STUDY OF FULLY CONTROLLED FULL WAVE RECTIFIER USING SCR

Experiment No.: 3

Experiment Name:

Study of Fully Controlled Full Wave Rectifier using SCR

Objective:

To study Fully Controlled Full Wave Rectifier using SCR

Theory:

The ac input voltage is fed to the input of a power transformer T_R which has centre-tapped secondary. Thus, the voltage V_i1 and V_i2 developed across the two halves of the secondary are equal in magnitude but opposite in phase. These are fed to two identical SCRs with identical gate control circuit. the current of each SCR flows through the load resistor R_L in the same direction. Hence one SCR conduct during the positive half cycles of the ac input voltage while the other conducts during the negative halves. Hence, the fullwave output voltage is obtained across R_L. Figure 2 shows the output voltage waveform. Here also a filter may be placed between the rectifier and the load resistor R_L to remove the ripple voltages.

Circuit Diagram:

Working of Single Phase Full Wave Controlled Rectifier using SCR:

During the positive half cycle of voltage V_i1, SCR₁ conducts during the period α to π radians where α is the firing angle. The negative half of V_i1, corresponds to the positive half of V_i2. During this positive half of V_i2, SCR₂ conducts during the interval α and π radians. The two outputs get added. The resultant output voltage V₀ has the shape shown in figure 2. The output is obtained during the shaded portion. The firing angle α may be changed by changing the gate current by varying the resistor R in the gate control circuit.

Magnitude of Output Volatge and Current:

Let the input voltage V_i1 be a sinusoidal voltage of amplitude V_m and frequency f Hz and let it be given by,

$V_{i1} = V_m sin\omega t = V_m sin\theta$ ………..(1)

Where ω is the angular frequency in radians/second and equals 2πf.

Let, α be the firing angle during the positive half cycle. Then SCR₁ conducts during angle α to π radians during the positive half cycle. Hence the average output voltage V_av1 contributed by SCR₁ is given by,

$V_{av1} = \dfrac{1}{2\pi}\int_{\alpha}^{\pi} V_m Sin \theta d\theta$ ………(2)

Similarly, the average voltage contributed by SCR₂ due to input voltage V_i2 is equal to V_av1. Hence, the total value of average output voltage V_av is given by,

$V_{av} = \dfrac{1}{\pi} \int_{a}^{\pi} V Sin \theta d\theta$ ……..(3)

= \dfrac{V_m}{\pi}[-Cos\theta]_{\alpha}^{pi}

$= \dfrac{V_m}{\pi}(1 + Cos \alpha)$ …….(4)

Average output current is,

$I_{av} = \dfrac{V_{av}}{R_L} = \dfrac{V_m}{\pi R_L}(1 + cos \alpha)$ ………(5)

We thus, find that fullwave SCR rectifier produces average output voltage and average output current twice those in half wave SCR rectifier. Further, removal of ripple voltage by filter circuit is more effective in full wave SCR rectifier.

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