Wireless power transfer (WPT) or wireless energy transmission is the transmission of electrical power from a power source to a consuming device without using solid wires or conductors.It is a generic term that refers to a number of different power transmission technologies that use time-varying electromagnetic fields. Wireless transmission is useful to power electrical devices in cases where interconnecting wires are inconvenient, hazardous, or are not possible. In wireless power transfer, a transmitter device connected to a power source, such as the mains power line, transmits power by electromagnetic fields across an intervening space to one or more receiver devices, where it is converted back to electric power and utilized.
Wireless power techniques fall into two categories, non-radiative and radiative.In near-field or non-radiative techniques, power is transferred over short distances by magnetic fields using inductive coupling between coils of wire or in a few devices by electric fields using capacitive coupling between electrodes.Applications of this type are electric toothbrush chargers, RFID tags, smartcards, and chargers for implantable medical devices like 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 player and portable computers without being tethered to a wall plug. With the radiative or far-field techniques, also called power beaming, electrical energy is transmitted 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.An important issue associated with all wireless power systems is limiting the exposure of people and other living things to potentially injurious electromagnetic fields (see Electromagnetic radiation and health).
How it is possible?
While the idea may sound futuristic, it isn’t particularly new. Nicola Tesla proposed theories of wireless power transmission in the late 1800s and early 1900s. Since then, researchers have developed several techniques for moving electricity over long distances without wires.
The wireless transmission of energy is common in much of the world. Radio waves are energy, and people use them to send and receive cell phone, TV, radio and Wi-Fi signals every day. The radio waves spread in all directions until they reach antenna that are tuned to the right frequency. A similar method for transferring electrical power would be both inefficient and dangerous.
We know the difference between direct and alternating currents as:
In a direct current circuit, the current is continuous; the fields are constant; which represents stored electric and magnetic energy. In an alternating current circuit, the fields also alternate; that is, with every half wave change of current and voltage, the magnetic and the electric field start at the conductor and run outwards into space with the velocity of light. These alternating fields impinge on another conductor a voltage and a current are induced. Here comes the concept of INDUCTIVE COUPLING.
While the idea may sound futuristic, it isn’t particularly new. Nicola Tesla proposed theories of wireless power transmission in the late 1800s and early 1900s. Since then, researchers have developed several techniques for moving electricity over long distances without wires.The wireless transmission of energy is common in much of the world. Radio waves are energy, and people use them to send and receive cell phone, TV, radio and Wi-Fi signals every day. The radio waves spread in all directions until they reach antenna that are tuned to the right frequency. A similar method for transferring electrical power would be both inefficient and dangerous.
We know the difference between direct and alternating currents as:
In a direct current circuit, the current is continuous; the fields are constant; which represents stored electric and magnetic energy. In an alternating current circuit, the fields also alternate; that is, with every half wave change of current and voltage, the magnetic and the electric field start at the conductor and run outwards into space with the velocity of light. These alternating fields impinge on another conductor a voltage and a current are induced. Here comes the concept of INDUCTIVE COUPLING.
Inductive coupling:
Electromagnetic induction is proportional to the intensity of the current and voltage in the conductor which produces the fields and to the frequency. The higher the frequency, the more intense the induction effect. Energy is transferred from a conductor that produces the fields (the primary) to any conductor on which the fields impinge (the secondary). Part of the energy of the primary conductor passes inductively across space into secondary conductor and the energy decreases rapidly along the primary conductor.
Inductive coupling uses magnetic fields that are a natural part of current’s movement through wire. Any time electrical current moves through a wire, it creates a circular magnetic field around the wire. Bending the wire into a coil amplifies the magnetic field. The more loops the coil makes, the bigger the field will be.
If you place a second coil of wire in the magnetic field you’ve created, the field can induce a current in the wire. This is essentially how a transformer works, and its how an electric toothbrush recharges. It takes three basic steps:
The above electric tooth brush is the perfect example.
