Subject : Power Electronics
Unit : AC to DC Converters
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 D_{2}. In the blocking state the voltage across D_{2 }is either v_{ac }or v_{bc}. Hence, D_{2 }can block only when these voltages are negative.
- Taking v_{bc }as the reference phasor (i.e., bcLv= 2Vsinωt) D_{2 }will block during 2π/3≤ωt ≤ 2π and will conduct in the interval 0 ≤ ωt ≤ 2π/3 .
- Similarly it can be shown that D_{4 }and D_{6 }will conduct during 2π/3 ≤ ωt ≤ 4π/3 and 4π/3 ≤ ωt ≤ 2π respectively.
- Considering conduction of T_{1}. The firing sequence of the thyristor is T_{1 }→ T_{3 }→ T_{5}. Therefore before T_{1 }comes into conduction T_{5 }conducts and voltage across T_{1 }is V_{ac} = √2Vsin (ωt π/3).
- If the firing angle of T_{1 }is α then T_{1 }starts conduction at ωt = α - π/3 and conducts upto α π/3. Similarly T_{3 }and T_{5 }conducts during α π/3 ≤ ωt ≤ α π and α π ≤ ωt ≤ 2π α - π/3.