Series Generators are generally NOT used because of their terminal voltage:

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ALP CBT 2 Electrician Previous Paper: Held on 22 Jan 2019 Shift 1

Answer 1

In the series dc machine, there is one field winding wound over the main poles with fewer turns and a large cross-sectional area.
Series winding is meant to be connected in series with the armature and naturally to be designed for rated armature current.
So, there will be practically no voltage or very small voltage due to the residual field under no-load condition (I_a = 0).
However, the field gets strengthened as the load will develop rated voltage across the armature with reverse polarity, is connected and terminal voltage increases.
Variation in load resistance causes the terminal voltage to vary. The terminal voltage will start falling, at saturation armature reaction effect becomes pronounced at large load current.
Hence, series generators are not used for delivering power at a constant voltage. Series generator found application in boosting up the voltage in d.c transmission system.