# Design of a Rotary Transformer for Installations on Large Shafts
## 5 Validation Over a Reduced-Scale Prototype  
Finally, the design procedure of rotary transformers proposed in this paper was validated using a small-size prototype due to the cost of realization of a full-scale prototype and the availability of materials. In particular, this test aimed to verify and investigate the possibility of transferring power contactless from primary to secondary with split C-ferrites forming the two cores. The prototype had inner and outer diameters equal to 46.5 mm and 93.5 mm, respectively, with a 1 mm air gap between the cores on the two sides. Therefore, forty-six manganese-zinc (MnZn by TDK, model B67350) C-ferrites were used: twenty of them were arranged to form the inner core, whereas the outer core was made with the remaining twenty-six. The size of each C-ferrite is reported in Figure 10.  
The windings had the same number of turns as the full-scale device, i.e., \(N_{1}=11\) and \(N_{2}=13\), and they were realized with a 0.2 mm enamelled copper wire. Three parallel wires were exploited in each coil to limit the values of \(R_{1}\) and \(R_{2}\). It is important to underline that a support both for keeping the ferrites and the wires in the correct position was needed. Thus, a small structure was created using nylon-fiber with a 3D printer. Figure 11a,b show some details regarding the realization of the cores and the geometry of the supports, as well. In addition to the practical realization of the prototype, a 3D model was created in ANSYS Maxwell (see Figure 12). This model was adopted to perform some predictions on the values of the inductances \(L_{1}\), \(L_{2}\) and \(M\), in addition to the transferred power \(P_{2}\), as reported in Table 2. Furthermore, the electrical parameters were identified using the LCR meter LCR-819 by GW-Instek. Figure 10: Geometrical dimensions of each C-ferrite adopted in the reduced-size prototype. Quotes are reported in mm. Figure 9: Electromagnetic model of the rotary transformer with C-shaped ferrites forming the cores: (**a**) top and (**b**) perspective views of the complete 3D model. It must be underlined that this device was designed to work with reduced supply voltage \(V_{\text{DC}}=8\,\text{V}\) and switching frequency \(f_{\text{sw}}=5\,\text{kHz}\) as well as load resistance \(R_{\text{L}}=5\,\Omega\) with respect to the full-scale transformer. A first set of tests was performed in these conditions. A square-wave generator was used in ANSYS Maxwell to emulate the behaviour of the H-bridge driven in square-wave modulation adopted in practice to test the power transfer of this device. On the other hand, the evaluation board TI C2000(tm) LaunchPad(tm) Piccolo F28069M, together with the converter board TI BOOSTYL-DRV8301 BoosterPack, was adopted in practice for running the power transfer test. Figure 12(a) shows the adopted setup. Results of both simulation and measurement show that both the inductance and power values were close, despite the fact that the materials could not be exactly characterized in the FEM software, the volume of the coils did not match the actual and the real supply voltage was far from being an ideal square wave. This mismatch in the harmonic content of the input voltages explains and partly justifies the big difference between the resulting shape of the voltage and current waveforms. Figure 12(a),b show the real test setup adopted for running these tests and a comparison between the measured waveforms, \(v_{1}(t)\), \(v_{2}(t)\), \(i_{1}(t)\) and \(i_{2}(t)\), and the simulated waveforms. In particular, this picture shows the mismatch in the harmonic content due to the different adopted voltage generators. Hence, to provide a better comparison between the virtual test and the measurements, the parameters identified in ANSYS Maxwell were used to run a simulation in Simulink(r). Namely, \(L_{1}\), \(L_{2}\) and \(M\) were inserted in a mutual inductor model and the equivalent circuit reported in Figure 3 was built.