The conversion of ac to dc with suitable examples and sketches are discuss here advanced. conversion of ac to dc is needed to read as engineer student.

## Semi-controlled Converters

Semi-controlled converter is a hybrid between uncontrolled and fullycontrolled converters. It is also known as βsemi-converterβ.

Semi-converter offers variable positive DC output voltage only. Regeneration or inverter mode operation is not available

## Three-phase Semi-converter with inductive load (With source side inductance)

Each thyristor-to-thyristor commutation involves with a conduction overlap angle π’π, and each diode-to-diode commutation involves with a conduction overlap angle π’π·. Here too, during overlap, the three recently active devices conduct simultaneously and outside overlaps the two recently active devices conduct simultaneously.

Waveforms of output voltage, input current, thyristor-1 voltage and voltage at PCC

Let consider the thyristor-overlap after triggering π1, during which π1,π·6,π5 conduct simultaneously. Following the same steps followed in the case of full-bridge before, we can show that,

Similarly, let consider the diode-overlap after π·2 becoming forward biased, during which π·2,π1,π·6 conduct simultaneously. Again, following the same steps as in thyristor-overlap, we can show,

Net reduction in π π ππππ due to conduction overlaps is,

As in the case without source-inductance, operating waveforms will be different depending on whether πΌ is below or above 60Β°. For πΌ > 60Β°, ο· π π has only 3 ripples per AC cycle. ο· During every overlap π π = 0 ο· Line voltage at the PCC contains 2 notches only, both are down to zero. However, expressions for π π ππππ , βπ π, π·ππ πΉ, π’π, π’π· all remain unchanged.

Waveforms of output voltage, input current, thyristor-1 voltage and voltage at PCC for Ξ± > 60Β°

Ex:

Derive expressions for βπ π,π’π and π’π· for a three-phase semi-converter operating at a delay angle above 60Β°. Hence verify that they are same as for delay angles below 60Β°.

Ans

Let consider the overlap after triggering π1. With respect to operating waveforms given before, π1,π·2,π5 all conduct simultaneously during this interval.

Net reduction of π π ππππ occurs due to thyristor-overlaps only, because diode overlaps are occurring while π π is already zero.

Considering diode overlap after π·4 becoming forward biased, during which π·4, π1, π·2 all conduct simultaneously,

Integrating,

Thus, the expressions of βπ π,π’π and π’π· for πΌ > 60Β° are same as those for πΌ < 60Β°.

The conversion of ac to dc with suitable examples and sketches are discuss here advanced. conversion of ac to dc is needed to read as engineer student.