Inductive charging
Inductive charging (also known as wireless charging or cordless charging) is a type of wireless power transfer. It uses electromagnetic induction to provide electricity to portable devices. Inductive charging is also used in vehicles, power tools, electric toothbrushes, and medical devices. The portable equipment can be placed near a charging station or inductive pad without needing to be precisely aligned or make electrical contact with a dock or plug.
"Wireless charging" redirects here. For other power transfer, see conductive wireless charging.
Inductive charging is named so because it transfers energy through inductive coupling. First, alternating current passes through an induction coil in the charging station or pad. The moving electric charge creates a magnetic field, which fluctuates in strength because the electric current's amplitude is fluctuating. This changing magnetic field creates an alternating electric current in the portable device's induction coil, which in turn passes through a rectifier to convert it to direct current. Finally, the direct current charges a battery or provides operating power.[1][2]
Greater distances between sender and receiver coils can be achieved when the inductive charging system uses resonant inductive coupling, where a capacitor is added to each induction coil to create two LC circuits with a specific resonance frequency. The frequency of the alternating current is matched with the resonance frequency, and the frequency is chosen depending on the distance desired for peak efficiency.[1] Recent improvements to this resonant system include using a movable transmission coil (i.e., mounted on an elevating platform or arm) and the use of other materials for the receiver coil such as silver-plated copper or sometimes aluminum to minimize weight and decrease resistance due to the skin effect.
History[edit]
Induction power transfer was first used in 1894 when M. Hutin and M. Le-Blanc proposed an apparatus and method to power an electric vehicle.[3] However, combustion engines proved more popular, and this technology was forgotten for a time.[2]
In 1972, Professor Don Otto of the University of Auckland proposed a vehicle powered by induction using transmitters in the road and a receiver on the vehicle.[2] In 1977, John E. Trombly was awarded a patent for an "Electromagnetically coupled battery charger." The patent describes an application to charge headlamp batteries for miners (US 4031449). The first application of inductive charging used in the United States was performed by J.G. Bolger, F.A. Kirsten, and S. Ng in 1978. They made an electric vehicle powered with a system at 180 Hz with 20 kW.[2] In California in the 1980s, a bus was produced, which was powered by inductive charging, and similar work was being done in France and Germany and Europe around this time.[2]
In 2006, MIT began using resonant coupling. They were able to transmit a large amount of power without radiation over a few meters. This proved to be better for commercial needs, and it was a major step for inductive charging.[2]
The Wireless Power Consortium (WPC) was established in 2008, and in 2010 they established the Qi standard. In 2012, the Alliance for Wireless Power (A4WP) and the Power Matter Alliance (PMA) were founded. Japan established Broadband Wireless Forum (BWF) in 2009, and they established the Wireless Power Consortium for Practical Applications (WiPoT) in 2013. The Energy Harvesting Consortium (EHC) was also founded in Japan in 2010. Korea established the Korean Wireless Power Forum (KWPF) in 2011.[2] The purpose of these organizations is to create standards for inductive charging. In 2018, the Qi Wireless Standard was adopted for use in military equipment in North Korea, Russia, and Germany.
Safety[edit]
An increase in high-power inductive charging devices has led to researchers looking into the safety factor of the electromagnetic fields (EMF) put off by larger inductor coils. With the recent interest in the expansion of high power inductive charging with electric cars, an increase in health and safety concerns has arisen. To provide a larger distance of coverage people would in return need a larger coil for the inductor. An electric car with this size conductor would need about 300 kW from a 400 V battery to emit enough charge in order to charge the vehicle. This much exposure of electromagnetic waves to the skin of a human could prove harmful if not met within the right conditions. Exposure limits can be satisfied even when the transmitter coil is very close to the body.[18]
Testing has been done on how organs can be affected by these fields when put under low levels of frequency from these fields. When exposed to various levels of frequencies, dizziness, light flashes, or tingling through nerves can be experienced. At higher ranges, heating or even burning of the skin can be experienced as well. Most people experience low EMF in everyday life. The most common place to experience these frequencies is with a wireless charger, usually on a nightstand located near the head.[19]
Standards refer to the different set operating systems with which devices are compatible. There are two main standards: Qi and PMA.[13] The two standards operate very similarly, but they use different transmission frequencies and connection protocols.[13] Because of this, devices compatible with one standard are not necessarily compatible with the other standard. However, there are devices compatible with both standards.
Medical implications[edit]
Wireless charging is making an impact in the medical sector by means of being able to charge implants and sensors long-term that is located beneath the skin. Multiple companies offer rechargeable medical implant (e.g. implantable neurostimulators) which use inductive charging. Researchers have been able to print wireless power transmitting antenna on flexible materials that could be placed under the skin of patients.[56] This could mean that under skin devices that could monitor the patient status could have a longer-term life and provide long observation or monitoring periods that could lead to better diagnosis from doctors. These devices may also make charging devices like pacemakers easier on the patient rather than having an exposed portion of the device pushing through the skin to allow corded charging. This technology would allow a completely implanted device making it safer for the patient. It is unclear if this technology will be approved for use – more research is needed on the safety of these devices.[56] While these flexible polymers are safer than ridged sets of diodes they can be more susceptible to tearing during either placement or removal due to the fragile nature of the antenna that is printed on the plastic material. While these medical based applications seem very specific the high-speed power transfer that is achieved with these flexible antennas is being looked at for larger broader applications.[56]