A 3-phase semi-converter works as a six-pulse converter when the firing angle α is:

A 3-phase semi-converter works as a six-pulse converter when the firing angle α is: Correct Answer between 0° to 60°

3-phase semi-converter:

Circuit diagram:

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Where, 

V_a V_b V_c are the input voltages

i_a i_b i_c are the input currents

T₁ T₃ T₅ are the thyristors

D₄ D₆ D₂ are the diodes

V₀ i₀ are output voltage and current respectively

R is the load resistance

• This is obtained by a series connection of a 3 pulse controlled converter and a 3 pulse uncontrolled one.
• The three arms of the former consist of thyristors and the three arms of the latter comprise diodes.
• The thyristors commutate at the phase angle at which they are fired. The diodes commu­tate at the natural firing instant α = 0.
• The thyristors conduct for 120° and are fired at intervals of 120°.
• A thyristor and diode conduct at any given time the diode is forward biased at the natural firing instant.
• A thyristor conducts, even if it is reverse biased until the next thyristor in the sequence is fired.
• Thus there is a natural free wheeling of the load current via the incoming diode and outgoing thyristor.
• The load voltage is zero during the freewheeling period.
• Freewheeling due to the conduction of the diode does not allow negative excursions of load voltage.
• This reduces the ripple content in the output voltage.
• The ripple frequency of the output voltage at α = 0 is 6f.
• For α < 60°, free-wheeling does not take place since the voltage is always positive on the dc side.
• A 3-phase semi-converter works as a six-pulse converter when the firing angle α is between 0° to 60°