Wireless power transfer(WPT) / wireless energy transmission

Wireless power transfer(WPT) or wireless energy transmission is the transmission of electrical energy from a power source to an electrical load, such as an electrical power grid or a consuming device, without the use of discrete man-made conductors. Wireless power is a generic term that refers to a number of different power transmission technologies that use time-varying electric,magnetic, or electromagnetic fields. In wireless power transfer, a wireless transmitter connected to a power source conveys the field energy across an intervening space to one or more receivers, where it is converted back to an electrical current and then utilized. Wireless transmission is useful to power electrical devices in cases where interconnecting wires are inconvenient, hazardous, or are not possible.

Wireless power techniques fall into two categories, non-radiative and radiative. In non-radiative techniques, power is typically transferred by magnetic fields using magnetic inductive coupling between coils of wire. Applications of this type include electric toothbrush chargers,RFID tags, smartcards, and chargers for implantable medical devices like artificial cardiac pacemakers, and inductive powering or charging of electric vehicles like trains or buses. A current focus is to develop wireless systems to charge mobile and handheld computing devices such as cellphones, digital music players and portable computers without being tethered to a wall plug. Power may also be transferred by electric fields using capacitive coupling between metal electrodes. In radiative far-field techniques, also called power beaming, power is transferred by beams of electromagnetic radiation, like microwaves or laser beams. These techniques can transport energy longer distances but must be aimed at the receiver. Proposed applications for this type are solar power satellites, and wireless powered drone aircraft.

Generic block diagram of an inductive wireless power system

Generic block diagram of a wireless power system that works by (non resonant) inductive coupling(electromagnetic induction). It works like a transformer. It consists of a "transmitter" unit consisting of an oscillator that produces an alternating current of the correct frequency in a "primary" coil. The alternating current produces an alternating magnetic field (B, green) which extends to the "secondary" coil in the receiver unit and induces a voltage in it by Faraday's law of induction. The alternating current in the secondary is rectified to DC (if DC is needed) and powers a load. This is a simplified version of how wireless charging units for portable devices like the Qi and PowerPad systems work. Wireless charging stands for tools such as electric toothbrushes operate at mains frequency (50/60 Hz) and so do not need the oscillator, but must use an iron core inside the coils in order to transfer power at low frequencies.

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