# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 387

375
Fig. 11.5
Single-phaseuncontrolled bridge rectifier B2
In the sameway, the voltage and current characteristics for three-phase uncontrolled as
well as controlled bridge rectifiers can be determined. For the ideal open circuit voltage
of theuncontrolledB6 rectifier it follows:
( )
LL
Str
6Bdi
U35.1 U34.2
U
= ⋅
=
Eq. 11.7
U
Str
stands for the phase voltage and U
LL
for the conductor voltage of the three-phase
syste
in aB6 rectifier.
For a B6H rectifier with full phase angle control of the thyristors it follow
as in
this case, the thyristors behave like diodes. In other cases, the firing angle
α
must be
considered and the following relationship follows for the ideal open circuit DC voltage
U
di
:
(
)
α ⋅
=α ⋅
=
cos U35.1
cos
U34.2
U
LL
Str
H6Bdi
Eq. 11.8
Because of the smoothing capacitor, there will be high peak currents resulting in
uncontrolled rectifiers at the time of connection, when the - so far uncharged - capacitor
is charged. To reduce the load on the semiconductors, usually a pre-charge (soft-
charge) unit is placed between the rectifier and the capacitor. The circuit can be
constructed as a series resistance, which, once the capacitor is charged to an
appropriate value, is shorted by a relay or contactor.
shows such a circuit and
the result of a simulation during the time of connection to a 400V/50Hz three phase
system. In the circuit the series resistance was shorted out by the relay at a capacitor
voltage of 510V. The load was connected when a capacitor voltage of 560V was
reached.
3
The inter-linking factor between phase voltage and conductor voltage in a three-phase system is
3
.
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