In recent years, EVs have become increasingly popular around the world, and research on wireless power supply technology for EV driving has progressed accordingly. In order to realize this wireless power transfer system for EVs on a real scale, it is necessary to lengthen the power transfer section per power transfer unit and to secure a sufficient power transfer section with a small number of power transfer units as a whole. To solve this problem, the single coil method, in which one coil is lengthened, has been proposed. However, as the coil length increases, resistance loss increases and efficiency decreases. Therefore, a repeater coil system has been proposed. In this in-transit power-feeding system, it is necessary to consider transient phenomena when the EV to be fed deviates from the feeding lane. Therefore, we propose a new repeater coil configuration on the transmission side that assumes a no-load condition. In this study, a new repeater coil system combining rectangular coils and orthogonal coils is proposed, which enables stable feeding of power to EVs by rectangular coils and response to transient phenomena. A comparison was also made with the single coil method that has been studied previously.

In recent years, electric vehicles (EVs), which do not emit harmful substances such as carbon dioxide while driving, have been gaining popularity around the world. EVs have long been known for their short cruising range, but one solution to this problem is the use of large-capacity batteries, which enable EVs to travel long distances on a single charge. However, this has also exposed new issues, such as higher vehicle prices and longer charging times. In particular, the recharging time is longer than that of conventional vehicles, which can be refueled in a few minutes, requiring the vehicle to remain stationary for a long period. In addition, most existing EVs use a wired power supply with a charging plug, but there are still issues to be solved, such as periodic maintenance and the risk of electric shock in the event of rain. As a solution to this charging method, a wireless power supply during vehicle stops has been proposed, which has recently been introduced to the market. Although this method improves maintainability and safety, it has yet to solve the problem of long charging times.

As a solution to these problems, a system that provides wireless power to a running EV has been proposed.1 Unlike power charging while the vehicle is stationary, this system differs from power charging while the load EV is moving at high speed, so the length of the charging lane must be long enough. However, by transmitting the power necessary for driving in a wireless manner, not only can the vehicle travel long distances with a small battery, but the charging time after the vehicle has stopped can be reduced. The single-coil method, in which a single coil connected to a power source is extended, is commonly used, but in order to implement this system, it is necessary to increase the length of a single coil or to arrange several coils connected to a power source. However, extending the coil length increases the resistance loss. Also, if a number of short coils are arranged in a row, the number of power supplies used will increase, and the cost of infrastructure construction will rise. The repeater coil method has been proposed as a way to solve all these problems at once. Although this method has been reported in other literature2–5 to be an effective means of extending the feed section per feed unit, several issues remain regarding its practical application. In addition, EVs do not necessarily deviate from the feed lanes, and transient phenomena must be considered in such cases.6,7 When the coupling coefficient between the transmitter and receiver is reduced, a large current or voltage is applied to the coil if the input impedance on the transmitter side is low, leading to coil damage. In this study, a new repeater scheme that takes into account the no-load condition is proposed, and a half-scale model for a full-scale system is developed and compared with a single-coil scheme, which is a common feeding scheme.

The wireless power charging system for EVs enables power to be supplied while the EV equipped with a power receiving coil passes through a power supply lane with a power transmission coil installed on the road surface. Compared to systems that supply power while the vehicle is stationary, the in-transit wireless power supply system needs to be installed in longer sections because EVs move at high speeds. However, longer power-feeding sections increase the number of power-feeding devices required, which increases the cost of infrastructure development. A single coil system, which attempts to extend the feeding section by extending a single coil connected to the power source, has been proposed, but the longer the length of a single coil, the greater the resistance loss and the lower the power-feeding efficiency.

The repeater coil method has been proposed to solve the above problem. Figure 1(a) shows a schematic diagram of the repeater coil system. This is an innovative system in which many LC resonant coils independent of the power supply are placed adjacent to the transmission coils connected to the power supply, thereby extending the power supply section per power supply unit. However, in a previous study,3,4,8–11 it was reported that there are some locations where power can hardly be supplied depending on the number of coils placed on the power transmission side. This phenomenon is confirmed at the even-numbered coils in the case of an odd number of coils and at the odd-numbered coils in the case of an even number of coils.

FIG. 1.

(a) Schematic diagram of the conventional repeater coil system, (b) Schematic diagram of the newly proposed repeater coil system, (c) Input impedance vs. coupling coefficient.

FIG. 1.

(a) Schematic diagram of the conventional repeater coil system, (b) Schematic diagram of the newly proposed repeater coil system, (c) Input impedance vs. coupling coefficient.

Close modal

The coil configuration of the conventional repeater coil system has the problem of a wide range of lanes that are not required for power supply due to the above phenomenon. Therefore, it is necessary to narrow the range of lanes where power can hardly be supplied and to expand the range of lanes where power can be supplied. In addition, it is necessary to assume the case where an EV deviates from a power supply lane and enters a no-load state. In the no-load state, the coupling coefficient between the power transmission and reception decreases. Therefore, if the input impedance is low, large currents and voltages are applied to the transmission side in a system designed for several kW, leading to damage to the power supply and coils. Therefore, we propose a new repeater system as shown in Fig. 1(b). This system uses orthogonally wound coils (Lin and L2) and rectangular coils (L1 and L3). In the present system, the odd-numbered coils, which have an even number of coils on the transmission side and thus significantly lower power-feeding efficiency, are orthogonally wound coils that are small in size and highly coupled with the rectangular coils. For the even-numbered coils, which are capable of supplying power, rectangular coils are used, which have been shown to be effective for stable power supply in previous studies.9  Figure 1(c) shows the relationship between the coupling coefficient and input impedance between the transmitter and receiver. It can be seen that the input impedance of the newly proposed repeater system is high in the low coupling state.

