Wireless (power transfer) transmission of electrical energy (electricity) intended for consumer purposes up to 50 W

POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH Wireless (Power Transfer) Transmission of Electrical Energy (Electricity) Intended for Consumer Purposes up to 50 W Marek PIRI, Pavol SPANIK, Michal FRIVALDSKY, Anna KONDELOVA Department of Mechatronics and Electronics, Faculty of Electrical Engineering, University of Zilina, Univerzitna 8215/1, 01026 Zilina, Slovak Republic [email protected], pav[email protected], michal.friv[email protected], anna.kondelov[email protected] DOI: 10.15598/aeee.v14i1.1573 Abstract. This project deals with Power Semiconductor Systems PSS for wireless transmission of electricity to the power of 50 W with regard to the distance and transmission efficiency. We decided to use electromagnetic resonance for electrical energy transmission. For experimental verification, we have wound two coils of identical dimensions. At a given power transmission solutions, we obtain the highest efficiency η= 70 %at a distance of 5 cm, where the transmitted power was 48 W. Keywords Coil, frequency, resonant, wireless power transfer. 1. Introduction Wireless transmission of electricity is a vision that circulates in the minds of inventors for over 100 years. Discovering of magnetic resonance opened the way for solving the problem of efficiency of electricity transmission in the near field. Thanks to this phenomenon, it is possible to transfer a high power capacity at high efficiency. The result is a prospective solution for many applications such as consumer electronics, automotive systems, medical equipment and many more. The aim of the paper is to design a topology of PSS for wireless transmission of electricity with the power up to 50 W with regard to the distance and transmission efficiency. Analysis of currents of main circuit for the wireless transfer system, has to help successful implementation of the task. The choice of the optimal design to achieve the specified parameters, the simulation of the designed system and its structure is based on this analysis. The article consists of several parts. The first is devoted to analysis and the current state of system solutions for the wireless transmission of electricity. The second part of the article describes the design of the main circuit for the wireless transmission. In the third part of the paper the simulation model is provided that is based on an earlier proposal and describes the behavior of the proposed system. The experimental verification of the designed solution the aim of which is an efficient transmission of electricity from the source to the load without the use of wires is performed in the fourth part of this work. 2. Applied Type of Coupling and its Analysis When the mutual inductance of two coils is low, the receiver coil induces the low voltage with a low efficiency. According to Eq. (1) we can see that the low Mvalue may be compensated by an increase in the angular frequency ω, or by an increase in the I1amplitude of the transmitting coil. up(t) = dφ dt =Mdi1(t) dt =MωI1·cos(ωt).(1) There are two types of power systems for wireless transmission-direct and indirect power supply (Fig. 1). At indirect power supply, transmitting and receiving coil is separated from the source and load to achieve higher quality factor Qat the transmitting and receiving part, whereby it is possible to achieve greater transmission distance. Coils L1and L2serve as binding coils, which transform the impedance of source and load. Reaching the higher quality factor Qcan increase c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 40 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH Indirect-fed Direct-fed Induction Coupled Resonance Induction Coupled Resonance Fig. 1: Wireless transmission in direct and indirect power supply. transmitted distance. However, the systems are more sensitive to the choice of parameters such as the inductance and the resonance frequency. Direct type of power supply indicates from the title that the source is directly connected to the transmitter section. A choice of this type of power supply is more suitable for practical applications because of the simplicity of the design, customization options, control and low cost. Its disadvantage is the reduction of the quality factor Q[1], [2], [3], [4]. 