# An Inverter Topology for Wireless Power Transfer System with Multiple Transmitter Coils
## 5 Experimental Results
Thus, the excitation switches S\({}_{1}\) and S\({}_{2}\) are off for efficient power transfer. On the other hand, the \(|I_{PT}|\) values of transmitter coils 3 and 4 in Figure 19c,d, are 0.72 A and 0.76 A, respectively. This is an indication that the receiver coil is close to both transmitter coils. Therefore, both switches S\({}_{3}\) and S\({}_{4}\) are turned on. From Table 1, switching pattern 3 is formed as the selected transmitter coil pattern. Figure 19: Experimental waveforms of the system under detection mode operation: (**a**) Transmitter coil number 1 is excited; (**b**) Transmitter coil number 2 is excited; (**c**) Transmitter coil number 3 is excited; (**d**) Transmitter coil number 4 is excited. Figure 18: Experimental results of the output voltage regulation under load change: (**a**) Without output voltage regulation; (**b**) With proposed controller. The measurement results of magnetic coupling coefficient, k, when the receiver coil was laterally moved along the X, Y, Z1, and Z2 directions, are shown in Figure 20. To illustrate the advantage of the proposed transmitter coil pattern selection, all the transmitter coils were first excited throughout the operation. This is switching pattern 15, and the magnetic coupling k obtained in this case is shown in Figure 20a. The coupling k is at the maximum value of 0.2, as the receiver coil is within a 5-cm radius. If the receiver coil is moved further, the coupling k is reduced to 0.01 at 25 cm on the Z1 and Z2 directions. With the proposed transmitter coil pattern selection, the resulted magnetic coupling is shown in Figure 20b. For the case of one-coil excitation, at 15-cm misalignment along the X and Y directions, the coupling coefficient increased from 0.18 to 0.32, which is a 77.78% improvement compared with results in Figure 20a. For the case of two-coil excitation, the coupling coefficient k increased from 0.16 to 0.23, which was a 43.75% improvement at 10-cm misalignment along the Z direction. Note that \"not applicable (N/A)\" is the case of switching pattern 0, in which all the transmitter coils were not activated. Figure 20: Measured magnetic coupling coefficient, k, at various receiver coil positions: (**a**) Without transmitter coil pattern selection; (**b**) With proposed transmitter coil pattern selection. System efficiency measurements when the receiver coil is laterally moved along the +X and +Z1 directions are shown in Figure 21a,b, respectively. The load resistance is fixed at 123.2 \(\Omega\). The output voltage is regulated at 200 V throughout the operation. The proposed multi-coil system with and without transmitter coil pattern selection is compared. At the center (zero horizontal misalignment), the measured efficiencies for both cases are at 83.63%, since the activated coil patterns are identical. When the misalignment in the +X direction is increased, the efficiency of the system with proposed coil selection is increased to its maximum at 90.1% before decreasing to 74.3% at 25 cm. On the other hand, the efficiency of the system without transmitter coil selection is gradually reduced to 38.21% at 25 cm. Similarly, for the +Z1 misalignment direction, the proposed coil selection yields improved efficiencies of 18.9% and 54.5% at 10 cm and 15 cm, respectively. The efficiency of a single-coil system (single transmitter coil and single receiver coil) is also illustrated. At the aligned position, the efficiency of the single-coil system is at 89.8%. That is 6.87% higher than the multi-coil system due to its higher coupling coefficient. As the horizontal misalignment is introduced, the single-coil system efficiency drastically decreases. At 15-cm misalignment, the efficiency is reduced to 44.1%, no matter the direction of the misalignment. With the proposed multi-coil system, the efficiency is increased to 90.1% (in the +X direction). For the +Z1 misalignment direction, the proposed multi-coil system yields an efficiency improvement of 19.6% and 22.6% at 10-cm and 15-cm misalignments, respectively.