Environmental Engineering Reference
In-Depth Information
Fig. 4.1
Power electronics applications
Different concepts require different variants of power electronic circuits.
Figure 4.1 indicates the tasks fulfilled by power electronic converters [Heu96].
Power electronic devices contain switching elements in form of semiconduc-
tors. The semiconductor elements are either not controllable (diodes), or control-
lable by switching on (thyristors) or by switching on and off (bipolar transistors,
MOS-FETs, IGBTs, GTO-thyristors). During operation currents are commutated
from one inverter leg to another. Depending on the source of the e.m.f. required
for the commutation process self-commutated and external commutated circuitry is
distinguished in inverter technology. External sources are grid, machine or a load as
source.
Depending on the task the following kinds of inverter are distinguished (see
Fig. 4.1):
•
A.c./d.c. inverters (rectifiers)
They transform a.c. current of a given voltage, frequency and number of phases
into d.c. current.
Uncontrolled devices contain diodes, normally in bridge arrangement. Most
used are the two-pulse (Graetz) bridge for single-phase input, and the six-pulse
bridge for three-phase input. Controlled a.c./d.c. inverters are used in external
commutated as well as in self-commutated schemes.
•
D.c./a.c. inverters
They transform d.c. current into a.c. current of a certain voltage, frequency and
number of phases.
These devices are either external or self-controlled. When the a.c. side is a
grid, these inverters can act also as a.c./d.c. inverters, allowing power exchange
in both directions.
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