**5. Experimental Results**

The proposed control strategy was validated experimentally by mechanically connecting a three-phase wound rotor, 11 kW, 4 poles, 400 V/50 Hz induction machine to a 27 kW DC machine acting as a prime motor. Both the electric machines were driven by power converters based on Semikron IGBT modules, while the system control was implemented on *dSpace 1103* hardware whose digital outputs were properly routed to the IGBT drivers' inputs. The whole experimental setup is shown in Figure 3. In order to evaluate the sensorless estimation errors, a 4000 pulse/round incremental encoder mechanically connected to the DFIG was used to determine the rotor speed and position.

**Figure 3.** Experimental setup.

The motor parameters used to build the observer matrix were estimated by means of standard blocked rotor/no-load IEEE 112 tests executed on the DFIG. Since no additional parameter tuning was carried out, the experimental results also highlight the robustness of the adaptive observer with respect to substantial parameter deviations in the characterization of the induction machine. Indeed, standard IEEE 112 tests are affected by non-negligible errors, especially when compared to more advanced off-line estimation methods [37]. This approach allows us, therefore, to portray the improved accuracy in rotor position estimation granted by the adaptive observer in comparison to a standard observer.

To validate the effectiveness of the conceived adaptation law both at different speed values and different torque values, the following test was performed:


The experimental results of the whole test are shown in Figure 4 (rotor speed), Figure 5 (rotor axes currents), and Figure 6 (rotor electric angle estimation error). From Figure 4, where both the measured and estimated rotor speed are plotted, it can be deduced that the DFIG observer was able to effectively track the real system speed with a negligible error, which stayed always under 0.5%. Naturally, the observable speed over- and undershoots are linked to the corresponding step variations of the DFIG reference torque. From Figure 5, where both the reference and actual rotor axes currents are plotted, it can be deduced that the control system (driven by the estimated values of the rotor speed and position) was able to effectively drive the rotor currents in the whole speed range. It should be pointed out that the ripples in the rotor axis current are visible only around the DFIG synchronous speed. This is indeed the result of the frequency injection in the rotor currents, which is used to keep the adaptation law sensible to the rotor voltage projection error when the rotor voltages become too small (in this instance, when the DFIG was around the synchronous operation mode). Finally, Figure 6 shows the rotor position error obtained with the proposed adaptation law versus that which would affect the observer when the adaptation law was not engaged. It can be deduced that the performance of the observer is appreciably improved: while the position error was kept between −5 degrees and 8 degrees with the adaptation law, it varied between 10 degrees and 20 degrees when the position error was not compensated. The maximum improvement can be noted at the low speed, where the error is 3 degrees, versus 17–20 degrees at the high speed.

**Figure 4.** Behavior of the estimated and actual rotor speed in the whole test.

**Figure 5.** Behavior of the reference and actual rotor axes currents in the whole test.

**Figure 6.** Behavior of the position estimation error with/without adaptation law in the whole test.

Figure 7 shows the behaviors of the first and second phase stator currents (green and violet lines) versus the corresponding rotor currents (blue and red lines) when the rotor speed was equal to 130% of the synchronous speed and the reference torque was set to 100% of the rated value. As expected, while the stator currents oscillated at the grid frequency (50 Hz), the frequency of the rotor currents were linked to the actual rotor speed. Given the value of the rotor speed, the resulting frequency was 15 Hz—this value is coherent with the difference between the synchronous speed and the actual one. It can also be noted that the first phase rotor current lagged after the second phase current—this is also expected since the DFIG was working with a rotor speed higher than the synchronous one.

**Figure 7.** Behavior of the DFIG stator and rotor currents at the rated torque in the high speed region.
