Electric vehicles are shaping the future of mobility and as almost all electric vehicles are proving to be more economical compared to any petrol car, there has been a shift in the car buying trend. It is expected that by 2030 there will be more electric cars on the roads than cars running on fossil fuels. Currently, typical electric cars have a range of 300 to 400 km on a full battery recharge. That's more than enough for your daily commute. An electric car battery is expected to work for at least five to eight years without the need for cell replacement or repair.
As more electric cars appear on the roads, the need for charging stations is also growing tremendously. In the US alone, there are 56,000 charging stations with 148,000 charging ports in total – and that's still not enough to satisfy the growing demand for EVs. Countries like India expect to need 700,000 electric vehicle charging stations by 2030. While currently a major challenge is finding effective ways to quickly charge electric cars, creating nationwide electric vehicle charging facilities is another challenge.
In an attempt to create an effective and elaborate electric vehicle charging infrastructure, car manufacturers are eager for new ways to charge electric vehicles. A viable solution for simultaneously charging multiple electric cars is wireless electric vehicle charging stations (WEVCS). These wireless charging stations work similarly to cell phone wireless chargers. These charging stations can be installed in parking lots, allowing electric cars to automatically charge during stops. Another option is dynamic wireless charging that allows you to charge the electric vehicle while it is on the move. While dynamic charging promises longer EV range, it poses environmental and health risks.
Static wireless charging
Static wireless charging is the most viable and safest wireless charging technology. Static charging means that the electric car is charged at a wireless charging station while it is stationary. Such a configuration can be easily installed in parking lots and garages. In this type of wireless charging, the transmitter is installed underground and the electric car has an integrated receiver at the bottom. When the car parks in the parking lot or inside the garage, the transmitter and receiver align and the car begins charging. The charging rate depends on the AC voltage level. While the car remains parked, it is charged wirelessly, without the need to connect cables. This is a practical, easy and efficient way to charge electric vehicles.
Dynamic wireless charging
Dynamic charging is another electric vehicle charging infrastructure concept. In this configuration, stationary power transmitters are installed on roads and highways. Electric vehicles charge while in motion as they approach stationary transmitters. This type of charging definitely promises longer EV range on the go. Setting up for dynamic charging, however, would come at a significant cost as it would involve building electric vehicle-equivalent charging facilities across entire roads and highways. The effectiveness of charging moving vehicles is another concern, as wireless charging requires perfect alignment of the transmitter and receiver. Furthermore, such a configuration also poses threats related to security, environmental effects and health risks.
How wireless charging works
Wireless charging was first demonstrated by Nicola Tesla when he developed the Tesla Coil. Wireless charging works on the same principle as a transformer. As in a transformer, the secondary coil comes out due to the magnetic field developed by the current flow in the primary coil – in the same way that a wireless charger has a transmitter and the charging device has a receiver. When alternating current flows through the transmitter, it creates an alternating magnetic field that transfers current to the receiver. This is the basic functioning of any wireless charger, whether it is designed for cell phones or electric cars. There is, however, no wireless charging method. There are four different recognized methods, as follows:
- Capacitive Wireless Charging System (CWCS)
- Permanent Magnetic Gear Wireless Charging System (PMWC)
- Inductive Wireless Charging (IWC) System
- Resonant Inductive Wireless Charging System (RIWC)
Capacitive Wireless Charging System (CWCS)
This type of charging is based on the principle of electrostatic induction observed in capacitors. The electric car has a receiving plate at the bottom, and the charging station has a transmitting plate on the ground. The air gap between the two plates acts as a dielectric medium. The electric car is charged by the displacement current on the receiving plate due to variations in the electric field across the transmitting plate. On the transmitter side, AC current is first supplied to a power factor correction circuit. The circuit maintains voltage levels and minimizes transmission losses. The voltage is then passed through an H-bridge generating a high-frequency AC voltage, which is applied to the transmission plate. High-frequency AC voltage – typically in the range of 100 to 600 KHz – causes an oscillating electric field. This generates a displacement current in the receiver plate. The amount of current received at the receiving plate depends on several factors such as the alignment of the transmitting and receiving plate, the air gap between the two plates, the applied AC voltage, the material used in the construction of the plates and the frequency of AC voltage. On the receiver side, the displacement current is used to charge the electric car battery with the help of a rectifier and filter circuit.
