Analysis of the 3 phase fully controlled bridge converter in the inverting mode
3 phase fully controlled bridge converter in Inverting mode:
Fig: Inverter mode of operation of the three phase fully controlled bridge converter- circuit diagram & Waveforms
- Analysis of the converter in the inverting mode is similar to its rectifier mode of operation. The same expressions hold for the dc and harmonic compounds in the output voltage and current.
- For values of α in the range 90° < α < 180° it is observed that the average dc voltage is negative and the displacement angle φ of the fundamental component of the input ac line current is equal to α > 90°. Therefore, power in the ac side flows from the converter to the source.
- An outgoing thyristor (thyristor T6)after commutation is impressed with a negative voltage of duration β = π – α. For successful commutation of the outgoing thyristor it is essential that this interval is larger than the turn off time of the thyristor i.e,
β ≥ ωtq, tq is the thyristor turn off time
Therefore, π-α ≥ ωtq or α ≤ π-ωtq.
Which imposes an upper limit on the value of α. In practice this upper value of α is further reduced due to Commutation overlap.
Questions of this topic
A three phase fully controlled converter operates from a 3 phase 230 V, 50 Hz supply through a delta/star transformer to supply a 220 V, 600 rpm, 500 A separately excited dc motor. The motor has an armature resistance of 0.02 ohms. What should be the transformer turns ratio such that the converter produces rated motor terminal voltage at 0 degreesfiring angle. Assume continuous conduction. The same converter is now used to brake the motor regeneratively in the reverse direction. If the thyristors are to be provided with a minimum turn off time of 100 microsecs, what is the maximum reverse speed at which rated braking torque can be produced?