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P a g e
We define three line neutral voltages (3 phase voltages) as follows
V
RN
= V
an
= V
m
sinwt where V
m
is the maximum voltage
V
YN
= V
bn
= V
m
sin (wt-
V
BN
= V
cn
= V
m
sin (wt-
The
3-phase half wave converter
combines three
single phase half wave controlled
rectifiers in one
single circuit feeding a common load. The thyristor T
1
in series with one of the
supply phase windings
'a-n'
acts as one half wave controlled rectifier The second thyristor T
2
in
series with
the supply phase winding
'b-n'
acts as the second half wave controlled rectifier. The
third thyristor T
3
in series with the supply phase winding acts as the third half wave controlled
rectifier.
The 3-phase input supply is applied through the star connected supply
transformer as shown in
the figure. The common neutral point of the supply is connected to one end of the load while the
other end of the load connected to the common cathode point.
When the thyristor T
1
is triggered at ω
t
=(∏/6 + α)=(30° + α)
, the phase voltage V
an
appears
across
the load when T
1
conducts. The load current flows through the supply phase winding
'a-
n'
and through thyristor T
1
as long as T
1
conducts.
When thyristor T
2
is triggered at ω
t
=(5∏/6α), T
1
becomes reverse biased and turns-off. The load
current flows through the thyristor and through the supply phase winding
'b-n'
. When
T
2
conducts the phase voltage
v
bn
appears across the load until the thyristor T
3
is triggered .
When the thyristor T
3
is triggered at ω
t
=(3∏/2 + α)=(270°+α) , T
2
is
reversed biased and hence
T
2
turns-off. The phase voltage V
an
appears across the load when T
3
conducts.
When T
1
is triggered again at the beginning of the next input cycle the thyristor T
3
turns off as it
is reverse biased naturally as soon as T
1
is triggered. The figure shows the 3-phase input supply
voltages, the output voltage which appears across the load, and the load current assuming a
constant and ripple free load current for a highly inductive load
and the current through the
thyristor
T
1
.
For a purely resistive load where the load inductance ‘L = 0’ and the trigger angle α >(∏/6) , the
load current appears as discontinuous load current and each thyristor is naturally commutated
when the polarity of the corresponding phase supply voltage reverses. The frequency of output
