03-01-2013, 04:48 PM
Three phase Cycloconverter
Three phase.ppt (Size: 2.05 MB / Downloads: 65)
Cycloconverter
The ac voltage controllers provide a variable output voltage, but the frequency of the output voltage is fixed.
In addition the harmonic content is high at low ac voltages (high α).
A variable output voltage at variable frequency can be obtained from a two stage conversion.
First the fixed ac is converted to a variable dc (controlled rectifier), and then the variable dc is converted to a variable ac at variable frequency (inverter).
However, the cycloconverter can eliminate the need of one or more intermediate converters.
A cycloconverter is a direct frequency changer that converts ac power at one frequency to ac power at another frequency by ac-ac conversion.
Three-Phase Cycloconverter
The circuit diagram of a three-phase/single phase cycloconverter is shown next.
The synthesis of output waveform for an output frequency of 12 Hz is also in this figure.
Reduction of Output Harmonics
The output voltage of cycloconverters is basically made up of segments of input voltages.
The average value of a segment depends on the delay angle for that segment.
If the delay angles of segments were varied in such a way that the average values of segments corresponds as closely as possible to the variations of desired sinusoidal output voltage, the harmonics on the output voltage can be minimized.
The delay angles for segments can be generated by comparing a cosine signal at the source frequency with an ideal sinusoidal reference voltage at the output frequency.