Circuit Operation

The circuit of a single-phase fully-controlled bridge rectifier circuit is shown in the figure above. The circuit has four SCRs. It is preferable to state that the circuit has two pairs of SCRs, with S₁ and S₃ forming one pair and, S₂ and S₄ the other pair. For this circuit, v_s is a sinusoidal voltage source. When it is positive, the SCRs S₁ and S₃ can be triggered and then current flows from v_s through SCR S₁, load inductor L, load resistor R, SCR S₃ and back into the source. In the next half-cycle, the other pair of SCRs conducts. Even though the direction of current through the source alternates from one half-cycle to the other half-cycle, the current through the load remains unidirectional.

The main purpose of this circuit is to provide a variable dc output voltage, which is brought about by varying the firing angle. Let v_s = E sin wt, with 0 < wt < 360^o. If wt = 30^o when S₁ and S₃ are triggered, then the firing angle is said to be 30^o. In this instance the other pair is triggered when wt= 210^o.

When v_s changes from a positive to a negative value, the current through the load does not fall to zero value at the instant wt = p radians, since the load contains an inductor and the SCRs continue to conduct, with the inductor acting as a source. When the current through an inductor is falling, the voltage across it changes sign compared with the sign that occurs when its current is rising. When the current through the inductor is falling, its voltage is such that the inductor delivers power to the load resistor, feeds back some power to the ac source under certain conditions and keeps the SCRs in conduction forward-biased. If the firing angle is less than the load angle, the energy stored in the inductor is sufficient to maintain conduction till the next pair of SCRs is triggered. When the firing angle is greater than the load angle, the current through the load becomes zero and the conduction through the load becomes discontinuous. Usually the description of this circuit is based on the assumption that the load inductance is sufficiently large to keep the load current continuous and ripple-free.

The operation of the circuit is illustrated by animating the functioning of this circuit. Key in the firing angle in degrees and click the button. The source voltage, and the bridge output voltage are also displayed. It is assumed here that the load inductance is quite large. The animation is correct only if the firing angle is less than 90^o. The programs under simulation section will run correctly for any firing angle.

 
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