CYCLOCONVERTERS*K.V.V.S.R.CHOWDARY

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Single-Phase Transformer Tap ChangersThyristors can be used as static switches for on-load tap changing of transformers.

The static tap changers have a very fast switching action.

The circuit diagram of a single-phase transformer tap changer is shown below.*K.V.V.S.R.CHOWDARY

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The gating pulses of thyristors can be controlled to vary the load voltage.*K.V.V.S.R.CHOWDARY

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CycloconvertersThe 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 ).

A variable output voltage at variable frequency can be obtained from a two stage conversion.*K.V.V.S.R.CHOWDARY

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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.

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Cycloconverters are naturally commutated and the maximum output frequency is a fraction of the source frequency.

Therefore, cycloconverters are low speed ac motor drives in ranges up to 15 MW with frequencies from 0 to 20 Hz.*K.V.V.S.R.CHOWDARY

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Single-Phase CycloconvertersThe principle of operation of single-phase cycloconverters can be explained with the following figure.

First, two single-phase controlled converters are operated as bridge rectifiers.

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Their delay angles are such that the output voltage of one converter is equal and opposite to that of the other converter.

If p is the delay angle of positive converter, the delay angle of the negative converter is: n = p

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Phase-Controlled Cycloconverter*K.V.V.S.R.CHOWDARY

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Three-Phase CycloconverterThe 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.*K.V.V.S.R.CHOWDARY

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The cycloconverter of previous figure can be extended to feed a three-phase load, by having six three-phase converters.

If six full-wave three-phase converters are used, 36 thyristors would be required.

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Reduction of Output HarmonicsThe 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.

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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.

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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.

The following figure shows generation of the gating signals for the cycloconverter.

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LIMITATIONS COMMUTATION FAILURECOMPLEX CONTROL CIRCUITPOWER FACTOR POORNOT SUITABLE FOR FREQUENCY*K.V.V.S.R.CHOWDARY

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