3. Design of the System of Selected Transmission Method The usage of the resonant circuit in the receiver and transmitter allows transmitting of the highest transmitted capacity at the highest possible distance. For this type of transmission, it is important to design a low-loss coils and pairing circuits. Fulfilment of the given conditions in the design allows achievement of the best transmission parameters. Topology design is based on the principle diagram for the resonant wireless system for electricity transmitting (Fig. 2). Fig. 2: Principles schematic of the resonant wireless system. DC voltage source supplies power amplifier (DC/AC), which produces rectangular voltage waveform. This voltage produces an alternating magnetic field in a transmission resonant circuit. Receiving resonant circuit is tuned to the same resonant frequency as the source frequency. Magnetic energy induces a sinusoidal voltage at the receiving side. The AC voltage is then rectified in a diode rectifier and DC voltage is led to the load [5], [6]. Based on the predicted performance, we set other parameters. Tab. 1: Other parameters. Uin 100 V Pout 50 W Uout 20 V Iout 2.5 A RL8Ω fSW 293 kHz 4. The Coil Design The coil design is one of the most important factors in the design of a system for wireless transmission. Important parameters such as quality factor Qand mutual inductance that determine the maximum transmission efficiency, maximum transmission distance and also the transmission capacity depends on the parameters of transmitter/receiver coil. Inductance calculation normally begins on the so-called pure inductor, when it is assumed that the solenoid coil is formed of infinitely thin wire without gaps between conductors (turns of wire are electrically isolated). The main characteristic of this coil is that at low frequencies it radiates uniform magnetic field over the whole length. As far as these conditions are met, we can write: Ls=µπD2N2 4h,(2) where µis the relative permeability of vacuum, Dis the diameter of the coil, Nis the number of turns and his the length of the coil Fig. 3. Pure inductor is a theoretical model, but we can use it after a small modification. The modification can be divided into two parts, frequency-dependent and frequency-independent. p h 2a D Fig. 3: Layout and dimensions of coil. At frequency independent modification, coefficient kLdescribes the irregularity of the field and is expressed in Eq. (3) [7], [8], [9], [10], [11]. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 41 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH kL=2h D· ln 4D h−1 2·1+0.393901 ·h D+ 0.017108 ·h D4 1+0.258952 ·h D2 +0.093842 ·h D2+ 0.002029 ·h D4−0.000801 ·h D6. (3) Then, an equation for inductance LScan be written according to Eq. (2). LS=µπD2N2 4h.(4) For real coils, it is needed to include the coefficient kS, which takes into account circular conductor crosssection and the coefficient kmfor the mutual inductance between the turns. ks=3 2−ln p a.(5) km= ln (2π)−3 2−ln (N) 6N−0.33084236 N −1 120N3+1 504N5−0.0011923 N7+0.0005068 N9. (6) L=Ls−µND 2 (ks+km).(7) Two other parasitic elements: the skin effect and proximity effect should be taken into consideration at high frequencies, respectively at frequency dependent modulation. The so-called internal induction, which is an imaginary contra equivalent of the skin effect, rapidly decreases with increasing frequency and is proportional to the length of the conductor, affects the calculation of the induction coil. The effect of the internal inductance, however, can be used only for short coils. Li= µ0δi  1−e(−"a 2δi#3,8)   1 3,8 4πa (1 −y)l, (8) where µ0is the permeability of vacuum, δidepth of penetration, athe radius of the conductor, ltotal length of the coil conductor. y=0.0239 1+1.67 (z0.036 −z−0.72)24.(9) z=a 2.552δi .(10) l=q(πND)2+h2.