A Study of Three-Phase Rectification Using Resistive Load

#LAB 3

STUDY OF THREE PHASE RECTIFICATION USING RESISTIVE LOAD

OBJECTIVE

To familiarize with the three phase rectification with resistive (R) load.

APPRATUS REQUIRED

1. Thyristor Kit

2. Connecting Wires

3. Oscilloscope

4. Multimeter

THEORY

Three phase rectifiers are preferred than single phase because of the following reasons:

- Higher dc output voltage and better input power factor

- Less ripple content and better load performance

- Lower size of filter required

- Better Transformer Utilization Factor (TUF)

Three-Phase Half Wave Rectification

The three phase half wave rectification using three diodes and [Abbildung in dieser Leseprobe nicht enthalten] transformer with resistive (R) load is shown in figure 1(a). The three diodes D1, D2 and D3 are connected in each phase, have their cathodes connected together to load R. The three phase voltage source is balanced and have phase sequence a-b-c. The input phase-to-neutral voltage, output voltage, output current and voltage across D1 waveforms are shown in figure 1(b).

The diode at the highest positive instantaneous voltage can only conduct. From ωt = 300 to ωt = 1500, ra is most positive voltage and thus it appears across load for this period and diode D1 is in conducting state. Similarly, From ωt = 1500 to ωt = 2700, rb is most positive and D2 is conducting with load voltage , rb. From ωt = 2700 to ωt = 3900, rc is most positive voltage and it appears across the load with diode D3 conducting. From ωt = 00 to ωt = 300, D3 conducts and load voltage is equal to rc. Each diode conducts for a period of 1200. The output voltage varies from maximum phase voltage (Vmp ) to 0.5 times Vmp. In one cycle of supply voltage, output voltage has three pulses and thus rectifier is also called 3-phase, 3-pulse rectifier.

The average value of output voltage is given by,

Abbildung in dieser Leseprobe nicht enthalten

The rms value of output voltage is given by,

Abbildung in dieser Leseprobe nicht enthalten

Figure 1(a): Three Phase Half Wave Rectifier

Abbildung in dieser Leseprobe nicht enthalten

Figure 2(b): Three Phase Half Wave Rectifier input voltages, output voltage and current, source current and voltage across diode D1

Three Phase Bridge Rectifier

Power circuit diagram for a 3 phase bridge rectifier using six diodes in shown in figure 2 (b). The diodes are arranged in three legs with each leg consisting two series-connected diodes. Upper diodes Dl, D3, D5 constitute the positive group of diodes. The lower diodes D2, D4, D6 form the negative group of ·diodes. This rectifier is also called 3 phase, 6 pulse diode rectifier, 3-phase full-wave diode rectifier.

The positive group diode (D1, D3, D5) conducts when they experience highest positive voltage and negative diode (D2, D4, D6) would conduct when they experience highest negative voltage. Each individual diode conducts for a period of 1200 and the width of output pulse is 600. The diode conduction period is shown in figure 2(a).

Abbildung in dieser Leseprobe nicht enthalten

Figure 2(a): Three Phase Bridge Rectifier diode conduction states

For a period of ωt = 00 to ωt = 300, diode D5 and D6 conducts. The diode D5 is connected with rc and diode D6 connected with rb, thus rcb voltage appears across the load. Similarly, for ωt = 300 to ωt = 900, diode D1 and D6 conducts and thus rab voltage appears across the load. The waveforms of input voltages, output voltage and voltage across D1 for resistive load are shown in figure 2(c). The average value of output voltage is given by,

Abbildung in dieser Leseprobe nicht enthalten

The rms value of output voltage is given by,

Abbildung in dieser Leseprobe nicht enthalten

Figure 2(b): Three Phase Bridge Rectifier using diodes

Abbildung in dieser Leseprobe nicht enthalten

Figure 2(c): Waveforms of source voltages, output voltage and voltage across diode D1

OBSERVATION

1. Input Parameter

a. Input voltage ..

b. Frequency ...

c. Cycle time period ...

2. Observation Table

3 Phase Half Wave Rectifier

Abbildung in dieser Leseprobe nicht enthalten

3 Phase Bridge Rectifier

Abbildung in dieser Leseprobe nicht enthalten