Developing a hybrid wireless power transfer system for electric vehicles

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Introduction

Analysis of storages for contemporary electric vehicles has identified the main disadvantages of batteries, including low energy density, power, and performance with increased current and low ambient temperature [1]. As a result, the electric vehicle has a range several times less than that of a similar wheeled vehicle with an internal combustion engine. In addition, the charging time of a traction battery is several hours, while it takes about a minute to fill the fuel tank of a passenger car. In the premises, the problem of developing an alternative traction voltage system is highly relevant.

A system of wireless power transfer to an electric vehicle could be a promising solution. Table 1 summarizes research and development related to wireless power transfer systems for electric vehicles [2–11].

 

Table 1. Some developments of wireless power transfer systems for electric vehicles

Табл. 1. Некоторые разработки в области бесконтактного электроснабжения электромобилей

R&D	Country	Year	Vehicle type	Power, kW	Efficiency, %	Frequency, Hz
Bombardier PRIMOVE	Canada	2018	Tram, electric bus	200	85	20
KAIST OLEV 4G	Republic of Korea	2015	Electric bus	100	80	20
Qualcomm FABRIC	USA	2017	Electric vehicle	20	85	85
ORNL	USA	2022	Tractor, electric bus, electric vehicle	200	85	85
Electreon	Israel	2023	Tractor, electric bus, electric vehicle	70	85	85
Toyota	Japan	2020	Electric bus, electric vehicle	10	85	85

 

Magnetic coupling resonant wireless power transfer system with one primary coil

Fig. 1 shows the circuit of an electric vehicle’s wireless power transfer system with one primary coil [1].

 

Fig. 1. Circuit diagram of an electric vehicle’s wireless power transfer system with one primary coil.

Рис. 1. Электрическая схема системы бесконтактного электроснабжения электромобиля с одной первичной обмоткой.

 

To compensate for reactive power, capacitors C_p and C_s and inductors L_s are used in the circuit. The primary coil L₁ is installed under the road surface and has inductive coupling with the secondary coil L₂ installed in the electric vehicle. The voltage to the primary winding is applied from the power source U1. The output voltage is converted by a single-phase full-wave rectifier to charge the traction battery.

The highest system efficiency is achieved by transferring power at the resonant frequency:

${\omega }_D=\frac{1}{\sqrt{L_{1s}{C}_{1p}}}=\frac{1}{\sqrt{L_{2s}{C}_{2p}}}$. (1)

Inductivity of compensating coils are determined based on the inductivity of primary and secondary coils [12]:

$L_{1s}={\alpha }_1{L}_1$; $L_{2s}={\alpha }_2{L}_2$, (2)

where α is the design factor randomly selected in the range 0<α≤1.

Capacity of series capacitors:

$C_{1s}=\frac{1}{{\omega }_d^2{L}_1(1-{\alpha }_1)}$, $C_{2s}=\frac{1}{{\omega }_d^2{L}_2(1-{\alpha }_2)}$. (3)

The inverter output voltage characteristic:

$U_1=\frac{2\sqrt{2}}{\pi }{U}_{\mathrm{1,}DC}\sin \left(\frac{\pi D}{2}\right)$, (4)

where D is the filling factor of the inverter.

The applicable rectifier input voltage is determined by formula (5):

$U_2=\frac{2\sqrt{2}}{\pi }{U}_{\mathrm{2,}DC}\approx \mathrm{0,9}{U}_{\mathrm{2,}DC}$. (5)

Primary and secondary currents:

$I_{L1}=\frac{U_1}{j{\omega }_d{\alpha }_1{L}_1}$, $I_{L2}=\frac{U_2}{j{\omega }_d{\alpha }_2{L}_2}$. (6)

Inverter output current:

$I_1=\frac{M{U}_1}{{\omega }_d{\alpha }_1{L}_1{\alpha }_2{L}_2}$. (7)

Rectifier output current:

$I_2=\frac{M{U}_2}{{\omega }_d{\alpha }_1{L}_1{\alpha }_2{L}_2}$. (8)

Thus, the transferred power can be calculated as:

$P_2=\frac{M{U}_1{U}_2}{{\omega }_d{\alpha }_1{L}_1{\alpha }_2{L}_2}$. (9)

The main advantages of single primary coil systems are ease of control and high specific primary power. The disadvantages include low specific secondary power.

Fig. 2 shows the circuit of an electric vehicle’s magnetic coupling resonant wireless power transfer system with multiple primary coils [1].

 

Fig. 2. Circuit diagram of an electric vehicle’s magnetic coupling resonant power transfer system with multiple coils.

Рис. 2. Электрическая схема резонансной системы электроснабжения электромобиля с несколькими первичными обмотками.

 

Magnetic coupling resonant wireless power transfer systems with multiple primary coils

In this system, inverters and compensators are connected in parallel, allowing to implement a system with multiple primary coils. Voltage is separately supplied to each coil. In all other respects, the design and the operating principle are similar to the system discussed above.

