Electric vehicles (EVs) are shaping the future of mobility, as nearly all EVs are more economical than those that run on gasoline. In fact, more electric vehicles are expected to be on the road than fossil fuel cars by 2030.
Most EVs range from 300 to 400 km on a full battery recharge, sufficient for daily commuting. Today’s battery packs are expected to run for at least five to eight years without the need for replacement.
With more EVs on the roads, the need for charging stations is increasing. In the U.S., there are some 160,000 charging stations — but more would be highly beneficial. One challenge is finding effective ways to fast-charge EVs for more effective turnaround times.
One solution is simultaneously charging EVs with wireless charging stations (WEVCS), so no cables are required. These wireless charging stations work similarly to mobile phone chargers. WEVCs could be conveniently set up in parking lots. Another option is dynamic wireless charging, which charges a vehicle while it’s in motion. However, this option comes with environmental and health risks.
Static wireless charging is currently the safest and most feasible wireless charging technology. Static charging means an EV is charged at a wireless charging station while parked, which is ideal for parking lots and garages.
In this type of wireless charging, a transmitter is installed underground, and an EV has an onboard receiver at the bottom of the car. The transmitter and receiver must be aligned for charging. The rate of charge depends upon the AC voltage level.
What’s convenient is the EV is charged while parked with zero need for cables. This is a practical, easy, and efficient manner of charging electric vehicles.
Dynamic wireless charging uses stationary power transmitters installed on roads and highways. EVs charge while they are in motion whenever they drive near the transmitters.
The advantage of this type of charging is a more extended range, so vehicles can go further without stopping. However, it would be costly to construct and install the charging structures across roads and highways. There’s also a safety concern. Charging a moving vehicle could be hazardous.
Also, there’s the alignment challenge because, based on current technology, the transmitter and receiver must be a perfect fit to charge properly.
How wireless charging works
Nicola Tesla first demonstrated wireless charging when he developed Tesla Coil, which works much like a transformer. The secondary coil is exited due to the magnetic field produced by the current flow in the primary coil — the same way a wireless charger has a transmitter and the charging device has a receiver.
When alternating current flows through the transmitter, an alternating magnetic field transfers the current to the receiver. It’s the same principle used for charging smartphones.
There are four different ways to charge EVs”
1. Capacitive wireless charging system (CWCS)
2. Permanent magnetic gear wireless charging system (PMWC)
3. Inductive wireless charging system (IWC)
4. Resonant inductive wireless charging system (RIWC)
Capacitive wireless charging system (CWCS)
CWCS is based on the principle of electrostatic induction as used in capacitors. In an EV, a receiver plate is placed underneath it, which connects to a charging station transmitter plate on the ground. The air gap between the two plates acts as a dielectric medium.
The EV is charged by the displacement current in the receiver plate due to variations in the electric field by the transmitter plate. AC is first supplied to a power factor correction circuit in the transmitter. This circuit maintains voltage levels and minimizes transmission losses.
The voltage is then passed through an H-bridge, generating high-frequency AC applied to the transmission plate. The high-frequency AC voltage — typically 100 to 600 KHz — causes an oscillating electric field, which generates a displacement current at the receiver plate.
The amount of current received at the receiver plate depends on several factors, including the:
- Alignment of the transmitter and receiver plates
- Air gap between the two plates
- Applied AC voltage
- Material used in the construction of the plates
- Frequency of the AC voltage
The displacement current is used to charge the EV’s battery pack with the help of a rectifier and filter circuit.
Permanent magnetic gear wireless charging system (PMWC)
PMWC is based on the principle of an electric motor. The transmitter and receiver consist of an armature winding with synchronized permanent magnets placed as the core within the windings. Mechanical torque is generated when AC voltage is applied to the transmitter winding, thanks to the permanent magnets.
This causes changes in the permanent magnetic field of the transmitter, inducing synchronized mechanical torque in the receiver’s permanent magnet and producing AC in the receiver winding. Essentially, 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 the magnetic gear. AC from the magnetic gear is rectified and filtered to charge the EV’s battery pack.
Unfortunately, this method of charging has several disadvantages. It’s costly because of the permanent magnets, which are also prone to damage under mechanical stress. So, this charging setup could also lead to high maintenance costs.
Inductive wireless charging system (IWC)
IWC is based on the principle of a transformer, where the transmitter and the receiver consist of coils. The transmitter coil is installed at the ground while the receiver coil rests at the bottom of an EV.
An AC voltage of 19~50 KHz is passed through the transmitter coil, producing a change in the magnetic field of the receiver coil. The AC inducted in the receiver coil is rectified and filtered to charge the EV’s battery pack using its battery management system (BMS). Currently, this is the most cost-effective method of wireless charging method.
Charging works like electricity transferred between the transformer’s primary and secondary winding. Transformers are one of the least costly electronic components and the most straightforward to construct. However, the coils must be aligned for wireless charging.
The rate of charging in an inductive wireless charging system depends on the distance between the transmitter and receiver coil, the mutual inductance between them, and the frequency of the applied AC supply.
Resonant inductive wireless charging system (RIWC)
RIWC is an improved method of inductive wireless charging and the most cost-effective and efficient wireless charging method.
When the primary and secondary coils in a transformer are tuned to the same resonant frequency, the electrical energy from the primary coil is quickly transferred to the secondary coil. When the coils are in resonance, the transfer of electrical energy happens even if the magnetic field between them is weak.
This charging method improves the power transfer efficiency and provides a high-quality factor. It offers the same cost benefits as inductive charging, providing higher efficiency, lower losses, and fast charging.
Compensation networks are added at both sides in series and parallel to match the resonant frequency at the transmitter and receiver coils. If the resonant frequency at both coils is matched, the electrical power is transferred despite the distance between the coils.
The operating frequency is 10 to 150 KHz. Additional compensation networks can be added to minimize any power losses, increasing the costs of this type of charging method. The additions are typically worth it, considering the improved power efficiency and faster charging rates.
EV manufacturing has several challenges, including safe battery technology and management, electric motor efficiency, efficient charging, and cost management.
In terms of charging, the biggest challenge is developing charging stations that are accessible, cost-efficient, and easy and quick to use.
Additionally, the environmental impact of wireless charging is an essential issue, as stations must comply with strict EMC and EMI standards.
Many international organizations like the Society of Automotive Engineers (SEA), the International Electro-technical Commission (IEC), and the Institute of Electrical and Electronics Engineers (IEEE) are actively working to develop a global standard for wireless EV charging.
Plug-in charging has a maximum efficiency of 94 to 94.5% but the latest SEA standards for wireless charging have shown up an efficiency of 90 to 92% — nearly eliminating that critical roadblock in adopting wireless charging. Let’s hope this continues.
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Filed Under: Charging