DC series and shunt generator characteristics

The characteristics of a generator show how terminal voltage V varies with load current I_L (external characteristic). V = E − I_aR_a (− series drop).

DC shunt generator

   Ia = IL + Ish
   +--(Ia)--+------+-----> IL
   |  Ra    |      |
  (G)      Rsh   [LOAD]
   |        |      |
   +--------+------+

External characteristic (V against I_L):

 V ^
   |*****
   |     ***
   |         ***
   |             **
   |               *   <- turning back
   +------------------> IL

• V falls gradually with load because of (i) the armature drop I_aR_a, (ii) armature reaction, and (iii) the fall of field current as V falls (a cumulative effect).
• Beyond the maximum current the curve bends back; at a short-circuit only a small current flows (due to residual flux).
• Voltage regulation is moderate (about 5 to 10 %).
• Applications: battery charging, lighting, general power supply, excitation of alternators.

DC series generator

   +--[Rse]--(Ia = Ise = IL)--> [LOAD]
  (G)

External characteristic:

 V ^            ***
   |        ***     ***
   |     **            **
   |   *
   |  *
   +--------------------> IL

• The field current equals the load current. At no load only the residual emf appears.
• As I_L increases, flux and emf increase, so V rises (almost linearly in the unsaturated region), then flattens due to magnetic saturation, and finally falls because of the I(R_a + R_se) drop and armature reaction.
• Applications: boosters in feeders, arc lamps (constant current), series lighting.