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Here is something I read:

Current in the field windings of a shunt-wound generator is independent of the load current (currents in parallel branches are independent of each other). Since field current, and therefore field strength, is not affected by load current, the output voltage remains more nearly constant than does the output voltage of the series-wound generator. In actual use, the output voltage in a dc shunt-wound generator varies inversely as load current varies. The output voltage decreases as load current increases because the voltage drop across the armature resistance increases (E = IR).

It seems that the "field current not being affected by load current (hence constant output voltage)" seems to contradict the next line ("In actual use...").

Why is armature resistance increasing with increase in load?

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The armature resistance isn't changing, except maybe through secondary effects such as heating. But the voltage drop across that resistance rises as the load current rises, causing a drop in voltage at the output terminals of the generator.

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  • \$\begingroup\$ Thank you, but what I'm still confused about is why does voltage drop increase across this resistance? E=IR so if resistance stays pretty much the same and current goes up, how I understand it voltage would also go up. \$\endgroup\$ – gw2500 Sep 1 '12 at 15:44
  • \$\begingroup\$ @gw2500 Exactly as you said, the voltage in one part of the circuit goes up, so the voltage in the rest of the circuit goes down. \$\endgroup\$ – immibis Nov 17 '17 at 5:31
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it is to do with inductance, impedance, and the general nature of DC generators.

the resistance isnt changing.thats just a length of wire wound up in a coil, the resistance is a constant. the impedance, on the other hand, is changing, depending on RPM and load. as the output current increases, so does the magnetic field it creates as it flows through the wire. the magnetic field happens to be of opposite polarity to the magnetic field that created the current in the first place. hence why a generator spins freely when "open circuit" but will lock up or have large rotational resistance, when "short circuit".

the reversed magnetic field creates its own voltage, that "fights" the applied voltage. +2volts + -1volt = 1 volt.

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    \$\begingroup\$ Welcome to SE.EE. Please consider using proper capitalization, your text is hard to read as it is. \$\endgroup\$ – Rev1.0 Aug 20 '14 at 10:09

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