Power Electronics
Massachusetts Institute of Technology MIT researchers demonstrated a process known as WiTricity to wirelessly transmit electrical power at ranges usable for indoor applications

Massachusetts Institute of Technology (MIT) researchers demonstrated a process known as WiTricity to wirelessly transmit electrical power at ranges usable for indoor applications.

Researchers Demonstrate Wireless Power

A combined research team at The Massachusetts Institute of Technology (MIT) has demonstrated a process referred to as WiTricity, to wirelessly transmit electrical power at ranges usable for indoor applications. This introduces the possibility of a practical system for eliminating wires for recharging portable electronics, the initial inspiration for the concept.

In addition to improving the convenience of charging portable devices, the elimination of cords enabled by WiTricity can also improve the safety and aesthetics of stationary appliances, such as floor lamps and table lamps. Furthermore, given the challenge of achieving regulatory compliance for electrical safety in medical power supplies (such as for patient-leakage requirements), the technology may have tremendous possibilities in bioinstrumentation. According to Robert Moffatt, research team member and undergraduate in physics at MIT, the system might even be made to work in underwater environments with careful engineering.

Massachusetts Institute of Technology <http://www.mit.edu/>  (MIT) researchers demonstrated a process known as WiTricity to wirelessly transmit electrical power at ranges usable for indoor applications.

The WiTricity system uses a process referred to as magnetically coupled resonance, in which non-radiating magnetic fields perform the transfer of energy. Because the system uses magnetic fields rather than electromagnetic radiation, such as microwaves, calculations show that it is possible to build a WiTricity system that complies with IEEE’s safety standards for general population exposure. The energy transfer process between the source and load is also omni-directional, so line-of-sight obstructions, such as normal room traffic, do not affect operation.

To emphasize these aspects of operation, the members of the research team that developed the concept are pictured in the Figure with the system in operation. Note that a few of the team members are directly in the line of sight between the transmitter and receiver as the system powers a conventional incandescent light bulb.

Just as a conventional resonant LC tank circuit has a mechanical analogy in a spring-mass system, the action of magnetically coupled resonance is similar to the action of synchronized pendulum clocks (see link). However, Moffatt states that to more closely resemble magnetically coupled resonance used in WiTricity systems, the pendulum of one clock would be driven by its movement, while the pendulum of the other clock would be mechanically loaded down (or damped). Also, the mechanical coupling between the clocks could be modeled as a light spring (as opposed to a rigid moving platform) to represent the strong magnetic coupling between the sending unit and the receiving unit.

The basic WiTricity system uses two coils and an oscillating magnetic field to transfer energy levels on the order of 100 W between a source (the sending unit) and a load (the receiving unit) across distances on the order of a few meters. The theoretical limit for efficiency approaches 100%. The magnetically coupled resonance used by the system is similar to magnetic coupling in conventional transformers, but with the key difference that the energy transfer is more a function of tuning the resonance, as opposed to simply reducing the distance (which would achieve higher magnetic flux linkage for conventional transformer action), between the two coils.

Moffatt states that the coil in the receiving unit produces an oscillating field around itself having a magnitude comparable to that of the coil of the sending unit. This field in the receiving unit loads the sending unit, allowing energy transfer to occur.

The magnetic field of the WiTricity system is similar to a superposition of two oscillating magnetic dipole fields, and the general shape resembles the field produced by two bar magnets separated by some distance. The oscillating fields of the two coils are also 90 degrees out of phase, so one magnetic field reaches its maximum strength when the other vanishes. Apart from its significantly higher power-transmission efficiency, this behavior of the composite magnetic field in magnetically coupled resonance is similar to that of a poorly coupled transformer formed by two separated coils.

Moffatt also states that the density of power flow through space (represented by the Poynting vector) diverges from the sending unit coil and converges on the receiving unit coil. This forms a shape that is similar to the static magnetic field between the two poles of a single long bar magnet.

Both the sending and receiving units of the demonstration system consist of copper coils with free ends, each a self-resonant system. The sending unit generates a non-radiating magnetic field oscillating at about 10 MHz. The receiving unit is carefully tuned to this frequency to enhance the transfer of energy from the magnetic field using magnetically coupled resonance.

The resonant nature of these systems ensures the strong interaction between the two units, while the interaction with the surrounding environment is weak. Efficiency varies with coil separation, and is about 50% in the demonstration system when a 60-W light bulb is powered at a range of about 2 m (this drops to 40% for 2.25 m). Methods to improve working efficiency, such as gold or silver plating for the coils, will be applied in the future.

According to Moffatt, an unusually high degree of stability in the sending unit and selectivity in the receiving unit is required for the WiTricity system to work. However, Moffatt states that the apparatus was manually tuned with only moderate effort.

While the demonstration system uses a simple load and crude form factors, it also has the potential to power a laptop using coils no larger than the laptop itself. This arrangement could eventually be adapted into a charging system for laptops and other portable electronic devices that is completely transparent to the user.

To implement such a charging system, capabilities besides the basic transfer of power might be needed. Along these lines, Moffatt states there is also ongoing research in which the sending and receiving units can exchange information, such as power management data, in a way that is similar to the operation of passive RFID systems. For example, the sending unit might slightly modify the amplitude of the transmitted magnetic field to deliver data to the receiving unit, while the receiving unit might modulate its load to transmit data, which would produce a detectable load modulation in the sending unit.

For more information on this system, please visit the special web site created by the leader of the WiTricity research team, Prof. Marin Soljacic: http://www.mit.edu/~soljacic/wireless_power.html.

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