Wireless power transmission, basic theory

BASIC THEORY

Nikola Tesla, at the beginning of the 19th century was probably the first person to discover and introduce the concept of sending electrical energy wirelessly to the world through his research. However, this did not necessarily make his findings enthusiastically accepted by scientists at the time. On the other hand, the concept that Tesla discovered at that time was considered as something that is harmful to living organisms, considering the side effects that could be caused by the emitted electromagnetic field could cause various kinds of diseases for the target organism. This is what gradually makes Tesla's research results generally tend to be forgotten for decades.

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Only later in 2007, a group of researchers from MIT (Massachusetts Institute of Technology) America conducted research on Wireless Power Transfer using almost the same principle as Tesla's concept but with fundamental differences. If what Tesla is researching is the transfer of electrical power using long-distance radiation techniques such as microwaves and radio waves, it is different from MIT, which uses the concept of a near field with a close distance between the sender and receiver. In the end the researchers succeeded in transferring electrical power wirelessly with a distance of more than 2 meters and a power of 60 watts, where the efficiency reached 40% (Kurs, 2007). The results of this study later became a turning point in the development of the WPT system.

Two researchers (Sibakoti & Hambleton, 2011), demonstrated the results of research that had been carried out on WPT and claimed to have succeeded in designing a system to wirelessly transmit power from the sending coil to the receiving coil capable of turning on 40 watt lamps with a distance between coils of 18 cm.

In addition to the research above, a researcher from South Korea (Hwang, 2011) also published a journal in which there are basic principles of this WPT technology. The basic principle of this technology (wireless power transfer) according to Hwang is that two separate coils with the same resonant frequency can form a resonance system based on high-frequency magnetic coupling and high-efficiency power exchange, where the effect of this coupling on nearby objects tends to weak because it works at different frequencies.

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Then in 2012, as stated in the final project, UNRAM Electrical Engineering Students have succeeded in transferring electrical power which is then used to turn on the LED and charge the Nokia N95 HP battery as a load with a charging power of 2.18 mW with a distance of 5 cm. Where to get maximum power transfer at longer distances, it can be done by modifying the dimensions of the coil, increasing the number of turns and the type of inductor material used and increasing the capacity of the capacitor value in the sending circuit (Kusuma, 2012).

Wireless power transfer is also known as WPT (Wireless Power Transfer). Wireless power transfer or wireless energy delivery is the process of sending energy from a power source to an electrical load without going through an intermediary cable. This wireless energy transfer has the potential to be used to power electronic equipment that requires relatively little power (Wikipedia.org, 2012).

WIRELESS POWER TRANSMISSION

The transfer of electrical power using air media can be used to distribute energy where the energy source and load are located far apart. One of the advantages of this technology is that it can penetrate objects in its path (except bimetallic materials) so that certain places that are generally not possible to transmit electrical power through cables can be reached with this technology. But in studies (MIT, LIPI, etc.), WPT still transmits energy in small amounts and at not very long distances. And its application is also in tools that require relatively small energy.

As another example, the principle of induction in the figure above, can transmit electric power from one coil without touching the other coil, even though the distance is still very close. In addition to transformers, the principle of electromagnetic radiation in radio waves can also transmit electrical energy wirelessly, but because of their low efficiency, these radio waves only play an important role in the telecommunications world in transmitting information and cannot be used to transmit large amounts of electrical power. Scientists have also tried to concentrate electromagnetic waves like lasers (doesn't propagate like electromagnetic waves in radio waves), but this is also impractical and can even damage and harm humankind. Finally found a way to be able to transmit electrical energy wirelessly by using the principle of magnetic resonance, where energy is transferred at the same frequency to the sender and receiver, so that it will not affect objects in the vicinity that have different frequencies.

In a transformer, electric current flows into the primary coil and induces the secondary coil, these two coils are not in contact, but are in very close proximity. The efficiency level of the transformer will be greatly reduced if these two coils are kept apart. In addition to the transformer, the electric toothbrush in the picture above also uses the same induction principle as the transformer, the electric toothbrush will recharge the battery if it is placed in its place.

The efficiency of electromagnetic induction can be increased by using a resonator circuit. This method is also known as resonance induction, which is widely used in medical equipment. Using this principle, a device has been created that can transmit electrical power wirelessly, over distances much different from traditional induction.