Electromagnetic induction is proportional to the intensity of the current and voltage in the conductor which produces the fields and to the frequency. The higher the frequency, the more intense the induction effect. Energy is transferred from a conductor that produces the fields (the primary) to any conductor on which the fields impinge (the secondary). Part of the energy of the primary conductor passes inductively across space into secondary conductor and the energy decreases rapidly along the primary conductor.Inductive coupling uses magnetic fields that are a natural part of current’s movement through wire. Any time electrical current moves through a wire, it creates a circular magnetic field around the wire. Bending the wire into a coil amplifies the magnetic field. The more loops the coil makes, the bigger the field will be.
If you place a second coil of wire in the magnetic field you’ve created, the field can induce a current in the wire. This is essentially how a transformer works, and its how an electric toothbrush recharges. It takes three basic steps:
- Current from the wall outlet flows through a coil inside the charger, creating a magnetic field. In a transformer, this coil is called the primary winding.
- When you place your toothbrush in the charger, the magnetic field induces a current in another coil, or secondary winding, which connects to the battery.
- This current recharges the battery.
The above electric tooth brush is the perfect example.
Let us consider the techniques used here…
Electro-dynamic induction method:
The electro-dynamic induction wireless transmission technique is near field over distances up to about one-sixth of the wavelength used. Near field energy itself is non-radiative but some radiative losses do occur. In addition there are usually resistive losses. With electro-dynamic induction, electric current flowing through a primary coil creates a magnetic field that acts on a secondary coil producing a current within it. Coupling must be tight in order to achieve high efficiency. As the distance from the primary is increased, more and more of the magnetic field misses the secondary. Even over a relatively short range the inductive coupling is grossly inefficient, wasting much of the transmitted energy.
This action of an electrical transformer is the simplest form of wireless power transmission. The primary and secondary circuits of a transformer are not directly connected. Energy transfer takes place through a process known as mutual induction. Principal functions are stepping the primary voltage either up or down and electrical isolation. Here the receiver must be directly adjacent to the transmitter or induction unit in order to efficiently couple with it.
The electro-dynamic induction wireless transmission technique is near field over distances up to about one-sixth of the wavelength used. Near field energy itself is non-radiative but some radiative losses do occur. In addition there are usually resistive losses. With electro-dynamic induction, electric current flowing through a primary coil creates a magnetic field that acts on a secondary coil producing a current within it. Coupling must be tight in order to achieve high efficiency. As the distance from the primary is increased, more and more of the magnetic field misses the secondary. Even over a relatively short range the inductive coupling is grossly inefficient, wasting much of the transmitted energy.This action of an electrical transformer is the simplest form of wireless power transmission. The primary and secondary circuits of a transformer are not directly connected. Energy transfer takes place through a process known as mutual induction. Principal functions are stepping the primary voltage either up or down and electrical isolation. Here the receiver must be directly adjacent to the transmitter or induction unit in order to efficiently couple with it.
Electromagnetic radiation:
Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The main reason for longer ranges with radio wave and optical devices is the fact that electromagnetic radiation in the far-field can be made to match the shape of the receiving area (using high directivity antennas or well-collimated Laser Beam) thereby delivering almost all emitted power at long ranges. The maximum directivity for antennas is physically limited by diffraction.
We have some more methods like microwave method, laser method, electrical conduction and so on. Though we have these many methods we still are unable to bring this practically because of the limitations they are having.
Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The main reason for longer ranges with radio wave and optical devices is the fact that electromagnetic radiation in the far-field can be made to match the shape of the receiving area (using high directivity antennas or well-collimated Laser Beam) thereby delivering almost all emitted power at long ranges. The maximum directivity for antennas is physically limited by diffraction.We have some more methods like microwave method, laser method, electrical conduction and so on. Though we have these many methods we still are unable to bring this practically because of the limitations they are having.
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