Experimental installation of wireless power transfer system based on the series resonance technology

Received Apr 21, 2020 Revised May 10, 2020 Accepted Jun 17, 2020 In this work, we aim to install a wireless power transfer (WPT) system experimentally. Series resonance technology was used to achieve zerovoltage switching (ZVS). We investigated the impact of the primary and secondary resonance frequencies (fp and fd), and inverter frequency switching (fch) on the efficiency (β) and maximum transfer power in a WPT system based on the inductive wireless power transfer (IWPT) technology. An ultrasonic device was utilized as a generator to excite the coil at the primary side. The experimental outcomes showed that there is an optimum unlike fp and fd can be got to match fch. It was found also that there is a trade-off between the power supplied to the load (PRL) and DC-DC efficiency (β). At an air-gap of 5 cm, the obtained results are recorded as follows; the peak recorded system β is 62% that was obtained at fp=19 kHz, fd=fch=24 kHz that is corresponding to 101.88W of PRL; whereas the highest PRL resulted i.e. 244W when fp=19 kHz, fd =24 kHz, fch=21 kHz at 61% of β; in such case, the maximum β* PRL multiplication was achieved i.e. 149. Moreover, the coils’ misalignment was studied. The outcomes showed that the lateral misalignment has worst effect on the PRL and β than the air-gap. The experimental results were validated with simulation ones.


INTRODUCTION
The interest in the wireless power transfer (WPT) technique has considerably increased in recent few years, because of the safety and conveniences over the wired charging technology. In general, the WPT technology for electric vehicles (EVs) and smartphones battery charging includes inductive and capacitive wireless power transfer (IWPT and CWPT) [1][2][3][4][5].
In this work, only the IWPT technique is presented, where the power transmits from the primary coil to the secondary one electromagneticaly. The coupling interface surface area between the coils and air-gap are parameters impacting the coupling factor (k). This parameter has an effect on the whole system's efficiency and performance. The air-gap between the coils is the most challenging factor. As the air-gap rises, the magnetizing current, leakage inductance, and input VA rating rise accordingly. Consequently, the system cost increases, system efficiency diminishes due to losses increasing [6][7][8].
A resonance circuit technique has been utilized to enhance the IWPT system efficiency and performance [2,9]. The resonance circuit is nothing but capacitor(s) linked with the coils in series/parallel to compensate its inductance. At the primary side, the resonance circuit minimizes reactive power, improves the power factor, less VA ratings which result in less voltage stress on the system's components of the system, and low inverter cost. At the secondary side, the resonance circuit improves the system efficiency and maximizes the power delivered to the battery. For the EVs applications, the normally utilized inverter switching frequency within the range of 10-200 kHz. The switching frequency is mainly based on the air-gap and coils' size. The high-quality factor of the coils results in high power transmission efficiency [6,10,11].
However, to make the most of resonance technology, the resonance frequency at the primary (f p ) should be correlated with the resonance frequency at the secondary (f d ) side; and both f p and f d should be correlated with the inverter switching frequency (f ch ) to achieve zero-voltage switching (ZVS). Recently, in [2,12,13], in a bid to achieve the highest power transfer level, the f p and f d were set to be equal to the fch. While in [14], it was found that at fch equal to the fd, the maximum power transfer efficiency was achieved [14]. It was proven that the transmitted power is not constantly an optimum at resonance case [15]. The power transfer level is mainly based on the coupled coils' inductance and its quality factor, switching frequency, the square of input current and mutual inductance. However, there is no interest has given to discover an optimum f ch , f p , and f d to accomplish the highest power transfer level and efficiency [16,17]. In this work, the impact of various f ch , f p , and f d on the IWPT level and efficiency was studied; to get resonance circuits, resonance capacitors were utilised to adjust f p and f d . This aims to accomplish a maximum WPT efficiency and load power by discovering an optimum f ch , f p , and f d . Moreover, since the the coupled coils misalignment is a common problem in the IWPT systems, we carried out a study to impact of this issue on the DC-DC system efficiency (β) and resistive load power (PL).
The transferred power and efficiency in a WPT system basically rely on the resonance frequency [18,19], magnetic coupling factor, k, and mutual inductance, M, which are mainly based on the coil configurations and varied with the air-gap (d) and coils' misalignment [18][19][20]. In this work, the spiral coil configuration of copper wires was built experimentally to deliver power wirelessly to a resistive load through an air-gap of 5 cm. The misalignment issue was investigated as well.
The next sections of the paper are arranged as follows: Section 2 presents the WPT system, which includes the experimental setup, mathematical model as well as the considered coil configuration and misalignment between the inductively coupled coils. Section 3 explains the methodology used to implement the experimental work and how the data are collected. Section 4 illustrates the experimental outcomes and discussion. Section 5 presents the conclusion.

