Abdul Lateef Dar

Ue = Rated operating voltage of Power Battery
Cess = Total Capacity of Battery (Ah)
WIRELESS CHARGING SYSTEM FOR ELECTRIC TRACTORS BASED ON MAGNETIC COUPLING RESONANT
As a green agricultural machinery, electric tractor will be widely promoted and applied. Wired charging is prone to cause poor contact and leakage during plug charging process, which brings potential safety hazard. Wireless charging can avoid above problems.
Energy shortage and environmental pollution have become an important factor restricting sustainable development of human society. The use of green environmental protection energy will be a new trend of the development of agricultural machinery in the future. As a new green agricultural machinery, electric tractors will be widely popularized and applied, which not only could solve the problem of exhaust emissions

effectively, but also alleviated the shortage of fossil fuels in China. In recent years, some problems were existed in traditional wired power transfer mode, such as: wire occupies a large space resource; overlapping wire will generate much electromagnetic interference; electric spark of wiring would cause severe damage; specifications of charging line was inconsistency. At the same time, charging of cable is affected by the length of charging cable, and charging range of electric tractor is limited, which seriously affects the promotion and application of electric tractor. Wireless power transfer technology greatly has reduced the risk of electric shock and friction, and improved the flexibility of charging correspondingly, and its quality and volume have also been reduced.
Wireless power transfer technology is based on electromagnetic coupling principle and induction, which using modern power electronic energy conversion technology, magnetic field coupling technology, high-power frequency conversion technology (including resonance conversion technology and electromagnetic compatibility design technology, etc.). With the help of modern control theory and methods, technology of non- contact power transmission of electric energy from stationary grid to mobile electrical equipment was realized. At present, wireless power transfer technology is mainly divided into three types: electromagnetic radiation type, electromagnetic induction type and electromagnetic coupling resonance type. Electromagnetic radiation type wireless energy transmission technology uses the far magnetic field for power transmission, and its transmission distance is much larger than physical size of its device, but directionality is poor, transmission efficiency is low, and transmission power is generally small. Transmission mechanism of electromagnetic induction type wireless energy transmission technology is similar to that of the separation transformer, and the core part is replaced by an air gap, which leads to cross- linking of non-directional channel of magnetic line and load side coil, so that only under the premise of a short distance High-power and high-
efficiency wireless power transmission can be realized. Electromagnetic coupling resonant wireless energy transmission technology utilizes the principle of electromagnetic resonance coupling, so that it can still obtain larger transmission power and bird transmission efficiency when transmitting medium-distance power, and it is hardly to be affected by space non-magnetic obstacles during transmissions. Performance of electromagnetically coupled resonant wireless energy transmission technology in terms of transmission distance, transmission power and transmission efficiency makes it particularly suitable for high-power wireless charging.

Fig.7:- Schematic diagram of wireless power transmission based on magnetically coupled resonant

Wireless power transmission based on magnetically coupled resonant is shown in Fig.7, coupling coil can be regarded as a resonant body consisting of inductance and compensation capacitance. Coupled transmitting coil oscillates received energy between electric field and magnetic field at its resonant frequency in space, and produces alternating electromagnetic fields nearby. Coupling receiving coil will generate induced electromotive force, which will also generate alternating electromagnetic fields nearby, and oscillate freely in space with the same resonance frequency. In this way, the two coils continue to transform magnetic field and electric field energy at the same frequency. Therefore, electric energy is transmitted from transmitter coil to receiving coil through time varying magnetic field by air.
Schematic diagram of electromagnetic coupling resonant wireless power transfer system is shown in Fig.8. It includes primary resonant circuit, secondary resonant circuit, rectifier circuit and voltage stabilizing circuit. Primary resonant circuit includes primary coil inductor L1, primary coil

resistor R1 and primary resonant compensation capacitor C1. High-frequency AC voltage source us(t) supplies power to primary resonant circuit, and secondary coil inductor L2, secondary coil resistor R2, and secondary resonant compensation capacitor C2 receive primary loop power through resonant coupling and are formed by diodes D1 to D4. Full-bridge rectifier circuit and voltage-stabilizing capacitor C, providing DC voltage udc(t). M to load Rload is mutual inductance of primary and secondary coils, i1(t) and i2(t) are divided into primary circuit and secondary circuit current, idc(t) is the DC terminal output current, ic(t) is current on Zener capacitor C and iL(t) is current on DC load Rload.

Fig.8 :- Schematic diagram of resonant wireless power transfer system

Power received by Load:

It can be known from above equation that increasing mutual inductance value can improve power transmission capability of the system. Magnitude of mutual inductance value depends on geometry, size, relative position, number of turns, and magnetic permeability of the primary secondary coil.