(11) The final formula to calculate the inductance of coil with all corrections is as follows: L=Ls−µND 2(ks+km) + Li,(12) where LSis inductance of the pure inductor, µ0is vacuum permeability, nis number of turns, ks,kmare correction factors and Liis internal coil inductance. So called Litzwire-high frequency cable is used to suppress the negative effects of frequency dependent part of resistance of a coil conductor in the high frequency systems. Its task is the suppression of skin effect and proximity effect [12]. High-frequency cable is made up of tangled thin insulated wires, the recommended diameter of which is: d≤2δ, (13) where dis the conductor diameter and δit is the depth of penetration. Litzwire should be used only for frequencies from 50 kHz to 3 MHz. If the two coils have the same radius, the same number of turns and are held in the same axis, their mutual inductance can be determined: M=µ0 D 2N2Zπ 0 cos x s2 (1 −cos (x)) + d D2dx. (14) Based on established parameters and relationships, the parameters of the being designed coil are calculated according to Tab. 2. Tab. 2: Calculated parameters of the designed coil. Par. Value Unit Describe D185 (mm) Coil average l60 (mm) Coil length a1.5 (mm) Wire average N6 (-) Numb. turns f300 (kHz) Frequency used in design p10 (mm) Pitch Φ1.06 (-) Proximity factor kL0.442 (-) C. f. inequalities field ks-1.34 (-) C. f. self ind. Of round wire km0.233 (-) C. f. mutual ind. Of round wire l3486 (mm) Physical length of were dmin <0.36 (mm) Recommended min. thickness cable wire NLW 32 (-) Num. of cable for litz-wire δl120 (µm) Penetration depth L9.34 (µH) Inductance R0.062 (Ω) Serial AC resistance C1020 (pF) Parasitic capacitance Q174 (-) Quality factor frez 22.652 (MHz) Self res. freq. of coil For the calculation of the mutual inductance Mthe Eq. (12) was used and the results are shown in Tab. 3. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 42 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH Tab. 3: Mutual inductance of two symmetrical coils. Distance (cm) 5 10 15 20 25 M(µ) 3.44 1.465 0.726 0.398 0.237 k(-) 0.368 0.157 0.078 0.043 0.025 Where kis the coupling factor which is calculated by the following formula: k=M √L1L2 .(15) 5. Calculation of Parameters and Circuit Elements For the analysis of topologies, it has been chosen a suitable test topology with series serial connection of the compensation capacitor, Fig. 4. Here the choice of topology determines the further calculations of elements and circuit parameters. DC C1L1 M L2C2 R Fig. 4: Serial capacitive compensating of capacitor. The following equation was used for the calculation of the transformation ratio: n=AV Uin 2 Uout = Uin 2 Uout |AV=1 = 2.5.(16) Transformation ratio between primary and secondary coil was chosen to 1 to simplify the design and the desired output voltage has been achieved with a frequency control [13]. Similarly, this solution is preferred in light of the coil structure and further design of the system. Value of compensation capacity C2of the secondary side is calculated from equation: C2=1 ω2 0L2 = 31.16 nF.(17) Next, the value of primary side compensation capacity C1was calculated: C1=L2C2 L1 = 31.16 nF.(18) Capacity values are rounded to the next higher production series C1,C2= 33 nF. Next, the efficiency for a given topology at a distance of 5 cm was calculated, where M= 3.44 µH. η=RL (RL+R2)1 + R1(R2+RL) ω2M2.(19) Then, the quality factor Qof transmitter (primary side) and of the receiver (secondary side) may be calculated: Q1=L1RL ω0M2= 3.43.(20) Q2=ω0L2 RL = 2.15.(21) Calculated values are decisive, but their values are only theoretical. The main reason is that the calculation was provided only for the resistance of the coils. Wire resistance, capacitors resistance and influence of disturbing elements have been neglected. 6. Time Dependent Analysis Mutual induction was calculated for five distances (Tab. 4) and used as a variable parameter. The simulations were solved for two cases. The first was the measurement of output voltage, current, and efficiency at a resonant frequency. The second simulation was aimed at changing the frequency and the achievement of constant output parameters Vout = 20 V and Iout = 2.5 A, in order to achieve the desired output power of 50 W. Voltage and current waveforms at individual components are displayed for one selected value of coils distance (5 cm). Tab. 4: The simulation results for a constant frequency at Av= 1 s. Equivalent circuit K_linear Distance (cm) U (V) I (A) η (%) U (V) I (A) η (%) 5 42 4.84 80 46.7 5.5 83 10 33.6 3.95 56 34 4 56 15 27.8 3.26 26 30 3.5 26 20 19.5 2.35 10 18.5 2.1 9 25 12.8 1.42 3 10.8 1.25 3.5 The value of resonance frequency was 286 675 Hz. From the previous simulation and the voltage transmission characteristics, it is known that a voltage gain is equal to 1 then. However, the operation mode of switching transistors at this point is not ideal and the suitable operating mode of switching at zero voltage (ZVS) is above the resonant frequency. For best results, the range from 286 kHz to 296 kHz was chosen, which is close to the resonant frequency and for the nine values the parametric simulation was c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 43 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH 5 10 15 20 25 0 20 40 60 80 100 η = f(D) D (cm) η (%) Equivalent circuit K_linear Fig. 5: Dependence of efficiency on the distance to the constant frequency. 