- Reduced electromagnetic interference in wireless charging systems for
electric buses
- Technology enables utilization of wireless charging system within limits of the
Radio Act of Japan
Toshiba has developed a method to enable a safe and simple means for frequently recharging batteries in electric buses through wireless power transfer. Toshiba’s technology has also enabled these power transfers within the limits of Japan’s Radio Act. This was achieved by reducing electromagnetic interference (EMI) from wireless rapid charging systems for electric buses.
Benefits of wireless power transfer
- Electricity can be transferred remotely without cables.
- Charging of electric buses become hassle-free as they are able to charge themselves while parked or as passengers are boarding.
- Energy can be exchanged more freely like information.
Limitations of current wireless rapid charging systems for electric buses
The wireless charging systems used to charge the batteries of electric buses utilize an 85 kHz frequency band, which is becoming the standard for wireless charging systems for electric vehicles, and deliver power at 44 kW.
The magnitudes of radiated EM waves for this high-power class of conventional wireless charging systems will become ten times larger than the specified EMI limits for the 85 kHz band, thus obtaining approval as an Industrial Facility Emitting Radio Waves under Japan’s Radio Act has been assumed to be difficult so far.
Toshiba’s wireless power transfer solution
Suppressing radiated waves is thus an important requirement to realize wireless charging systems for electric buses. Toshiba’s technology achieves this by employing two discrete 22-kW blocks that transfer power in opposite phases, cancelling EM waves from each other and reducing undesirable EM waves.
Employing this technology reduces the strength of the EM field 10m away from the device, to one-tenth or less of its unreduced strength, enabling 44 kW power transfers within the limits of the Radio Act.
This wireless charging solution has been proven through a field test campaign with actual operation of two electric buses in a joint project with Professor Yushi Kamiya of Waseda University. The joint project is commissioned by Japanese Ministry of the Environment.
How Toshiba’s technology works
- Interference coupling between the two blocks causes variation in the magnitude and phase of the radiated EM waves, degrading the cancellation effect.
- Identifying the optimal positioning of the two blocks to minimize interference between them is thus essential.
- Rotating the relative position of the two blocks on a plane will eventually cause interference coupling to reach zero.
- A theoretical analysis and an EM field simulation are used to find the relative angle at which this happens, realizing interference suppression between the two blocks.
