*4.2. Experiment*

The experimental setup is shown in Figure 8, the experimental parameters of compensation circuit are generally consistent with Tables 1 and 2. The coils are planner spiral coil wound by Litz wire, and tightly wound with the turn spacing *d* = 2*a*, the conductor radius *a* = 1.8 mm, the inner radius of the transmitter and receiver coil *<sup>r</sup>*1\_*min* = *<sup>r</sup>*2\_*min* = 2 cm, the out radius of the transmitter *r*1*\_max* = 20 cm, the out radius of the receiver *r*2*\_max* = 19.28 cm. The quality factor of the transmitter and receiver coil are 612 and 623, respectively. The gate drive signal of full-bridge inverter circuit can achieve frequency adjustment, and its duty cycle is 0.5. In the resonance state, the input power can be obtained from the input voltage value and input current value of rectifier bridge. The output power is obtained by measuring the load voltage with a differential probe and measuring the output current with a current probe. Considering that the voltage on the capacitor at resonance is Q (quality factor) times the voltage on the circuit, the tuning capacitor is composed of CBB capacitor series and parallel.

**Figure 8.** Experimental set up.

A comparative experiment is performed for SS and LCC compensation topologies. Figure 9 shows the experimental waveforms of SS and LCC under the system's optimal efficiency load. In Figure 9b, the primary current shows wonky parts. This is because LCC compensation topology is more complicated than SS, and the coil winding and capacitance matching errors are larger, which results

in the failure to realize ideal resonance at the rated frequency. Therefore, the *IP* waveform of LCC structure is not as stable as the SS structure.

**Figure 9.** The input and output waveform of optimal state, (**a**) SS-type; (**b**) LCC-type.

As can be seen from Table 4, the actual system operating efficiency and output power are basically consistent with the design goals. It illustrates the correctness of the above design method. The error between experimental results and simulation values is due to the fact that the actual system cannot fully work in the ideal state, the coil and resonant capacitor cannot be perfectly matched at the rated frequency in experiment. It is verified through experiments that the output power is increased by 13.5% under the same input voltage without a significant decrease in efficiency.

Table 5 shows the experimental data of the power and efficiency of the SS and LCC compensation topology under different loads. It can be seen from Table 5 that both SS and fixed LCC compensated WPT system achieve maximum efficiency at 50 ohms load, which is consistent with theoretical analysis. By adjusting the parameters of LCC compensation topology according to the load resistances, the output power and transfer efficiency of the adjusted LCC compensated WPT system are improved and more stable under different loads.


**Table 4.** System experimental results and simulation values.

**Table 5.** System power and efficiency under different load resistances.


Based on the experiment of transmission distance change under SS and LCC structures, Figure 10 was produced. When the transmission distance changes, the system resonance frequency remains unchanged, so as to observe the vertical offset stability of the system under SS and LCC structures. It can be seen from Figure 10 that for the SS resonant topology, the input current and output current increase with distance; for the LCC structure, the input and output current decrease with distance. In practical applications, this positive correlation of the SS structure is very dangerous.

**Figure 10.** (**a**) Input current and (**b**) output current with respect to distance.

There is a deviation between the theoretical and experimental current values, the main reason is that the mutual inductance measured by the actual wound coil is smaller than the theoretical value. However, the current trend is basically the same.
