# Experimental Investigation of Controlled and Uncontrolled Rectifiers for Low-Power Wind Turbines
## 6 Silicon-Controlled Rectifier  
In the previous experiments for single-phase half-wave and full-wave bridge rectifiers, it was established that it is difficult to control the output voltage of the rectifier if the AC source is intermittent, or during varying load conditions. Therefore, a fully controlled  
Figure 11: Effect of load change on output voltage regulation: (**a**) voltage vs. Power (**b**) output voltage ripples vs. power (**c**) output voltage ripples vs load. Figure 10: Experimental results of full-wave rectification: (**a**) DC components (**b**) the voltage ripples across the output voltage. \begin{table}
\begin{tabular}{c c c c} \hline
**Load (\(\Omega\))** & **Voltage (V)** & **Power (W)** & **Ripple (mV)** \\ \hline
25 & 25.8 & 26.55 & 175 \\
75 & 28.59 & 10.89 & 73 \\
137.5 & 29.55 & 6.288 & 45 \\
200 & 29.98 & 4.47 & 33 \\
250 & 30.2 & 3.595 & 29 \\ \hline \end{tabular}
\end{table} 
Table 3: Parameters during load change experiment. bridge rectifier with three semiconductor switches was used to precisely control the output voltage through firing angle control of the switches [21]. Thyristor switches are utilized in this experiment. Thyristors work the same as diodes, except they have an additional gate terminal. Firing a voltage pulse at the gate terminal turns on the thyristors, provided the anode has a higher potential than the cathode. There are many types of three-phase controlled rectifiers. However, a three-phase controlled half-wave rectifier is preferable for small wind-energy systems because of its simplicity, precise output voltage control, high efficiency, and low ripples [20]. The implemented topology is shown in Figure 12.  
The three-phase AC power from a three-phase wind turbine or any other source is connected to the three anodes of the thyristors. Next, the cathodes of the thyristors are shorted to make a common-cathode configuration, and finally, the load is attached. The amplitude of the DC voltage at the load can be controlled by a firing angle delay (\(\alpha\)). The theoretical load voltage can be expressed in terms of delay angle \(\alpha\)[12], as shown in Equations (13) and (14). \[V_{dc}=\frac{3\sqrt{3}V_{pk}\cos(\alpha)}{2\pi} 0<\alpha<\frac{\pi}{6} \tag{13}\]  
\[V_{dc}=\frac{3V_{pk}}{2\pi}\Big{[}1+\cos\Big{(}\alpha+\frac{\pi}{6}\Big{)} \Big{]} \frac{\pi}{6}<\alpha<\frac{5\pi}{6} \tag{14}\]  
Simulations are performed to verify the theoretical values of Equations (13) and (14). MATLAB SIMULINK with the PLECS toolbox is utilized for the simulations. First, the three-phase AC supply is stepped down using a transformer with the following turns ratio. \[Turns\ ratio=\frac{N_{primary}}{N_{secondary}}=\frac{208\sqrt{3}}{28} \tag{15}\]  
The theoretical, simulation, and experimental values of the output DC voltage for different values of firing angles are shown in Table 4. Data from the experiments show that the delay angle can precisely control the output voltage. As the firing pulse delay increases the output, the DC voltage decreases, as shown in Figure 13.  
Figure 12: Three-phase half-wave fully controlled rectifier. The experimental results were collected by building a three-switch fully controlled rectifier. The Figure 14 shows the oscilloscope trace of various waveforms captured during the experiments. Channel 1 shows the stepped-down AC input voltage, channel 2 shows the load current, and channel 3 shows the rectified DC voltage. The pulsating DC voltage results from all three phases rectified and applied to a fixed resistive load. When there is no delay in firing the thyristors (Figure 14a), the rectifier acts as an uncontrolled rectifier and produces pulsating DC of 28.65 VRMS at channel 3. Next, the firing angle is delayed by 15 degrees, and a corresponding DC voltage of 28.28 VRMS appears in channel 3 in Figure 14b. Figure 14c shows that the output DC voltage reduced to 23.40 VRMS with 30 degrees of delay. At the same time, Figure 14d reports its value as 20.20 VRMS when the delay is 45 degrees.