21] and [22], neither a controller nor a power converter at 4.0/). the secondary-side of the IPT system is used. In contrast to these studies, the solution

Energies 2021, 14, 1412. https://doi.org/10.3390/en14051412 https://www.mdpi.com/journal/energies Energies 2021, 14, 1412 2 of 14

presented in this paper provides a fully functional drive system, which is integrated with motor housing. Importantly, the system is powered via a contactless plug which makes it suitable for specific applications. It can easily be adapted to all types of electric motors [25]. The developed system is composed of an electric motor connected to a dedicated drive, powered wirelessly by and receiver modules—a substitute for a regular plug. Therefore, the system is fully hermetic and dedicated to harsh environments, e.g., the food and beverage (F&B) industry. The average lifetime of motors in the F&B industry is about one month [26]. The main failure reasons are bearings or windings of the motors. Motors that have been in operation for more than a month are often replaced by new ones as part of preventive maintenance [27,28]. The most popular motors are rated in the range of 1 to 2 kW and are used in pumps, conveyors, compressors, and others. The F&B industry (similarly to the chemical industry) has very high requirements for clean and reliable operation due to the continuous production process. Any unpredicted stops and delays may damage semi-products used in production and increase operating costs. The system modularity is a valuable advantage, allowing for simple installation and quick replacement of a damaged module (drive with motor). Due to the complexity of the whole system, this paper is mainly focused on presenting the general concept of the developed system and in more detail on its IPT subsystem. This IPT subsystem has a fully load-independent gain. Similar solutions are analyzed in [29–31], but the design method of the IPT subsystem given in this paper is simplified and appropriately adapted. At the same time, additional feedback is applied to ensure the required fully load-independent voltage gain. Section2 provides an overview of the integrated motor drive system and its structure. The selection of magnetically coupled resonant circuit components is given in Section3 . Section4 describes the prototype of the integrated motor drive system, presenting the applied IPT subsystem and its control in detail. The results of experimental tests of the prototype are shown in Section5. Finally, the conclusions are drawn in Section6.

2. Concept of IPT-IMD System The proposed IPT-IMD system includes the IPT subsystem powering the integrated motor drive (IMD), which consists of a variable- drive (VFD) with an electric motor (M). First, it should be clarified that the IPT and IMD subsystems can be considered individually. The IPT subsystem provides the required power to the IMD subsystem under a variable load, and its DC output voltage aligned with to the DC input voltage contains the information about the set angular velocity of the motor. On the other hand, the IMD subsystem based on this voltage is responsible for obtaining the desired angular velocity. A 3D visualization of the system is presented in Figure1 and its schematic is depicted in Figure2. The applied magnetically coupled coils are designed to be identical and to fit within the plastic casings. They are single-layer and spiral, wound with litz-wire. Additionally, the coils are shielded with 3F3 ferrite and aluminum plates. The casings are built to have the same dimensions as the motor housing. The IPT-IMD system can be divided into the following sections: the input (IN) consisting of the DC supply UI with bank CI, the primary-side GaN FET inverter (INV), the magnetically coupled resonant circuit (MCR circuit), the secondary-side bridge rectifier (REC) and the output load (OUT/IMD) consisting of a DC link with capacitor bank CO and the VFD with the electric motor (M). The equivalent components for the MCR circuit are also shown on the schematic. Namely, the AC sinusoidal source u1 on the primary-side, and the AC load resistance ROAC equivalent to the DC load resistance RODC on the secondary-side. Energies 2021, 14, x FOR PEER REVIEW 3 of 14 Energies 2021, 14, 1412 3 of 14 Energies 2021, 14, x FOR PEER REVIEW 3 of 14

electric motor secondary primary electric motor secondary casingsecondary casing primary VFD primary secondarycoil casing casing coilprimary VFD magnets coil coil

optical link inverter rectifier magnetic shields optical link inverter aluminummagnetic plates shields (a) aluminum plates (a)