5 10 15 20 25 0 10 20 30 40 50 U = f(D) D (cm) U (V) Equivalent circuit K_linear Fig. 6: Dependence of secondary voltage on the distance to the constant frequency. performed from which the most suitable frequency was determined in terms of efficiency. It was 290 kHz. Maximum efficiency was 83 %for the simulation model using K_linear block. The difference between using a transformer equivalent circuit and K_linear block is minimal, so the results can be considered correct. The voltage and current waveforms at each component are displayed for 5 cm value. Fig. 7: The time waveforms of the voltage Uds and current Id of transistor T1for 5 cm. Tab. 5: Simulation results for the constant Uout and Iout. ZVS ZCS Distance (cm) η(%)f(Hz) η(%)f(Hz) 5 73 326250 30.5 196078 10 53 305510 26 229357 15 26 294117 11.5 251889 20 8.7 289885 5.2 265252 25 3 287356 2.5 277777 Fig. 8: The time waveforms of the voltage at resonant elements of transmitting side for 5 cm distance. Fig. 9: The time waveforms of the voltage and current at the load without a rectifier bridge for 5 cm distance. 5 10 15 20 25 0 20 40 60 80 η = f(D) D (cm) η (%) ZVS ZCS Fig. 10: Dependence of the effectiveness on the distance for constant Uout and Iout. 5 10 15 20 25 150 200 250 300 350 f = f(D) D (cm) f (kHz) ZVS ZCS Fig. 11: Dependence of Tws change on the distance for constant Uout and Iout. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 44 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH As the transformation ratio of coils was 1:1, the change in the output voltage and current was ensured with the change in switching frequency. The Tab. 5 shows that zero voltage switching (ZVS) is more preferred in terms of efficiency than switching at zero current. From the measured values it is confirmed that the system is more sensitive to changes of frequency at a longer distance. The chart of frequency dependence on the distance (Fig. 10) shows that with the increasing distance it is necessary to approach to the resonance frequency to obtain a sufficient gain. 7. Experimental Verification on Physical Model We have created a physical model to verify the correctness of the designed solutions on the basis of theoretical analysis and simulation analysis in the previous chapters. Design of physical model is based on several parts: on the choice of topology from theoretical documents, on the type of circuit power supply, on choice of switching transistors, suitable capacitors, on construction of transmitter and receiver coils. The whole system is divided according to the block diagram Fig. 7, which was created in the theoretical design of the system. A half bridge connection of transistors is used as a DC/AC inverter similarly to simulation model. For this purpose, the wiring on the universal board for a half bridge circuit was used. Transistors FDPF17N60NT are used for switching. Their selection has been made on the basis of simulation analysis, from which we see that the transistors current Idat lower distances is 8 A and it is growing with increasing distance. The transistors are suitable for the maximum allowable voltage. Tab. 6: Basic parameters. Id17 A Uds 600 V Rds(on) 340 mΩ The physical model was powered by a DC system source Agilent N5771A. During the design, it was considered a production series of capacitors MKP or MKT having low ESR values. As in the simulations, in the experimental physical model 33 nF value was used for the transmitter and the receiver side too. To reduce the voltage and current load, the capacitor of transmitting (primary) side was made up of series parallel capacitors Fig. 12. For the proposed system, two identical coils were made, the dimensions of which can be found in Section 5. The coil design. Wire of coil is formed of 32 tangled thin insulated wires with a diameter of 0.18 mm. A non-conductive