Wireless charging allows an electric vehicle (EV) to be charged by parking it over a wireless charging pad containing an electrical coil that generates an alternating electromagnetic field when an electric current flows through it. The EV is equipped with a secondary coil, typically mounted underneath the vehicle. When the vehicle is parked over the charging pad, the secondary coil aligns with the primary coil.
Figure 1. Convenience is one of the main advantages of wireless charging for EV drivers. There’s no need for charging cables or to plug-in vehicles at charging stations, greatly alleviating range anxiety.
The alternating electromagnetic field generated by the primary coil induces an electric current in the secondary coil. This is the same principle as wireless phone charging or a transformer — electromagnetic induction.
Induction is the relationship between electrical current and magnetism, which can transfer power from one electrical circuit to another disconnected circuit. EVs can be dynamically charged in much the same way by driving over roads.
Understanding electromagnetic induction
When an electrical current flows through a wire, it generates a magnetic field around the wire. For a constant current, the wire’s resistance consumes the energy but the magentic field that occurs does not. Rather, the magnetic field stores the energy.
Imagine an electrical current as water flowing through a pipe The voltage is analogous to pressure and the current is like the flow rate. The magnetic field can be compared to the mass of the water. At a steady flow rate, the mass has no effect, but when the flow rate changes, the acceleration creates a force. In electromagnetism, this inertia-like effect is known as inductance. A change in current produces a corresponding change in voltage.
The magnetic fields circling a wire can be added together by forming the wire into a coil. This is how electromagnets are made to generate strong magnetic fields and how inductors are made to store energy in an electrical circuit. In this application, they generate voltages that resist changes in current.
Using ac to transfer power by induction
Just as there is a two-way relationship between force and acceleration, there is a two-way relationship between storing and releasing energy in a magnetic field. A change in current causes energy to be stored in the magnetic flux, and a change in the magnetic flux can induce a current in a wire.
When an alternating current flows into a coil, continuous changes in the direction of the current pump release a lot of energy into the magnetic flux. If another coil is located within the magnetic field, this will cause an alternating current (ac) to flow through that second coil, transferring power between the two electrical circuits.
In fact, this is how transformers work, with different windings between the coils resulting in different voltages in the two circuits.
Figure 2. Wireless charging of an EV is similar to how a transformer works in that both use electromagnetic induction to transfer energy. In both systems, a primary coil generates a magnetic field when an alternating flows through it, which then induces an electric current in a secondary coil to transfer power without direct electrical contact.
Typically, transformers have a core that helps direct the magnetic field between coils. However, a core is not essential, as a magnetic field can propagate through air or even a vacuum. Wireless charging operates as a transformer, with each coil located in a different device.
For an EV, power is supplied to one coil located in a charging pad in the road, while the other is attached to the bottom of the vehicle. The alternating electromagnetic field generated by the primary coil induces an electric current in the secondary coil, which is then converted back into direct current (dc) by an onboard inverter in the EV. The dc power is used to charge the vehicle’s battery.
For optimal efficiency, the alignment between the primary and secondary coils is critical. Some systems include mechanisms to support precise alignment or may have multiple coils to cover a larger area. Additionally, wireless charging systems include safety features to prevent accidental exposure to electromagnetic fields, such as detecting objects between the coils and shutting off the power.
Overall, static wireless charging points can provide greater convenience for drivers, allowing them to park over the charging point without attaching charging cables. This is particularly useful for busses and taxis, allowing rapid charging while passengers are boarding. Frequent, convenient charging also means smaller batteries can be employed, reducing vehicle costs and bottlenecks to scaling EV manufacturing.
Wireless EV charging technology is continually evolving, with ongoing research aimed at improving efficiency, reducing costs, and expanding its applicability.
Advantages of wireless EV charging
- Convenience: No need to plug and unplug cables at charging stations, reducing range anxiety
- Reduced wear and tear: Less wear on charging connectors and cables
- Enhanced safety: Reduces the risk of tripping over cables and exposure to electrical contacts
- Weather resistance: Protects charging components from weather-related damage since there are no exposed connectors
- Aesthetics: Eliminates the need for visible cables and connectors
- Potential for autonomous vehicles: Simplifies the charging process for autonomous vehicles
Filed Under: FAQs, Wireless charging