In this experiment, a half-scale model was created to compare the two schemes, the newly proposed repeater scheme, and a common single-coil scheme, as expected in a real scale. The coil configurations of each method used in the experiments are shown in Fig. 2. The repeater coil method (Fig. 2(a)) uses two orthogonal coils and two rectangular coils. The overall length was 5.3 m. Only Lin is connected to the power supply; the other coils are independent. Ferrites were placed on top of each orthogonally wound coil to enhance the coupling between the rectangular coils. L2 is divided into L2a and L2b, but they are one coil connected by a single Litz wire. The single coil system (Fig. 2(b)) was created using only one rectangular coil, with an overall length of 5.3 m. A spiral coil was used for the receiving coil, and its outer diameter was 0.25 m, the same as the width of the rectangular coil. The parameters of each coil are shown in Table I. Note that the transmission coil of the single-coil system is denoted as Lin’.

FIG. 2.

(a) Schematic diagram of the repeater coil system used in the experiment, (b) Schematic diagram of the single coil system used in the experiment.

FIG. 2.

(a) Schematic diagram of the repeater coil system used in the experiment, (b) Schematic diagram of the single coil system used in the experiment.

Close modal
TABLE I.

Parameters of each coil.

Coil parametersLinL1L2aL2bL3LinLR
Inductance (µH) 49.05 6.43 51.50 49.97 6.18 11.74 77.99 
Internal resistance (mΩ) 37.27 18.10 33.50 35.61 20.50 30.74 54.61 
702.92 188.82 820.97 749.50 160.99 204 762 
Coil parametersLinL1L2aL2bL3LinLR
Inductance (µH) 49.05 6.43 51.50 49.97 6.18 11.74 77.99 
Internal resistance (mΩ) 37.27 18.10 33.50 35.61 20.50 30.74 54.61 
702.92 188.82 820.97 749.50 160.99 204 762 

In this experiment, the current values flowing through the transmitter coil under no load conditions were first compared between the two systems. Experimental conditions were set to a frequency of 85 kHz, a gap of 75 mm, and a load of 10 Ω. The input voltage was set to a value of 30 W when the receiving coil was installed. Therefore, the input voltage was 18 V for the repeater coil system and 6.5 V for the single coil system. Next, the power supply efficiency was measured when the receiving coil was moved from end to end of the transmitter coil under the same conditions as before. The coupling coefficients of the transmitter and receiver coils are shown in Table II. Note that the coupling coefficient between Lin′ and LR is kinR′.

TABLE II.

Coupling coefficient between transmission and reception.

k1Rk3RkinR
Coupling coefficient 0.058 0.059 0.043 
k1Rk3RkinR
Coupling coefficient 0.058 0.059 0.043 

The results of each experiment are shown in Fig. 3. Figure 3(a) shows that the repeater coil method reduces the current flowing in the power transmission side at no load. The power factor at this time is 0.82 for the repeater coil method and 0.32 for the single coil method, indicating that a further difference in current values between the two methods may be generated when the power factor is improved. Next, Fig. 3(b) shows that the newly proposed repeater method provides a stable power supply and narrows the section where power cannot be supplied. Comparing the average efficiencies of the two methods, the repeater coil method and single coil method have slightly lower average efficiencies (62.1% and 64.9%, respectively), but the newly proposed repeater coil method, which can reduce the coil current when no load is applied, is considered superior for actual-scale applications.

FIG. 3.

(a) Current flowing through each coil at no load, (b) Feeding efficiency of each method, (c) Current value of each coil at 30W power supply, (d) Schematic diagram of a system applying power feeding characteristics.

FIG. 3.

(a) Current flowing through each coil at no load, (b) Feeding efficiency of each method, (c) Current value of each coil at 30W power supply, (d) Schematic diagram of a system applying power feeding characteristics.

Close modal

Figure 3(c) shows the current values flowing through each coil when 30 W is supplied. The single-coil system consists of a single transmission coil, so the current value remains almost constant. However, in the repeater coil method, the value of I2 changes significantly when the receiving coil is on L1 and when it is on L3. For full-scale implementation, both methods are expected to use multiple power transmission units to construct a power feed lane. In this case, it is necessary to switch the coil to be used using a vehicle detection system, etc. However, considering the EV's running speed, switching must be performed at high speed, and there is concern about damage to the transmission-side coil due to transient phenomena. Therefore, we propose a vehicle detection system that uses the aforementioned change in L2 current value. A schematic diagram of the system is shown in Fig. 3(d). Unit 1 and Unit 2 are each equipped with a current monitoring system. First, when the power receiving coil is on L1 of Unit1, the switch of Unit2 is turned off. Then, Unit 2 is turned on when the receiving coil moves to L2 and the current value of L2 exceeds the set threshold value. In this case, even when there is no load, a large current does not flow to the transmission coil because Unit 2 uses the newly proposed repeater method. Therefore, Unit 2 can be turned on in advance and the switching time lag can be reduced.

In this study, we approached two of the issues that need to be addressed in wireless power transfer systems for EVs in motion: reducing the number of power sources used when building the infrastructure and transient phenomena when EVs deviate from their original course of operation. The newly proposed repeater coil system was compared with the commonly considered single coil system, and its superiority was confirmed.

We plan to conduct full-scale experiments using actual EVs. We are also planning to study the power supply for multiple vehicles.

The authors have no conflicts to disclose.

A. Saito: Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Y. Oishi: Investigation (equal). S. Miyahara: Supervision (equal). F. Sato: Formal analysis (equal); Project administration (equal); Supervision (equal). H. Matsuki: Supervision (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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