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Toyo Denki Announces Successful Test of Electric Vehicle Equipped withWorld’s First Wireless-Power-Transfer In-Wheel Motor
Tokyo, May 20, 2015 — Toyo Denki Seizo K.K. (hereinafter Toyo Denki) announced today that it
has conducted a successful test of an experimental electric vehicle (EV) equipped with a wirelessin-wheel motor (W-IWM) employing wireless power transfer. The W-IWM, the first of its kind inthe world, was jointly developed by a research group led by Associate Professor Hiroshi Fujimotoof the Graduate School of Frontier Sciences at The University of Tokyo, NSK Ltd, and Toyo Denki.
Toyo Denki is committed to the advancement of next-generation EVs through the ongoingdevelopment of vehicle electrical products that employ new motor drive technologies.

1. Details of the Invention
Conventional in-wheel motors (IWMs)1 for EVs are powered by connecting electrical wires to thevehicle power source in order to drive the wheels. The joint research sought to develop a wirelessin-wheel motor (W-IWM) in which both power and control signals are transmitted wirelessly(Figure 1). This led to the world’s first successful test of an experimental EV equipped with theW-IWM (Figure 2).
The W-IWM, the first of its kind in the world, uses magnetic resonance coupling2 to achieve
wireless power transfer between two coils separated by a gap of 10 centimeters. This configuration
fully eliminates electrical wires between the vehicle chassis and the wheels.

Figure 1 Wireless In-Wheel Motor (W-IWM)
Figure 2 Experimental Vehicle with W-IWM

2. W-IWM System
Due to the motion of suspension links, the relative position of the IWM and the vehicle chassischanges, requiring the use of a magnetic resonance coupling that is not affected by changes in thepower coil alignment. Figure 3 shows a simplified configuration diagram of the W-IWM.
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DC-DC
converter

InverterConverter

Inverter
Motor

Vehicle chassis
Primary

Wheel
Secondary

Figure 3 Configuration Diagram of W-IWM

There are a few possible circuit configurations for magnetic resonance coupling, depending on how
the resonance capacitor3 is inserted. The W-IWM does not require circuit switching during power
regeneration, enabling the resonance capacitor and coil to be in series on both the primary andsecondary side. The primary power conversion circuit uses a DC-DC converter4 and inverter.5 Afterthe battery voltage is changed to a suitable voltage, the supply voltage is converted by the inverter
to a high-frequency voltage, with the same frequency as the resonance frequency of the coil andresonance capacitor. The converted power is wirelessly transferred by magnetic resonance coupling.
The received power is converted to DC by the converter 6 in the secondary circuit, driving apermanent magnet synchronous motor7 using a motor drive inverter. The system can regenerate
electric power without any problems.

3. Research Background (IWM Potential)
EVs offer various advantages due to their superior driving performance. Conventional IWMs arehoused in the vehicle wheel and supplied with power through wires connected to the vehicle chassis,
which involves a complex connecting configuration of power cables and control wiring. There isalso the risk of an interruption in the wiring due to bending from vibration, freezing in cold
environments, and impact from debris. Vehicle motors are warranted to last more than 10 years, andthe risk of wiring interruption has been a barrier to the adoption pf IWMs in vehicles.
To overcome these shortcomings by completely eliminating wires, the research group sought todevelop a W-IWM that uses wireless power transfer employing magnetic resonance coupling, andpower conversion employing SiC devices.8 The successful W-IWM that was developed removes therisk of wiring interruption, significantly improving safety and reliability.

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4. Social RelevanceEVs deliver outstanding environmental performance, while also achieving superior drivingperformance due to their quick motor response. By housing the drive source in the wheels, IWMs
enable power to be directly transferred to the wheel, which is ideal as a drive system. EVs equippedwith an IWM directly transfer the drive power to all four wheels independently using drive power
control, effectively eliminating the mechanical loss that comes with using a drive shaft, while also
reducing the vehicle weight and enabling independent control of each wheel. This improves safety
by preventing tire slippage, and realizes other advantages such as extending the maximum driving
distance by enabling drive power to be optimally adjusted to each wheel.
However, conventional IWMs have to use cables to receive power from the vehicle chassis. TheW-IWM eliminates the risk of wiring interruption between the IWM and inverter,3 to improvesafety and reliability. The development of the W-IWM promises to pave the way for the widespread
adoption of IWMs.

5. Future Research
The W-IWM offers the potential for charging while on the move, which could extend maximum
driving distance and eliminate the need for large-capacity batteries. The experimental EV that wasdeveloped has a lower output than EVs that are available to consumers today. Future research willconcentrate on increasing the power output and realizing vehicle power supply on the move fromground sources.

Figure 4 Concept for Power Supply on the Move(on Designated Highways)

6. Possibilities for W-IWM
The W-IWM addresses the risk of wiring interruption to improve reliability, and has the potential todeliver outstanding driving performance in EVs equipped with the motors. In principle, the W-IWM
can also be used in fuel cell vehicles and hybrid vehicles. Looking further ahead, the developmentof infrastructure using ground coils opens the door to EVs that receive power externally without
relying on batteries.

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7. Terminology
1.
2.

3.
4.
5.
6.
7.

8.

In-wheel motor: A motor that is housed inside a wheel.
Magnetic resonance coupling: A wireless power transfer system that uses a resonancecapacitor on the primary (power supply) side and secondary (power receiving) side to achieve
resonant transfer. The system enables power transfer across a larger distance than is possible
with magnetic induction, and offers superior transfer efficiency.
Resonance capacitor: A capacitor that is connected in series with the supplying coil andreceiving coil, for magnetic resonance coupling.
DC-DC converter: A switching power supply that converts DC voltage to another DC voltage,using a regenerating circuit.
Inverter: A device that changes DC voltage to a three-phase AC voltage at a specific frequency
and amplitude, supplying power to drive the motor.
Converter: A device that converts AC voltage to a specific DC voltage.
Permanent magnet synchronous motor: A synchronous motor that, in place of a winding, uses apermanent magnet (a magnet that retains its magnetic properties for an extended period,
without receiving an external magnetic field or current).
SiC device: A device made from silicon carbide (SiC), which has roughly 10 times thedielectric breadown field strength of silicon and delivers high resistance and low loss foradvanced power devices.

Contact
PR, IR, and CSR Department, Management Planning Division,
Toyo Denki Seizo K.K.
Phone: +81-3-5202-8122
Email: contact@toyodenki.co.jp
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