(b) (b) FigureFigure 1. Visualization 1. Visualization of IPT-IMD of IPT-IMD system: system: (a) (disassembled,a) disassembled, (b) ( bassembled.) assembled. Figure 1. Visualization of IPT-IMD system: (a) disassembled, (b) assembled. control loop of IPT subsystem control loop of IMD II control loop of IPT subsystem control loop of IMD IO II T1 T3 C1 R R i1 p1 p2 i2 IO T1 T3 C1 R R D1 D3 i1 p1M p2 i2 CO C D1 D3 I M CO UI UO CI u1 L1 L2 u2 M T2 T4 R ~ UI ROAC ODCUO ~ u1 L1 L2 u2 ~ M T2 T4 D2 D4 R ~ ROAC ODC ~ D2 D4 INV MCR circuit REC VFD M IN INV IPT MCRsubsystem circuit REC OUTVFD / IMD M IN IPT subsystem OUT / IMD Figure 2. Schematic of the IPT-IMD system. Figure 2. SchematicSchematic of of the the IPT-IMD IPT-IMD system. The applied magnetically coupled coils are designed to be identical and to fit within the plasticTheThe casings.applied angular Theymagnetically velocity are single-layer of the coupled motor ancoils isd controlledspiral, are de woundsigned by the towith be VFD, identicallitz-wire. proportionally andAdditionally, to fit within to the thetheoutput coils plastic are voltage shieldedcasings.UO ,Theywith which 3F3are varies ferritesingle-layer with and theal anuminum inputd spiral, voltage plates. wound UTheI .with Incasings turn, litz-wire. theare built IPT Additionally, subsystem to have thetheis same controlled coils dimensions are shielded to ensure as withthe the motor 3F3 equality ferrite housing. of andUI Thealanduminum IPT-IMDUO despite plates. system load The can variation.casings be divided are Therefore, built into to thehave the followingtheMCR same circuit sections: dimensions is designed the input as the accordingly, (IN) motor consisting housing. to make of The the the IPT-IMDDC output supply voltage system UI with stable can capacitor be in divided a wide bank rangeinto C Ithe, of thefollowingload primary-side conditions sections: andGaN the to FET beinput approximately inverter (IN) consisting (INV), 10% the of higher themagnetically DC than supply the inputcoupled UI with voltage resonantcapacitor in case circuitbank of open- C I, (MCRtheloop primary-sidecircuit), operation. the secondary-side Additionally, GaN FET inverter an bridge optical (INV), rectifier link the is (REC) used magnetically asand feedback the output coupled information load resonant (OUT/IMD) about circuit the consisting(MCRoutput circuit), voltage of a DC theU linkO secondary-sidefor with the IPTcapacitor subsystem bridge bank re controlCctifierO and in (REC)the case VFD ofand closed-loopwith the theoutput electric operation. load motor (OUT/IMD) Further (M). Theconsistingconsiderations equivalent of componentsa DC are link made with underfor capacitorthe theMCR assumption circuibank tC areO and thatalso the theshown VFD primary-side onwith the the schematic. electric inverter motorNamely, switching (M). theThefrequency AC equivalent sinusoidal is 150 components source kHz and u1 on the for the inputthe primary-side, MCR and circui output tand are thealso AC shown shouldload on resistance the change schematic. R inOAC the equiva- Namely, range of lentthe100 to AC tothe 300sinusoidal DC V, load which resistance source relates u1 R toonODCthe the on fullprimary-side, the range secondary-side. of angular and the velocity AC load of resistance the considered ROAC equiva- motor lent(motorThe to angularthe parameters DC load velocity resistance are of given the motorR inODC Section on is thecontro 4.3 secondary-side.).lled The by selected the VFD, switching proportionally frequency to the ensures out- putfeasibility voltageThe angular U andO, which acceptable velocity varies powerof with the motorthe losses input is of contro the voltage IPT-IMDlled U byI. In system.the turn, VFD, the It propor has IPT a subsystem directtionally impact to is the con- on out- the trolledputparameters voltage to ensure U ofO , the thewhich magneticallyequality varies of with U coupledI andthe input UO coils, despite voltage which load UI should. Invariation. turn, be the flat Therefore, IPT and subsystem properly the MCR is fill con- the trolled to ensure the equality of UI and UO despite load variation. Therefore, the MCR

Energies 2021, 14, x FOR PEER REVIEW 4 of 14

circuit is designed accordingly, to make the output voltage stable in a wide range of load conditions and to be approximately 10% higher than the input voltage in case of open- loop operation. Additionally, an optical link is used as feedback information about the output voltage UO for the IPT subsystem control in case of closed-loop operation. Further considerations are made under the assumption that the primary-side inverter switching Energies 2021, 14, 1412 4 of 14 frequency is 150 kHz and the input and output voltages should change in the range of 100 to 300 V, which relates to the full range of angular velocity of the considered motor (motor parameters are given in Section 4.3). The selected switching frequency ensures feasibility andplastic acceptable casings. power Hence, losses their of relatively the IPT-IMD high sy qualitystem. factorsIt has a anddirect strong impact magnetic on the couplingparam- etersare obtained.of the magnetically coupled coils, which should be flat and properly fill the plastic casings.An Hence, additional their componentrelatively high of thequality system factors is a setand of strong magnets magnetic placed coupling in the corners are ob- of tained.both casings. In the event of a failure on either side of the system, it can be quickly plugged or unpluggedAn additional without component having of to the use system any wires. is a set of magnets placed in the corners of both casings. In the event of a failure on either side of the system, it can be quickly plugged or3. unplugged MCR Circuit without Component having to Selection use any wires. A schematic of the MCR circuit is shown in Figure3. It is assumed that passive 3.components MCR Circuit of Component the circuit areSelection linear, invariant in time and ideal, except for the parasitic resistancesA schematic of the of coils. the MCR Overall, circuit two is series–series shown in Figure compensation 3. It is assumed capacitors that arepassive included com- in ponentsthe circuit. of the The circuit analysis are islinear, carried invariant out using in fundamentaltime and ideal, harmonic except approximationfor the parasitic in re- the sistancescomplex of domain—sinusoidal the coils. Overall, two voltages series–ser and currents.ies compensation The primary-side capacitors inverter are included is replaced in theby circuit. a voltage The source analysisU 1is(1), carried while out the usin rectifierg fundamental is substituted harmonic by an approximation equivalent resistance in the complexROAC (2)—Figure domain—sinusoidal2. voltages and currents.√ The primary-side inverter is replaced U U 2 2 by a voltage source U1 (1), while the rectifier1 = 2 is= substituted by an equivalent resistance(1) π ROAC (2)—Figure 2. UI UO

-jXC1 -jXC2 I1 Rp1 Rp2 I2 jXM

U1 jXL1 jXL2 ROAC U2

FigureFigure 3. 3.SchematicSchematic of ofMCR MCR circuit. circuit.

U1 and U2 stand for root mean square (RMS) values of the input and output voltages. 𝑈 𝑈 2√2 = = (1) 𝑈R 𝑈 8π OAC = 2 (2) U1 and U2 stand for root mean squareRODC (RMS) valuπ es of the input and output voltages.

The MCR circuit is described by the𝑅 following8 equations: = (2) 𝑅 π  √ U = I Rp1 + j(XL1 − XC1) − jkI XL1XL2 The MCR circuit is 1described1 √ by the following equations:2  U2 = jkI1 XL1XL2 − I2 Rp2 + j(XL2 − XC2) , (3) U = R I 2𝑈 =𝐼OAC𝑅2+ 𝑗𝑋 −𝑋−𝑗𝑘𝐼𝑋𝑋 (3) where Rp1 and Rp2 are𝑈 the = parasitic𝑗𝑘𝐼𝑋 resistances𝑋 −𝐼 𝑅 of+ the𝑗 𝑋 coils. −𝑋 The , mutual reactance XM is expressed by the coils’𝑈 self-reactance =𝑅𝐼 values XL1, XL2 and the coupling coefficient k. Previous works [29–31] show that in the analyzed series-series compensated circuit where Rp1 and Rp2 are the parasitic resistances of the coils. The mutual reactance XM is it is possible to obtain the output voltage U2, which is nearly invariant with changes in expressed by the coils’ self-reactance values XL1, XL2 and the coupling coefficient k. the load resistance ROAC and proportional to the input voltage U1. Complete invariance Previous works [29–31] show that in the analyzed series-series compensated circuit can only be obtained if the circuit is lossless (Rp1 = Rp2 = 0). If the coils’ quality factors Q1 it is possible to obtain the output voltage U2, which is nearly invariant with changes in the and Q2 (4) are high, the output voltage does not change much. Additionally, if the coils are loadstrongly resistance coupled ROAC (k and> 0.8—[ proportional32]) the efficiency to the input is also voltage high. U1. Complete invariance can

