Fig: Dual converter circuits (a) non-circulating type (b) output V-I plane (c) circulating current type
- A three phase fully controlled converter although can produce both positive and negative voltage it cannot supply current in both directions. However, some applications such as a four quadrant dc motor drive require this capability from the dc source.
- This problem is solved by connecting another three phase fully controlled converter in anti-parallel. Converter-I supplies positive load current while converter-II supplies negative load current.
- In other words converter-I operates in the first and fourth quadrant of the output v – i plane whereas converter-II operates in the third and fourth quadrant.
- Thus the two converters taken together can operate in all four quadrants and is capable of supplying a four quadrant dc motor drive. The combined converter is called the Dual converter.
- Obviously since converter-I and converter-II are connected in antiparallel they must produce the same dc voltage. This requires that the firing angles of these two converters be related as α2 = π – α1.
- The dc voltages produced by these converters are equal the output voltages do not match on an instantaneous basis. Therefore to avoid a direct short circuit between two different supply lines the two converters must never be gated simultaneously.
- Converter-I receives gate pulses when the load current is positive. Gate pulses to converter-II are blocked at that time. For negative load current converter-II thyristors are fired while converter-I gate pulses are blocked. Thus there is no circulating current flowing through the converters and therefore it is called the non-circulating current type dual converter.
- It requires precise sensing of the zero crossing of the output current which may pose a problem particularly at light load due to possible discontinuous conduction.
- To overcome this problem an interphase reactor may be incorporated between the two converters. With the interphase reactor in place both the converters can be gated simultaneously with α2 = π – α1. The resulting converter is called the circulating current type dual converter.
Questions of this topic
A 220V, 750 RPM, 200A separately excited dc motor has an armature resistance of 0.05 ohms. The armature is fed from a three phase non circulating current dual converter. If the forward converter operates at a firing angle of 70 degrees i) At what speed will the motor deliver rated torque. ii) What should be the firing angle in the regenerative braking mode when the motor delivers half the rated torque at 600 rpm. Assume continuous conduction. Supply voltage is 400 V.