THREE-PHASE SEMI CONVERTER

AIM:

To study the waveforms for three phase semiconverter with RLE load.

APPARATUS REQUIRED:

Sl.No. Item Specification Quantity1 SCR SN 106 32 Power diode IN 4001 43 D C Motor(Separately excited) 1 KW 14 CRO - 15 Bread board - 16 3 phaseTriggering Kit 1

THEORY:

Figure 1 shows the circuit diagram of three phase semi converter supplying an R-L-E load.The output voltages vo across load terminals is controlled by varying the firing angles of SCRs T1,T2 and T3.The diodes D1,D2 and D3 provide merely a return path for the current to the most negative line terminal.Figure 2 shows voltage and current waveforms for a 3-phase semiconverter for different firing angles At any instant thyristor connected to most positive line terminal and diode connected to most negative line terminal conducts.For 0≤α≤π/3 each diode and thyristor conducts for 2π/3 radians.Thefree wheeling diode does not conduct for α<π/2.The free wheeling diode conducts for π/6 radians in each pulse when α=π/2.For α≥π/2, when output voltage tends to become negative free wheeling diode become forward biased which makes output voltage zero.The output voltage thus never becomes less than zero in the case of three phase semi converter supplying an R-L-E load .The output voltage is discontinuous when firing angle α=π/2,2π/3.The output current is discontinuous for α=2π/3.For α=2π/3 when all the energy stored in inductance is discharged,FD diode stops conducting and as a result,load voltage rises to load counteremf E.A three phase semi converter has the unique feature of working as a six-pulse converter for α>π/3 and as a three-pulse converter for α≥ π/3.Average output voltage for three phase semiconverter for α<π/3 is

V0=(3Vml(1+cos α))/2π

Where α=firing angle of SCR,Vml=maximum value of line-line voltage.

PROCEDURE:Connections are done as shown in diagram.Switch on triggering kit.Switch on three phase semi converter circuit.Vary firing angle and note down output voltage using a CRO for different firing angles.

CONNECTION DIAGRAM:

Figure 1:Connection diagram for 3 phase semi converter with RLE load.

WAVEFORM:

Figure 2:Voltage and current waveforms for a 3-phase semiconverter for different firing angles.

Figure 3:Waveforms of 3 phase semi converter when α=π/6 radians.

Figure 4:Waveforms of 3 phase semi converter when α=π/2 radians.

RESULT:

Connections were made as given in the diagram and the wave forms were observed in the CRO.