Operating principle of three phase half controlled converter
Operating principle of Three Phase Half controlled converter:
Fig: Circuit diagram and conduction table of a three phase half converter and controlled converter (a) circuit diagram (b) Conduction table
Fig: waveforms of three phase half controlled converter (a) α= π/6 (b) α= π/2
- In the continuous conduction mode only one thyristor from top group and only one diode from the bottom group conduct at a time. However, unlike fully controlled converter here both devices from the same phase leg can conduct at the same time.
- Considering the conducting and blocking state of D2. In the blocking state the voltage across D2 is either vac or vbc. Hence, D2 can block only when these voltages are negative.
- Taking vbc as the reference phasor (i.e., bcLv= 2Vsinωt) D2 will block during 2π/3≤ωt ≤ 2π and will conduct in the interval 0 ≤ ωt ≤ 2π/3 .
- Similarly it can be shown that D4 and D6 will conduct during 2π/3 ≤ ωt ≤ 4π/3 and 4π/3 ≤ ωt ≤ 2π respectively.
- Considering conduction of T1. The firing sequence of the thyristor is T1 → T3 → T5. Therefore before T1 comes into conduction T5 conducts and voltage across T1 is Vac = √2Vsin (ωt π/3).
- If the firing angle of T1 is α then T1 starts conduction at ωt = α - π/3 and conducts upto α π/3. Similarly T3 and T5 conducts during α π/3 ≤ ωt ≤ α π and α π ≤ ωt ≤ 2π α - π/3.