INDUCTIVE WIRELESS POWER TRANSFER SYSTEM
As mentioned earlier, the d between the coupled coils is playing a major role in any IWPT system performance. The self-inductances of the primary and secondary (L1 and L2) coils, k and M are parameters utilized to describe the inductive coupler; The relation between these parameters is expressed in (1) [20].

Experimental Setup
The adapted IWPT system includes an ultrasonic generator (see technical characteristics in Table) that was utilized to excite the primary coil; based on it favorable features among other topologies, the seriesseries (SS) resonance technology was utilized [2]. The experimental setup of the presented IWPT system is depicted in Figure 1. Where C p1 , C p2 , R 1 , and R 2 , are the equivalent parallel capacitances and the equivalent series resistances, respectively. From Figure 1, it can be noted that the ultrasonic generator includes the followings components: − Generator unit, − Remotely controlled panel which used for parameterization, − Adapter which is used for external control; − A half-bridge inverter which is made from IGBT transistors The f ch is controlleded by utilizing the handheld control unit which linked through RS485-232 adapter to the generator unit. The ultrasonic generator's technical characteristics are mentioned in Table 1 [21]. However, due to its most robust to the rotation misalignment and uniform flex distribution, the spiral configuration was adapted to create the coils: − The copper wire coils; − Capacitors (WIMA type) were utilized for the SS resonance technology, the f p and f d are calculated theoretically as in equation (2). − A full-bridge rectifier made of four power diodes of type Schottky (DSS 2x61-01 A). To soak up the generated heat and wasted it. The rectifier mounted over the heatsink with dimension 20x12.5x2.5 cm.

Mathematical model
To validate the empirical outcomes with the theoretical ones, the mathematical IWPT system model is presented here. The dependent voltage source was used to model the inductive coupler, the equivalent circuit of the IWPT system that mentions in section 2.1 is shown in Figure 2, where the output of the ultrasonic generator is modeled as a square voltage source with an RMS value of (u 1 ).
Based on Figure 2, the expression of the RMS input current at the primary circuit i 1 and RMS secondary current i 2 are described in equations (7) and (8), respectively.
From equations (7) and (8), the voltage gain (G v ) can be written as; However, the DC load power (P RL ) which delivered to load is described as follows;

Inductively coupled coils
Generally, the k is utilized to predict the IWPT performance [2,10,11]; The DC-DC efficiency can be computed as follows; β = (11) where the input DC power (Pin) was calculated using approximation formula: =the input power (AC)no-load losses (i.e. ultrasonic generator) (12) In this work, the well-aligned coils, as shown in Figure 3(a), air-gap fluctuation as well as the lateral misalignment, Figure 3(b), with L>0 between the coils were investigated.
Based on the equation (13), the k was measured experimentally as follows, − A function generator with a set voltage of V 1 was utilized to energize the primary coil; − Utilizing an oscilloscope, the induced EMF (V 2 ) was measured at the secondary coil.

MATERIALS AND METHODS
In this paper, an IWPT system was implemented experimentally. This work involves the study of the impact of the f ch, f p , and f d on the P RL & β and investigate the optimum β*P RL condition. The f ch is controlled using the ultrasonic generator's control unit. The f p and f d were controlled utilizing the resonance capacitors that are connected in parallel and/or series to obtain the desired resonant frequency. Based on the frequency characteristics, the f p and f d were computed utilizing the LCR HiTESTER of model HIOKI 3532-50.
The power analyzer connects the ultrasonic device to the 230 V/50 Hz of electricity plug. The function of the power analyzer is to compute the AC input electric power. The ultrasonic device's no-load losses were gauged; these losses were assumed to be the same at all loading conditions; this presumption was utilized to compute the input DC power (equation (12)).
A spiral shape was considered to constitute the coils: − The length each copper coil wire is 17 m, − The cross-section area of the copper wire is1.5 mm 2 , − r i =R i =2.5 cm, r o =R o =11.88 cm with 2 mm of coil pitch, and − The turns number of the coils is the same i.e. 34.
Utilizing digital RLC meter, the coils' parameters were determined. Several experiments were carried out at well-alignment coils at d=5cm with various f p , f d , and fch; the β was identified accordingly at each situation. Moreover, the impact of the d and L on the PRL was researched. It was presented a case to research the effect of fch on PRL. The imperical data were validated with the theoretical omes that was obtained based on the mathematical model implemented in MATLAB.

