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What is the effect of the increase in rotor speed on the open circuit characteristic?
If the rotor speed of a DC generator is increased keeping other parameters unchanged, the slope of the OCC increases. This is because higher speed leads to increased EMF generation according to Faraday's law of electromagnetic induction.
Why does the internal characteristic lie below the open circuit characteristic?
The internal characteristic considers the effect of the armature reaction which lowers the main magnetic flux from the no-load value. Hence, the emf generated (E) on load is less than the no-load emf (E0), causing the internal curve to lie below the OCC.
How can we draw the internal characteristic from the external characteristic?
The internal characteristic can be obtained by raising perpendiculars from points on the external load curve to the OCC and marking lengths along them equal to the IaRa voltage drops at those points. The points of intersection will lie on the internal characteristic curve.
What happens when the field resistance of a shunt generator is increased beyond the critical value?
If the field resistance of a shunt generator is increased beyond the critical resistance value, the generator will not be able to build up voltage during self-excitation and will fail to work.
Why does the external characteristic of a separately excited DC generator not droop much with load?
In a separately excited DC generator, the field current and thereby the main flux can be controlled independently of the load current from an external source. Therefore, the terminal voltage remains almost constant over a wide range of loads, unlike self-excited generators whose characteristics droop noticeably.