DC generator types; compound loading characteristics

Types of DC generator

Generators are classified by the way the field is excited.

1. Separately excited: the field is supplied from an independent DC source. Field current does not depend on the load; the terminal voltage is almost constant (it drops slightly due to $I_aR_a$ and armature reaction).
2. Self-excited: the field is fed from the generator's own armature.
   • Shunt: field winding across the armature. $I_a = I_L + I_{sh}$. The voltage drops slightly with load.
   • Series: field winding in series with armature and load. $I_a = I_L = I_{se}$. The voltage rises with load current, so it is used as a booster.
   • Compound: both fields. It is short shunt (shunt field across armature only) or long shunt (shunt field across armature plus series field), and cumulative (series aids shunt) or differential (series opposes shunt).

Loading (external) characteristic of a compound generator

It is the plot of terminal voltage $V$ against load current $I_L$ at constant speed.

 V |   over-compound
   |      ___----
   |  ___/---- flat
   | /  ----______ under-compound
   |/            \___
   |               \____ differential
   +----------------------------- IL

• Cumulative compound: the series field adds flux as load increases, which compensates for the drop due to armature resistance and armature reaction.
  • Over-compounded: voltage at full load is higher than at no load (series ampere-turns are large).
  • Flat (level) compounded: full-load voltage equals no-load voltage.
  • Under-compounded: full-load voltage is less than no-load voltage, but the fall is less than for a shunt generator.
• Differential compound: the series field opposes the shunt field, so flux decreases with load and the voltage falls rapidly (drooping). It has a nearly constant-current characteristic and is used for arc welding.

Over-compounded generators are used to supply loads at the end of long feeders, because the rise in voltage makes up for the line drop. Flat-compounded generators supply a nearly constant voltage to loads close to the generator (such as lighting).