XL1 XL2 Q1 = , Q2 = (4) Rp1 Rp2

Further analysis is done for the special case of the IPT subsystem—the output voltage UO should be approximately 10% higher than the input voltage UI in open-loop operation and the coils should be roughly similar. Initially, the analysis is simplified by assuming the Energies 2021, 14, x FOR PEER REVIEW 5 of 14

only be obtained if the circuit is lossless (Rp1 = Rp2 = 0). If the coils’ quality factors Q1 and Q2 (4) are high, the output voltage does not change much. Additionally, if the coils are strongly coupled (k > 0.8—[32]) the efficiency is also high.

𝑋 𝑋 𝑄 = ,𝑄 = (4) 𝑅 𝑅 Energies 2021, 14, 1412 Further analysis is done for the special case of the IPT subsystem—the output voltage5 of 14 UO should be approximately 10% higher than the input voltage UI in open-loop operation and the coils should be roughly similar. Initially, the analysis is simplified by assuming the circuit to be lossless (Rp1 = Rp2 = 0). By applying Thevenin’s theorem to the MCR circuit, circuit to be lossless (Rp1 = Rp2 = 0). By applying Thevenin’s theorem to the MCR circuit, thethe equivalent equivalent schematic schematic shown shown in inFigure Figure 4 4is is obtained. obtained.

ZE I2

E ROAC U2

FigureFigure 4. Thevenin’s 4. Thevenin’s equivalent equivalent of ofMCR MCR circuit. circuit.

TheThe equivalent equivalent circuit circuit is isdescri describedbed by by the the following following equations: equations: √ U𝑈𝑘k𝑋XL1𝑋XL2 E𝐸== 1 , ,(5) (5) 𝑋XL1−𝑋− XC1 𝑘k2𝑋X𝑋X 𝑍 = 𝑗𝑋 −𝑋 − 𝑗 L1 L2. (6) ZE = j(XL2 − XC2) − j𝑋 −𝑋 . (6) XL1 − XC1

It isIt obvious is obvious that that the the output output voltage voltage U2 isU independent2 is independent of the of load the loadresistance resistance ROAC R(UOAC2 = E(U) when:2 = E) when: ZE = 0. (7) 𝑍 =0. (7) Condition (7) is met, among others, for: Condition (7) is met, among others, for:  X = X 1 −k2 , 𝑋C1 =𝑋L11−𝑘 , (8) = (8) X𝑋C2 =0.0.

TheThe output output to toinput input voltage voltage ratio ratio UUO/OU/I UisI equalis equal to tothe the ratio ratio of ofU2U to2 toU1U (1).1 (1). By By using using EquationsEquations (5), (5), (7), (7), and and (8), (8), it is it isobtained: obtained: s 𝑈 𝑈 𝐸 1 𝑋 UO =U2 = E = 1 X. L2 (9) 𝑈 = 𝑈 = 𝑈 =𝑘 𝑋 . (9) UI U1 U1 k XL1

Therefore,Therefore, if ifthe the coupling coupling coefficient coefficient kk isis in thethe orderorderof of 0.9 0.9 and and the the coils’ coils’ self-reactance self-reac- tance XL1 and XL2 are similar, the output voltage will be approximately 10% higher than XL1 and XL2 are similar, the output voltage will be approximately 10% higher than the theinput input voltage. voltage. EquationEquation (8) (8) will will be beused used for for the the design design process process of ofthe the MCR MCR circuit. circuit. In Inthis this case case the the inputinput impedance impedance of ofthe the circuit circuit is: is: 𝑈 𝑅𝑘 𝑋𝑋 𝑅𝑘 𝑋 𝑍 =U = R k2X X + 𝑗 R2 k2X (10) = 𝐼1 = 𝑅OAC +𝑋L1 L2 +𝑅 OAC+𝑋 L1 Z1 2 2 j2 2 (10) I1 ROAC + XL2 ROAC + XL2 and remains inductive for any resistance value. This is beneficial due to the use of a reso- nantand voltage remains source inductive inverter for (Figure any resistance 2). Considering value. the This non-zero is beneficial parasitic due resistances to the use of a theresonant coils, the voltage efficiency source of the inverter MCR (Figurecircuit 2can). Considering be estimated the using non-zero the following parasitic equation: resistances of the coils, the efficiency of the MCR circuit can be estimated using the following equation:

2 2 2 2 ∆P Rp1 I + Rp2 I Rp2 Rp1 R + X η = 1 − = 1 − 1 2 = 1 − − OAC L2 (11) P Re[U I∗] R R k2X X 1AC 1 1 OAC OAC L1 L2 Energies 2021, 14, x FOR PEER REVIEW 6 of 14