Wireless capacitive charging operation in electric vehicles
Permanent Magnetic Gear Wireless Charging System (PMWC)
This type of wireless charging is based on the operating principle of an electric motor. Both the transmitter and receiver consist of an armature winding with synchronized permanent magnets placed as a core within the windings. Mechanical torque is generated when AC voltage is applied to the transmitter winding due to permanent magnets. Changes in the transmitter's permanent magnetic field cause synchronized mechanical torque to be induced in the receiver's permanent magnet, producing AC current in the receiver's winding. The receiver is converted into a power generator as the mechanical torque in the receiver's permanent magnet is converted to alternating current in its winding. The coupling of rotating permanent magnets is called magnetic gear. On the receiver side, the AC current from the magnetic gear is rectified and filtered to charge the electric car battery.
This wireless charging method has several disadvantages. Firstly, due to the use of permanent magnets, the charging setup becomes expensive. Secondly, permanent magnets are prone to breaking under mechanical stress. Therefore, the charging configuration may incur high maintenance costs.
Wireless charging operation with permanent magnet gear in electric vehicles
Inductive Wireless Charging (IWC) System
This type of wireless charging is based on the operating principle of a transformer. Both the transmitter and receiver consist of coils. The transmitting coil is installed on the ground while the receiving coil is at the bottom of the electric car. An AC voltage of frequency in the range of 19 to 50 KHz passes through the transmitter coil. This causes a change in the magnetic field in the receiving coil, producing AC current. The AC current induced in the receiving coil is rectified and filtered to charge the electric car battery using its battery management system. This is the most economical method of wireless charging for electric cars. Charging works in the same way that electricity is transferred between the primary and secondary windings of a transformer. Since transformers are one of the cheapest and easiest electronic components to build, this wireless charging method is the most cost-effective and simple to implement. The coils need to be aligned for wireless charging. The charging rate in a wireless inductive charging system depends on the distance between the transmitting and receiving coil, the mutual inductance between them, and the frequency of the applied AC power.
Wireless inductive charging operation in electric vehicles
Resonant Inductive Wireless Charging System (RIWC)
Resonant wireless charging is an improved inductive wireless charging method and the most economical and efficient wireless charging method. When the primary and secondary coils of a transformer are tuned to the same resonant frequency, electrical energy from the primary coil is transferred to the secondary coil at a much faster rate. When the coils are in resonance, the transfer of electrical energy takes place even if the magnetic field between them is weak. The resonant inductive wireless charging method improves power transfer efficiency and provides a high quality factor. It has the same cost benefits as inductive charging, as well as providing greater efficiency, lower losses and enabling faster charging. If the resonant frequency in both coils is the same, electrical energy will be transferred despite the greater distance between the coils. Compensation networks are added on both sides in series and parallel to match the resonant frequency in the transmitting and receiving coil into a resonant inductive load system. Some additional compensation networks can be added to further minimize energy losses. The operating frequency for this type of charging remains 10 to 150 KHz. Integrating additional circuitry to match the resonant frequency adds some additional cost to the wireless charging setup on both the transmitter and receiver side. The additions are worth it considering the greater energy efficiency and faster charging rates.
Functioning of resonant inductive wireless charging in electric vehicles
Challenges in wireless charging of electric vehicles
Many car manufacturers are working towards wireless charging as it will be of great benefit to the EV ecosystem. There are, however, many challenges in the EV segment such as fast charging, safe battery technology, electric motor efficiency, battery management, safety and implementation of EV technology in different automobile segments.
The biggest challenge for wireless charging of electric vehicles is its adoption by car manufacturers and companies' involvement in the development of charging stations. Wireless charging needs to prove to be much more economical and efficient than wired charging to drive adoption. The environmental impact of wireless charging is another important issue, as charging stations need to meet strict EMC and EMI standards to overcome these concerns.
WEVCS Standards
Many international organizations, such as the Society of Automotive Engineers (SEA), the International Electrotechnical Commission (IEC), and the Institute of Electrical and Electronics Engineers (IEEE), are actively working together to develop a global standard for wireless electric vehicle charging. . Plug-in charging has a maximum efficiency of 94 to 94.5 percent, but the latest SEA standards for wireless charging have shown an efficiency of 90 to 92 percent – nearly eliminating this critical hurdle in wireless charging adoption.
