An electric vehicle (EV) can be conveniently charged while parked over a wireless charger without plugging in a cable. It’s also possible to dynamically charge while driving if the vehicle has a wireless electric road system.
Wireless power transfer technology uses electromagnetic induction, with an electrical current flowing through one device to create a magnetic field, which then induces an electrical current in another device.
Magnetic fields are continually generated around electric currents. The magnetic fields circle the current, traveling in a perpendicular direction. When a dc voltage causes a constant current to flow through a wire, the magnetic field is also constant. If the wire is coiled, then the magnetic fields combine and become stronger. Generating a constant magnetic field doesn’t consume energy, but energy is stored in it.
Imagine an electrical current is analogous to water flowing through a pipe. The voltage is like the pressure, and the magnetic field is like the mass of the water. When the current changes, the magnetic field creates a voltage that resists the change in current — much like the pressure generated by an acceleration of water in a pipe.
This inertia-like force is known as inductance. The faster the change in current, the greater the voltage generated.
Just as every force has an equal and opposite reaction, a change in the magnetic field can induce a current. A change in the current induces a change in the magnetic flux.
Suppose two coils of current-carrying wire are arranged so they’re electrically insulated from one another but share a common magnetic field. In this case, electrical power can be transferred without current flowing directly between the two circuits. This is the basics of wireless energy transfer for EVs (and other applications, such as wireless phone chargers).
However, energy is only transferred when the current changes, requiring an alternating current to work. When one of the wire coils is energized by an alternating current, the current (which is continuously changing direction) pushes a lot of energy into the magnetic field. Another coil of wire within the magnetic field will absorb this energy by causing an alternating current to flow.
Transformers use this principle, with each circuit containing a different number of coils and voltages. While transformers use a ferrous core to provide an easy path for the magnetic field, the effect works with nothing between the coils, so long as they are close enough. Therefore, a wireless charger can be thought of as a transformer, with no core and potentially equal numbers of coils in both circuits.
Static and dynamic wireless charging have been trialed for many years, but have yet to be deployed at scale. An early example of static wireless charging was the Qualcomm Halo, and current leaders in the technology include Volvo and Genesis.
Dynamic charging has the greatest potential to radically improve the convenience of EVs, with coils embedded below the road surface. This means vehicles could use smaller batteries without stopping to charge. It would only be economical to fit such systems on main roads, so some batteries would still be required to reach your final destination.
Dynamic wireless charging of city buses has been trialed on public routes in Korea since 2013, and several other companies are developing this technology, including Toyota, Enrx, and Electron.
Filed Under: Charging, FAQs