3.1. Wireless In-Wheel Motor
We have developed a novel driving system named the wireless in-wheel motor (WIWM). It integrates the driving system and dynamic charging system, which include motor, inverter, rectifier, control unit, cooling system, and receiving coil in the unsprung area. The latest one, named the third generation of wireless in-wheel motor (WIWM-3), is shown in
Figure 2.
The uniqueness of WIWM is the position of the receiver coil. There is a regulation for minimum under clearance of body side parts to avoid obstacles. However, the parts set into the unsprung area are not regulated, because they move with the tire due to its unsprung setting, and it can avoid road obstacles with the tire itself. Therefore, the air gap with the transmitter coil is kept constant even when the suspension is displaced and the air gap can be minimized. Moreover, the receivable energy can be chosen by a number of the receiver coils and it can adjust a lot of variations of vehicles.
WIWM adopts magnetic resonance coupling, and thus, minimizing the air gap is effective in improving the WPT efficiency. Moreover, the air gap affects parametric optimization to maximize the efficiency of the system, and the constant air gap is suitable for efficient control. This DWPT system has a series–series (SS) type WPT system. SS type can realize high efficiency with robust control [
22]. The equivalent circuit of SS type WPT system is displayed in
Figure 3. The equivalent load
when WPT system transmits with maximum efficiency is described as below:
Here,
R is resistance,
L is inductance, and
is the frequency of transmitter current. The index number 1 is the transmitter side, and 2 is the receiver side.
is mutual inductance between the transmitter coil and the receiver coil. Then, the equivalent load
with voltage load like battery is described as below:
Here,
V is voltage, and
I is current. When the voltage of the transmitter side
and circuit parameters are given, the equivalent load is given by the voltage of the receiver side alone. System configuration of WIWM-3 is shown in
Figure 4. There is an up-converter between the battery and WIWM-3, and the equivalent load can be controlled to
.
WIWM-3 has six states of DWPT. Received energy from the transmitting coil can be used for driving directly or charging the battery or for both these operations. They are decided by the battery state and balance of driving output and output energy from WPT .
- (a)
Driving with Energy of Battery and WPT
When is bigger than , driving output is supplied by both battery and WPT receiving energy.
- (b)
Driving with only Energy of WPT
When and are same, driving output is supplied by only WPT receiving energy. WIWM-3 has an active rectifier, thus can be controlled by pulse width control of power device. Even though is much bigger than the received energy, it can be controlled just as . Therefore, when the battery cannot be charged, for example, when the battery state of charge (SOC) is high enough, driving output is also supplied by the WPT receiving energy alone.
- (c)
Driving with only battery power
When there is no transmitter coil, WIWM-3 drives only with battery power.
- (d)
Driving and Charging using WPT energy
When is smaller than and the battery is chargeable, the received energy from WPT may be applied to both driving and battery charging.
- (e)
Charging with Energy of Regeneration and WPT
When the motor is regenerating, and the battery is chargeable, both regenerating energy and WPT energy are used for charging the battery.
- (f)
Charging with Energy of WPT
When is zero, the vehicle is stopping or coasting down, WPT energy supplies to only the battery.
- (g)
Not Driving or Charging
When the battery is not rechargeable, power devices of the active rectifier short circuit with the receiver coil and stop charging.
All the states are displayed in
Figure 5.
3.2. Efficiency of WPT
In this study, the parameters of the WPT system are based on WIWM-3. The specification of the WTP system is shown in
Table 2.
is the voltage of battery output. These parameters are the measured values set at a nominal position of 85 kHz, which is in the regulated frequency range of SAE J2954. SAE J2954 is a standard of stationary WPT system for electric vehicles. Coil size consists of the wire, coil case, and ferrite blocks. The transmitter coil and receiver coil are displayed in
Figure 6. The maximum voltage of the resonant capacitor of the transmitter side will be 7 kV, then the board of resonance capacitors is set into the center of the coil to decrease line voltage outside of the transmitter coil case. Charging energy of DWPT is decided by the WPT power, vehicle speed, and the receivable area. Then the transmitter coil is bigger than the receiver coil due to the increase in the received energy. The effective voltage of WPT
in the form of a rectangular wave, and the minimum
is the fundamental wave of the battery DC voltage
. Transmit efficiency is evaluated in the WPT bench displayed in
Figure 7. The output of AC/DC converter which is supplied AC voltage from the distribution board is connected to both transmitter side and receiver side. Then,
and
are the same during evaluation and are not optimized to maximum efficiency. The inverter of the transmitter consists of an SiC power device due to high-frequency switching. The specification of SiC power device of the transmitter is in
Table 3 [
23]. The rectifier can be used for synchronous rectification. The test result at 85 kHz is shown in
Figure 8. DC Voltage supply is 499.3 V, and thus,
and
are in the range of the specification. DC-to-DC efficiency which includes the inverter loss, copper loss, iron loss, and rectifier loss reached 92.5% with 20 kW output.
Reduction of theoretical maximum efficiency caused by lateral misalignment, which is calculated with equation described below, is also evaluated.
Here,
k is the coupling factor, and
Q is the quality factor of the coils. The result of theoretical maximum efficiency change by misalignment is displayed in
Figure 9. With 60 mm lateral misalignment, there is 0.6% reduction of theoretical efficiency.