Wireless Power and Wireless Charging Ready to Start

Prof. Markus Rehm von der Hochschule Furtwangen.
Prof. Markus Rehm from Hochschule Furtwangen University.

The success of the Qi standard for wireless charging of smartphone batteries is contagious. Other industries are discovering the technology and manufacturers of components are expanding their portfolios. Prof. Markus Rehm finds, now is just the right moment to take a closer look at wireless power.

The breakthrough for wireless power transmission is in full swing, as current projects and the lively interest at the wireless power session of the embedded world Conference in March 2019 show. Expectations and opportunities for product innovations have significantly increased the acceptance of Wireless Power Transfer (WPT).

The term »wireless« is somewhat misleading because it implies that there are no cables. The transmitter, however, is supplied via a power cable and the receiver often supplies its energy to the consumer via a cable. There are some applications where the transmitter is installed in such a way that no cables are visible and the receiver is located in the consumer. Nevertheless, I would rather speak of contactless energy transmission, because this property is always present and emphasizes the essential advantage over plug contacts.

Many market leaders and hidden champions want to maintain their position and try to make their products even more user-friendly and fail-safe or to facilitate maintenance work, for example with WPT. Significant and practicable improvements are in demand, as are innovations to keep demand high. Cables and contacts are often annoying, sometimes even hated or simply impossible to use.

Is there a customer benefit?

In contrast to artificial intelligence, where many people do not know exactly what to think about it, contactless energy transfer has clear advantages and disadvantages for many applications. Many new developments are conceivable and make extended useful functions possible.

Here are two examples: In order to have electrical energy available on rotating parts, slip rings are used as standard wherever possible. Limited lifetime and reliability as well as mechanical tolerance intolerance – e.g. with a large shaft stroke – are the main disadvantages. Energy harvesting is also used, but this is only possible with very low power. Contactless energy transmission with a distance of up to a few centimeters is the solution for many such applications.

It is hard to believe that even plug contacts are not as reliable as users would like them to be. Unfavourable environmental conditions such as dirt, dust, damp or salty air quickly make the contact surfaces highly resistive and lead to system failure. Vibrations and constant movements cause cable breaks. The contactless power transmission, with completely dense and robust surfaces, also ensures a reliable power supply here.

What are the disadvantages?

Wireless power is not easy to buy to install it. Wired power supplies, on the other hand, are ready-made, and there is often no way around a customer-specific design. Contactless power supplies have even one more dimension! That's why the working groups find it difficult to standardize. Wireless power catalogue goods are manageable and will probably remain so. The chance of purchasing a suitable wireless power supply at the click of a mouse will also be very low in the future. Nevertheless, it is worth a try, e.g. with the Qi standard. For most developers, the path to the finished product with wireless power will be long and difficult!

The installation situation is never the same. The combination of input voltage and output power offers many possibilities – and the regulations regarding electromagnetic compatibility and safety to be observed depend on the area of application.

How much distance is required between the transmitting and receiving coils? Every millimeter more increases the leakage inductance and reduces the coupling. This has a negative effect on power losses and interference radiation. A larger distance forces higher qualities in the resonant circuit. This means more reactive power, which is shifted back and forth with the resonant frequency in the resonant circuit and further increases the losses. A larger coil diameter in turn increases the coupling, but also the required installation space. This list shows: The combination possibilities are endless. And: If humans or animals can get between energy transmitter and receiver, then the safety effort increases extremely.

The efficiency of the power transmission with cable, plug and socket is unbeatably high, almost 100 percent. Connectors and cables achieve very long livetimes under uncritical conditions, and electronic components are not able to achieve this. That's why power cables are of course still used – wherever possible.