The output power:

$P_2=\left|{\sum }_{n=1;3;\mathrm{5...}}\frac{M_{n2}{U}_n{U}_2}{{\omega }_d{\alpha }_1{L}_1{\alpha }_2{L}_2}\right|$. (10)

Analyzing (10), we may conclude that the transferred power is determined as the modulus of the sum of powers of primary coils. Thus, turning on any primary coil does not necessarily lead to increased power in the secondary one as the magnetic induction vector may be directed against the main flux. In this case, the inverter controlling the negatively coupled coil must be switched off.

Fig. 3 shows the temporal variation curves of system efficiency [12]. The characteristic indicates that the system efficiency is very high (about 92%).

 

Fig. 3. Temporal variation curves of system efficiency: DC to DC efficiency (%) is the energy transfer efficiency from inverter to battery in percent; time t, (s) is the time in seconds; η_DC-DC is the efficiency; moving average of η_DC-DC is the average efficiency.

Рис. 3. Кривые изменения КПД системы во времени.

 

The main advantage of the system is its high efficiency. Disadvantages include a complex control system and low specific primary power.

Electric coupling wireless power transfer system

The above-mentioned vehicle power transfer systems include the air-core transformer. Thus, their operation is based on Faraday’s law of induction:

$\epsilon =w\frac{d\Phi }{dt}$, (11)

where w is the number of coil turns; Ф is the magnetic flux (Wb).

This section discusses power transfer using an air capacitor.

The electric coupling power transfer system [13] is shown in Fig. 4.

 

Fig. 4. Electric coupling power transfer system of an electric vehicle: 1: aelectric vehicle; 2: power supply; 3: secondary coil of transformer; 4: primary coil of transformer; 5, 8, 9: power supply cables; 6: neutral plate; 7: switching converter; 10: bearing surface; 11: on-board energy storage; 12: air gap; 13: wheel.

Рис. 4. Устройство системы электроснабжения транспортного средства методом электростатической индукции: 1 — электромобиль; 2 — источник электроэнергии; 3 — вторичная обмотка трансформатора; 4 — первичная цепь трансформатора; 5, 8, 9 — сетевые кабели; 6 — нейтральная пластина; 7 — преобразовательно-коммутационная аппаратура; 10 — опорная поверхность; 11 — бортовой накопитель энергии; 12 — воздушный промежуток; 13 — колесо.

 

The power supply feeds a transformer connected to power supply cables. An electrostatic field is generated between them and the neutral plate of the capacitor. To match the input voltage and the charging voltage of the on-board storage, a switching converter is used.

Power transferred through the air capacitor to the vehicle:

$P=2K_0\pi f_{0}C{V}^2$, (12)

where ƒ₀ is the resonance frequency (Hz); С is the capacitance of the air capacitor (Ф); V is the line voltage (V); К₀ is the coupling factor.

Advantages:

• High efficiency;
• High controllability of the electric vehicle;
• Able to supply other vehicles, including those with dielectric propellers.

Disadvantages:

• Large dimensions;
• Low energy density;
• High power supply requirements.

Developing a structural diagram of a traction voltage system of an electric vehicle with a hybrid wireless power transfer system

The review and analysis of various wireless power transfer systems and relative R&D showed multiple prospects for using inductive coupling systems with one primary coil. Their advantages include small dimensions, high energy density, possible use of power supplies with low parameters, ease of operation, and high efficiency.

Electric traction systems for vehicles have some advantages [14]. However, the specific energy of traction batteries does not exceed 576 J/g, which is about 100 times lower than that of gasoline or diesel fuel. In the premises, electric vehicles are equipped with batteries weighing about 30% of the vehicle’s gross weight with the range equal to a similar vehicle [1]. On the contrary, electric vehicles are now mainly used in cities, where the driving cycle involves constant alternation of acceleration, steady movement, braking, and stopping.

The cycle provided by UNECE Regulation No. 83 [14] is taken as the fuel efficiency and exhaust toxicity test cycle. According to the rules, the cycle consists of two parts, namely urban and highway. The research and calculations will be based on the urban electric vehicle; thus, it is advisable to use the first part of the cycle only (Fig. 5). The total duration of the urban driving cycle is 195 s.

 

Fig. 5. Estimated urban driving cycle as provided by UNECE Regulation No. 83.

Рис. 5. Расчётный городской цикл движения согласно Правилам ЕЭК ООН № 83.

 

Let us analyze the urban driving cycle. The distance between stops is small (about 300–500 m) as there are multiple traffic obstacles, including traffic lights, pedestrian crossings, speed bumps, etc. In the premises, an electric vehicle requires a relatively small amount of power to cover the distance from stop to stop. However, frequent acceleration requires high power consumption. Overcoming the vehicle’s inertia requires up to 80% of the storage capacity [1]. Therefore, power storages shall have a high specific power to have acceptable weight and dimensions.