material (extruded polystyrene) was used as a frame of coil. The calculated inductance value in the design of coil at the designed frequency of 100 kHz was L= 9.34 µH and its quality factor Q= 94.7. The RLC meter was used to verify the design. Following parameters were measured at the 100 kHz. L1= 9.57 µH, Q1= 69 and L2= 9.12 µH, Q2= 59. The resulting values are influenced mainly by certain construction elements such as cable lengths, the tendency of individual turns, and by the fact that the resulting coil is not a perfect circle. However, the goal was achieved and the difference between the calculated and measured inductance is within a standard. Fig. 12: View of the series-parallel connection of capacitors - 33 nF. Fig. 13: View on a designed coil. The output rectifier was connected as bridge rectifier. It was designed from Schottky diodes STPS10H100CT due to the high working frequencies. Used load had only resistive character. It was built of four non-inductive resistors connected in parallel and its resulting value was 8.25 Ω. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 45 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH 8. Measurements on Physical Model Created physical model for the wireless transmission of electrical energy has been subjected to measurement. Time courses were recorded on an oscilloscope Tektronix TDS 3024B that allows the storage of scanned waveform in the data file. The current probe Tektronix TCP A306 and the differential voltage probe HZ100 HAMEG were applied for taking the time waveforms of voltage and current on the load. The resulting waveforms were processed in a spreadsheet program and graphically displayed. The resulting measured values of voltage and current on the load were truncated for one period for graphical representation. The resulting graph was interpolated by trend line formed with the polynomial of the sixth degree because of variability and large amounts of data. The equation of the trend line is shown in the Fig. 14 where yUis an equation for voltage and yIis an equation for current. For each measurement input, output power and efficiency were then calculated. An example calculation is measuring for distance 5 cm between the coils. Fig. 14: Dependence of output voltage and output current for a distance of 5 cm at constant switching frequency. Tab. 7: Measurement at a distance of 5 cm. Uin (V) Pin (W) IM (A) ϕ () Iin (A) UM (V) T (µs) η () 99.9 67.93 3.43 0 0.68 27.92 3.4388 Pout =1 TZT 0 UMsin(ωt)·Imsin (ωt +ϕ)dt. (22) Pout =1 3.4388 ·10−6 ·ZT 0 27.92 ·sin (2π·290799 ·t) ·3.43 ·sin (2π·290799 ·t)=4. (23) η=Pout Pin =47.88 67.93 = 0.705.(24) Tab. 8: Measurement at a distance of 10 - 20 cm. Measurement at a distance of 10 cm Uin (V) Pin (W) IM (A) ϕ () Iin (A) UM (V) T (µs) η () 99.9 63.94 1.83 13.49 0.64 14.74 3.43 0.21 Measurement at a distance of 15 cm Uin (V) Pin (W) IM (A) ϕ () Iin (A) UM (V) T (µs) η () 99.9 62.94 1.05 4.22 0.63 8.04 3.43 0.08 Measurement at a distance of 20 cm Uin (V) Pin (W) IM (A) ϕ () Iin (A) UM (V) T (µs) η () 99.9 62.94 0.63 1.51 0.63 4.8 3.43 0.02 9. Conclusions Design of systems for wireless transmission is currently promising area of research and development, in respect of the wide range of applications where it is possible to use this technology. For the design and construction today there is still no strict procedure for achieving the desired resultant parameters, therefore solving of the given issue is not uniform, however, it is based on the phenomenon of magnetic resonance. In this paper we have set a target to design the PSS topology for wireless transmission of electricity with power up to 50 W. In the process solutions, we divided the work into three parts - theoretical, theoreticalpractical and practical. In them, we focused on important individual design analysis. In the theoretical part we went into the history of wireless transmission and we described the various options of wireless transmission of electricity. From this initial theoretical analysis, we decided for transmission by means of electromagnetic resonance. Explanation of important factors that enter into this type of transmission and influence it was a continuation of theoretical analysis. Theoretical-practical part was used for summarizing of possible solutions and for choosing of the appropriate system topology for wireless transmission. We