 # Three-phase Semi-converter with inductive load

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

## Semi-controlled Converters

Semi-controlled converter is a hybrid between uncontrolled and fullycontrolled converters.  It is also known as “semi-converter”.

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 α.   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.

𝑉 𝑜 𝑚𝑒𝑎𝑛 is variable but positive only for the 0° to 180° range of α.

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

Mathematically,

Order of harmonics in 𝐼𝑠 is = 5, 7, 11, 13, …..=  6𝑘 ±1 ,𝑘 = 1,2,3…

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

Once 𝐷𝑖𝑠𝐹 is known,  𝐼𝑠,𝐹𝑢𝑛𝑑 𝑟𝑚𝑠 can be found by equating input and output power too.

Operating waveforms of three-phase semi-converter will be different depending on whether 𝛼 is below or above 60°.  For example, below 60°, 𝑉 𝑜 has 6 ripples per AC cycle but above 60°, 𝑉 𝑜 has only 3 ripples per AC cycle.   However, the expressions of the  𝑉 𝑜 𝑚𝑒𝑎𝑛 , 𝐷𝑖𝑠𝐹Operating waveforms of three-phase semi-converter will be different depending on whether 𝛼 is below or above 60°.  For example, below 60°, 𝑉 𝑜 has 6 ripples per AC cycle but above 60°, 𝑉 𝑜 has only 3 ripples per AC cycle.   However, the expressions of the  𝑉 𝑜 𝑚𝑒𝑎𝑛 , 𝐷𝑖𝑠𝐹 and (Is,Fund)rms are valid for all values of α.

Operating waveforms for α = 60°

Operating waveforms for α >  60°

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

## Reference ac to dc converter diagram

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