Circuit Operation

The operation of a 3-phase fully-controlled bridge rectifier circuit is described in this page. A three-phase fully-controlled bridge rectifier can be constructed using six SCRs as shown below.

The three-phase bridge rectifier circuit has three-legs, each phase connected to one of the three phase voltages. Alternatively, it can be seen that the bridge circuit has two halves, the positive half consisting of the SCRs S₁, S₃ and S₅ and the negative half consisting of the SCRs S₂, S₄ and S₆. At any time when there is current flow, one SCR from each half conducts. If the phase sequence of the source be RYB, the SCRs are triggered in the sequence S₁, S₂, S₃ , S₄, S₅, S₆ and S₁ and so on.

The operation of the circuit is first explained with the assumption that diodes are used in place of the SCRs. The three-phase voltages vary as shown below.

Let the three-phase voltages be defined as shown below.

It can be seen that the R-phase voltage is the highest of the three-phase voltages when q is in the range from 30^o to 150^o. It can also be seen that Y-phase voltage is the highest of the three-phase voltages when q is in the range from 150^o to 270^o and that B-phase voltage is the highest of the three-phase voltages when q is in the range from 270^o to 390^o or 30^o in the next cycle. We also find that R-phase voltage is the lowest of the three-phase voltages when q is in the range from 210^o to 330^o. It can also be seen that Y-phase voltage is the lowest of the three-phase voltages when q is in the range from 330^o to 450^o or 90^o in the next cycle, and that B-phase voltage is the lowest when q is in the range from 90^o to 210^o. If diodes are used, diode D₁ in place of S₁ would conduct from 30^o to 150^o, diode D₃ would conduct from 150^o to 270^o and diode D₅ from 270^o to 390^o or 30^o in the next cycle. In the same way, diode D₄ would conduct from 210^o to 330^o, diode D₆ from 330^o to 450^o or 90^o in the next cycle, and diode D₂ would conduct from 90^o to 210^o. The positive rail of output voltage of the bridge is connected to the topmost segments of the envelope of three-phase voltages and the negative rail of the output voltage to the lowest segments of the envelope.

At any instant barring the change-over periods when current flow gets transferred from diode to another, only one of the following pairs conducts at any time.

Period, range of q	Diode Pair in conduction
30o to 90o	D1 and D6
90o to 150o	D1 and D2
150o to 210o	D2 and D3
210o to 270o	D3 and D4
270o to 330o	D4 and D5
330o to 360o and 0o to 30o	D5 and D6

If SCRs are used, their conduction can be delayed by choosing the desired firing angle. When the SCRs are fired at 0^o firing angle, the output of the bridge rectifier would be the same as that of the circuit with diodes. For instance, it is seen that D₁ starts conducting only after q = 30^o. In fact, it can start conducting only after q = 30^o , since it is reverse-biased before q = 30^o. The bias across D₁ becomes zero when q = 30^o and diode D₁ starts getting forward-biased only after q =30^o. When v_R(q) = E*Sin (q), diode D₁ is reverse-biased before q = 30^o and it is forward-biased when q > 30^o. When firing angle to SCRs is zero degree, S₁ is triggered when q = 30^o. This means that if a synchronizing signal is needed for triggering S₁, that signal voltage would lag v_R(q) by 30^o and if the firing angle is a, SCR S₁ is triggered when q = a + 30^o. Given that the conduction is continuous, the following table presents the SCR pair in conduction at any instant.

Period, range of q	SCR Pair in conduction
a + 30o to a + 90o	S1 and S6
a + 90o to a + 150o	S1 and S2
a + 150o to a + 210o	S2 and S3
a + 210o to a + 270o	S3 and S4
a + 270o to a + 330o	S4 and S5
a + 330o to a + 360o and a + 0o to a + 30o	S5 and S6

The operation of the bridge-rectifier is illustrated with the help of an applet that follows this line. You can set the firing angle in the range 0^o < firing angle < 180^o and the instantaneous angle. The applet displays the SCR pair in conduction at the chosen instant. The current flow path is shown in red colour in the circuit diagram. The instantaneous angle can be either set in its text-field or varied by dragging the scroll-bar button. The rotating phasor diagram is quite useful to illustrate how the circuit operates. Once the firing angle is set, the phasor position for firing angle is fixed. Then as the instantaneous angle changes, the pair that conducts is connected to the thick orange arcs. One way to visualize is to imagine two brushes which are 120^o wide and the device in the phase connected to the brush conducts. The brush that has "Firing angle " written beside it acts as the brush connected to the positive rail and the other acts as if it is connected to the negative rail. This diagram illustrates how the rectifier circuit acts as a commutator and converts ac to dc. The output voltage is specified with the amplitude of phase voltage being assigned unity value.

click here to open the applet

 
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