24-01-2013, 10:54 AM
THREE PHASE HALF CONTROLLED BRIDGE CONVERTER WITH R& RL LOADS
THREE PHASE HALF.docx (Size: 15.37 KB / Downloads: 55)
AIM:
To obtain the output waveforms of three-phase full wave half-controlled bridge rectifier
with R and RL load and with or without commutating or freewheeling Diode.
APPARATUS:
S.No Name of the Equipment Type Ramge Quantity
1. There phase Half
controlled bridge
converter power circuit
2. Firing Unit 1
3. Voltmeter MI (0-100V) 1
MC (0-100V) 1
4. 3-ph Variac 415V/(0-415V) 1
5. Rheostat Wire wound 100ohm/5A 1
6. Inductive load 0-150mH 1
7. CRO
8 Patch Chords
Theory :
For large power dc loads, 3-ph ac to dc converter are commonly used. The various types
of three phase controlled converter are 3-ph half wave converter, 3-ph half wave converter is
rarely used in industry because it introduces de component in the supply circuit. If diodes are
replaced by 3-thyristors, a semi converter bride is obtained.
Free wheeling mode of operation of bridge connected rectifiers can be realized half of its
thyristor with diodes. Therefore, circuit of three phase half-controlled bridge converter contains
three thyristor in three arms and diodes in the other three arms.
For α<600 the continuous conduction mode is possible. For firing angles α>600 the
discontinuous conduction mode occurs. It can be observed from the waveforms that the output
voltage becomes zero during a part of the output voltage period, because of the free wheeling
action. It is easily noted from the waveforms that the freewheeling period is . Therefore
the supply current flows for the period (Π-α) in each half cycle. As α increase the duration of the
supply current pulse decreases. Therefore, the harmonic content in the source current increases
as the firing angle increases.
For large firing angle delays, commutation failure may take place due to the limited time
available in symmetrical half controlled converter circuit configuration, if the current is assumed
to be continuous. This may result in half weaving effect.
Procedure:
1. Make the connections as per the circuit diagram.
2. Verify the connections before switching on the supply
3. Keep the rheostat position and variac positions as the values given by the instructor.
4. Switch on power circuit and firing circuit.
5. Switch on the CRO and calibrate it with input voltage.
6. Observe output voltage waveform on CRO.
7. Note down readings of firing angle and output voltage.
8. Also calculate theoretical and practical values of output voltages and compare.
9. Repeat above steps for various firing angles.
10. For RL-load connect Inductance in series with resistance.
RESULT:
Observed and drawn the output waveforms of 3-phase half-controlled bridge
converter with R and RL loads.