Question

Design three phase AC/DC converter with the following parameters - Vin (Line-Line) = 400 rms - Frequency = 50 Hz - Vout = 0 to 450V

Answer 1

Here in the question the range of output voltage is given form 0V to 450V ,so here we require controlled operation of rectifier.

I am considering three phase half wave controlled bridge rectifier as shown.

here Van,Vbn,Vcn are phase voltages shifted by 120 degree apart.

Vo is the output voltage and Io is the output current wavefoms.

is the firing angle which controls the magnitude of the output voltage,it plays an important role in the controlled operation of the rectifiers.

TO DERIVE AN EXPRESSION FOR THE AVERAGE OUTPUT VOLTAGE OF A 3-PHASE HALF WAVE CONVERTER FOR CONTINUOUS LOAD CURRENT

The reference phase voltage is v_RN=v_an=V_msinωt. The trigger angle is measured from the cross over points of the 3-phase supply voltage waveforms. When the phase supply voltage V_an begins its positive half cycle at ωt=0 , the first cross over point appears at ωt=(∏/6)radians30°.

The trigger angle α for the thyristor T₁ is measured from the cross over point at . The thyristor T₁ is forward biased during the period ωt=30° to 150° , when the phase supply voltage v_an has higher amplitude than the other phase supply voltages. Hence T₁ can be triggered between 30° to 150°. When the thyristor T₁ is triggered at a trigger angle α, the average or dc output voltage for continuous load current is calculated using the equation

on solving the above equation using trignometric formulas we get,

---------------------------(i)

----------------------------------(ii)

where

now for the required value of output voltage we can calculate the triggring value theoretically and can apply practically.

*for example

we require 350V in the output as DC ,so here =350v and =400/0.707v

substitute in the above equation (i) you get 41.57

hope you understand,thank you.