Excitation methods and types of DC generator

Excitation is the process of producing the main magnetic flux in a DC generator by passing direct current through the field winding on the poles. The way this field current is supplied decides the type of generator.

Methods of excitation

1. Separate excitation: field current from an independent external DC source (battery, another generator, rectifier).
2. Self-excitation: field current taken from the generator's own armature. Voltage builds up from residual magnetism: the small residual flux induces a small emf, which sends a small field current, which increases the flux, and so on until the voltage settles where the field resistance line cuts the OCC.

Conditions for self-excitation: residual magnetism must exist, field connection must aid residual flux, field resistance must be less than critical resistance, and speed must be above critical speed.

Types of DC generators

            DC generators
           /             \
   Separately          Self-excited
    excited          /     |       \
                 Shunt  Series  Compound
                                /       \
                         Long shunt  Short shunt
                     (cumulative or differential)

1. Separately excited

Field fed from outside; $I_a = I_L$, $E_g = V + I_a R_a + V_b$. Gives good control of voltage over a wide range; used in Ward-Leonard drives and testing.

2. Shunt generator

Field (many turns, thin wire, high resistance) in parallel with the armature. $I_{sh} = V/R_{sh}$, $I_a = I_L + I_{sh}$, $E_g = V + I_a R_a + V_b$. Voltage falls slightly with load. Used for battery charging, lighting, and as exciters.

3. Series generator

Field (few turns, thick wire, low resistance) in series with armature and load. $I_a = I_{se} = I_L$, $E_g = V + I_a (R_a + R_{se}) + V_b$. Voltage rises with load. Used as boosters in DC feeders.

4. Compound generator

Has both shunt and series fields.

• Long shunt: shunt field across (armature + series field). $I_a = I_{se} = I_L + I_{sh}$.
• Short shunt: shunt field across armature only. $I_{se} = I_L$, $I_{sh} = (V + I_L R_{se})/R_{sh}$.
• Cumulative: series flux aids shunt flux; can be over-, flat- or under-compounded. Flat compound gives nearly constant voltage for supply.
• Differential: series flux opposes shunt flux; voltage drops sharply with load. Used for arc welding.