Δ𝑃 𝑅𝐼 +𝑅𝐼 𝑅 𝑅 𝑅 +𝑋 𝜂=1− =1− ∗ =1− − (11) 𝑃 Re𝑈𝐼 𝑅 𝑅 𝑘 𝑋𝑋

where ΔP is equal to power losses in the coils, P1AC is equal to the input power of the Energies 2021circuit,, 14, 1412 I1 and I2 are RMS values of input and output currents. The output to input voltage6 of 14 ratio is given as:

where ∆P is equal𝑘 to𝑋 power𝑋 𝑅 losses−𝑅 in the−𝑅 coils, P1AC𝑋 is equal+𝑅 to the𝑅 input power of the 𝑈 𝑈 (12) circuit, I1 and I2 are RMS values of input and output currents. The output to input voltage = = ratio𝑈 is𝑈 given as: 𝑅𝑘 𝑋 q √   2 2 2 U U k XL1XL2 ROAC − Rp2 − Rp1XL2 + ROACRp1 4. Prototype of IPT-IMD SystemO = 2 = 2 (12) UI U1 ROACk XL1 4.1. MCR Circuit 4. Prototype of IPT-IMD System The MCR circuit has been built based on the analysis results presented in Section 3. 4.1. MCR Circuit The single-layer and spiral coils (outside diameter: 83 mm, inside diameter: 35 mm) were The MCR circuit has been built based on the analysis results presented in Section3 . wound with 24 turnsThe single-layer of litz wire and (Figure spiral coils 5a). (outside They are diameter: well fitted 83 mm, to inside the plastic diameter: casings. 35 mm) 3F3 were ferrite plates (PLT38,wound E withseries—25 24 turns × of 38 litz × wire3.8 mm (Figure3) were5a). They used are to well shield fitted and to the increase plastic casings.both the self- and mutual3F3 ferriteinductances plates (PLT38, of the E coils. series—25 The ×magnetic38 × 3.8 mm shields3) were were used placed to shield outside and increase of the coils (Figure both5b). Additionally, the self- and mutual aluminum plates of were the coils. used The on magnetic the outside shields to wereimprove placed outside of the coils (Figure5b). Additionally, aluminum plates were used on the outside to the shielding effectimprove and theat the shielding same effect time and pr atovide the same more time effective provide more cooling effective (Figure cooling 5c). (Figure The5c ). aluminum platesThe were aluminum cut accordingly plates were to cut avoid accordingly additional to avoid power additional losses power due losses to eddy due tocur- eddy rents. currents.

(a)

(b)

Figure 5. Cont.

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(c) Figure 5. Coils of MCRFigure circuit: 5. Coils (a) ofprimary MCR circuit: coil, ( ab)), primary coils with coil, magnetic (b), coils with shields, magnetic (c) coils shields, with (c) mag- coils with netic shields and aluminummagnetic shieldsplates. and aluminum plates. The coils, magnetic shields and aluminum plates were placed in the dedicated plastic The coils, magneticcasings shields (Figure5 ).and The aluminum casings ensure plates a stablewere positionplaced in of the coilsdedicated and their plastic coaxial casings (Figure 5).arrangement. The casings The ensure distance a ofstable 12 mm position between of the the coils coils results and directly their fromcoaxial the appliedar- rangement. The distancecasings andof 12 their mm mounting between position. the coils The results parameters directly of the coilsfrom were the measuredapplied cas- with an ings and their mountingAgilent 4294Aposition. impedance The paramete analyzer atrs the of frequencythe coils ofwere 150 kHz.measured The measurement with an Ag- results are summarized in Table1. ilent 4294A impedance analyzer at the frequency of 150 kHz. The measurement results are summarized in Table 1. Table 1. Parameters of magnetically coupled coils.

L1, µH Q1 Rp1, Ω L2, µH Q2 Rp2, Ω k Table 1. Parameters of magnetically coupled coils. Coils without shielding 76.8 240 0.30 76.5 247 0.29 0.65

L1, µCoilsH with magneticQ1 shieldsRp1, Ω 234L2, µH 305 0.72Q2 231Rp2, Ω 313 0.70k 0.87 Coils without shielding Coils with 76.8 magnetic shields 240 and aluminum 0.30 plates 229 76.5 218 0.99 247 227 0.29 223 0.96 0.65 0.87 Coils with magnetic 234 305 0.72 231 313 0.70 0.87 shields Using Equation (8) and assuming the frequency (f = 150 kHz), the coupling coefficient Coils with magnetic (k = 0.87) and the primary coil (L1 = 229 µH), the required primary-side com- shields and aluminum 229 218 pensation capacitance0.99 was227 calculated (C1 =223 20.2 nF). Based0.96 on Equation (12)0.87 and Table1, plates assuming the input voltage (e.g., UI = 300 V) and the output power (e.g., PO = 1200 W), the output voltage (UO = 330 V, UO/UI = 1.1) was iteratively calculated. This operation point corresponds to RODC = 90.8 Ω and ROAC = 73.6 Ω (2), respectively. Finally, according to Using EquationEquation (8) and (11), assuming the efficiency the (frequencyη = 97%) was (f found. = 150 kHz), the coupling coefficient (k = 0.87) and the primary coil inductance (L1 = 229 µH), the required primary-side com- 4.2. Control Method pensation was calculated (C1 = 20.2 nF). Based on Equation (12) and Table 1, The control method of the IPT subsystem is explained in Figure6. Figure6a shows assuming the input voltage (e.g., UI = 300 V) and the output power (e.g., PO = 1200 W), the a simplified diagram of the IPT subsystem control. The control includes a proportional- O O I output voltage (Uintegral = 330 (PI) V, controllerU /U = that1.1) alignswas iteratively the DC output calculated. voltage UO Thiswith theoperation DC supply point voltage corresponds to RODCUI. The= 90.8 voltage Ω andUI is R measuredOAC = 73.6 and, Ω using (2), anrespectively. additional rate Finally, limiter, according is entered into to the Equation (11), the PIefficiency controller ( asη = a 97%) reference was signal found.UOREF . In turn, the feedback signal (UO) is provided via an optical link. The output of the PI controller is the phase shift angle Φ between control signals of both the inverter half-bridges (pulse width modulation (PWM) control 4.2. Control Method method) as explained in more detail in Figure6b. The primary-side inverter (T 1,T2,T3, The control methodT4) is controlled of the IPT by a field-programmablesubsystem is explained gate array in (FPGA)-basedFigure 6. Figure system. 6a The shows switching a simplified diagramfrequency of the IPTf is set subsystem to 150 kHz. control. The inverter The is co inductivelyntrol includes loaded, a thus proportional-in- hard commutations are avoided. The VFD microprocessor system controls the optical link transmitter according tegral (PI) controller that aligns the DC output voltage UO with the DC supply voltage UI. to the PWM method. The signal has a fixed frequency of 20 kHz and the duty cycle carries I The voltage U is themeasured information and, about using the output an additional voltage UO .rate The opticallimiter, link is is entered placed in into the center the PI of the controller as a referencecoils, within signal the primaryUOREF. In and turn, secondary the feedback casings. The signal VFD integrated(UO) is provided on the secondary-side via an optical link. The output of the PI controller is the phase shift angle Φ between control signals of both the inverter half-bridges (pulse width modulation (PWM) control method) as explained in more detail in Figure 6b. The primary-side inverter (T1, T2, T3, T4) is con- trolled by a field-programmable gate array (FPGA)-based system. The switching fre- quency f is set to 150 kHz. The inverter is inductively loaded, thus hard commutations are avoided. The VFD microprocessor system controls the optical link transmitter according to the PWM method. The signal has a fixed frequency of 20 kHz and the duty cycle carries the information about the output voltage UO. The optical link is placed in the center of the coils, within the primary and secondary casings. The VFD integrated on the secondary-

