Switch - Over of Control from Armature to Field

Field control is necessary above the base speed of the motor and the field current has to vary only over a limited range, say from 0.6 pu to 1 pu. The signal that sets the active limits on the firing angle for armature control can be used also for field control. The block diagram related to field control is shown in Fig. 14.

When the active limit is not set, the firing angle of the field controller is set such that the field current remains at the nominal value. When the active limits are applied to the armature control circuit, the firing angle is varied such that the field current gets adjusted to the value required for the speed reference set.

 
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