EXPERIMENTAL OUTCOMES AND DISCUSSION
This section shows the experimental outcomes which include the parameters of the coils and adjusting f ch , f p , and f d to achieve an optimum β and P RL . Based on the ultrasonic generator's technical characteristics (Table), only 15% of the device power was utilized with no-load losses 18W.

Coils parameters
The coils' resistance, inductance, and quality factor were determined experimentally by utilizing the digital RLC meter as shown in Table 2. The f p and f d were calculated utilizing the frequency characteristics in order to take into account the coils' distributed capacitances. The obtained outcomes showed that

Resonance
As mentioned in section 4.1, the f p and f d were regulated utilizing resonance capacitors. The of L 1 and L 2 at 1 kHz were assumed to have constant values at higher frequencies when the f p and f d computed. Therefore, for various f ch , at R L =32 Ω, d=5 cm, the impact of the f p and f d on the β was investigated, the outcomes are shown in Figure 4. As can be observed from Figure 4, the maximum achieved β at f ch of 28 kHz is 62% (at this efficiency the obtained P RL is 101.8 W) that was achieved at f p =19 kHz, f d =24 kHz. The next highest β i.e. 61% was obtained at f p =19 kHz, f d =24 kHz, and f ch =21 kHz (at this efficiency the obtained P RL is 244.13 W). Based on the obtained results, the maximum β values were occurred at f d ≈ f ch . At this condition, the power reflected from the secondary side to the primary side removed. The obtained outcomes are correlated with the ones presented in [14]. The maximum achievable P RL is 244.13 W was obtained at f ch > f p . At this condition, the ZVS occur since the inverter inductively loaded; this makes the losses of the halfbridge inverter are near to zero [14,24].  Figure 4. β against f ch for various f p and f d As it is known, high β with P RL is an eligible outcome. Therefore, the optimum β*P RL factor was studied. The obtained outcomes as shown in Figure 4 indicate that the peak value i.e 149 (P RL ≈ 244 W & β≈ 61%) accomplished at f p =19 kHz, f d =24 kHz, and f ch =21 kHz where f d ≈ f ch . The slight difference between f ch and f d because the resistive load made of copper wire that has a small inductance which effects the f d value. In other words, according to equation (2), the real f d is a bit below than 24 kHz i.e. f ch ≈ f d ; where at this condition zero (or very slight amount) reflected power obtained.
The frequency characteristics behavior of P RL and P in are analogous. Therefore, only the P RL is presented here. At f p =19 kHz, f d =24 kHz condition (peak β*P RL ), the impact of the f ch on the P L was investigated imperically when d=5 cm, P RL =32 Ω; the measure u 1 experiment values were used in the IWPT system's mathematical model to compute the P L for validation purpose, the outcomes are shown in Figure 5.
As can be seen, Figure 5, the P RL declines considerably as the f ch rise; for each situation, the peak P RL was resulted when f p a bit less than f ch . The difference between the experimental and simulation curves due to the losses of the ultrasonic generator which is not considered in the mathematical model. Figure 5. P RL against f ch at f p =19 kHz, f d =24 kHz

Misalignment
At peak β* P RL situation i.e. f p =19 kHz, f d =24 kHz, f ch =21 kHz, the air-gap and lateral misalignment variations impact on the P RL were researched; the outcomes are illustrated in Figure 6. As can be noted, Figure 6, the two curves have same behavior; where, as the d rises the PRL sharply get down. Similarly, as the L rises the PRL considerably decreases. However, to supply a fixed power of good quality and high efficiency the misalignment should be avoided. A mechanic or magnetic trap have been used by some IWPT systems to avoid the misalignment [25].

CONCLUSION
This work has researched the impact of the f ch , f p , and fd in an IWPT system on the P RL and β. The outcomes proved an optimum f p , fd, and f ch values are existe to achieve peak reachable β. Meanwhile, there are other f p , fd, and f ch values to achieve peak reachable P RL . The imperical outcomes determined that f ch and fd should have same values (or almost equal) with f ch >fd and both should be a bit more than f p to achieve peak β* PRL. The most we can talk about that these results highlight the enhancements in our work by studying the best f p and fd values to be tuned with the f ch as well as regulate the f p and fd in a bit to improve the β* P RL factor. The ultrasonic device with adjustable f ch has utilized in the primary side to supply its coil; while the f p and fd have been regulated by utilizing resonance capacitors. However, some ultrasonic generator limitations are worthy of note that the f ch cannot exceed the limit (17.5-28 kHz). The coils' misalignment has been researched as well; the outcomes identified that as the d has a more negative impact on PRL than L. However, the future work should include a study of higher frequency ranges, for example, 120 kHz.