The wireless power transfer technology referred to in this article is a technology that does not transmit and refers to the near-field concept. Many other techniques in the field of sending electrical energy wirelessly are based on radiation techniques, both for information purposes such as radio waves, laser beams (narrow beams) and light waves. Air radiation from frequencies in radio waves is widely used to transmit information wirelessly because information can be transmitted in any direction for use by multiple users. The power received at any radio or wireless receiver circuit is very small, and must be amplified again in the receiving circuit using an external power source. Since most of the radiation power is wasted in the open air, this radio transmission is very inefficient if it functions to transmit large amounts of electrical power. To increase the amount of energy that the receiver circuit can capture, the transmitter side of the circuit can be given higher power as well, but this is unsafe and may even interfere with other devices that also use radio frequencies.

Direct radiation, using an antenna that is directed directly from the source to the receiver without any obstacles to shoot energy using radio frequencies. In this way, the energy that can be received by the receiving circuit is increased, but this method also has a direct impact on organisms and can be harmful. Therefore, this method also cannot be used in the distribution of electrical energy with large power such as for industry, or for the consumption of everyday electronic equipment. But in reality this is still being studied and explored to be able to shoot energy from outer space to earth according to the concept of "solar space power" (Asimov, 1941), and for defense needs as a lethal weapon that can shoot energy. from the sky to the battlefield.

As explained above, the concept of transmitting electrical power used in this article is very different from radio waves or direct radiation, because in the process of transmitting electrical power it does not require conditions that require that there is no partition between the transmitter circuit and the receiver circuit.

ELECROMAGNETIC INDUCTION PRINCIPLE

Magnetism and electricity are two natural phenomena whose processes are reversible. When H.C. Oersted (1777-1851) proved that around a current-carrying wire there is a magnetic field (meaning that electricity creates magnetism), scientists began to think about the relationship between electricity and magnetism. In 1821 Michael Faraday (1791-1867) proved that a changing magnetic field can cause an electric current (meaning a magnet produces electricity) through a very simple experiment (Figure 2.3). A magnet that is moved in and out of a coil can produce an electric current in the coil.

Galvanometer is a tool that can be used to determine whether there is an electric current flowing. When the magnet is moved in and out of the coil, the galvanometer needle deviates to the right and to the left. The movement of the galvanometer needle indicates that the magnet being moved in and out of the coil creates an electric current. Electric current can occur if there is an emf at the ends of the coil (electromotive force). The emf that occurs at the ends of the coil is called the induced emf. Electric current only appears when the magnet moves. If the magnet is stationary in the coil, then there is no electric current at the ends of the coil.

THE PROCESS OF INDUCTION MOVEMENT

When the north pole of the bar magnet is moved into the coil, the number of magnetic lines of force in the coil increases. This increase in the number of lines of force causes an induced emf at the ends of the coil. The induced emf causes an electric current to flow moving the galvanometer needle. The direction of the induced current can be determined by observing the direction of the magnetic field it generates. When the magnet enters, the lines of force on the coil increase. As a result, the magnetic field generated by the induced current reduces the lines of force. Thus, the end of the coil is the north pole so that the direction of the induced current is shown in Figure 2.3(a).

When the north pole of the bar magnet is removed from the coil, the number of magnetic lines of force present in the coil decreases. This reduction in the number of lines of force also causes an induced emf at the ends of the coil. The induced emf causes an electric current to flow and moves the galvanometer needle. The same is true when the bar magnet enters the coil. When the magnet is out, the lines of force in the coil decrease. As a result, the magnetic field generated by the induced current adds to the lines of force. Thus, the end of the coil is the south pole, so the direction of the induced current is shown in Figure 2.4 (b).

When the north pole of the bar magnet is stationary in the coil, Figure 2.4(c), the number of magnetic lines of force in the coil does not change (fixes). Since the number of lines of force is constant, there is no induced emf at the ends of the coil. As a result there is no electric current and the galvanometer needle does not move. So, an induced emf can occur at both ends of the coil if there is a change in the number of lines of magnetic force (magnetic flux) in the coil. The emf that arises due to a change in the number of magnetic lines of force in the coil is called induced emf. The electric current generated by the induced emf is called the induced current. The occurrence of induced emf and induced current due to changes in the number of magnetic lines of force is called electromagnetic induction.

