The document discusses the operation and characteristics of a three-phase semi-converter in power electronics, focusing on the firing angles of SCRs and their conduction intervals. It outlines how each SCR is fired in succession, producing various output voltages based on the firing angles, particularly with an emphasis on α=60°. Additionally, it includes equations for phase and line-to-line voltages associated with the operation of the converter.

1. Three phase semi converter
Power electronics -I
PRESENTED BY:-
Rami nirav (161310109048)
Arpit raval (161310109050)
Sem.:-5
Electrical engineering

2. Fig.:- three phase semi converter with RL load

3.  the circuit of three phase semi converter consists three SCRs,
three diodes and load of RL type.
 The firing angle should be measured from ωt = 60°, scr1 is fired at
ωt = 60°+0°= 60° and for α= 60°, scr1 is fired at ωt = 60°+60°=
120°.
 Before turning off the scr next scr will not be fired so scr id fired
after turning off the previous scr.
 For example for α=60° if scr is fired it start conducting from
ωt=120° and it conduct from ωt=120°to ωt=240°
 Now next scr is fired at ωt=240° and at same instant previous scr
gets turned off. scr will be conducting from
ωt=240°to ωt=360°.
 Next scr is fired at ωt=360°and at the same instant the previous scr
will get turned off.

5.  Scr3 is already conducting through D2. scr1 conduct
for 120° and it fired at ωt=60°
 The output voltage vab and vac are obtained when
scr1 is conducting. Similarly vbc and vba are obtained
when scr2 is conducting. Again scr2 conducts for
120°and it is fired at ωt=180°.
 At ωt=300°, when scr3 is fired, voltage vca and vcb
are obtained. Scr3 conduct for 120° from ωt=300°to
ωt=420° the above cycle repeat with scr1.

6.  The output voltage and current waveforms for α=60°
 scr1 is fired at ωt=120° and scr3 is already conducting through
d2.scr1 and d3 conduct simultaneously for 120° and voltahe vac is
obtained. Scr2 is fired at ωt=240° and it conducts through diode
d1.
 In this case voltage vba is obtained. Similarly the scr3 conducts
and vcb is obtained. here it should be noted that freewheeling
diode does not come into play for α≤60° and voltage vab ,vbc ,vca
do not appear in the output voltage waveform for α=60°.

7.  The phase voltage are ;
van = vm sinωt
vbn = vm sin(ωt-2∏/3)
vcn = vm sin(ωt+2∏/3)
 And corresponding line to line voltage are
vac = van – vcn = √3vm sin(ωt-∏/6)
vcb = vcn – vbn = √3vm sin(ωt+∏/6)
vab = van – vbn = √3vm sin(ωt+∏/6)

8. For α≤ 60° ( continues output
voltage);the average voltage is given by