Wireless charging for EVs (Part 2): Electromagnetic induction enters the practical stage

Electromagnetic induction enters practical use

Among the three methods mentioned above, the wireless power supply system using our electromagnetic induction method has entered the demonstration stage. It is scheduled to be introduced for use on actual bus routes after the fall of 2011. In this sense, it can be said that the only wireless power supply method that can be immediately put into practical use is the electromagnetic induction method.

The electromagnetic induction method is described in detail below. The electromagnetic induction method can basically be considered a type of transformer (Figure 4). In other words, it is similar to a transformer that outputs 50Hz frequency after inputting 50Hz frequency when the gap between the cores is zero.

Figure 4: Same as a transformer
Contactless charging is similar to a transformer with a larger gap.

If it is an ideal transformer, the gap is very small and the magnetic lines of force will not leak, so the coupling coefficient (k) is basically 1. However, when used for wireless power supply, a certain gap is required, so the magnetic lines of force will leak. Therefore, k will be less than 1.

In principle, a static circuit with a primary coil and a secondary coil facing each other is equipped with an inverter in any case, and power is received from the inverter via the primary coil to the secondary coil and then connected to the load (RL) (Figure 5). Of course, it can also be converted to DC, but rectification is required.

Figure 5: Capacitors are required for resonance
The static electromagnetic induction method requires the use of capacitors for resonance in order to improve transmission efficiency.

However, the efficiency of the coil alone cannot be improved. The optimal load (ZL) that maximizes the transmission efficiency is expressed as ZL = RL - jωL. As can be seen from this formula, there is a negative reactance on the back side of RL, so a resonance part based on the capacitance component is required. Therefore, whether the capacitor is installed in series or in parallel, resonance is obtained through the capacitance component of the capacitor.

In addition, in order to prevent the fluctuation of the secondary coil from negatively affecting the voltage fluctuation rate of the primary coil, a capacitor is often installed in the primary coil to improve the power factor of the system. So, where should the capacitor be installed? This article lists about nine installation examples and introduces four representative ones in detail.

When the k value is large, the mutual inductance M is also large, as shown in the "/series mode" of Figure 5, sometimes it is sufficient to configure only a series capacitor in the secondary coil to compensate for magnetic leakage. When the k value is small, as shown in the "/parallel mode", a capacitor with the resonant frequency of the secondary side self-inductance as the power supply frequency is configured in parallel in the secondary coil part.

In order to improve the power factor of the power supply, a series capacitor is generally configured on the primary side like the "series/parallel method", but sometimes a parallel capacitor is configured to provide reactive excitation from the primary coil to the air gap like the "parallel/parallel" method.

In fact, the inter-coil transmission efficiency (η) of electromagnetic induction and magnetic resonance methods is proportional to the square (α) of the product of the coupling coefficient (k) and the resonance peak value (Q) (Figure 6). Our 30kW electromagnetic induction method has an inter-coil efficiency of about 92% when α is about 103.

Figure 6: Transmission efficiency proportional to α
The transmission efficiency η between coils in all cases, including the electromagnetic induction method, is proportional to the square of the kQ product α. The black dots are the experimental results of the electromagnetic induction method and the blue dots are the experimental results of the magnetic resonance method.

We are still developing the magnetic resonance method, and have already reached the level of the black dots in Figure 6. At this level, even at a distance of about 60 cm to 1 m, the inter-coil efficiency can be achieved by increasing the Q value to about 60%.

Debuted in the 1980s

Automobiles have a long history, and EVs actually appeared earlier than engines. As early as the dawn of automobiles, there was a period when EVs were popular. Later, due to the convenience of gasoline as a fuel, EVs gradually withdrew from the stage of history, but recently, the era of EVs is finally expected to return to the stage.

Wireless power supply for cars was born in the 1980s. At that time, cheap small inverters appeared, and the development of wireless power supply was put on the right track. In 1995, the PSA Peugeot Citroen Group implemented the "Tulip (Transport Urbain, Individual et Public)" plan for a non-contact charging system using electromagnetic induction in France, which is the prototype of our current wireless power supply system. This system sets a primary coil on the ground. When a car equipped with a secondary coil under the floor stops on the primary coil, information is exchanged between the primary and secondary sides to control the charging amount according to the power required by the vehicle. The output power at that time was relatively small, at 6kW.

The wireless power supply system that can transmit electricity with a higher output power is the "IPT (Inductive Power Transfer)" of Germany's Wampfler. Currently, dozens of units have been imported overseas, and 4 units have been imported in Japan. Its principle is the same as the Tulip project, but the output power can reach 30kW. Wampfler has developed a system that communicates with the BMS (Battery Management System) that allows the vehicle battery to reach the optimal charging state and obtains the values ​​required for charging. IPT is equipped on buses and completes charging from the bottom of the vehicle when passengers get on and off the bus at bus stops. It can be said to be an early practice of the basic concept of wireless power supply for electric buses.

Japan is also starting to conduct empirical tests

Showa Aircraft Industry also manufactured the "WEB-1 (Waseda Electric micro Bus No. 1)" micro electric bus for Waseda University in 2004 with the cooperation of NEDO, and equipped it with the IPT of Winfull (Figure 7). The on-board battery used the sodium molten salt battery "ZEBRA Battery" of the Swiss company MESDEA, and later changed to a lithium-ion rechargeable battery, but both methods have the problem of high battery costs.

Figure 7: Micro electric bus "WEB-1" equipped with a wireless power supply system
The initial contactless power supply system of WEB-1 used IPT from Winfull, and the on-board battery was equipped with sodium molten salt batteries and capacitors. Later, it was changed to a self-developed wireless power supply system and lithium-ion rechargeable batteries.

Therefore, we reduced the initial introduction cost by reducing the number of batteries as much as possible, and also reduced the weight of the vehicle to improve fuel efficiency. However, reducing the number of batteries will shorten the driving distance per charge, so instead of fully charging the batteries at the terminal and station, the basic model is to charge the batteries multiple times in a short period of time.

The test results conducted by IPT at Waseda University Honjo Campus showed that even small batteries can be used. In particular, the CO2 emission reduction effect is very significant.

However, wireless power supply systems manufactured overseas have many problems, such as large size, high price, and inability to ensure large gaps. Therefore, from 2005 to 2008, we started to work on the localization of wireless power supply systems with the cooperation of NEDO.