Figure 2.3

ELECTROMAGNETIC FIELD FACTOR

The magnitude of the induced emf can be seen in the size of the galvanometer needle angle deviation. If the angle of deviation of the galvanometer needle is large, the induced emf and the resulting induced current are large. There are three factors that affect the induced emf, namely:

1. The speed of motion of the magnet or the rate of change of the number of lines of magnetic force (magnetic flux)
2. Number of turns (N)
3. Magnetic field (B)

Like electric flux, magnetic flux can also be described as the number of field lines that pass through a surface.

The electric flux produced by the field B on a surface with area dA is:

Experiments conducted by Faraday showed that a change in the magnetic flux on a surface bounded by a closed path will cause an emf. Faraday concluded that the magnitude of the resulting emf is:

LENZ LAW

The negative sign of Faraday's law corresponds to the direction of the induced emf. Lenz's law (1804-1865) states that the direction of the induced current is such that it creates an induced magnetic field that is opposite to the direction of the changing magnetic field.

From Figure 2.6 (a) above, it can be seen that if the magnetic field increases (up), the induced magnetic field will appear in the opposite direction to the main magnetic field (down), this induced field will produce an induced emf in the coil in the opposite direction. adjusted according to the right-hand rule as shown in Figure 2.6 (b).

SELF INDUCTION

From the explanation of Biot-Savart's law and Ampere's law (Jean Baptista Biot (1774–1862), Victor Savart (1803–1862), it has been shown that the presence of an electric current flowing in a conductor causes the magnetic field around the magnetic conductor to be produced proportional to the magnitude electric current flowing, for example:

Equations (2.8) to (2.10) show that B is proportional to I, and since from equation (2.2) it is obtained that B is proportional to theta , the magnetic flux is also proportional to the value of I. Therefore, a proportional constant can be obtained, namely :

Where L is the proportional constant between the flux and I which is called the (self) inductance of the system. Because in Faraday's law, changes in electric flux can cause emf, then equation (2.7) can be expressed by:

MUTUAL INDUCTION

The current in coil 1 (Figure 2.9), will produce a magnetic field whose magnetic flux will affect coil 2. If it changes, then the magnetic field in coil 1 will also change, and this will cause an induced emf in coil 2. flows in coil 2 and will produce a magnetic field which will also affect coil 1, this is called mutual inductance (M), which according to Faraday's law is:

ELECTROMAGNETIC RESONANCE PRINCIPLE

The phenomenon of resonance is widespread in nature. Different types of resonance also contain different energies. The sound of a tuning fork results from resonance, just as an earthquake results from resonance, but the energy of the earthquake is much greater than the sound of a tuning fork.

Resonance is a symptom of a system which in a certain frequency tends to absorb more energy from the environment. In other words, resonance is a phenomenon where if an object or object vibrates, then another object with the same frequency will also vibrate. Resonance can transmit energy. As a simple example, if there are 2 tuning forks with the same frequency and sufficient distance, then if our tuning fork presses A so that it makes a sound, then when we hold the tuning fork A until the sound stops, the tuning fork B will also be heard even though it is not. we hit. This is the phenomenon of acoustic resonance. The energy that makes tuning fork B vibrate is generated from sound waves from tuning fork A whose medium is a sound field. It can be said that the essence of this vibration propagation is energy transmission. Similar to sound fields, this can also happen in electromagnetic fields.

ELECTROMAGNETIC RESONANCE

Electromagnetic resonance exists widely in electromagnetic systems. The electromagnetic field itself is an energy field that can provide energy for use in the process of electric current. Given the danger to humans and other organisms in electric fields, magnetic fields are safe and more suitable for use as energy delivery media in magnetic resonance energy transfer.

Electromagnetic waves are waves that can propagate even though there is no medium and consist of an electric field and a magnetic field as illustrated in Figure 2.11. Electromanetic wave radiation itself contains energy. No matter whether there is a receiver or not, the energy of electromagnetic waves is continuously consumed. If we can create a non-radiating magnetic field with a certain resonant frequency, the inductance coil will continue to collect energy, the receiving voltage will increase, and the received energy can be transmitted to the load after being converted by an additional circuit.

In general, electromagnetic systems with the same resonant frequency have weaknesses within a certain range. Two systems with the same resonant frequency will produce a strong magnetic resonance and form a magnetic resonance system. If there are more than two resonant generators within the effective range, they can also join this magnetic resonance system. One resonator can be connected to a continuous power supply to act as an energy source and the other consumes energy, so that this energy delivery system can be realized. In other words, this system can transmit energy from one place to another through an invisible magnetic field (wireless), rather than in the usual way through visible electrical wires.

