APPARATUS 1) 3- φ Half controlled convertor power circuit and firing circuit, SCR - SKKT106B14E 2) CRO and Probes, 3) Patch Chords, 4) Load : Lamp load 5) DC Voltmeter CIRCUIT DIAGRAMS Fig2. 3-φ Half controlled circuit for R-laod MVGR College of Engineering 39
Department of EEE Power Electronics Lab Manual Fig2. 3-φ Half controlled circuit for RL-load THEORY The semi converter bridge operation for different firing angles is shown in Fig.3 in the form of voltage and current waveforms. The conduction angles for SCRs, shown. For a firing angle delay of α=0°, thyristors T 1 , T 2 , T 3 would behave as diodes and the output voltage of semi converter would be symmetrical six-pulse cycle as shown in Fig.3(a).The output voltage consisting of pulses v cb , v ab , v ac , v bc are shown in Fig.3(a). In Fig. 3(a), v cb is the load voltage from ωt=0° to 60°. As the first subscript indicates conducting element in the positive group, v cb shows that T 3 is already conducting through diode B 2 of negative group. Voltages v ab , v ac indicate that, according to the first subscript, T 1 conducts for 120° and it begins to conduct at ωt=60° for α=0°.Similarly v bc , v ba indicate that T 2 conducts for 120° and it begins to conduct at ωt=180° for α=0°. An SCR with zero firing angle behaves like a simple diode. Thus, as per the definition of firing angle, it should be measured from ωt=60° for T 1 , from ωt=180° for T 2 , from ωt=300° for T 3 and so on. For α=60° Fig.3 (b), the thyristors are fired so that current returns through one diode during each 120° conduction period. For voltage v ac , T 1 and T 3 conduct simultaneously for 120° as shown. Similarly, other elements conduct. Further note that voltage pulses v ab ,v bc , v ca do not appear in the output voltage waveform for α=60°. It will be seen that for α ≥ 60°, voltage pulses v ab ,v bc , v ca are eliminated. The load current, assumed continuous for α=60° is not shown in Fig.3 PROCEDURE 1) Make connections as per the circuit diagram. 2) Give trigger pulse from firing circuit to related SCR’s in power circuit. 3) Give AC supply from 3- φ available supply to the terminals of 3- φ Half controlled convertor power circuit by using proper regulator. MVGR College of Engineering 40
Department of EEE Power Electronics Lab Manual 4) Connect resistive load at load terminals. 5) After checking all connections switch ON the power circuit and trigger output pulse. 6) Now observe the waveforms on CRO across load and SCR for different values of delay angles and also measure the output voltage for same delay angles. 7) By using digital multimeter measure the output voltage at different firing angles and compare it with theoretical values for same delay angles. 8) Now, add inductance to the resistive load and repeat the steps 6 & 7. PRECAUTIONS 1) Connect the circuit tightly without any loose connections.
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- Summer '20
- Alternating Current, Power Electronics Lab, MVGR College of Engineering