DKE Homepage



Wireless power transmission - but with acceptable EMC levels

Large numbers of cable connections in the home is a universal problem, and one which surely everyone would welcome a solution to. There are now many different projects aimed at doing away with electrical cables by transmitting the electrical energy wirelessly to devices. Some mobile phones, household appliances such as shavers or electric toothbrushes, but also larger applications such as electric vehicles, buses or trams are already making use of the new technology in which the power is transmitted with the aid of electromagnetic fields.


There are further applications in the pipeline, including some for industry, where wireless power transmission will be used to supply energy to equipment and facilities. Wireless power transmission will result in a few cables disappearing from the home, but it opens up a whole realm of possibilities for electric vehicles: There are now scenarios in which vehicle batteries can be charged while waiting at traffic lights, for example. Highly futuristic models will charge the battery while on the move with the aid of short but powerful electromagnetic pulses. This raises the hope of increasing the currently limited range of electric vehicles, without drivers having to seek out a charging station to charge up the battery in a time-consuming process.

How does wireless power transmission work?

All wireless energy transmission methods involve sending energy from a transmitter, such as a charging station, via electromagnetic fields and collecting it in a receiver fitted to the device or vehicle. The charging process can be inductive or capacitive: the former involves inducing a current in a receiver coil by means of an alternating magnetic field. The current is then rectified and fed to the device as a supply current. The energy transfer normally takes place over short distances, i.e. in the near field and can reach comparatively high efficiency levels of up to 90%. In capacitive charging, on the other hand, an electric field is created between two metal plates which act as two halves of a capacitor and transfer energy across the gap between two plates. Transmission across longer distances using electromagnetic waves or light waves is also possible. Depending on the purpose, the wireless power transmission can also be complemented by wireless communication. This ranges from a simple transmission of control signals through to more extensive information such as consumption and billing data.

Effective wireless power transmission is dependent upon a number of technical and organizational preconditions (system interoperability, electrical safety etc.) which also includes ensuring electromagnetic compatibility (EMC). It should be noted that the electromagnetic frequency spectrum deployed for such practical applications is also used for the operation of broadcast and radio services, which must not be interfered with. This requires defining appropriate radiation limits for the relevant measurement and detection methods.

The international standard CISPR 11, which includes limits on radio equipment electromagnetic emissions, is used here as a guideline for EMC standardization. The limits were also cited e.g. in the standard on general requirements for electric vehicle charging stations. The relevant international subcommittee is in the process of drawing up specific provisions for the reduction of emissions in wireless energy transmission installations which will be implemented in a future edition of CISPR 11. The EMC standards for wireless energy transmission equipment can also be used to support regulatory requirements for these facilities and systems.

Bretz, Klaus-Peter
+49 69 6308-220