Three Phase Converter Circuit:
For large sized (rating exceeding 15 kW) motors 3-phase drive is employed. The output voltage of a 3-phase converter has less ripple contents than that of single-phase converter. So the filtering requirements for smoothing out the load current are less and the current is mostly continuous. Three phase converter could be half-wave, semiconverter, full-converter and dual converters. The 3-phase half-wave converter drives are only of theoretical importance and are generally not employed in industrial applications. This is because of presence of dc component in supply currents. Semi- and full-converters are widely used. A dual converter is employed in case of reversible drives with a power rating up to several MW.
1. Three-Phase Semiconverter Drives: It is a one quadrant drive and is employed for motors of rating up to about 100 kW. The field converters may be 3-phase or single phase. The armature voltage Va is given as
where Vmax is the peak value of line-to-neutral voltage and αa is the firing angle of the converter in the armature circuit.
The circuit diagram for a separately excited dc motor supplied from 3-phase ac supply through a three-phase fullconverter is shown in Fig. 27.21.
2. Three-Phase Full Converter Drives: 3-phase full converter drive is a two quadrant drive and is employed up to 1,500 kW motors. The converter in the field circuit may be single phase or 3-phase. When 3-phase full converters are used for both armature and field, the armature and field voltages are given as
3. Three-Phase Dual Converter Drives: Three-phase dual converters consists of two converters and can be operated in all the four quadrants. Either converter 1 is operated to supply armature voltage Va or converter 2 is operated to supply armature voltage – Va. It is used for motors of rating up to 1,500 kW. The armature voltage supplied by converter 1 is given as
where αa1 is the firing angle for converter 1.
For converter 2, the voltage Va is given as
If the field is also supplied by a 3-phase full converter,