Published on November 6th, 2018 | by Andy Miles0
Wireless Charging At 120kW By Electromagnetic Induction
November 6th, 2018 by Andy Miles
The principle of electromagnetic induction is that a current flowing through a conductor will generate a magnetic field, and conversely, moving a conductor in a magnetic field will generate a current. In the case of a continuous stable magnetic field, the conductor has to be physically moved to create a current in it. A single piece of wire moving through a magnetic field will produce only a small current. Where a wire is wrapped into a coil of wire, with many thousands of turns, that small current is repeated in each part of the coil, and so combined, becomes a substantial amount of electrical power. That is how a simple dynamo works, where a coil of wire is moved in a strong magnetic field to create a current.
AC & DC
There are 2 different types of electrical current. One is called direct current (DC), and the other alternating current (AC). A DC source, like a battery, has one positive terminal and one negative terminal. Current will flow in an electrical circuit joined to the battery continuously, from one terminal to the other. An AC current source has one neutral terminal and a live terminal. The live terminal reverses polarity a certain number of times every second, usually around 50 times for domestic supplies, known as 50 cycle current, or 50Hz (Hertz). The voltage at the live terminal starts off as, say, 340 V, which reduces smoothly down to 0 and then continues to minus 340 V in what is known as a sine wave pattern. The sine wave is just the graph plot of the voltage over time which has a smooth letter “S” shape. That sine wave shape is often used as a symbol to denote AC current on wiring diagrams. Although the voltage is constantly changing, a nominal voltage is attributed, as a kind of equivalent to a DC voltage. The nominal voltage on a domestic supply might be 240V, and the peak voltage to get that would be 240 times the square root of 2, which is the 340 V I mentioned earlier.
In an AC circuit, instead of the current flowing continuously in one direction, it vibrates backwards and forwards according to the number of cycles per second. One analogy is a bicycle chain, which behaves like DC current when the pedals are turned continuously, but like AC current when the pedals are dithered backwards, and forwards, rapidly. You can sometimes hear this 50 cycle buzzing coming from devices that use AC current. In AC current moving through a conductor, the magnetic field is expanding out as the voltage increases and collapsing down to zero when the voltage is zero. So, in AC current the magnetic field is pulsing, and an AC current will be induced in an adjacent conductor which is not moving, by the movement of the magnetic field as it expands and collapses.
This is how transformers work, which most people are familiar with. They transform the high voltage from the mains supply to a lower voltage used by an electrical device. A coil of wire known as the primary coil is connected to the mains supply, and a coil of wire known as the secondary coil is connected to the output terminals of the transformer . There is no physical connection between the 2 coils, but they are wrapped around a metal core which carries the magnetic fields. For instance, the primary field might have 10 times the number of turns in the coil as in the secondary coil, in which case, the voltage in the secondary coil will also be 10 times less. If the voltage in the primary coil was 100 V, then the voltage induced in the secondary coil would be only 10 V. So, a transformer is a good example of a device which works by electromagnetic induction, where current in one coil of wire is induced by magnetic fields from the current in an adjacent coil of wire.
Induction Charging for EVs
This technology is nothing new, and has been around for a very long time. Novelty comes in new applications of the induction technology. One of those possible applications is to induce a charging current in an electric vehicle without using any connecting wires. Instead of wires, we have a primary coil set into the floor of a garage, or the surface of the driveway. The secondary coil is attached to the underside of the electric vehicle. When the vehicle is parked with the secondary coil directly above the primary, and the AC current switched on, a current is induced in the secondary coil and that current is passed through the onboard charging system of the electric vehicle, just as it would normally when connected with a standard Mennekes Level 2 charger connector.
The technical difficulties with that are:
- Efficiency decreases as the distance increases between the two coils.
- Partly related to the above, the efficiency is also decreased if the coils are not perfectly aligned.
- The primary coil will induce a current in any conductor in proximity to it, so, if left switched on, is a potential electrical hazard.
An inefficiency with any AC charging current is that the charging system has to convert it to DC to charge the batteries, and some power is lost in the process. That inefficiency is the same whether connected by induction or by a lead. Most of the fast charging systems using 50 kW of power or more use DC current, as that is more efficient, in that it doesn’t need to be converted from AC current. Charging via electromagnetic induction requires AC current, so DC fast charging has to be cable connected.
Induction charging being possible does not necessarily make it desirable. It is possibly more convenient to charge without having to plug in, but the car has to be manoeuvred to an exact location to achieve that, so I am not convinced it is something I would ever want on my electric vehicle, especially as I would have to have a lead and socket in any case, in order to be able to charge up at other locations where no inductive charging is available. It all means extra weight and extra systems for very little advantage.
Keep Charging on
A more interesting possibility is to have the primary coil or coils under a road surface, which are continuous. Electric vehicle drivers would be able to charge up without stopping. This could be a separate charging lane, and people billed for using it via number plate recognition as they enter the lane. However, at the normal charging rate of adding about 15 or 30 miles of range for every hour of charging, this would provide less new range than would be consumed in the process of attaining it. It would take induction charging at high rates of power to actually add useful range during a relatively short drive along a charging lane.
Breakthrough at Oak Ridge
Researchers at the US Department of Energy’s Oak Ridge National Laboratory, (ORNL) have now come up with an induction charging system capable of operating at a power of 120 kW, with up to a 6-inch gap between the primary and secondary coils, and at an efficiency of about 97%. This would not only require a secondary coil built into the car, but also a specialist charging system to convert the high power AC current to DC current for charging the battery. Such a system could also be used in parking bays or at rest-stops on the highway to enable quick and convenient fast charging. This would have positive advantages, as leads for fast charging are thick, cumbersome, and heavy, and the car has to be fairly precisely positioned in the parking bay to charge up even when using a lead. It could not be used with any current electric vehicle, because it would require specialist equipment both inside and outside the car.
For the future, having achieved 120 kW of power, the team at ORNL hopes to go on to develop devices which will transfer up to 350 kW, enabling long-range batteries to be charged up to 80% in only 15 minutes or less.
In the report on the ORNL website, they state:
“This breakthrough significantly advances the technology needed to encourage greater adoption of electric vehicles by increasing their range and the ease of recharging, and in turn supports an energy-efficient mobility system for the nation’s economic success,” said Moe Khaleel, associate laboratory director for Energy and Environmental Sciences at ORNL.
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