Energies 2021, 14, x FOREnergies PEER2021 REVIEW, 14, 1412 8 of 14 8 of 14

side with the motorwith is the a standalone motor is a standaloneunit, controlled unit, by controlled a microprocessor by a microprocessor system, ensuring system, ensuring the the stabilizationstabilization of the angular of velocity the angular of the velocity motor of in the a wide motor range in a of wide load range variations. of load variations.

INV REC VFD C1 ~ M

UI UO M ~ ~ ~

Rate UOREF Ф Limiter + UO PI optical link ...

(a)

OutputEnable i1 Enable O-C Controller Level

Over-Current Protection T1 PWM Signal Generator

UI Rate UOREF Limiter PI Ф s (0) T2 Ф u1 Logic & PWM to Dead Time Voltage 150 kHz f s (Ф) Converter T3

Voltage to UO PWM T4 Converter Optical Link (20 kHz PWM) OutputEnable (b)

Figure 6. Control blockFigure diagrams: 6. Control (a) simplified, block diagrams: (b) detailed. (a) simplified, (b) detailed.

A soft-start of theA soft-startsystem is ofrequired the system to safely is required pre-charge to safely the capacitors pre-charge C theO on capacitors the CO on the VFD. This is doneVFD. when This the is connected done when IPT-IMD the connected system is IPT-IMD started for system the first is time, started or forafter the first time, or the primary coil afteris put the in its primary place (as coil for is a put regular in its plug). place The (as forpre-charge a regular process plug). is The realized pre-charge process is by limiting the raterealized of the by reference limiting signal the rate UOREF of the at referencethe input signalof the PIUOREF controller.at the inputAn over- of the PI controller. current protection,An over-currentwhich turns protection,off the invert whicher when turns offits theload inverter current when threshold its load is currentex- threshold is ceeded, is also implemented.exceeded, is also This implemented. can occur, for This example, can occur, when for the example, motor whenis overloaded the motor is overloaded or when the coilsor are when accidentally the coils are decoupled. accidentally The decoupled. settings of Thethe settingsPI controller of the are PI controllernot crit- are not critical ical as the responseas the time response is also timedeliberately is also deliberately limited by limitedthe rateby limiter the rate during limiter normal during op- normal operation. eration. It is an orderIt is an of ordermagnitude of magnitude faster than faster for the than system for the soft-start. system soft-start.This solution This has solution has been been applied dueapplied to the relative due to thely large relatively capacitance largecapacitance of the rectifier of thefilter rectifier to avoid filter unneces- to avoid unnecessary sary overloadingoverloading of the IPT system. of the IPT system.

4.3. IPT-IMD System

Energies 2021, 14, x FOR PEER REVIEW 9 of 14

The IPT-IMD system prototype is depicted in photographs in Figure 7. Figure 7a pre- sents its primary-side: the inverter (INV), the resonant capacitor (C1), the primary casing (PC) with the shielded coil and the receiver-side of the optical link (OL). The inverter uses TP65H035WS cascode GaN FETs as power semiconductors. The inverter control system is based on an Artix-7 FPGA (100 MHz, 32 MB). The optical link is placed outside the casings for measurement purposes. Figure 7b presents the secondary-side of the proto- type: the electric motor (M) with the VFD and the capacitors CO, the secondary casing (SC) with the shielded coil, the rectifier (REC) and the transceiver-side of the optical link (OL). The rectifier printed circuit board (PCB) is placed outside the casing for measurement purposes only. It is actually mounted on the aluminum plate inside the casing. The main Energies 2021, 14, 1412 9 of 14 system parameters are summarized in Table 2.

Table 2. Parameters of IPT-IMD system. 4.3. IPT-IMD System Parameter Value Description UI The100–300 IPT-IMD V system prototype is depicted DC power in photographs supply, 300 V/5 in A Figure 7. Figure7a CI presents its 810 primary-side: µF the inverter (INV), 3 the × 270 resonant µF on INV capacitor PCB (C1), the primary T1–T4 casing (PC) 650 V/29.5 with theA shielded coil and the receiver-side TP65H035WS, of the GaN optical FETs link (OL). The inverter C1 uses TP65H035WS 20.85 nF cascode GaN FETs as power semiconductors. MLCC C0G The inverter control D1–D4 system is 650 based V/10 onA an Artix-7 FPGA (100 MHz, STPSC20H065 32 MB). The SiC optical Diodes link is placed outside CO the casings 940 for µF measurement purposes. Figure 2 ×7 470b presents µF on VFD the PCB secondary-side of the RODC prototype:35–900 the electric Ω motor (M) with the VFD andlaboratory the capacitors resistorC O, the secondary casing VFD (SC) with the shielded 3-phase coil, the inverter, rectifier 6 × (REC)TP65H035WS, and the GaN transceiver-side FET-based of the optical link 3-phase, 1.5 kW, 1445 rpm, 230/400 V, M3AA090LD4 (delta configuration with nominal voltage of Motor (OL). The rectifier printed circuit board (PCB) is placed outside the casing for measurement purposes only. It is actually mounted on the230 aluminum V) plate inside the casing. The main system parameters are summarized in Table2.

