What are wireless power coils?

More than 120 years ago, a genius by the name of Nicholas Tesla conducted some experiments that laid down the technology of distributing electricity without wires. Today, we are using Tesla’s technology to charge our mobile phones. More than 50 million mobile phones remain alive through inductive charging. We use induction stovetops in our kitchens. The technique of wireless transfer of power is certainly proliferating.

A large part of the world population is completely dependent on smartphones and tablets. Personal encounters have given way to texting, emails and video chats. Spare times involve playing games on mobiles or watching videos. However busy people may be, there is always some time to browse the social networks and post in them. In fact, this gadget takes up most of the time we stay awake. With so much drain of energy, it is no surprise that mobile batteries do not last through the day.

Now, thanks to wireless power coils integrated into bedroom dressers or restaurant tables, people can charge their smartphones and other gadgets by simply placing them on the table. Device manufacturers adopt one of three major standards for Wireless Energy Transmission – WPC, A4WP and PMA.

The basis of Wireless Energy Transmission lies in Faraday’s Law of Induction. According to this law, a current flowing through a primary coil generates magnetic flux. If a secondary coil is present, the magnetic flux from the primary coil induces a voltage in the secondary coil. During wireless power transfer, the quality of the coils is of prime importance in determining the coupling and efficiency of the process.

The quality of a coil depends on its internal resistance and its reactance. Various factors affect both the parameters. For example, with a ferrite pot core, the coil can tightly bundle its magnetic field, reducing outward radiation while increasing the coil distance. Mounting space required is also smaller, since the coil is compact and round. Since the ratio of the area between transmitter and receiver coils affects the efficiency of energy transmission significantly, smaller receiver coils result in small transmitter coils as well.

Manufacturers reduce the internal resistance of their coils by using materials of very high purity. However, wireless energy transfer calls for the use of very high frequencies, for example, in the MHz range. At such high frequencies, the nature of current flow through the wire changes dramatically. High frequency current flows mostly along the outer periphery of the wire, with very little current flowing through the central core. As a result, the resistance of the wire increases tremendously as the frequency of the current passing through it goes up. This is called Skin Effect.

To reduce the resistance of a single wire carrying high frequency current because of Skin Effect, manufacturers replace it with a bundle of thinner wires insulated from each other. This type of wire is called Litz wire. Since the wires are thin and insulated from each other, the high frequency current is forced to travel equally through all of them. The overall resistance is therefore much lower.