Discussion
When the value of the field excitation current is varied from a low value to the normal value, which of the following depicts the variation of the armature current?
Answer: C
The armature current decreases from a large value to a minimum value.
This describes a synchronous motor on constant load (its V-curve). With low excitation the motor is under-excited and draws a large lagging current to supply its own magnetisation. As the field current is raised towards normal, the reactive component falls, and at normal excitation the motor runs at unity power factor, where the armature current is at its minimum.
Increasing the excitation further would make the current rise again, now leading, which is what option D describes. The current never falls to zero, because the active component needed to drive the load remains.
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