MUTUAL RESONANCE PRINCIPLE

The basic principle of electromagnetic induction is when an alternating current passes through a coil, around the coil will produce a magnetic field. If in this condition another coil is placed near the coil, a magnetic field from the first coil will also appear around the second coil. This is the reason why wireless energy transfer can occur between the two coils. As explained earlier, mutual resonance is a special state of wireless energy delivery. A special location is that all the coils used to resonate together operate under resonant conditions.

Figure 2.12

Resonance occurs when the self-resonant frequency of the coil is equal to the frequency of the alternating current source, when the equivalent circuit of the coil at high frequency has the smallest impedance. At times like this, most of the energy can be sent through the resonance path. Figure 2.12 shows the occurrence of a combined magnetic resonance process, the yellow color indicates the coil has the same resonant frequency, the blue and red colors indicate the magnetic field generated in the coil, both are identical to each other, this is a simple description of mutual resonance.

LC CIRCUIT

The LC circuit (Figure 2.13) is a resonant circuit consisting of an inductor (L) and a capacitor (C). LC circuits are usually used to generate alternating current sources or as signal generators.

Gambar 2.13

LC CIRCUIT PRINCIPLES

The working principle of the LC circuit in order to produce alternating or oscillating signals is to use capacitors and inductors. Capacitors store energy in an electric field between two plates, based on the magnitude of the voltage between the two plates, while inductors store energy in a magnetic field, based on the magnitude of the current through the inductor. Figure 2.14 explains the working principle of the LC circuit.

Gambar 2.14

In Figure 2.14 above, the far left position shows the start, t = 0 or t = T, where the capacitor value is maximum, and no current flows. When the switch starts to close, that is, between t = 0 to t = T/4, a closed circuit occurs, the capacitor begins to discharge, and the current flows counterclockwise to the inductor and continues to increase.

At condition t = T/4, the capacitor has a minimum value, maximum current flows and is still counterclockwise. From t = T/4 to t = T/2, the current continues to flow charging the capacitor with the opposite side, and the current flowing begins to decrease.

At time t = T/2, no more current flows in the circuit, and the capacitor is maximum. From t = T/2 to t = 3T/4, the capacitor begins to discharge, and the current flows clockwise and continues to increase.

At time t = 3T/4, the capacitor is empty, the maximum current flows through the inductor clockwise.

From t = 3T/4 to t = T, the capacitor begins to recharge, current flows into the capacitor with the same side as the starting side clockwise and continues to decrease until the capacitor is full.

This continues to repeat to the beginning, so that an alternating signal is obtained.

OSCILATION LC CIRCUIT

Figure 2.15

Referring to Figure 2.15, when the switch is closed, the voltage across the capacitor and inductor is the same.

Adjust both sides, and we know that i = -dq/dt, so that equation 2.19 can be written,

If we give a constraint where i = 0, when t = 0, because the current (the inductor cannot change directly), then we can determine the value of the capacitance of the capacitor when it is charging by integrating the current (i) and multiplying -1, we get ,

WIRELESS POWER TRANSMISSION WIRING STRUCTURE

Figure 2.17

In Figure 2.17 above, a simple schematic diagram of a wireless power delivery system is shown using the principle of magnetic resonance induction. The block on the left (marked by the dotted line) is the transmitter circuit, while the block on the right is the receiver circuit for the system. At the transmitter, the alternating current power source is rectified first with the DC module, then enters the LC circuit, in this case Ls and Cs, to create a non-radiative alternating magnetic field signal generator. The resonant frequency of this LC circuit is called.

On the side of the receiver circuit there is also an LC circuit, where Lt and Ct function to generate resonance from the magnetic field generated by the transmitter circuit to receive electrical power. The frequency of the receiving circuit is called.

The value of the magnetic resonance frequency at the receiving end is largely determined by the magnetic resonance frequency in the transmitter circuit. The closer the value or equal to, the greater the value of the resonant current, the stronger the magnetic field and the greater the electrical power that can be received or transmitted.

In the receiving circuit it is necessary to emphasize that the LC circuit does not have to be the same to transmit electric power, as long as the resonant frequency is the same ( = ), the wireless electric power distribution system using magnetic resonance induction can still work.

The farther the distance from the transmitter circuit to the receiver circuit, the smaller the electrical power that can be received by the receiving circuit.

The process of sending electric power without wires can still transmit electric power even though it is blocked by various non-metallic objects, but if it is blocked by metal, then the electrical power cannot be received by the receiving circuit.