Self-excited vs separately excited DC generators

A DC generator needs a magnetic field from its field winding. If the field current comes from an external DC source, the generator is separately excited. If the field current comes from the generator's own armature, it is self-excited; it builds up voltage from the residual magnetism of the poles.

Self-excited vs separately excited

Classification of self-excited generators

Depending on how the field winding is connected to the armature:

1. Shunt generator
2. Series generator
3. Compound generator – (a) long shunt, (b) short shunt; each can be cumulative or differential.

1. Shunt generator

Field winding (many turns of thin wire, high resistance) is connected in parallel with the armature.

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

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

2. Series generator

Field winding (few turns of thick wire, low resistance) is in series with the armature and load.

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

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

3. Compound generator

Has both shunt and series fields.

Long shunt: shunt field across the series combination of armature and series field.

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

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

Short shunt: shunt field directly across the armature only.

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

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

In a cumulative compound generator the series flux aids the shunt flux; in a differential one it opposes it. Here $V_b$ is the total brush drop.