What is Single Phase Semi Converter? Working, Circuit Diagram, Waveform & Applications

Single Phase Semi Converter provides only a one-quadrant operation. Most commercial applications need only controlled rectification and for this a Single Phase Semi Converter is enough.

Types of Single Phase Semi Converter

Generally, there are two types of configurations of semi-converter.

1. Symmetrical Semi Converter.
2. Asymmetrical Semi Converter.

Single Phase Symmetrical Semi Converter with R Load

(a) Circuit Diagram.

 

(b) Input, output voltage and current waveforms

Fig. 1: Single Phase Symmetrical Semi Converter with R Load

In Fig. 1 (a), shows the circuit diagram of single-phase half-controlled bridge rectifier with R load. In this rectifier two SCRs and two diodes are connected in symmetrical configuration.

The operation of this converter mainly consists of three modes:

Mode 1 :(α to π):

During positive half-cycle of AC input voltage, thyristor T₁ and diode D₁ conduct and thyristor T₁ is fired at ωt = α. Therefore the average output voltage is equal to the instantaneous supply voltage and load current flows through T₁ → R → D₁ and back supply again.

Mode 2 : (π to π + α):

At instant ωt = π, the supply goes through zero and after π supply voltage reverses its polarity. Due to reverse supply across conducting devices T₁ and D₁, they turned off at ωt = π and this type of turn-off is called as “natural” or “line commutation”. Therefore, the average output voltage is zero (i.e. V₀ = 0 volt) and load current is also zero.

Mode 3: (π + α to 2π):

At instant ωt = π + α, the supply voltage becomes completely negative i.e. during negative half cycle of AC input voltage, thyristor T₂ and diode D₂ are forward biased, thyristor T₂ is fired at ωt = π + α. Thus the average output is equal to instantaneous supply voltage because due to conduction of T₂ and D₂, the load is directly connected to supply (i.e. V₀ = V_S). The load current flows through T₂ → R → D₂. Thyristor T₂ and diode D₂ conduct upto 2 π, at ωt = 2π commutation takes place due to natural zero appears across supply.

Advantages of Single Phase Semi Converter

1. Improved power factor.
2. Cost is reduced as 2 SCRs are replaced by 2 diodes.
3. Reactive input power is reduced.
4. Average output d.c. voltage is higher for same firing angle.

Disadvantages of Single Phase Semi Converter

1. It can be operated only in rectification mode.
2. Energy feedback from load to source is not possible.

Applications of Single Phase Semi Converter

Since it has a higher power factor, it can be used in high power applications. It is also widely used in main line a.c. traction where large d.c. motors are applied from a single-phase a.c. supply.

Formulas of Single Phase Semi Converter

Average value of output voltage (V₀)

\[{{\text{V}}_{\text{0}}}=\frac{{{\text{V}}_{\text{m}}}}{\text{  }\text{ }\!\!\pi\!\!\text{ }\text{    }}\left( 1+\cos \alpha  \right) \]

RMS value of output voltage (V_rms)

\[{{\text{V}}_{\text{rms}}}=\frac{{{\text{V}}_{\text{m}}}}{\sqrt{2}}{{\left[ \frac{\text{1}}{\text{  }\pi \text{  }}\left( \text{  }\pi \text{ }-\alpha +\text{ }\frac{\text{sin 2}\alpha }{\text{ 2    }}\text{ } \right) \right]}^{\text{1/2}}}\]