Commutation

Owing to the inductance present in the circuit, the current cannot change instantaneously from Id to 0 in one anode and from 0 to Id in the next. Hence, two anodes conduct simultaneously over a period known as the commutation time or overlap angle (g). When the valve in phase (b) commences to conduct, it short- circuits the (a) and (b) phases, the current eventually becoming zero in the valve of phase (a) and Id in the valve of phase (b). This is shown in Figure 9.9. It can be seen from the diagram that the overlap angle shifts the current peak with respect to the voltage peak by power factor angle (this is zero when g = 0 as shown in Figure 9.8).

Gate Control

A positive pulse applied to a gate situated between anode and cathode controls the instant at which conduction commences, and once conduction has occurred the gate exercises no further control. In the voltage waveforms shown in Figure 9.10 the conduction in the valves has been delayed by an angle a by suitably delaying the application of positive voltage to the gates. Ignoring the commutation angle g, the new direct-output voltage with a delay angle of a is,

Waveforms of rectifier with instant of firing delayed by an angle a by means of gate control

Figure 9.10 Waveforms of rectifier with instant of firing delayed by an angle a by means of gate control

where Vo is the maximum value of direct-output voltage as defined by equation (9.1).

 
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