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

Three Phase Converter Circuit

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.

Three Phase Converter Circuit

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

Three Phase Converter Circuit

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,

Three Phase Converter Circuit

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