Dual converter, the name itself says two converters. It is really an electronic converter or circuit which comprises of two converters. One will perform as rectifier and the other will perform as inverter. Therefore, we can say that double processes will occur at a moment. Here, two full converters are arranged in anti-parallel pattern and linked to the same dc load. These converters can provide four quadrant operations. The basic block diagram is shown below.
Modes of Operation of Dual Converter
There are two functional modes: Non-circulating current mode and circulating mode.
Non Circulating Current Mode
- One converter will perform at a time. So there is no circulating current between the converters.
- During the converter 1 operation, firing angle (α1) will be 01o; Vdc and Idc are positive.
- During the converter 2 operation, firing angle (α2) will be 02o; Vdc and Idc are negative.
Circulating Current Mode
- Two converters will be in the ON condition at the same time. So circulating current is present.
- The firing angles are adjusted such that firing angle of converter 1 (α1) + firing angle of converter 2 (α2) = 180o.
- Converter 1 performs as a controlled rectifier when firing angle be 01o and Converter 2 performs as an inverter when the firing angle be 90o2o. In this condition, Vdc and Idc are positive.
- Converter 1 performs as an inverter when firing angle be 90o1o and Converter 2 performs as a controlled rectifier when the firing angle be 02o In this condition, Vdc and Idc are negative.
- The four quadrant operation is shown below.
Ideal Dual Converter
The term ‘ideal’ refers to the ripple free output voltage. For the purpose of unidirectional flow of DC current, two diodes (D1 and D2) are incorporated between the converters. However, the direction of current can be in any way. The average output voltage of the converter 1 is V01 and converter 2 is V02. To make the output voltage of the two converters in same polarity and magnitude, the firing angles of the thyristors have to be controlled.
Average output voltage of Single-phase converter =
Average output voltage of Three-phase converter =
For converter 1, the average output voltage,
For converter 2, the average output voltage,
The firing angle can never be greater than 180o. So,
Types of Dual Converters
They are of two types: Single-phase dual converter and three-phase dual converter.
Single Phase Dual Converter
The source of this type of converter will be single-phase supply. Consider, the converter is in non-circulating mode of operation. The input is given to the converter 1 which converts the AC to DC by the method of rectification. It is then given to the load after filtering. Then, this DC is provided to the converter 2 as input. This converter performs as inverter and converts this DC to AC. Thus, we get AC as output. The circuit diagram is shown below.
Three Phase Dual Converter
Here, three-phase rectifier and three-phase inverter are used. The processes are similar to single-phase dual converter. The three-phase rectifier will do the conversion of the three-phase AC supply to the DC. This DC is filtered and given to the input of the second converter. It will do the DC to AC conversion and the output that we get is the three-phase AC. Applications where the output is up to 2 megawatts. The circuit is shown below.
Application of Dual Converter
- Direction and Speed control of DC motors.
- Applicable wherever the reversible DC is required.
- Industrial variable speed DC drives.