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## Semi-controlled Converters

Semi-controlled converter is a hybrid between uncontrolled and fullycontrolled converters. It is also known as βsemi-converterβ.

Semi-converter offers variable positive DC output voltage only. Regeneration or inverter mode operation is not available

## Three-phase Semi-converter with inductive load

Diodes in the lower-half conduct at default forward-bias points. Thyristor conduction is delayed by delay angle Ξ±. It is assumed that load current is approximately constant due to load inductance.

The assumption that load current is continuous and constant is quite valid when the delay angle is not large. For larger delay angles in semiconverters, load current can become discontinuous, in which case the operating waveforms will be somewhat different.

Output voltage, input current and thyristor-1 voltage waveforms

At any point of time, the two previously active devices conduct and pass the corresponding line-voltage to the output.

π π ππππ is variable but positive only for the 0Β° to 180Β° range of Ξ±.

With respect to the case of three-phase diode converter, the input current waveformβs positive half-cycle is shifted to the right by angle Ξ± but the negative halfcycle remains un-shifted.

Mathematically,

Order of harmonics in πΌπ is = 5, 7, 11, 13, β¦..= 6π Β±1 ,π = 1,2,3β¦

From a qualitative view point too, we can deduce input displacement angle. If positive half-cycle only shifts to the right by angle πΌ, the fundamental component will be shifted to the right by angle πΌ 2.

Once π·ππ πΉ is known, πΌπ ,πΉπ’ππ πππ can be found by equating input and output power too.

Operating waveforms of three-phase semi-converter will be different depending on whether πΌ is below or above 60Β°. For example, below 60Β°, π π has 6 ripples per AC cycle but above 60Β°, π π has only 3 ripples per AC cycle. However, the expressions of the π π ππππ , π·ππ πΉOperating waveforms of three-phase semi-converter will be different depending on whether πΌ is below or above 60Β°. For example, below 60Β°, π π has 6 ripples per AC cycle but above 60Β°, π π has only 3 ripples per AC cycle. However, the expressions of the π π ππππ , π·ππ πΉ and (Is,Fund)rms are valid for all values of Ξ±.

Operating waveforms for Ξ± = 60Β°

Operating waveforms for Ξ± >Β 60Β°

The ac to dc converter diagram with suitable examples and sketches are discuss here advanced. ac to dc converter diagram is needed to read.