Subject : Power Electronics
Unit : AC to DC Converters
Single phase fully controlled bridge converter supplying an R-L-E load- Circuit Diagram & Conduction Table
Single phase fully controlled bridge converter with RLE load:
- It is one of the most popular converter circuits and is widely used in the speed control of separately excited dc machines. Indeed, the R–L–E load may represent the electrical equivalent circuit of a separately excited dc motor.
- The single phase fully controlled bridge converter is obtained by replacing all the diode of the corresponding uncontrolled converter by thyristors.
- Thyristors T_{1 }and T_{2 }are fired together while T_{3 }and T_{4 }are fired 180º after T_{1 }and T_{2}. From the circuit diagram of Fig above, it is clear that for any load current to flow at least one thyristor from the top group (T_{1}, T_{3}) and one thyristor from the bottom group (T_{2}, T_{4}) must conduct.
- It can also be argued that neither T_{1}T_{3 }nor T_{2}T_{4 }can conduct simultaneously. For example whenever T_{3 }and T_{4 }are in the forward blocking state and a gate pulse is applied to them, they turn ON and at the same time a negative voltage is applied across T_{1 }and T_{2 }commutating them immediately.
- Similar argument holds for T_{1 }and T_{2}.For the same reason T_{1}T_{4 }or T_{2}T_{3 }cannot conduct simultaneously. Therefore, the only possible conduction modes when the current i_{0 }can flow are T_{1}T_{2 }and T_{3}T_{4}.
- It is possible that at a given moment none of the thyristors conduct. This situation will typically occur when the load current becomes zero in between the firings of T_{1}T_{2 }and T_{3}T_{4}. Once the load current becomes zero all thyristors remain off. In this mode the load current remains zero. Consequently the converter is said to be operating in the discontinuous conduction mode.
- Conduction table shows the voltage across different devices and the dc output voltage during each of these conduction modes.
- It is to be noted that whenever T_{1 }and T_{2 }conducts, the voltage across T_{3 }and T_{4 }becomes –v_{i}. Therefore T_{3 }and T_{4 }can be fired only when v_{i }is negative i.e, over the negative half cycle of the input supply voltage. Similarly T_{1 }and T_{2 }can be fired only over the positive half cycle of the input supply.
- The voltage across the devices when none of the thyristors conduct depends on the off state impedance of each device.
- Under normal operating condition of the converter the load current may or may not remain zero over some interval of the input voltage cycle.
- If i_{0 }is always greater than zero then the converter is said to be operating in the continuous conduction mode. In this mode of operation of the converter T_{1}T_{2 }and T_{3}T_{4 }conducts for alternate half cycle of the input supply. However, in the discontinuous conduction mode none of the thyristors conduct over some portion of the input cycle. The load current remains zero during that period.