Working of Wireless Power Transfer for Electrical Devices

We are living in the world of wireless transfer wherein data is transmitted wirelessly over very long distances covering some thousands of kilometers as with the case of internet, mobile phones and other communication systems. In addition to this, electrical power is also transferred to other circuits without using any electrical conductors. One can observe this in wireless chargers, wireless power connectors, wireless sensor networks, power slip rings in motors, and so on.

Wireless power transfer is not a new concept as it was invented by Nikola Teslas in the latter half of the 19th century after several attempts and experiments. He found that high-frequency transfer of power results in high efficiency than the low-frequency transfer. Wireless power transfer concept is same as the working principle of a transformer wherein the power is transferred from the primary winding to the secondary winding without any conducting medium in between them.

Wireless Power Transfer and its Working

Wireless power transfer involves transmission of electrical energy from one place to another without using any wires. This concept can be implemented when there is a least possibility of using wires to transfer electrical power, such as in hazardous places. There are three basic methods to send this power wirelessly: electromagnetic induction, microwave or laser electromagnetic radiation and electrical conduction as in the case with a wardenclyfee tower.

Electromagnetic induction based transfer is the most common method of wireless power transfer that uses two coils for transferring and receiving power. As it is a known fact that when one coil is energized by an alternating current, it produces a magnetic field that prompts to induce voltage in the other nearby coil as in case with a transformer. The extent of this transfer depends on the frequency of power to be transmitted. This concept of efficient transfer introduces term resonance that enhances a range of transferring power over different positions and orientations.

The condition for this resonance is to match the inductive reactance with the capacitive reactance of a coil. Hence choosing capacitors and inductance of coil should be in such a way that XL = XC. To match with this condition, the requirement is to find the inductive and capacitive reactance of coils.

Inductive Reactance XL = 2π x f x L
Capacitive Reactance XC = 1/2π x f x C
Where f is the resonance frequency and is given as
f = 1/2π sq.rt of LC

The formula for inductance of the coil depends on the type of coil whether it is circular or square or of any other shape. So the capacitors’ insertion according to the inductance of coil makes to match resonance which results in maximum power transfer. And also the coil size and number of turns depend on the current carrying capacity and the amount of power transfer.

The electromagnetic induction-based wireless power transfer operation involves in a set of functional blocks which are given below. In such a setup, the mains AC power to be transmitted is converted into a DC by a rectifier circuit. This DC is again converted into an AC at a high frequency with the use of an inverter circuit. The frequency is around some hundreds of Hertz and this high frequency supply matches the resonance condition with the use of an impedance matching circuit so that an efficient transfer of power takes place.

Subsequently this high- frequency supply is transferred to a device resonator wirelessly by producing magnetic lines of force. The energy coupled with the resonator is again rectified to a DC with the use of a rectifier based on load requirement such as battery charging. Hence, this application of resonance with normal wireless transfer increases the transmission range.

The induction transfer incorporates for smaller distances, whereas microwave transfer using radio frequency electromagnetic fields supports greater distance transfer of power over multiple kilometers. In the microwave transfer, the signals are transferred from the transmitting circuit over a power beam with shorter wavelengths.

The antenna collects this microwave energy and sends it to other rectifying circuit to drive the loads. Laser method of transferring power is involved in electromagnetic radiation in the form of laser beam transferred to the photovoltaic cell. But radiation of laser is hazardous so it is not permissible to use laser anywhere.

As a practical example of this concept, the below description about the concept of wireless power transfer is of immense help to the readers.

Wireless Power Transfer for Energizing DC Motor

This proposed project eliminates the current carrying conductors to turn a DC motor. The project can be implemented if a motor operation requires wireless power, as in the case of rotational objects. Powering rotational objects is a little bit complex process involving zigzag turning of wires to prompt rotation, which may lead to the breakdown of the wires.

Apart from the operational blocks of a wireless power transfer, as discussed above, this system can also be constructed with the use of a principle wherein a rectifier, an inverter, a high- frequency transformer, a high-frequency rectifier, a filter, a regulator and a DC motor fan are used.

In this project, the power from the AC mains is rectified to a DC at 12V with the use of a diode bridge rectifier, and the capacitor filters this pulsed DC to constant DC. This DC supply is again inverted to an AC at a high frequency of around 40 KHz by switching the transistors at corresponding time intervals. This high frequency 12V AC in the primary coil produces a magnetic field around it.

The Secondary coil is placed at a considerable distance in close proximity to the primary coil, approximately 3cm apart due to the resonance frequency matching. When this field, in the primary couples with the secondary coil, it induces a voltage so that the current passes through the load. As the load is DC, induced voltage is rectified again, filtered and regulated so that it drives the DC motor.

These primary and secondary resonant coils are connected with a set of capacitors to match the resonant condition for a maximum power transfer. A wireless power transfer in the 3D space is also possible by increasing the number of turns in the coils, by using high gauge coils and high rated capacitors.

In this way, one can design a wireless-power-transfer system for simple electronic gadgets like mobile phones chargers which not only reduces the risk of shock, but also the efforts to plug repeatedly into the sockets. We hope that this article might have provided some basic insights about the wireless power transfer to you. Furthermore, for any technical assistance on this topic as well on other electrical projects you can contact us by commenting below.

Photo Credits:

• Wireless Power Transfer by applemagazine
• Wireless Power Transfer Working by hori