Final answer:
A separately-excited generator can have a constant, changing, or zero field strength.
Step-by-step explanation:
A separately-excited generator can have a. constant field strength, b. changing field strength, or c. zero field strength. In a separately-excited generator, the field winding is separate from the armature winding, allowing for the control of the field strength. For example, in a constant field strength generator, the field strength remains the same regardless of the current flowing through the armature winding. On the other hand, in a changing field strength generator, the field strength can vary depending on the current or voltage applied to the field winding. Finally, a generator with zero field strength will not produce any electrical output even with current flowing through the armature winding.
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