Comparison of DC shunt and series generators

A shunt generator has its field winding connected in parallel with the armature, while a series generator has its field winding connected in series with the armature and load. This one difference changes their field design, characteristics and uses.

Circuit diagrams and equations

Shunt generator

   +---Ra---+------+
   |        |      |
  (Eg)     Rsh   Load  V
   |        |      |
   +--------+------+

$$I_{sh} = \frac{V}{R_{sh}}, \quad I_a = I_L + I_{sh}, \quad E_g = V + I_a R_a + V_b$$

Series generator

   +---Ra---Rse---+
   |              |
  (Eg)          Load  V
   |              |
   +--------------+

$$I_a = I_{se} = I_L, \quad E_g = V + I_a (R_a + R_{se}) + V_b$$

Characteristic curves

Open-circuit (magnetisation) characteristic: $E_0$ vs $I_f$. For a shunt generator it is taken by exciting the field separately; its shape is like a B-H curve starting at the residual voltage. For a series generator it is also taken with separate excitation of the series field.

External characteristic ($V$ vs $I_L$):

 V
 |                         Shunt
 |----___
 |       ----____
 |               ---__
 |                    \  (breakdown,
 |       ___           \  curls back)
 |     _/   \__ Series  \
 |   _/        \_        |
 | _/            \       |
 |/________________\_____|_______ IL

• Shunt: $V$ drops slowly as load rises because of $I_a R_a$ drop, armature reaction, and the resulting fall in field current. Beyond a critical load the curve turns back and voltage collapses (short circuit current is small).
• Series: at no load $V$ is only the residual voltage. As load current rises, field current (= load current) rises, so $V$ rises steeply; at very high load, saturation and armature reaction make $V$ fall again.

Comparison

Example: a shunt generator charging a battery keeps nearly constant voltage; a series generator in a long DC feeder raises its voltage as load current rises, compensating line drop.