It is a well-known fact that capacitive storages (CS) have high specific power, but relatively low energy density, while properties of chemical accumulators are exactly the opposite [1]. In addition, electric double layer capacitors offer simple design, ease of use, safety, environmental friendliness, and, most importantly, the ability to quickly accumulate and release stored energy. CS may be operated at low below-zero temperatures and withstand deep discharges, overcharges, and short circuits. In particular, molecular energy storages (MES) have more than 500,000 battery cycles, which is three orders of magnitude longer than that of batteries [15–18].

Fig. 6 shows a traction voltage system for an electric vehicle with a hybrid wireless power transfer system developed by the Trucks Laboratory (Land-Based Vehicles Department) of Moscow Polytechnic University.

 

Fig. 6. The traction electric system of a prototype: 1: secondary coil; 2: capacitor; 3: rheostat; 4: rectifier; 5: traction motor; 6: axle drive.

Рис. 6. Система тягового электрооборудования ходового макета: 1 — вторичная обмотка; 2 — конденсаторная батарея; 3 — реостат; 4 — выпрямитель; 5 — тяговый электродвигатель; 6 — главная передача.

 

The primary coil of the air-core transformer, to which the alternating supply voltage is applied, is located under the bearing surface. The primary current flow generates an alternating magnetic field that penetrates the turns of the secondary coil, which can be installed either inside the tires or at the bottom of the electric vehicle. In the secondary coil, a self-induction EMF is generated and supplied to the rectifier input. If the secondary coil is installed inside the buses, brushes and slip rings are used to supply the EMF. A single-phase full-wave rectifier is provided to convert alternating current to direct current and control the charging voltage of the capacitor. During acceleration or steady movement of the electric vehicle, the capacitor voltage is supplied to both the armature coil and the excitation coil of the traction motor. The DC motor torque is transferred to the drive wheel via the drive gear. During regenerative braking, the torque from the drive wheel is supplied to the DC machine using the above systems, and the DC machine is switched to the generator mode.

Fig. 7 shows the operational algorithm of the traction voltage system (TVS) in the urban driving cycle.

 

Fig. 7. Operational algorithm of the traction voltage system in the urban driving cycle: КБ: the capacitor is used for driving; PT: regenerative braking.

Рис. 7. Алгоритм работы СТЭО в городском цикле движения: КБ — использование энергии конденсаторной батареи для движения; РТ — рекуперативное торможение.

 

Conclusion

Thus, the study allowed to build a diagram of the traction voltage system of an electric vehicle with hybrid wireless power transfer system and its operational algorithm for the urban driving cycle based on UNECE Regulation No. 83.

This work will be continued in the form of the implementation of a mathematical model of the developed system. The introduction of traction electrical equipment system with inductive energy transfer and its capacitive storage will improve the energy efficiency of electric vehicles.

Additional information

Author contributions: E.M. Klimov: writing the text of the manuscript; A.M. Fironov: expert opinion, search for publications on the topic of the manuscript; R.A. Maleev: approval of the final version; S.M. Zuev: creation of images. All the authors approved the version of the manuscript to be published and agreed to be accountable for all aspects of the work, ensuring that issues related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Ethics approval: N/A.

Funding sources: No funding.

Disclosure of interests: The authors have no relationships, activities, or interests for the last three years related to for-profit or not-for-profit third parties whose interests may be affected by the content of the article.

Statement of originality: No previously obtained or published material (text, images, or data) was used in this study or article.

Data availability statement: The editorial policy regarding data sharing does not apply to this work as no new data was collected or created.

Generative AI: No generative artificial intelligence technologies were used to prepare this article.

Provenance and peer review: This paper was submitted unsolicited and reviewed following the standard procedure. The peer review involved two external reviewers, a member of the editorial board, and the in-house scientific editor.

作者简介

Egor Klimov

Moscow Polytechnic University

编辑信件的主要联系方式.
Email: klimov.mami@yandex.ru
ORCID iD: 0009-0004-9739-0267
SPIN 代码: 2759-7425

Lecturer at the Electrical Equipment and Industrial Electronics Department

俄罗斯联邦, Moscow

Anatoly Fironov

Moscow Polytechnic University

Email: a.m.fironov@mospolytech.ru
ORCID iD: 0000-0003-2683-9958
SPIN 代码: 8824-5702

Cand. Sci. (Engineering), Assistant Professor, Assistant Professor of the Land Vehicles Department

俄罗斯联邦, Moscow

Ruslan Maleev

Moscow Polytechnic University

Email: 19rusmal@gmail.com
ORCID iD: 0000-0003-3430-6406
SPIN 代码: 7801-3294

Cand. Sci. (Engineering), Assistant Professor, Professor of the Electrical Equipment and Industrial Electronics Department

俄罗斯联邦, Moscow

Sergey Zuev

MIREA-Russian Technological University; Central Research Automobile and Automotive Engines Institute NAMI

Email: sergei_zuev@mail.ru
ORCID iD: 0000-0001-7033-1882
SPIN 代码: 6602-6618

Cand. Sci. (Physics and Mathematics), Assistant Professor, Head of the Department for Training Highly Qualified Personnel and Continuing Professional Education, Assistant Professor of the Optical-Electronic Devices and Systems Department

俄罗斯联邦, Moscow; Moscow

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