SC PC PC SC INV CI CO

M REC OL VFD C1 OL

(a) (b)

FigureFigure 7.7. PhotographsPhotographs ofof IPT-IMDIPT-IMD systemsystem prototype:prototype: ((aa)) primary-side,primary-side, ((bb)) secondary-side.secondary-side.

5. ExperimentalTable Results 2. Parameters of IPT-IMD system. The measurements were carried out in a laboratory setup corresponding to the sche- Parameter Value Description matic shown in Figure 2. The main aim of the experimental tests focused on the IPT sub- UI system100–300 was to V determine its AC/AC and DC/D DCC power efficiencies supply, and 300 V/5the Adependence of its CI output810 voltageµF UO on the output power PO. As this 3 × subsystem270 µF on INV canPCB be considered individ- ually for much easier and more comprehensive testing, its DC output was loaded with a T1–T4 650 V/29.5 A TP65H035WS, GaN FETs laboratory resistor RODC. This equivalent resistor replaces the IMD subsystem. Addition- C 20.85 nF MLCC C0G 1 ally, the VFD was also connected to the DC output, but only its input capacitor bank was D1–D4 used.650 All V/10 measurements A were conducted using a STPSC20H065 Yokogawa WT5000 SiC Diodes power analyzer. The CO following940 µquantitiesF were recorded to illustrate the 2 × system470 µF on performance VFD PCB (Figure 2):

RODC 35–900 Ω laboratory resistor VFD 3-phase inverter, 6 × TP65H035WS, GaN FET-based

Motor 3-phase, 1.5 kW, 1445 rpm, 230/400 V, M3AA090LD4 (delta configuration with nominal voltage of 230 V)

5. Experimental Results The measurements were carried out in a laboratory setup corresponding to the schematic shown in Figure2. The main aim of the experimental tests focused on the IPT subsystem was to determine its AC/AC and DC/DC efficiencies and the dependence of its output voltage UO on the output power PO. As this subsystem can be considered individually for much easier and more comprehensive testing, its DC output was loaded with a laboratory resistor RODC. This equivalent resistor replaces the IMD subsystem. Energies 2021, 14, 1412 10 of 14

Additionally, the VFD was also connected to the DC output, but only its input capacitor Energies 2021, 14, x FOR PEER REVIEW 10 of 14 bank was used. All measurements were conducted using a Yokogawa WT5000 power analyzer. The following quantities were recorded to illustrate the system performance (Figure2): • DC parameters (PI = UIII—DC input power, PO = UOIO—DC output power, ηDC = • DC parameters (PI = UIII—DC input power, PO = UOIO—DC output power, ηDC = PO/PI—DC/DC efficiency), PO/PI—DC/DC efficiency), • AC parameters (P1AC = R T𝑢 𝑖 𝑑𝑡—AC input power, P2AC = R T𝑢 𝑖 𝑑𝑡—AC output • AC parameters (P = 1 u i dt—AC input power, P = 1 u i dt—AC output 1AC T 0 1 1 2AC T 0 2 2 power, ηAC = P2AC/P1AC—AC/AC efficiency, where T = —switching1 period). power, ηAC = P2AC/P1AC—AC/AC efficiency, where T = f —switching period). FigureFigure 88 presents presents the the efficiency efficiency measurements measuremen asts aas function a function of the of output the output power. power. Both I BothDC/DC DC/DC and AC/ACand AC/AC efficiencies efficiencies were were recorded recorded at a constantat a constant input input voltage voltage (UI = (U 300 = and300 and150 V)150 in V) open-loop in open-loop (ηDC_O (ηDC_O, ηAC_O, ηAC_O) and) and closed-loop closed-loop (η DC_C(ηDC_C, ,η ηAC_CAC_C)) operations. operations. Due to the additionaladditional power losses caused by the converters,converters, the DC/DC efficiency efficiency is is approximately approximately 1 percentage percentage point point (p.p.) (p.p.) lower lower than than the theAC/A AC/ACC efficiency. efficiency. Similarly, Similarly, the efficiency the efficiency meas- uredmeasured in open-loop in open-loop operation operation is approximately is approximately 1 p.p. higher 1 p.p. than higher in closed-loop than in closed-loop operation. Theoperation. PWM control The PWM of the control primary-side of the primary-side inverter is responsible inverter is responsible for this efficiency for this reduction. efficiency Inreduction. simple terms, In simple the terms,conduction the conduction time of the time transistors of the transistors is shorter is shorter and the and inverter the inverter zero outputzero output voltage voltage interval, interval, where where energy energy is not is notconverted converted but butpower power losses losses are aregenerated, generated, is longer.is longer. The The depicted depicted efficiency efficiency curves curves have have a typical a typical shape shape for IPT for IPTsystems systems [28]. [The28]. out- The putoutput power power changes changes are caused are caused by load by load changes, changes, resulting resulting in energy in energy conversion conversion for a for dif- a ferentdifferent voltage–current voltage–current relationship. relationship. For For the the input input and and output output voltages voltages of of 300 300 V V and and the DCDC output power ofof 12001200 WW inin closed-loop closed-loop operation, operation, the the distribution distribution of of power power losses losses is asis asfollows: follows: 12 12 W W in thein the inverter inverter (including (including 4 W 4 forW for its control),its control), 24 W 24 in W the in MCRthe MCR circuit, circuit, and and12 W 12 in W the in rectifier. the rectifier. The AC/AC The AC/AC efficiency efficiency was also was calculated also calculated analytically analytically using Equations using Equations(11) and (12) (11) for and the (12) output for the power outputPO inpower the range PO in ofthe 200 range to 1200 of 200 W atto input1200 W voltage at inputUI voltageequal to U 300I equal V. These to 300 theoretical V. These results theoretical were inresults the range were of in 97–98%, the range corresponding of 97–98%, corre- to the spondingmeasurements. to the measurements.

