Three Phase Cycloconverter
There are
two kinds of three-phase to single-phase (3f-1f) cycloconverters: 3f-1f
half-wave cycloconverter (Fig. 1) and 3f-1f bridge cycloconverter (Fig. 2).
Like the 1f-1f case, the 3f-1f cycloconverter applies rectified voltage to the
load. Both positive and negative converters can generate voltages at either
polarity, but the positive converter can only supply positive current and the
negative converter can only supply negative current. Thus, the cycloconverter
can operate in four quadrants: (+v, +i) and (-v, -i) rectification modes and
(+v, -i) and (-v, +i) inversion modes. The modulation of the output voltage and
the fundamental output voltage
The
polarity of the current determines if the positive or negative converter should
be supplying power to the load. Conventionally, the firing angle for the
positive converter is named aP, and that of the negative converter is named aN.
When the polarity of the current changes, the converter previously supplying
the current is disabled and the other one is enabled. The load always requires
the fundamental voltage to be continuous. Therefore, during the current polarity
reversal, the average voltage supplied by both of the converters should be
equal. Otherwise, switching from one converter to the other one would cause an
undesirable voltage jump. To prevent this problem, the converters are forced to
produce the same average voltage at all times. Thus, the following condition
for the firing angles should be met
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