The three phase fully controlled rectifier allows conversion of Three phase AC into DC. Normally this is used in various applications such as battery charging for a larger UPS, speed control of large DC motors, Arc Furnaces, HVDC transmission and front end large SMPS for telecom towers.
All 6 devices used are thyristors. The turn-on instants of these devices are dependent on the firing signals that are given. The firing angle is calculated as the instant at which thyristor 1 (T1) is fired from VAC zero crossing. Turn-off happens when the current through the device reaches zero and it is reverse biased at least for a duration equal to the turn-off time of the device specified in the data sheet. The firing angle can be varied from 0° to a maximum of 180°.
Fig. 2.1. Fully controlled 3-phase converter
The firing of the devices T1,T2,T3,T4,T5 and T6 are staggered from each other by 60° in a 60/ 50 Hz cycle. That is if the delay angle α is 40°, then T1 is fired at an angle of 30° from VAC zero crossing. T4 is fired exactly 180° away from T1 as shown in the Fig.2.2. The positive current of A phase (Ia) is carried by T1 and the negative current is carried by T4.
At any time, at least 2 devices have to conduct. For eg. When 6 and 1 are conducting simultaneously, from A phase source, the current flows through T1, R load, L load and returns through T6 to B phase. When T2 is fired after another 60°, then T6 goes OFF and T2 takes over making the return path as C phase rather than B phase.
When the source inductance is negligible the current from one phase to another shifts without any time delay. But if there is a perceivable amount of source inductance, it opposes the current being transferred from , for eg. , A phase to B phase, when T3 is fired and T1 is supposed to go OFF. This causes 3 devices to conduct simultaneously for a short-while; i.e., T1, T2 and T3 –all three conduct simultaneously causing the forward current to be shared between A and B phases (T1 and T3) and the return current being carried by T2 or C phase. This causes a reduction in the output voltage. If 1 and 2 were conducting, the output would have been VAC. If 2 and 3 are only conducting, the output would have been VBC. Because all three of them are conducting the output Dc voltage will be an average of these two line to line voltages i.e.,
This causes a reduction in the net output voltage as shown in the 4th waveform of Fig.2.4. It is also clearly seen that the phase currents rise and fall abruptly in Fig.2.3 whereas they rise slowly in Fig.2.4.
Fig.2.2 Firing instants of T1 and T4 with respect to VAC(L-L voltage); T1 fired at positive going Ia and T4 at negative going Ia.
Fig.2.3. Waveforms of a 3-phase converter without source inductance effect – Phase currents, output DC current and DC voltage
Fig.2.4 Waveforms of phase currents Ia and Ib, Dc link current and voltage in a 3-phase converter with large Ls