100 100 U =150V UI = 300 V I 98 98

, % % , 96 9 5 96 8 Ο 94 AC_O DC_O _ 0 AC_O DC_O AC_C DC_C 9 Χ AC_C DC_C 94 92 200 400 600 800 1000 1200 60 100 200 300 400 500 600 P , W P ,W O O

FigureFigure 8.8.AC/AC AC/AC and DC/DC efficiencies efficiencies as as a a function function of of output output power power ( (UUII == 300 300 and and 150 150 V). V).

TheThe output voltage measurements under analogous experimental conditions are U U shownshown in in Figure Figure 9.9. Both Both open-loop open-loop (U (O_O)O_O and) andclosed-loop closed-loop (UO_C () operationsO_C) operations are included. are in- cluded. As predicted by the MCR circuit analysis, U is approximately 10% higher than As predicted by the MCR circuit analysis, UO_O is approximatelyO_O 10% higher than UO_C. U . The output voltage U was also calculated using Equation (12). These theoretical TheO_C output voltage UO was alsoO calculated using Equation (12). These theoretical charac- teristicscharacteristics are within are within1% of the 1% measurements of the measurements (Figure (Figure 9). 9).

Energies 2021, 14, 1412 11 of 14 EnergiesEnergies 2021 2021, ,,14 14, ,,x xx FOR FORFOR PEER PEERPEER REVIEW REVIEWREVIEW 1111 ofof 1414

340 180 340 180 UUO_OO_O UO_O U 330330 UO_CO_C 330 170170 O_C 170 UUOO (theoretical) (theoretical) UO (theoretical)

320320 UUO_OO_O 320 UO_O ,V ,V ,V UU ,V 160 ,V O_C ,V 160

O O_C O O UO_C O 160 O O U U U UU (theoretical)(theoretical) U U 310310 OO U 310 UO (theoretical) 150150 300 150 300 UU == 150150 VV UUI I == 300300 VV UI I = 150 V UI = 300 V I 290 290 140140 200 400 600 800 1000 1200 200 400 600 800 1000 1200 6060 100100 200 200 300 300 400 400 500 500 600 600 P , W P , W POO,, WW POO,, WW O O

Figure 9. Output voltage as a function of output power (UI I= 300 and 150 V). FigureFigure 9. 9.Output Output voltage voltage as as a a function function of of output output power power ( U(UI I= == 300 300300 and andand 150 150150 V). V).V).

FigureFigure 101010 presentspresentspresents the the efficiencyefficiency efficiency measurementsmeasurements measurements as asas as a aa a function functionfunction function of ofof of the thethe the input inputinput input voltage, voltage,voltage, voltage, at atat at a constant load resistanceODC (R = 72 Ω) in closed-loop operation. The measurements aa constantconstant loadload resistanceresistance ((RRODCODC = == 72 7272 Ω Ω)) inin closed-loopclosed-loop operation.operation. TheThe measurementsmeasurements werewere were recorded for the input voltageI U in the range of 100–300 V. In this case, the AC/AC recordedrecorded forfor thethe inputinput voltagevoltage UU II in inin the thetheI range rangerange of ofof 100–300 100–300100–300 V. V.V. InIn thisthis case,case, thethe AC/ACAC/AC effi-effi- ciencyefficiencyciency isis independentindependent is independent ofof the ofthe the inputinput input voltage.voltage. voltage. InIn turn, Inturn,turn, turn, the thethe the DC/DC DC/DCDC/DC DC/DC efficiency efficiencyefficiency efficiency varies variesvaries varies due duedue due to toto the thethe to usetheuse useof of PWMPWM of PWM control.control. control.

100100 ΡΡ RROO=72=72Ρ RO =72

9898

, % , 96 , % , 96

, % , 96 2 2 2

9494 __ 94 _AC_CAC_C AC_C 88 8DC_CDC_C DC_C 9292 100100 150 150 200 200 250 250 300 300 U , V UI I,, VV I FigureFigure 10.10. AC/ACAC/AC andand and DC/DCDC/DC DC/DC efficienciesefficiencies efficiencies asas as aa afunction function function ofof of inputinput input voltagevoltage voltage inin in closed-loopclosed-loop closed-loop operationoperation operation ((RRODCODC == 7272 ΩΩ).). (R(RODCODC = 72 72 ΩΩ).).

FigureFigure 1111 showsshows thethe preliminarypreliminary experimentalexperimentexperimentalal resultsresults forfor thethe wholewhole IPT-IMDIPT-IMD system system prototype—aprototype—a graph graphgraph ofof thethe the motormotor motor angularangular angular velocityvelocity velocity (no (no(no (no load loadload load condition) condition)condition) condition) as asas a asaa function functionfunction a function of ofof the thethe of inputtheinputinput input voltage voltagevoltage voltage in inin closed-loop closed-loopclosed-loop in closed-loop operation. operation.operation. operation. According AccordingAccording According to toto the thethe to adopted adoptedadopted the adopted assumptions, assumptions,assumptions, assumptions, this thisthis rela- rela- thisrela- tionshiprelationshiptionshiptionship is isis linear. linear.linear. is linear. Detailed DetailedDetailed Detailed experimental experimentalexperimental experimental studies studies studies ofof of thethe the IMDIMD IMD subsystemsubsystem subsystem andand the thethe IPT-IMD IPT-IMD systemsystem prototypeprototype willwill be be presented presented inin inin aa aa separately separately separatelyseparately prepared prepared preparedprepared paper. paper. paper.paper.

16001600

12001200

800800

,rpm 800 ,rpm ,rpm n n n

400400

00 100100 150 150 200 200 250 250 300 300 U ,V UI I,V,V I Figure 11. Motor angular velocity (no load condition) as a function of input voltage in closed-loop FigureFigure 11.11. MotorMotor angularangular velocityvelocity (no(no loadload condition)condition) asas aa functionfunction of input voltage in closed-loop operation.operation. operation.operation.

