The gap between an electric vehicle’s (EV) transmitter and receiver coils during wireless power transfer (WPT) affects six parameters:
- Coupling coefficient
- Mutual inductance
- Alignment sensitivity
- Magnetic field strength
- Resonant frequency
- Safety/EMI
This article briefly discusses the effect of each parameter, followed by a few case studies to understand the practical implications.
Anyone who has seen and understood how WPT works, even for basic applications, knows that the transmitter and receiver coils must be as close as possible to each other for a higher power transfer. The same applies to WPT EV charging.
Let’s consider the six parameters shown in Figure 1 and determine if they are directly or indirectly proportional or have some other relationship with the performance of WPT in EV charging.
Figure 1. The six parameters affected by the distance between an EV’s charging coils during WPT. (Image: Rakesh Kumar, Ph.D.)
Coupling coefficient
The coupling coefficient measures the magnetic coupling between the transmitter and receiver coil. Its value ranges from ‘0’ to ‘1,’ where ‘0’ denotes no coupling and ‘1’ represents perfect coupling.
It’s important to note that the coupling coefficient decreases as the spacing between the coils increases. This decrease directly affects the power flow from the transmitter to the receiver coil, illustrating the direct relationship between coil spacing and power transfer efficiency.
Mutual inductance
Mutual inductance is the ability of one coil to induce an electromotive force in the other through the shared magnetic field. Mutual inductance and self-inductance of the charging coils are also directly related.
The mutual inductance decreases as the spacing between the coils increases. Like the coupling coefficient, reduced mutual inductance means decreased power flow between the charging coils.
Alignment sensitivity
As the spacing increases, the system becomes more sensitive to misalignments. Interestingly, as the outer diameter of the charging coils decreases, the effect of misalignment on power transfer is more sensitive, and reduced power flow is observed at greater distances.
Misalignments can further reduce the coupling coefficient and mutual inductance, thus increasing the efficiency loss due to increased spacing.
Magnetic field strength
The strength of the magnetic field generated by the transmitting coil diminishes with distance. A larger air gap means the magnetic field reaching the receiving coil is weaker, leading to slower charging times and reduced overall system performance.
Resonant frequency
The resonant frequency of the WPT system may need to be adjusted based on the spacing between coils to maintain efficient power transfer. This is because the coil spacing affects the inductance and capacitance values that determine the resonant frequency.
The capacitance at the receiver coil profoundly affects the power transfer efficiency. The higher the capacitance value within a particular range, the better the power transfer efficiency.
Safety and EMI
Larger spacing can lead to increased electromagnetic interference (EMI) leakage and safety concerns due to stray magnetic fields. Proper shielding and system design are necessary to mitigate these issues, especially as coil spacing increases.
Case study
Researchers from Curtin University, Malaysia, have investigated the relationship between the above-stated parameters and spacing between the charging coils, revealing interesting numbers.
Before we look into the numbers, the standard test condition for the investigation is below:
- Number of turns in the transmitter and receiver coil = 15
- Spacing between each turn of the coil = 1 mm
- Charging coils outer diameter = 250 mm
- Charging coil inner diameter = 100 mm
- Self-inductance of the coils = 44.41 uH
- Wire diameter of the coils = 4 mm
Figure 2 shows the variations in coupling coefficient and mutual inductance for variations in distance between the charging coils from 100 to 250 mm in steps of 50 mm. It clearly shows that as the distance increases, both the parameters have seen a noticeable decrease in value.
The data suggests that a distance of 100 mm offers only a 0.15 coupling coefficient, and for a better result, it is understood that the distance has to be much smaller.
Figure 2. This graph shows the decreasing coupling coefficient and mutual inductance with the increase in space between the coils. (Data: Wui et al. / Image: Rakesh Kumar, Ph.D.)
Figure 3 highlights the receiver capacitance value for better power transfer efficiency. Under the above-mentioned standard test condition, a receiver capacitor value of 50 nF raises power transfer efficiency above 80%. The distance between the charging coils also affects power transfer efficiency.
Figure 3. This graph shows the increase in power transfer efficiency between the transmitter and receiver coils for an increase in the capacitor value in the receiver coil for 150 and 200 mm spacing between the coils. (Data: Wui et al. / Image: Rakesh Kumar, Ph.D.)
Summary
Coupling coefficient, mutual inductance, and magnetic field strength are the three parameters linked to each other. Improving any of them naturally improves the other two parameters and thereby enhances the power transfer efficiency of the EV WPT.
Resonant frequency has more to do with the proper choice and design of circuit elements in the WPT system. Safety and EMI cannot be neglected because flux leakage is possible for a large distance between the charging coils.
However, the most sensitive and often underrated parameter regarding the distance between the coils is the outer diameter of the charging coils. Although the outer coil diameter can be increased to increase the EV charging power levels, it comes at the cost of improved sensitivity of coil alignment and affects power transfer efficiency.
References
- A Study on Transmission Coil Parameters of Wireless Power Transfer for Electric Vehicles, Serbian Journal Of Electrical Engineering
- Wireless Charging of Electric Vehicle While Driving, IEEE Access
- The Impact of Coil Position and Number on Wireless System Performance for Electric Vehicle Recharging, MDPI
Images
- Figure 1-3, Rakesh Kumar, Ph.D.
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Filed Under: FAQs, Wireless charging