Three phase dc drives

• Large dc motor are always three phase converters for their speed control.
• A three phase controlled converters feed power to the armature circuit for containing speeds below base speed.
• Another three phase controlled converter is inserted in the field circuit for getting speeds above base speed.
• The output frequency of three phase converter is higher than those single phase converter.
• Therefore, for  reducing the armature current ripple, the inductor in a three phase dc drive is of lower value than that in a single phase dc drive.
• As the armature current is mostly continuous, the motor performance in 3 phase dc drives is superior to those in single phase dc drive.

The three phase dc drive, as in single phase dc drive, may be subdivided an under :

• Three phase half wave converter drives
• Three phase semi-converter drive
• Three phase full converter drives
• Three phase dual converter drives