Energies 2021,, 14,, x 1412 FOR PEER REVIEW 1212 of of 14 14

RepresentativeRepresentative voltage voltage and and current current waveforms waveforms of ofthe the primary-side primary-side inverter inverter and andthe secondary-sidethe secondary-side rectifier rectifier of the of IPT the subsystem IPT subsystem in the inclosed the closedloop case loop are case presented are presented in Fig- urein Figure12. For 12 the. Fortested the voltages tested voltages and powers, and powers,the phase the shift phase angle shift Φ anglemeasuredΦ measured with an oscilloscopewith an oscilloscope changed from changed 37° to from 51°, 37 which◦ to 51 corresponded◦, which corresponded to the duty to factor the dutyD (D factor= 1 − ◦ ΦD/180°)(D = 1in− theΦ/180 range) inof the 0.79 range to 0.72. of 0.79 These to 0.72. changes These are changes minor are due minor to the due applied to the applieddesign methoddesign methodof the MCR of the circuit—natural MCR circuit—natural load-independent load-independent voltage voltagegain. gain.

UI=300 V UI=300 V Φ f=150 kHz f=150 kHz i1 i1

1 μs 1 μs u1 u1

u2 u2 i2 i2

200V/div 200V/div 5A/div 5A/div

(a) (b)

UI=150 V UI=150 V f=150 kHz f=150 kHz i1 i1

u1 1 μs u1 1 μs

i2

u2 u2

i2 200V/div 200V/div 5A/div 5A/div

(c) (d)

FigureFigure 12. 12. VoltageVoltage and current waveforms of primar primary-sidey-side inverter inverter and and secondary-side secondary-side rectifier rectifier in in closed-loop closed-loop operation: operation: (a) UI = 300 V, PO = 1200 W, (b) UI = 300 V, PO = 600 W, (c) UI = 150 V, PO = 600 W, (d) UI = 150 V, PO = 300 W. (a) UI = 300 V, PO = 1200 W, (b) UI = 300 V, PO = 600 W, (c) UI = 150 V, PO = 600 W, (d) UI = 150 V, PO = 300 W.

TableTable 33 summarizes summarizes the the performance performance of of three three IPT IPT systems, systems, two two representative representative ones ones de- describedscribed in in [29 [29,31],31] and and the the third third one one presented presente ind thisin this paper. paper. Their Their geometric geometric parameters parameters are aresimilar. similar. The The measured measured coupling coupling coefficients, coeffici coils’ents, self-inductances,coils’ self-inductances, switching switching frequencies, fre- quencies,output voltages, output andvoltages, maximum and maximum output powers output and powers maximum and maximu efficienciesm efficiencies are tabulated. are tabulated.Unfortunately, Unfortunately, the efficiencies the efficiencies in [25] are in omitted. [25] are The omitted. IPT subsystem The IPT subsystem developed devel- in this opedpaper in is this compact paper and is integratedcompact and with integrat motored housing. with motor Its performance housing. Its are performance comparable with are comparableothers, also with in closed-loop others, also operation in closed-loop (* in Table operation3). (* in Table 3).

TableTable 3. Comparison of IPT systems.

k k LL11~~L2,L2, µHµ H f, kHzf, kHz UO, VU O,V PO_maxP, O_maxkW , kW ηAC_maxηAC_max ηηDC_maxDC_max [29][ 29]0.71 0.71 180 180 124.5 124.5 300–400 300–400 1 kW 1 kW- - - - [31][ 31]0.8 0.8 63 63 81 8190–150 90–150 1.4 kW 1.4 kW 99% 99% 95% 95% ThisThis paper paper 0.87 0.87 77 77 150 150 100–300 100–300 1.2 kW 1.2 kW 98.6%; 98.6%; 98.2% 98.2% * * 97.5%; 97.5%; 96.9% 96.9% * *

6.6. Conclusions Conclusions TheThe concept concept of of an an integrated integrated motor motor drive drive has has been been presented presented and and examined. examined. The Thede- velopeddeveloped system system is safe, is safe, wirelessly wirelessly powered powered (“wireless (“wireless plug”), plug”), and andallows allows the angular the angular ve- locity of the motor to be changed by setting the input voltage. The system is more complex

Energies 2021, 14, 1412 13 of 14

velocity of the motor to be changed by setting the input voltage. The system is more complex than a regular motor drive and this complexity results in lower efficiency and can make the system more prone to failure. However, this can be accepted in an industrial environment with an additional benefit. This benefit is the introduction of galvanic isolation, where the IPT transmitter and receiver modules can be freely connected and disconnected by workers not fully qualified to service electrical equipment. For the given assumptions, the simplified procedure for selecting MCR circuit pa- rameters is explained. A compact prototype of the IPT-IMD system has been built and the IPT subsystem has been experimentally tested. The results confirm feasibility of the system. The AC/AC and DC/DC efficiencies of 96–98.6% and 93–97.5% were recorded for the input voltage of 300 and 150 V, respectively. These conditions corresponded to the output power in the ranges of 200–1200 W (300 V) and 60–600 W (150 V). The efficiencies are approximately 1 p.p. lower in closed-loop operation than in the open-loop case. This is due to the PWM control of the primary-side inverter. Continuation of studies on the IPT-IMD system will be presented in a separate publi- cation. At the same time, further research should be focused on the system optimization, in particular on its reliability and control methods.

Author Contributions: Conceptualization, A.R., M.K. and Z.K. with contributions from all co- authors; methodology, Z.K. and M.K.; software, K.P., K.K. and K.S.; validation, K.P., K.K., P.Z. and M.K.; formal analysis, Z.K.; investigation, K.P., K.K. and M.K.; resources, A.R., K.S. and Z.K.; writing— original draft preparation, P.Z.; writing—review and editing, P.Z., Z.K., A.R. and K.S.; visualization, M.K., K.P., Z.K., P.Z. and K.S.; supervision, M.K. and A.R.; project administration, M.K. and Z.K.; funding acquisition, M.K. and A.R. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by ABB Corporate Technology Center, Cracow, Poland. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest.

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