have created a block diagram of the circuit and in the same part we have made the design of system and design of the coil. In the practical part we have created the simulation model first, which we used to predict the behavior of the designed system. It also gave us the c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 46 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH results that we then compared with measurements on a physical model. We have spooled two coils with identical dimensions for the experimental verification of the system. Compared to the theoretical calculation, deviation of their inductance was 3 %, but the quality factor was lower than 38 %. We have then created an experimental wiring according to the simulation model and we performed measurements for four distances. The highest achieved efficiency of 70 %was for 5 cm distance and the transmitted power was 48 W. We have met the main aim of this paper We have designed PSS topology for wireless transmission of electric energy with power up to 50 W and we have experimentally verified the solution’s correctness. Based on the knowledge obtained during paper solutions we have written some recommendations for further development of the system design for wireless transmission of electric energy. Acknowledgment The authors wish to thank to Slovak grant agency VEGA for project no. 1/0184/13 - Research of indirect computing algorithms and tools for evaluation of power loss in power electronic device’s component with support of physical model simulation postprocesing. References [1] PANKRAC, V. The Algorithm for Calculation of the Self and Mutual Inductance of ThinWalled Air Coils of General Shape With Parallel Axes. IEEE Transactions on Magnetics. 2012, vol. 48, iss. 5, pp. 1875–1889. ISSN 0018-9464. DOI: 10.1109/TMAG.2011.2177854. [2] WEISSTEIN, E. W. Elliptic Integral of the Third Kind. 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In: International Power Electronics, Electrical Drives, Automation and Motion. Ischia: IEEE, 2008, pp. 721–726. ISBN 978-1-4244-16639. DOI: 10.1109/SPEEDHAM.2008.4581085. [12] KOVACOVA, I. and D. KOVAC. Inductive Coupling of Power Converter’s-EMC. Acta Polytechnica Hungarica. 2009, vol. 6, no. 2, pp. 1–53. ISSN 1785-8860. [13] RADVAN, R., B. DOBRUCKY, M. FRIVALDSKY and P. RAFAJDUS. Modelling and Design of HF 200 kHz Transformers for Hardand Soft-Switching Application. Elektronika IR Elektrotechnika. 2011, vol. 110, no. 4, pp. 7–12. ISSN 2029-5731. DOI: 10.5755/j01.eee.110.4.276. About Authors Marek PIRI was born in Sahy, Slovak Republic. He graduated study at University of Zilina (2006). c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 47 POWER ENGINEERING AND ELECTRICAL ENGINEERING VOLUME: 14 |NUMBER: 1 |2016 |MARCH Nowadays study at Ph.D. grade at Department of Mechatronics and Electronics at University of Zilina. He is interesting in the field of power electronics-switch mode power supplies, simulations, design of power supplies. Pavol SPANIK graduated at University of Transport and Communications in Zilina (1978), in the field of Electrical traction and energetics in transport. Nowadays works at Department of Mechatronics and Electronics of Faculty of Electrotechnical Engineering at University of Zilina. He is interested in the field of power electronics, mechatronics and control systems. Michal FRIVALDSKY was born in Stara Lubovna, Slovak Republic. He graduated study at University of Zilina (2006). He finished his Ph.D. Study in the field of power electronics at the University of Zilina (2009) and became assoc. prof. in 2014. Nowadays he works at the Department of Mechatronics and electronics, Faculty of Electrical Engineering, at the University of Zilina. His research interests include power electronics, simulations (FEM, time-domain, multilevel) and power converters optimization, design and application. Anna KONDELOVA was born in Trstena, Slovak Republic. She graduated at Slovak Technical University in Bratislava (1983). She finished her Ph.D. study in the field of Process Control at the University of Zilina (2013). Nowadays she works at the Department of Mechatronics and Electronics, Faculty of Electrical Engineering, at the University of Zilina. Her research interests include programmable circuits, electronics, and simulations. c 2016 ADVANCES IN ELECTRICAL AND ELECTRONIC ENGINEERING 48